碱性玄武岩中幔源岩石包体内尖晶石成分特征及其岩石学成因意义

The Characteristics of spinel compositions in mantle derived xenoliths from alkali basalts and their petrogenetic significance

通过对我国东部北起黑龙江省南至海南省３１个产地的新生代碱性玄武岩中橄榄岩类 （纯橄岩、方辉橄榄岩及二辉橄榄岩） 包体和辉石岩类包体中的１６０ 多个尖晶石电子探针分析数据， 论述了尖晶石的Ｃｒ－Ａｌ， Ｆｅ－Ｍｇ， Ｎｉ－Ｍｇ 主元素及有关比值和端员组分之间的关系， 分析研究了不同岩类包体中尖晶石的颜色、成分及成因之间的联系。并利用所分析的有关橄榄石、斜方辉石、单斜辉石共生矿物的电子探针分析数据， 采用新的、较合理的Ｂｒｅｙ－Ｋｏｈｌｅｒ温度计及Ｋｏｈｌｅｒ－Ｂｒｅｙ 压力计和Ｂａｌｌｈａｕｓ ｅｔａｌ． 氧逸度计算方法进行了计算， 探讨了尖晶石成分与其形成温度、压力和氧逸度状态的关系， 指出了尖晶石成分所反映的地幔亏损程度及部分熔融程度的变化规律及尖晶石 （相） 稳定的温度、压力和氧逸度范围。

The mantle derived xenoliths are widely present in Cenozoic alkali basalts in eastern China A great numler of works regarding them have been published, but systematic studies on spinels of mantle derived xenoliths in this area are scarce; especially the research concerning their equilibrium temperatures, pressures and oxygen fugacities has not yet been undertaken On the basis of 160 electron microprobe analyses of spinels and their coexisting olivines, orthopyroxenes and clinopyroxenes in peridotite and pyroxenite xenoliths from 31 areas of eastern China and using Brey Kohler's T P calculation method and Ballhaus' f o 2 calculation method, their T P and f o 2 states have been calculated and the differences between the colours and chemical compositions of the spinels in peridotites and pyroxenites and their significance in petrogenesis as well as the relationships between the chemical compositions of the spinels and between the compositions and their equilibrium T P f O 2 states have been studied in detail In addition, the stable field of the spinels in the upper mantle is presented From the study the following conclusions can be drawn:(1) With increasing degree of partial melting of the upper mantle the colour of the spinels in residual peridotites changes from light brown to dark brown and their Cr and Fe contents increase gradually and their Al and Mg contents decrease progressively (2) The spinels of pyroxenite xenoliths in Cenozoic basalts from eastern China differ greatly in colour and composition from those of peridotite xenoliths, implying that their origins are different (3) The spinels of mantle derived xenoliths mainly contain Cr, Al, Fe and Mg There exists wide isomorphic substitution between Cr and Al and between Fe 2+ and Mg, but the isomorphic replacement between their Fe 3+ and Cr are limited (4) During the Cenozoic, the equilibrium temperatures for the spinels in the upper mantle conditions of eastern China are 1100°K～1500°K (about 827℃～1227℃), and their equilibrium pressures are 0 12～6 0GPa, but mostly less than 4 0GPa (corresponding to a formation depth of less than 125km) The majority of relative oxygen fugacities for their formation range from -2 to +1 (5) In the upper mantle, the Cr 2O 3 content, Cr/(Cr+Al) ratio and end member FeCr 2O 4 show a tendency of positive correlation with their formation temperatures and pressures (6) The equilibrium temperatures of the spinels tend to show a positive correlation with their relative oxygen fugacities and a negative correlation with their formation pressures, i e with increasing formation depths the relative oxygen fugacities have a tendency to decrease