| The northeastern Jiangxi is located at the junction of the Yangtze plate and Cathaysian plate,and on the geotectonic belongs to the Jiangnan orogenic belt in the margin of the Yangtze plate.Northeastern Jiangxi ophiolite is located in the southwest of JIANGSHAO fault,along the edge of zhangshudun in the northeast of Jiangxi and exposed at MaoQiao,was extended to the northeast.Based on the fully collection of the research production of predecessors,this paper carries out the research work of the field geological exploration,the indoor rock mineralogy?geochronology and whole rock geochemistryon of the gabbro in the ophiolitic melange of northeastern Jiangxi Province,the purpose is to find out the geochemical characteristics and formation age of gabbros,and attempts to explore the tectonic evolution history of northeastern Jiangxi ophiolite melange.This paper has obtained the following understanding:(1)The gabbro in the ophiolitic melange belt in northeastern Jiangxi Province is grayish green,massive structure,medium-coarse-grained structure,euhedral mineral particles.The primary minerals of gabbro are clinopyroxene and plagioclase,and there will be small and fresh or weakly altered pyroxene and feldspar inclusions in the clinopyroxene grains.The alteration of plagioclase is more serious,but polymer double crystal structure still can be seen in some of the altered plagioclase.(2)The study of mineral chemistry indicates that the chemical composition of plagioclase contained in gabbro varies little,with the characteristics of high Al2O3,CaO,low Na2 O and K2 O,is calcium feldspar plagioclase.Clinopyroxene is diopside,in which Na2 O and Al2O3 were low,the content of TiO2 is 0.03%~0.59%,the average is 0.34%,It can be initially determined that formation environment of gabbro maybe the expansion of the mid-ocean ridge by the characteristic of high Si and Ti.(3)The zircon SHRIMP U-Pb dating results of the gabbro in the ophiolitic melange of zhangshudun in the northeastern Jiangxi Province showed that the weighted mean age of 207 Pb / 206 Pb of the gabbro is 1029 ± 13 Ma,which belongs to the late Mesoproterozoic era,indicating that the northeastern Jiangxi is still in the initial expansionary phase of the basin.(4)Geochemical analysis showes that gabbro belongs to subalbine gabbro,with low K2O(0.09% ~ 2.62%),TiO2(0.14% ~ 0.25%),high Na2O(0.58% ~ 2.46%),Al2O3(14.51% ~ 16.72%)and other characteristics.The Mg # value of the gabbro is between 73 and 83,Mg# is low,the content of SiO2,K2 O,Na2O,CaO and Al2O3 is higher than that of the original mantle,and MgO is lower than that of the original mantle,which indicates that the mantle loss is low.The rare earth content is between 3.91 × 10-6 and 7.41 × 10-6,REE curve is flat,Light rare earth weak loss,the light and heavy rare earth fractionation is not obvious,LREE/HREE=0.74~1.17,(La/Yb)N is 0.18~0.49,the Eu shows obvious positive anomalies,indicating that the gabbro comes from the depleted mantle.Gabbro with enrichment in LILE such as Sr?K?Rb?Ba and depletion in HFSE like Ce?P?Zr?Hf?Sm?Ti?Y?Yb,TNT negative anomaly.Based on the results of mineralogy,geochronology and geochemistry of gabbro,this paper believes that the formation of ophiolitic melange in the northeastern Jiangxi Province has experienced several stages of tectonic evolution,the gabbros is probably formed in the expansion process of ancient Huanan ocean,the mineral composition of plagioclase is calcareous plagioclase,An is more than 95;The clinopyroxene is diopside and contains high Si and Ti,and has the characteristics of the source region of weakly depleted mantle,and the rare earth element has MORB type distribution pattern.In the process of subduction of the paleo ocean in the South China sea,the fluid released during the subduction process account of the mantle wedge subduction zone,which affects the micro-element composition of gabbro,leeding to the enrichment in LILE such as Sr?K?Rb?Ba and depletion in HFSE like Ce?P?Zr?Hf?Sm?Ti?Y?Yb.Therefore,it is considered that the ophiolitic melange in the northeastern Jiangxi Province is MORB-type ophiolite,formed in the mid-ocean ridge,with the subsequent superposition fluid transformation process,and affeced by the terrestrial material. |
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