The determination of Zhenggehe Devonian volcanic rocks in Nurt area, southern Altay

Abstract: The Nurt area is located in the southern part of Altay, where Late Paleozoic volcanic rocks are well developed. There exists a set of rock assemblages of altered rhyolite and tuff dacite rhyolite in Zhenggehe area, which are in zoned distribution from northwest to southeast and were originally assigned to member a of Hongshanzui Formation. The LA-ICPMS zircon U-Pb dating has revealed that the volcanic rocks formed in the Early Devonian (the zircon 206Pb/238U weighted average age is (400.4±3.6) Ma), unlike Hongshanzui，whose age is of Carboniferous. The lithologic characteristics are obviously different from members b, c of Hongshanzui Formation, and are extensively stably distributed, so this set of rhyolites was re-determined, and tentatively assigned to Zhenggehe volcanic rocks. The SiO2 values range from 69.77% to 75.16%, and σ from 1.26 to 1.70, Na2O+K2O values range between 5.90% and 7.28%, A/CNK ratios range between 0.88 and 1.16, with sub-alkaline, calc-alkaline and meta-aluminum characteristics. The pluton is characterized by enrichment of trace elements Rb, Th and other large LILE, loss of high field strength elements Nb, Ta, Sr, Hf. The content of REE changes considerably, ΣREE range between 189.20×10-6 and 308.47×10-6, light and heavy REE show strong fractionation ((La / Yb) N being 5.82-7.86), Eu exhibits negative anomalies (δEu = 0.45-0.75), and the REE characteristics are similar to features of island-arc volcanic rocks. Combined with the data of southerner margin of Altay, the authors hold that the Zhenggehe volcanic rocks were formed in an active continental margin tectonic environment.