苏鲁造山带胶南区段片麻岩原岩的成因：地球化学及Nd同位素证据  

Geochemical and Nd isotopic evidence for the genesis of the gneisses from the southern Shandong Peninsula,Sulu orogen.

作　　者：薛怀民[1] 刘福来[1] 孟繁聪[1] 

机构地区：[1]中国地质科学院地质研究所,北京100037

出　　处：《岩石学报》2007年第12期3239-3248,共10页Acta Petrologica Sinica

基　　金：国家重点基础研究发展规划项目"973"(编号2003CB716502);国家自然科学基金重大项目(编号40399143);中国地质调查局地质大调查项目(121201056606)资助成果

摘　　要：出露于苏鲁造山带胶南区段的片麻岩主要包括三种类型:黑云斜长片麻岩、二长花岗片麻岩和A型花岗质片麻岩,地球化学判别结果表明它们的原岩均为火成岩。不同类型片麻岩的地球化学特征各不相同,反映它们的原岩具有不同的形成与演化历史。其中黑云斜长片麻岩以负Eu异常弱,甚至出现明显的正异常(δEu=0.60～1.45)、强的Ba正异常、无明显的Sr异常为特征而明显不同于其它类型的片麻岩。其地球化学的总体特征类似于扬子克拉通北缘新元古代双峰式火山岩的酸性端元;与黑云斜长片麻岩类似,二长花岗片麻岩中轻、重稀土元素之间的分馏程度也较强(La_N/Yb_N=6.3～17.2),但以强的负Eu异常(δEu=0.27～0.54)、强的Sr负异常和弱的Ba负异常为特征明显不同于黑云钭长片麻岩;A型花岗片麻岩的稀土模式及蛛网图的形态类似于二长花岗片麻岩,所不同的是前者Nb、Ta负异常相对较弱。区内几种类型片麻岩的ε_(Nd)(t)值差别不大,反映它们的原岩之间可能有一定的成因联系。它们的Nd同位素模式年龄T_(DM)都集中在2.0Ga左右,表明它们的源区主要为早元古代的地壳物质,且岩性比较均一。推测二长花岗片麻岩与黑云斜长片麻岩的原岩是同源异相,而A型花岗岩是在大规模Ⅰ型花岗质岩浆形成后,由脱水的紫苏辉石质残留下地壳物质再次发生部分熔融形成的。The gneisses outcroped in the southern part of the Shandong peninsula, Sulu orogen include three types: biotite-plagiogneisses, monzogranitic gneiss and A-type granitic gneiss, their protoliths are all belong to igneous rocks based on geochemical discriminations. The three types of gneisses have different geochemical characteristics, which reflecting different evolutionary processes of their protoliths. Compared to other gneisses, the biotite-plagiogneisses are characterized by feeble negative to large positive Eu anomalies (delta Eu = 0.60 similar to 1.45), significant positive Ba anomalies, no negative Sr anomalies, which is characteristic of rocks that have high model concentrations of plagioclase. The bulk geochemical features make them similar to the acidic end-member (rhyolitic pyroclastic rocks) of the Neoproterozoic bio-modal volcanic rocks that outcrop along the northern margin of the Yangtze Craton, implying that the gneisses may be metamorphosed from the rhyolitic pyroclastic rocks; The monzogranitic gneisses are characterized by strong fractionation between LREE and HREE (La-N/Yb-N = 6.3 similar to 17.2) which similar to the biotite-plagiogneisses, while strong negative Eu anomaly (delta Eu = 0.27 similar to 0.54), strong negative Sr anomalies and feeble negative Ba anomalies which obvious different to the biotite-plagiogneisses; The A-type granitic gneisses have similar REE and multielement spidergram patterns with the monzogranitic gneisses, while have relatively feeble negative Nb and Ta anomalies compared to the monzogranitic gneisses. The epsilon(Nd) (t) values are homogeneous among these three types of gneisses, which may reflecting genesis relationship among their protoliths. Their model ages of Nd isotope (T-DM) are focused at about 2.0Ga, implying that they formed by partial melting of homogeneous Paleoproterozoic rocks in the Neoproterozoic. It can be considered that the protoliths of the biotite-plagiogneisses and the monzogranitic gneisses are comagmatic and out-of-phases, and the A-type g

关 键 词：花岗片麻岩 斜长片麻岩 地球化学 新元古代 胶南 

分 类 号：P588.345[天文地球—岩石学] P595[天文地球—地质学]