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首页> 外文期刊>Contributions to Mineralogy and Petrology >Oxygen isotope zoning in garnets from Franciscan eclogite blocks: evidence for rock-buffered fluid interaction in the mantle wedge
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Oxygen isotope zoning in garnets from Franciscan eclogite blocks: evidence for rock-buffered fluid interaction in the mantle wedge

机译：方济会的榴辉岩块中的石榴石中的氧同位素分区：地幔楔中岩石缓冲流体相互作用的证据

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摘要

The oxygen isotope compositions of eclogite and amphibolite garnets from Franciscan Complex high-grade blocks and actinolite rinds encasing the blocks were determined to place constraints on their fluid histories. SIMS oxygen isotope analysis of single garnets from five eclogite blocks from three localities (Ring Mountain, Mount Hamilton, and Jenner Beach) shows an abrupt decrease in the δ~(18)O value by ~ 1-3 %o from core to rim at a distance of ~ 120 ± 50 um from the rim in nine out of the 12 garnets analyzed. In contrast, amphibolite garnets from one block (Ring Mountain) analyzed show a gradual increase in δ~(18)O value from core to rim, implying a different history from that of the eclogite blocks. Values of δ~(18)O in eclogite garnet cores range from 5.7 to 11.6 %o, preserving the composition of the eclogite protolith. The abrupt decrease in the δ~(18)O values of the garnet rims to values ranging from 3.2 to 11.2 %o suggests interaction with a lower δ~(18)O fluid during the final stages of growth during eclogite facies metamorphism (450-600℃ ). We hypothesize that this fluid is sourced from the serpentinized mantle wedge. High Mg, Ni, and Cr contents of actinolite rinds encasing the blocks also support interaction with ultramafic rock. Oxygen isotope thermometry using chlorite and phengite versus actinolite of rinds suggests temperatures of 185-240℃ at Ring Mountain and Mount Hamilton. Rind formation temperatures together with the lower δ~(18)O garnet rims suggest that the blocks were in contact with ultramafic rock from the end of garnet growth through low-temperature retrogression. We suggest a tectonic model in which oceanic crust is subducted at the initiation of subduction and becomes embedded in the overlying mantle wedge. As subduction continues, metasomatic exchange between high-grade blocks and surrounding ultramafic rock is recorded in low δ~(18)O garnet rims, and later as temperatures decrease, with rind formation.
机译：确定了来自方济各会综合体高级块体的榴辉辉石和角闪石石榴石的氧同位素组成，以及包裹这些块体的阳起石外皮对它们的流体历史施加了限制。 SIMS氧同位素分析来自三个地区（环山，汉密尔顿山和詹纳海滩）的五个榴辉岩块的单个石榴石，显示从中心到边缘，δ〜（18）O值突然降低了1-3％o。在分析的12个石榴石中，有9个与边缘之间的距离约为120±50 um。相比之下，从一个区块（环山）分析的角闪石石榴石显示，从核心到边缘的δ〜（18）O值逐渐增加，这意味着与榴辉岩区块的历史不同。榴辉石石榴石芯中的δ〜（18）O值范围为5.7至11.6％o，保留了榴辉岩原石的成分。石榴石边缘的δ〜（18）O值突然下降至3.2至11.2％o，表明在榴辉岩相变质作用的生长的最后阶段与较低的δ〜（18）O流体相互作用（450- 600℃）。我们假设这种流体来自蛇形化的幔楔。包围块体的阳起石皮的高Mg，Ni和Cr含量也支持与超镁铁质岩石的相互作用。使用亚氯酸盐和变质铁矿与阳起石的外皮进行氧同位素测温表明，环形山和汉密尔顿山的温度为185-240℃。裂谷的形成温度以及较低的δ〜（18）O石榴石边缘表明，从石榴石生长结束到低温倒退，这些块体与超镁铁质岩石接触。我们建议一种构造模型，其中洋壳在俯冲开始时就被俯冲并嵌入上覆地幔楔中。随着俯冲的继续，在低δ〜（18）O石榴石边缘记录了高级块体与周围超镁铁质岩石之间的交代交换，随后随着温度的降低，形成了果皮。

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