Monazite as a monitor of melting, garnet growth and feldspar recrystallization in continental lower crust
Monazite as a monitor of melting, garnet growth and feldspar recrystallization in continental lower crust
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DOI：
10.1111/jmg.12150
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发表时间：
2015-09
影响因子：
3.4
通讯作者：
G. Dumond;P. Goncalvès;Michael L. Williams;M. Jercinovic
中科院分区：
文献类型：
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作者：
G. Dumond;P. Goncalvès;Michael L. Williams;M. Jercinovic
独居石是加拿大西部地盾阿萨巴斯卡麻粒岩上甲板域暴露的长英质麻粒岩带糜棱岩中常见的副相。na20 - cao - k20 - feo - mgo - al2o3 - sio2 - h2o - tio2 - fe2o3体系的场关系、整体岩石地球化学和相平衡模拟与代表大陆下地壳基性岩浆内镀驱动含黑云母副长岩超高温熔融残余产物的富含石榴石的岩石一致。石榴石中含有的c. 2.64-2.61 Ga富Y的吸收独居石岩心被解释为c. 2.61 Ga之后沉积在地球表面的碎屑颗粒的残留物。在从≤0.8 GPa到≥1.4 GPa的加载过程中，石榴石中的钇贫独居石域在Sm和Gd中被耗尽，并与黑云母+斜长石+石英±硅线石的无流体熔融有关。c. 2.61 ~ 2.55 Ga贫Y富Th独居石域在石榴石+三元长石±正辉石+过铝熔体存在下结晶。在地壳增厚过程中，c. 2.58-2.52 Ga贫Th + Ca和富集Eu的独居石边缘与局部熔体萃取有关，同时伴随着以斜长石为牺牲的高压（HP）富镁石榴石的生长，最终在bbb950°c达到顶峰。1.9 Ga温度下的再加热和右旋下地壳再活化导致富（La + Ce）的独居石和第二代石榴石同步生长，同时在1.0-1.2 GPa和600-700℃温度下长石和正辉石再结晶。本研究中的独居石颗粒相对于球粒陨石具有正Eu异常。独居石中的Y、Sm、Eu和Gd与大陆下地壳石榴石和斜长石两大相有直接联系。下地壳独居石中与模式丰富的石榴石相关的Eu‐正异常似乎与Eu‐富集和Y、Sm和Gd的耗尽直接相关，这是在过铝体成分高温熔融过程中石榴石生长和斜长石破裂的结果。
Monazite is a common accessory phase in felsic granulite ribbon mylonites exposed in the Upper Deck domain of the Athabasca granulite terrane, western Canadian Shield. Field relationships, bulk rock geochemistry and phase equilibria modelling in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3 system are consistent with the garnet‐rich rocks representing the residual products of ultrahigh temperature melting of biotite‐bearing paragneisses driven by intraplating of mafic magma in continental lower crust. The c. 2.64–2.61 Ga Y‐rich resorbed monazite cores included in garnet are interpreted as relicts of detrital grains deposited on the Earth's surface after c. 2.61 Ga. Yttrium‐poor monazite domains in garnet are depleted in Sm and Gd and linked to fluid‐absent melting of biotite + plagioclase + quartz ± sillimanite during a prograde loading path from ≤0.8 to ≥1.4 GPa. The c. 2.61–2.55 Ga Y‐depleted, Th‐rich monazite domains crystallized in the presence of garnet + ternary feldspar ± orthopyroxene + peraluminous melt. The c. 2.58–2.52 Ga monazite rims depleted in Th + Ca and enriched in Eu are linked to localized melt extraction synchronous with growth of high‐pressure (HP) grossular‐rich garnet at the expense of plagioclase during crustal thickening, culminating at >950 °C. Re‐heating and dextral transpressive lower crustal reactivation at c. 1.9 Ga resulted in syn‐kinematic growth of (La + Ce)‐enriched monazite and a second generation of garnet, concurrent with recrystallization of feldspar and orthopyroxene at 1.0–1.2 GPa and 600–700 °C. Monazite grains in this study are marked by positive Eu‐anomalies relative to chondrite. A direct link is implied between Y, Sm, Eu and Gd in monazite and two major phases in continental lower crust: garnet and plagioclase. Positive Eu‐anomalies in lower crustal monazite associated with modally abundant garnet appear to be directly related to Eu‐enrichment and depletions of Y, Sm and Gd that are consequences of garnet growth and plagioclase breakdown during HP melting of peraluminous bulk compositions.