Study On The Early Permian Ophiolite In Guoganjianian Mountain Area, The Central Qiangtang In Tibet

Early Permian ophiolite of Guoganjianian mountain is a new discovery in the middle part of the Qiangtang,which is located in The Longmu Co-Shuanghu-Lancang River suture zone. According to field geological investigation and preliminary study on laboratory petrology, geochronology and geochemistry, we found Guoganjianian mountain Permian ophiolite exposed in the middle part of the Guoganjianian mountain. The scale of ophiolite is larger and outcrops is better. Early Permian ophiolite is mainly composed of cumulate rocks, mafic dyke swarms and basalt, in which the scale of mafic dyke swarms is the largest.The majority of ophiolite has developed distortion and deterioration, and output of tectonic slices as inequality in size, since the study area suffered varying degrees of regional metamorphism and dynamic metamorphism transformation. Gabbro dyke swarms under the basalt and cut it in some area. The breadth of single dyke ranges from 10 centimeter to 150 centimeter, and large grain and fine-grained is clear.There is developed gabbro of cumulatic texture in addition, and better layering of rocks.The average of SiO2 content in basalts and gabbros is 48.86% and 49.86%,and the average of K2O content is 0.1% and 0.9% respectively, Na2O content is 3.03% and 2.58%, CaO content is 11.21% and 9.23%, Al2O3 content is 15.11% and 14.06%. The study showed that the ophiolite is tholeiite series.The major elements of ophiolite geochemical characteristics is something similar to the MORB-type rock in general. The REE distribution patterns of basalts and gabbros is near flat,and different depletion LREE ,and no fractionation of HREE, and low light and heavy rare earth fractionation. Compared with mid-ocean ridge basalt REE distribution pattern, it's similar in distribution pattern. In the trace element spider diagram, gabbro and basalt showed loss of high field strength elements Nd, Ta, enriched in large ion elements Rb and U. The loss of Nb,Ta indicates that it may be subjected to terrestrial contamination and the influence of subduction during the formation of the rock.It shows that the genesis is related to subduction. In general, the Early Permian ophiolite trace element abundances indicates that it is close to the island arc basalt and ocean ridge basalt by contrast abundance of trace elements with the typical area.In addition, the research result of the ophiolite zircon SHRIMP U-Pb geochronology indicated that the zircon SHRIMP U-Pb age of basalt is 279±3.6 Ma, zircon SHRIMP U-Pb age of gabbro is 274±3.9 Ma. The age obtained is broadly identical, which is more accurate determination the age of the ophiolite in the Guoganjianian Mountain region.It is an important tool for geologists to judge the tectonic setting of basalt by discrimination diagram. This article completely uses the trace element distinction chart and adopts the multi- pattern synthesis method to discuss the formation of ophiolite environment for taking into account the discriminant diagrams of multiple solutions and determine the accuracy of the results diagram.According to Ophiolite gabbro and basalt trace elements tectonic discrimination diagrams, we can see that Early Permian ophiolite gabbro and basalt of Guoganjianian mountain not only being characteristics of mid-ocean ridge basalt , but also island-arc tholeiite. According to a variety of tectonic discrimination diagrams of the comprehensive comparison and study of the geochemical characteristics, indicate that Early Permian ophiolite tectonic environment may subduction zone of back-arc basin in the Guoganjianian mountain.