北祁连山石灰沟奥陶纪岛弧火山岩系岩浆性质的确定

Determination of Magmatic Nature of Ordovician Island Arc Volcanic Series in the Shihuigou Area in the Northern Qilian Mountains

北祁连山石灰沟地区发育一套完整的奥陶纪岛弧火山岩系。中奥陶世始岛弧形成,初期为拉斑玄武质岩浆喷发,尔后以钙碱性火山活动为主,至中奥陶世末岛弧演化成熟,产生橄榄玄武质火山作用。

The Middle Ordovician submarine spilite-alkaline volcanic rock association in the Shihuigou area in the Northern Qilian mountains was the product of island arc volcanism.Based on the studies of petrology, mineralogy, geochemistry of trace elements, magmatic inclusions in minerals and Sr isotope, the nature of the 'source' magma series of the spilite-alkaline volcanic rock association is determined and the sequence of the island arc volcanism is established in this paper.At first erupted a small amount of tholeiitic magma which was produced by the partial melting of the mantle wedge above subduction zone, then a lot of calc-alkaline magma. At the end of Middle Ordovician, the shoshonitic magma formed by a high pressure (1 GPa) fractional crystallization of calc-alkaline basalt magma in Moho-level chamber erupted, and resulted in the formation of shoshonite rock association. Shoshonitic rocks in the Shihuigou area are stratigraphically highest and consist mainly of trachybasalt, leucite-tephrite, leucite-analcitite and leucite-trachyte porphyry. The development of the shoshonite rock association suggests that the palaeo-island are system of the Northern Qilian mountains had reached to the mature stage at the end of Middle Ordovician.