Rutile records for the cooling history of the Trans-North China orogen from assembly to break-up of the Columbia supercontinent

Abstract

The amalgamation of the North China Craton along the Trans-North China Orogen at ~1.85 Ga has been widely discussed, but its thermal evolution after amalgamation is relatively poorly constrained. This study presents U-Pb ages, trace element compositions, and Hf isotopes of rutile xenocrysts carried by the Yangyuan Cenozoic carbonatites, revealing an episodic cooling history of the Trans-North China Orogen. The Zr-in-rutile thermometer records formation of the rutiles under granulite facies at 743–932 °C. Cr and Nb concentrations in rutiles support a pelitic protolith, which is consistent with 176Hf/177Hf ratios that are close to zircons from pelitic granulites in the orogen.U-Pb ages of the rutiles are cooling ages, for which three populations can be clearly delineated. In combination with dating results from other systems, three episodes of cooling can be identified: [1] active cooling after the ~1.85 Ga peak metamorphism, with a cooling rate increasing from 7.9 °C/Ma in the period 1850–1810 Ma to 15.3 °C/Ma at 1810–1792 Ma; [2] active cooling after mantle upwelling at ~1.78 Ga, with cooling rate increasing from 8.7 °C/Ma at 1780–1735 Ma to 17.2 °C/Ma between 1735 and 1719 Ma; [3] passive cooling after a rifting event at ~1.68 Ga, with cooling rate decreasing from 20.6 °C/Ma at 1680–1673 Ma to 6.9 °C/Ma in the period 1673–1633 Ma. The first two episodes correspond to exhumation-related cooling, while the last episode resulted from the rift-to-drift transition of the North China Craton due to increasing distance from the heat source during break-up of the Columbia supercontinent.