Late Early Cretaceous Extension along the Southeastern Coast of China: Evidence from the Yunqicun Diorites, Northern Fujian

Abstract. Abstract. The timing and geodynamic implications of late Early Cretaceous extension along the southeastern continental margin of China remain debated. To better constrain this issue, we investigate the Yunqicun diorites from northern Fujian using geochronological, geochemical, isotopic, and mineralogical approaches. Zircon U-Pb dating yield an emplacement age of 114.3 ± 2.1 Ma. The Yunqicun diorites exhibit isotopically enriched source signature, reflected by the elevated (⁸⁷Sr/⁸⁶Sr)_i ratios (0.7119–0.7122) and negative ε_Nd(t) values (-8.7 to -8.4). Their high La/Nb (3.35–4.28), Th/Ta (17.41–18.94), and La/Ta (53.47–66.67) ratios suggest modification of the mantle source by subduction-related components. Geochemical mixing trends and Sr-Nd isotopic modeling indicate involvement of ancient crustal components during subsequent magma evolution. The Yunqicun magmas underwent substantial fractional crystallization of plagioclase, amphibole, biotite, and clinopyroxene, as evidenced by low Mg^# values (40.09–40.61), depleted Cr (14.1–20.3 ppm) and Ni (6.37–7.60 ppm) contents, negative Eu anomalies, and whole-rock geochemical trends. Mineral thermobarometric calculations further constrain the crystallization history, revealing polybaric crystallization from deeper levels (~8–13 km) recorded by clinopyroxene to shallower levels (~1–3.5 km) indicated by amphibole and biotite. Zircon and hydrous minerals indicate that the Yunqicun magmas evolved under relatively oxidizing and water-rich conditions. Regional magmatic activity and crustal-thickness variations support coastal extension during ca. 125–105 Ma and demonstrate its persistence in northern Fujian. This extension likely reflects lithospheric reworking associated with Paleo-Pacific plate subduction.