The Formation And Evolution Of The Archaean Greenstone Belt In The Anshan-Benxi Area

Anshan-Benxi area is located in Liaoning Province, NE China, and is the place where consecutive geological records ranging from 3.8 Ga to 2.5 Ga are preserved. It is an ideal place for studying the evolution of North China Craton in Archean. Chronology, petrography, lithogeochemistry and structure have been undertaken on it. Then, combining with the previous research results, the formation and evolution of the greenstone belt in Anshan-Benxi area will be discussed in this paper.The metamorphic supracrustal rocks outcropped in Anshan-Benxi area is Anshan Group. It consists of Cigou Formation, Dayugou Formation and Yingtaoyuan Formation. High-precision SHRIMP U-Pb zircon dating of the amphibolites collected from Cigou Formation indicate that they were Neoarchean rocks and formed in 2571±5.3Ma. The Formations of metamorphic supracrustal rocks in Anshan-Benxi area have the similar sequence: Metabasaltic rocks on the bottom, metamorphic sedimentary rocks was local; magnetite quartzite in the middle(contain a small amount of metabasaltic rocks), and metamorphic sedimentary rocks in the upper. The major difference between them is the metabasalts and metamorphic sedimentary rocks represent different proportion in different Formation. The metabasaltic rocks of greenstone belt in Anshan-Benxi area have flat(TH1 type) or right dipping(TH2 type) REE patterns. TH2 type shows Nb and Ti depletion, and TH1 type has no Nb and Ti depletion. These geochemical characteristics indicate that TH2 type is similar to Island Arc Basalt(IAT), and TH2 type is similar to Middle Ocean Ridge Basalt(MORB). The association and temporal–spatial distribution of IAT and MORB and the results of tectonic discrimination diagrams plotting indicate that the metamorphic supracrustal rocks outcropped in Anshan-Benxi area probably formed in back-arc basin. The Formation which is rich in metabasaltic rocks was formed in the area far away from the continent or volcanic arc(Cigou Formation, Dayugou Formation), and the Formation rich in metamorphic sedimentary rocks was formed in the area closed to the continent or volcanic arc(Yingtaoyuan Formation). The association and temporal-spatial distribution of greenstone belt in North China Craton are collected in this paper. Then greenstone belt can be partitioned in the light of following:(1)Whether there are >2.55 Ga granitoids in this area;(2)Which type dose these metabasaltic rocks in the greenstone belt belong to, CAB type, IAT type, or MORB type?(3)Whether there are high-magnesium andesites;(4)The <2.55 Ga granitoids belong to TTG or potassium-rich granite;(5)Whether the provenance of metamorphic sedimentary rocks is volcanic arc or both continent and volcanic arc. This research concludes that the outcropping area of greenstone belt can be divided into three belts: Western Island-arc Belt(It contains Northern Liaoning, Western Liaoning, Eastern Hebei, Wutai area, and Dengfeng area), Middle Micro-blocks Belt(It contains Qianhuai Micro-block and part of Jiaoliao Micro-block), and Eastern Back-arc Basin and Continental Margin Belt(It contains Southern Jilin, Anshan-Benxi area, Eastern Shandong, Western Shandong, Wuyang area, and Lushan area).The Western oceanic crust subducted towards the east during 2.60Ga~2.53 Ga. This resulted in the formation of the Western Island-arc Belt and Eastern Back-arc Basin and Continental Margin Belt. Calcium Alkaline Basalt(CAB), Island Arc Tholeiite(IAT), Andesite(High-magnesium andesites are typical representatives), and TTG formed in the Western Island-arc Belt, and Middle Ocean Ridge Basalt(MORB) and Island Arc Tholeiite(IAT) formed in Eastern Back-arc Basin and Continental Margin Belt. There are many >2.55 Ga granitoids during the Eastern Back-arc Basin and Continental Margin Belt. They are continent crustal remnants separated from the mainland.The back-arc basin began to close around 2.53 Ga. It led to the oceanic crust subduction toward the Eastern Block. The combination of CAB, TTG and potassium-rich granite formed in continental margin in this stage. The oceanic crust occurred deformation and formed a series of NW-SE tending folds, ductile shear zone and faults during 2.52Ga~2.47 Ga. The basic tectonic pattern of basement structure in Anshan-Benxi area formed in this stage. Meanwhile, low amphibolite or high green-schist facies also occurred while deformation occurred.The second alteration occurred around 1.80 Ga. Previous faults revived due to closure and post-orogeny of the Liao-Ji rift. Mg-rich hydrothermal fluids transported along the faults, and they reform the magnetite quartzite and wall rocks around the fault. Finally, hydrothermal fluids altered the amphibolite into weakly altered rocks, chloritite, and garnetite, and altered the magnetite quartzite into cummingtonite rock and rich iron ore.