Petrogenesis of the Early Cretaceous adakite-like porphyries and associated basaltic andesites in the eastern Jiangnan orogen, southern China

Abstract

Early Cretaceous quartz porphyries are associated with mantle-derived mafic rocks (basaltic andesites) in the eastern segment of the Neoproterozoic Jiangnan orogen, southern China. Zircons from two quartz porphyries yield identical U–Pb ages at 142±1Ma by the LA–ICP–MS method. Hornblende and augite are the dominant phenocrysts in the basaltic andesites and the groundmass plagioclase has a high anorthite content (∼80), both of which indicate high H2O (∼6wt.%) contents in the magma. The basaltic andesites have low TiO2 (0.68–0.78wt.%) and Nb/La (0.23–0.28), Nb and Ta depletions, high initial 87Sr/86Sr ratios (0.710) and unradiogenic Nd isotopes (εNd(t)=−3.4 to −3.7), implying that they were generated from the partial melting of an isotopically enriched lithospheric mantle source that had been metasomatized by subduction-related fluids. The quartz porphyries (SiO2=65.6–70.5wt.%) show adakite-like geochemical features, with high Sr/Y (18.1–53.4) and La/Yb (18.6–39.4) ratios but low Y (9.44–15.2ppm) and Yb (0.84–1.43ppm) contents. They have low MgO (0.72–1.46wt.%), Ni (1.04–14.7ppm) and Cr (2.34–36.7ppm) contents and lower Nd isotopic ratios (εNd(t)=−6.02 to −7.64) than the basaltic andesites, suggesting that they may have been generated from the partial melting of the lower part of a thickened continental crust, without interaction with the (underlying) lithospheric mantle. The mafic magmas ascended from the metasomatized and hydrated lithospheric mantle and heated the lower part of thickened continental crust, triggering partial melting to form the adakite-like porphyries. The moderate levels of Sr (204–546ppm) and negative Eu anomalies (Eu/Eu*=0.69–0.86) in the porphyries require the presence of minor plagioclase as residual phase in the magma source, indicating the continental crust may be not as thick (>50km) as previously suggested. The new recognition of adakite-like porphyries and associated mafic rocks in the eastern Jiangnan orogen reveals the significant role of Neoproterozoic orogen-related crust and oceanic sediments in the formation of Mesozoic adakite-like rocks of Central-Eastern China.