P-T-t-d evolution paths within the Gander Zone, NE Newfoundland
The Gander Lake Subzone of northeast Newfoundland preserves a complex tectonothermal evolution resulting from continental collision of Gondwana and Laurentia following closure of the Iapetus ocean. Field, petrographic, geothermobarometric studies and isotopic age data define five northeastsouthwest trending domains, each with a characteristic P-T-t-d evolutionary path, which reveal elements of the overall tectonothermal evolution in this sector of the Appalachians. Domain I preserves deformed low grade metasediments and east vergent flat-lying 02 folds formed at c. 470 Ma. Domain 2 preserves focusing of later progressive deformation (D3wEST) into a steep, predominantly sinistral high strain zone characterised by andalusite ~ kyanite ~ sillimanite indicative of a clockwise metamorphic path (peak conditions c. 650°C, 5.5 kbar). In domain 3, deformed metasediments (D2-D3EAsT) display an eastward increase in structural complexity and metamorphic grade to a peak of c. 600°C. Domain 4 displays progressive amphibolite facies deformation (D3EAsT) characterised by prograde andalusite ~ sillimanite-bearing (c. 425 Ma) migmatites with peak conditions of c. 700°C, 4.5 kbar. Retrograde 04 deformation and metamorphism is concentrated in steep narrow high strain zones. S4WES~amphibolite to greenschist facies shear fabrics (predominantly dextral) overprint prograde fabrics (S3WEST)within domain 2 and are cross-cut by the c. 427 Ma Middle Brook Granite, Locally in domains 3 and 4 prograde (D3EAsT) fabrics are overprinted by amphibolite to upper greenschist facies S4EAST fabrics which also form the dominant fabric in c. 417 Ma syntectonic granites. D5-06 retrogressive deformation is pervasive in a c. 2 km wide mylonitic zone adjacent to the Dover Fault. D5 dextral greenschist-facies ductile structures are cut by the c. 385 Ma Newport Granite which in tum is cut by 06 sub-greenschist facies brittle dextral faults. In combination, the domains preserve A) low grade deformation (Ordovician?) associated with easterly thrusting of the Dunnage Zone over the Gander Zone, B) Silurian rIletamorphism and deformation progressively partitioned into high strain zones and, C) Devonian retrograde ductile-brittle shearing and brittle faulting local to the Dover Fault. The spatial and temporal coincidence of transpressive deformation, moderate to high grade metamorphism and voluminous granite magmatism in the east portion of the Gander Zone is taken to relate to sinistrally oblique collision between two major crustal blocks during the Silurian. Devonian reactivation juxtaposed part of the high grade Gander Zone against the low grade Avalon block across the brittle-ductile Dover Fault.
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