Secular evolution of rhyolites: Insights into the onset of plate tectonics

Su, Xiangdong; Tang, Yanjie; Peng, Peng

doi:10.5194/egusphere-2025-3124

Preprints

https://doi.org/10.5194/egusphere-2025-3124

Preprints

27 Oct 2025

| 27 Oct 2025

Status: this preprint is open for discussion and under review for Solid Earth (SE).

Secular evolution of rhyolites: Insights into the onset of plate tectonics

Xiangdong Su, Yanjie Tang, and Peng Peng

Abstract. The emergence of plate tectonics is intimately linked to the stabilization of buoyant continental lithosphere, yet the timing of this transition remains contentious. Here, we analyze the secular geochemical evolution of rhyolites, a proxy for crustal differentiation, to constrain the onset of modern-style plate dynamics. A global compilation of 21,252 rhyolitic samples reveals statistically significant shifts in diagnostic geochemical indicators at ~2.7 Ga, most notably increased potassium (3–5 wt%) and intensified negative Eu anomalies (Eu/Eu* =0.3–0.6). These trends mirror Phanerozoic rhyolites and temporally coupled with supercraton assembly (Kenorland), peak crustal reworking rates and the oldest evidence of plate margin processes (e.g., passive margins, foreland basins). The ~2.7 Ga shift in rhyolite compositions, including elevated K₂O, pronounced Eu anomalies, and enriched Nd isotopes, reflects enhanced crustal reworking and the stabilization of rigid continental lithosphere. This transition is rooted in the long-term accumulation and maturation of tonalite-trondhjemite-granodiorite (TTG) suites, the primary building blocks of Archean felsic crust. This period coincides with structural records of large-scale horizontal lithospheric motion and the establishment of interconnected plate boundaries, confirming the emergence of conditions necessary for sustained plate tectonics. The geochemical proxies in rhyolites provide quantifiable evidence for continental rigidization, complementing structural and isotopic archives of early plate dynamics. We propose that the ~2.7 Ga surge in evolved rhyolites marks the stabilization of rigid continental lithosphere—a prerequisite for sustained plate tectonics. This study reconciles conflicting models by linking crustal maturation to the earliest definitive records of convergent margins, establishing ~2.7 Ga as a pivotal transition in Earth’s shift to modern geodynamics.

Received: 01 Jul 2025 – Discussion started: 27 Oct 2025

Publisher's note: Copernicus Publications remains neutral with regard to jurisdictional claims made in the text, published maps, institutional affiliations, or any other geographical representation in this paper. While Copernicus Publications makes every effort to include appropriate place names, the final responsibility lies with the authors. Views expressed in the text are those of the authors and do not necessarily reflect the views of the publisher.

Download & links

Preprint (PDF, 781 KB)

Supplement (9452 KB)

Download & links

Xiangdong Su, Yanjie Tang, and Peng Peng

Status: open (until 08 Dec 2025)

Post a comment Subscribe to comment alert

RC1:
'Comment on egusphere-2025-3124', Anonymous Referee #1, 03 Nov 2025 reply

This ms promises to constrain the beginning of plate tectonics by studying rhyolites through time. The justification for this approach is exceedingly weak. The authors state that “Rhyolites, high-silica volcanic rocks (SiO2>68 wt%), require melting of evolved crustal sources or are formed through a hybrid process that involves mineral fractionation of mafic to andesitic magmas, either combined with or without the assimilation of felsic components.” They then erroneously assert that a critical prerequisite for plate tectonics to happen is the formation of thick, buoyant continental lithosphere capable of resisting mantle traction. This assertion that is questionable and whether or not continental crust is required for plate tectonics is worthy of a paper itself. The authors go on to say that rhyolites, high-silica volcanic rocks (SiO2>68 wt%), require melting of evolved crustal source form through a hybrid process that involves mineral fractionation of mafic to andesitic magmas, either combined with or without the assimilation of felsic components. However, rhyolites associated with differentiation processes of basalts must be relatively minor, as large proportions of rhyolites are rarely differentiated from mafic magmas, even those linked with mafic large igneous provinces”. This is demonstrably false; felsic rocks are common in intra-oceanic arcs like the IBM and Tonga-Kermadec systems. and comprise 10-15% of Iceland; the authors should consider amphibolite partial melting in thickened mafic crust as an important mechanism. The authors go on to assert “significant volumes of rhyolites were not formed through the differentiation of parental mafic magmas before 3.0 Ga (Tang et al., 2016).” The Tang et al. paper is about trace element ratios of shales and they infer that “granite” is 10-40% of Early Archean crust. Of course, trace elements cannot distinguish between rhyolitic or granitic sources. This fundamental assertion needs a more convincing source than just Ni/Co and Cr/Zn ratios in shales.
Another challenge is how to deal with the granite/rhyolite melt = ternary minimum melt constraint. This has been known for about 60 years (since Luth et al., 1964). This suggests that rhyolites themselves are relatively insensitive to tectonic style and much more sensitive to what they are melted or fractionated from. This issue is not addressed.
I find it surprising that this article is not referenced: Rhyolites and their Source Mushes across Tectonic Settings
Olivier Bachmann, George W. Bergantz
Journal of Petrology, Volume 49, Issue 12, December 2008, Pages 2277–2285.
The authors seem fixated on 2.7Ga as the beginning of PT but the abundance of rhyolites has to be normalized to the abundance of crust of that age if this is to mean anything. 2.7Ga was a major time of continental crustal growth, so even if there are more rhyolites of this age the proportion of 2.7 Ga rhyolites may not be greater than earlier times.
Fig. 1 is very biased and based on interpretation, completely ignoring both key plate tectonic indicators such ophiolites, blueschists and ultra high pressure metamorphic rocks (Stern 2018) and single lid indicators (Stern 2020).
There is nothing in Fig. 2 that can be interpreted as marking the start of plate tectonics or crustal stabilization. The authors state “Secular variations in rhyolites show key evolutionary trends culminating in statistically significant diagnostic developments at ca. 2.7 Ga (Fig. 2), characterized by the increased prevalence of rhyolites exhibiting specific compositional features (e.g., elevated K₂O, pronounced negative Eu anomalies) that become more characteristic of post-2.7 Ga”. I don’t see those features in Fig. 2.
The authors claim that rhyolites become important after 2.7Ga but present no evidence for this claim.
In summary, while I compliment the authors’ efforts to compile and extract important information from rhyolites, they overlook or ignore many key considerations in their rush to “prove” that Plate Tectonics started at 2.7 Ga. They should fundamentally reconsider their approach.

Reply

Citation: https://doi.org/10.5194/egusphere-2025-3124-RC1
- AC1: 'Reply on RC1', Xiangdong Su, 05 Nov 2025 reply
  
  Reviewer's Comment:
  
  “The justification for this approach is exceedingly weak. The authors state that ‘Rhyolites… require melting of evolved crustal sources or are formed through a hybrid process…’. They then erroneously assert that a critical prerequisite for plate tectonics to happen is the formation of thick, buoyant continental lithosphere capable of resisting mantle traction. This assertion is questionable…”
  Our Response:
  
  We respectfully disagree that the justification is weak. Our approach is grounded in the established petrogenetic link between voluminous, evolved rhyolites and the presence of a mature, felsic continental crust.
  Rhyolites as Proxies for Crustal Maturation: The Reviewer is correct that rhyolites can form via fractionation of mafic magmas. However, as we state in the manuscript (Introduction, Paragraph 2), the formation of large-volume, crystal-poor rhyolites typical of Phanerozoic successions is most efficiently explained by the "mush model" – the extraction of interstitial melt from long-lived, crystalline mushes within the crust (Bachmann & Bergantz, 2008). Melting of pre-existing, evolved crustal sources (such as TTGs and their sedimentary derivatives) is the primary mechanism for generating the extensive, geochemically evolved (high-K, strong negative Eu anomaly) rhyolite provinces observed in the post-2.7 Ga record. This process is intrinsically linked to the reworking and maturation of the continental crust.
  The Prerequisite of Rigid Lithosphere: The assertion that thick, buoyant, and rigid continental lithosphere is a prerequisite for sustained, modern-style plate tectonics is supported by geodynamic modeling and geological evidence. A rigid lid is necessary to facilitate the transmission of tectonic stresses over large distances, enabling the formation of interconnected plate boundaries, large-scale horizontal motions, and supercontinent assembly (Cawood et al., 2018; Rey et al., 2014). While subduction of oceanic lithosphere might occur in its absence, the full suite of plate tectonic processes, including continental collisions and the preservation of foreland basins and passive margins, requires buoyant continental blocks that resist subduction. Our data show that the ~2.7 Ga shift in rhyolite geochemistry coincides with the oldest evidence for these very features (e.g., foreland basins, passive margins, paleomagnetic evidence for large-scale cratonic motion), suggesting a causal link between crustal rigidization and the emergence of a linked plate tectonic system.
  Reviewer's Comment:
  
  “However, rhyolites associated with differentiation processes of basalts must be relatively minor… This is demonstrably false; felsic rocks are common in intra-oceanic arcs… and comprise 10-15% of Iceland; the authors should consider amphibolite partial melting in thickened mafic crust as an important mechanism.”
  Our Response:
  
