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Edited by Peter B. Kelemen, Lamont-Doherty Earth Observatory, Palisades, NY, and approved December 11, 2020 (received for review August 14, 2020)
Significance
On Earth, igneous, metamorphic, and sedimentary rocks are recycled from the surface to upper mantle depths and subsequently returned to the surface as a result of tectonic and sedimentary processes. Within active plate boundaries, the rock cycle involves lithospheric deformation to exhume rocks to the surface. We show that trapped inclusions in garnet sand from the actively exhuming high- and ultrahigh-pressure metamorphic terrane of eastern Papua New Guinea preserve a record of crustal subduction and rapid exhumation linking upper mantle and surface processes operative on short geologic timescales (<10 My). Detrital garnet geochemistry and their inclusion suites from both modern sediments and stratigraphic sections can be used to reveal rock recycling processes throughout Earth’s history.
Abstract
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 CO2 inclusions also provide observational constraints for geochemical cycling of carbon and volatiles during subduction. Additional discoveries include polyphase inclusions of metastable polymorphs of SiO2 (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.
Footnotes
- ↵1To whom correspondence may be addressed. Email: sbaldwin{at}syr.edu.
↵2Present address: Department of Earth and Environmental Sciences, University of Pavia, 27100 Pavia, Italy.
↵3Present address: Research School of Earth Sciences, The Australian National University, Canberra, ACT 2000, Australia.
Author contributions: S.L.B. designed research; S.L.B. and H.D. led field expeditions to Papua New Guinea; and S.L.B., J.S., J.P.G., H.D., and H.v.E. performed research, analyzed data, and wrote the paper.
The authors declare no competing interest.
This article is a PNAS Direct Submission.
This article contains supporting information online at https://www.pnas.org/lookup/suppl/doi:10.1073/pnas.2017231118/-/DCSupplemental.
Data Availability.
All study data are included in the article and supporting information.
Published under the PNAS license.
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Garnet sand reveals rock recycling processes in the youngest exhumed high- and ultrahigh-pressure terrane on Earth
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- Earth, Atmospheric, and Planetary Sciences
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