The origin of mafic microgranular enclaves and their host granodiorites from East Kunlun, Northern Qinghai-Tibet Plateau: implications for magma mixing during subduction of Paleo-Tethyan lithosphere

摘要

Voluminous granodioritic magmatism is recorded in the East Kunlun, Northern Qinghai-Tibet Plateau. Here we present mineralogical, petrological, geochemical and Sr-Nd-Hf isotopic data and zircon U-Pb ages for the Naomuhun pluton and its numerous mafic microgranular enclaves (MMEs). Whole-rock geochemical data and regional geological studies indicate that this pluton consists of subduction-related high-K calc-alkaline metaluminous, I-type granodiorite. The MMEs have plagioclase xenocrysts and disequilibrium textures, such as oscillatory zoning and resorbed rims, indicating magma mixing. Compositions of plagioclase (An₃₀-An₄₉), amphibole (Mg# = 0.62 ~ 0.68), and biotite (Mg# = 0.52–0.56) of MMEs are similar to or very slightly different from equivalent minerals in the host granodiorites, suggesting nearly complete equilibration between the mafic-and felsic magmas. The zircon U-Pb age of the MMEs (263 ± 2 Ma) is identical, within analytical error, to that of the host granodiorites (261 ± 2 Ma). The MMEs have ε_Hf(t) values of −6.83 to −3.15 (average = −4.68), whereas those of the granodiorites range from −9.00 to −3.20 (average −5.63), which is identical within analytical uncertainty. Combined with relatively homogeneous Sr-Nd isotopic compositions, we suggest the MMEs were derived from magma mixing, and their source is similar to an enriched mantle composition. The granodiorites have T_DM2(Hf) model ages ranging from 1.49 to 1.86 Ga, consistent with the Nd model ages (T_DM2), implying that the host magma was derived from Paleo- or Meso-proterozoic rocks, probably the Xiaomiao Group, which forms the basement of East Kunlun. We propose a model for magma formation and magma mixing in a subduction zone environment, in which subduction of an oceanic slab at ca. 260 Ma led to fluid metasomatism, inducing partial melting of an enriched lithospheric mantle to form the voluminous mafic magma. The mafic magma underplated the overlying lower crust, resulting in its partial melting to form felsic magma. The mafic magma then mixed with the felsic magma at lower crustal levels to form the MMEs by convective motion, or forceful injection into the host felsic magma. The MMEs and their host magma were then emplaced at a depth of ca. 12 km, where they crystallized at a temperature of ca. 700–770°C.