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岩石学报  1990 


Keywords: Gray gneiss,Low-Al_2O_3 type,Granitic gneiss,Early Precambrian basement,West Sichuan and Mid Yunnan

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Yakou gneiss distributed in the border region of Sichuan and Yunnan Province is the oldest basement rock of the area. Its relevants stretch northwards to Xichang and Mianning, West Sichuan and southwards to Yuanmou, Mouding and Xinping, Mid Yunnan. Perhaps it might extend down to North. Vietnam (Songchay Gneiss). The isotopic dating demonstrates that they might form during Late Archaean. The gneiss is composed of trondhjemitic-tonalitic-granodioritic gneiss (gray gneiss) and granitic gneiss. The latter might result from reformation (potassium feldsparization) of the gray gneiss. The linear relations can be found between silica and other oxides of the gneisses. The decreasing Na_2O K_2O with increasing MgO can be observed in both cases. The relationships between Ca and K/Na ratio show a tonalite-trondhjemite trend in the gray gneiss, and a more typical calc-alkaline one in the granitic gneiss, respectively. The mineral controlling the change of Al_2O_3, Na_2O,. K_2O and CaO contents in gneiss is feldspar. The chemical composition and mineral assemblage of the gray gneiss reveal that they are compatible with gabbro-trondhjemite trend of Archaean TTG suite. The gray gneiss belongs to the low-Al_2O_3 type. Compared with high-Al_2O_3 type, the contents of Al_2O_3, MgO, CaO, P_2O_5, Co and Sr are lower, while Zr, La, Ba, Rb and Th higher. Its REE distribution is characterized by slight Eu negative anomaly, different from the Eu positive anomaly in high-Al_2O_3 type gneiss. In addition, an increasing Y with increasing SiO_2 of the gneiss confirms the gray gneiss low-Al_2O_3 type. The REE pattern of the granitic gneiss, whose Al_2O_3 is as low as the tronhjemitic one, shows a clear Eu negative anomaly.


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