西藏当惹雍错富钾和富钠碱性火山岩的矿物学研究及其成因指示

Mineralogical Characteristics of the Sodium- and Potassic-rich Alkaline Volcanic Rocks at Tangra Yumco, SE Tibet: Implications for Petrogenesis

对青藏高原西南部当惹雍错地区的中新世超钾质粗面岩及共存的富钠质方沸石(霞石)响岩进行了详细的岩石学、矿物学研究,揭示出两套岩石具有不同的矿物学特征。钠质方沸石(霞石)响岩中橄榄石斑晶的熔体包裹体与钾质火山岩的成分相当,应属于早期残余的钾质岩浆,表明了钠质方沸石(霞石)响岩的形成晚于超钾质粗面岩。两类岩石的全岩化学成分、矿物组合及其单斜辉石核部的化学成分、Ti-Al比值的明显差异,表明钠质方沸石(霞石)响岩不是钾质粗面岩岩浆演化的产物,它们来自于不同的岩浆源区,即富钾和富钠的地幔源区。

The Miocene sodium-rich analcime （nepheline） phonolite and coexisting ultrahigh-potassium trachytes at Tangra Yumco area in southwestern Qinghai-Tibet Plateau show distinct petrological and mineralogical characteristics. The magmatic inclusions within olivine phenocrysts in the sodium analcime （nepheline） phonolite are compositionally similar to the ultrapotassic volcanic rocks, indicating that the potassium magma was formed prior to the sodium magma. Because of the different whole rock components, mineral assemblages, mineral compositions and Ti-Al ratios in the clinopyroxene cores, the sodium analcime （nepheline） phonolites were likely derived from different mantle magmatic sources rather than fractional results of a potassic trachytic magma.