Scapolite pegmatite from the Minas fault, Nova Scotia: tangible manifestation of Carboniferous, evaporite-derived hydrothermal fluids in the western Cobequid highlands?
Jv. Owen et Jd. Greenough, Scapolite pegmatite from the Minas fault, Nova Scotia: tangible manifestation of Carboniferous, evaporite-derived hydrothermal fluids in the western Cobequid highlands?, MINERAL MAG, 63(3), 1999, pp. 387-397
Pegmatite cutting chlorite schist in the Minas fault at McKay Head, Nova Sc
otia, consists of Cl-rich (2.7-3.8 wt.% Cl) marialitic scapolite (EqAn(21-3
2)) with interstitial, apparently primary analcite, hematite and rutile, an
d later (including vug-lining) analcite, pyrite, chlorite, titanite and cal
cite, and cross-cutting epidote veins. Some of the latter phases might have
crystallized from residual pegmatitic fluids. Unlike many other primary sc
apolite-bearing igneous rocks, the McKay Head occurrence has compositional
affinities with mafic (rather than felsic) systems: it is enriched in trans
ition metals (e.g. Cr less than or equal to 53 ppm), and has very low LILE
concentrations (e.g. Rb<10 ppm; U<1 ppm; Th<2 ppm; Ba<20 ppm) and Rb/Sr rat
ios (similar to 0.05). The presence of interstitial rutile and hematite rat
her than ilmenite indicates that the pegmatitic fluid was oxygenated late (
T similar to 400 degrees C) in its crystallization history.
The pegmatite is interpreted to be related to highly sodic hydrothermal sol
utions derived from (or affected by) early Carboniferous evaporites of the
Windsor or Horton groups. Compositionally-similar fluids, perhaps also rela
ted to an evaporite source, may be responsible for a regional, early Carbon
iferous Na-metasomatic event that altered a suite of alkaline granitoid int
rusions shortly after their emplacement.