Baldwin Suzanne L, Schönig Jan, Gonzalez Joseph P, Davies Hugh, von Eynatten Hilmar
Department of Earth and Environmental Sciences, Syracuse University, Syracuse, NY 13244-1070;
Department of Sedimentology and Environmental Geology, University of Göttingen, D-37077 Göttingen, Germany.
Proc Natl Acad Sci U S A. 2021 Jan 19;118(3). doi: 10.1073/pnas.2017231118.
Rock recycling within the forearcs of subduction zones involves subduction of sediments and hydrated lithosphere into the upper mantle, exhumation of rocks to the surface, and erosion to form new sediment. The compositions of, and inclusions within detrital minerals revealed by electron microprobe analysis and Raman spectroscopy preserve petrogenetic clues that can be related to transit through the rock cycle. We report the discovery of the ultrahigh-pressure (UHP) indicator mineral coesite as inclusions in detrital garnet from a modern placer deposit in the actively exhuming Late Miocene-Recent high- and ultrahigh-pressure ((U)HP) metamorphic terrane of eastern Papua New Guinea. Garnet compositions indicate the coesite-bearing detrital garnets are sourced from felsic protoliths. Carbonate, graphite, and CO inclusions also provide observational constraints for geochemical cycling of carbon and volatiles during subduction. Additional discoveries include polyphase inclusions of metastable polymorphs of SiO (cristobalite) and K-feldspar (kokchetavite) that we interpret as rapidly cooled former melt inclusions. Application of elastic thermobarometry on coexisting quartz and zircon inclusions in six detrital garnets indicates elastic equilibration during exhumation at granulite and amphibolite facies conditions. The garnet placer deposit preserves a record of the complete rock cycle, operative on <10-My geologic timescales, including subduction of sedimentary protoliths to UHP conditions, rapid exhumation, surface uplift, and erosion. Detrital garnet geochemistry and inclusion suites from both modern sediments and stratigraphic sections can be used to decipher the petrologic evolution of plate boundary zones and reveal recycling processes throughout Earth's history.
俯冲带前弧内的岩石循环涉及沉积物和水化岩石圈俯冲到上地幔、岩石出露到地表以及侵蚀形成新的沉积物。电子微探针分析和拉曼光谱揭示的碎屑矿物的成分和包裹体保留了可与岩石循环过程相关的岩石成因线索。我们报告了在巴布亚新几内亚东部新近纪晚期至现代正在隆升的高压和超高压变质岩区一个现代砂矿中，发现超高压指示矿物柯石英作为碎屑石榴石中的包裹体。石榴石成分表明含柯石英的碎屑石榴石源自长英质原岩。碳酸盐、石墨和一氧化碳包裹体也为俯冲过程中碳和挥发物的地球化学循环提供了观测约束。其他发现包括二氧化硅（方石英）和钾长石（科克切塔夫石）亚稳多晶型物的多相包裹体，我们将其解释为快速冷却的原熔体包裹体。对六个碎屑石榴石中共存的石英和锆石包裹体应用弹性热压测定法表明，在麻粒岩相和角闪岩相条件下出露过程中发生了弹性平衡。石榴石砂矿保留了在小于1000万年地质时间尺度上运行的完整岩石循环记录，包括沉积原岩俯冲到超高压条件、快速出露、地表隆升和侵蚀。现代沉积物和地层剖面中的碎屑石榴石地球化学和包裹体组合可用于解读板块边界带的岩石学演化，并揭示地球历史上的循环过程。
