Forty new 40Ar/39Ar age spectra on micas and amphiboles from both granitic and metamorphic rocks show that two geologically distinct terranes in the Cape Breton Highlands (Nova Scotia) have had contrasting thermal histories. Some plutons in the Bras d'Or Terrane in the southeastern highlands apparently cooled through the hornblende (or muscovite) closure temperature immediately following Precambrian to Cambrian intrusion. Other rock units in this terrane, particularly in the west, have been variably overprinted by a Silurian tectonothermal event, probably associated with juxtaposition of the Bras d'Or Terrane with the Aspy Terrane to the northwest. Gneisses at Kellys Mountain in the southeastern Bras d'Or Terrane apparently were not overprinted by this event.Argon data from the Aspy Terrane suggest that Silurian deposition, deformation, and metamorphism were followed by rapid cooling through hornblende and biotite closure temperatures in the Middle Devonian. This probably resulted from uplift and exhumation of the terrane as it collided with the Appalachian Orogen to the northwest. The Cheticamp Pluton in the western part of the Aspy Terrane, which does not appear to have been significantly affected by Silurian–Devonian tectonothermal events, may represent an outlier of the Bras d'Or Terrane. The Aspy Terrane records an Acadian tectonothermal history similar to that of the Gander Zone in southwestern Newfoundland.
The Ming's Bight Group of northwestern Newfoundland, an outlier of Humber Zone continental margin rocks, is entirely surrounded by ophiolitic rocks of the Dunnage Zone. Structures in the Ming's Bight Group and adjacent units record three main phases of deformation. The earliest structures relate to Silurian sinistral transpression previously documented in the region. Two later phases of extensional deformation produced a series of dextral obliquenormal shear zones and faults that now separate the Ming's Bight Group in the footwall from ophiolitic and granitoid rocks in the hangingwall. 40 Ar/ 39 Ar and U-Pb data constrain the times of oblique-normal shear and cooling. Metagabbro in the Point Rousse Ophiolite Complex, which lies in the hangingwall, preserves disturbed Ordovician hornblende 40 Ar/ 39 Ar ages, whereas adjacent shear zones record Devonian ages. Hornblendes in Pacquet Harbour Group amphibolites within extensional shear zones mainly record 40 Ar/ 39 Ar ages of 390-380 Ma. Synkinematic titanite and rutile porphyroblasts from an extensional shear zone on the northwestern margin of the Ming's Bight Group have been dated by the U-Pb method at 388 and 380 Ma, interpreted as growth and cooling ages, respectively. The titanite and hornblende ages suggest that the main phase of ductile oblique-normal shear was underway at 405-385 Ma. Ming's Bight Group schists and pegmatites produced concordant muscovite 40 Ar/ 39 Ar ages averaging 362 Ma, interpreted as the time of footwall cooling below 350ЊC. We suggest that the Ming's Bight Group is a mid-Devonian symmetrical core complex formed within a local transtensional regime developed during dextral oblique transcurrent movement along the Baie Verte Line. The timing and tectonic setting of extension do not support recent models for "extensional collapse" in the northern Appalachians.
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