广东良口亚髻山霞石正长岩地球化学特征及大地构造意义

Petro-geochemistry characteristics and tectonic significance of the Yajishan nepheline-syenite at Liangkou,Guangdong Province

广东良口亚髻山霞石正长岩岩体内带为霞石角闪正长岩,w(SiO_2)平均值60.45%,w(K_2O)/w(Na_2O)平均值0.97,w(Nb)/w(Ta)平均值22.68,w(Zr)/w(Hf)平均值39.96,w(ΣCe)/w(ΣYb)平均值6.32,δEu(0.30)负异常明显。外带为黑云角闪正长岩,w(SiO_2)平均值60.34%,w(K_2O)/w(Na_2O)平均值1.50,w(Nb)/w(Ta)平均值21.23,w(Zr)/w(Hf)平均值43.07,w(ΣCe)/w(ΣYb)平均值7.85,δEu(0.47)负异常明显。同位素研究显示,岩体形成时间约134.6±8.9 Ma(Rb-Sr同位素测年),ISr平均值0.706 75,εNd(t)平均值-2.8,暗示了岩石的源区为上地幔-下地壳重熔型物质,其高碱特征可能与上地幔岩浆分异有关,它形成于白垩纪早期古太平板块俯冲后撤诱发的陆-陆碰撞造山后拉张构造环境,这一过程对华南地区构造演化具有一定重要的地质意义。

The internal zone of nepheline syenite in Liangkou Yajishan, Guangdong Province is composed of amphibole bearing nepheline syenite, with SiO2 averaging 60 45%, K2O/Na2O averaging 0 97, Nb/Ta averaging 22 68, Zr/Hf averaging 39 96,ΣCe/ΣYb averaging 6.32,and δEu（0 .30） The external zone, Bi Am syenites zone,have average SiO2 of 60.34%, average K2O/ Na2O of 1.50, average Nb/Ta of 21. 23, average Zr/Hf of 43.07,average ΣCe/ΣYb of 7.85,and negative anomaly in Eu（δEu=0.47）. Isotopic studies show that the rock formation time is about 134 6 ± 8 9 Ma （Rb Sr whole rock isochron method）, with average ISr of 0.06 75, average εNd（t） of 2.8. Isotope features suggest that the magmatic rocks were derived from the upper mantle. The intrusive rocks formed at a continent continent post collisional extension setting, which was significant to the early Cretaceous tectonic evolution of the South China Block.