摘要
广东河源白石冈岩体位于近东西向展布的佛冈花岗岩带的东端,主体岩性为中粗粒黑云母花岗岩,主要组成矿物为石英(25%～35%)、微纹长石(45%～50%)、斜长石(An=20～30,15%～20%)和黑云母(5%～10%)。锆石U-Pb定年结果表明其形成年龄为148.5±1.6Ma,属晚侏罗世岩浆活动的产物。化学成分上,该岩体铝弱过饱和,A/NKC值主要变化于1.0～1.1之间;富硅,富钾(K2O/Na2O=1.31～1.70),全碱含量中等偏低(K2O+Na2O=7.44%～8.48%),碱铝指数(AKI值)为0.75～0.88,可归为高钾钙碱性岩系。微量和稀土元素组成上,岩体富Rb、Th、U、Pb和轻稀土,贫Ba、Sr、P、Ti,Rb/Sr比值高,K/Rb比值低,铕负异常显著(δEu=0.05～0.28),Nb、Ta、Zr、Hf等高场强元素含量及104×Ga/Al比值(2.43～3.26)较之典型A型花岗岩均偏低。岩体的εNd(t)值为-5.99～-7.51,TDM值偏低(1.42～1.54Ga),综合地球化学资料指示其应属高分异的I型花岗岩。结合对区域动力地质背景的全面分析,表明白石冈岩体形成于后造山的伸展引张环境,是由底侵的幔源基性岩浆及其诱发的长英质岩浆在深部岩浆房混合,并经高程度分离结晶的产物。
The Baishigang Pluton is located at the eastern end of the E-W-trending Fogang granite belt, and has an outcrop area of about 440km^2. It intruded into the Lower Jurassic Lantang Group and is made up mainly of medium- to coarse-grained biotite granites. The major mineral phases of the granites are quartz （25%～35%）,microperthite （45%～50%）, plagioclase （An=20～30, 15%～20%） and biotite （5%～10%）. Zircon U-Pb isotopic dating yields an age of 148.5±1.6Ma, indicating that it was formed in the Late Jurassic. Chemical analyses show that the granites have high SiO2（=73.37%～76. 05%） and moderate Al2O3（12. 00%～13. 16%）. They are slightly peraluminous with A/NKC values ranging mainly from 1.0 to 1.1 and normative corundum 〈1%. The rocks also have moderate to low alkaline contents （K2O＋Na2O=7.44%～8.48%,NK/A [molecular （K2O＋Na2O）/Al2O3]=0. 75～0. 88） and high potassium contents （K2O=4. 22%～5.29%, K2O/Na2O=1.31～1.70）, thus can be ascribed to the rocks of the high-K calc-alkaline series.Trace and rare earth elements of the granites are characterized by rich Rb, Th, U, Pb and LREE, and poorBa, St, P, Ti, and have high Rb/Sr （2. 36～20. 82） and low K/Rb ratios （〈150 for most of the samples）.Chondrite-normalized REE patterns are right-inclined and display significant negative europium anomalies （δEu=0.05～0.28）. Their HFSE （e. g. , Nb, Ta, Zr, Hf） concentrations and Ga/Al ratios are lower than that of the respective values of A-type granites. The granites have a more enriched Nd isotopic composition with initial εNd values from -5. 99 to -7.51, and show younger depleted-mantle model ages （TDM=1. 42～1. 54Ga）.Integrated geochemical data suggest that the rocks are highly fractionated Ⅰ-type granites. By combining with comprehensively analyses of the tectonic settings, we suggest that the Baishigang pluton was formed in a postcollisional tensile environment, and the granites were generated by a two-stage process including mixing of underplating basaltic magma with the induced felsic melts in the source region and subsequent high-degree fractional crystallization.
出处
《地质学报》
EI
CAS
CSCD
北大核心
2005年第4期503-514,共12页
Acta Geologica Sinica
基金
教育部新世纪优秀人才支持计划和国家自然科学基金项目(编号40132010
40221301)联合资助成果。
