西藏达如错地区晚侏罗世高镁安山岩——班公湖—怒江洋壳俯冲消减的证据

The Late Jurassic high-Mg andesites in the Daru Tso area, Tibet: Evidence for the subduction of the Bangong Co-Nujiang River oceanic lithosphere

对出露于拉萨地块北部的达如错接奴群安山岩进行了年代学、元素地球化学和锆石原位Hf同位素研究。用LAICP-MS技术测得的安山岩锆石206Pb/238U年龄为163.3±1.7Ma,即火山活动时限为晚侏罗世早期。安山岩地球化学研究结果表明,其具有典型高镁安山岩的地球化学特征,所有样品均具有较高的Mg O、Mg#值,低的TFe O/Mg O值(小于1.5);所有样品都显示轻稀土元素富集,具有Eu负异常,富集大离子亲石元素、亏损高场强元素的特征;锆石εHf(t)值为较均一的负值(εHf(t)=-8.5^-6.7)。研究表明,达如错高镁安山岩很可能是班公湖—怒江洋壳在俯冲消减背景下,由俯冲沉积物熔融产生的熔体与地幔橄榄岩发生交代作用的产物,形成于与班公湖—怒江洋壳俯冲消减有关的活动大陆边缘(安第斯型)的构造环境。由此认为,晚侏罗世接奴群火山岩是班公湖—怒江洋壳俯冲消减的产物,从而为班公湖—怒江洋壳的俯冲消减提供了直接的火山岩证据。

This paper reports zircon U-Pb and Hf isotopic data of major and trace elements for the andesites of Jienu Group from the Daru Tso area in the northern Lhasa block. LA-ICP-MS U-Pb zircon analyses suggest that the andesite were formed at 163.3 ± 1.7Ma, and that the volcanic rocks were formed in early Late Jurassic period. Geochemical studies show Daru Tso andesites have typi-cal geochemical characteristics similar to the high-Mg andesite （HMA）. They are characterized by high MgO content, Mg # values, and low TFeO/MgO ratios. They are enriched in LREE in the chondrite-normalized REE patterns with negative Eu anomalies, and are enriched in large ion lithophile elements （LILEs） and depleted in high field strength elements （HFSEs）. Zircon εHf（t） values are from-8.5 to-6.7. Geochemical features of the Daru Tso high-Mg andesites are considered to have been generated by the subduc-tion of Bangong Co-Nujiang River oceanic lithosphere, and can be genetically regarded as resulting from partial melting of subduc-tion sediment melts that had been metasomatized by mantle peridotite under the condition of oceanic crust subduction. They were formed at the tectonic setting of the active continental margin （Andeans-type） associated with Bangong Co-Nujiang River oceanic crust subduction. It is therefore held that the Late Jurassic Jienu Group volcanic rocks in the Daru Tso area are the products of subduc？ tion of the Bangong Co-Nujiang River oceanic lithosphere. These results provide direct evidence of the volcanic rocks for the sub-duction of the Bangong Co-Nujiang River Tethys Ocean.