Geochronology And Geochemistry And Petrogenesis Of The Granites In Putian Area, Fujian Province

Granites are widely spreading in the southeastern of South China Block, which have been produced by multi-stage and multi-source magmatism. The igneous rocks in this area have been intensive studied by geologists, but some aspect about the petrogenesis and geological setting during the period of this area are still uncertain and controversial, The systematic study of late Mesozoic granites in Putian where is located on the southeast coast is not enough. This paper reports the results of peotrology, zircon U-Pb geochronology, zircon trace elements, Hf isotope geochemistry and whole-rock geochemistry, trying to provide more information of tetonic settings, petrogenesis, the crustal evolution in this area.The samples are from Xiayang pluton, Pinghai pluton and Chishi pluton.The main rock types are biotite granite,granite porphyry, granodiorite, monzonite granite. Zircon U-Pb dating results indicate that the age of Xiayang pluton is ca.109 Ma and 100Ma; the age of Pinghai pluton is ca.122Ma; the age of Chishi pluton is ca.119 Ma and 90 Ma, all plutons in Putian area have crystallized during the Cretaeous in Late Mesozioc.The Cretaceous granites are belonging to different degree of differentiation, metaluminous to peraluminous,high-K, calc-alkaline series. They are enrich in LREE and depleted in HREE, with moderate to strong degree of Eu(δEu=0.11~0.85) negative annoumalies. They show similar characteristics in the spider diagram: they are enriched in Rb, K, Th, U, Pb and depleted in Ba, Nb,Ta, Sr, P, Ti. The rocks belong to different degrees of differentiation of I-type granites.The zircons in Xiayang pluton show negative εHf(t) value(-4.2~-0.7), The zircons in Pinghai pluton show differant εHf(t) value(-1.6~+0.6), The zircons in Xiayang pluton show positive εHf(t) value(+0.3~+3.8). The granitic magma are derived from proterozoic and neoproterozoic crust, which is younger than the crystalline age of ancient cathaysia block. The rocks are derived from partial melting of old lower crust and mantle, and underplating are getting stronger from inland to the coast.As to the petrogenesis, the cretaeous granites in Putian could be the results of comprehensive shaping of many factors mainly including the increasing angle and faster speed of the pacific plate subduction, partial melting of pre-exit underplating basaltic rock, mixed with magmas from mantle, changes of tectonic environment and magmatic evolution.