地质学报（英文版）2002，Vol.76Issue(1)：15-30,16.
A Petrographic and Mineralogical Study of Volcanic Rocks from the Mayaxueshan Area, North Qilian Fold Belt, NW China
A Petrographic and Mineralogical Study of Volcanic Rocks from the Mayaxueshan Area, North Qilian Fold Belt, NW China
摘要
Abstract
The Ordovician volcanic rocks in the Mayaxueshan area have been pervasively altered or metamorphosedand contain abundant secondary minerals such as albite, chlorite, epidote, prehnite, pumpellyite, actinolite, titanite, quartz,and/or calcite. They were denoted as spilites or spilitic rocks in terms of their petrographic features and mineral assem-blages. The metamorphic grades of the volcanic rocks are equivalent to that of the intercalated metaclastic rocks. Thisindicates that both the spilitic volcanic rocks and metaclastic rocks in the Mayaxueshan area have formed as a result ofCaledonian regional metamorphism. We suggest that the previously denoted spilitic rocks or altered volcanic rocks shouldbe re-denoted as metabasalts or metabasaltic rocks. The metamorphic grade of the volcanic rocks increases with their age:prehnite-pumpellyite facies for the upper part of the Middle Ordovician volcanic rocks, prehnite-pumpellyite to lowergreenschist facies for the lower part of the Middle Ordovician volcanic rocks, and lower greenschist facies for the LowerOrdovician volcanic rocks. The P-T conditions are estimated as T = 240 - 290C and P = 1.5 - 4.5 kbar for the lower partof the Middle Ordovician rocks, and T = ～ 300~C for the Lower Ordovician rocks. The variations of mineral assemblagesoccurring at different domains of the volcanic rocks were controlled by the variations of the effective bulk composition inthose domains during metamorphism. The geochemical characteristics of Mg-Al chromite in the Mayaxueshan volcanicrocks are consistent with an origin of island arc environment.关键词
Qilian Fold Belt/alteration/regional metamorphism/volcanic rock/spilite/basaltKey words
Qilian Fold Belt/alteration/regional metamorphism/volcanic rock/spilite/basalt分类
天文与地球科学引用本文复制引用
徐达伟,萧炎宏..A Petrographic and Mineralogical Study of Volcanic Rocks from the Mayaxueshan Area, North Qilian Fold Belt, NW China[J].地质学报（英文版）,2002,76(1):15-30,16.基金项目
The authors are very grateful to Professor Wu Hanquan at the Xi'an Institute of Geology and Mineral Resources and Professor Zuo Guochao at the Gansu Bureau of Geology and Mineral Resources, who offered guidance and many helps during the field investigatio （）
