Geochronologic And Geochemical Constraints On The Paleoproterozoic Crustal Evolution Of Zhongtiao Mountains From Shanxi Province

In the last few years, great progress has been made in understanding the tectonicdivision and evolution of the North China Craton. Collisional models have been applied tothe Craton, including recognition of a Paleoproterozoic collisional orogen, called theTrans-North China Orogen, which separates the Craton into two distinct blocks, called theEastern and Western Blocks. Zhongtiao Mountains is situated in southern margin of northChina Block where the Paleoproterozoic supracrustal rocks is well exposed and igneousrocks is well developed. Detail geological and geochronological studies suggest thatArchean and Paleoproterozoic custal growths in Zhongtiao have episodic characteristics.Controversy has surrounded the classification of Sushui Complex,with some peopleproposing that Sushui Complex should be Archean age, whereas others classified it intoPaleoproterozoic age. Detail structural, geochemical and geochronological investigationsindicated that the previous Sushui Complex contains both of Archean rocks andPaleoproterozoic rocks in which the Paleoproterozoic rocks consists of Lengkoumetamorphic volcanics and 3 suites of metamorphic granitoids. The age of Tongkuangyuvocanics is a controversial issue in Zhongtiao Mountains. Combining mineral studies,SHRIMP dating for zircon from acid volcanics is 2274±14Ma(1σ), inferring an age ofPaleoproterozoic nor Archean.2300Ma island arc igneous rocks has been identified from the geochemical studies,which includes Lengkou metamorphic volcanics with age of 2333Ma and Zhaizimetamorphic tonalitic gneisses with age of 2321Ma. Geochemical character indicates thatthe volcanics is derived by partial melting of weakly depleted mantle and the TTG isformed by partial melting of of mafic rock in deep crust at high pressure. Thepost-collisional magmas between 2274Ma and 2250Ma is recognized, includingTongkuangyu volcanics, Henglingguan calcalkaline granites and Yanzhuang A-typegranites. The Tongkuangyu vocanics is the products of partial melting from metasomaticlithosphere mantle at shallow depth. The granites were derived by partial melting of TTG indeep crust. The geological and geochemical of igneous rocks indicates that the orogenicevent may be occurred between 2350-2250Ma in Zhongtiao Mountains.The nature and compostions of Paleoproterozoic lithosphere of Zhongtiao Mountainshave been discussed from comprehensive studies on geology, geophysics and geochemistry.Geochemical evidences provided a new idea that the lithospheric mantle exhibits weaklydepleted to primitive nature in composition. The lower part of lower crust is composedchiefly of mafic granulites and minor metamorphic supracrustal rocks and the upper partcontains mainly intermediate granulites, whereas the middle and upper crust consisits ofmetamorphic supracrustal rocks and igneous plutons ranging metamorphism fromamphibolite to greenschist phases. Detail geological and geochronological studies indicatesthat the primary crust of Zhongtiao Mountains occurred at 3.3-3.0Ga, considerable crustformed between 2.8-2.5Ga, a great deal of crust grows with time span from 2350Ma to2250Ma and final cratonization(Zhongtiaonian Movement) occurred at the age of 1900±50Ma.