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Full text release has been delayed at the author's request until September 01, 2027

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Petrology and Geochemistry of the Neoproterozoic Blueschists of Anglesey, Wales

LaCroix, Connor
http://orcid.org/0009-0003-6209-3898
http://rave.ohiolink.edu/etdc/view?acc_num=ohiou1722526192389411

Abstract Details

2024, Master of Science (MS), Ohio University, Geological Sciences.
Blueschists are an important petrogenetic indicator of subduction and the first appearance of blueschists in the geologic record is commonly taken as evidence for the start of modern-style plate tectonics on Earth. The oldest known blueschists are Neoproterozoic in age, so studies of these blueschists may provide information about the processes and conditions associated with the initiation of plate tectonics on Earth. Of the known Neoproterozoic blueschist localities, the Anglesey (Wales, UK) blueschists are of particular importance because they contain the mineral lawsonite and are thought to be the oldest known lawsonite-bearing rocks in the geologic record. Petrographic, bulk-rock geochemical, and mineral composition and zoning data were acquired to better understand the protolith of the Anglesey blueschists, the conditions (P-T) under which they formed, and their metamorphic evolution. The bulk-rock major and trace element compositions of the metamafic rocks in the Anglesey blueschist belt are generally consistent with a mid-ocean ridge basalt (MORB) protolith, although some samples display variable enrichments in the large ion lithophile elements (LILEs; Cs, Ba, Rb), as well as Sr, Pb, and U. Incompatible trace element systematics suggest most samples underwent seafloor alteration prior to subduction. The most common amphiboles in the Anglesey blueschist belt are glaucophane, winchite, actinolite, riebeckite, and magnesio-riebeckite. Amphiboles in the blueschist facies rocks have core-to-rim zoning and record a transition from calcic/sodic-calcic amphiboles (winchite and/or actinolite cores) to sodic amphiboles (glaucophane rims). Riebeckite and magnesio-riebeckite are most common in greenschist and transitional greenschist-blueschist facies rocks, where they display patchy zoning and compositional variations in Fe3+ and Al. Differences between the amphiboles developed in the greenschist and blueschist facies rocks likely reflect variations in the pressure-temperature conditions of metamorphism, as well as bulk-rock composition. Epidotes also display systematic variations in Fe3+ and Al content, characterized by a core-to-rim decrease in Fe3+ content. These amphibole and epidote zoning trends are consistent with growth along the prograde path. Chlorite compositions indicate metamorphic temperatures of ~200 °C to in excess of 350 °C and the compositions of co-existing amphibole and chlorite pairs suggest metamorphic pressures between 0.5 - 0.7 GPa.
Katherine Fornash (Advisor)
146 p.
Geochemistry; Geological; Geology; Plate Tectonics
Petrology; Geochemistry; Blueschists; Anglesey; Wales; Neoproterozoic

Recommended Citations

Citations

  • LaCroix, C. (2024). Petrology and Geochemistry of the Neoproterozoic Blueschists of Anglesey, Wales [Master's thesis, Ohio University]. OhioLINK Electronic Theses and Dissertations Center. http://rave.ohiolink.edu/etdc/view?acc_num=ohiou1722526192389411

    APA Style (7th edition)

  • LaCroix, Connor. Petrology and Geochemistry of the Neoproterozoic Blueschists of Anglesey, Wales. 2024. Ohio University, Master's thesis. OhioLINK Electronic Theses and Dissertations Center, http://rave.ohiolink.edu/etdc/view?acc_num=ohiou1722526192389411.

    MLA Style (8th edition)

  • LaCroix, Connor. "Petrology and Geochemistry of the Neoproterozoic Blueschists of Anglesey, Wales." Master's thesis, Ohio University, 2024. http://rave.ohiolink.edu/etdc/view?acc_num=ohiou1722526192389411

    Chicago Manual of Style (17th edition)

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This open access ETD is published by Ohio University and OhioLINK.