The Geological Significance for Geochemical Composition Analysis of Marine Volcanic Rock from Dacha-Daban Areain North Qilian Mountains

Early Paleozoic marine volcanic rocks were well developed in the North Qilian Mountains. The paper focuses on discussing the formation process and origin of Na element for Ordovician marine volcanic rocks during the process of spilite-keratophyre alteration. There is more Na₂O in spilite-keratophyre than normal volcanic rock,because of the basaltic magma react with seawater. Therefore, the discrimination diagram involved with Na element cannot be used as the discrimination criteria for distinguishing the original magma series of keratophyre and related geotectonic setting, but the immobile elements can be used as distinguishing basis. Meanwhile, the Na₂O-SiO₂ diagram is proposed for reflecting the degree of spilite-keratophyre alteration. In this graphic, the boundary line for the percentage of Na element between spilite-keratophyre and normal volcanic rock is a slightly up-convex, which is stand for the content of Na element ranging from 3.5% to 4%. For the marine volcanic rocks in Dacha-Daban area, most of them fall into the distribution region of spilite-keratophyre, suggesting that the convection of seafloor hot brine was very strong at Ordovician back-arc and island-arc spreading ridge during intervolcanic eruption stage, and this area provides very favorableconditions for mineralization.