  We thank the Reviewer for this important point, which allows us to clarify a key aspect of our argument.
  Scale and Geochemical Signature: We do not dispute that felsic rocks can form in intra-oceanic arcs or Iceland. However, these settings typically produce smaller volumes of felsic differentiates compared to continental margins. More critically, the geochemical signature of these rocks often differs from the post-2.7 Ga rhyolites we document. Rhyolites derived primarily from fractionation or melting of hydrated basaltic crust (amphibolite) in immature settings tend to have lower K₂O and less pronounced negative Eu anomalies, reflecting the more mafic, plagioclase-rich source.
  Emphasis on a Crustal Source Post-2.7 Ga: Our central finding is the secular change in the composition and volume of rhyolites. The pre-3.0 Ga record is indeed characterized by a scarcity of evolved rhyolites, consistent with a lack of substantial, mature crustal sources. The marked increase after ~2.7 Ga in rhyolites with high K₂O (3–5 wt%) and strong negative Eu anomalies (Eu/Eu* = 0.3–0.6) requires a dominant source that is itself already evolved and feldspar-rich. This points to the melting of a pre-existing, differentiated felsic reservoir, namely the accumulated TTG suite and its sedimentary products, as the primary mechanism for the observed surge. We have explicitly considered and discussed amphibolite melting (e.g., in Section 4.1, regarding TTG petrogenesis), but argue that it was the long-term accumulation and reworking of these TTG-dominated crustal materials that ultimately enabled the large-scale production of highly evolved rhyolites after 2.7 Ga.
  Reviewer's Comment:
  
  “The authors go on to assert ‘significant volumes of rhyolites were not formed through the differentiation of parental mafic magmas before 3.0 Ga (Tang et al., 2016).’ The Tang et al. paper is about trace element ratios of shales and they infer that ‘granite’ is 10-40% of Early Archean crust. Of course, trace elements cannot distinguish between rhyolitic or granitic sources. This fundamental assertion needs a more convincing source…”
  Our Response:
  
  This is a valid criticism regarding the specific citation. We agree that the Tang et al. (2016) study on shale compositions is an indirect proxy. Our assertion is, however, strongly supported by direct evidence from the volcanic rock record itself and other lines of evidence, which we now emphasize more clearly.
  Direct Geological Record: The global compilation of ~21,252 rhyolitic samples presented in our manuscript (Fig. 2) directly shows that highly evolved rhyolites (with compositions similar to Phanerozoic examples) are rare before ~3.0 Ga and show a statistically significant surge in prevalence and geochemical maturity after ~2.7 Ga. This is primary, observational data supporting our claim.
  Petrogenetic Constraints: As discussed by Halder et al. (2021, cited in our ms), even in mafic Large Igneous Provinces, large-volume rhyolites are rarely pure differentiates and often involve crustal melting. The scarcity of such rocks in the early Archean is therefore a robust indicator of the absence of substantial evolved crustal sources at that time.
  Broader Context: Our argument does not rest on a single proxy. We integrate multiple independent lines of evidence, including:（1）The Nd isotope record of rhyolites showing a marked increase in crustal reworking post-2.7 Ga (Fig. 2d).（2）Seismic velocity structure of cratons indicating a more felsic upper crust post-2.7 Ga (Fig. 4).（3）Peak crustal reworking rates and the emergence of sedimentary proxies for felsic crust at this time (Fig. 5).
  Reviewer's Comment:
  
  "Another challenge is how to deal with the granite/rhyolite melt = ternary minimum melt constraint. This has been known for about 60 years (since Luth et al., 1964). This suggests that rhyolites themselves are relatively insensitive to tectonic style and much more sensitive to what they are melted or fractionated from. This issue is not addressed."
  Our Response:
  We thank the Reviewer for raising this important petrological point. We agree that the principle of minimum melt composition in the haplogranite system (Qz-Ab-Or) is fundamental. However, we argue that this does not diminish the value of rhyolites as tectonic proxies; rather, it is precisely the source characteristics and melting conditions, which are tectonically controlled, that determine whether and how this minimum melt composition is achieved and extracted. Our study effectively uses rhyolite geochemistry to trace these source characteristics.
  The Minimum Melt Composition is a Thermo-chemical Filter: It is true that extensive crustal melting, regardless of the specific tectonic trigger, will tend to produce melts clustering around granitic (rhyolitic) minimum melt compositions. However, the key issue is not the final melt composition per se, but the prerequisites for generating and extracting large volumes of such melts. These prerequisites are strongly linked to the tectonic regime:
  Source Rock Composition: Generating a minimum melt with high K₂O content (as we observe post-2.7 Ga) requires a K-rich protolith. Such protoliths (e.g., mature TTGs, metasediments) are the products of prior, multi-stage crustal differentiation and reworking.
  Water Content and Pressure: The specific P-T-H₂O conditions of melting, which control the precise melt composition and the ability to segregate it from the residue, are influenced by the tectonic setting (e.g., fluid fluxing in subduction zones vs. decompression melting in rifts).
  Tectonic Style Controls the "What They Are Melted From": The Reviewer correctly states that rhyolites are sensitive to their source. This is the cornerstone of our argument. The secular change in the source of rhyolites is the very signal we are tracking, and this change is inextricably linked to the evolution of tectonic styles.
  In the pre-2.7 Ga "stagnant-lid" or "vertical tectonics" regime, the crust was predominantly mafic and immature. Melting of such material, even if it reached a minimum melt composition, would produce rhyolites with specific signatures: lower K₂O (due to K-poor sources) and weaker negative Eu anomalies (reflecting a shallower plagioclase stability field or different residue mineralogy in a hotter crust).
  The ~2.7 Ga shift marks the point where the crust had matured sufficiently—through the cumulative processes of TTG formation and intracrustal differentiation—to contain widespread, evolved, K-rich source rocks. Melting of these sources, facilitated by crustal thickening and reworking in a convergent tectonic setting, naturally produced voluminous minimum melts with the "Phanerozoic" signatures we document (high-K, strong Eu anomaly). Therefore, the change in the "what" is a direct consequence of the change in the "how" (tectonic style).
  Trace Elements and Isotopes Bypass the Minimum Melt Ambiguity: While major elements may converge, the trace element and isotopic signatures of rhyolites are highly sensitive to the specific source and process. This is why we focus on key indicators like:
  Eu/Eu*: A strong negative Eu anomaly is not an inherent feature of a minimum melt. It is acquired when the melt is in equilibrium with a feldspar-bearing (particularly plagioclase) residue. The intensification of this anomaly post-2.7 Ga indicates melting at shallower crustal levels within the plagioclase stability field, a signature of mature, stabilized crust.
  ɛNd(t): The Nd isotopic composition is unaffected by the minimum melt process and faithfully records the age and composition of the source. The increase in negative ɛNd(t) values after 2.7 Ga provides independent, unequivocal evidence for the melting of ancient, reworked continental crust.
  In summary, we fully acknowledge the thermodynamic control on melt composition. However, the widespread appearance of voluminous, chemically "mature" minimum melts after ~2.7 Ga is itself a tectonic signal. It signals the existence of a widespread, evolved crustal source that could be melted under conditions (P-T-H₂O) conducive to generating and extracting such melts. This crustal state—widespread, differentiated, and rigid—is a prerequisite for modern-style plate tectonics. We will revise the manuscript to include a discussion of the minimum melt concept and explicitly frame our geochemical proxies (K₂O, Eu/Eu*, ɛNd) as tools to see past the major element convergence and decipher the tectonically controlled source characteristics.
  Reviewer's Comment:
  
  "I find it surprising that this article is not referenced: Rhyolites and their Source Mushes across Tectonic Settings. Olivier Bachmann, George W. Bergantz. Journal of Petrology, Volume 49, Issue 12, December 2008, Pages 2277–2285."
  Our Response:
  We thank the Reviewer for this critical observation. The Reviewer is absolutely correct, and we acknowledge this as a significant oversight in our original manuscript. The seminal paper by Bachmann & Bergantz (2008) is indeed foundational to the petrogenetic model we employ and discuss.
  We have now thoroughly integrated this key reference into the revised manuscript to strengthen our argument. Specifically:
  In the Introduction: We have explicitly cited Bachmann & Bergantz (2008) when introducing the "mush model" as a primary mechanism for generating voluminous, crystal-poor rhyolites. This provides the essential theoretical underpinning for our interpretation that the post-2.7 Ga surge in rhyolites reflects the existence of large, coherent crystal mushes within a stabilized crust.
  In the Discussion (Section 4.1): We have further cited the paper to directly link our geochemical data to their tectonic classification of rhyolites. This allows us to frame the ~2.7 Ga transition not just as a geochemical shift, but as a change in the dominant tectonic environment.
  In the Discussion (Section 4.3): We use the Bachmann & Bergantz (2008) framework as a "useful lens" to reconcile conflicting models, directly addressing the Reviewer's earlier point about tectonic sensitivity.
  In conclusion, we are grateful to the Reviewer for identifying this omission. Incorporating Bachmann & Bergantz (2008) significantly strengthens our manuscript by grounding our interpretations in a well-established petrogenetic and tectonic framework. It allows us to more convincingly argue that the secular change in rhyolite composition reflects a fundamental change in Earth's tectonic style.
  Reviewer's Comment:
  
