Two-stage contamination during crustal assimilation: isotopic evidence from volcanic rocks in eastern Nevada
Two-stage contamination during crustal assimilation: isotopic evidence from volcanic rocks in eastern Nevada
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地壳同化过程中的两阶段污染：内华达州东部火山岩的同位素证据
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发表时间：
1992
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影响因子：
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通讯作者：
A. Grunder
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作者：
A. Grunder
数千立方公里的中间成分占主导地位的变辉岩浆在大约35 Ma时在内华达州东部爆发。根据对最早期火山岩的详细研究，推断出两个阶段的地壳混染。从玄武岩到流纹岩的细粒镁铁质岩石和混合结构岩石被富含Nd但贫Sr的地壳成分污染。上覆的斜长富安山岩和英安岩具有较高的Sr和87 Sr/86 Sr，但Nd较少，Nd含量较低，被解释为被具有相反元素特征的地壳成分污染（即，贫Nd但富Sr）。第一种污染物代表地壳的部分熔融，斜长石作为残留相，第二种污染物是部分熔融事件的残留物或大块地壳。净效应是大量的地壳同化，但在两个“咬”。地壳源分离成两种元素不同的污染物，导致87 Sr/86 Sr与εNd的变化趋势不同，这可能被误解为存在同位素不同的地壳储层。泥质岩中计算的污染物与熔融关系的比较与代表熔融物和约30%熔融的残留物的两种污染物一致。大块地壳的模型年龄约为2.2 Ga，与其他工作者推断的早元古代地壳省一致。
Thousands of cubic kilometers of dominantly intermediate composition, metaluminous magma erupted at approximately 35 Ma in eastern Nevada. Two stages of crustal contamination are inferred from detailed study of the earliest vocanic rocks. Fine-grained mafic rocks and rocks with mixed textures, ranging from basalt to rhyolite, were contaminated with a crustal component rich in Nd but poor in Sr. Overlying plagioclaserich andesites and dacites have greater Sr and 87Sr/86Sr, but less Nd and lower ɛNd and are interpreted to have been contaminated by a crustal component with the opposite elemental signature (i.e., poor in Nd but rich in Sr). The first contaminant represents a partial melt of the crust with plagioclase as a residual phase and the second contaminant is the residue of the partial melting event, or bulk crust. The net effect is bulk crustal assimilation, but in two “bites”. The separation of the crustal souree into two elementally distinct contaminants causes divergent trends with respect to the variation of 87Sr/ 86Sr versus εNd that could be misinterpreted to indicate the existence of isotopically distinct crustal reservoirs. Comparison of the calculated contaminants to melting relationships in pelites is consistent with the two contammants representing melt and residue at about 30% melting. The model age of the bulk crust is approximately 2.2 Ga, consistent with an early Proterozoic crustal province inferred by other workers.