Devonian Extension in Northwestern Newfoundland: ⁴⁰Ar/³⁹Ar 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 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.

Taconic Metamorphism Preserved in the Baie Verte Peninsula, Newfoundland Appalachians: Geochronological Evidence for Ophiolite Obduction and Subduction and Exhumation of the Leading Edge of the Laurentian (Humber) Margin During Closure of the Taconic Seaway

“…Previous geochronological studies of the Fleur de Lys Supergroup and surrounding units did not obtain clear age constraints on the Taconic metamorphic imprint (Dallmeyer 1977;Mattinson 1977;Hibbard 1983;Dallmeyer and Hibbard 1984;Vance andO'Nions 1990, Cawood andDunning 1993;Cawood et al 1994;Anderson et al 2001). 40 Ar/ 39 Ar analyses of the Fleur de Lys Supergroup rocks west of the Baie Verte Line indicate north-younging results of 428 ± 5 to 393 ± 5 Ma for hornblende, 424 ± 5 to 398 ± 5 Ma for muscovite, and 394 ± 5 to 374 ± 5 Ma for biotite (Dallmeyer 1977; recalculated using revised isotopic ratios, decay constants and standard ages in Dallmeyer and Hibbard 1984 and in this study).…”

“…1; D 2 of Anderson et al 2001). D 4 is well developed along the contacts between the Ming's Bight Group and overlying BVOT units, where it is associated with an amphibolite-facies metamorphic assemblage (Anderson 1998;Anderson et al 2001). The D 4 regional deformation is associated with an overall dextral strike-slip (transpressional to transtensional) regime during the Acadian orogeny (Waldron et al 1998;Anderson et al 2001).…”

“…1). The Ming's Bight Group comprises metapelite, psammite, amphibolite and rare ultramafic fuchsite-actinolite schist that are exposed in the core of a tectonic window (Anderson et al 2001).…”

“…East of the Baie Verte Line, D 4 structures are mainly recognized in the northern part of the peninsula as shallowly inclined to recumbent folds, cogenetic with northwest-and southeast-dipping ductile-brittle extensional shear zones ( Fig. 1; D 2 of Anderson et al 2001). D 4 is well developed along the contacts between the Ming's Bight Group and overlying BVOT units, where it is associated with an amphibolite-facies metamorphic assemblage (Anderson 1998;Anderson et al 2001).…”

Thermochronology in Orogenic Systems

“…A more direct approach to dating deformational features involves the application of isotopic geochronology to minerals that grew initially or were dynamically recrystallized during deformation at temperatures below their nominal closure temperatures (Anderson et al, 2001;Coleman and Hodges, 1995;Dallmeyer et al, 2001;Dunlap, 1997;Gromet, 1991;Mulch et al, 2005a;Mü ller et al, 2001;Resor et al, 1996;Reuter and Dallmeyer, 1989;West and Lux, 1993). Many deformed rocks display multiple generations of tectonite fabrics, and techniques such as laser microprobe 40 Ar/ 39 Ar dating of polished thin sections permit the dating of specific minerals in structural context and, thus, determination of the absolute ages of different fabric-forming events.…”