山东岚山Ⅰ型与A型复合片麻状变质花岗岩:年代学、地球化学及其构造指示意义  

I- and A-type composite gneissic metagranites in Lanshan area, Shandong Province; Geochronology, geochemistry and tectonic implications.

作　　者：胡建[1] 邱检生[1] 徐夕生[1] 于津海[1] 李真[1] 

机构地区：[1]南京大学成矿作用国家重点实验室,南京210093

出　　处：《岩石学报》2009年第2期282-296,共15页Acta Petrologica Sinica

基　　金：本项研究得到国家自然科学基金(40772036);国家重点基础研究发展规划项目(2006CB403508);高等学校博士学科点专项科研基金(20070284023)的资助

摘　　要：在山东日照岚山地区,出露有原岩为新元古代岩浆活动产物的I型与A型复合的片麻状变质花岗岩体,前者特征矿物组合为石英+钾长石+斜长石+黑云母,而后者则为石英+钾长石+斜长石+霓石+亚铁钠闪石+黑云母.锆石LA-ICP-MS U-Pb定年结果表明,I型片麻状变质花岗岩的原岩成岩年龄为795±32Ma,而A型片麻状变质花岗岩的原岩年龄则为772 ±26Ma.二者均经历了早中生代的超高压变质作用,并在220Ma左右遭受强烈的角闪岩相退变质作用的改造.地球化学上,I型和A型片麻状变质花岗岩均具有高硅、富铁和贫镁的特征,但A型片麻状变质花岗岩更富碱贫铝,其铝饱和指数仅为0.88～0.94,表现为准铝质,而I型片麻状变质花岗岩则具有准铝-弱过铝的特点(A/CNK=0.94～1.01).在微量和稀土元素方面,A型片麻状变质花岗岩较I型稀土总量高,且具有高场强元素富集的特征,富Ga,104×Ga/Al值为2.77～3.89.二类片胡建等:山东岚山I型与A型复合片麻状变质花岗岩:年代学、地球化学及其构造指示意义麻状变质花岗岩具有不同的Nd同位素组成,εNd(t)值分别为-6.26～-8.00和-3.73～-5.20,指示它们应具不同的岩浆源区,元素结晶分异模拟显示二者岩浆演化的历程也存在差异.岚山片麻状变质花岗岩叠加有'弧'和'裂谷'双重岩浆活动的地球化学印记,它们与由地幔柱上隆所引发的主动裂谷背景下岩浆作用产物的特征存在明显差剐.I型与A型复合片麻状变质花岗岩的产出指示扬子板块北东缘在片麻状变质花岗岩原岩形成时(795～770Ma)应处于弧后伸展引发的被动裂谷环境的初始阶段.The I- and A-type composite gneissic metagranites with Neoproterozoic protolith ages are outcropped at Lanshan of Rizhao, Shandong Province. Mineral assemblages for the I-type gneissic metagranites are quartz + K-feldspar + plagioclase + biotite, and for the A-type rocks are quartz + K-feldspar + plagioclase + aegirine + arfvedsonite + biotite. Zircon LA-ICP-MS U-Pb dating gives protolith ages of 795 +/- 32Ma for I-type gneissic metagranites, and of 772 +/- 26Ma for A-type rocks. Both the I- and A-type gneissic metagranites had been experienced ultrahigh-pressure metamorphism in Early Mesozoic and were subsequently overprinted by intensive amphibolite-facies retrograde metamorphism at about 220Ma. Geochemically, the I- and A-type gneissic metagranites are all characterized by rich in silicon and iron, and poor in magnesium, however, there are still some obvious differences between them. Compared with the I-type gneissic metagranites, the A-type rocks contain higher alkalis and lower aluminum, with alumina saturation index (ASI) values generally ranging from 0.88 to 0.94, thus showing a metaluminous signature. In contrast, the I-type rocks with ASI values of 0.94 similar to 1.01 are commonly metaluminous to weakly peraluminous. The A-type gneissic metagranites also have higher contents of BEE, HFSE (e. g., Nb, Ta, Zr, Hf, Y, Ga), and display higher 10(4) x Ga/Al ratios (= 2.77 similar to 3.89). The two types of rocks have different Nd isotopic compositions, with epsilon(Nd) (t) values of -3.73 similar to -5.20 for the I-type, and of -6.26 similar to -8.00 for the A-type, suggesting that they were generated from different sources. Geochemical modeling also indicates that the I- and A-type gneissic metagranites evolve along different trends. The gneissic metagranites at Lanshan show both are-and rift-like geochemical signatures, which are quite different from those developed under an active rifting setting in association with upwelling of mantle plume. The occurrence of I- and A-type composite gneissic metagranites

关 键 词：Ⅰ型和A型片麻状变质花岗岩 锆石LA-ICP-MS U-PB定年 矿物化学 地球化学 岩石成因 构造意义 

分 类 号：P588.121[天文地球—岩石学] P597.3[天文地球—地质学]