A petrological, geochemical, isotopic and fluid-inclusion study of 370 Ma pegmatite-aplite sheets, Peggys Cove, Nova Scotia, Canada

The Peggys Cove area of southern Nova Scotia contains abundant, flat-lying, zoned, tourmaline-bearing aplite-pegmatite sheets cutting leucomonzogranite of the peraluminous, 372 Ma South Mountain Batholith, locally enriched in metasedimentary xenoliths. Muscovite extracted from aplite, pegmatite and quartz - muscovite - tourmaline pocket zones give flat 40Ar/39Ar age spectra with 370 Ma plateau ages. Petrographic observations indicate rapid post-crystallization cooling and minimal deuteric alteration, although erratic Al/Si ordering in feldspar suggests heterogeneous strain, also indicated by undulose textures in quartz; hence, pegmatite formation occurred in a dynamic setting. Whole-rock and mineral compositions indicate that the aplite-pegmatites are not chemically evolved compared to global geochemical databases for such rocks. Rather, they may reflect locally generated volatile-rich melts extracted from the leucomonzogranitic magma. Whole-rock REE data show that the aplite cannot be derivatives of the leucomonzogranitic magma by simple fractional crystallization. Whole-rock (WR) and mineral delta(18)O data (WR: +11.7 to +14.7parts per thousand) are high compared to data for the SMB (WR: +8.5 to +12.5parts per thousand) and reflect equilibration of the pegmatite-forming melt with a metasedimentary source-material. Fluid-inclusion studies indicate pegmatite crystallization from a melt that exsolved a moderately saline (20-25 wt.% equiv. NaCl) fluid and constrain pegmatite formation to ca. 600-650degreesC at 3-3.5 kbar pressure. Abundant decrepitation-induced textures for the inclusions indicate pegmatite formation in an environment undergoing rapid changes in pressure, consistent with undulose extinction in quartz and Al-Si ordering in K-feldspar. The observations and data support a model of pegmatite-aplite formation via devolatilization of the metasedimentary xenoliths, abundant in the host rocks, and local generation of melts enriched in B during periods of cycling fluid pressure, in part related to regional stresses. Thus these tourmaline-bearing pegmatites are not related to conventional processes of late-stage fractionation of granitic magmas. Textural features of the pegmatites (e.g., comb-textured K-feldspar) and aplites indicate a single period of dilatancy and infilling to form the sheets.