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.