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

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 +/- 10Ma and 1839 +/- 13Ma for alumina-rich gneiss and a metamorphic age of 1919 +/- 17Ma for gneissic garnet granite. The mineral assemblages of progressive metamorphism stage (Ml) 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.