This paper reports new results on the petrology, geochronology and geochemistry of two kmscale 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: M 1 =Pl+Qtz+Bt, M 2 =Cpx+Grt+Pl+Qtz+Atp±Opx, and M 3 =Amp+Ep+Pl+Bt+Qtz, in contrast to a two-stage metamorphic history of the Yushugou granulite: M 1 =Cpx+Grt+Pl±Hyp, M 2 =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 40 Ar/ 39 Ar 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.

Four international standards, Ga1550, MMhb-1, Lp-6, Bern 4M, and one domestic standard BT-1 have been intercalibrated. The repeated measurements on MMhb-1 with different mass demonstrate that MMhb-1 is inhomogeneous in age and its average age is 519.8 Ma. The results of Bern 4M and Lp-6 reflect that they have an invariable value of 40 Ar*/ 39 Ar k (F) and the ages we obtained are consensus with their K-Ar age: Lp-6=127.7Ma; Bern 4M=18.2 Ma. Analyses of BT-1 age spectra, Ca/K and Cl/K spectra as well as inverse isochrons indicate that the sample is homogeneous and invariable and keeps close chemically, with its trapped argon isotope composition close to the atmosphere. The dating results show that age values are reproducible and steady, total fusion age, step-heating age, plateau age and isochron age are in accord with each other within the error range (2σ). Therefore, we recommend 28.7 Ma as the calibrated age of BT-1.We also discuss the variation in neutron flux gradients of Beijing 49-2 reactor. It was found that the neutron flux gradient varies considerably, and more monitors (standard samples) are needed to fix the trend of variation. The coefficient of the 49-2 reactor that transfers the ratio of production rate of 37 Ar Ca / 39 Ar K into Ca/K ratio is 1.78. This is different from that reported earlier, 2.0, which may be caused by the reconstruction of the reactor.

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