A Petrographic and Mineralogical Study of Volcanic Rocks from the Mayaxueshan Area, North Qilian Fold Belt, NW China

HSU, Ta-Wei and SHAU, Yen-Hong

Department of Marine Resources, National Sun Yat-Sen University, Kaohsiung 80424, Republic of China

The Ordovician volcanic rocks in the Mayaxueshan area have been pervasively altered or metamorphosed and 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 assemblages. The metamorphic grades of the volcanic rocks are shown to be equivalent to that of the intercalated metaclastic rocks. This indicates that both the spilitic volcanic rocks and metaclastic rocks in the Mayaxueshan area have formed as a result of the Caledonian regional metamorphism. We suggest that the previously denoted spilitic rocks or altered volcanic rocks should be 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 lower greenschist facies for the lower part of the Middle-Ordovician volcanic rocks, and lower greenschist facies for the Lower-Ordovician volcanic rocks. The P-T conditions are estimated as T = 240 ~ 290¢XC and P = 1.5 ~ 4.5 kb for the lower part of the Middle-Ordovician rocks, and T = ~300¢XC for the Lower-Ordovician rocks. The variations of mineral assemblages occurred at different domains of the volcanic rocks were controlled by the variations of effective bulk composition in those domains during metamorphism. The geochemical characteristics of the Mg-Al chromite in the Mayaxueshan volcanic rocks are consistent with an origin of island arc environment.

Key words: Qilian Fold Belt, alteration, regional metamorphism, volcanic rock, spilite, basalt

Acta Geologica Sinica, 76, 15-30, 2002