This paper reports new results on the petrology, geochronology and geochemistry of two km-scale high-pressure granulite exposures, namely the Weiya and the Yushugou granulites, that occur in the central and southern Tianshan Paleozoic sub-belts, respectively. Petrological and geochemical results suggest that the Weiya and the Yushugou granulites are different in both the rock types and geological structures. The protolith of the Weiya granulite is basic to intermediate rocks, and that of the Yushugou granulite is basic rocks; the former is distributed as fault-bounded blocks within the Precambrian schist-gneiss, and the latter occurs as thrust slabs in the allochthonous ophiolitic mélange. Observation of deformation structures provides some evidence for two-phase ductile deformation of the Tianshan granulites, characterized by deformed garnet-pyroxene and deformed feldspar-quartz microstructures, corresponding to rheological conditions of the lower crustal level and of the middle crustal level, respectively. The deformation structures also indicate that the Weiya granulite was involved in the late stage retrograde metamorphism forming new amphibole, following a thrusting movement toward the north and ductile deformation of feldspar-quartz grains. The Yushugou granulite underwent strong ductile shearing during the Silurian–Devonian interval. Three stages of metamorphism are recorded by mineral assemblages of the Weiya granulite: M1=Pl+Qtz+Bt, M2=Cpx+Grt+Pl+Qtz+Atp±Opx, and M3=Amp+Ep+Pl+Bt+Qtz, in contrast to a two-stage metamorphic history of the Yushugou granulite: M1=Cpx+Grt+Pl±Hyp, M2=Amp+Pl+Qtz±Bt. The P–T condition estimates suggest that the Weiya granulite was formed between 910–1025 °C and 1.08–1.12 GPa, and underwent retrograde metamorphism at 650–670 °C, while the peak metamorphism of the Yushugou granulite took place between 800–870 °C and 0.88–1.13 GPa. Geochemical characteristics (major, REE and incompatible elements) of the Weiya and Yushugou granulites indicate that their protoliths were formed in a volcanic arc setting, implying that a late Precambrian active continental margin may have existed in the study area. The Sm–Nd isochron dating for the Weiya granulite suggests that the peak metamorphism occurred at 538±24 Ma, which was likely related to the ancient plate subduction. The 40Ar/39Ar dating on amphibole grains separated from the Weiya granulite yielded a plateau age of 432±1 Ma for the retrograde metamorphism, which represents an exhumation age, suggesting a significant early Paleozoic tectonothermal event.
Geological, geochronological and geochemical features of granulites in the Eastern Tianshan, NW China