Microstructure, metamorphism, thermochronology and P‐T‐t‐deformation history of the Port aux Basques gneisses, south‐west Newfoundland, Canada*

Abstract

The Port aux Basques gneisses comprise three lithostratigraphic units separated by major fault zones: the Grand Bay Complex; the Port aux Basques Complex; and the Harbor le Cou Group. A similar regionally developed polyphase history of penetrative deformation characterizes each of these units. Thickening during D1 produced rare recumbent folds (Fl) and an axial planar schistosity (Sl), overprinted by D2 recumbent folds (E), and transposed during development of a locally penetrative, differentiated crenulation cleavage (S2). In western sectors of the area, D2 was associated with NW-directed reverse shearing. The NE-trending structural grain reflects D3 transpression, partitioned into dextral transcurrent movement along major shear zones and development of upright-to-steeply inclined, periclinal folds (€3) and a variably penetrative schistosity (S3).Amphibolite facies metamorphism increases in grade from west to east across the area. Microstructures, including porphyroblast-matrix foliation relations and internal textural unconformities in garnet, indicate episodic porphyroblast nucleation and growth, which reflect a prograde traverse sequentially across univariant reactions during syntectonic metamorphism. Garnet, kyanite and staurolite porphyroblasts are wrapped by the S2 foliation, but each may contain trails of inclusions that define S1; commonly these trails preserve early stages of S2 crenulation cleavage development. Progressive and sequential reaction out of kyanite, staurolite and muscovite in favour of sillimanite, garnet, biotite and K-feldspar, and the development of an increasing volume of anatectic migmatite in south-eastern sectors of the area record syn-to late-D2 peak metamorphic conditions. Microstructural relationships and petrogenetic grid considerations indicate clockwise trajectories in P-T space for units of the Port a m Basques gneisses. Peak metamorphic conditions are estimated to have been 620-650°C at 28kbar in the west and 700-750" C at 5 8 kbar in the east.Titanite from an upper amphibolite facies calc-silicate gneiss yields U-Pb ages of c. 420 Ma, interpreted to date cooling shortly after the thermal peak in these gneisses. Variable D3 strain was associated with some recrystallization of hornblende and micas. 40Ar/39Ar hornblende plateau isotope correlation ages range from c. 419 to c. 393 Ma, from east to west across the area, and are interpreted to record cooling through c. 500°C coeval with or soon after D3 deformation. The range in ages may record the effects of heterogeneous D3 deformation and differential uplift from south-east to north-west associated with displacement on major shear zones. 40Ar/'9Ar muscovite plateau ages cluster at c. 390 Ma, and date cooling through c. 375" C during regional exhumation. Cooling rates are moderate to fast and may indicate a component of tectonic exhumation.The Port aux Basques gneisses are a product of Silurian collisional tectonics. The higher grade of metamorphism in comparison with adjacent areas of the Canadian Appalachians is interpreted ...

Pre-Carboniferous, episodic accretion-related, orogenesis along the Laurentian margin of the northern Appalachians

“…Its extent and geometry at depth was largely constructed on the basis of seismic profiles (van der Velden et al 2004). Rocks of the allochthon reached peak metamorphism at 418 -414 Ma (van Staal et al 1994;Burgess et al 1995;Valverde-Vaquero et al 2000) and must have travelled a fair distance and for a considerable time interval, because they are generally emplaced above low-grade rocks and locally were even transported above unmetamorphosed Lower Devonian sediments . Attainment of Acadian high-grade metamorphism in these Gander margin rocks so shortly after the onset of the Acadian orogeny suggests that these rocks were already hot, either as a result of being situated in the Acadian backarc (see below) prior to collision and establishment of the flat-slab or were already metamorphosed to high-grade conditions during the immediately preceding Salinic orogeny and remained buried and relatively hot until their involvement in the Acadian.…”

“…Burgess et al 1995). Metamorphic isograds generally seem to dip shallowly to the NW or north and high-grade rocks have been juxtaposed locally with low-grade rocks along major faults, suggesting significant and complex postmetamorphic movements along some of the belt-bounding faults.…”

“…Acadian folds deform an earlier layerparallel metamorphic foliation, which could be an early Acadian or Salinic structure (van Staal et al 1992). Development of the recumbent or overturned folds and associated shear zones partly overlapped with high-grade metamorphism, dated at c. 418-414 Ma (Burgess et al 1995;van Staal et al 1994), and were intruded late syn-kinematically by Upper Silurian (421 -418 Ma) biotite + muscovite granites such as the Gaultois (c. 421 Ma, Piasecki 1988;Dunning et al 1990) andRose Blanche plutons (c. 418 Ma, van Staal et al 1992;ValverdeVaquero et al 2000). Correlative Gander margin rocks in southern New Brunswick (St. Croix belt) also may contain early southerly overturned structures (Fyffe et al 1991), although the dominant Acadian structural transport direction here and in adjacent Maine appears to be towards the NW (Tucker et al 2001;Castonguay et al 2003).…”

Time-Transgressive Salinic and Acadian Orogenesis, Magmatism and Old Red Sandstone Sedimentation in Newfoundland

“…In addition, a number of different lines of evidence indicate that they constitute two kinematically and dynamically distinct events that followed each other closely in time (e.g. Dunning et al 1990;West et al 1992;van der Pluijm et al 1993;Hibbard 1994;Holdsworth 1994;Cawood et al 1994;Burgess et al 1995;Kerr et al 1995;D'Lemos et al 1997;Schofield and D'Lemos 2000;O'Brien 2003;van Staal et al 2003;McNicoll et al 2006;Zagorevski et al 2007; van Staal et al 2009;Wilson and Kamo 2012). As such we espouse a tectonic-based definition of orogeny or mountain building in this manuscript.…”

Connecting Cape Breton Island and Newfoundland, Canada: Geophysical Modeling of pre-Carboniferous 'Basement' Rocks in the Cabot Strait Area

“…Schofield et al 1993Schofield et al , 1998Valverde-Vaquero et al 2000) and is generally considered to be part of the Gander Zone based on its metasedimentary components (Hibbard et al 2006). Although SilurianDevonian metamorphic history shows similarities to that of the Aspy terrane (Burgess et al 1995;, rock types such as the ca. 475 Ma Margaree orthogneiss are not present in the Aspy terrane, and the two areas have not been directly correlated in previous studies .…”