Xianquan Ping

Lower Crustal Accretion and Reworking Beneath the North China Craton: Evidences from Granulite Xenoliths

How has the Earth’s deep continental crust evolved? Most of our knowledge is derived from surface exposures, but xenoliths carried in igneous rocks can be an important source of information. The North China Craton (NCC) is one of the oldest cratons in the world and Phanerozoic igneous rocks with abundant xenoliths are widespread, making it an ideal area to study the formation and evolution of continental crust. Abundant data of U–Pb ages and Hf isotopes in zircons were obtained for lower crustal xenoliths from over ten localities to constrain the history beneath the craton. The oldest components of the NCC may be ~4.0 Ga. The craton experienced complex accretion and reworking processes in its deep crust, accompanied by the formation and differentiation of the ancient continental nucleus. The small size of the NCC, compared with many other cratons worldwide, made it more susceptible to the effects of marginal subduction and collision with surrounding blocks. In the lower crust, the ancient components of the craton were reworked in Paleoarchean (3.80–3.65 Ga) time. The craton also experienced two important accretionary episodes, in the Neoarchean (2.8–2.5 Ga) and the Paleoproterozoic (2.3–1.8 Ga). Asthenospheric upwelling in Neoproterozoic time (0.6 Ga) locally modified the lower crust. Subduction and collision of the surrounding blocks, such as the Yangtze Craton, in Paleozoic and in early Mesozoic time also strongly modified the lower crust, especially along the cratonic margins. Accretion and modification of the lower crust during late Mesozoic–Paleogene were obvious due to the addition of depleted-mantle materials (underplating).

Derivation of A1-type granites by partial melting of newly underplated rocks related with the Tarim mantle plume

The granitic rocks of the Tarim large igneous province (LIP) are temporally and spatially related to mafic intrusions. However, their tectonic setting and genetic relationship are debated. Here, we report geochemical, and zircon U–Pb–Hf isotopic results for three alkali feldspar granitic plutons in the Halajun area, western margin of the Tarim Block. Zircon U–Pb ages suggest these plutons were emplaced at 268–275 Ma, coeval with the neighbouring mafic–ultramafic complexes and syenitic rocks. These granitic rocks have high contents of SiO 2 , alkalis, Rb, Th, Zr and REE (except Eu), and high ratios of FeO*/MgO and Ga/Al, and show strong depletions in Ba, Sr, Eu, which are commonly observed in the A1-type granites. Zircon Hf isotopes reveal a limited range of e Hf ( t ) values from −1.0 to +3.5 for different samples from three granitic plutons, obviously higher than those (mostly