藏北南羌塘陆块北缘毕洛错地区古近纪纳丁错组火山岩的特征及构造环境

Characteristics and tectonic setting of volcanic rocks of the Paleogene Nading Co Formation in the Biluo Co area on the northern margin of the southern Qiangtang block, northern Tibet, China.

藏北南羌塘陆块北缘毕洛错地区古近纪纳丁错组火山岩由灰色安粗岩、深灰色安粗岩、深灰色杏仁状橄榄玄武岩(气孔发育,气孔充填物为石英、方解石等,致使杏仁状橄榄玄武岩岩石化学定名偏中性,故采用镜下定名)组成。其中,安粗岩的SiO2含量介于60.80%～61.83%之间,Al2O3含量介于16.30%～～17.22%之间,Na2O/K2O>1,MgO含量介于0.49%～1.42%之间。岩石轻稀土元素较富集,∑Ce/∑Y介于8.20～9.60之间,δEu介于0.70～～0.91之间,呈弱负异常。岩石学和岩石地球化学特征表明,纳丁错组火山岩形成于板内裂谷构造环境,其岩浆可能来自壳幔过渡带的部分熔融的物质,并与张性深断裂有关。根据样品K-Ar同位素年龄值32.60Ma,确定该火山岩的地质时代为古近纪。

Volcanic rocks of the Paleogene Nading Co Formation in the Biluo Co on the northern margin of the southern Qiangtang block, northern Tibet, consist of gray latite, dark gray latite, dark gray amygdaloidal olivine basalt and crypto-htite, The geochemical composition of the latite is characterized by SiO2=60.80-61.83%, Al2O3=16.30-17.22%, Na2O/K2O〉1 and MGO=0.49-1.42%. The volcanic rocks are LREE-enriched with ∑ Ce/∑Y=8.2O-9.60 and show weak negative Eu anomalies with δEu=0.70-0.91. These petrological and geochemical characteristics suggest that the volcanic rocks in the study area formed in an intraphte rift environment. The magma of the volcanic rocks was probably derived from partial melting of the materials in the crust-mantle transition zone and related to deep extensional faults. The volcanic rocks have a K-Ar age of 32.60 Ma. According to this age, the geologic age of the volcanic rocks may be determined to be Paleogene.