A-type granites in northeastern China: age and geochemical constraints on their petrogenesis

Abstract

A-type granites are widely distributed in northeastern China (NE China). They were emplaced during three major episodes (the Permian, late Triassic to early Jurassic, and early Cretaceous) and evolved in different tectonic regimes. According to their mineralogical and geochemical characteristics, two subgroups of A-type granites (aluminous and peralkaline) can be recognized. The peralkaline subgroup contains alkali mafic minerals, such as riebeckite, arfvedsonite and sodic pyroxene, while the aluminous subgroup contains annite and Fe-rich calcic- or sodic-calcic amphibole. With respect to the aluminous subgroup, the peralkaline granites contain higher Rb, Ga and total rare earth elements (REE), but lower MgO, CaO, Al 2O 3, Ba and Sr. Based on the discrimination criteria of Eby [Geology 20 (1992) 641], the Permian and late Triassic to early Jurassic A-type granites belong to the A 2 (post-orogenic) type, whereas the early Cretaceous granites are of the A 1 (anorogenic) type. Nd isotopic compositions of these A-type granites indicate their derivation from a dominantly juvenile crustal source. Their origin is thought to have involved partial melting of an underplated lower crustal source. Because the generation of A-type granites requires high melting temperature, we propose models involving slab break-off, lithospheric delamination and extension. The Permian A-type granites have the same age range as those in eastern Junggar, southern Mongolia and central Inner Mongolia. They occur along a major suture and form a narrow belt between the north China and Siberian Cratons. We suggest that their formation was associated with post-collisional slab break-off. The late Triassic to early Jurassic A-type granites are likely to be the product of lithospheric delamination after the final collision of the major crustal blocks in the late Paleozoic to early Triassic. The early Cretaceous A-type granites have an anorogenic affinity and were possibly associated with rifting in eastern China at this time, associated with the onset of paleo-Pacific subduction. Consequently, we conclude that the A-type granites in NE China were generated at three different times, involving multiple processes operative in different tectonic environments.