鞍山地区太古宙早期地壳生长及重熔——来自始—古太古代片麻岩杂岩的岩石学及年代学证据

Early Archean Crustal Growth and Re-Melting in the Anshan Area:Evidence from Petrology and Geochronology of the Eo-Paleoarchean Gneiss Complex

本文报道了鞍山地区新厘定的一套始—古太古代片麻岩杂岩的岩石学特征及锆石U-Pb年代学数据,以探讨太古宙早期地壳的形成及演化过程。杂岩在全区广泛分布,野外以包体形式产于约3.1Ga细粒奥长花岗岩之中。主要包括始太古代奥长花岗岩/石英闪长岩和古太古代片麻岩杂岩两个岩石单元,前者呈透镜体产出于后者之中。始太古代奥长花岗岩/石英闪长岩部分遭受变质作用改造,但整体较好地保留了岩浆结构及构造,其年龄为3.77～3.81Ga,锆石εHf（t）值大于0,表明其为玄武质岩石经部分熔融形成的新生地壳。古太古代片麻岩杂岩由条带状片麻岩、浅色奥长花岗质片麻岩、黑云母片岩及少量斜长角闪岩等组成,岩石类型多样且组构复杂、不均一,受到塑性流变变形作用的改造,为地壳深部层次下变质-深熔作用的产物,各岩石锆石U-Pb年龄相近,为3.29～3.36Ga,锆石εHf（t）值小于0,表明其来源于始太古代古老地壳的重熔。杂岩记录了～3.8Ga、3.7～3.4Ga以及～3.3Ga等多期岩浆-构造热事件,其中～3.8Ga和～3.3Ga分别代表本区主要的两期地壳生长和地壳重熔事件。多期次地壳生长和重熔可能是早期地壳演化的主要机制及过程,这与全球多个太古宙地体类似。

The petrological characteristics and zircon U-Pb geochronologic data of a newly recognized EoPaleoarchean gneiss complex in the Anshan area was presented in this study,aiming to investigate the formation and evolution process of the early Archean crust.The complex were widely distributed across the region,mainly as enclave within the ～3.1 Ga fine trondhjemite.The complex were composed of Eoarchean trondhjemite/quartz-diorite and Paleoarchean gneiss complex.The former is enclosed by the latter as enclaves.The Eoarchean trondhjemite/quartz-diorite mainly preserved igneous textures,but was partly altered by subsequent metamorphism.Their ages are dated to be 3.77～3.81 Ga and their zirconεHf（t）values are positive,indicating that they represent juvenile crust that originated from partial melting of basaltic rocks.Paleoarchean gneiss complex are composed of banded gneiss,light-color trondhjemitic gneiss,biotite schist and a small amount of amphibolite.They show complex and heterogeneous textures,and were altered by ductile deformation,which possibly formed in metamorphism-anatexis event at a deep crustal level.The Paleoarchean gneiss complex formed simultaneously at 3.29～3.36 Ga and their zirconεHf（t）values are all negative,indicating that they formed from re-melting of an Eoarchean ancient crust.In conclusion,the Eo-Paleoarchean gneiss complex recorded multiple magmatic-tectonic thermal events at～3.8Ga,3.7～3.4Ga and ～3.3Ga,in which ～3.8Ga and ～3.3Ga respectively represent two major phases for crustal growth and re-melting.Result in this study implies that multistage crustal growth and re-melting was possibly a dominant mechanism and process for early-stage crustal evolution,which is similar to that revealed from Archean terranes worldwide.