Ultrahigh-temperature metamorphism under isobaric heating: New evidence from the North China Craton

Abstract

The Khondalite Belt within Inner Mongolia Suture Zone (IMSZ) in the North China Craton (NCC) preserves evidence for extreme crustal metamorphism under ultra-high temperature (UHT) conditions at ca. 1.92Ga, associated with the subduction–collision tectonics between the Yinshan and Ordos Blocks. Here we report a new locality in Hongsigou where cordierite- and spinel-bearing granulites record UHT metamorphism. The prograde, peak, and retrograde mineral assemblages in these pelitic granulites have been identified based on petrography and mineral chemistry as: Bt1+Grt1+Sil1+Kfs1+Pl1+Ilm+Qtz1, Grt1+Sil2+Kfs2+Pl2+Spl+Ilm+Qtz2+Liq, and Crd+Grt2+Sil3+Kfs2+Pl2+Ilm+Qtz2 respectively. The peak metamorphic conditions of the pelitic granulite were estimated as 930–1050°C and 6.5–7.5kbar based on pseudosection analysis in the system NCKFMASHTO, suggesting extreme thermal metamorphism. We report LA-ICPMS zircon U–Pb data from the granulite which show weighted mean 207Pb/206Pb age of 1881±6.6Ma, marking the timing of UHT metamorphism. Lu–Hf analyses of the zircons show εHf(t) values within a restricted range of −4.2 to 0.3 and together with Hf model ages, a Paleoproterozoic arc magmatic source is inferred for the detrital zircons. The estimated P–T path for the UHT granulite suggests isobaric heating followed by cooling and decompression along a clockwise trajectory, different from the anti-clockwise P–T paths defined in earlier studies for the 1.92Ga UHT rocks from the IMSZ. The younger age and the isobaric heating trajectory suggest that the Hongsigou UHT rocks are related to heat input from underplated mafic magmas following continental collision.