Petrogenesis and tectonic setting of Early Paleozoic granites and high-Mg diorites in the Northern Qilian Orogen, China

摘要

The Northern Qilian Orogenic Belt (N-QOB) was formed by oceanic subduction and collision between the Alashan and Central Qilian Belt in the Early Paleozoic. New zircon U-Pb ages, Hf isotopic and whole-rock elemental data of the intrusive rocks in the central N-QOB have been conducted to constrain the tectonic evolution of N-QOB. Zircon dating results revealed that the Ladonggou, Suyougou granitic plutons and Xichagou dioritic pluton were emplaced at 505 +/- 3 Ma, 471 +/- 3 M and 425 +/- 3 Ma, respectively. The Ladonggou monzogranite displays depletion of Nb, Ta, Ti, P and negative epsilon(Hf)(t) values (-6.5 to -0.3), and old Hf crustal model ages (1.5-1.9 Ga), suggesting generation from melting of ancient crust above subducted North Qilian oceanic slab. The Suyougou granite possesses geochemistry of I-type granite. The relative enrichment of LILEs, depletion of HFSEs and negative Eu anomalies, as well as prominent Hf isotope heterogeneities and old Hf crustal model ages (2.1-2.5 Ga) suggest a magma source of ancient crust mixed with minor mantle-derived melts in a subduction-related setting. Diorites from the Xichagou plutonare characterized by high Mg# values, enrichments in Cr, Ni, Y, Yb and high Sr/Y ratios, and convincing subduction-related features, suggesting generation from partial melting of subducted sediments mixed with mantle components. Taking all these geochemistry and geochronology together with the regional geology, we interpreted the Ladonggou monzogranite, Suyougou granite and Xichagou diorite were related to the long-term northward subduction of North Qilian oceanic lithosphere during ca. 505 and 425 Ma.