New constraints from garnetite on the P-τ path of the khondalite belt: Implications for the tectonic evolution of the North China craton

Garnetite pods containing >60 mol % garnet in association with garnet-bearing quart z-rich lenses are sporadically distributed in the Khondalite Belt, North China Craton. The mineral assemblages and microtextures of garnetite occurring in quartzo-feldspathic sillimanite-garnet gneiss in the Xiaoshizi area allow definition of the metamorphic history of the area. Formation of garnet poikiloblasts coupled with their enclosing and matrix minerals quartz, sillimanite, plagioclase, biotite, rutile and ilmenite defines an M₁ assemblage consistent with partial melting and subsequent melt separation. Thermodynamic modeling of bell-shaped Ca zoning in garnet poikiloblasts confirms that this occurred during decompression. A retrograde M₂ assemblage replacing M₁ garnet or sillimanite is defined by two microdomains: coronas of Grt2+Crd (M_2a) where quartz is present, and Spl + Crd symplectite (M_2b) where quartz is absent. The final metamorphic stage led to the coronal assemblage Grt3 + Opx + Crd + Pl + Bt (M₃) surrounding M₁ garnet. Comparison of the observed mineral proportions and compositions with the predictions of phase equilibria modeling in the NCKFMASHTO system for the effective bulk compositions, in combination with Zr-in-rutile thermometry, results in the following P-T conditions for the metamorphic events: 820-850⁰C (up to 950⁰C) and 8·5-9·5 kbar for M₁; 850-865⁰C and 7·4-7·6 kbar for M₂; 710-720⁰C and 6·4-6·6 kbar for M₃.The clockwise P-T path thus defined for M₁ to M₃ evolution is interpreted to reflect the onset of extension and exhumation of the Khondalite Belt at ~1890 Ma, following the emplacement of S-type granitoids at ~1920-1890 Ma and ultrahigh-temperature metamorphism at ~1920 Ma that resulted from collision of the Yinshan and Ordos Blocks at ~1950 Ma.