吉林南部新太古代末地壳深熔作用——来自变质石英闪长岩及淡色花岗岩的证据

Late Neoarchean crustal anatexis in the southern Jilin Province: Evidence from metamorphic quartz diorite and associated leucogranite

深熔作用是大陆地壳分异、元素迁移富集和混合岩化作用的主要机制和关键地质过程。吉南地区出露的太古宙基底普遍经历了角闪岩相-麻粒岩相变质及深熔作用,长英质淡色体及淡色花岗岩广泛分布。吉南和龙花岗-绿岩地体出露的太古宙变质石英闪长岩及相关的长英质浅色体和含斜方辉石(角闪石)淡色伟晶花岗岩的野外地质特征、相互关系及岩相学特征指示变质石英闪长岩经历了部分熔融,熔体发生了原地分凝聚集。锆石CL图像、U-Pb年代学、微量元素及Hf同位素分析显示,变质石英闪长岩与淡色伟晶花岗岩具有相同的岩浆锆石和变质重结晶锆石为主的锆石类型组成,其中后者以变质重结晶锆石为主,变质石英闪长岩的岩浆结晶年龄为2 535±14 Ma,二者的岩浆锆石具有统一的Hf同位素组成,变质重结晶锆石具有统一的U-Pb年龄,分别为2 495±22 Ma和2 491±10 Ma,统一的锆石微区Th、U含量及Th/U值,并且具有与岩浆锆石统一的Hf同位素成分,指示两者变质重结晶锆石与岩浆锆石具有形成于同一岩浆源区的初始结晶锆石,进一步指示变质石英闪长岩在约2.5 Ga经历了变质深熔作用,含斜方辉石(角闪石)淡色伟晶花岗岩源自变质石英闪长岩的原地部分熔融。结合华北克拉通及吉南地区新太古代末准同时的强烈岩浆作用和具有逆时针p-T轨迹的角闪岩相-麻粒岩相变质作用,其可能反映了华北克拉通东部陆块新太古代末在形成稳定克拉通过程中地幔物质上涌的热效应。

Anatexis is the main mechanism and key geological process of continental crustal differentiation, element migration and enrichment and migmatization. The Archean basement exposed in the southern Jilin Province, in which felsic leucosome and leucogranites are extensively distributed, widely experienced amphibolite-granulite facies metamorphism and anatexis. The field geological characteristics, correlation and petrographic characteristics of the Archean metamorphic quartz diorite and associated felsic leucosome and orthopyroxene(Opx)-and amphibole(Amp)-bearing pegmatite granite occurred in Helong granite-greenstone terrane in the southern Jilin Province indicate that the metamorphic quartz diorite underwent partial melting and in-situ segregation and accumulation of melt. The integrated analyses of zircon cathodoluminescence(CL) images, in-situ U-Pb geochronology, trace elements and Hf isotope on the metamorphic quartz diorite and pegmatite granite show that they have the same types of magmatic and metamorphic recrystallized zircons,the latter is dominated by metamorphic recrystallized zircons,that the metamorphic quartz diorite has magmatic crystallization age of 2 535 ± 14 Ma,and that their magmatic zircons have consistent Hf isotope composition. Their metamorphic recrystallized zircons yield consistent metamorphic ages of2 495±22 Ma and 2 491 ± 10 Ma,respectively,and have identical Hf isotope compositions,which are consistent with that of magmatic zircons,Th and U contents and Th/U ratios,indicating that both metamorphic recrystallized and magmatic zircons have initial crystalline zircons formed in the same magmatic source,and further demonstrating that the metamorphic quartz diorite experienced metamorphic anatexis at ca. 2.5 Ga and that the Opx-and Ampbearing pegmatite granite derived from in-situ partial melting of the metamorphic quartz diorite,in combination of the coeval strong and extensive magmatism and amphibolite-granulite facies metamorphism with counterclockwise p-T path at the end of Neoarchean in the North China Craton as well as the southern Jilin Province,which may reflect the thermal effect of mantle material upwelling on the continental crust during formation and stabilization of the craton lithosphere.