Continental melting and growth through subduction processes: Evidence from the petrogenesis of Paleozoic granitoids and gabbros in the Qinling Orogen

Abstract

The formation and evolution of the continental crust are critical to understanding Earth's geological history. This study investigates the genesis of continental adakitic rocks and the genetic relationships between granitic and gabbroic magmatism in the Lujiaping and Cha-an districts of the North Qinling Terrane, central China. Using integrated whole-rock elemental geochemistry and SrNd isotopes, as well as zircon trace elemental and U-Pb-Lu-Hf isotopic geochemistry, we identify two distinct magmatic stages: Stage I (∼437 Ma) and Stage II (∼390 Ma). Stage I is characterized by the emplacement of the Lujiaping biotite monzogranite, which exhibits adakitic signatures, including high Sr/Y ratios (35.8–48.0), elevated SiO₂ (69.84–71.79 wt%), and low Y (6.10–8.50 ppm) and MgO (0.44–0.59 wt%). These features suggest partial melting of juvenile, mantle-derived lower crust during post-collisional crustal thickening and slab break-off in the Silurian. Stage II involves the emplacement of the Lujiaping gabbro and Cha-an muscovite granite. The gabbro is alkaline, with high MgO (7.31–11.90 wt%) and CaO (10.30–12.80 wt%), indicating derivation from a subduction-modified lithospheric mantle. In contrast, the peraluminous Cha-an granite shows flat rare earth element (REE) patterns, strong negative Eu anomalies, and isotopic evidence for mixing between lower crust and mantle-derived melts. Zircon geothermometry reveals high crystallization temperatures for the biotite monzogranite (∼996 °C) and gabbro (∼994 °C), compared to lower temperatures for the muscovite granite (∼729 °C). These thermal and geochemical signatures indicate that the Lujiaping biotite monzogranite crystallized first (∼437 Ma), followed by gabbroic intrusions (∼390 Ma), which triggered partial melting of the lower crust and the subsequent formation of the muscovite granite. This magmatic evolution reflects a dynamic tectonic setting. Thickening of the continental crust during the early Silurian, related to the collision between the Erlangping and North Qinling terranes, was followed by late Silurian crustal thinning and mantle upwelling. This transition facilitated the injection of mafic magmas and juvenile crust formation. By the Early Devonian, slab rollback and Paleo-Tethys subduction further promoted partial melting of the thickened crust and lithospheric mantle, driving the generation of compositionally diverse magmas. This study highlights the interplay between crustal thickening, thinning, and mantle dynamics in intracontinental orogens, providing new insights into the processes of continental crust formation and evolution.