Constraints on assembly of Tongariro and Ruapehu andesite magmas based on Sr-isotope compositions of plagioclase and groundmass,Lithos

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Constraints on assembly of Tongariro and Ruapehu andesite magmas based on Sr-isotope compositions of plagioclase and groundmass
Lithos ( IF 3.5 ) Pub Date : 2021-08-05 , DOI: 10.1016/j.lithos.2021.106406
Kate Cocker ₁ , Phil Shane ₁ , Claudine H. Stirling _{2,

3} , Malcolm R. Reid _{2,

3}

Affiliation

Sub-crystal scale Sr/Sr analysis of plagioclase phenocrysts and groundmass in andesites reveal cryptic disequilibrium that is not observable in bulk rock-based studies, shedding light on magma assembly. Isotopic, chemical and textural features demonstrate that plagioclase phenocrysts in continental arc andesites from Tongariro and Ruapehu volcanoes are a heterogenous population of various cognate and non-cognate origins. The studied rocks span a wide age range (~270 ka) and have different vent locations but their plagioclase share many characteristics. Most phenocrysts comprise of a relic core surrounded by one or more mantling overgrowths of various textures demonstrating a multi-stage growth during crustal transit. The total compositional range found in a single thin section is wide (~An). Some zoned phenocrysts have a comparable wide range. Inter-crystal Sr/Sr diversity is considerable with differences up to 0.7044–0.7058 within a hand-sized sample. Individual phenocrysts can be isotopically zoned with rim-core differences up to 0.0005, and contrasting isotopic zonation can occur in phenocrysts of the same sample. Overall, more than half the phenocrysts are non-cognate as indicated by their isotopic differences. Phenocrysts with relic calcic (>An) interiors and low Sr/Sr ratio (<0.7050) are probably antecrysts and xenocrysts from deep mafic cumulates and/or forerunner intrusions. Other phenocrysts have relic sodic interiors (

中文翻译：

基于斜长石和基质的锶同位素组成对汤加里罗和鲁阿佩胡安山岩岩浆组装的约束

安山岩斜长石斑晶和基底的亚晶尺度 Sr/Sr 分析揭示了在大块岩石研究中无法观察到的神秘不平衡，为岩浆聚集提供了线索。同位素、化学和结构特征表明，汤加里罗和鲁阿佩胡火山大陆弧安山岩中的斜长石斑晶是具有各种同源和非同源起源的异质群体。研究的岩石年龄范围很广（~270ka），并且具有不同的喷口位置，但它们的斜长石具有许多共同特征。大多数斑晶由一个遗迹核心组成，周围环绕着一个或多个不同质地的地幔过度生长物，表明地壳运输过程中的多阶段生长。在单个薄片中发现的总成分范围很宽（〜An）。一些分区斑晶具有相当宽的范围。晶间 Sr/Sr 多样性相当大，在手掌大小的样品中差异高达 0.7044–0.7058。单个斑晶可以进行同位素分区，其边缘核心差异高达 0.0005，并且同一样品的斑晶中可以出现对比的同位素分区。总体而言，超过一半的斑晶是非同源的，如它们的同位素差异所示。具有遗迹钙质 (>An) 内部和低 Sr/Sr 比 (<0.7050) 的斑晶可能是来自深层镁铁质堆积物和/或先行侵入体的前晶和异晶。其他斑晶具有残留的钠内部（

更新日期：2021-08-05

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