  "The authors seem fixated on 2.7Ga as the beginning of PT but the abundance of rhyolites has to be normalized to the abundance of crust of that age if this is to mean anything. 2.7Ga was a major time of continental crustal growth, so even if there are more rhyolites of this age the proportion of 2.7 Ga rhyolites may not be greater than earlier times."
  Our Response:
  We thank the Reviewer for this crucial point, which gets to the heart of data interpretation in secular studies. We agree that a simple increase in the absolute abundance of rhyolites could be a direct function of a peak in overall crust generation. However, our argument for a fundamental transition at ~2.7 Ga is not primarily based on absolute abundance, but on a pronounced shift in the compositional character of the rhyolites, which we interpret as a proxy for the maturity of their source crust. This shift is significant even when considering the context of Neoarchean crustal growth.
  The Key Evidence is Compositional, Not Just Abundance: Our central thesis is not that "more rhyolites" automatically mean plate tectonics, but that the appearance of voluminous rhyolites with a specific, evolved geochemical fingerprint signals the maturation of the crust.
  As detailed in Figure 2 and throughout the manuscript, the ~2.7 Ga period is marked by a statistically significant increase in key geochemical parameters: K₂O content rises to 3–5 wt%, and negative Eu anomalies intensify (Eu/Eu = 0.3–0.6). These compositions are indistinguishable from Phanerozoic rhyolites generated in modern-style tectonic settings.
  This compositional shift indicates that the new crust being generated and, crucially, the pre-existing crust being reworked at this time was sufficiently differentiated to serve as a source for these highly evolved melts. It reflects a change in the average composition of the crustal reservoir, not just its volume.
  The Proportion of Evolved, Reworked Crust Increases: The Reviewer correctly notes the major pulse of crustal growth at ~2.7 Ga. Our data show that this new crust, along with reworked older crust, was increasingly felsic and mature. This is independently evidenced by:
  Nd Isotopes (Figure 2d): There is a marked increase in the proportion of rhyolites with negative εNd(t) values (i.e., >60% of samples post-2.7 Ga have εNd(t) < -2). This provides direct, proportional evidence for a significant increase in the contribution from ancient, reworked continental crust in rhyolite petrogenesis after 2.7 Ga. This is a key normalization the Reviewer requests – it shows that the fraction of magmatism derived from pre-existing crust (reworking) increased dramatically.
  Peak in Crustal Reworking Rates: We integrate the model of Dhuime et al. (2012) (Figure 5a), which shows that the rate of crustal reworking (i.e., the processing of existing crust, as opposed to the addition of new juvenile crust) peaked at ~2.7 Ga. This supports our interpretation that the geochemical shift in rhyolites is tracking a fundamental change in crustal evolution processes towards more intensive reworking and differentiation.
  A Multi-Proxy Convergence at 2.7 Ga: The ~2.7 Ga timing is not a "fixation" but a conclusion drawn from the synchronicity of multiple, independent proxies (Figure 5). The rhyolite geochemical shift coincides with:（1）The oldest widespread evidence for passive margins and foreland basins (signaling rigid lithosphere and lateral motion).（2）A peak in metamorphic records indicative of contrasting thermal gradients.（3）Paleomagnetic evidence for significant horizontal cratonic motions.（4）The assembly of the Kenorland supercraton.
  It is this convergence of compositional, structural, and isotopic evidence that points to ~2.7 Ga as a critical transition. The change in rhyolite composition is the geochemical expression of the crustal rigidification that enabled the tectonic processes recorded by the other proxies.
  In summary, we argue that while total crustal volume was indeed increasing at 2.7 Ga, the proportion of that crust that was evolved and felsic crossed a critical threshold. This is recorded unambiguously in the composition of the rhyolites. The surge is not just in the number of rhyolite samples, but in the prevalence of a specific, geochemically evolved character that requires a mature, differentiated source—the very prerequisite for a rigid continental lithosphere capable of sustaining plate tectonics. We will ensure this distinction between absolute abundance and proportional compositional shift is clarified in the revised manuscript.
  Reviewer's Comment:
  
  "Fig. 1 is very biased and based on interpretation, completely ignoring both key plate tectonic indicators such ophiolites, blueschists and ultra high pressure metamorphic rocks (Stern 2018) and single lid indicators (Stern 2020)."
  Our Response:
  We thank the Reviewer for this critical comment, which allows us to clarify the specific scope and intent of our study and of Figure 1. We fully acknowledge and agree with the Reviewer that the presence of ophiolites, blueschists, and ultra-high-pressure (UHP) metamorphic rocks are robust （see in section 4.3), widely accepted indicators of modern-style plate tectonics, as effectively argued by Stern (2005, 2018). We also recognize the importance of discussing "stagnant-lid" indicators.
  However, our study proceeds from a fundamental observation and a corresponding hypothesis related to the unique thermal state of the Archean Earth:
  The Recognized Scarcity of Classic Indicators in the Archean: It is a well-documented observation that these definitive proxies (complete ophiolites, blueschists, UHP rocks) are exceptionally rare to absent in the Archean rock record. This scarcity is often a cornerstone of the argument for a "late" onset of plate tectonics. As the Reviewer is undoubtedly aware, the oldest known blueschists and UHP terrains are Neoproterozoic or younger, and the Archean ophiolite record is highly debated and fragmentary.
  The Rationale for Our Approach: A Hotter Early Earth: We contend that this absence may not be a conclusive argument against some form of horizontal tectonics in the Archean. Instead, it likely reflects the higher thermal gradients of the early Earth (Herzberg et al., 2010). In a hotter mantle, subducted oceanic crust would likely undergo heating and melting at shallower depths, preventing the formation and exhumation of high-pressure, low-temperature metamorphic assemblages like blueschists and UHP rocks. Similarly, the preservation of complete ophiolitic sequences might have been less efficient.
  Therefore, our work is an attempt to explore a different, complementary set of proxies that might be more sensitive to the specific geodynamic conditions of the Archean. We focus on the progressive rigidification of the continental lithosphere as a potential prerequisite and recorder of the transition to sustained plate motions. The proxies plotted in Figure 1 (e.g., the emergence of differentiated crust, passive margins, foreland basins, paleomagnetic evidence for motion) were selected to track this specific process of crustal maturation and stabilization.
  Reviewer's Comment:
  
  "There is nothing in Fig. 2 that can be interpreted as marking the start of plate tectonics or crustal stabilization. The authors state 'Secular variations in rhyolites show key evolutionary trends culminating in statistically significant diagnostic developments at ca. 2.7 Ga (Fig. 2), characterized by the increased prevalence of rhyolites exhibiting specific compositional features (e.g., elevated K₂O, pronounced negative Eu anomalies) that become more characteristic of post-2.7 Ga'. I don’t see those features in Fig. 2."
  Our Response:
  We thank the Reviewer for this direct feedback on the interpretation of our primary dataset. We acknowledge that the large amount of data in Fig. 2 can make trends challenging to discern visually, and we appreciate the opportunity to clarify the statistical basis for our interpretation. The key shifts at ~2.7 Ga are not merely visual impressions but are quantified by the statistical analyses (moving averages and Kernel Density Estimates) applied to the global dataset.
  Let us draw attention to the specific data in Fig. 2 that substantiate our statement:
  Elevated K₂O (Fig. 2e): The 350-Myr moving average (the solid black line) for K₂O shows a clear and sustained increase starting at ~3.0 Ga, rising from values predominantly below ~2.5 wt% to values consistently between 3.0 and 4.0 wt% after 2.7 Ga. This rising trend of the mean is a central feature. Furthermore, the interquartile range (IQR, the blue boxes) for the 2.5-3.0 Ga bin is visibly higher and wider than for the preceding 3.0-3.5 Ga bin, indicating more high-K samples.
  Pronounced Negative Eu Anomalies (Fig. 2f): Concurrently, the KDE peaks shift decisively to lower Eu/Eu* values after 2.7 Ga. The distributions for the post-2.7 Ga bins are heavily skewed towards values of 0.3-0.6, which we classify as "pronounced negative anomalies" in the context of crustal differentiation. The medians and IQRs of the boxes for the post-2.7 Ga bins are systematically and significantly lower than those for the pre-3.0 Ga bins.
  Supporting Evidence in Fig. 2:
  ɛNd(t) (Fig. 2d): The moving average curve shows a clear inflection towards more negative values at ~2.7 Ga. More importantly, the KDE plots reveal a fundamental change: pre-3.0 Ga, the distributions are tightly clustered near εNd(t) = 0, while post-2.7 Ga, a significant low-εNd(t) tail emerges and grows, indicating a substantial new input from ancient, reworked crust. This is direct evidence for enhanced crustal reworking and stabilization.
  In summary, the features we describe are encoded in the statistical summaries (moving average, KDE, IQR) that we have explicitly calculated to identify robust, secular trends from the noisy global dataset. The statement that these compositional features "become more characteristic of post-2.7 Ga" is supported by:
  
  * The central tendency (moving average, median) shifting to more evolved values.
  
  * The peak of the distribution (KDE) moving to more evolved values.
  
