青藏高原东南缘腾冲后碰撞粗面安山岩形成的深部岩浆过程:来自辉长质物质的启示

Gabbroic xenoliths and glomerocrysts in the post-collisional trachyandestic rocks from Tengchong,SE Tibet:Implications for the magma chamber processes

位于青藏高原东南缘的腾冲火山区发育大量的后碰撞高钾钙碱性岩浆岩(8Ma至今),形成了近连续的玄武岩-玄武安山岩-粗面安山岩-英安岩系列。在晚更新世(0.3~0.4Ma)粗面安山岩中发现了大量的辉长岩包体与辉长质矿物聚晶。辉长岩包体分为两类:I类为辉长苏长岩(直径2~4cm),主要由斜长石(50%~60%)、单斜辉石(20%~30%)和斜方辉石(5%~10%)组成,矿物间呈高角度接触的开放结构并包含少量的基质玻璃;Ⅱ类辉长岩包体(直径2~12cm)主要由斜长石(40%~50%)与单斜辉石(30%~40%)组成,含少量铁钛氧化物(5%~10%),矿物间见少量的基质玻璃与微晶斜方辉石(粒径<50μm)。矿物聚晶(粒径2~5mm)由斜长石(40%~60%)、单斜辉石(20%~30%)和斜方辉石(5%~10%)组成,矿物间呈高角度接触的开放结构,其间贯入基质玻璃。辉长岩包体、矿物聚晶与寄主粗面安山岩斑晶具有相同的矿物组成(斜长石+单斜辉石+斜方辉石);单斜辉石具有相似的微量元素组成特征;辉长岩包体全岩主量元素成分落在玄武岩-玄武安山岩-粗面安山岩-英安岩的演化序列中,它们与寄主粗面安山岩具有一致的Sr-Nd-Pb同位素组成。上述观测结果表明,辉长岩包体和矿物聚晶与寄主岩浆同源,它们均来自粗面安山质岩浆房。矿物温度与压力计算结果表明,辉长岩包体与矿物聚晶的结晶温度低于斑晶,形成深度位于粗面安山质岩浆房的中上部。这些具有开放结构的矿物聚晶与辉长岩包体可能代表粗面安山质岩浆喷发前形成晶粥的"碎片":矿物聚晶与I类辉长岩包体矿物间基质含量较高,矿物成分与寄主粗面安山质熔体(全岩成分)平衡,可能代表寄主粗面安山质岩浆形成的晶粥;Ⅱ类辉长岩包体矿物间紧密程度较高,矿物与较演化的岩浆平衡,可能代表早期较演化的岩浆形成的经历压实作用的晶粥。粗面安山质岩浆的快速上升将这些晶粥破碎并以辉长岩包体与矿物聚晶的方式运移至地表。岩浆房中基性岩浆的补给是导致安山质岩浆喷发的重要诱发机制。

Post-collisional high-K calc-alkaline magmatic activities(8Ma to present)with eruption of basalts,basaltic andesites,trachyandesites and dacites are extensively distributed in the Tengchong volcanic field,SE Tibet.Plenty of gabbroic xenoliths and glomerocrysts were entrained by the Late Pleistocene(0.3~0.4Ma)trachyandesites.There are two types of gabbroic xenoliths:Type I xenoliths(2~4cm in size)are in open structures and consist of plagioclase(50%~60%),clinopyroxene(20%~30%)and orthopyroxene(5%~10%)with interstitial glass;TypeⅡxenoliths(2~12cm in size)contain plagioclase(40%~50%),clinopyroxene(30%~40%)and Fe-Ti oxide(5%~10%)with interstitial orthopyroxene and glass.The glomerocrysts(2~5mm in size)are also in open structures,and they are composed of plagioclase(40%~60%),clinopyroxene(20%~30%)and orthopyroxene(5%~10%)with interstitial glass.The gabbroic xenoliths and glomerocrysts display similar mineral assemblage(plagioclase+clinopyroxene+orthopyroxene)with phenocrysts of the host lava.The clinopyroxenes from xenoliths,glomerocrysts and phenocrysts show similar trace element patterns.Furthermore,gabbroic xenoliths display similar whole-rock Sr,Nd and Pb isotopes to the host lava.We interpret these to indicate a genetic link between xenoliths(plus glomerocrysts)and the trachyandesitic magma,suggesting that the gabbroic xenoliths and glomerocrysts belong to the magmatic system in the Tengchong volcanic field.Results of P-T estimation indicate that the glomerocrysts and gabbroic xenoliths appear to have crystallized at the upper environment of the trachyandesitic magma reservoir.The glomerocrysts and gabbroic xenoliths with open structures contain residual interstitial glass,suggestting that they are fragments of partially solidified crystal mush:the glomerocrysts and Type I xenoliths which are in equilibrium with the melt appear to have crystallized from the host trachyandesitic melts;TypeⅡxenoliths may have crystallized from a more evolved magma based on the lower Mg#of the clinopyroxene.The crystal mush may have been disaggregated by the rapid ascending trachyandesitic magma and carried to the surface as glomerocrysts and gabbroic xenoliths.Recharge by mafic magmas in magma reservoir may be the main trigger for eruption.