Eclogitic and ultrahigh-pressure crustal garnets and their relationship to Phanerozoic subduction diamonds, Bingara area, New England fold belt, eastern Australia

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doi: 10.2113/gsecongeo.100.8.1565
Authors:Barron, B. Jane; Barron, L. M.; Duncan, G.
Author Affiliations:Primary:
7 Fairview Avenue, St. Ives, N.S.W., Australia
New South Wales Department of Primary Industries, Geological Survey, Australia
Rimfire Pacific Mining, Australia
Volume Title:Economic Geology and the Bulletin of the Society of Economic Geologists
Source:Economic Geology and the Bulletin of the Society of Economic Geologists, 100(8), p.1565-1582. Publisher: Economic Geology Publishing Company, Lancaster, PA, United States. ISSN: 0361-0128
Publication Date:2005
Note:In English. Includes appendix; digital supplement available online at 80 refs.; illus., incl. 4 tables, sketch maps
Summary:At Bingara-Copeton in the Phanerozoic New England fold belt, New South Wales, Australia, about two million diamonds were mined from Tertiary alluvial deposits that are more than 1,500 km distant from the nearest craton. An exploration program of the modern drainage system and soils in the Bingara district was undertaken to search for a hard-rock diamond source. Eight prospects were identified by regional magnetic anomalies, and 118 samples were processed for heavy minerals. Five diamonds were recovered; two rounded macrodiamonds and three small diamonds that retain some delicate features. More importantly, abundant high P and ultrahigh-P (UHP) metamorphic minerals also were recovered, particularly garnet. These were not derived from the low metamorphic grade country rock in the district. Using a major element classification scheme Bingara garnets were separated into "crustal" and "mantle-derived" groups and the following subgroups: eclogite (Group I diamond and Group II lower grade), peridotite (lherzolite), and Cr-poor megacrysts. Major element chemistry and REE analysis of selected garnets identifies their deeply subducted protoliths, including MORB and picrite, and arc-related basalt. Also defined here are a new subgroup of distinctive UHP crustal garnet from the crustal group above with almandine-spessartine compositions and enrichment in Na2O and heavy REEs. These compare with garnets from exhumed UHP schist and/or orthogneiss from Dabie Shan, central China, which formed from deeply subducted leucocratic continental crustal material, and also with rare garnet inclusions in diamonds from Sloan diatremes in Colorado and Wyoming and the Finsch kimberlite pipe, South Africa. Decompression microstructures, such as crystallo-graphic exsolutions of rutile, apatite, or ilmenite in some eclogitic and Fe-Mn-enriched Bingara garnets, confirm their partial exhumation from mantle depth. This work has calibrated an exploration technique for locating sources of unusual subduction-formed diamonds in collisional tectonic settings using eclogitic and UHP-crustal garnet as an indicator mineral. [G.L.B.]
Sections:Mineral data; Petrology
Subsections:Metamorphism: regional, dynamic
Mineral Groups:Ortho-and ring silicates
Subjects:Absolute age; Alluvium; Amphibole group; Ar/Ar; Cenozoic; Chain silicates; Clastic sediments; Crust; Dates; Diamond deposits; Eclogite; Electron probe data; Garnet group; Gems; Geodynamics; High pressure; Major elements; Metals; Metamorphic rocks; Mineral exploration; Models; Nesosilicates; New England Orogeny; Orthosilicates; Phanerozoic; Pressure; Rare earths; Sediments; Silicates; Soils; Solution; Stream sediments; Subduction; Tertiary; Trace elements; Ultrahigh pressure; Xenocrysts; Australasia; Australia; New South Wales Australia; Bingara-Copeton mining district
Abstract Numbers:06M/2893
Record ID:2006026737
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United Kingdom, Twickenham, United Kingdom
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