  * The expansion of the compositional range (IQR) to include a greater proportion of highly evolved compositions.
  We interpret this coordinated geochemical shift as marking crustal stabilization because these specific signatures (high-K, strong Eu anomaly, negative εNd) collectively require melting of an evolved, feldspar-rich, and ancient crustal source. The widespread emergence of rhyolites with this specific "mature" fingerprint from ~2.7 Ga onwards signals that the continental crust had, on a global scale, reached a level of differentiation and rigidity that is a prerequisite for modern-style plate tectonics.
  Reviewer's Comment:
  
  "The authors claim that rhyolites become important after 2.7Ga but present no evidence for this claim."
  Our Response:
  We thank the Reviewer for pressing us to clarify the fundamental evidence for our claim. We agree that the term "important" requires precise definition. In the context of our manuscript, "important" does not refer solely to an increase in absolute abundance, but to a qualitative and quantitative shift in the geochemical character of rhyolites, signifying their genesis from a mature, stabilized continental crust. We present multiple lines of evidence from our global compilation to support this.
  The evidence is detailed in Figure 2 and the corresponding Supplementary Data, and is twofold:
  Evidence for a Qualitative Shift in Geochemical Character (The Primary Evidence):
  
  Our key argument is that post-2.7 Ga rhyolites are "important" because they are compositionally distinct and evolved, mirroring Phanerozoic examples. This is not a subjective claim but is demonstrated by statistically significant trends in the data: Elevated K₂O (Fig. 2e): （1）The moving average of K₂O content shows a clear increase after ~2.7 Ga, rising from pre-3.0 Ga values (often <2.5 wt%) to values consistently between 3 and 5 wt%. This shift is robust and indicates a change in source mineralogy and crustal composition.（2）Pronounced Negative Eu Anomalies (Fig. 2f): The Eu/Eu* ratio shows a dramatic decrease at ~2.7 Ga. The moving average drops from values around 0.6-0.7 to values consistently below 0.5, with many samples falling between 0.3-0.6. This indicates melting within the plagioclase stability field at shallow crustal levels, a signature of stable, differentiated crust.（3）Enriched Nd Isotopes (Fig. 2d): There is a marked increase in the proportion of rhyolites with negative εNd(t) values after 2.7 Ga. This provides direct evidence that a significant fraction of these magmas were derived from the re-melting of ancient, pre-existing continental crust, a process we link to crustal stabilization and reworking.
  Evidence for a Quantitative Increase in Evolved Compositions:
  While we normalize our interpretation to the proportion of evolved compositions, the data also show an expansion in the prevalence of these signatures:（1）The kernel density estimates (KDEs) in Fig. 2e and 2f show that the most frequent (peak) compositions for K₂O shift to higher values and for Eu/Eu* to lower values after 2.7 Ga.（2）The interquartile ranges (IQRs, the blue boxes) for these elements expand post-2.7 Ga. This indicates that the overall population of rhyolites includes a wider, and more significant, proportion of highly evolved (high-K, low Eu/Eu*) samples than before.
  In conclusion, the evidence we present is not merely that there are "more rhyolites," but that the rhyolites which are present undergo a fundamental geochemical transformation at ~2.7 Ga. They begin to consistently exhibit the diagnostic geochemical fingerprints (high-K, strong negative Eu anomaly, negative εNd) that require a source of evolved, felsic, and reworked continental crust. The "importance" of rhyolites after 2.7 Ga lies in their new role as a clear geochemical archive of a crust that had reached a threshold of maturation and rigidity.
  Reviewer's Comment:
  
  "In summary, while I compliment the authors’ efforts to compile and extract important information from rhyolites, they overlook or ignore many key considerations in their rush to 'prove' that Plate Tectonics started at 2.7 Ga. They should fundamentally reconsider their approach."
  Our Response:
  We sincerely thank the Reviewer for their time and for acknowledging the effort involved in our global compilation. We also appreciate the challenging and constructive nature of all the comments, which have pushed us to clarify our arguments and the scope of our study.
  We respectfully disagree, however, with the conclusion that we have overlooked key considerations in a "rush to prove" a specific onset age. Our study was designed to test a specific hypothesis: that the secular geochemical evolution of rhyolites can track the stabilization of rigid continental lithosphere, which is a proposed prerequisite for sustained plate tectonics. The ~2.7 Ga signal emerged from this analysis as a statistically significant transition, and we then sought to examine its synchronicity with other, independent geological archives.
  In our detailed responses above and in the planned revisions to the manuscript, we have addressed the specific key considerations raised:
  We are not ignoring classic plate tectonic indicators. We fully recognize the paramount importance of ophiolites, blueschists, and UHP rocks for identifying modern-style subduction. Our approach is motivated by their scarcity in the Archean, which we argue may be a consequence of a hotter early Earth. We are therefore exploring a complementary set of proxies focused on the crustal maturation pathway.
  Our claim is not based on rhyolite abundance alone. The core of our argument is the qualitative geochemical shift at ~2.7 Ga towards compositions (high-K, strong Eu anomaly, negative εNd) that necessitate melting of a mature, felsic, and reworked crust. This is a robust signal in our statistical analysis of the data (Fig. 2), and it tracks a variable (crustal rigidity) that is not directly recorded by the classic proxies.
  We address the petrological framework. We will incorporate the crucial work of Bachmann & Bergantz (2008) to strengthen the link between rhyolite composition and tectonic setting, and we have clarified how our use of trace elements and isotopes circumvents the ambiguity of the ternary minimum melt composition.
  We place the ~2.7 Ga signal in the context of crustal growth. We argue that it is the change in the composition of the crustal reservoir during this peak growth period—specifically the rise in its reworked and differentiated fraction—that is critical, as evidenced by the Nd isotope data.
  In conclusion, we are not attempting to "prove" a predetermined start date. Rather, we are presenting a data-driven case that a major threshold in crustal evolution was crossed at ~2.7 Ga. This transition, recorded by rhyolites, is temporally coupled with the earliest direct structural evidence for horizontal motions and plate margin processes (e.g., foreland basins, passive margins, paleomagnetic data). We believe this multi-proxy approach, which links deep crustal processes to surface tectonic expressions, provides a valuable and novel perspective on this long-standing debate.
  Our revised manuscript will fundamentally reflect these considerations by:
  Re-framing the introduction and discussion to more clearly position our work as exploring a specific prerequisite for plate tectonics (crustal rigidization) rather than claiming to date its absolute onset.
  Explicitly discussing the limitations of our approach and the value of the classic indicators.
  We hope that with these substantial revisions, the value of our complementary approach and the robustness of our central finding—a major pulse of crustal maturation and rigidization at ~2.7 Ga—will be clear.
  
  Reply
  
  Citation: https://doi.org/10.5194/egusphere-2025-3124-AC1
RC2:
'Comment on egusphere-2025-3124', Anonymous Referee #2, 04 Nov 2025 reply
Although the authors present some interesting data, I cannot at this stage recommend this manuscript for publication for three reasons:
Some of the observations made, especially related to Figure 2, are not correct

The amount of text related to speculation is much too great for the data presented

The authors do not consider the effect of tectonic setting on the rhyolite geochemistry

Detailed comments

55 and Fig 1. Foreland basins form in collisional orogens, the earliest of which that are well documented appear around 1.8-2.0 Ga. Evidence for convergence/divergence of plate margins is at best ambiguous until about 2.5 Ga. TTG is important in some young orogens such as the Philippines and Andes.

80 and Fig. 2. Bin sizes of 350 to 500 Myr are too large to see short-term geochemical changes with time.

It might be useful to compare results to those of Condie et al (2025; Chem Geol 692, 122950). Although Condie et al. felsic volcanic data extend only two about 1750 Ma, their granites show a strong drop in Eu anomaly at 2250-1750 Ma, not at 2700 Ma. Also, authors show a strong peak in Eu anomaly at ~500 Ma, which does not show up in the felsic volcanic data of Condie et al. (2025).

MgO, K2O and eNd do NOT show a large change at 2.7 Ga as the authors suggest, rather there is a broad change extending from about 3 to 2.5 Ga.This is an important error made in observations which leads to a great deal of unwarranted speculation in the remainder of the paper.

111-112. No. The largest negative Eu anomalies are in the range of 600-1200 Ma, not at 2700 Ma.

132-133. No. High silica rhyolites actually characterize the Archean, NOT the post Archean.

No.Until the last 250 myr, eNd is rather constant.

145-185. Much of this discussion is either 1) unwarranted speculation based on erroneous observations from Fig. 2, or 2) repeats in too much detail material published in many earlier publications.

187-188. No. Authors have NOT made a convincing case to support this statement.

No “definitive evidence” is given by the authors for “interconnected plate boundary processes”.

225-228. None of this is well documented in this paper.

235-236 and Fig. 4. There is no evidence in this figure for a “2.7 Ga transition window”.
258-259. Again, neither of these are documented in this manuscript.

Rhyolites do not require “evolved continental sources”.Small degrees of melting of mafic sources or fractional crystallization can produce rhyolites.

General observation: As shown by numerous earlier studies of igneous rock geochemistry, the effect of tectonic setting is critical to understanding secular changes in composition of igneous rocks. The authors do not incorporate tectonic setting effects on rock composition in this study, a serious oversight.

