The timescale and mechanism of granulite formation at Kurunegala, Sri Lanka
The timescale and mechanism of granulite formation at Kurunegala, Sri Lanka
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斯里兰卡库鲁内格勒麻粒岩形成的时间尺度和机制
DOI：
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发表时间：
1990
期刊：
影响因子：
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通讯作者：
R. O’nions
中科院分区：
文献类型：
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作者：
Kevin W. Burton;R. O’nions
斯里兰卡库鲁内加拉早期紫苏花岗岩形成的特征是，在原本均匀的片麻岩中，斜方辉石以角闪石和黑云母为代价生长。紫苏花岗岩组合中的矿物平衡记录了738±60° C和6.9±1.2 kbar的压力-温度（P-T）条件，-17.0 ±1.2 log fO 2和aH 2 O =0.18±0.16。全岩微量元素和同位素测定表明，紫苏花岗岩形成时伴随着Sm>Rb>Pb>U>Sr>Nd的系统亏损，Rb/Sr、Sm/Nd和Th/U比值发生分馏，紫苏花岗岩组合在535±5 Ma时结晶。矿物间的主量元素（Fe−Mg−Ca）和Sm−Nd平衡发生在524±9 Ma，而Pb和Rb−Sr交换分别持续到501±5 Ma和486±1 Ma。角闪岩和紫苏花岗岩矿物的微量元素数据表明，消耗在整个岩石规模可以占矿物模式或微量元素丰度的变化，在直接的脱水区域。Sm/Nd的分馏在整个岩石尺度上是由角闪石的分解，没有一个主要的新的主相Sm的紫苏花岗岩的增长控制。铷和锶取决于黑云母、斜长石和碱性长石的相对行为。脱水熔融涉及角闪石，黑云母，碱长石的故障建模再现所观察到的Sm/Nd和Rb/Sr分馏，并表示小熔体馏分的损失，在厘米尺度上，从紫苏花岗岩。这些观察结果表明，部分熔融是最合理的手段，影响脱水和消耗，伴随紫苏花岗岩的形成。
Incipient charnockite formation at Kurunegala in Sri Lanka is characterized by the growth of orthopyroxene at the expense of amphibole and biotite in an originally homogeneous gneiss. Mineral equilibria in the charnockite assemblage record pressure-temperature (P-T) conditions of 738±60° C and 6.9±1.2 kbar at-17.0±1.2 log fO2 and aH2O=0.18±0.16. Wholerock trace-element and isotopic measurements show that charnockite formation was accompanied by a systematic depletion of Sm>Rb>Pb>U>Sr>Nd, with a fractionation of Rb/Sr, Sm/Nd and Th/U ratios, and crystallization of the charnockite assemblage at 535±5 Ma. Major element (Fe−Mg−Ca) and Sm−Nd equilibration between minerals occurred at 524±9 Ma, whereas, Pb and Rb−Sr underwent continued exchange to 501±5 Ma and 486±1 Ma, respectively. Trace-element data for both amphibolite and charnockite minerals show that depletion on a whole-rock scale can be accounted for either by changes in mineral modes or trace-element abundances, within the immediate area of dehydration. The fractionation of Sm/Nd on a whole-rock scale is controlled by the breakdown of amphibole, without the growth of a major new host-phase for Sm in the charnockite. Rubidium and Sr are dependent on the relative behaviour of biotite, plagioclase and alkali-feldspar. Modelling of dehydration-melting involving the breakdown of amphibole, biotite, and alkali-feldspar reproduces the observed Sm/Nd and Rb/Sr fractionation, and indicates the loss of small melt fractions, on a cm scale, from the charnockite. These observations suggest that partial melting is the most plausible means of effecting both the dehydration and depletion that accompanies charnockite formation.