Late Silurian mafic and felsic magmatism in the South Qilian Belt, northern Tibet Plateau: Response to slab breakoff

Abstract

The North Qaidam Belt (NQDB) is an Early Paleozoic continental ultrahigh-pressure (UHP) metamorphic belt between the Qaidam Block and South Qilian Belt (SQB) in northwest China, and the UHP metamorphic rocks recorded differential/diachronous exhumation and anatexis histories. To constrain the multi-stage exhumation and the associated magmatism of the NQDB and its adjacent areas, an integrated study of petrology, geochemistry, zircon UPb and LuHf isotopes was conducted on the OIB-like dolerite and A-type dacite porphyry from the SQB. The OIB-like dolerite is enriched in light rare-earth elements (LREEs), large ion lithophile elements (LILEs), and high field strength elements (HFSEs), and depleted in heavy rare-earth elements (HREEs), with a crystallization age of 425 ± 7 Ma and zircon εHf(t) values ranging from −4.2 to 0.3. Given that the oceanic/continental deep subduction occurred in this area during Early Paleozoic, the fertile mantle source of OIB-like dolerite was likely formed by the crust-mantle interaction between the oceanic crust and juvenile lithospheric mantle at the instability field of rutile above the subduction zone. The A-type dacite porphyry is enriched in Al, Fe, LILEs and LREEs, depleted in Ca, Mg, Ba, Sr, Eu and HFSEs, and is characterized by high Zr concentrations (231–271 ppm), 10,000*Ga/Al ratios (2.51–2.73) and crystallization temperatures (738–836 °C). Zircon UPb and LuHf isotopic data reveal the dacite porphyry crystalized at 423 ± 5 Ma with zircon εHf(t) values ranging from −8.9 to −3.3. The inherited magmatic zircons gave a formation age of 472 ± 8 Ma with εHf(t) values ranging from −3.9 to −1.9, contemporaneous with the oceanic subduction-related magmatism in the NQDB, indicating the dacite porphyry may be derived from the Early Paleozoic arc. Combined with the previous studies on the UHP metamorphism (ca. 458–420 Ma), retrograde metamorphism (ca. 432–397 Ma), and anatexis (ca. 436–417 Ma) in the NQDB, we conclude that the breakoff of the oceanic lithosphere from the continental lithosphere triggered the thermal perturbation from the asthenosphere upwelling at ca. 425 Ma and the subsequent partial melting of the fertile metasomatized mantle and Early Paleozoic arc, forming the OIB-like dolerite and A-type dacite porphyry in the SQB. Moreover, the coeval mafic and felsic magmatism indicates an important tectonic transformation that the subducted continental crust experienced a localized detachment/exhumation before ca. 425 Ma and entire exhumation after slab breakoff.