The Hanna Farm pluton, Cobequid Highlands: petrology and significance for motion on the Kirkhill fault

Abstract

The Hanna Farm pluton is a complex granite -gabbro pluton which appears to be truncated by the Kirkhill fault zone, probably by predominantly dip-slip motion. A leucogranite has been intruded into the Kirkhill fault zone at the southern end of the Hanna Farm pluton. The main Hanna Farm pluton contains secondary biotite and other evidence of hydrothermal alteration, and is petrographically and geochemically similar to a series of Early Carboniferous (ca. 345 Ma) granites in the western Cobequid Highlands. Secondary biotite within country rock has been radiometrically dated at 303±11 Ma. The leucogranite within the Kirkhill fault zone lacks evidence of hydrothermal alteration. It shows local ductile deformation, but is not noticeably offset by the Kirkhill fault zone.The Kirkhill fault zone is one of the major east-west trending fault zones in the western Cobequid Highlands. It cuts and mylonitises late Hadrynian sedimentary and plutonic rocks, and probable Early Carboniferous plutons. The Hanna Farm pluton appears to post-date the last major phase of strike-slip motion on the Kirkhill fault.Le Pluton de Hanna Farm est un bati complexe de granite et gabbro que semble recouper la zone de failles de Kirkhill, probablement sous l'effet d 'un movement a forte composante selon le rejet-pente. Un leucogranite s'est mis en place au sein de la zone de failles de Kirkhill aux confins meridionaux du Pluton de Hanna Farm. Le pluton principal de Hanna Farm contient une biotite secondaire ainsi que d 'autres preuves d 'une alteration hydrothermale. II ressemble, par sa petrographie et son geochimisme, a une serie de granites eocarbonifcres (environ 345 Ma) de l'ouest des Monts Cobequid. Une datation radiom^triquedelabiotite secondaire querenferm el'encaissant, lasitue a303±l 1 Ma. On ne rencontre aucune trace d 'alteration hydrothermale dans le leucogranite sis au sein de la zone de failles de Kirkhill. Celui-ci presente une deformation ductile locale mais n 'est pas d&ale de fafon notable par la zone de failles de Kirkhill.Cette demiere est 1 'une des principales zones de failles a tendance Est-Ouest dans 1 'ouest des Monts Cobequid. Elle recoupe et mylonitise des roches plutoniques et sedimentaires tardihadryniennes ainsi que des plutons probablement eocarboniferes. Le Pluton de Hanna Farm semble postdater le dernier episode majeur de decrochement sur la faille de Kirkhill.[Traduit par le journal]

A synopsis of the geology of the Cobequid Highlands, Nova Scotia

“…pluton in the north central Cobequid Highlands is probably of similar age. Detailed descriptions of the Cape Chignecto Pluton are given by and Waldron et al (1989), the Hanna Farm Pluton by Pe- Piper and Turner (1988b), the West Moose River Pluton by Pe-Piper et al (1991), the North River Pluton by Pe-Piper (1991), the Wyvern and Gilbert Hills plutons by Pe-Piper et al (1994), the Pleasant Hills Pluton by Pe-Piper et al (1998), and the Wentworth Pluton by Pe-Piper (1998) and Koukouvelas et al (2002).…”

Granite and associated mafic phases, North River pluton, Cobequid Highlands, Nova Scotia

“…These textural relationships suggest that the biotite resulted from a late hydrothermal event entering fractured rocks. Similar alteration has been recognised in other granites along the Cobequid Fault zone (Pe-Piper and Turner, 1988;Waldron et al, 1989;Pe-Piper et al, in press). …”

“…Studies in nearby plutons show that this biotite can be distinguished from primary biotite by its colour and mode of occurrence; it also has a different chemical composition (Pe-Piper and Turner, 1988). In the gran ites, it is associated with small amounts of chlorite, muscovite and epidote.…”

“…(1989). (Pe-Piper and Turner, 1988), the West Moose River pluton (PePiper et al, in press), and the Pleasant Hills pluton (Pe-Piper et al, 1989b). All these plutons contain greater or lesser amounts of gabbro, diabase and diorite, some of which predated and some of which postdated granite intrusion.…”

“…1) and have yielded early Carboniferous Rb-Sr isochrons (Pe-Piper etal., 1989a) and latest Devonian zircon dates (Doig et al, 1990). The Cobequid Fault is part of the major fault zone (Cobequid-Chedabucto Fault zone, or Minas Geofracture) that marks the boundary between the Meguma and Avalon terranes of the Appalachian Orogen (Keppie, Murphy et al (1988), Pe-Piper and Turner (1988) and Waldron etal. (1989).…”