PETROCHEMICAL CHARACTERISTICS OF CENOZOIC VOLCANIC ROCKS IN FUJIAN PROVINCE AND ITS RELATIONSHIP TO TECTONICS
Sun Weihan and Lai Zhimin(Geologic Team No. 2, Mingxi, Ftijian Province)
Cenozoic volcanic rocks in Fujian are of Pliocene and Pleistocene ages and can be divided into three zones: the east volcanic rock zone consists mainly of tholeiite, the west zone is limburgite and the intermediate zone is transitional in character between the former two zones with a little prevalance of olivine basalt. Two rock types can be distinguished, i.e., the limburgites containing no feldspars and the ba-salts including olivine basalt and tholeiitic basalt. In terms of petrochemistry, the volcanic rocks can be classified as strong alkaline, alkaline and sub-alkaline suites. Most of the strong alkaline rocks are distributed in the west zone, while the alkaline is in the intermediate zone and the subalkaline occurs predominantly in the east zone. With increasing distance from the coastline, the rocks become progressively alkaline and the potassium content and K/Si ratio increase. Significant amount of mantle material, including garnet 1herzolite xenoliths and pyrope xenocrysts, has been noticed in the above-mentioned volcanic rocks. The garnet Iherzolite bears strong resemblance to "mantle rocks", especially with respect to the high values of Mg and Fe. Garnets in these rocks are Cr-bearing pyrope, with pyrope component in excess of 60 percent. It is suggested that the original basaltic melt might have been resulted from partial melting of garnet Iherzolite in the upper mantle and then extruded onto the surface along the ultra-deep fault in the Pacific plate.