Kimberlite-borne garnet xenocrysts reveal complex lithospheric structure between intact and destroyed parts of the North China Craton

Abstract

The North China Craton (NCC) consists of a destroyed eastern part underlain by a sharply thinned lithosphere and an intact western part supported by a deep mantle root. Lithospheric structure under the transitional belt remains poorly constrained but critical for a thorough understanding on the mechanism for the differential destruction of the craton. Here, we explore this problem by lithospheric tomography using newly found kimberlite-borne garnet xenocrysts in the Hebi area, central NCC. The garnets include a high-Mg# (> 85.5, atomic Mg/(Mg + Fe^T) *100) group mainly equilibrated at high temperatures (1040–1160 °C) and restricted pressures (24–26 kbar), and a low-Mg# (< 85.5) group with a wide range of equilibration temperatures (790–1000 °C) and pressures (20–30 kbar). The inverted thermochemical mantle profile suggests the presence of a ~ 120–130 km thick lithosphere with an overheated and refractory mantle (olivine Mg# > 92) interior. The elevated heavy rare-earth element (HREE, from Ho to Lu) contents of the high-Mg# garnets (∑HREE > 20 ppm) relative to the low-Mg# garnets (∑HREE < 15 ppm) further suggest an element re-distribution due to the heating-induced garnet breakdown within the lithosphere. The selective heating of the refractory lithospheric interior likely reflects preferential asthenospheric upwelling through fossil lithospheric weak zones possibly related to the amalgamation of craton nuclei. Together with the knowledge on the eastern and western NCC, we suggest that the unevenly distributed lithospheric weak zones and the circum-craton tectonics should have jointly shaped the uneven loss of deep root of the craton.