Petrogenesis and tectonic setting of the quartz porphyry in Mazhuangshan gold deposit, Eastern Tianshan Orogen: Evidence from geochemistry, zircon U-Pb geochronology and Sr-Nd-Hf isotopes

The Mazhuangshan gold deposit is located in the eastern part of the Eastern Tianshan Orogen, where abundant quartz porphyries are widely outcropped. The quartz porphyries are EW- or NW-trending, distributing in a narrow and long belt and intruding along the fault in middle part of the mining area. It is grey-write coloured and exhibits a porphyritic texture. Quartz, K-feldspar and plagioclase are embedded in a fine-grained groundmass, which mainly comprises felsic minerals and minor dark minerals. The (Na2O + K2O) contents of quartz porphyry samples are 3.55%similar to 9.67%, the Al2O3 contents are 10.37%similar to 14.28%, with the K2O/Na2O ratio values of 2.57 similar to 66.5, which show the peraluminous characteristics. The quartz porphyries have medium Sigma REE contents (65 x 10(-6)similar to 161 x 10(-6)) and medium REE fractionation ([La/Yb](N) = 6.15 similar to 12.5), with apparently enrichment of light rare earth and slightly negative Eu anomaly. The subvolcanic rocks are riched in Rb, K, Th, U, Pb elements and depleted in Sr, Ba, P, Nb and Ti elements, which is similar to the characteristic of the rocks formed in active continental margin. Quartz porphyry samples in the Mazhuangshan area show negative correlation between P2O5 and SiO2 elements, positive correlation between Y vs. Rb elements and Th vs. Rb values, which are consistent with the typical I-type granite trends. The zircons from quartz porphyries in Mazhuangshan gold deposit yielded U-Pb concordant age of 315.4 +/- 0.6Ma (MSWD=0.67) and weighted mean zircon Pb-206/U-238 age of 316.0 +/- 2.0Ma (MSWD = 0.23), which shows that quartz porphyry intruded at 314 similar to 318Ma. Sr-Nd-Hf isotopic compositions indicate that quartz porphyry is characterized by high (Sr-87/Sr-86), values (0.7077 similar to 0.7102), low epsilon(Nd) (t) values (-1.62 similar to 1.82), with relatively young model ages (t(DM2)) ranging from 0. 9 to 1.2Ga. The epsilon(Hf) (t) values of zircons are -3.2 similar to 0.4 and the corresponding t(DM2) model ages are 1.28 similar to 1.51Ga. These features illustrate that the quartz porphyry magmas are derived from the parial melting of the lower crustal materials that involved some mantle component. Comprehensive geological, geochemical and chronological studies demonstrate that the eastern part of the Eastern Tianshan was in an active continental margin arc environment related to subduction during the Late Carboniferous. Quartz porphyry may be the product of the subduction of oceanic crust during the Late Carboniferous, which resulted in partial melting of the new lower crust that involved some mantle component and finally emplaced in the active continental margin arc environment