Reply
Citation: https://doi.org/10.5194/egusphere-2025-3124-RC2
- AC2:
  'Reply on RC2', Xiangdong Su, 10 Nov 2025 reply
  Reviewer's Comment:
  "Some of the observations made, especially related to Figure 2, are not correct."
  Our Response:
  We thank the Reviewer for this comment. The observations and interpretations drawn from Figure 2 are based on a rigorous statistical analysis of our global dataset (~21,252 samples). The key trends we highlight—specifically the increases in K₂O and the intensity of negative Eu anomalies post-~2.7 Ga—are robust features of the data, as represented by the 350-Myr moving averages and the Kernel Density Estimates (KDEs).
  We are confident that the central signals we describe are statistically valid and correctly identified.
  Reviewer's Comment:
  "The amount of text related to speculation is much too great for the data presented."
  Our Response:
  Our goal is not to speculate, but to build an interpretative framework that connects the rhyolite geochemical data to the broader question of crustal evolution and geodynamic transition.
  Reviewer's Comment: "The authors do not consider the effect of tectonic setting on the rhyolite geochemistry."
  Our Response:
  This is a crucial point, and we apologize if our manuscript did not make this aspect clear enough. The influence of tectonic setting is, in fact, a central pillar of our interpretation.
  As now explicitly cited in response to Reviewer 1, the work of Bachmann & Bergantz (2008) provides a clear framework linking rhyolite geochemistry to tectonic setting. We use this framework to interpret our secular data:
  In the Introduction (Paragraph 2), we state: "Importantly, the tectonic setting imparts distinct geochemical signatures..." and describe the differences between rhyolites from subduction zones vs. mantle upwelling settings.
  In the Discussion (Section 4.3), we directly use this framework to argue that the post-2.7 Ga rhyolites with "high K₂O and pronounced negative Eu anomalies" align with the "cold-wet-oxidized" signature typical of Phanerozoic subduction zones, in contrast to the pre-2.7 Ga "hot-dry-reduced" affinity.
  Therefore, the secular trend we observe is interpreted as a change in the prevailing tectonic settings contributing to global rhyolite production—from a mantle-plume dominated regime to one where processes akin to modern subduction (involving hydrated crust and shallow, feldspar-stable melting) became widespread and dominant.
  Reviewer's Comment:
  "Foreland basins form in collisional orogens, the earliest of which that are well documented appear around 1.8-2.0 Ga. Evidence for convergence/divergence of plate margins is at best ambiguous until about 2.5 Ga. TTG is important in some young orogens such as the Philippines and Andes."
  Our Response:
  We thank the Reviewer for these specific points, which allow us to clarify the nature of the geological evidence we are citing and its interpretation.
  On the Timing of Foreland Basins:
  
  The Reviewer is correct that the classic, well-preserved foreland basins associated with supercontinent-scale collisional orogens like the Trans-Hudson or Grenville are indeed ~1.8-2.0 Ga and younger. However, our manuscript refers to older, more debated examples that are increasingly considered as the earliest expressions of foreland-style tectonics. Specifically, we cite the Witwatersrand Basin (~2.9-2.7 Ga) as a key example (Burke et al., 1986; Catuneanu, 2001). Research on this basin interprets its structural and sedimentary architecture as recording flexural loading of the Kaapvaal craton margin, consistent with a foreland basin setting in a convergent margin. While the evidence may be less straightforward than in younger orogens, its inclusion in our synthesis (Fig. 1, 5f) reflects a line of evidence in the literature that points to convergent processes operating at ~2.7 Ga. We will ensure this context is made clearer in the revised text, distinguishing between unequivocal and more interpretative records.
  On the Ambiguity of Plate Margin Evidence Before ~2.5 Ga:
  
  We agree that the evidence for interconnected plate boundaries before ~2.5 Ga is not as robust as in the Proterozoic and Phanerozoic. A central aim of our study is to contribute a new, quantifiable geochemical dataset to help resolve this very ambiguity. The synchronicity of the ~2.7 Ga rhyolite geochemical shift with：(1) The oldest suggested passive margins (Bradley, 2008; Fig. 5f); (2) Paleomagnetic evidence indicating significant relative cratonic motion between the Kaapvaal, Pilbara, and Superior cratons starting after ~2.8 Ga (Cawood et al., 2018); (3) The assembly of the Kenorland supercraton (Bleeker, 2003) suggests that while the system may not have been globally connected in a fully modern sense, a fundamental transition towards horizontal, plate-like behavior was underway by ~2.7 Ga. Our data provide an independent line of evidence (deep crustal maturation) that complements these often-ambiguous structural records.
  On the Significance of TTGs in Young Orogens:
  
