内蒙古崇根山蛇绿岩中铬铁矿特征及其构造意义

Characteristics and Tectonic Significance of Chromites from Chonggenshan Ophiolite in Inner Mongolia

为探讨内蒙古贺根山蛇绿岩形成时的物化条件和大地构造环境,选取崇根山蛇绿岩块中未蚀变副矿物铬铁矿进行矿物学和矿物地球化学分析。结果表明:铬铁矿成分均一,不发育环带结构;铬铁矿中的Cr2O3质量分数为38.86%~39.12%,Al2O3质量分数为26.56%~26.92%,TFeO质量分数为17.37%~17.50%,Cr#为49.34~49.58,Mg#为68.82~69.73,Yfe为6.62~6.97,Fe2+#为30.27~31.18,为富镁铝铬铁矿,其寄主岩石为蛇绿岩;铬铁矿结晶平均温度为1 396.88℃,平均压力为3.35 GPa,推断其形成深度约为103.72 km;相对于FMQ(fayalite magnetite quartz)缓冲剂的地幔氧逸度为FMQ+1.41~FMQ+1.45 lg单位(平均值为FMQ+1.43 lg单位);地幔熔融程度为20.93%~20.95%(平均值为20.94%)。推测崇根山铬铁矿原岩橄榄岩单元源区为石榴石二辉橄榄岩,形成于亏损地幔。崇根山蛇绿岩为消减带型蛇绿岩,可能产自俯冲带环境中的洋内弧后盆地环境。

In order to analyze the formation physicochemical conditions and tectonic environment of the Hegenshan ophiolite in Inner Mongolia,the unaltered chromites from the Chonggenshan ophiolite were chosen to study their mineralogy and mineral geochemistry.The result shows that the chromites consist of Cr2 O3(38.86%-39.12%),Al2 O3(26.56 %-26.92%),and TFeO(17.37 %-17.50%),Cr#values of 49.34-49.58,Mg#values of 68.82-69.73,Yfe values of 6.62-6.97,and Fe2+#values of 30.27-31.18,without texture zoning.They are aluminum-rich chromites.The average crystallization temperature and pressure of the chromite are estimated to be 1 396.88℃and 3.35 GPa,respectively,indicating its formation depth of about 103.72 km.It is inferred that the Chonggenshan chromite-hosting peridotite was derived from the garnet lherzolites in depleted upper mantle with oxygen fugacities from FMQ+1.41 to FMQ+1.45 lg units(average FMQ+1.43 lg units),and partial melting degree of the mantle from 20.93% to 20.95 %(average 20.94%).The chromites show the characteristics of supersubduction zone.We believe that the Chonggenshan ophiolite was formed in the background of intraoceanic back-arc basin in the subduction zone.