Publication Type:

Journal Article

Authors:

Kontak, Daniel J.; Kyser, T. Kurtis

Source:

The Canadian MineralogistThe Canadian Mineralogist, Mineralogical Association of Canada, Ottawa, ON, Canada, Volume 47, Number 4, p.745-764 (2009)

ISBN:

0008-4476

Keywords:

Albitization, alkali feldspar, alkali metals, Canada, cesium, chain silicates, Clinopyroxene, deuterium, eastern canada, feldspar group, framework silicates, garnet group, hydrogen, isotope ratios, Isotopes, K-feldspar, lithium, magmas, Maritime Provinces, Meguma terrane, metals, metasomatism, mica group, muscovite, nesosilicates, Nova Scotia, O-18/O-16, orthosilicates, oxygen, Paleozoic, pyroxene group, ring silicates, sheet silicates, silicates, Silurian, sodium, spodumene, Stable isotopes, tantalum, tourmaline group, Yarmouth County Nova Scotia, Yarmouth Nova Scotia

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.

Notes:

GeoRef, Copyright 2018, American Geological Institute.<br/>2009-089827<br/>Brazil Lake<br/>LCT-type pegmatite<br/>White Rock Group