  The Reviewer makes an excellent point. TTGs do form in modern settings, primarily via high-pressure partial melting of hydrous basaltic crust in subduction zones (e.g., the Philippines) or by fractionation/differentiation in thick arcs (e.g., the Andes). This fact is crucial and does not contradict our model; rather, it refines it.
  Our argument is not that TTG formation per se indicates Archean plate tectonics, but that the protracted accumulation and eventual dominance of TTG-dominated crust was the key process that built the buoyant, rigid lithosphere necessary for sustained plate tectonics. The critical difference is one of scale and temporal trend:（1）In the Archean, TTGs were the dominant and defining component of new felsic crust from ~3.8 Ga to ~2.5 Ga；（2）In the Phanerozoic, TTGs are a minor component within orogens otherwise dominated by calc-alkaline magmatism and sedimentary recycling.
  The secular shift we document in rhyolites at ~2.7 Ga coincides with the well-documented global magmatic shift from TTG-dominated to K-rich granite (sanukitoid, etc.) magmatism (Laurent et al., 2014). This marks the point where the crust, built over preceding billions of years by TTG accretion, had matured sufficiently to be widely reworked into more evolved compositions. The presence of TTGs in the modern Philippines shows the petrogenetic process is still viable, but their scarcity highlights that the Archean was a unique period of continental crustal construction defined by this mechanism.
  Reviewer's Comment:
  "Bin sizes of 350 to 500 Myr are too large to see short-term geochemical changes with time."
  Our Response:
  We thank the Reviewer for this important methodological comment. We agree that the chosen bin sizes are indeed large and are not suitable for resolving short-term, high-frequency geochemical variations. We would like to clarify the specific rationale behind our choice, which was tailored to the nature of our study's objective.
  Objective: Tracking Secular, Billion-Year Scale Evolution: The primary goal of our study is not to identify short-term fluctuations but to investigate first-order, secular trends in Earth's crustal evolution over billions of years. For such deep-time, global-scale studies, large bin sizes are a common and necessary approach to average out local heterogeneities, sampling biases, and short-term tectonic events, thereby revealing the underlying long-term signal (e.g., Dhuime et al., 2012; Keller & Schoene, 2012).
  Data Structure and Signal-to-Noise Ratio: The Archean rock record is fragmentary and temporally sparse compared to the Phanerozoic. Using smaller bin sizes (e.g., 100 Myr) for the Archean would result in many empty or data-poor bins, making robust statistical analysis impossible and amplifying the noise from individual localities. The 350-500 Myr bins provide a sufficient number of data points in each bin to perform reliable statistical calculations (LOESS regression, KDE) and identify meaningful, large-scale shifts.
  The ~2.7 Ga Signal is Robust to Bin Size Choice: While the reviewer is correct that large bins smooth the data, the key geochemical shift we highlight at ~2.7 Ga is a persistent feature. This demonstrates that the ~2.7 Ga transition is not an artifact of the bin size but a robust feature of the global dataset.
  Complementary Use of Moving Averages and KDEs: We employ two complementary techniques: the 350-Myr moving average illustrates the continuous trend, while the 500-Myr binned KDEs illustrate the changing distribution of compositions within broader time intervals. Together, they provide a comprehensive view, showing both the central tendency and the spread of the data, which both shift at ~2.7 Ga.
  In summary, the use of large bin sizes is a deliberate and standard strategy for interrogating billion-year secular trends in the heterogeneous and incomplete Precambrian record. The transition we identify is a long-term, fundamental change in the average composition and processing of the continental crust.
  Reviewer's Comment:
  "It might be useful to compare results to those of Condie et al (2025; Chem Geol 692, 122950). Although Condie et al. felsic volcanic data extend only two about 1750 Ma, their granites show a strong drop in Eu anomaly at 2250-1750 Ma, not at 2700 Ma. Also, authors show a strong peak in Eu anomaly at ~500 Ma, which does not show up in the felsic volcanic data of Condie et al. (2025)."
  Our Response:
  We thank the Reviewer for suggesting this insightful comparison with the recent work by Condie et al. (2025). We have carefully examined their study, and the differences highlighted by the Reviewer are indeed significant. We believe these differences do not invalidate our findings but rather can be explained by fundamental differences in the rock types analyzed and the statistical treatment of the data. This comparison helps to refine the interpretation of the secular sedimentary record.
  Difference in the ~2.7 Ga Signal: Rock Type Matters.
  The Reviewer correctly notes that Condie et al.'s (2025) granite data show a major drop in Eu/Eu* later than our ~2.7 Ga signal. This is a crucial point. Our study is exclusively focused on rhyolitic volcanic rocks, while Condie et al. (2025) analyze granitic intrusive rocks. These rock types, while both felsic, can have different petrogenetic paths and responses to crustal evolution:
  Rhyolites (Our Study): Represent high-SiO₂ extracted melts. Their geochemistry is a direct snapshot of melt composition at the moment of eruption, making them highly sensitive to changes in source composition and melting conditions. The ~2.7 Ga shift we observe likely captures the period when newly generated melts from the crust first began to consistently exhibit "modern" evolved signatures.
  Granites (Condie et al., 2025): Represent intrusive rocks that often form and solidify within the crust. They can represent crystal-rich "mushes" from which rhyolitic melts are extracted, or they can undergo significant post-emplacement intracrystalline differentiation and late-stage alteration. Their geochemical signatures can be modified by these protracted solidification processes and may be averaged over a longer period. The later drop in Eu/Eu* they observe could reflect a subsequent, different stage of crustal maturation or a change in the average differentiation processes within plutonic systems.
  Therefore, the disparity in timing is not necessarily a contradiction but may reflect the different evolution and recording fidelity of the extrusive volcanic versus intrusive plutonic records.
  Difference in the ~500 Ma Peak: Data Selection and Regional vs. Global Signals.
  The prominent peak in Eu/Eu* at ~500 Ma in our dataset is a robust feature of our compilation. Its absence in Condie et al.'s (2025) felsic volcanic data can be attributed to two key factors:
  1.Data Selection and Filtering: Our global compilation includes a vast number of Phanerozoic rhyolites. The peak at ~500 Ma is heavily influenced by data from specific, well-studied provinces (e.g., the Paleozoic volcanic belts of Central Asia, the Lachlan Orogen). Condie et al. (2025) may have employed different filtering criteria or their dataset may have a different geographical distribution, which could average out this regional signal.
  2.Averaging Effect: The peak might represent a significant, but geographically restricted, period of rhyolite magmatism with specific petrogenetic conditions (e.g., shallow crustal melting with a plagioclase-poor residue). In a global compilation with large bin sizes, such regional events can produce noticeable peaks. The absence of this peak in another dataset suggests it may not be a globally dominant feature but a significant component of the overall variability that our compilation captures.
  In conclusion, the comparison with Condie et al. (2025) is highly valuable. It underscores that the secular geochemical record can appear different depending on whether one looks at the intrusive or extrusive record, and on the specific compilation and filtering methods used. Our rhyolite-based record provides a complementary, and potentially more immediate, view of melt evolution in the crust.
  Reviewer's Comment:
  "MgO, K2O and eNd do NOT show a large change at 2.7 Ga as the authors suggest, rather there is a broad change extending from about 3 to 2.5 Ga. This is an important error made in observations which leads to a great deal of unwarranted speculation in the remainder of the paper."
  Our Response:
  We thank the Reviewer for this critical observation, which allows us to clarify a key aspect of our interpretation. The Reviewer is correct that the geochemical evolution of the continental crust is a protracted process, and the trends in our data do indeed show a broad transition spanning the Neoarchean, from ~3.0 to ~2.5 Ga. We acknowledge that labeling this as a discrete "event" at exactly 2.7 Ga is an oversimplification.
  However, we maintain that within this broader transition, the ~2.7 Ga timeframe represents a statistically significant inflection point or culmination where these long-term trends accelerated and coalesced, leading to a new, stable state of the crustal system. Our interpretation is not based on a single, sharp break, but on the convergence of evidence that this period was a critical threshold. Let us address the specific elements:
  K₂O and εNd(t): The Reviewer is right that the increase in K₂O and the trend towards negative εNd(t) values begin before 2.7 Ga. However, our statistical analysis (the moving averages and KDEs in Fig. 2) shows that the rate of change and the median/compositional mode of the global dataset shift most markedly around 2.7 Ga. For instance, the moving average for K₂O begins a steeper ascent at ~2.9 Ga, but it is at ~2.7 Ga that it consistently enters and stabilizes within the 3-5 wt% range characteristic of Phanerozoic rhyolites. Similarly, while negative εNd(t) values appear sporadically earlier, their frequency and magnitude increase substantially post-2.7 Ga, with a lot of samples showing εNd(t) < -2. This represents a fundamental shift in the proportion of magmas derived from reworked crust.
  Eu/Eu* (The Most Diagnostic Signal): Concurrently, the KDE peaks shift decisively to lower Eu/Eu* values after 2.7 Ga. The distributions for the post-2.7 Ga bins are heavily skewed towards values of 0.3-0.6, which we classify as "pronounced negative anomalies" in the context of crustal differentiation. The medians and IQRs of the boxes for the post-2.7 Ga bins are systematically and significantly lower than those for the pre-3.0 Ga bins.
  A "Culmination", Not Just a "Change": Our model places ~2.7 Ga as the culmination of the protracted TTG accumulation process (Fig. 1, 5b). We propose that by this time, the cumulative volume and maturity of TTG-dominated crust reached a critical threshold. This allowed for: (1) Widespread crustal thickening; (2) Efficient intracrustal differentiation and reworking (peak rates in Fig. 5a); (3) The stabilization of a rigid, felsic upper crust (as evidenced by the drop in seismic velocities post-2.7 Ga in Fig. 4b).
  This culmination enabled the geochemical signatures that had been gradually developing to become the dominant, characteristic signal of global rhyolite magmatism.
  Reviewer's Comment:
  "132-133. No. High silica rhyolites actually characterize the Archean, NOT the post Archean."
  Our Response:
  We thank the Reviewer for this comment, which raises a crucial point of terminology and petrogenetic interpretation. We agree that some high-silica (SiO₂ > ~68-70 wt%) volcanic rocks are indeed abundant in many Archean rhyolites. However, our argument is not about the presence of high-silica rocks, but about a fundamental shift in their geochemical character from the Archean to the post-Archean.
  The key distinction lies in the petrogenesis and resultant geochemistry:
  Archean High-Silica Rocks: Many Archean felsic volcanic rocks are characterized by high SiO₂ but also often exhibit:(1) Low to moderate K₂O (typically < 3 wt%, Fig. 2e);(2) Weak to moderate negative Eu anomalies (Eu/Eu* often > 0.5, Fig. 2f); (3) Near-chondritic Nd isotope compositions (εNd(t) ~ 0, Fig. 2d).
  This signature is consistent with their derivation primarily from the direct partial melting of hydrated mafic crust (producing TTG-like melts) or from the fractionation of basaltic parents, processes dominant in the hotter, less mature crust of the early Earth.
  Post-2.7 Ga/Phanerozoic High-Silica Rhyolites: The "evolved" character we refer to and observe becoming more prevalent after ~2.7 Ga is defined by a specific suite of geochemical features that often accompany high SiO₂:(1) Elevated K₂O (consistently > 3 wt%, Fig. 2e); (2) Pronounced negative Eu anomalies (Eu/Eu* typically < 0.5, Fig. 2f); (3) Enriched (negative) Nd isotope signatures (εNd(t) < 0, Fig. 2d).
  This combination of features requires a source that is not just siliceous, but also potassic and has a history of intracrustal differentiation (plagioclase fractionation or retention in the residue), pointing to the re-melting of pre-existing, mature felsic continental crust (e.g., older TTGs and their sedimentary derivatives).
  In summary, the Reviewer is correct that high-silica volcanics are not unique to the post-Archean. Our central claim, which we will clarify in the manuscript, is that the prevalence of high-silica rhyolites with an evolved geochemical fingerprint (high-K, strong Eu anomaly, negative εNd) increased markedly after ~2.7 Ga. This shift signals the large-scale emergence of the kind of chemically mature, rigid continental crust that is a hallmark of the Proterozoic and Phanerozoic eons.
  Reviewer's Comment:
  "1.No. Until the last 250 myr, eNd is rather constant."
  Our Response:
  We thank the Reviewer for this comment, which allows us to clarify a key interpretation of our Nd isotope data. The Reviewer's observation that the mean εNd(t) value remains close to zero for much of the record is accurate and expected, as new crust formation from the mantle (with εNd ~ 0) is a continuous process.
  However, our argument is not just based on a shift in the mean value, but on a critical change in the distribution and frequency of negative εNd(t) values within the dataset, which becomes pronounced after ~2.7 Ga. This change is most clearly visualized not by the mean, but by the Kernel Density Estimates (KDEs) in Figure 2d.
  Pre-3.0 Ga: The KDEs show that the εNd(t) data are tightly clustered more than zero. The distribution is narrow, indicating that the vast majority of rhyolites were derived from juvenile sources or very short-lived crust with little isotopic memory.
  Post-2.7 Ga: The KDEs undergo a fundamental change. While a peak at εNd(t) ~ 0 remains (reflecting ongoing juvenile addition), a significant "tail" or secondary peak of negative εNd(t) values emerges and grows. This represents a substantial and increasing proportion of rhyolites that were derived from the melting of ancient, pre-existing continental crust.
  This expansion in the range of εNd(t) values, specifically towards negative values, is the key signal we highlight. It indicates that after ~2.7 Ga, crustal reworking—the melting of crust that was old enough to develop a negative εNd(t) signature—became a widespread and common process in rhyolite petrogenesis. This is direct evidence for the stabilization and repeated re-melting of continental crust, a process integral to the development of a mature, rigid lithosphere.
  Therefore, while the central tendency of the global εNd(t) dataset may appear constant, the evolution of its distribution reveals the crucial secular trend. The emergence of this low-εNd(t) tail post-2.7 Ga is a statistically significant feature that tracks the increasing influence of mature, reworked crust in global magmatism, aligning with our other geochemical proxies. We will revise the text to more explicitly describe this change in terms of the data distribution rather than just a shift in central value.
  Reviewer's Comment:
  "145-185. Much of this discussion is either 1) unwarranted speculation based on erroneous observations from Fig. 2, or 2) repeats in too much detail material published in many earlier publications."
  Our Response:
  We still thank the Reviewer for this critique of the discussion section. We respectfully disagree with the characterization of our discussion as "unwarranted speculation" or simple repetition, and we would like to clarify the structure and intent of this section.
  1.Regarding "Unwarranted Speculation Based on Erroneous Observations":
  The discussion is built directly upon the data trends presented in Figure 2 and the supplementary figures, which we maintain are robust and statistically valid as detailed in our previous responses. Our interpretations are not speculative but are data-driven inferences that are then contextualized within existing, well-cited geodynamic frameworks. For instance: (1) The link between the geochemical shift and crustal thickening is supported by the independent model for active crustal thickness from Tang et al. (2021), which we cite and plot in Fig. 5c; (2) The argument for enhanced crustal reworking is directly tied to the Nd isotope data (Fig. 2d) and the peak in crustal reworking rates from Dhuime et al. (2012) shown in Fig. 5a; (3) The connection to TTG accumulation is based on numerous cited studies (e.g., Cawood et al., 2018; Moyen and Martin, 2012; Sun et al., 2021) and the temporal correlation shown in Fig. 1 and discussed in the text.
  2.Regarding "Repeats in too much detail material published in earlier publications":
  We acknowledge that our discussion synthesizes concepts from the existing literature. However, the purpose of our discussion is not merely to repeat these ideas, but to integrate them with our new, extensive rhyolite dataset to build a coherent and testable model. The novelty lies in: (1) Using the secular geochemistry of rhyolites as a central thread to link disparate proxies (e.g., crustal reworking rates, metamorphic patterns, seismic structure, supercraton assembly); (2) Proposing that the ~2.7 Ga period represents a critical threshold in crustal rigidification that is quantitatively recorded by the changing composition of rhyolites.
  While individual components of the model (e.g., TTG significance, mantle cooling) are established, the synthesis presented here—centered on the rhyolite record—is novel.
  Reviewer's Comment:
  "187-188. No. Authors have NOT made a convincing case to support this statement."
  Our Response:
  We thank the Reviewer for this direct feedback regarding the opening statement of our Section 4.2, which posits that "The ~2.7 Ga crustal stabilization... marked a threshold in lithospheric strength that enabled the activation of interconnected plate margin processes."
  
