Late Carboniferous high εNd(t)– εHf(t) granitoids, enclaves and dikes in western Junggar, NW China: Ridge-subduction-related magmatism and crustal growth

Abstract

We report results of petrologic, geochronological and geochemical investigation of the Late Carboniferous diorites, granodiorites, amphibole (Am)-bearing granites, and associated dioritic and monzonitic enclaves and mafic and granitic dikes in the Keramay area, of the western Junggar region of Central Asian Orogenic Belt (CAOB). Zircon U–Pb dating suggests that they were generated in the Late Carboniferous (316–304 Ma). The diorite and granodiorite compositions extend over a wide range of SiO2 (53–70 wt.%), Sr (240–602 ppm), and Mg# (41–58) values, and are characterized by moderately fractionated rare earth element (REE) patterns, Nb–Ta depletion and relatively low Y and Yb contents. The mafic dikes consist of dolerites, diorite porphyries and minor granodiorite porphyries, and have variable SiO2 (51–59 wt.%) and high Mg#, Cr and Ni values. With the exception of two samples with relatively high heavy REE (HREE) contents, the mafic dikes exhibit trace element characteristics similar to diorites and granodiorites. The Am-bearing granites and a granite porphyry dike sample have high levels of SiO2 (73–77 wt.%), HREEs (e.g., Yb=3.46–15.7 ppm) and low Mg#, Cr and Ni contents, along with clearly negative Eu, Ba and Sr anomalies, similar to typical A-type granites. All granitoids, enclaves and dikes in this region have high positive εd(t) (+7.13 to +9.74) and zircon εHf(t) (+10 to +16) values and moderate initial 87Sr/87Sr ratios (0.7004–0.7049).Mineral composition data suggest that the parental magmas for mafic dikes are similar to Cenozoic sanukitoids in the Setouchi arc area (Japan) and were possibly generated under water-rich and high oxygen fugacity (NNO+1.5 to NNO+2.7) conditions. They most likely originated from partial melting of a mantle source variably modified by subducted oceanic crust-derived melts and minor fluids and subsequently underwent fractional crystallization. The diorites and granodiorites were possibly generated by magma mixing between enriched lithospheric mantle and juvenile lower crust-derived magmas coupled with minor crystal fractionation. The Am-bearing granites and granite porphyry dike were produced by partial melting of juvenile crustal materials at shallow crustal levels. Taking into account widespread contemporaneous magmatism including “MORB-type” basalts and slab-derived adakites in western Junggar, we suggest that the Keramay intrusive rocks were generated in a special arc setting related to ridge subduction and resultant slab window, which played an important role in the crustal growth of the CAOB.