New approach to constraining Cr-mineralization and forearc processes: U-Pb dating and geochemistry of titanites in chromitites of the Dun Mountain ophiolite belt (New Zealand)

IF 3.2 2区 地球科学 Q1 GEOLOGY
Daniel Patias , Renjie Zhou , Xin Wang , Jonathan C. Aitchison
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Abstract

Ophiolites provide important insights for understanding of subduction zones and forearc mantle-wedge geodynamics. Specifically, within the mantle section, podiform chromitites may record processes associated with development of highly depleted supra-subduction ophiolites, chromium mineralization and subsequent metasomatism. This study explores the first ever U-Pb isotope and trace-element data obtained from titanites in chromitites of the Dun Mountain ophiolite belt in New Zealand. Titanite is unusual in such geochemically depleted rocks and a low recovery rate attests to its rarity (ca. 190 grains from 100 kg of material from two chromitite samples). Results of U-Pb geochronology and geochemistry for the titanites constrain both the mid-Permian timing of chromitite crystallization (∼283 Ma) and later metasomatism (∼272 Ma). Older titanite grains are depleted in HFSE and have flat REE chondrite-normalized patterns reflecting a high-degree of partial melting in depths greater than the plagioclase stability field (>15 km). Younger titanites show enrichment in HFSE, depletion in LREE and negative Eu anomalies, revealing melt extraction at shallower depths (<15 km) that likely record refertilization of the mantle-wedge. Results highlight the potential of titanites in the acquisition of direct geochronological and geochemical constraints from rocks commonly devoid of minerals suitable for U-Pb dating.

Abstract Image

制约铬矿化和弧前过程的新方法:邓山蛇绿岩带(新西兰)铬铁矿中榍石的 U-Pb 测定和地球化学研究
蛇绿岩为了解俯冲带和弧前地幔楔地球动力学提供了重要的启示。具体来说,在地幔部分,荚状铬铁矿可能记录了与高度贫化的超俯冲蛇绿岩、铬矿化和随后的变质作用有关的发展过程。本研究首次探讨了从新西兰敦山蛇绿岩带铬铁矿中的榍石获得的 U-Pb 同位素和痕量元素数据。在这种地球化学贫化的岩石中,榍石并不常见,低回收率也证明了它的稀有性(从两个铬铁矿样品的 100 千克材料中回收了约 190 粒榍石)。榍石的铀-铅地质年代学和地球化学研究结果确定了铬铁矿结晶的二叠纪中期(283 千兆年前)和后期变质期(272 千兆年前)。较老的榍石晶粒贫化了高频闪长岩,具有平坦的REE软玉归一化模式,反映出在大于斜长石稳定场(15千米)的深度内部分熔融的程度较高。较年轻的榍石显示出HFSE富集、LREE贫化和Eu负异常,揭示了在较浅深度(15千米)的熔融萃取,很可能记录了地幔边缘的再肥化。研究结果凸显了榍石在从通常不含适合进行U-Pb定年的矿物的岩石中获取直接地质年代和地球化学约束方面的潜力。
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来源期刊
Ore Geology Reviews
Ore Geology Reviews 地学-地质学
CiteScore
6.50
自引率
27.30%
发文量
546
审稿时长
22.9 weeks
期刊介绍: Ore Geology Reviews aims to familiarize all earth scientists with recent advances in a number of interconnected disciplines related to the study of, and search for, ore deposits. The reviews range from brief to longer contributions, but the journal preferentially publishes manuscripts that fill the niche between the commonly shorter journal articles and the comprehensive book coverages, and thus has a special appeal to many authors and readers.
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