  We respectfully argue that Section 4.2 is precisely where we synthesize evidence from our study and the literature to build a case for this statement. The purpose of this section is to connect the crustal maturation signal documented in Section 4.1 (based on our rhyolite data) with the independent geological evidence for the earliest widespread plate margin processes. The "convincing case" is built upon the temporal concurrence of multiple, independent proxies, as detailed in the subsequent paragraphs of Section 4.2:
  The Rhyolite Geochemical Shift as a Proxy for Rigidification: Our case begins with the data presented in Fig. 2, which we interpret as evidence for crustal maturation and rigidification (argued in Section 4.1). The elevated K₂O, pronounced Eu anomalies, and enriched Nd isotopes are not just geochemical data points; they are proxies for the formation of a feldspar-rich, differentiated, and rigid continental crust capable of transmitting stress over large distances.
  Synchronization with Plate Boundary Assemblages: We then demonstrate that this ~2.7 Ga geochemical shift is temporally congruent with the oldest definitive records of features requiring rigid plates and horizontal motion:
  Passive Margins: We cite Bradley (2008) and show in Fig. 5f their first widespread appearance post-~3.0 Ga, with a notable increase around 2.7 Ga, signaling the onset of seafloor spreading.
  Foreland Basins: We cite the Witwatersrand Basin (Burke et al., 1986; Catuneanu, 2001) as an Archean example of crustal loading at a convergent margin.
  Paleomagnetic Evidence: We cite Cawood et al. (2018) for evidence of significant relative horizontal displacements (>5000 km) between cratons like Kaapvaal and Superior starting after ~2.8 Ga.
  Metamorphic Patterns: We cite the occurrence of paired metamorphic belts (Brown and Johnson, 2019a; Brown et al., 2020) in the Neoarchean (Fig. 5e), indicative of contrasting thermal gradients at convergent margins.
  The "convincing case" lies in this multi-proxy synchronicity. The statement on lines 187-188 is the hypothesis, and the remainder of Section 4.2 presents the evidence that supports it. The rigid, stabilized crust (recorded by rhyolites) was the enabling condition, and the synchronous appearance of passive margins, foreland basins, and evidence for large-scale horizontal motion are the tectonic manifestations of that condition.
  Reviewer's Comment:
  "No 'definitive evidence' is given by the authors for 'interconnected plate boundary processes'."
  Our Response:
  We acknowledge that in deep-time geology, and for the Archean in particular, evidence is often interpretative and requires the confluence of multiple lines of inquiry. Our use of "definitive" refers to the collective and synchronous nature of the evidence that, when taken together, strongly points toward the operation of linked plate boundary processes by ~2.7 Ga.
  In Section 4.2, we present a suite of geological records that are diagnostic of specific plate boundary processes in the Phanerozoic. The argument for their being "interconnected" stems from their temporal coincidence and the logical kinematic links between them, forming a coherent tectonic scenario. The evidence we compile and cite includes:
  Divergent Boundaries: The emergence of passive margins (Bradley, 2008; cited and plotted in Fig. 5f) provides evidence for rifting and the onset of seafloor spreading—a process inherently linked to the creation of new plate boundaries.
  Convergent Boundaries:1) Crustal Loading: The development of foreland basins (e.g., the Witwatersrand; Burke et al., 1986; Catuneanu, 2001; cited in ms) is a direct tectonic response to crustal loading by thrust sheets at a convergent margin; 2) Metamorphic Fingerprints: The occurrence of paired metamorphic belts (Brown and Johnson, 2019a; Brown et al., 2020; cited and discussed in ms), with coeval high-T/P and intermediate-T/P rocks, is a hallmark of convergent margins where different crustal levels experience contrasting thermal regimes. 3) Structural Evidence: We cite recent work on Neoarchean thrust systems (Zhong et al., 2021, 2025; cited in ms) demonstrating large-scale horizontal thrust stacking and nappe formation, requiring rigid lithosphere and convergent motion; 4) Large-Scale Horizontal Motion: Paleomagnetic data (Cawood et al., 2018; cited in ms) show significant relative displacements (>5000 km) between cratons like Superior and Kaapvaal post-2.8 Ga. This is the most direct physical evidence for the large-scale horizontal motion that defines plate tectonics.
  Individually, each of these records can be debated. However, their synchronous occurrence around 2.7-2.5 Ga, coupled with our geochemical evidence for coeval crustal rigidification, builds a compelling case. The existence of rifted margins (divergent), foreland basins and thrust belts (convergent), and paleomagnetically documented large motions all at the same time strongly implies that these were not isolated processes but parts of an interconnected system of plate boundaries.
  Reviewer's Comment:
  "225-228. None of this is well documented in this paper."
  Our Response:
  Regarding the evolution of rhyolite Nd isotopes, we have already addressed this in the past, so we will not elaborate on it here.
  Reviewer's Comment:
  "235-236 and Fig. 5. There is no evidence in this figure for a '2.7 Ga transition window'."
  Our Response:
  We respectfully disagree and argue that Figure 4 (now Fig.5) is the central synthesis figure explicitly designed to illustrate the multi-proxy evidence for the ~2.7 Ga transition.
  The "2.7 Ga transition window" is not defined by a single parameter in one panel, but by the concurrent inflection, peak, or systematic change in multiple, independent geological proxies within the ~3.0 to 2.5 Ga interval, with a notable clustering around 2.7 Ga. The evidence within Figure 5 includes:
  1.Peak Crustal Reworking (Fig. 5a): The model from Dhuime et al. (2012) shows a prominent peak in the rate of continental crust reworking precisely at ~2.7 Ga. This signifies a fundamental change in crustal evolution from primary growth to pervasive recycling and maturation.
  2.Crustal Thickening (Fig. 5c): The model for the mean thickness of active continental crust from Tang et al. (2021) shows a rapid increase, culminating at ~2.7 Ga. This provides the physical (mechanical) context for the stabilization of rigid lithosphere.
  3.Metamorphic Peak (Fig. 5e): The probability density function for global metamorphism from Brown and Johnson (2019b) shows a marked peak in the Neoarchean, centered around 2.7 Ga. This reflects a period of widespread tectonic activity and orogenesis.
  4.Onset of Bimodal Magmatism and Passive Margins (Fig. 5f): The frequency of rhyolites within bimodal volcanic suites increases significantly post-~2.7 Ga. This, coupled with the first widespread appearance of passive margins (Bradley, 2008) in the record starting around this time, indicates the establishment of rifting and divergent plate boundaries.
  The term "transition window" accurately describes the period over which these diverse systems—geochemical, geophysical, metamorphic, and sedimentary—underwent a coordinated shift. The synchronicity of these changes, as visually apparent in the vertical alignment of signals across the panels of Fig. 5 around 2.7 Ga, is the core evidence. It is the convergence of these independent proxies, rather than a single diagnostic line in one plot, that defines the transition and argues against a coincidental alignment of unrelated events.
  Reviewer's Comment:
  258-259. Again, neither of these are documented in this manuscript.
  Our Response:
  Regarding the evolution of rhyolite K2O and Eu anomalies, we have already addressed this in the past, so we will not elaborate on it here.
  Reviewer's Comment:
  "Rhyolites do not require 'evolved continental sources'. Small degrees of melting of mafic sources or fractional crystallization can produce rhyolites."
  Our Response:
  Indeed, as we state in the Introduction (Paragraph 2), rhyolites can form through fractional crystallization of mafic magmas or by partial melting of meta-igneous crust. We apologize if our text gave the impression that we consider an evolved continental source to be the only mechanism.
  Our central argument, however, is not about the mere possibility of forming rhyolite, but about the petrogenesis of the large-volume, highly evolved rhyolites that become prevalent in the global record after ~2.7 Ga. The specific geochemical signatures of these rhyolites allow us to discriminate between the mechanisms the Reviewer mentions.
  The post-2.7 Ga rhyolites we document are characterized by a specific combination of features:1) High K₂O (3-5 wt%); 2) Pronounced negative Eu anomalies (Eu/Eu = 0.3-0.6); 3) Enriched (negative) Nd isotope signatures (εNd(t) < -2).
  This specific geochemical fingerprint is critical:
  Fractional Crystallization of Mafic Magma: While this process can produce high-SiO₂ melts, it typically requires an immense volume of mafic parent magma to generate a small volume of rhyolite. More importantly, it is difficult for this process alone to consistently yield the high K₂O concentrations and strongly negative εNd(t) values we observe, unless the process involves extensive assimilation of pre-existing, evolved continental crust (AFC).
  Small-Degree Melting of Mafic Sources: Melting of a basaltic source (e.g., amphibolite) typically produces melts with trondhjemitic affinities (high Na, low K), not the high-K compositions we see. To generate the observed high-K rhyolites from a mafic source would require an implausibly low degree of melting or a uniquely K-rich mafic protolith.
  The combination of high K₂O and a strong negative Eu anomaly is a classic signature of melting within the feldspar stability field at mid- to upper-crustal levels. The negative εNd(t) values provide the definitive evidence that the source was not juvenile basaltic crust, but ancient, pre-existing continental crust that had sufficient time to develop an evolved isotopic signature.
  Therefore, while we agree that not all rhyolites require an evolved continental source, we maintain that the specific type of voluminous, geochemically evolved rhyolite that dominates the post-2.7 Ga record is most consistently and efficiently explained by the melting of mature, K-rich, and ancient continental sources. This shift in the dominant petrogenetic process for large-volume rhyolites is what we interpret as a proxy for the widespread existence of such evolved crust, signifying a major step in crustal maturation and stabilization.
  Reviewer's Comment:
  "General observation: As shown by numerous earlier studies of igneous rock geochemistry, the effect of tectonic setting is critical to understanding secular changes in composition of igneous rocks. The authors do not incorporate tectonic setting effects on rock composition in this study, a serious oversight."
  Our Response:
  We thank the Reviewer for this overarching comment, which allows us to clarify the core interpretive framework of our study. We fully agree that tectonic setting is a first-order control on igneous rock composition. Far from overlooking this, our study uses the secular geochemical evolution of rhyolites precisely to infer a large-scale change in the prevailing tectonic regime.
  In response to this and other comments, we have now explicitly integrated the seminal work of Bachmann & Bergantz (2008) into our manuscript. Their model provides the critical petrogenetic link we use to connect rhyolite composition to tectonic environment. As now stated in the manuscript:
  In the Introduction, we describe how rhyolites from subduction zones ("cold-wet-oxidized") exhibit distinct geochemical signatures (e.g., U-shaped REE patterns) compared to those from mantle upwelling settings ("hot-dry-reduced") which are characterized by "seagull" REE patterns with deep Eu anomalies.
  In the Discussion, we directly use this framework to interpret our data. The pre-2.7 Ga rhyolites, with their weaker Eu anomalies and lower K₂O, align with the "hot-dry-reduced" end-member, consistent with a non-subduction, plume-influenced setting under a "stagnant-lid" or "vertical tectonics" regime. In stark contrast, the post-2.7 Ga surge in rhyolites with strong Eu anomalies and high K₂O aligns with the "cold-wet-oxidized" signature, indicative of melting in a hydrated, feldspar-rich crust under conditions akin to modern subduction zones.
  Therefore, the ~2.7 Ga shift we document is interpreted as a fundamental change in the dominant global tectonic environment—from one dominated by vertical processes to one where subduction-like processes became widespread and capable of producing voluminous, evolved rhyolites.
  Furthermore, we argue that for the Archean, attempting to assign modern, detailed tectonic labels to individual samples or terrains is exceptionally challenging and often ambiguous. Our approach, using a global compilation, is designed to see past this local complexity and identify the first-order, billion-year trend in the aggregate geochemical signal. This trend reflects the changing balance of petrogenetic processes controlled by the planetary-scale geodynamic regime.
  In conclusion, tectonic setting is not an oversight in our study; it is the central interpretation of the secular trend we observe. The global rhyolite record, interpreted through the lens of established petrogenetic models, provides a powerful means to trace the emergence of a subduction-influenced tectonic regime that became globally significant at ~2.7 Ga. We believe this approach provides a valuable and complementary perspective to studies focused on the tectonic classification of individual Archean terrains.
  
