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首页> 外文期刊>Geochemical Journal >Petrology and geochemistry of the back-arc lithospheric mantle beneath eastern Payunia (La Pampa, Argentina): Evidence from Agua Poca peridotite xenoliths
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Petrology and geochemistry of the back-arc lithospheric mantle beneath eastern Payunia (La Pampa, Argentina): Evidence from Agua Poca peridotite xenoliths

机译：Payunia东部（阿根廷La Pampa）下方的弧后岩石圈地幔的岩石学和地球化学：来自Agua Poca橄榄岩异岩的证据

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This paper presents the results of new petrochemical studies carried out on mantle xenoliths hosted in Pleistocene basaltic rocks from the Agua Poca volcano in central-western Argentina. Mantle xenoliths studied are shown to be mainly anhydrous spinel lherzolites with minor amounts of harzburgite and banded pyroxenite, showing highly variable equilibrium temperatures ranging from 820°C to 1030°C at 1.0 to 2.0 GPa. This constitutes evidence that the mantle xenoliths are representative of a large portion of the lithospheric mantle column and that the geothermal gradient is not very elevated as reported in some other Patagonian provinces. Geochemical characteristics of clinopyroxene in the mantle xenoliths allow classification into two groups; Groups 1 and 2. Group 1 contains most of the lherzolites and has light-REE depletion, with slightly positive anomalies of Eu in some samples and extreme Nb and Ta depletion. Group 2 consists of two harzburgitic samples, has flat REE patterns with lower Sm to Lu concentrations, with enriched Sr and negative HFSE anomalies. Based on mineral and residua compositions estimated assuming equilibrium with clinopyroxenes, Group 1 can be considered to be refractory residua after up to 7%, non-modal, near-fractional melting of a spinel-facies Primitive Mantle. Group 2 can be considered to be after ca. 13% of partial melting. It is inferred that partial melting events in the lithospheric mantle beneath the Agua Poca occurred in different ages since the Proterozoic, but compared with Group 1, the metasomatic overprint is dominant in Group 2 mantle xenoliths. The calculated melt compositions from Group 2 are interpreted to be transient liquid compositions developed during melt-peridotite interaction, and are different from the host alkaline basalts. The HFSE-depleted composition estimated for the rising melt suggests the presence of a slab-derived component, although the possibility cannot be disregarded (on the basis of present data) that such a geochemical feature is due to segregation of HFSE-bearing minerals during the interaction with the peridotite. Thus, we attribute the metasomatic agent to a basaltic melt and to a minor amount of slab-derived fluids.
机译：本文介绍了对阿根廷中西部阿瓜波卡火山（Agua Poca）火山更新世玄武岩中的地幔异岩进行新的石化研究的结果。研究表明，地幔中的异种岩主要是无水尖晶石锂沸石，具有少量的尖晶石和带状的辉石岩，在1.0至2.0 GPa的平衡温度范围内变化很大，为820°C至1030°C。这构成了地幔异岩代表岩石圈地幔柱的很大一部分的证据，并且地热梯度没有像其他巴塔哥尼亚省所报道的那样升高得很高。地幔异岩中斜辉石的地球化学特征可分为两类。第1组和第2组。第1组包含大多数锂沸石，并具有轻稀土元素消耗，某些样品中Eu的异常异常轻微，而Nb和Ta的消耗极度。第2组由两个哈茨形样品组成，具有平坦的REE模式，具有较低的Sm到Lu浓度，富集的Sr和负HFSE异常。根据假定与斜发rox烯达到平衡而估计的矿物和残渣成分，在尖晶石相原始地幔经过7％的非模态，近分数熔融之后，第1组可以被认为是难熔残渣。组2可以认为在ca之后。 13％的部分熔融。据推测，自古生代以来，阿瓜波卡下面岩石圈地幔的部分融化事件发生在不同的年龄，但与第一组相比，交代叠印在第二组地幔异岩中占主导地位。从组2计算得出的熔体成分被解释为是在熔体-橄榄岩与橄榄石相互作用期间形成的瞬时液体成分，与主体碱性玄武岩不同。尽管上升的熔体中估计有HFSE的贫化成分表明存在板状成分，但是（根据现有数据）不能忽略这样的地球化学特征是由于含HFSE的矿物在分离过程中的偏析的可能性。与橄榄岩的相互作用。因此，我们将交代剂归因于玄武质熔体和少量的平板状流体。

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