华南诸广山一带桂东岩体加里东期花岗岩的成岩时代研究及其地质意义

Geochronology and Geological Significance of Caledonian Granites in Guidong Batholith from Zhuguangshan Area,South China

桂东岩体位于诸广山—万洋山花岗质岩浆岩带的中部,沿NE向呈不规则条带状分布于湘赣交界处,该岩体作为诸广山—万洋山加里东期花岗质岩浆岩带的核心部分,记录着该岩浆岩带的形成时间、岩浆物质来源、壳幔相互作用等典型特征。在详细地质调查基础上,本文对桂东岩体主要岩石类型进行了岩相学、LA-ICP-MS锆石U-Pb年龄及Hf同位素组成的分析研究。结果表明,桂东岩体是一个复式岩体,岩浆活动持续>60Ma(474—411Ma),经历了4个脉动阶段:474—464Ma、452—442Ma、435—425Ma、415—411 Ma,其中,474—464 Ma、452—425 Ma、415—411 Ma期间的岩浆活动与华南加里东造山运动三个构造幕密切吻合。4个阶段岩浆活动对应岩石类型分别为黑云母花岗岩-花岗闪长岩、黑云母花岗岩-二云母花岗岩、黑云母花岗岩-二长花岗岩及角闪黑云花岗岩-黑云母花岗岩。原位锆石Hf同位素特征表明,大部分样品εHf(t)值位于球粒陨石演化线下方,主体集中于–2.20~–9.84之间,Hf同位素两阶段模式年龄TDM2为1969~1577Ma。其岩浆物质主要由华南造山带下地壳物质的一次或多次重熔而成,且幔源物质参与程度经历了由弱—强—弱,并与研究区太古宙—古元古代发生的重要地壳增生事件时间相呼应。

The Guidong Batholith is located in the middle of the Zhuguangshan–Wanyangshan granitic magmatic rock belt.It forms an irregular strip in a NE direction from the junction of Hunan and Jiangxi provinces.The batholith,which records the formation and magmatic origin of the Zhuguangshan–Wanyangshan rock belt and the evolution of the interaction between the regional crust and the crust-mantle,is the main component of the Zhuguangshan–Wanyangshan granitic magmatic rock belt.Based on the detailed geological survey of the Guidong Batholith,in this study we have carried out petrography,LA-ICP-MS zircon chronology and Hf isotope composition studies.The results show that the Guidong Batholith is a compound batholith,with magmatic activity that lasted>60 Ma(474–411 Ma).The magmatic activity can be divided into four stages:474–464 Ma,452–442 Ma,435–425 Ma,and 415–411 Ma,and the main rock types of these stages are granodiorite-biotite granite,biotite granite-two-mica granite,biotite granite-biotite monzogranite and hornblende biotite granite-biotite granite.The magmatic activities during 474–464 Ma,452–425 Ma and 415–411 Ma are consistent with the three tectonic episodes of the Caledonian orogeny in South China.The Hf isotope characteristics of the in-situ zircon show that,theεHf(t)values of most of the samples located below the chondrite evolution line,between-2.20 and-9.84,indicating that the diagenetic material may be mainly derived from the lower crust of South China orogenic belt.The diagenetic material or may also include a small amount of mantle-derived material or the partial melting of early magmatic rocks.The Hf isotope two-stage model age(TDM2)is 1969–1577 Ma.Therefore,the source magma mainly formed by the remelting of lower crust materials in the South China Orogenic Belt,and the degree of participation of these mantle-derived materials is a process from weak to strong to weak,which coincides with the important crustal accretion events in the study area during the Archean–aleoproterozoic time period.