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Metasomatism and the crystallization of zircon megacrysts in Archaean peridotites from the Lewisian complex, NW Scotland
Contributions to Mineralogy and Petrology ( IF 3.5 ) Pub Date : 2018-11-17 , DOI: 10.1007/s00410-018-1527-5
John W Faithfull 1 , Tim J Dempster 2 , John M MacDonald 2 , Monica Reilly 2 ,
Affiliation  

Zircon megacrysts are locally abundant in 1–40 cm-thick orthopyroxenite veins within peridotite host rocks in the Archaean Lewisian gneiss complex from NW Scotland. The veins formed by metasomatic interaction between the ultramafic host and Si-rich melts are derived from partial melting of the adjacent granulite-facies orthogneisses. The interaction produced abundant orthopyroxene and, within the thicker veins, phlogopite, pargasite and feldspathic bearing assemblages. Two generations of zircon are present with up to 1 cm megacrystic zircon and a later smaller equant population located around the megacryst margins. Patterns of zoning, rare earth element abundance and oxygen isotopic compositions indicate that the megacrysts crystallized from crustal melts, whereas the equant zircon represents new neocryst growth and partial replacement of the megacryst zircon within the ultramafic host. Both zircon types have U–Pb ages of ca. 2464 Ma, broadly contemporaneous with granulite-facies events in the adjacent gneisses. Zircon megacrysts locally form > 10% of the assemblage and may be associated to zones of localized nucleation or physically concentrated during movement of the siliceous melts. Their unusual size is linked to the suppression of zircon nucleation and increased Zr solubility in the Si-undersaturated melts. The metasomatism between crustal melts and peridotite may represent an analog for processes in the mantle wedge above subducting slabs. As such, the crystallization of abundant zircon in ultramafic host rocks has implications for geochemistry of melts generated in the mantle and the widely reported depletion of high field strength elements in arc magmas.

中文翻译:

苏格兰西北路易斯安杂岩中太古代橄榄岩的交代作用和锆石巨晶结晶

锆石巨晶在苏格兰西北部太古代刘易斯片麻岩复合体橄榄岩寄主岩中 1-40 厘米厚的正辉石脉中局部丰富。超基性基质与富硅熔体交代相互作用形成的脉是由相邻麻粒岩相正方麻岩的部分熔融形成的。这种相互作用产生了大量的斜方辉石,并且在较粗的矿脉中产生了金云母、磷灰石和长石轴承组合。存在两代锆石,具有高达 1 cm 的巨晶锆石和稍后位于巨晶边缘周围的较小的等距锆石群。分区模式、稀土元素丰度和氧同位素组成表明巨晶是从地壳熔体中结晶出来的,而等值锆石则代表了新晶生长和超镁铁质宿主内巨晶锆石的部分替代。两种锆石类型的 U-Pb 年龄都约为。2464 Ma,与相邻片麻岩中的麻粒岩相事件大致同期。锆石巨晶在局部形成 > 10% 的组合,并且可能与局部成核区相关或在硅质熔体运动期间物理浓缩。它们不寻常的尺寸与抑制锆石成核和增加 Zr 在 Si 不饱和熔体中的溶解度有关。地壳熔体和橄榄岩之间的交代作用可能代表俯冲板片上方地幔楔中的过程的类似物。因此,
更新日期:2018-11-17
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