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Nature and origin of an LCT-suite pegmatite with late-stage sodium enrichment, Brazil Lake, Yarmouth County, Nova Scotia; II, Implications of stable isotopes (delta (super 18) O, delta D) for magma source, internal crystallization and nature of sodium metasomatism

Daniel J. Kontak and T. Kurtis Kyser
Nature and origin of an LCT-suite pegmatite with late-stage sodium enrichment, Brazil Lake, Yarmouth County, Nova Scotia; II, Implications of stable isotopes (delta (super 18) O, delta D) for magma source, internal crystallization and nature of sodium metasomatism
The Canadian Mineralogist (August 2009) 47 (4): 745-764

Abstract

The 395 Ma Brazil Lake granitic pegmatite, a rare occurrence of an LCT-type (Li-Cs-Ta) pegmatite in the Meguma Terrane of Nova Scotia, Canada, is hosted by metasedimentary and metavolcanic rocks of the Silurian White Rock Group. Previous mineralogical studies showed the pegmatite formed in two distinct stages, an early spodumene--K-feldspar-quartz + or - muscovite stage that was followed by a pervasive metasomatic stage in which albite and secondary muscovite grew at the expense of earlier K-feldspar. The economically important oxide phases (Ta-Nb) are intimately associated with secondary albite. Determination of the stable isotopic (delta (super 18) O, D) values for silicate phases (quartz, K-feldspar, spodumene, albite, muscovite, garnet, tourmaline) provide insight into the origin of the pegmatite-forming melt and its subsequent internal evolution. A primary delta (super 18) O signature for the melt of +8 per mil is calculated using the delta (super 18) Ovalues of quartz and Delta quartz--whole-rock values from studies on other granitic suites. Interpreted in the context of the geological setting, the delta (super 18) O data preclude a dominantly metasedimentary source and suggest, instead, that the pegmatite-forming melt originated via partial melting of a mixed igneous (i.e., volcanic)- sedimentary source. The calculated delta (super 18) O (sub H2O) -delta D (sub H2O) values, at 500 degrees C, correspond to the field for magmatic waters, but some isotopic data, including delta D for fluid-inclusion extracts, suggest late-stage incursion of metamorphic water that had equilibrated with the surrounding metasedimentary rocks. Mineral-mineral fractionation (e.g., Delta Qtz-Ms) does not reflect equilibrium within the pegmatite, which may have resulted from several processes, including non-equilibrium crystallization, variable degrees of isotopic exchange during protracted cooling, or interaction with an external reservoir(s). The data for albite (delta (super 18) O = +9.1 per mil, n = 6) are consistent with a magmatic origin, albeit late in the evolution of the pegmatite, on the basis of textural arguments. A model is proposed for the origin of the extensive albite-enriched zones, which involves introduction of a sodic melt derived at depth after protracted fractionation of the same parental melt from which the Li-rich parts of the pegmatite originated.


ISSN: 0008-4476
EISSN: 1499-1276
Coden: CAMIA6
Serial Title: The Canadian Mineralogist
Serial Volume: 47
Serial Issue: 4
Title: Nature and origin of an LCT-suite pegmatite with late-stage sodium enrichment, Brazil Lake, Yarmouth County, Nova Scotia; II, Implications of stable isotopes (delta (super 18) O, delta D) for magma source, internal crystallization and nature of sodium metasomatism
Affiliation: Laurentian University, Department of Earth Science, Sudbury, ON, Canada
Pages: 745-764
Published: 200908
Text Language: English
Publisher: Mineralogical Association of Canada, Ottawa, ON, Canada
References: 96
Accession Number: 2009-089827
Categories: Igneous and metamorphic petrologyIsotope geochemistry
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 2 tables, sketch map
N43°49'60" - N43°49'60", W66°07'60" - W66°07'60"
Secondary Affiliation: Queen's University, CAN, Canada
Country of Publication: Canada
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, copyright, Mineralogical Association of Canada. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 200949
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