Thermal history of the southwestern Meguma zone, Nova Scotia, from an ⁴⁰Ar/³⁹Ar and fission track dating study of intrusive rocks

Abstract

Two geologically distinct regions within the Meguma terrane have been studied in detail and are shown to have contrasting thermal histories. This conclusion is based on 31 new 40Ar/39Ar age spectra on micas, K-feldspars, and hornblendes from several of the plutons in southern Nova Scotia, which are satellites to the much larger South Mountain batholith (SMB). In addition, we report four K-feldspar age spectra from a locality in the northeastern part of the SMB and seven fission track ages from both the southern and northern areas.The complex age spectrum of hornblende from a mafic phase associated with the Barrington Passage pluton suggests a minimum intrusive age of 385 Ma, a result consistent with geologic evidence that this pluton represents an early, less evolved magmatic pulse. Later intrusive activity appears to have been coeval with the intrusion of the SMB at ca. 370 Ma ago. In the southern region, argon clocks in feldspars were completely reset and mica clocks variably reset by a later thermal event. The cumulative geochronologic and geologic evidence constrains this event to 300–320 Ma ago. In the SMB, this Alleghanian/Hercynian thermal disturbance is much less pronounced and appears to be localized to small areas that are often associated with economic mineralization.K-feldspars from the southern plutons record an episode of argon loss 220–230 Ma ago, which is less evident in the SMB to the north. Upper Triassic dike injection in the southern region, associated with the initial rifting phase of the Canadian Atlantic margin, accompanied this milder thermal pulse.Apatites in the two regions record a mean fission track age of ca. 180 Ma, which we attribute to the final cooling of the terrane below about 100 °C. The timing of this event coincides with regional uplift recorded in sedimentary sequences along much of the Maritime continental shelf.

Offshore continuation of Meguma Terrane, southwestern Nova Scotia

“…The magnetite granites also resemble peraluminous granites of the Piedmont Zone of South Carolina (Fullagar and Butler 1979;Speer et al 1980 ing event. This date is substantially younger than other ages from the plutons of southwestern Nova Scotia (Reynolds et al 1987). Dallmeyer and Keppie (1987) recognized a late thermal event in sillimanite gneisses in the region of Cape Sable, dating from about 287 Ma, which they ascribed to an unknown young offshore pluton.…”

Carboniferous barite-fluorite mineralization in the Late Devonian Kinsac Pluton, southern Nova Scotia

“…A thermal event of similar age has been recorded in the partial resetting o f argon age spectra at other localities in the central Meguma Terrane, including Gays River (Ravenhurst 1987;Kontak et al 1994) and West Gore (Kontak et al 1996), and the western Meguma Terrane Kontak et al 1995;Keppie and Dallmeyer 1996). Reynolds et al (1987) also record disturbed argon age spectra for feldspars in the SMB, but they were unable to distinguish between a Late Carboniferous or Permo-Triassic age for the event based on modelling. The second constraint, which is in fact of indirect nature, comes from the fact that sulphur in the barite must have originated within the Carboniferous evaporites, as discussed above, since there is no other reasonable source reservoir.…”

Identification of geochemical factors in regression to mineralization endogenous variables using structural equation modeling

“…The study area (≈ 7780 km 2 ) is mainly underlain by Cambro-Ordovician greenschist to amphibolites grade metamorphosed sedimentary rocks and Devonian granitoid rocks. The South Mountain Batholith (SMB) is a complex of multi-phase granites covering nearly one-third of the entire study area (MacDonald et al, 1992;Reynolds et al, 1987). A number of Au, U, W and Sn deposits have been found in the area.…”