Precambrian Crustal Evolution In The North China Craton

The North China Craton(NCC) is among the ancient cratonic cores of Asia with rock records that preserve the evidence for Precambrian crustal evolution history and related tectonic processes. The research undertaken as part of this Ph D thesis focused on selected corridors in the NCC including the Inner Mongolia Suture Zone, the Trans-North China Orogen and the Eastern Block. The major objectives of this work are to understand the Neoarchean and Paleoproterozoic tectonic events and their implications on crustal evolution history. The petrologic(petrography, EPMA, fluid inclusion), geochemical(major, trace and REE) and geochronologic(zircon U-Pb and Lu-Hf) investigations undertaken in this study have led to the identification of prominent magmatic events during late Neoarchean, early Paleoproterozoic and late Paleoproterozoic, suggesting crust building and recycling events during this time in the NCC. The age data suggest multiple magmatic events at ca. 2.5-2.4 Ga, 2.1 Ga, 1.9 Ga and 1.79-1.75 Ga in Xinghe-Jining area with the metamorphism during ca 1.9-1.83 Ga. Magmatic suites in the Trans-North China Orogen were emplaced at ca. 2.2-2.0 Ga, 1.87-1.85 Ga and 1.76-1.73 Ga, associated with pre-, syn- and post-collisional events related to the final collision between East and West Blocks. Studies on the Qianxi Complex within the Jiaoliao microblock show arc-related magmatism at ca. 2.55 Ga followed by metamorphism at ca. 2.50 Ga. The Lu-Hf isotopic data from all the rock types in different tectonic belts show both negative and positive values, suggesting that the magma sources are derived from both juvenile components and reworked crustal material. The geochemical data from the various rock suites suggest that the dominant process of crustal evolution involved arc magmatism in convergent margin settings. The high temperature and dominantly anhydrous charnockites from the Luyashan Complex along the western margin of the TNCO(showing CO2 as the dominant fluid phase from the fluid inclusion studies) suggest magmatism associated with a ridge subduction event during late Paleoproterozoic. Widespread post-collisional magmatism occurred in the NCC generating a range of ultramafic, mafic, intermediate and felsic rock suites. Among the Neoarchean and late Paleoproterozoic metamorphic events, the latter reached up to ultra-high temperature conditions with the peak P-T conditions of 930-1050 °C and 6.5-7.6 kbar.This study also reports for the first time the occurrence of incipient charnockites in the NCC, and models the pressure-temperature regimes and fluid flow revealing that the patch- and vein-type of charnockites were formed within TTG(tonalite-trondhjemite-granodiorite) gneisses through CO2 advection at high temperatures. Infiltration of CO2-rich fluids during high grade metamorphism is also traced from carbon stable isotope studies of graphite from metapelites(khondalites) where the dominant category of graphite is of biogenic origin with a subordinate variety of fluid-precipitated type.In conclusion, the data obtained in this study trace juvenile crust formation in the NCC throughout Paleoproterozoic, in addition to the major pulses during Neoarchean, suggesting that there was no global plate tectonic shut down during the Siderian period. The mechanisms of formation of the Precambrian rock suites in the NCC are broadly similar to those generated through modern-style plate tectonic processes.