Devonian extension in Northwestern Newfoundland: Ar-40/Ar-39 and U-Pb data from the Ming's Bight area, Baie Verte Peninsula

Abstract

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 oblique-normal shear zones and faults that now separate the Ming's Bight Group in the footwall from ophiolitic and granitoid rocks in the hangingwall. Ar-40/Ar-39 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 Ar-40/Ar-39 ages, whereas adjacent shear zones record Devonian ages. Hornblendes in Pacquet Harbour Group amphibolites within extensional shear zones mainly record Ar-40/Ar-39 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 Ar-40/Ar-39 ages averaging 362 Ma, interpreted as the time of footwall cooling below 350 degreesC. 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.