A Devonian Shoshonitic Appinite–Granite Suite in the North Qinling Orogenic Belt: Implications for Partial Melting of a Water-Fluxed Lithospheric Mantle in an Extensional Setting

Abstract

AbstractAppinite–granite suites commonly occur in the final stage of collisional orogenic processes, providing a unique opportunity to reveal the properties of continental lithospheric mantle and crust–mantle geodynamics. In this paper, we present a systematic study of the petrology, mineral chemistry, whole-rock geochemistry and geochronology of the Xiong’erling pluton and adjacent appinite dikes in the northern margin of the North Qinling orogenic belt. The pluton is mainly composed of diorites, quartz monzonites and minor granites. The diorites and appinites have LA-ICP-MS zircon U–Pb ages of c. 389 Ma with variable εHf(t) values of −5.58 to +3.36 and TDM1 model ages peaking at c. 1133 Ma. These rocks belong to the shoshonitic series with high Ba–Sr content and were emplaced in an intraplate extensional environment. The quartz monzonites and granites are oxidized A-type granites and have zircon U–Pb ages of c. 396 and c. 378 Ma with negative εHf(t) values of −11.99 to −1.18 and TDM2 model age peaks at 1434, 1720, and 1888 Ma. The in situ87Sr/86Sr ratios of plagioclase from the mafic and felsic rocks are 0.70565-0.70752 and 0.70695-0.71008, respectively. The Xiong’erling pluton and the coeval appinite dikes represent a Devonian appinite–granite suite in the North Qinling orogenic belt. Asthenospheric upwelling during the intraplate extension triggered partial melting of a phlogopite- and amphibole-bearing garnet lherzolite mantle source that had been previously metasomatized by the subducted oceanic slab, leading to the formation of a primitive hydrous mafic magma. The rapid ascent of the water-rich magma along deep-seated active faults with fast crystallization of amphibole resulted in emplacement of the c. 389 Ma appinite dikes. Replacement of anhydrous minerals by amphibole and biotite via hydrous reaction during magma cooling resulted in the formation of the coeval diorites. The mafic magma underplating triggered episodic remelting of a late Paleoproterozoic to early Mesoproterozoic crustal source, leading to the generation of the c. 396–378 Ma quartz monzonites and granites. Combining our results with existing data, we identify a sequence of (1) northward subduction of the Shangdan oceanic crust beneath the Qinling block at c. 524–438 Ma resulting in island-arc calc-alkaline magmatism, (2) closure of the Shangdan Ocean indicated by collision between the Qinling block and the South Qinling terrane and slab failure magmatism at c. 438–410 Ma, and (3) post-collisional to intraplate extension with alkaline magmatism at c. 410–370 Ma. The Devonian extensive intraplate magmatism marks the end of the Paleozoic orogenesis in the North Qinling belt.