Pressure-temperature paths and exhumation of Late Ordovician-Early Silurian blueschists and associated metamorphic nappes of the Salinic Brunswick subduction complex, northern Appalachians

Abstract

The Late Ordovician-Early Silurian Bruns wick subduction complex in northern New Brunswick preserves a nearly intact southeast-facing forearc terrane with clear links among Silurian subduction, underplating, exhumation, and forearc sedimentation. The forearc terrane grew over time due to successive accretion and underplating of seamounts and isolated ribbons made up of continental and transitional crust concomitant with foreland migration of the trench. A sliver of strongly deformed epidote-blueschists derived from an accreted seamount was stacked together with slightly lower-pressure winchite-bearing tectonites and medium-to high-pressure greenschists of the Fournier, California Lake, and Tetagouche blocks into a series of D 1 metamorphic nappes during Salinic closure of the Tetagouche-Exploits backarc basin. The blueschists (~375 °C, 7.2 kbar) locally preserve mineralogical (zoning from actinolite or barroisite core to a rim of glaucophane) and microstructural evidence for a counterclockwise pressure-temperature (P-T) path ascribed to underthrusting beneath the young oceanic lithosphere of the Fournier block shortly after the inception of subduction at ca. 450 Ma. The next phase of accretion is represented by underplating of the California Lake block at ca. 442 Ma. This may have caused emplacement of the blueschists above the slightly lower metamorphic grade Canoe Landing Lake winchite-bearing nappe (350 °C, 6.5 kbar). Underplating of the Tetagouche block at ca. 435 Ma extruded the blueschist sliver and underlying nappes of the California Lake block immediately above greenschistfacies nappes (~5.8 kbar) and beneath lowerpressure (4-5 kbar) greenschist-facies basalt and gabbro of the ophiolitic Fournier block, which was deformed and imbricated earlier, probably during accretion of the California Lake block. Shear sense indicators confi rm that the hanging-wall shear zone of the blueschist sliver accommodated Early Silurian normal motion. D 1 is generally time-transgressive due to down-stepping of the underplating-related deformation. Metamorphic relicts, as well as zoning in amphibole and epidote, indicate that the Canoe Landing Lake and other nappes underwent clockwise P-T paths related to subduction of the cool oceanic lithosphere and/or accretion of the California Lake and Tetagouche blocks.

Disparate Paths in the Geologic Evolution of the Northern and Southern Appalachians: A Case for Inherited Contrasting Crustal/Lithospheric Substrates

“…Hibbard et al 2010); likewise, the Salinic orogeny is a northern Appalachian event (e.g. van Staal et al 2008). The areal limitation of tectonic imprints of the Cherokee and Salinic events to the south and north, respectively, strongly supports the idea that the modern spatial partitioning of Carolinia and Ganderia is mainly a reflection of their pattern of accretion.…”

“…The timing of accretion of these two elements is similar, although Carolinia appears to have accreted first, initiating the Late Ordovician southern Appalachian Cherokee orogeny (Hibbard et al 2010, whereas Ganderia was accreted in the Early Silurian during the northern Appalachian Salinic orogeny (van Staal et al 2008). The effects of the Cherokee orogeny inboard of Carolinia are confined to the southern Appalachians (e.g.…”

A Review of Proterozoic to Early Paleozoic Lithotectonic Terranes in New Brunswick, Canada and their Tectonic Evolution during Penobscot, Taconic, Salinic and Acadian Orogenesis

“…In the later stages of subduction, the leading edge of the more buoyant, ensialic part of the basin entered the subduction zone and was exhumed by the Middle Silurian (Brunswick subduction complex of de van Staal (1994) and van Staal and de Roo (1995)). Deformation related to this exhumation was accompanied by blueschist metamorphism and stacking of a series of major nappe structures to form the Brunswick subduction complex van Staal 1994;de Roo and van Staal (1994); van Staal and de Roo 1995;van Staal et al 2008). This metamorphism has been dated by 40Ar/39Ar on crossite and phengite to have occurred in the Early Silurian, between 447 ± 6 Ma and 430 ± 4 Ma .…”

“…De Roo and van Staal (1994), van Staal and de Roo (1995), West et al (2003), and van Staal et al (2008van Staal et al ( , 2009) extended this definition to include diachronous tectonic events that occurred within Ganderia between the Late Ordovician and Late Silurian. The various stages of Salinic uplift (Figs.…”

Pb-Isotopic Study of Galena by LA-Q-ICP-MS: Testing a New Methodology with Applications to Base-Metal Sulphide Deposits

“…The Bathurst Supergroup originally formed distinct rift-related basins within the greater Tetagouche-Exploits back-arc basin [21][22][23][24][25][26][27] and were subsequently tectonically juxtaposed in a west-dipping subduction complex. The BMC deposits have been affected by the Salinic Orogeny [22] and subsequent Acadian Orogeny [23][24][25][26][27]. Metamorphism varies from lower-to upper greenschist facies with peak metamorphic conditions from 325 to 400 • C and 6 to 7 kbars [28].…”