Geophysical and structural signatures of syntectonic batholith construction: the South Mountain Batholith, Meguma Terrane, Nova Scotia

Abstract

The Late Devonian South Mountain Batholith is a very large (7000 km2) composite peraluminous granitoid complex situated within the Meguma Terrane of the northern Appalachians. It is made up of two suites of granodioritic to leucogranitic plutons emplaced at approximately 380–370 Ma during the Acadian Orogeny, i.e. during the collision of Gondwana with the eastern margin of North America. A significant geophysical and geological database makes the South Mountain Batholith a type example of a very large syntectonic batholith emplaced within a collisional orogen. Gravity models reveal the plutons have flat or gently dipping floors at approximately 7.0 km depth and aspect ratios >6:1. They are underlain by deeper (>10 km) elongate northeast–southwest‐trending roots that may indicate magma feeder zones. Dyke transport of granitic magma and the progressive construction of plutons by sheet injections are supported by field observations and by mapping of the anisotropy of magnetic susceptibility at the pluton scale. The very narrow deformation aureole within the country rocks suggests lateral spreading of the plutons was not the main space creation mechanism during emplacement; space was mostly created by vertical displacements of country rocks. The data are consistent with a laccolithic model for syntectonic batholith assembly. The laccolithic plutons may have been emplaced at the base of the Meguma Supergroup metasedimentary rocks, suggesting a maximum thickness of approximately 7.0 km for the supracrustal rocks in the Meguma Terrane.

Insights on Archean continental geodynamics from gravity modelling of granite–greenstone terranes

“…The tabular shapes of the post-2.8 Ga batholiths are consistent with the common pluton and batholith shapes documented in many Phanerozoic terranes (Benn et al, 1999;Petford et al, 2000) and that are attributed to sheet-like emplacement. Therefore, the model results for post-2.8 Ga granite-greenstone belts suggest the dominant tectono-magmatic processes in the Late Archean may have more closely resembled those of modern day accretionary tectonics than the crustal-scale diapirism model, which is may be more applicable to the pre-2.8 Ga terranes.…”

“…Plutonic and batholithic root zones are interpreted to indicate feeder zones traversed by ascending granitic material (Vigneresse, 1995;Benn et al, 1999).…”

The sources of granitic melt in Deep Hot Zones

“…The development of a temperature anomaly and the presence of melt at the level of intrusion of the first emplaced sills might act as a rheological trap for the successive intrusions, inducing a feedback effect and the focusing of magma injection. In the case of shallow magmatic bodies, geochronological, geophysical and structural data suggest underaccretion is a common emplacement style (Benn et al 1999;Bridgwater et al 1974;Coleman et al 2004;Cruden 1998;Cruden & McCaffrey 2001;Harrison et al 1999;Saint-Blanquat (de) et al 2001). The sandwiching of crustal rock by sills is common.…”

Magmatic history and geophysical signature of a post-collisional intrusive center emplaced near a crustal-scale shear zone: the Plechý granite pluton (Moldanubian batholith, Bohemian Massif)

“…Gravimetric surveys have been routinely used to provide some indirect clues to the three-dimensional subsurface shape of granitoid intrusions (e.g., Améglio and Vigneresse 1999;Aranguren 1997;Aranguren et al 1996Aranguren et al , 2003Benn et al 1999;Cruden et al 1999;Vigneresse 1990). Here we use gravimetry to complement our field observations and structural data and approximately constrain the gross shape and the minimum vertical dimension of the Plechý pluton.…”