Across-arc geochemical variations in the Southern Volcanic Zone, Chile (34.5–38.0°S): Constraints on mantle wedge and slab input compositions
Across-arc geochemical variations in the Southern Volcanic Zone, Chile (34.5–38.0°S): Constraints on mantle wedge and slab input compositions
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DOI：
10.1016/j.gca.2013.05.016
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发表时间：
2012-12
影响因子：
5
通讯作者：
G. Jacques;K. Hoernle;J. Gill;F. Hauff;H. Wehrmann;D. Garbe‐Schönberg;P. Bogaard;I. Bindeman
中科院分区：
文献类型：
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作者：
G. Jacques;K. Hoernle;J. Gill;F. Hauff;H. Wehrmann;D. Garbe‐Schönberg;P. Bogaard;I. Bindeman
地壳同化作用（如Hildreth和Moorbath， 1988）和/或俯冲侵蚀作用（如Stern， 1991, Kay et al., 2005）被认为控制了智利南部火山区北部的地球化学变化。为了验证这些假设，我们提供了一个完整的地球化学数据集（主要元素和微量元素以及O-Sr-Nd-Hf-Pb同位素），这些数据集来自于34.5°S ~ 38.0°S之间的弧上的全新世主要含橄榄岩火山岩，包括从Tinguiririca到Callaqui的火山锋面中心，Infernillo火山场、Laguna del Maule和Copahue的弧后中心，并延伸至弧后300 km。我们还提供了该剖面外智利海沟沉积物的等效数据集。火山弧（包括火山前后弧）样品主要为玄武岩和安山岩/粗玄武岩，弧后岩石为低硅碱玄武岩和粗玄武岩。所有样品均表现出一定的俯冲带特征，但弧后样品表现最少。弧后玄武岩的Ce/Pb、Nb/U、Nb/Zr和Ta/Hf较高，Ba/Nb和Ba/La较低，表明弧后玄武岩的板源成分较少，因此地幔熔融程度较低。橄榄石和斜长石斑晶（火山弧= 4.9 ~ 5.6‰，弧后= 5.0 ~ 5.4‰）的δ18O呈地幔状，δ18O与分异指数及其他同位素比值缺乏相关性，说明地壳同化作用不明显。火山弧和弧后样品在Sr和Nd同位素组成上几乎完全重叠。高精度（双尖峰）Pb同位素比值紧密相关，排除了较老的硅质地壳的明显同化作用，但表明南大西洋中洋脊玄武岩（MORB）源与俯冲沉积物和蚀变洋壳形成的板块组分之间存在混合作用。Hf-Nd同位素比值确定了火山弧和弧后的独立线性阵列，两者都不倾向于俯冲沉积，可能反映了火山弧的主要软流圈地幔阵列和弧后富集元古代岩石圈地幔的参与。我们提出了一个火山弧下混合源、板块衍生熔体和非均质地幔之间的定量混合模型。在假定板坯内部水饱和条件下，该模型与局部地动力参数一致。
Crustal assimilation (e.g. Hildreth and Moorbath, 1988) and/or subduction erosion (e.g. Stern, 1991, Kay et al., 2005) are believed to control the geochemical variations along the northern portion of the Chilean Southern Volcanic Zone. In order to evaluate these hypotheses, we present a comprehensive geochemical data set (major and trace elements and O–Sr–Nd–Hf–Pb isotopes) from Holocene primarily olivine-bearing volcanic rocks across the arc between 34.5°S and 38.0°S, including volcanic front centers from Tinguiririca to Callaqui, the rear arc centers of Infernillo Volcanic Field, Laguna del Maule and Copahue, and extending 300 km into the backarc. We also present an equivalent data set for Chile trench sediments outboard of this profile. The volcanic arc (including volcanic front and rear arc) samples primarily range from basalt to andesite/trachyandesite, whereas the backarc rocks are low-silica alkali basalts and trachybasalts. All samples show some characteristic subduction zone trace element enrichments and depletions, but the backarc samples show the least. Backarc basalts have higher Ce/Pb, Nb/U, Nb/Zr, and Ta/Hf, and lower Ba/Nb and Ba/La, consistent with less of a slab-derived component in the backarc and, consequently, lower degrees of mantle melting. The mantle-like δ18O in olivine and plagioclase phenocrysts (volcanic arc = 4.9–5.6‰ and backarc = 5.0–5.4‰) and lack of correlation between δ18O and indices of differentiation and other isotope ratios, argue against significant crustal assimilation. Volcanic arc and backarc samples almost completely overlap in Sr and Nd isotopic composition. High precision (double-spike) Pb isotope ratios are tightly correlated, precluding significant assimilation of older sialic crust but indicating mixing between a South Atlantic Mid Ocean-Ridge Basalt (MORB) source and a slab component derived from subducted sediments and altered oceanic crust. Hf–Nd isotope ratios define separate linear arrays for the volcanic arc and backarc, neither of which trend toward subducting sediment, possibly reflecting a primarily asthenospheric mantle array for the volcanic arc and involvement of enriched Proterozoic lithospheric mantle in the backarc. We propose a quantitative mixing model between a mixed-source, slab-derived melt and a heterogeneous mantle beneath the volcanic arc. The model is consistent with local geodynamic parameters, assuming water-saturated conditions within the slab.