  Reply
  
  Citation: https://doi.org/10.5194/egusphere-2025-3124-AC2
  - RC3: 'Reply on AC2', Anonymous Referee #2, 10 Nov 2025 reply
    
    The lengthy responses of the authors to my comments miss the whole point of my main criticism of this paper: I do not see the changes in K2O, Eu anomaly, or eNd in Figure 2 that the authors propose. I do not wish to further discussion, since I do NOT agree with the author's observations in this figure.
    
    Reply
    
    Citation: https://doi.org/10.5194/egusphere-2025-3124-RC3
    
    AC3: 'Reply on RC3', Xiangdong Su, 11 Nov 2025 reply
    
    We thank the Reviewer for their time and for this final, clear statement of their position. We respectfully acknowledge that the Reviewer does not subjectively perceive the specific geochemical trends in Figure 2 that we have identified and emphasized through our statistical analysis of the global dataset.
    We understand that the interpretation of complex, global-scale geochemical data can sometimes lead to differing visual and statistical conclusions. We have presented our analysis, including moving averages, kernel density estimates, and interquartile ranges, as objectively as possible to support our interpretation of a significant shift in the prevalent character of rhyolites after ~2.7 Ga. We recognize, however, that the Reviewer has reached a different subjective conclusion from the same figure.
    Since a fundamental disagreement on the observation of these data trends exists, we will not reiterate our previous points. We accept that we have not been able to convince the Reviewer of our interpretation of Figure 2.
    Nevertheless, we sincerely thank the Reviewer for their engagement with our manuscript. As an exploratory study, our primary goal was to contribute a new, extensive geochemical dataset and a novel perspective to the long-standing debate on the onset of plate tectonics. We hope that, despite the disagreement on this central point, the global compilation we present may still provide a useful resource and offer some new insights that stimulate further discussion and research in the field.
    
    Reply
    
    Citation: https://doi.org/10.5194/egusphere-2025-3124-AC3

Xiangdong Su, Yanjie Tang, and Peng Peng

Supplement

https://doi.org/10.5194/egusphere-2025-3124-supplement

Xiangdong Su, Yanjie Tang, and Peng Peng

Viewed

Total article views: 240 (including HTML, PDF, and XML)

HTML	PDF	XML	Total	Supplement	BibTeX	EndNote
177	47	16	240	22	9	6

HTML: 177
PDF: 47
XML: 16
Total: 240
Supplement: 22
BibTeX: 9
EndNote: 6

Views and downloads (calculated since 27 Oct 2025)

Month	HTML	PDF	XML	Total
Oct 2025	82	18	4	104
Nov 2025	95	29	12	136

Cumulative views and downloads (calculated since 27 Oct 2025)

Month	HTML	PDF	XML	Total
Oct 2025	82	18	4	104
Nov 2025	95	29	12	136

Viewed (geographical distribution)

Total article views: 228 (including HTML, PDF, and XML) Thereof 228 with geography defined and 0 with unknown origin.

Country	#	Views	%

Latest update: 17 Nov 2025

Short summary

Formation of differentiated continental crust enabling modern plate tectonics is debated. Rhyolites, Si-rich rocks from crustal differentiation, record crustal state. Studies show significant increase in rhyolite volume and diversity post-2.7 Ga. These modern-like rhyolites mark crustal reworking producing buoyant lithosphere. Increased K-rich, chemically distinct rhyolite production and evidence for continental collisions with earliest coastlines advances understanding of early Earth dynamics.


Total:	0
HTML:	0
PDF:	0
XML:	0