苏鲁地体南部高压变质带岩石地球化学特征及其地质意义

Geochemical characteristics of metamorphic rocks in the HPM belt along the southern margin of Sulu terrane and their geological implications

苏鲁地体南部的高压变质带由中温高压变质岩片和低温高压变质岩片两部分组成,前一岩片自下而上依次为含砾白云石英岩、云母二长石英片岩、大理岩磷块岩、白云钠长石英片岩;后一岩片的主要岩石类型为二长石英片岩钠长石英片岩。两岩片中片岩内的斜长石端员组成为钠长石,长石斑晶具有明显的火山晶屑特征,锆石具有岩浆成因韵律环带,其Th/U>0.5。此外,它们都具有高硅、高碱、低钙,轻稀土元素富集、分馏程度高,重稀土元素亏损、分馏程度低,Eu强到中度亏损及Nb、P、Ti负异常的特征,在Si O2A·R图解上均位于碱性岩区,在铝饱和指数图解中分布在偏铝质过铝质区,在(K2O+Na2O)/CaO Zr+Nb+Ce+Y图解上落在A型岩区。这些特征说明其原岩为火山岩,其岩石类型为过铝质偏铝质碱性流纹岩类,其成因类型为A型。常量元素和微量元素组成特征及其判别图解反映其原岩形成于板内拉张环境,是新元古代Rodinia大陆裂解、扬子地块陆内裂谷形成过程中伴生的产物。

The Hp metamorphic belt along the southern margin of Sulu terrane is made up of a middle temperature-high pressure metamorphic slice and a low temperature-high pressure metamorphic slice. The former slice consists of gravel-bearing phengite feldspar quartzite, mica feldspar quartz schist, marble-phosphate rock and phengite albite quartz schist in upward succession, whereas the latter consists of feldspar quartz schist and albite quartz schist. In schists, the end-member component of plagioclase is albite, the feldspar phenocrysts have features of volcanic crystal fragments, and the zircon has magmatic rhythm crystallization zoning and high Th/U ratio （〉0.5）. In addition, the schists exhibit high SiO2 and Na2O＋ K2O, low CaO, enrichment of LREE with strong fractionation, depletion of HREE with weak fractionation, 8Eu between 0.13 and 0.47, and significant negative Nb, P and Ti anomalies. The schists are plotted in the alkaline field in a SiO2 versus A·R discrimination diagram, in the A-type field in a （K2O＋ Na2O）/ CaO versus Zr ＋ Nb ＋ Ce ＋ Y discrimination diagram. The schists in the medium temperature-high pressure metamorphic slice and in the low temperature-high pressure metamorphic slice are plotted in the peraluminous field and the metaluminous field respectively in a A/CNK versus A/NK discrimination diagram. It is thus considered that the protoliths of the three sorts of schists are acid volcanic rocks, and that the rock type and genetic type of the volcanic rocks are peraluminous to metaluminous alkaline rhyolite and A type igneous rocks, respectively. According to the major and trace element discrimination diagrams, the volcanic rocks were formed in an extensional plate environment and might have been an accessory product during the breakup of Rodinia continent and the formation of Yangtze intracontinental rift in Neoproterozoic.