Derivation of some A-type magmas by fractionation of basaltic magma: an example from the Padthaway R
Derivation of some A-type magmas by fractionation of basaltic magma: an example from the Padthaway R
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DOI：
10.1016/0024-4937(92)90029-x
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发表时间：
1992-07
期刊：
影响因子：
--
通讯作者：
S. Turner;J. Foden;R. S. Morrison
中科院分区：
文献类型：
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作者：
S. Turner;J. Foden;R. S. Morrison
对于A型花岗岩和火山岩，已经提出了各种岩石成因方案，其中许多涉及I型花岗岩的熔融或其残余来源。这些模型可以被证明有几个短期的，虽然可能有各种类型的A-型岩浆，其高温表明，地幔熔融调用任何成岩模型。此外，我们认为，双峰协会与镁铁质岩石，化学特征和典型的低初始87 Sr/86 Sr比值的许多A型岩石是完全一致的起源，通过扩展分馏的玄武岩岩浆。层状镁铁质侵入体中的填隙长英质物质也为这种起源提供了证据，这里提供的南澳帕德威海岭A型花岗岩和火山岩套件的新数据也是如此。矿物学，化学和同位素的参数表明，Padthaway套件演化的玄武岩浆，也形成了同期辉长岩岩体本身包含A型长英质分馏。这些超固溶花岗岩和火山岩的橄榄石和辉石组合证明温度为900-1000°C，水不饱和条件下，最终H2O< 3重量%。锆石地球化学趋势和负Eu异常表明辉石和长石的长期分馏历史。与它们的高Nd含量（+2至−3）和低初始87 Sr/86 Sr（0.703-0.706）相一致，这些A型岩浆被很好地模拟为现代幔源母岩浆结晶的产物。我们的计算，重力数据的支持下，表明A型套房，通常侵入在非挤压设置，可能标志着地壳生长的情节，在此期间，相当多的地幔物质被添加到地壳。
Various petrogenetic schemes have been proposed for A-type granitic and volcanic rocks, many involving melting of I-type granites or their residual sources. These models can be shown to have several short-comings and whilst there may be various kinds of A-type magmas their high temperatures suggest that mantle melts be invoked in any petrogenetic model. In addition, we suggest that the bimodal association with mafic rocks, the chemical characteristics and typically low initial87Sr/86Sr ratios of many A-type rocks are entirely consistent with an origin by extended fractionation of bsaltic magmas. Interstitial felsic material from layered mafic intrusions also provides evidence for this origin, as do new data presented here for suite of A-type granites and volcanics from the Padthaway Ridge in South Australia. Mineralogical, chemical and isotopic arguments show that the Padthaway suite evolved from the basaltic magma that also formed contemporaneous gabbroic plutons which themselves contain A-type felsic fractionates. Olivine and pyroxene assemblages of these hypersolvus granites and volcanics attest to temperatures of 900–1000°C and water undersaturated conditions with final H2O< 3 wt.%. Curvilinear geochemical trends and negative Eu anomalies indicate a history of protracted fractionation involving pyroxene and feldspar. Consistent with their high ϵNd (+ 2 to −3) and low initial87Sr/86Sr (0.703-0.706), these A-type magmas are well modelled as the products of ∼90% crystallization of contemporary, mantle-derived, parental magma. Our calculations, supported by gravity data, suggest that A-type suites, which typically intrude in non-compressional settings, may mark episodes of crustal growth during which considerable mantle material is added to the crust.