Late Silurian–early Devonian adakitic granodiorite, A-type and I-type granites in NW Junggar, NW China: Partial melting of mafic lower crust and implications for slab roll-back

Abstract

Late Silurian–early Devonian magmatism of the NW Junggar region in the Central Asian Orogenic Belt provides a critical geological record that is important for unraveling regional tectonic history and constraining geodynamic processes. In this study, we report results of Zircon U–Pb ages and systematic geochemical data for late Silurian–early Devonian largely granitic rocks in NW Junggar, aiming to constrain their emplacement ages, origin and geodynamic significance. The magmatism consists of a variety of mafic to felsic intrusions and volcanic rocks, e.g. adakitic granodiorite, K-feldspar granite, syenitic granite, gabbro and rhyrolite. U–Pb zircon ages suggest that the granitoids and gabbros were emplaced in the late Silurian–early Devonian (420–405Ma). Adakitic granodiorites are calc-alkaline, characterized by high Sr (407–532ppm), low Y (12.2–14.7ppm), Yb (1.53–1.77ppm), Cr (mostly <8.00ppm), Co (mostly <11.0ppm) and Ni (mostly <4.10ppm) and relatively high Sr/Y (31–42) ratios, analogous to those of modern adakites. K-feldspar granites and rhyolites are characterized by alkali- and Fe-enriched, with high Zr, Nb and Ga/Al ratios, geochemically similar to those of A-type granites. Syenitic granites show high alkaline (Na2O+K2O=8.39–9.34wt.%) contents, low Fe# values (0.73–0.80) and are weakly peraluminous (A/CNK=1.00–1.07). Gabbros are characterized by low MgO (6.86–7.15wt.%), Mg# (52–53), Cr (124–133ppm) and Ni (84.7–86.6ppm) contents. The geochemical characteristics of the gabbroic samples show affinity to both MORB- and arc-like sources. All granitoids have positive εNd(t) (+3.9 to +6.9) and zircon εHf(t) (+9.8 to +15.2) values and low initial 87Sr/86Sr ratios (0.7035–0.7043), with young TDM(Nd) (605–791Ma) and TDM(Hf) (425–773Ma) ages, suggesting significant addition of juvenile material. The adakitic granodiorites probably resulted from partial melting of mafic lower crust, leaving an amphibolite and garnet residue. The K-feldspar granites, rhyolites and syenitic granites probably formed from partial melting of the Xiemisitai mid-lower crust, while the gabbroic intrusion was probably generated by interactions between asthenospheric and metasomatized lithospheric mantle. Voluminous plutons of various types (adakites, A-type granites, I-type granites, and gabbros) formed during 420–405Ma, and their isotopic data suggest significant additions of juvenile material. We propose that a slab roll-back model can account for the 420–405Ma magmatic “flare up” in NW Junggar as well as an extensional setting.