湘西通道地区新元古代长界橄榄辉石岩中单斜辉石矿物学特征及其板内裂谷成因论证

摘要

湘西通道地区长界橄榄辉石岩主要组成矿物为单斜辉石、橄榄石和斜长石,还具有少量的Ti—Fe氧化物.本文利用电子探针对江南造山带西段湘西通道地区长界橄榄辉石岩单斜辉石进行了详细的矿物学研究,研究结果表明单斜辉石属于顽透辉石和普通辉石.长界橄榄辉石岩单斜辉石较富镁而贫铁,并且还具有高硅、贫碱、低钙的特征,其母岩浆属于拉斑玄武系列.单斜辉石温压计算结果显示,橄榄辉石岩的形成温度为1260 ～ 1383℃,压力为1.5 ～2.6GPa,深度为50～86km,平均深度为73km.整体来看,长界橄榄辉石岩形成的温度基本与软流圈地幔的温度(1280 ～ 1350℃)相当,其形成深度为约73km的地幔深处,且在这种温压条件下,软流圈地幔可以发生减压熔融.单斜辉石的成分特征显示示长界橄榄辉石岩是板内拉斑玄武岩,指示其构造背景为裂谷环境.另外,单斜辉石具低的Al2O3(0.02％ ～ 5.51％)、TiO2(0 ～0.46％)和Na2O(0.01％ ～ 1.44％)的特征,这可能是单斜辉石寄主岩石岩浆源区受到俯冲熔体/流体的交代改造作用所导致.结合作者和前人的研究结果,我们推测长界橄榄辉石岩源区为软流圈地幔,其形成可能与裂谷作用导致软流圈物质上涌并发生熔融有关,且在形成长界橄榄辉石岩之前,软流圈地幔已经遭受过俯冲熔体/流体的交代改造作用.%Objectives:The Changjie olivine pyroxenolite in Tongdao County,western Hunan Province contains clinopyroxene,olivine and plagioclase with a small amount of Ti—Fe oxide minerals.In this paper,we report systematic mineralogical and mineral chemical studies on the clinopyroxene of the Changjie olivine pyroxenolite.Based on the new results,we discuss the physical and chemical condition for the formation of the Changjie olivine pyroxenolite,constraining on the deep material compositions,thermal state and geodynamic setting.Methods:We determine the chemical compositions of the clinopyroxene of the Changjie olivine pyroxenolite by electronic microprobe method in detail from the Tongdao area in western Hunan Province,the western segment of Jiangnan (south Yangtze) Orogen Belt.Results:The results show that the clinopyroxene of the Changjie olivine pyroxenolite is belonging to endiopside and augite.The clinopyxene is relatively enriched in magnesium but poorer in iron with high silicon,low alkali and calcium contents.The parental magma of the Changjie olivine pyroxenolite should belong to tholeiitic series.The crystallization temperature and pressure of clinopyroxene of the Changjie olivine pyroxenolite have been estimated by the clinopyroxene—melt equilibrium.The results show that the crystallization temperature of the Changjie olivine pyroxenolite is 1260 ～ 1383 ℃,and pressure is 1.5 ～ 2.6 GPa with the depth of 50 ～ 86 km (the average depth of 73 km).Generally,the formation temperature (1260 ～1383 ℃) of the Changjie olivine pyroxenolite is compared with the temperature of the asthenospheric mantle (1280 ～ 1350 ℃),and the formation depth is about 73 km.And at those conditions,the asthenospheric mantle can occur decompression melting.Moreover,the compositions of the clinopyroxene of the Changjie olivine pyroxenolite show island arc-like characteristics with relatively low A12O3 (0.02％ ～ 5.51％),TiO2 (0 ～ 0.46％) and Na2O (0.01％ ～ 1.44％) contents,indicating that the source of magma had been metasomatized by slab-derived fluids/melts.Conclusions:we consider that the Changjie olivine pyroxenolite were produced by the partial melting of the asthenospheric mantle which had been matasomatized by slab-derived fluits/melts.In addition,the compositions of clinopyroxene of the Changjie olivine pyroxenolite indicate that the Changjie olivine pyroxenolite is tholeiitic series produced in intraplate setting,suggesting an intraplate rifting tectonic setting.In combination with previous researches,we infer that the mechanism for the Changjie olivine pyroxenolite refers to the partial melting of the upwelling asthenospheric mantle due to the rifting.