Geochemical Characteristics Of Early Carboniferous Volcanic Rocks In The Kalamaili Region In East Junggar, Xinjiang And Their Geological Significance

The Kalamaili ophiolite belt and rock association related to ancient Asia oceanic subduction, collisional and post-collisional process located in east Junggar indicate it is a key tectonic unit for understanding tectonic evolution of the Central Asian Orogenic Belt. This paper, based on the theory of plate tectonics for academic instruction, and systematical petrology and geochemistry of basaltic rocks conformally interbedded in Shanlianglishi formation of Lower Carboniferous in Kalamaili area, reveals the magmatic processes and tectonic settings of volcanic rocks of Shanlianglishi formation. New studied resulets as following:1. Basaltic volcanic rocks in the study area mainly composed of compact block and almond-shaped basalts occur discontinuously within Shanlianglishi Group of Lower Carboniferous as interbed, south side of Kalamaili fracture zones. They are products of volcanic eruption in early Carboniferous.2. The basaltic rocks from study area have 50.3-54.44 wt.% SiO2,15.54-16.39 wt.% Al2O3, 9.32-13.01 wt.% TFe2O3, low and narrow 4-4.2 wt.% MgO. The volcanic rocks are ranged from basalt to andesitic basalt in the silicon alkali (TAS) diagram and belong to calc-alkaline basalt in SiO2-K2O diagram. These samples in the chondrite-normalized REE pattern are slightly enriched in LREEs (LaN/YbN=2.75-2.96) and possess weak Ce and weak negative Eu anomalies (0.99-1.03,0.87-095, respectively), with high SREEs (112.42-136.2ppm) contents. They are enriched in LILEs but depleted in HFSEs in the primitive mantle-normalized trace elements pattern, with negative Nb, Ta, Ti, positive Rb, Ba, K, U and different levels of pronounced positive Pb anomalies. These characteristics indicate a certain degree of involvement of crust, suggesting that they were from partial melting of lithospheric mantle metasomatized by previous subduction slab.3. Comprehensive results of petrology and geochemistry show that basaltic rocks (andesitic basalts, basalt), in the the study area, were homologous magmatism product formed in post-collisional tectonic environment, and this characteristic imply the ancient ocean basin, Kalamaili ophiolite belt represented, had been closed in early Carboniferous, and entered the post-collisional tectonic environment.