华北地台北缘集宁地区古元古代片麻状石榴花岗岩的深熔成因及地质意义

Anatectic origin and geological significance of the Paleoproterozoic gneissic garnet granite in the Jining area,northern margin of the North China Craton

本文对华北克拉通北缘集宁地区空间上密切共生的片麻状石榴花岗岩和孔兹岩系富铝片麻岩的岩相学、地球化学及年代学特征进行了对比研究。SHRIMP锆石U-Pb定年方面,在富铝片麻岩中获得了1910±10Ma和1839±13Ma变质锆石年龄,在片麻状石榴花岗岩中获得了1919±17Ma的变质重结晶锆石年龄。在石榴花岗岩的石榴石包裹体中识别出与富铝片麻岩相对应的进变质阶段(M1)和峰期阶段(M2)的矿物组合,由此确认富铝片麻岩的变质作用和导致石榴花岗岩形成的深熔作用是同一构造热事件的产物。通过对二者变质作用演化及特征变质矿物的对比,认为深熔作用主要发生在峰期后等温降压阶段(M3),石榴花岗岩中的石榴石为深熔作用过程中的残留矿物相或转熔矿物相,而石榴花岗岩则是混合有大量残留矿物相的熔体结晶的产物。对片麻状石榴花岗岩和富铝片麻岩的地球化学组成特征进行了对比分析,片麻状石榴花岗岩既有一定的继承性,又有十分明显的变异性。变异性表现为:1)石榴花岗岩主量和微量元素含量分布极不均匀,微量元素含量普遍低于源岩(Cs、Rb、Th、U、Nb、Ta、Zr、Hf等);2)大离子亲石元素Cs和生热元素U、Th亏损明显,Sr相对富集;3)高场强元素Nb、Ta、P、Ti的明显亏损;4)铕异常变化大,存在铕富集型、铕平坦型和铕亏损型共存的稀土配分曲线的岩石,这是深熔成因石榴花岗岩最突出的表现,也可能是原地-半原地深熔花岗岩的主要地球化学标志。综合区域上的地质资料,认为深熔作用与碰撞后伸展构造背景下基性岩浆底侵事件有关。

The authors carried out a comparative study on petrography,geochemistry and geochronology of garnet granite gneiss and khondalite（ mainly alumina-rich gneisses） in the Jining area,northern margin of the North China Craton. SHRIMP U-Pb dating revealed two metamorphic zircon ages of 1910 ± 10 Ma and 1839 ± 13 Ma for alumina-rich gneiss and a metamorphic age of 1919 ± 17 Ma for gneissic garnet granite. The mineral assemblages of progressive metamorphism stage（ M1） and peak metamorphism stage（ M2） in alumina-rich gneiss are also identified in the inclusions in garnets of the garnet granite gneiss. It is concluded that formation of the garnet granite is due to anatexis of the alumina-rich gneiss,as a result of one and the same tectono-thermal event. It is also affirmed that the anatexis mainly occurred at the post peak isothermal depressurization stage（ M3） based on comparison of characteristic metamorphic minerals between them. The garnets in the garnet granite are residual mineral facies or peritectic in anatexis,and the garnet granite is a product of leucosome melt with a large amount of residual minerals. The garnet granite shows geochemical characteristics of the alumina-rich gneiss,but with some variations,such as： 1） Unevenly distribution of major elements and elements;2） Strong depletion of large ion lithophile element Cs,heat production element U and Th,and high field strength elements Nb,Ta,P and Ti; 3） Showing variations in Eu anomalies,as indicated by Eu-enriched,Eu-flat and Eu-depleted patterns,which is considered to be a prominent feature of garnet granite of anatectic origin and may be the mainly geochemical characteristic of in-situ and semi-situ anatectic granite. It is proposed that the anatexis was closely related to the underplating of basic magma in a post-collisional extensional tectonic setting.