Petrographic And Zircon Geochronological Studies Of The Taihua Group On The Southern Margin Of The North China Craton

The "Taihua Group" is a collective term for a series of old terranes scattered along the southern margin of the North China Craton. The timing of formation and thermal overprinting of the Taihua Group have long been contentious, and its relationship with the Qinling orogenic belt has been unclear. In order to resolve these questions, a detailed study of geological investigations, whole-rock major and trace elements analysis, zircon U-Pb age dating and zircon Hf isotopic analysis, and whole-rock geochemical analysis were conducted for an amphibolite (from the Xiong'ershan terrane) and a biotite gneiss (from the Lantian-Xiaoqinling terrane).Samples collected from Xiong'ershan terrane are interbanded amphibolites of the Taihua Group. The representative Qiyugou amphibolite (Q1) is equigranular and shows typical metamorphic equilibrium microtextures. It is composed of plagioclase and amphibole in equal proportions. Major-element and trace-element analyses show that this amphibolite is rich in Na2O (3.62%), K2O (1.81%), and REE (∑EE=67.13 ppm) contents. The representative banded biotite gneiss (QL0701) in the Taihua Group of the Lantian-Xiaoqinling terrane shows a granoblastic microtexture and gneissic foliation. It is composed of plagioclase (An =23,60～65%), quartz (10～15%), biotite (15～20%), amphibole (5～10%) and minor potassium feldspar. A typical metamorphic "triple junction" equilibrium microtexture is well developed. Its REE pattern is different from that of the amphibolite in showing significant depletion in the heavy rare earth elements, with (La/Yb)N=42.98.The data of integrated in-situ U-Pb dating and Hf isotope analysis of zircons from these rocks indicate that the Upper Taihua Group formed during the Paleoproterozoic (2.3-2.5 Ga) and thus was originally part of the southern edge of North China Craton, detached during the Mesozoic Qinling orogeny and displaced about 100 km north from its original location. This suggests that the Taihua Group became part of the tectonic terrane associated with the Qinling orogeny and now forms part of the overthrust basement section of the Qinling belt. Before the Qinling orogeny, the Taihua Group was metamorphosed at 2.1 Ga. The initial Hf-isotope compositions of zircons mainly distribute between the CHUR and Depleted Mantle (DM) lines in the plot of (176Hf/177Hf)i vs. age, together with their positive whole-rockεNd(t) values (+2.7～+3.2), implying that the original magmas were derived from a juvenile source with some assimilation of an Archean crustal component.