Tectonic evolution of the North Qinling Orogenic Belt, Central China: Insights from metamafic rocks of the Songshugou Complex

Abstract

High‐pressure (HP) and ultrahigh‐pressure (UHP) rocks have been widely recognized in the North Qinling Orogenic Belt (NQOB). The nature and age of their protoliths can provide insights into the formation and evolution of the NQOB. The Songshugou Complex, which crops out along the Shangdan Suture Zone, mainly consists of high‐grade metamorphosed mafic and ultramafic rocks, and thus, this area is an important study site for understanding the tectonic evolution of the NQOB. The metamafic rocks that enclose the ultramafic rocks are overwhelmingly composed of amphibolites that primarily exhibit amphibolite‐facies metamorphism, and minor garnet amphibolites (retrograde eclogite) and HP mafic granulites occur as discontinuous lenses or blocks within these amphibolites. Laser inductively coupled plasma mass spectrometry (LA‐ICPMS) U–Pb dating on zircons yielded ages of 499 ± 6, 507 ± 5, and 504 ± 10 Ma for the HP mafic granulite, garnet amphibolite (retrograde eclogite), and amphibolite, respectively. Those ages were interpreted to represent timing of the HP granulite or amphibolite metamorphism that occurred during later exhumation. Relict omphacites discovered in both garnet and zircon as well as in the matrix of the garnet amphibolite indicate that the rock underwent eclogite‐facies metamorphism and subsequent rapid exhumation. However, the timing of the different metamorphic stages cannot be distinguished from the zircon ages obtained using the LA‐ICPMS technique. The protolith age of the complex is constrained to be the early Neoproterozoic. Geochemical data indicate that the protoliths of the Songshugou metamafic rocks are of continental origin. These Neoproterozoic continental mafic rocks most likely formed during the breakup of the supercontinent Rodinia and were mainly derived from low‐degree partial melting of the asthenosphere with some lithospheric mantle contribution, within an extensional regime. All the available data reveal that the NQOB was one of the components of the Rodinia supercontinent and that it was likely involved in the Grenville orogenic belt and subsequently separated from the supercontinent at approximately 830–740 Ma.