Lithospheric evolution of the northern Trans-North China orogen from Paleoproterozoic to Cenozoic
A consensus has been reached that eastern part of the North China Craton (NCC) experienced lithospheric thinning and transformation while the western part remained stable. Studies of the Trans-North China Orogen (TNCO, also called the Central Orogenic Belt), which lies between the western and eastern blocks of the NCC, are critical for understanding the mechanism of lithosphere evolution. Here, systematic researches were conducted on various of samples from the northern TNCO, including peridotite and pyroxenite xenoliths in Cenozoic basalts, carbonatite dykes, and rutile xenocrysts in carbonatites. These samples provide deep insights into the lithospheric evolution of the northern TNCO.
The first part of the thesis investigates mantle xenoliths in the Cenozoic basalts. New insitu analyses of major and trace elements and Sr isotopic compositions are presented for spinel harzburgite and lherzolite in basalts from Datong. The results show that the lithospheric mantle beneath the northern NCC experienced varying degrees of metasomatism by carbonatite melts related to subduction of the Paleo-Asian oceanic plate (PAOP) during late Paleozoic. In-situ analyses of spinel peridotite and pyroxenite xenoliths in the Y angyuan basalt provide constraints on the subduction style of the PAOP, revealing roll-back of the southward subducted P AOP in the late stages of subduction. Before this, low-angle subduction of the PAOP had triggered hydrous metasomatism of the overlying NCC lithospheric mantle at 295±38 Ma according to mineral Rb-Sr isochron dating.
Despite the great potential of Rb-Sr isochrons for dating metasomatism, this method is difficult for in-situ analysis because of isobaric and polyatomic interferences on Sr isotopes, especially 87Rb. This problem can be solved by using laser ablation triple-quadrupole ICP-MS (LA-QQQ-ICP-MS) and a method development study has been completed here, optimising this new technique for samples that include potassium-rich phases. After overcoming limitations in PIA calibration and Rb-Sr fractionation during analysis, in-situ Rb-Sr dating was successfully performed on samples with a wide range of geological ages. This technique was then applied to phlogopite-bearing xenoliths in the Hannuoba basalts: the Rb-Sr isochrons show that the age of mantle metasomatism in the northern TNCO is ~47 Ma with radiogenic initial Sr isotopic composition (0.71205±0.00039). These data allow the conclusion to be made that subduction of the Pacific plate triggered metasomatism at the crust-mantle boundary beneath the northern TNCO.
The Yangyuan carbonatite dykes, which intrude and thus post-date Cenozoic basalts, contain rutile xenocrysts that provide further constraints on the crustal evolution of the northern TNCO. Geochemical analyses of these carbonatites show them to be derived from limestones that were melted by asthenospheric upwelling beneath the TNCO during the Cenozoic. The Phanerozoic limestones were deposited on the NCC and thrust into the lower continental crust during the development of the Yinshan-Y anshan fold and thrust belt during the Mesozoic, providing the source for the carbonatites. In-situ analyses of major and trace elements and UPb-Hf isotopic compositions of rutile xenocrysts show that they were captured from lower crustal pelitic granulites in the basement of the northern TNCO. Combination of the U-Pb ages of these rutiles with dating results from zircons and biotites permit the identification of three episodes of cooling (1850-1792 Ma; 1780-1719 Ma; 1680-1633 Ma). The first two episodes correspond to crustal exhumation, while the last episode resulted from the rift-to-drift transition of the NCC due to increasing distance from the heat source during break-up of the Columbia supercontinent.