Geochronology, geochemistry and Sr-Nd isotopes of the granitic rocks associated with tungsten deposits in Beishan district, NW China, Central Asian Orogenic Belt: Petrogenesis, metallogenic and tectonic implications

Abstract

The Beishan orogenic belt is an important tectonic unit and a suitable target to investigate and understand the Paleozoic tectonic framework of the Central Asian Orogenic Belt. Paleozoic granitic rocks are widely distributed in the area and closely related to the tungsten deposits at Hongjianbingshan, Guoqing, Yingzuihongshan and Yushan. To understand the petrogenesis of the intrusions, we performed SIMS zircon U-Pb, O and whole rock major-trace element contents and Sr-Nd isotopic analyses. Two major suites of granitic intrusions associated with tungsten mineralization have been recognized during 424–314Ma and 286–244Ma. The former suite shows variable Sr-Nd and O compositions (average (87Sr/86Sr)i=0.7149 or 0.7034, εNd(t)=−6.3 or 7.6, T2DM=1.68 or 0.41Ga, δ18O=9.3 or 5.9‰) indicating an ancient crust or a juvenile lower crust origin, whereas the latter suite shows A-type affinity, and the Sr-Nd and O compositions (average (87Sr/86Sr)i=0.7125, εNd(t)=−4.5, T2DM=1.42Ga, δ18O=6.9‰) suggest the Precambrian metasedimentary strata may have exerted a significant role in the magma source. The proposed magma sources, combined with the geochemical differences between these two suites of intrusions, indicating the Silurian-Carboniferous mineralization such as Hongjianbingshan, Guoqing and Yingzuihongshan are closely associated with subduction-accretion and generation of Gongpoquan arc-accretionary system, while the Permian mineralization including Yushan deposit are attributed to continuing accretion and subsequent post collision. These tungsten-related granitic rocks are enriched in LREE and depleted in HREE and Ba, Sr, Nb, P, Eu and Ti, suggesting they may have experienced advanced fractional crystallization. Furthermore, a spectacular tetrad effect in their REE distribution patterns has been discovered, showing the granites are highly evolved rocks with strong hydrothermal interaction. The prolonged fractional crystallization and magmatic-hydrothermal interactions have contributed to the formation of the Silurian-Permian tungsten mineralization.