High-magnesian andesite from Mount Shasta: A product of magma mixing and contamination, not a primitive mantle melt
High-magnesian andesite from Mount Shasta: A product of magma mixing and contamination, not a primitive mantle melt
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DOI：
10.1130/g23286a.1
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发表时间：
2007-04
期刊：
影响因子：
5.8
通讯作者：
M. Streck;W. Leeman;J. Chesley
中科院分区：
文献类型：
--
作者：
M. Streck;W. Leeman;J. Chesley
有人提出，来自沙斯塔山地区的高镁安山岩（HMAs）可能代表近原生地幔熔体，携带板状熔体与卡斯卡迪亚地幔楔相互作用的签名。我们提出了强有力的证据表明，它们的形成涉及混合的英安质和玄武质岩浆和夹带的超镁铁质晶体材料，因此，他们不能代表原始岩浆。岩石中含有（1）含英安质熔体包裹体的低Mg #（65-72）单斜辉石（cpx）和斜方辉石（opx）斑晶核，以及（2）高Mg # opx和橄榄石捕虏晶，它们都被cpx或opx的自形过生物镶边，类似于Mg#（87）中的骨架橄榄石斑晶。结构关系表明，超镁铁质捕虏晶与英安质液体发生反应，然后污染的岩浆与玄武质液体混合，产生混合的HMA整体成分。高镁，铬，镍来自后者的输入，而高的Sr/Y和整体埃达克岩的亲和力是继承英安岩端员，这可以说是地壳的起源。我们认为，开放系统的过程可能是更重要的HMAs的岩石成因比一般公认的，他们的镁成分并不一定意味着他们是原始地幔熔体。
It has been proposed that high-Mg andesites (HMAs) from the Mount Shasta area may represent near-primary mantle melts, carrying signatures of slab melt interaction with the Cascadia mantle wedge. We present strong evidence that their formation involved mixing of dacitic and basaltic magmas and entrainment of ultramafic crystal material, and thus they cannot represent primitive magmas. The rocks contain (1) low-Mg# (65–72) clinopyroxene (cpx) and orthopyroxene (opx) phenocryst cores containing dacitic melt inclusions, and (2) high-Mg# opx and olivine xenocrysts, all of which are rimmed by euhedral overgrowths of cpx or opx similar in Mg# (87) to skeletal olivine phenocrysts. Textural relations indicate that ultramafic xenocrysts reacted with dacitic liquid, after which the contaminated magma mixed with basaltic liquid to produce a hybrid HMA bulk composition. High Mg, Cr, and Ni derive from the latter inputs, whereas high Sr/Y and overall adakite affinity is inherited from the dacite end member, which is arguably crustal in origin. We suggest that open system processes may be more important in the petrogenesis of HMAs than generally recognized, and that their magnesian compositions do not necessarily imply that they are primitive mantle melts.