Geochronology and trace element geochemistry of zircon, monazite and garnet from the garnetite and/or associated other high-grade rocks: Implications for Palaeoproterozoic tectonothermal evolution of the Khondalite Belt, North China Craton

Abstract

It is well documented that the Khondalite Belt of the North China Craton formed by the collision of the Yinshan and Ordos Blocks during ca. 1.95 Ga, but the onset of the post-collisional exhumation stage has not been well dated. The garnetite and garnet-bearing quartz-rich lens that are regarded as residue-melt phases occur occasionally in the foliated metasedimentary rocks in the Jining terrane of the Khondalite Belt. They are free of regional gneissosity, and may be generated during the post-collisional exhumation stage. Zircon U–Pb dating, monazite chemical dating and trace element analyses of zircon, monazite and garnet have been carried out on the garnetites and associated other high-grade metamorphic rocks. The geochronological results reveal a major age group of ca. 1890 Ma from the zircon grains/overgrowths and inclusion-type monazites within the garnet poikiloblasts in the garnetites. The trace elements analyses show that zircon hosted in the mantle of the garnet poikiloblasts in the garnetite contains the highest HREE and Y contents, whereas zircon hosted in the garnet rims shows higher HREE and Y contents than matrix zircon grains. Inclusion-type monazite in the garnetite shows higher LaN/SmN ratio than matrix monazite grains. Large garnet poikiloblasts in the garnetite exhibit bell-shaped HREE and Y zoning profiles attributed to Rayleigh fractionation during their growth. Compared to garnet poikiloblasts, coronal garnet shows higher Gro, Cr and Li contents, but lower XMg, Sm, HREE, Zn and Zr contents and weaker Eu negative anomalies, reflecting their different formation environments. Based on evidences from microstructural analyses, REE concentrations and patterns, HREE partition coefficients and the Ti-in-zircon thermometer, it is proposed that most of the zircon grains/overgrowths in the garnetite formed coevally with garnet poikiloblasts. Consequently, ca. 1890 Ma represents the timing of garnet poikiloblast growth during the partial melting of the protolith. Coronal garnet or Zn-rich spinel plus cordierite coupled with the matrix-type monazite formed after garnet poikiloblasts growth during near-isothermal decompression. Given the high metamorphic temperature and pressure revealed by the garnetite (820–850 ◦C and up to 950 ◦C; 8.5–9.5 kbar), it is suggested that the ca. 1890 Ma age group represents the beginning of extension/exhumation event in the Khondalite Belt, which is also recorded in the associated other high-grade rocks such as the garnet-bearing quartz-rich lenses, sillimanite–garnet-bearing quartzo-feldspathic gneisses, garnet-bearing quartzo-feldspathic gneisses and a pure quartz vein.