滇西阿拉斯加型岩体的主要特征

MAIN CHARACTERISTICS OF ALASKAN-TYPE INTRUSIVE BODIES IN WEST YUNNAN

澜沧江镁铁-超镁铁岩侵位于二叠系和中上三叠统岛弧型碎屑沉积岩和火山岩中。依丰度为序,主要岩石类型有辉长岩、橄榄岩和单辉岩。岩石主要是堆晶岩。橄榄岩由橄榄石、单斜辉石和少量(或无)斜方辉石组成,而没有斜长石。辉石岩主要是单斜辉石,而缺少斜方辉石和斜长石。与其它地区蛇绿岩的同类岩石相比,超镁铁岩富 FeO 和 CaO 而贫 MgO,辉石岩富TiO_2,和 CaO 而贫 Ni,辉长岩的 Al_2O_3和总碱含量在 AFM 图中显示钙碱性岩浆的趋势。吉岔和半坡岩体辉长岩的球粒陨石标准化型式显示 LREE 富集或 LREE 亏损,而近于平坦型。这些特征说明上述深成岩是阿拉斯加型的镁铁-超镁铁岩体。

Lancangjing mafic- -ultramafic complexes intruded into clastic-sedimentary and volcanic rocks of island-arc type in P and T2-3. The main rock types in their abundance order are gabbros, peridotites and clinopyroxenites. The rocks are mainly cumulates. Petrographically, Peridotite consists of olivine, clinopyroxene and minor (or not) orthopyroxene without plagioclase; pyroxenite is mainly clinopyroxene and devoid of orthopyroxene and plagioclase. The chemical compositions indicate that the ultramafic rock are rich in FeO and CaO, and poor MgO, and pyroxrnites are rich in TiO2, CaO and poor in Ni in comparison to those of the same rocks in ophiolites from defferent regions, and the gabbros have high Al2O3and total alkali contents and show a trend of calc-alkaline magma in AFM diagram. Chondrite-normalized rare earth elements pattern in gabbros in Jicha and Banpo bodies show slight enrichment in LREE or slight depletion in LREE and close to flat, but different from the same rocks of depleted Alpine-type and enriched layered intrusions. All of these indicate that the plutons ementioned above are mafic-ultramafic complexes of Alaskan-tepe and not of Alpine-type and layered intrusions.