牛鼻子梁镁铁质-超镁铁质杂岩体岩石特征

Rock characteristics of Niubiziliang mafic-ultramafic complex

牛鼻子梁岩体位于柴达木地块的北缘,出露面积约8 km2,平面形态呈长条状,主要由斜长二辉橄榄岩、斜长单辉橄榄岩、角闪二辉橄榄岩、角闪橄榄岩、角闪橄榄二辉岩、黑云母化二辉岩、角闪辉石岩、橄榄辉石角闪石岩、角闪橄榄辉长岩、细粒辉长岩、似斑状辉长岩、暗色辉长岩、辉长岩、淡色辉长岩、石英闪长岩和英云闪长岩组成。文章通过岩石学、矿物学、地球化学研究,得到锆石U-Pb年龄为(361.5±1.2)Ma,Sm-Nd等时线年龄为(347±26)Ma。研究认为,牛鼻子梁基性-超基性岩体含矿岩石产于大陆边缘环境。岩体形成于泥盆纪晚期。岩浆分异充分,岩石类型丰富,岩浆演化过程中主要发生了橄榄石和斜长石的分离结晶/堆晶作用。岩体的母岩浆应属于拉斑玄武岩质岩浆。从目前发现的矿化情况来看,牛鼻子梁基性-超基性杂岩体为含矿岩体,有很好的找矿前景。

The ore-prospecting work in Niubiziliang mafic-ultramafic complex has achieved good results; nevertheless, the exploration methods have been rather blind and lack of systematic and professional means. In this paper, based on studying the genesis, formation environment, morphology, and differentiation of the rock body, the authors evaluated the vista of the prospecting work in this area. Microscopic identification shows that the rocks are composed mainly of plagioclase lherzolite, plagioclase wehrlite, lherzolite amphibole, hornblende peridotite, olivine amphibole, biotitized websterite, hornblende pyroxenite, olivine pyroxene amphibolite, hornblende olivine gabbro, fine-grained gabbro, porphyritoid gabbro, dark gabbro, gabbro, light gabbro, quartz diorite and tonalite. Electron microprobe analyses of olivines from different rock types show that their modes of occurrence are not identical. In peridotites, they mostly occur in subhedra｜ granular or rounded crystal accumulation forms; in pyroxene, they mostly occur as reaction pyroxenite remnant ; Fo values range between 78.54 and 85.23, sug- gesting chrysolite. Pyroxene occurs in various kinds of rocks and assumes anhedral form with wrapped olivine. Graphic analysis shows that almost all the alkaline rocks are located in subalkaline region, suggesting that it be- longs to native sub-alkaline magma series. All samples are located in the tholeiitic basalt region, indicating that parental magmas should belong to the tholeiitic magma. As for hornblende, peridotite phases contain brownhornblende, with the amount up to 15 %. There is a considerable amount of olivine hornblende rocks, whose amphibole content is up to 60 % ～ 75 %. These phenomena fully show that the magma was rich in volatiles. Dia- gram analysis shows that all amphiboles fall into the mantle zone, indicating that they belong to native amphi- boles. As to plagioclase, An grade plagioclase varies widely from 13 to 65, and appears from bytownite to ande- sine. Silicate sample analyses show that SiO2 content of all samples ranges from 39.91% to 41.35 %, suggesting mafic-ultramafic rocks, m/f values of all ultrabasic rock samples vary between 3.84 and 4.90, being lower than 6.5 and higher than 2.5, implying iron ultrabasic rocks, which are favorable for the formation of copper-nickel sulfide deposits. REE analyses show that total REE values of various rocks are 17.3 × 10^-6- 83.77 × 10^-6. There exists strong internal fractionation between heavy rare earth elements and between light and heavy rare earth elements, and the distribution patterns belong to LREE enrichment. All samples fail to show obvious Eu anomalies or Eu anomalies. The U-Pb isotopic ages are exclusively （361.5 ± 1.2） Ma or so, and the Sm-Nd isochron age is （347 ± 26） Ma, implying Late Devonian. Studies suggest that ore-bearing rocks of Niubiziliang mafic-ultramafic complex were produced in the continental margin, and the rock body was formed in the late De- vonian. Magma experienced full differentiation, with rich rock types. During magmatic evolution, there mainly occurred olivine and plagioclase fractional crystallization/cumulate action. Magmatic intrusions should belong to tholeiitic basaltic type. Judged from the known mineralization situation, Niubiziliang mafic-ultramafic complex is an ore-bearing rock body with promising ore-prospecting potential.