The Early Paleozoic Pillow Basalt in Northern Altyn,Western China and it Tectonic Implications

This paper reports petrologic and geochemical study results of Early Paleozoic pillow basalt from ophiolite melange of the Hongliuquan section in the Northern Altyn Tagh,western China.The content of SiO2ranges from 43.34% to 52.55%;Mg-index vary from 39 to 58.They are high in TiO2(2.13%~5.49%)and Na2O(3.97%~7.36%),low in Al2O3(9.02%~13.77%)and K2O(0.27%~0.94%),which resembles to those of spilite.Their trace elements are different from MORB but comparable to the OIB or seamount basalt.They display varied 87Sr/86Sr(0.7043~0.7052),143Nd/144Nd(0.51253~0.51265).Our study interprets the pillow basalt originated from upwelling asthenospheric mantle with enriched components.Considering comprehensively the sedimentary rock assemblage on field profile,it is assumed that the magma extruded into the back-arc basin.System of back-arc basin in the northern Altyn Tagh was in the same unit as that in the northern Qilian Mountain,and likely it was the northernmost margin of proto-Tethys in China before the formation of the Altyn strike-slip fault.This is of great importance for understanding the evolution of the Qinghai-Tibetan Plateau and tectonic patterns of western China.