Geochemical Characteristics and Tectonic Environment Studies of Altay Group Volcanic Rocks of Middle Devonian Epoch in Kalamanzhe Area of the Fuyun Region, Xinjiang

Field survey and geochemical study indicate that volcanic rocks of Aletai Formation, formed during Middle Devonian, outcropped in Fuyun county, Kalamanzhe area at the south margin of Altay Orogenic Belt. It generally consists of basalts and rhyolites, which are typical rock assemblages of bimodal volcanic rocks. Basaltic rocks are usually altered into albite-actinolite schist and epidote-tremolite schist, which belong to tholeiite series with low SiO₂, K₂O content, and high Ti content. Its REE distribution pattern is flat-type with LREE slightly enriched. Without any obvious Eu anomaly (δEu=0.92-1.08), basalt is rich in Th and U, with an Nb negative anomaly. In comparison, the rhyolitic rocks belong to the calc-alkaline series with high SiO₂ and low K₂O, which are the agpaitic type. The REE distribution pattern goes rightward with an Eu negative anomaly(δEu=0.29-0.58). The MORB-normalized spider diagram shows evident enrichment in Th, U, La, Ce and Hf and loss in Sr, Nb, Ta, Zr, P and Ti. In terms of REE and trace elements characteristics, there are prominent differences between the basalts and rhyolites, showing that the rhyolites may derive from different degrees of partial melt in the mafic lower crust rather than the fractional crystallization of the basic magma. Whereas, the basalts may derive from partial melt of depleted mantle which was altered by subduction fluid. Geochemical researches of the bimodal rocks reveal that the rocks have geochemical features of both OIB and arc-island, but combined with the regional geological setting, kalamanzhe bimodal volcanics are probably the products of island-arc rifts, and belongs to mature island arc instead of the back-arc basin.