Research On Early Palaeozoic Tectonic Characters Of DHNS, GS

Danghe nanshan uncovering Eopaleozoic layer there was in the area of Subeicounty,Gansu province. Strata from old to new are Wu Ligou group,Yan Chiwanformation,Duo Suoqu formation and Ba Longgonggaer formation.Wu Ligou group with three different lithologic member belongs to EarlyOrdovician. In the lower part of Wu Ligou group, the rock association includeskeratophyric rubble moltenlava,spilite, basalt-moltenlava and so on, both of whichcontain tholeiitic series rock sand calc-alkaline series rocks. In the second part of WuLigou group , the rock association includes andesite porphyry, quartz andesite, whichwas dominated by calc-alkaline series. In the third part of Wu Ligou group, the rockassociation includes crystalline limestone. The chemical and geochemicalcharacteristics of rocks indicate that the basic volcanic rocks and oceanic tholeiitehave the same characteristics. Intermediate and basic rocks have similar major elementcontents(ΣREE39.29~1205.41μg/g,LREE/HREE=3.42~10.51) and the similiar weakEu anomaly. In terms of characteristics of trace elements, the rock is rich in Sr, Rb, Ba,Th selectively, especially in Th, but poor in Nb, Zr, Ti, Y, Yb. Moreover, it's aevolutive relationship between Wu Ligou group and basic igneous rock in terms ofchemical composition.Therefore, this conclusion can be that the basic igneous rocksof Wu Ligou group show the oceanic circumstance and volcanic island arc based onoceanic crust.Yan Chiwan formation belongs to Middle Ordovician. The rock associationincluding green pelitic siltstone, arkose quartzite siltstone, argillaceous rockd andmany others are a flysch formation . The matrix and Calcium content in this rockcomposition is high, but the grade of maturity and composition is at lower level. Thesub sea and the deep sea with turbidites and flysch deposition shale portfolioconstruction reflects its sedimentary environment in the formation of continental slope to the abyssal plain transition .Duo Suoqu formation belongs to Late Ordovician. Major elements of volcanicroc still have the characteristics of oceanic tholeiite. According to the geochemicalcharacteristics of Late Ordovician volcanic roc in South Qilian, I realize that thevolcanic roc of Duo Suoqu formation belongs to igneous island arc and exsit adescending plate in this area.Ba Longgonggaer formation belongs to Silurian, the rock group is composed ofthe construction of deep-sea flysch phasethe consisted by lower part of Thinnersandstone, fine sandstone gravel, change sand siltstones, calcareous lithic sandstone,lithic sandstone, silty slate, sericite slate, chlorite slate and Sea - continental molasseconsisted by up part of the formation. In space, the northern deep sea sedimenttransition to shallow marine and deposits to the south indicates a more classic featuresof the foreland basin.There are two different periods granites in the study area. Early, I type granitesand adakites formed in the Ordovician and respond to the process of oceanic platesubduction to north at the same period in South f Qilian District. Late, I-S typegranites formed in Silurian of Devonian is a prodouct of collision orogenesis. Inaddition, there are basic dykes formed in 365Ma in this area, representing theprocess of continental crust uplift and extension after orogeny.According to environmental features of this rock at different times, reflectedestablished tectonic evolution of models early Paleozoic in this area:①In the EarlyOrdovician ,the south Qilian Ocean extend environment into subduction,andformatted of intra-oceanic arc on ocean crust②In the Middle Ordovician there is aquiet period for the tectonic movement,;③In the Middle Ordovician, the oceanic crustsubduct in large-scale to the continental crust again. The trench-arc-back arc systemin south of danghe mountain-Laji mountain- Wusu mountain was formed④As earlyas the beginning of the Silurian, the south of qilian oceanic basin is closured.andturned to the collisional stage. Foreland basin formed in the south of danghe mountain.Finally, in the late Silurian, formationed collision orogenesis and extend to the Early -Middle Devonian. In the late Paleozoic , a new period of tectonic evolution wascoming.