扬子板块西缘黄草坝A型复式花岗岩体的成因及其地质意义

Genesis and geological significance of Huangcaoba A-type granitoid pluton,western margin of Yangtze Block

扬子板块西缘分布有一系列中元古代晚期—新元古代中期的岩浆岩体,是探究Rodinia超大陆聚合和裂解的重要窗口。黄草坝复式花岗岩体出露于扬子西南缘的云南牟定戌街地区,岩性包括有片麻状花岗岩、二长花岗岩和黑云母花岗岩。这些花岗岩在主量元素组成上均具有高SiO_(2)(70.77%~78.79%)、高碱(Na_(2)O+K_(2)O=7.15%~8.73%)、高FeO^(T)/(FeO^(T)+MgO)(0.70~0.98)值和高10000×Ga/Al(2.68~6.84)值,以及低P_(2)O_(5)(0.01%~0.06%)、低CaO(0.07%~0.90%)、低MgO(0.02%~0.88%)的地球化学特点。在稀土元素组成上均表现为轻稀土相对富集、具明显的负Eu异常特征。在微量元素组成上表现为Rb、Th、U、K、Nd、Sm等元素相对富集,Nb、Zr、Hf等元素相对亏损,Ba、Sr、P、Ti等元素强烈亏损。主微量元素特征表明黄草坝复式花岗岩体属于典型的A型花岗岩。锆石原位微区U-Pb定年结果显示片麻状花岗岩(1090±24 Ma)和二长花岗岩(1063±5 Ma)均形成于中元古代时期,ε_(Hf)(t)=+4.3~+10.3,T_(DM2)年龄介于1.33~1.62 Ga,表明成岩物质来自新生地壳的部分熔融。黑云母花岗岩的结晶年龄为782.4±8.2 Ma(ICP-MS U-Pb法),属于新元古代,其ε_(Hf)(t)=-7.3~-2.1,T_(DM2)年龄介于1.80~2.14 Ga,表明成岩物质可能来源于古老地壳。结合前人研究资料分析,笔者等认为黄草坝A型复式花岗岩体形成于中元古代—新元古代时期多阶段造山旋回中的碰撞造山后伸展环境中。

A series of late Mesoproterozoic to middle Neoproterozoic granitoids are distributed in the western margin of the Yangtze Block,which is an important window to explore the convergence and disintegration of the Rodinia supercontinent.The Huangcaoba granitoid pluton,which consists of gneissic granite,monzonitic granite,and biotite granite,is located in the Xujie area of the Mouding County,Yunnan Province along the western margin of the block.These granites are characterized by high Si(SiO_(2)70.77%~78.79%),high alkali(Na_(2)O+K_(2)O)(7.15%~8.73%),high ratio of FeO^(T)/(FeO^(T)+MgO)(0.70~0.98)and 10000×Ga/Al(2.68~6.84),low Ca(CaO 0.07%~0.90%),and low Mg(MgO 0.02%~0.88%).Their LREE compositions are relatively enriched and show obviously negative Eu anomalies.Moreover,they are enriched in Rb,Th,U,K,Nd,and Sm,but are relatively depleted in Nb,Zr,and Hf,and are strongly depleted in Ba,Sr,P,and Ti.These geochemical features indicate that the Huangcaoba granitoid pluton belongs to the A-type granite.In-situ zircon U-Pb dating results show that the gneissic granite(1090±24 Ma)and monzonite granite(1063±5 Ma)formed in the Mesoproterozoic with theε_(Hf)(t)values of+4.3~+10.3 and T_(DM2)ages of 1.33~1.62 Ga.All lines of evidence suggest that the diagenetic material came from the partial melting of new crusts.The crystallization age of the biotite granite was dated to 782.4±8.2 Ma,belonging to the Neoproterozoic period.Theirε_(Hf)(t)values vary from-7.3 to-2.1,all of which fall below the chondrite evolutionary line.Combined with their T_(DM2)age of 1.80~2.14 Ga,this biotite granite was derived from ancient crusts.Based on the previous research,the Huangcaoba A-type granitoid pluton was generated in a post-orogenic extensional environment during the Mesoproterozoic to Neoproterozoic period in response to the contemporary multi-stage orogenic cycle.