Ultrahigh metamorphic temperatures over 1050 °C recorded by Fe-Ti oxides and implications for Paleoproterozoic magma-induced crustal thermal perturbation in Jining area, North China Craton

Abstract

A pseudobrookite, approximately Mg0.04Fe2+0.85Ca0.01Al0.05Fe3+0.16Ti1.89O5, was discovered in Fe-Ti oxide intergrowths in a spinel–garnet-bearing granulite from the Xuwujia Paleoproterozoic ultrahigh-temperature (UHT) metamorphic region within the Khondalite Belt of the North China Craton. The presence of this pseudobrookite indicates a peak temperature possibly over 1120 °C, when compared with experimental data. Ferric pseudobrookite grains (Fe3+ up to 0.58 per formula unit) from the same granulite outcrop record retrograde temperatures as low as 940 °C. Titanomagnetite and ilmenite intergrowths that resulted from reduction-exsolution, which was buffered by the decomposition of high-Ti pseudobrookite grains, recorded a post peak temperature up to 1050 °C. The thermal gradient (>100 °C/kbar) required to generate the UHT metamorphic bodies in an area over 10,000 km2 around the Jining area was modeled by the finite element method. The thermal modeling results show that, (1) lithospheric thermal erosion induced by a mantle plume with potential temperature of >1550 °C could not have provided enough thermal perturbation to allow the observed UHT metamorphism; (2) underplating at the scale of even the largest known horizontal magma intrusion on Earth would not be able to elevate the geotherm to the gradients required for the observed UHT metamorphism; and (3) intraplating of dykes and intrusions at the depth of this UHT metamorphism was able to increase the local temperature field to the required gradients. These modeling results suggest that a modern back-arc may not be able to provide sufficient heat at the middle crust level for UHT metamorphism. The UHT metamorphism in the study area was probably accompanied by an orogeny–plume interaction event, which simultaneously supplied heat through thermal erosion, magma underplating and magma intraplating.