Petrogenesis of the Late Mesozoic granitic rocks in southeast coastal areas of Fujian Province and its geological significance

Baiyunshan, Gushan and Shiniushan are all located in coastal areas of Fujian Province, and granitoids are widely distributed in these areas. LA-ICP-MS zircon U-Pb dating shows that the petrogenetic ages of Baiyunshan, Gushan(Kuiqi) and Shiniushan are respectively 99.3Ma±1.8Ma, 99.4Ma±2.3Ma and 94.7Ma±1.4Ma, suggesting that the granites were formed in the early Late Cretaceous period. The granites are characterized by rich silicon and alkali, poor calcium and magnesium, and high fractionation indexes, thus belonging to metaluminous to weakly peraluminous granites. REE are characterized by intermediate to strong negative Eu anomalies and exhibit right-oblique "V" patterns with LREE enrichment. Trace elements show that granites are strongly enriched in Rb, U, Th and La while relatively depleted in Ba, Sr, P and Ti. The petrographic and geochemical characteristics indicate that the granites belong to typical A-type granitoids, in which Kuiqi granites belong to alkaline A-type granites whereas the other rock bodies belong to aluminous A-type granites. Studies have shown that the two kinds of A-type granites may have similar sources; magma originated from the partial melting of crustal material, with the probably addition of some mantle material; however, alkaline A-type granites might have had more added mantle material than the aluminous A-type granites. In combination with geochemistry, field geology, regional background and geochronology, it is suggested that the two kinds of A-type granites in southeast coastal areas of China should be the products of back-arc extension background in the Paleo-Pacific plate subduction system.