东天山大草滩蛇纹岩中铬铁矿特征及其构造意义

Characteristics and tectonic significance of chromites from Dacaotan serpentinite of East Tianshan Mountains

东天山大草滩蛇纹岩中发育副矿物铬铁矿,部分铬铁矿具环带结构,核部为铝铬铁矿,富Al贫Fe,Cr2O3为39.27%~42.65%,Al2O3为24.58%~26.21%,Ti O2含量很低(<0.20%),Fe O为12.18%~18.14%,而Fe2O3含量为0.33%~7.30%,Cr#为50.6~53.5,Mg#为53.3~68.2,Fe2+#为31.8~46.7。核部铝铬铁矿结晶温度平均约为1 385℃,结晶压力平均为2.77 GPa,推断其形成深度约为86 km,相当于软流圈位置;相对于FMQ缓冲剂的地幔氧逸度为FMQ-4.38~FMQ+0.63 log单位,平均值为FMQ-1.15 log单位;地幔熔融程度F为21.04%~21.28%。推测大草滩蛇纹岩的原岩橄榄岩单元源区为石榴石二辉橄榄岩,形成于亏损的软流圈地幔,可能产自俯冲带环境中的岛弧环境。大草滩铬铁矿的环带结构特征表明其经历了温度由高到低、氧逸度由低到高的过程,反映了大草滩蛇纹岩由最初的高温高压地幔环境被推覆到地壳,随后经历了低温低压的浅程度区域变质作用。

The disseminated and banded chromites in the serpentinized ultrabasic rocks of the Dacaotan area, East Tianshan Mountains, are helpful to clarifying the characteristics and evolutionary process of the rocks. Some chro- mites have zoned texture, with rich A1 but poor Fe in the core. The geochemical characteristics of the core of chromites, with the data Cr203 39.27% -42.65%, A1203 24.58% -26.21%, TiO2 less than 0. 20%, FeO12. 18% - 18.14%, Fe203 0. 33% -7.30%, Cr# 50. 6 - 53.5, Mg# 53.3 -68.2, and Fe2 ＋#31.8- 46. 7, are in accord with the features of ophiolites serpentinite and may be a member of ophiolites. Moreover, the crystallization temperature and pressure of the chromite are estimated to be 1 385℃ and 2. 77 GPa on average, respectively, with a formation depth of about 86 kin, which is similar to features of the asthenosphere. The oxygen fugacity relative to FMQ buffer is FMQ - 4. 38 - FMQ ± 0. 63 log units, FMQ - 1.15 log units on average. Furthermore, the degree of partial melting of the mantle is estimated to be in the range of 21.04% - 21.28%, 21.18% on average, in the light of its relationship with Cr^#. These parameters provide some clues to the physicochemical conditions： these peridotites were derived from garnet lherzolites located in the asthenosphere of the upper mantle, and Dacaotan serpentinite was formed in the depleted island-arc setting of supra-subduction zone. The compositional zoning suggests that the peridotites have undergone a thermal path of decreasing temperature and increasing oxygen fugacity. It can be in- ferred that the mantle peridotites experienced the process from the stage of high temperature and high pressure mantie with lower oxygen fugacity to the thrusting slice stage to reach the crustal surface, where they were subjected to regional metamorphism with higher oxygen fugaeity.