关键词: 六盘山南段, 花岗斑岩, 中-新生代, 古元古代, A型花岗岩

Abstract:

The granite porphyry in southern section of Liupanshan and eastern section of Longxian Guguan Zhen was considered as
Mesozoic-Cenozoic high-K calc-alkaline granites that record Paleoproterozoic magma activities. The granite porphyry contains high levelof SiO2 (71.14%~73.33%), alkaline-rich (Na2O+K2O=7.61~8.70), K-high （K2O/Na2O=1.21~2.52，with an average of 2.03） , fallinginto the range of metaluminous to weakly peraluminous (A/CNK=0.93~1.09，with an average of 1.0). The intenseenrichment in LREEasymmetrical right-inclined rare earth element pattern, and the obvious negative Eu anomaly （δEu=0.44~0.47） suggest that the rocksare enriched in LILE (K, Rb, Ba, Th) and depleted in the high field strength elements (Ti, Nb, Ta). They are characterized by crustal derived A-type granites in geochemistry. Zircon U-Pb LA-ICP-MS dating of the magmatic genetic zircon yields an age of 1846±32 Ma,which should represent the emplacement age of the granite porphyry, suggesting that the granite porphyry was formed in Paleoproterozoic, not Mesozoic-Cenozoic as previously thought. The calcirudites of Beiguoshan Formation in Upper Ordovician wereactually deposited on the granite porphyry of Paleoproterozoic. This study shows that the southwestern margin of North China Craton in Paleoproterozoic was in an extensional tectonic environment. It may provide direct petrological evidence for Paleoproterozoic aulacogen of Helan mountain that extends southward to the southern section of Liupanshan.

Key words: southern section of Liupanshan, granite porphyry, Mesozoic-Cenozoic, Paleoproterozoic, A-type granites