Characteristic And Tectonic Significances Of Zonal Spinel In Northeastern Jiangxi Ophiolite

The study object is zonal spinel mineral in the northeast of Jiangxi ophiolitic, by detailed geochemical analyses of the mantle peridotite and basalt, In combination with the depth study in zoned spinel features, and composition analysis in association mineral.The purpose is that reveal the relation between micro-area zonal features and geological significance. Mainly considerable progress as follows:(1) Spinel in harzburgite can divided into three types: â‘  part alteration spinel of black and brown irregular shaped, has the feature of low Cr# value(20.25~38.39). These characteristics indicate the residual of deep-sea peridotite body and a partial melting degree of about 8%~11%; â‘¡ zonal spinel that a granular distribution in the serpentine,Cr# value(between 45.36 to 49.53) indicate the subduction environment; â‘¢ porous spinel that is disseminated and granular, high Cr# values(between 48.39 to 69.90)indicate island arc environment, reflect the late collision orogeny;(2) Trace elements of zonal spinel shows that FeO, Ga, Ni and Ti content increase,Al2O3, MgO, Cr2O3,Co and Mn content decreases from core to rim,andlinear relationship bewteen Mg# and Cr#, the core of Mg# is greater,Cr# is less than the rim;(3) The zonal spinel formed by the subduction metasomatic process, and re-ordering of trace element from core to rim. Due to the fluid metasomatism Al2O3 and MgO transfer into the spinel, chlorite may fill in the pore during the reaction,with the formation of porous spinel.The oxidizing fluids infiltrated the pores in the Cr2+- and Fe2+-rich porous spinel, and the re-ordering of Fe2+ and Fe3+ into octahedral and tetrahedral sites which promoted Zn, Co and Mn, together with Fe2+ enter the tetrahedral sites vacated by Mg2+, the entrance of Ni2+, Ga3+, Sc3+ and Ti4+ into the octahedral sites.While magnetite settle in the solution pore, thus promote the Fe2+ and Fe3+ diffusion in the spinel, diffusion from core to rim results in the formation of zona spinel;(4) Fluid play an important role in the formation of zonal spinel, and the source offluid in the mantle peridotite is closely related to tectonic environment. Studies have shown that reated ophiolitic melange of mantle peridotite characteristics of depleted mantle source area.The mantle peridotite of northeast Jiangxi ophiolite has characteristics of depleted mantle source area, original mantle melting residues, and showing some land-based sources of contamination. It may be affected by subduction.(5) According to the characteristics of zonal spinel and the regional tectonic process,understanding of spinel ring causes are as follows:Cathaysia and Yangtze plate first collage in the southwest Yangtze during 1100~1000Ma, formed the feature of MORB basalt and mid-ocean ridge deep peridotite characteristics of alteration spinel.During1000 ~ 890 Ma, with subduction of ancient south China oceanic to Yangtze block, a higher degree of melting occurred by the mantle wedge, which results in the formation of oxidation edge and high Cr# values zoned spinel, also namely SSZ type ophiolites. At the same time, the oxidizing fluids infiltrated the pores in the Cr- and Fe2+-rich porous spinel,and the re-ordering of Fe2+ and Fe3+ which promoted the Complex transformation of Ni2+,Ga3+, Sc3+ and Ti4+. Cathaysia and Yangtze plate collaged in Shaoxing-Jiangshan-Pingxiang area during 890~850Ma, the northeast Jiangxi ophiolitic formed by the tectonic stress action, indirect cause spinel mineral internal fissure development, and basalt has the feature of low Ti, Nb and Ta affected by terrigenous material contamination and reduce; Region occured magma activity during 850~800Ma,influence on spinel micro zone is unknown.Spinel has the instruction significance of rock formation, which discuss the causes of ophiolite mineralogy provide direct evidence. This study on zonal spinel in the northeast of jiangxi ophiolitic melange belt indicate that spinel have recording function on the formation and evolution of ophiolite, also can reflect rock deep geological process.In addition, in this paper, the zonal spinel is just a preliminary study, in the subsequent research can have a further system analysis and research on the trace elements, isotopes,fluid source of zonal spinel micro, reveal distribution regularity of elements in the spinel micro, and limit fluid properties and mineral formation temperature, pressure, oxygen fugacity and other physical and chemical information.