New precise timing constraint for the Keketuohai No. 3 pegmatite in Xinjiang, China, and identification of its parental pluton

Abstract

The Keketuohai pegmatite field is one of the pegmatite-rich regions in the Altay Orogen, with abundant pegmatites and voluminous granite magmatism. The Keketuohai No. 3 pegmatite is renowned for its deposits of rare metals, but despite a lot of in–depth research, the geochronology of the pegmatite remains the subject of dispute, and there is a lack of precise geochronological evidence to support the view that the Aral granite represents the parental magma of the pegmatite. To further our understanding of this pegmatite field, one sample was collected from the Aral granite in the northern part of the ore district, and six molybdenite samples were taken from the marginal zone of the Keketuohai No. 3 pegmatite. The geological context of the molybdenite samples indicates an origin by hydrothermal metasomatism in the zone between the margin of the pegmatite and the wall rock, and the time of its formation should coincide with or immediately follow the formation of the pegmatite margin. This allows the lower time limit of the pegmatite-formation to be set. LA–MC–ICP–MS dating of zircons from the Aral granite gives 206Pb/238U ages that range from 209.0 to 215.6Ma (from 16 spot analyses), and the plotted data are mainly distributed on or around the concordant line. The weighted mean age is 211.4±0.8Ma (MSWD=1.4), which may represent the crystallization age of the granite. The Re–Os isotopic model ages for the six molybdenite samples range from 207.8 to 212.2Ma. The weighted mean age of 209.9±1.3Ma (MSWD=0.69) and the Re–Os isochron age of 208.8±2.4Ma (MSWD=0.89) are consistent within error. If decoupling of the Re–Os isotope system in molybdenite is neglected, this consistency in the age data may indicate the time at which molybdenite was deposited from fluids. After researching in detail the previously published chronological data, we propose that earlier views on the ages of the Keketuohai No. 3 pegmatite and the Aral granite may be incorrect, and we now suggest that the pegmatite started to form around 209–210Ma, in the Late Triassic. The agreement between the newly determined U–Pb ages for the Aral granite and the Re–Os ages of the molybdenite in the pegmatite strongly indicates a close relationship between the crystallization of the Aral granite and the formation of the pegmatite and its rare-metals deposits. It also points to the Aral granite being the parent of the No. 3 pegmatite.