Compositional zone investigation of clinopyroxene phenocryst in the Cenozoic ultra-potassic rocks from the middle-southern Lhasa block
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摘要: 在青藏高原拉萨地块中南部的米巴勒、麦嘎乡、赛利普地区分布有新生代的超钾质火山岩,这些火山岩的斑晶中广泛发育单斜辉石正环带,其记录了超钾质岩浆的早期结晶历史。这些单斜辉石的正环带核部到边部Mg#和镁、铬的氧化物含量降低,而铁、钠、铝、钛的氧化物含量升高,它反映了岩浆正常结晶的顺序。米巴勒地区超钾质岩中的单斜辉石还出现反环带和少量的韵律环带,可能与该地区所发现的共生钠质过碱性火山岩有关,韵律环带揭示了其可能是相同成分的超钾质母岩浆从岩浆源区多次补给形成,而反环带则可能是与钠质过碱性火山岩岩浆存在一定程度的混合造成的。尽管三个地区超钾质火山岩的喷发时间稍不同,但是三地区的单斜辉石正环带核部成分基本相同,它们具有相似的Al-Ti值,指示出超钾质岩形成在相同的构造环境并具有相同的岩浆成因。Abstract: The ultra-potassic volcanic rocks in Mibale, Maigaxiang, Sailipu areas occur in middle-southern Lhasa block, Tibetan Plateau. A large number of normal zones in clinopyroxene phenocrysts have been recognized in these ultra-potassic volcanic rocks, which have recorded the early crystallizing history of the magmas. The normal zones show FeO, Na2O, Al2O3, TiO2 content increasing with the value of Mg#, MgO, Cr2O3 reducing from the core part to the edge, which reflects the normal sequence of magmatic crystallization. In addition, the reverse zones and the oscillatory zones have been found only in Mibale area, implying that ultra-potassic magmas experienced the magmatic mixing with the associated sodium-peralkalic magmas, and replenishment of a more primitive melt into an evolved magma body. Although erupted times of ultra-potassic rocks in the three areas were slightly different, core compositions of clinopyroxene normal zones are same each other, and they show a similar Al-Ti variation, indicating that the ultra-potassic rocks from three areas were formed in a tectonic setting and had a same magmatic genesis.
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Key words:
- Clinopyroxene zones /
- Ultra-potassic rocks /
- Middle-southern Lhasa block
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