浙江沐尘石英二长岩及其镁铁质包体的锆石U-Pb年龄和Hf同位素组成——对岩浆混合作用的示踪  

Zircon U-Pb Age and Hf Isotopic Compositions of Quartz Monzonite and Enclosed Mafic Enclaves in Muchen Pluton,Zhejiang Province:Tracing Magma Mixing in Their Petrogenesis

作　　者：刘亮[1] 邱检生[1] 李真[1] 

机构地区：[1]南京大学成矿作用研究国家重点实验室,南京210093

出　　处：《地质论评》2011年第3期327-336,共10页Geological Review

基　　金：国家自然科学基金资助重点项目(编号40730313)的成果

摘　　要：浙江沐尘石英二长岩体中普遍发育形态各异的暗色镁铁质微粒包体。本文运用LA-ICP-MS锆石U-Pb定年技术,对该石英二长岩和其中的暗色镁铁质微粒包体进行了精确的年龄测定,获得的年龄分别为112·1±1·0Ma和112·4±1·2Ma,二者在误差范围内一致,说明它们同时形成,这为指示包体属岩浆混合作用成因提供了关键证据。包体野外多呈塑性形态,包体中可见与寄主岩中相似的钾长石斑晶,包体中的斜长石发育复杂的核边环带结构,并普遍出现针状磷灰石,这些特征也指示包体的形成经历了岩浆混合作用过程。包体与寄主岩样品具有相似的Hf同位素组成,其共同特点主要表现为εHf(t)值散布于正值与负值之间。综合分析表明,沐尘岩体中的暗色镁铁质微粒包体最可能为亏损的地幔组分与其诱发的地壳物质部分熔融形成的长英质岩浆经混合作用的产物。Numerous mafic microgranular enclaves（MMEs） with different shapes are found in quartz monzonites of the Muchen pluton in Zhejiang Province.Precise LA-ICP-MS U-Pb dating was conducted on zircon grains extracted from the host quartz monzonites and the MMEs,yielding crystallization ages of 112.1±1.0Ma and 112.4±1.2Ma,respectively.The consistent ages within analytical errors suggest that the host rocks and MMEs were coeval,providing crucial evidence for magma mixing during their petrogenesis.The MMEs commonly show plastic forms and contain similar K-feldspar megacrysts to the host rocks.Furthermore,plagioclases with core-rim zoning texture and apatites with euhedral acicular shape occur widely in the MMEs.These micrographic features are also consistent with the involvement of magma mixing in their petrogenesis.The MMEs and host quartz monzonites show similar zircon Hf isotopic signatures with εHf（t） values ranging from negative to positive.Integrated geological and geochemical characteristics suggest that the MMEs in Muchen pluton were most likely produced by magma mixing between mafic magmas derived from a depleted mantle source and induced felsic magmas generated from partial melting of crust materials.

关 键 词：镁铁质微粒包体 石英二长岩 锆石U-Pb年龄 HF同位素组成 岩浆混合作用 浙江沐尘岩体 

分 类 号：P597.3[天文地球—地球化学]