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Anatectic granitic pegmatites from the Eastern Alps; a case of variable rare metal enrichment during high-grade regional metamorphism; II, Pegmatite staurolite as an indicator of anatectic pegmatite parent melt formation; a field and experimental study

Juergen Konzett, Christoph Hauzenberger, Thomas Ludwig and Roland Stalder
Anatectic granitic pegmatites from the Eastern Alps; a case of variable rare metal enrichment during high-grade regional metamorphism; II, Pegmatite staurolite as an indicator of anatectic pegmatite parent melt formation; a field and experimental study (in A tribute to Milan Novak, Radek Skoda (editor), Pietro Vignola (editor), Axel Mueller (editor) and Lee A. Groat (editor))
The Canadian Mineralogist (July 2018) 56 (4): 603-624

Abstract

Staurolite was found in two granitic pegmatites emplaced in crystalline basement rocks of the Eastern Alps in Northern Italy. The pegmatites form decimeter- to meter-sized concordant veins with quartz + albite + muscovite + paragonite as major phases along with minor garnet + beryl and accessory Nb-Ta-Sn-Ti phases, zircon, and magmatic phosphate minerals. U-Pb dating of cassiterite from one of the pegmatites yielded Permian ages of approximately 290 Ma. The pegmatite country rocks are garnet-biotite-muscovite-plagioclase gneisses with accessory paragonite + or - staurolite + or - kyanite. The pegmatite staurolites are present as an accessory groundmass phase with sizes typically < or = approximately 200-300 mu m and as tiny ( approximately 10-50 mu m) inclusions in large beryl crystals. When enclosed in beryl staurolite is typically intergrown with quartz and is part of a complex solid phase assemblage including tapiolite, chrysoberyl, cordierite, and Zn-Al-spinel. With few exceptions, the staurolites from the pegmatites are perfectly euhedral and free of solid inclusions. They are Fe-rich with X (sub Mg) =0.02-0.20 and contain 0.6-7.2 wt.% ZnO, <0.05-0.4 wt.% TiO (sub 2) , 0.48-0.69 wt.% Li (sub 2) O, and approximately 1.8 wt.% H (sub 2) O. With respect to trace elements, high Be (96-171 mu g/g), Nb (18-49 mu g/g), and Ta (10-72 mu g/g) contents are noteworthy, as is a very low Nb/Ta ratio of 0.6-1.9. Staurolites from the pegmatite country rock show a comparable range in XMg of 0.16-0.23 and an identical H (sub 2) O content but are significantly different in TiO (sub 2) (0.49-0.92 wt.%), ZnO (0.1-2.4 wt.%), and Li (sub 2) O (0.06-0.31 wt.%) contents. Also, Be (13-48 mu g/g), Nb (0.4-4.7 mu g/g), and Ta (0.04-1.2 mu g/g) contents are much lower and Nb/Ta ratios of 4.0-10.9 are higher than in the pegmatite staurolites. In both pegmatites and country rocks staurolite is the major Li carrier and one of the major Ta-fractionating silicate phases. The unusual paragenetic and textural features of the pegmatite staurolites suggest that they are magmatic rather than metamorphic in origin. Partial-melting experiments at 7 kbar and 650-750 degrees C confirm the potential of the pegmatite country rocks to generate strongly peraluminous granitic melts in equilibrium with staurolite and may produce staurolite textures very similar to those found in the pegmatites. Hence, both field and experimental data are consistent with magmatic formation of the pegmatite staurolites and an origin of the pegmatite parent melts by anatexis of metapelites during the Permian high-T/low-P tectonometamorphic event, which affected significant portions of the Eastern Alpine crystalline basement.


ISSN: 0008-4476
EISSN: 1499-1276
Coden: CAMIA6
Serial Title: The Canadian Mineralogist
Serial Volume: 56
Serial Issue: 4
Title: Anatectic granitic pegmatites from the Eastern Alps; a case of variable rare metal enrichment during high-grade regional metamorphism; II, Pegmatite staurolite as an indicator of anatectic pegmatite parent melt formation; a field and experimental study
Title: A tribute to Milan Novak
Author(s): Konzett, JuergenHauzenberger, ChristophLudwig, ThomasStalder, Roland
Author(s): Skoda, Radekeditor
Author(s): Vignola, Pietroeditor
Author(s): Mueller, Axeleditor
Author(s): Groat, Lee A.editor
Affiliation: University of Innsbruck, Institute of Mineralogy and Petrology, Innsbruck, Austria
Pages: 603-624
Published: 201807
Text Language: English
Publisher: Mineralogical Association of Canada, Ottawa, ON, Canada
Meeting name: PEG2017; 8th international symposium on Granitic pegmatites
Meeting location: Kristiansand, NOR, Norway
Meeting date: 20170613June 13-15, 2017
References: 60
Accession Number: 2019-003884
Categories: Igneous and metamorphic petrologyGeochemistry of rocks, soils, and sediments
Document Type: Serial Conference document
Bibliographic Level: Analytic
Illustration Description: illus. incl. 9 tables
N36°30'00" - N47°30'00", E06°30'00" - E19°00'00"
Secondary Affiliation: University of Graz, AUT, AustriaHeidelberg University, AUT, Austria
Country of Publication: Canada
Secondary Affiliation: GeoRef, Copyright 2019, American Geosciences Institute. Abstract, copyright, Mineralogical Association of Canada. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201902
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