安徽庐枞地区中生代A型花岗岩类及其岩石包体：在碰撞后岩浆演化过程中的意义  

Mesozoic A-Type Granitoids and Xenoliths in the Lujiang-Zongyang Area,Anhui Province:Significance in Post-Collisional Magmatic Evolution

作　　者：曹毅[1] 杜杨松[1] 蔡春麟[1] 秦新龙[1] 李顺庭[1] 向文帅[1] 

机构地区：[1]地质过程与矿产资源国家重点实验室,中国地质大学,北京100083

出　　处：《高校地质学报》2008年第4期565-576,共12页Geological Journal of China Universities

基　　金：国家自然科学基金项目(40672045);教育部科学技术研究重大项目(308006)联合资助

摘　　要：安徽庐枞地区位于下扬子断陷带内,区内中生代岩浆活动强烈,壳幔交换频繁,形成了一系列A型花岗岩类,其中产有一些同源岩石包体。这些A型花岗岩类以富碱富钾为特征,为准铝质硅饱和岩石,具有高的104×Ga/Al比值和REE含量,明显富集Rb,Th,K等大离子亲石元素,而Nb,Ta,Ti和Zr等高场强元素和Sr,P相对亏损。与寄主岩相比,岩石包体SiO2和全碱含量偏低,Cr,Co,Sc,V等元素明显偏高,Zr和Eu的负异常不明显。包体和寄主岩的(87Sr/86Sr)i值为0.7053~0.7089,εNd(t)值为-2.2^-8.66。这些资料表明,庐枞地区中生代A型花岗岩类是起源于富集岩石圈地幔的玄武质岩浆与地壳物质发生轻度同化混染作用,并经历结晶分异作用的产物,在岩浆演化过程中,结晶分异作用发挥着主导作用。从岩石组合来看,庐枞地区的A型花岗岩类主要由石英正长斑岩、正长斑岩、辉石二长岩和碱长花岗岩组成,属于碰撞后准铝质镁铁质-长英质岩套的一部分。岩石样品分析数据在Nb-Y-Ce,Nb-Y-3Ga和Rb/Nb-Y/Nb图上的投影结果表明,庐枞A型花岗岩类为碰撞后环境结束阶段的产物。结合区域地质背景分析,可以认为庐枞地区A型花岗岩类形成于岩石圈伸展背景下的碰撞后岩浆活动的末期,其出现可能标志着碰撞后环境的结束。The Lujiang-Zongyang area in Anhui Province is located in the fault-depression zone of Lower Reaches of Yangtze River. A series of A-type granitoids with various autoliths formed by intensive magrnatic activities and frequent crust-mantle interaction during J3-K1 are exposed in this area. Chemically, the A-type granitoids are rich in alkalies, REE and LILE （e.g., Rb, Th and K）, but poor in HFSE （e.g., Nb, Ta, Ti and Zr）, Sr and P, and have higher ratios of 10^4 × Ga to A1. In contrast, the autoliths are poor in SiO2 and alkalies, but rich in Cr, Co, Se, V, Zr, and have higher ratios of Eu to Eu^＊. The autoliths and their host rockshave similar （^87Sr/ ^86Sr）i values （0.7053 - 0.7089） and eNd（t） values （-2.2 - -8.66）. These data support the formation of these A-type granitoids through slight crustal material contamination combined with intensive fractional crystallization of basaltic magma derived from an enriched lithospherie mantle. Lithologies of the A-type granitoids include quartz syenitie porphyry, syenite porphyry, pyroxene monzonite and alkali feldspar granite, and constitute a part of the post-collisional metaluminous mafic-felsie igneous suites. The distribution of all analyzed samples in a small area near the boundary of A1 and A2 on the plots of Nb-Y-Ce, Nb-Y-3Ga and Rb/Nb-Y/Nb for subgroup discrimination of A-type granitoids is consistent with their formation at the end of post-collisional magmatie activity in a setting of lithospheric extension. Therefore, their occurrence marks the end of the post-collisional tectonic setting.

关 键 词：碰撞后 岩浆演化 A型花岗岩 岩石包体 安徽庐枞地区 

分 类 号：P588.121[天文地球—岩石学]