The main rock types of the Boluonuo and Daguangding plutons are diorite, quartz diorite, tonalite, granodiorite, and subordi- nate plagioclase-bearing hornblendite and hornblende gabbro. Zircon SHRIMP U-Pb dating for a quartz diorite of the Boluo- nuo pluton suggests that the pluton was emplaced at about 296 ± 4 Ma. Plagioclase-bearing hornblendites show typical cumu- lative textures, which, in combination with their convex-upward REE patterns and the large variation of compatible elements such as Co, V and Sc, suggests that these hornblendites formed through accumulation of hornblende during magma evolution. Microgranular mafic enclaves (MMEs) are common in the Boluonuo and Daguangding intermediate to felsic plutons. Many plagioclase grains show compositional and textural disequilibrium, with calcium-rich cores (An46-50) mantled abruptly by sodium-rich plagioclase (An26-33). Whole-rock samples of the plutons are characterized by quite negative εNd(t) values (-16.5 to -11.8) and εNt(t) values (-22.5 to -16.8), and the εNd(t) values are negatively correlated with silica contents. All these fea- tures suggest that the intermediate to felsic plutons formed through magma mixing of enriched mantle-derived, evolved basal- tic magma with granitic, crustal melts, followed by fractional crystallization of mainly hornblende and small amounts of py- roxene, apatite and zircon. The hornblende-dominated fractionation contributed significantly to the adakite-like features of the intermediate to felsic plutons, like the high Sr and Sr/Y ratios and low Yb abundance. In addition, the Boluonuo and Daguang- ding plutons are highly enriched in LILEs (e.g., Ba and Sr), but depleted in HFSEs (e.g., Nb, Zr and Ti), which is typical of arc magmas. Therefore, the formation of Boluonuo and Daguangding plutons was probably related to the subduction of the Paleo-Asian oceanic slab beneath the North China Craton in the Paleozoic.
Metapelite is one of the predominant rock types in the high-pressure–ultrahigh-pressure(HP–UHP) metamorphic belt of western Tianshan, NW China; however, the spatial and temporal variations of this belt during metamorphism are poorly understood. In this study, we present comparative petrological studies and 40^Ar/39 ^Ar geochronology of HP and UHP pelitic schist exposed along the Habutengsu valley. The schist mainly comprises quartz, white mica, garnet, albite and bluish amphibole. In the Mn O–Na2O–Ca O–K2O–Fe O–Mg O–Al2O3–Si O2–H2O(Mn NCKFMASH) system, P–T pseudosections were constructed using THERMOCALC 333 for two representative pelitic schists. The results demonstrate that there was a break in the peak metamorphic pressures in the Habutengsu area. The northern schist has experienced UHP metamorphism, consistent with the presence of coesite in the same section, while the southern one formed at lower pressures that stabilized the quartz. This result supports the previous finding of a metamorphic gradient through the HP–UHP metamorphic belt of the Chinese western Tianshan by the authors. Additionally, phengite in the northern schist was modelled as having a Si content of 3.55–3.70(a.p.f.u.) at the peak stage, a value much higher than that of oriented matrix phengite(Si content 3.32–3.38 a.p.f.u.). This indicates that the phengite flakes in the UHP schist were subjected to recrystallization during exhumation, which is consistent with the presence of phengite aggregates surrounding garnet porphyroblast. The 40^Ar/39^ Ar age spectra of white mica(dominantly phengite) from the two schists exhibit similar plateau ages of ca. 315 Ma, which is interpreted as the timing of a tectonometamorphic event that occurred during the exhumation of the HP–UHP metamorphic belt of the Chinese western Tianshan.