| The northeastern Jiangxi province is located at the junction of the Yangtze plate and Cathaysian plate,it belongs to the Jiangnan orogenic belt in the southeast margin of the Yangtze plate.The Ophiolitic mélange in northeastern Jiangxi is mainly distributed in the"Northeast Jiangxi deep fault zone"of Zhangshudun,Xiwan,Luowan,Maoqiao.The ophiolitic mélange alongs the NNE-orientated structure,extending northward to Zhejiang and from west to Yiyang.This article is based on the previous data,a comprehensive geological survey of ophiolitic mélange which exposed in northeastern Jiangxi Province,combined with the identification of rock microsection,geochemical analysis and LA-ICP-MS zircon U-Pb dating,expoundcharacteristics petrological,geochemical and geochronology,and discussed the the tectonic evolution of ophiolitic mélange The following achievement was obtained by the research:?1?The Zhangshudun ophiolitic mélange in northeastern Jiangxi is composed of more than 20 different sizes of rocks,including ophiolitic olivine pyroxenite,Serpentite,gabbro,basalt,diabase and so on.The serpentine is dark green-black green,scale blastic structure,block structure,the mineral composition is mainly serpentine,olivine,magnetite and calcite.Serpentinized olivine pyroxenite is greyish green,gray black,fibrous blastic structure and massive structure,and its main mineral components are serpentine,pyroxene and olivine.Gabbro is dark gray,black gray,gabbro structure,block structure,the main mineral composition is common pyroxene and plagioclase.Basalt is black,dark green,porphyry structure,block structure,the phenocryst are mainlyincludepyroxene and plagioclase.Ophiolite is black,dark green,cryptocrystalline structure,block structure,the mineral compositions mainly include plagioclase,pyroxite and pyrite.?2?Petrogeochemistry of ophiolitic mélange in northeastern Jiangxi shows the characteristics of low MgO?5.30%7.94%?and K2O?0.52%1.51%?,medium TiO2?1.48%2.05%?and high Na2O?2.89%4.47%?,which belongs to the series of tholeiite.Reated ophiolitic mélange sample of rare earth elements?REE range in56.64 x 10-6-74.59 x 10-6,LREE range in 41.08 x 10-6-52.84 x 10-6,HREE range in13.97 x 10-6-21.75 x 10-6,LREE/HREE=2.433.02,enrichment of light rare earth elements,LaN/YbN ratio between 1.692.63,significantly greater than 1,the delta Eu=0.831.12,there is no obvious europium anomalies.Its trace elements are expressed as large ionic lithophile elements?LILE?Rb,Ba,K,Sr,Nd and Pb enrichment,indicating that the ophiolitic hybrid rocks are affected by the regression.High field strength elements?HFSE?what them rest of the degree enrichment and E-MORB,obvious differences to OIB and N-MORB,is similar to the characteristics of enrichment type ridge basalt?E-MORB?.To sum up,it is believed that the ophiolitic mélange in northeast jiangxi is formed in the initial expansion stage of the mid-ocean ridge,and is influenced by the fluid metasomatic enrichment effect of subduction of the plate.?3?the cathodoluminescence pictures and U-Pb age of LA-ICP-MS zircon from the gabbro of Zhangshudun shows:The zircons are mostly colourless,transparent and short columnar,and the crystal particles are not completely nongrind.most of the zircon has a clear belt structure,which is a typical cause of magma crystallization.The Th/U ratios of zircons are between 0.220.53,which are all greater than 0.1,indicating that zircon is characterized by magmatic zircon.The weighted average age(206Pb/238U)of the 40 measure point in the 38 zircon is 1094+6.2 Ma?MSWD=0.81?.This age represents the crystalline age of gabbro,indicating that the time of the formation of the oceanic crust of Zhangshudun ophiolitic mélange is Middle proterozoic era.?4?The tectonic evolution in the northeast of Jiangxi Province is as follows:1100950Ma:during the ocean expansion stage,The E-MORB ophiolite is formed;950870Ma:oceanic crust subduction stage,at this stage the island arc had formed as Shuangxiwu.870820Ma:The development stage of the island arc,and had formed large scale volcanic rocks which is called Zhangcun Group in Northeast Jiangxi.At the same time,the back arc were expanding,had formed a large scale unconformity interface.and the same orogenic S type granite and post orogenic A type granite are also developed. |
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