Hongjing Xie

The petrographic characteristics and micro‐area composition of altered minerals in pale microgranular masses (PMMs) that were discovered in the chromite mines of northern Xinjiang, China, were studied via laser ablation inductively coupled plasma mass spectrometry (LA‐ICP‐MS). There are six lithotypes of PMMs, namely, rodingitized vitrobasalt, gabbro‐diabase, ultramafite, tuff‐clastic rock, rodingite or skarn, and metasomatite. The texture, composition, and geochemical affinities of a series of altered minerals, such as clinopyroxenes, garnets, serpentines, and chlorites, show that the incompatible elements actively migrate with the Fe, Mg, alkali leaching losses, and Ca aggregates during the course of the alteration process. The geochemical characteristics of the major trace elements in the altered minerals are mostly influenced by the types of protoliths and minerals and have roughly inherited those properties from the protoliths. A marked difference exists between the serpentine geochemical characteristics of the major trace elements in the surrounding peridotites and the PMMs and/or metasomatite. This observation suggests that the two differences are derived from the altered protolith minerals instead of from the fluid metasomatism. Furthermore, the chlorite in the metasomatites inherits the properties of the chlorite component in the PMMs. This study examines the development of PMMs that might undergo two different mechanisms or (at least) two different periods of genesis, i.e., early ultramafic magma xenoliths and metasomatism, as indicated by the rock fabric, and late rodingitization, as indicated by the assemblage of altered minerals and the suite of geochemical affinities. The mechanism by which PMMs develop in peridotites reveals that some podiform chromite in this area may also be xenoliths that formed after chromite was captured by ultramafic magmas in early times.

The Qiarbahete complex in NW China consists of gabbroic diorite, granodiorite, and late-stage quartz diorite porphyry veins. Zircon sensitive high-resolution ion microprobe (SHRIMP) U–Pb analyses show that the gabbroic diorite and granodiorite formed at 368 ± 5.2 Ma and 354 ± 4.1 Ma, respectively, indicating that the complex was emplaced in the Late Devonian–Early Carboniferous. The gabbroic diorites, characteristic of Sanukitoids, exhibit high Mg^# (62 average), MgO (6.84% average), Cr (195 ppm average), and Ni (61.4 ppm average) contents. The rocks show moderately fractionated rare earth element (REEs) patterns and weak negative Eu anomalies (δEu: 0.83–0.89), enrichment of large ion lithophile elements (LILEs), and depletion of high field strength elements (HFSEs), with low ϵ_Nd(t) values (1.46–1.73). The gabbroic diorites originated from partial melting of a hydrous mantle wedge followed by assimilation of crust during ascent. The granodiorites show a geochemical affinity with adakitic rocks, e.g. SiO₂ (64.95–67.87%) > 56%, Al₂O₃ (15.88–16.56%) > 15%, MgO (1.79–2.31%) < 3%, Sr (315–375 ppm) > 300 ppm, and Yb (1.84–2.06 ppm). They are enriched in light rare earth elements (LREEs) and LILEs and depleted in HFSEs, with weak negative Eu anomalies (δEu: 0.78–0.87). The granodiorites were mainly derived by the partial melting of a subducted oceanic slab, followed by subsequent melt–mantle interaction and crustal rocks contamination. All these indicate that the Qiarbahete complex was emplaced in a continental arc setting attending the southward subduction of the Junggar Ocean during the Late Devonian–early Carboniferous, generating the lateral accretion of continental crust in NW Tianshan.