Devonian-Carboniferous igneous intrusions and their deformation, Cobequid Highlands, Nova Scotia

Abstract

Field relationships and regional distribution patterns of igneous rocks in the Cobequid Highlands provide information on sequential styles of deformation during the later Devonian and Carboniferous formation of the Magdalen Basin. New mapping and aeromagnetic data show that large diorite plutons have orthogonal outlines, suggesting that they occupy pullapart space. Diorites both cut and are cut by granite plutons, with magma intermingling relationships suggesting co-existing magmas. Limited dating suggests that most plutonism occurred at about the Devonian-Carboniferous boundary and was preceded by mid to late Devonian volcanism in the eastern Cobequids; younger ages reflect alteration and/or deformation events. Granite veins in diorite plutons show strike-slip syn-magmatic deformation. Ductile thrust deformation of the west ern granite plutons and Fountain Lake Group was followed by brittle faulting and emplacement of dykes along zones of weakness showing east-west pull-apart. Magnetic data reveal that rocks similar to the major diorite plutons underlie much of the central and western Cobequid Highlands and the northeastern part of the Minas sub-basin. The diorite plutons may be an exposed representative of mafic magma that underplated much of the extensional Magdalen Basin. The structure of the Cobequid Highlands is dominated by east-west faults parallel to the Cobequid-Chedabucto Fault that at times acted as transfer faults for major extensional structures in the Magdalen Basin.Les relations de terrain et les types regionaux de distribution des roches ignees dans les hautes terres de Cobequid fournissent des informations sur les styles sequentiels de deformation pendant la formation du bassin de la Madeleine. Une cartographie et des donnees aeromagnetiques recentes montrent que de grands plutons de diorite ont des contours orthogonaux, suggerant qu'ils occupent des espaces de transtension. Les diorites a la fois coupent des plutons de granite et sont recoupes par eux, avec des relations d'enchevetrement de magmas suggerant leur coexistence. Quelques datations suggerent que la plupart du plutonisme a eu lieu pres de la limite Devonien-Carbonifere et qu'il a ete precede par le volcanisme du Devonien moyen a tardif dans les Cobequids orientales; les ages plus jeunes refletent des evenements d'alteration ou de deformation. Des veines de granite dans les plutons de diorite montrent une deformation syn-magmatique en decrochement. La deformation ductile en chevauchement des plutons granitiques occidentaux et du Groupe de Fountain Lake a ete suivie par un faillage cassant et la mise en place de dykes le long de zones de faiblesse montrant des rhombochasmes est-ouest. Les donnees magnetiques revelent que des roches similaires aux plutons majeurs de diorite sont sous-jacentes a une grande partie des hautes terres de Cobequid centrales et occidentales et a la partie nord-est du sous-bassin de Minas. Les plutons de diorite pourraient etre un echantillon affleurant du magma mafique qui s'est accumule sous une grande parti...

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

“…Pe- Piper et al 1996b). The Whirley Brook intrusion is marked by a strong magnetic anomaly (Piper et al 1993), suggesting the presence of voluminous mafi c rocks at shallow depth. The granitic rocks have a high Zr content, similar to distinctive high-Zr rhyolite found only at the extreme top of the Byers Brook Formation , with an inferred age of about 355 Ma (earliest Carboniferous).…”

“…Some felsic dykes have yielded U-Pb zircon ages of 356 Ma (Dunning et al 2002); some mafi c dykes appear to be as young as 328 Ma (Nearing et al 1996). Abundant gabbro and diabase dykes occur south of Squally Point cutting Fountain Lake Group (Piper et al 1993). South of the Rockland Brook Fault, gabbro sills make up perhaps 50% of the section in the Nuttby Formation and intrusive pods of gabbro are common in the Neoproterozoic rocks westward to Folly River.…”

“…Regional magnetic (Kiss et al 1989;Piper et al 1993) and gravity (Mazzocco 1996) data suggest that plutonic rocks extend south of the Cobequid Fault. At Clark Head, near Parrsboro, blocks of Devonian-Carboniferous gabbro-granulite outcrop in a melange variously interpreted as a fault breccia (Donohoe and Wallace 1985, p. 50-53;Gibbons and Murphy 1995;Gibbons et al 1996) or the cap rock on a Carboniferous evaporitic diapir (R.K. Stevens, personal communication, 1998).…”

Tectonostratigraphie du carbonifère de la Gaspésie Québec Canada

“…They are related to regional shear at the level of the Iapetan suture while the Theic Ocean, southern extension of the then-already-closed Iapetus Keppie, 1985Keppie, , 1992Briden et al, 1988;Kent and Keppie, 1988;Reed et al, 1993), was still in the process of closing Barr, 1984;Fyffe and Barr, 1986;Waldron et al, 1989;Pe-Piper et al, 1991;MacDonald et al, 1992;Piper et al, 1993;Kontak, 1994). They are interpreted as horst structures induced by plutonic activity and would have served as intermittent sources for clastic sedimentation during Late Devonian and Mississipian times (St. Peter, 1993).…”

The La Coulée Formation, a new post-Acadian continental clastic unit bearing groundwater calcretes, Gaspé Peninsula, Quebec

“…Many other similar highlands occupied by Devonian to Permian plutonic complexes are found throughout the Maritime provinces (Fyffe et al 1981;Barr 1990;Fyffe and Barr 1986;Waldron el al. 1989;Pe-Piper 1991;Pe-Piper et al 1991;McDonald et al 1992;Piper et al 1993;Kontak 1994). They are interpreted as horst structures induced by plutonic activity and would have served as intermittent sources for clastic sedimentation during Late Devonian and Mississipian times (St. Peter 1993).…”