Archean Genesis Of Granitic Rocks And Crustal Evolution, In Eastern Hebei And Western Liaoning Area

The North China Craton is the largest and oldest known cratonic block in China. The Precambrian rocks distributed abroad, and developed complete. It recorded the very complex evolution process on earth with more than 30 hundred million years in geology history. In recent years, the North China Craton is tectonically divided into two major Archean to Paleoproterozoic blocks, named Eastern blocks and western blocks, separated by a Paleoproterozoic orogen, named the Central zone. The eastern Hebei and western Liaoning area belongs to the Eastern blocks in tectonic attribute, with large area of late Archean granitic rock exposure. This is an idea area for the evolution research of Archean lithosphere in North China Craton.In eastern Hebei and western Liaoning area, the Archean granitic rocks mainly are Suizhong granites, Anziling gneisses, Wangchang gneisses, Liugezhuang diorites and Qinhuangdao granites. Based on petrography and geochemistry characteristics, the granitic rocks in this area could mainly be divided into three types: tonalite-granodiorite(TTG), biotite adamellite and biotite granite. Based on field investigation, the distribution ranges of kinds of rocks are preliminary divided. Tonalite-granodiorite is distributed in west Anziling and Wangchang area. Biotite granite is distributed in north Qinhuangdao and south Suizhong area. And biotite adamellite is distributed between the two of them, in Heshangfangzi- Kuanbang area. There is a gradual change in the three of granitic rocks, without obviously boundary.In eastern Hebei and western Liaoning area, the zircon in granitic rocks generally have obviously fine-scale oscillatory growth zoning and high Th/u ratios, with the mode of depleted in light rare earth and enriched in heavy rare earth, suggesting magmatic origin. LA-ICPMS zircon U–Pb age shown that the granitic rocks age is 2500-2560Ma, indicated the mainly era of the continental crust accretion in this area is in late Archean, which is consistent with the mainly tectonic-magma events in North China Craton. Simultaneously, the age indicated that the three mainly granitic rocks basically have accordant form age, without obviously time interval.Geochemical characteristics indicated that tonalite-granodiorite and biotite adamellite all have high Mg#, low Rb/Sr ratios, Y and Yb, enriched in large ion lithophile elements and depleted in high field strength elements, shown as the product of basaltic crust partial melted at high pressure. However, tonalite-granodiorite have higher MgO, Cr, Ni, V, and lower SiO2, low light-heavy rare earth fractionation, suggesting early magmatic evolution. Biotite granite have high SiO2, K2O, low MgO, Cr, Ni, and high Rb/Sr ratios, suggesting a higher degree evolution of K-rich granite. And biotite granite have Eu negative anomaly,and depleted in Ba, Sr, suggesting the molten residuum are plagioclase. In addition, rock and zircon's rare earth characteristics all shows relatively strong light-heavy rare earth fraction, and high Yb, Y. All these geochemistry characters indicated the product of partial melting salic crustal at low pressure.To discuss the source region characteristics and evolution of different granitic rocks in late Archean, especially discuss the continental crust accretion mechanism and the old continental crust's characteristics, based on the zircon U-Pb chronology, corresponding zircons are carefully tested of Hf isotopes in situ. The test result indicated that: 1) Four periods of crust-mantle differentiation events as 3.7Ga, 3.0Ga-3.4Ga, 2.7-2.9Ga and 2.5-2.6Ga existed, the mainly event happed between 2.7Ga and 2.9Ga. 2) Partial zircon's crystallization age are consistent with Hf model age (2.5-2.6Ga), indicated new crust substance from mantle differentiation added when granite formed. 3) Confirmed inconsistency of the crust-mantle differentiation from the three granite rocks. Biotite granite reflected Eoarchean-mo crust-mantle differentiation events (3.7Ga, 3.0-3.4Ga, 2.9Ga). Tonalite-granodiorite and biotite adamellite reflected middle-late Archean crust-mantle differentiation events (2.7-2.9Ga,2.5-2.6Ga). 4) Presumed mainly crust-mantle differentiation and a few products of granitic magma in 2.7-2.9Ga, mainly granic magma generated in 2.5-2.6Ga and a few crust-mantle differentiation happened.According to the previous research results, the paper speculated that: 1) The Archean granitic rocks in this area distributed in planar, with a lot of amphibolite, pyroxenite and other mafic enclaves. 2) The formation age of the Archean granitic rocks in this area is 2.50-2.56Ga, indicated a large-scale crust transformation event mainly happed in 2.50-2.56Ga. 3) Metamorphsim rocks in this area generally have equal pressure cool-down type anticlockwise P-T path characteristics, suggesting a additional heat source from out crust (mantle). This may related with lots of magmatic emplacement from mantle differentiation. 4) The Hf isotope model age of zircon shown mantle differentiation simultaneously happed when large-scale granitic rocks generated in 2.5-2.6Ma. According to above characteristics, it is speculated that the large-scale magmatic events in late Archean in this area are related with mantle magma's underplating.