Structure, metamorphism and tectonic setting of metasedimentary rocks and mafic-ultramafic inclusions in the Dashwoods subzone of the Dunnage zone, southwestern Newfoundland

Fox, Don (Donald Laughlin) (1992) Structure, metamorphism and tectonic setting of metasedimentary rocks and mafic-ultramafic inclusions in the Dashwoods subzone of the Dunnage zone, southwestern Newfoundland. Masters thesis, Memorial University of Newfoundland.

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Abstract

The Dashwoods subzone of the Dunnage zone in southwestern Newfoundland is characterized by high grade metasedimentary schists and gneisses and disrupted ophiolite complexes (Annieopsquotch Complex and Long Range Mafic-Ultramafic Complex) all intruded by hornblende-biotite granodiorite to tonalite. A previously reported age of the granitoid rocks is 456 ± 3Ma. This study focuses on an area east of the Long Range Fault, on the western side of the Dashwoods subzone and contains all the major lithostratigraphic units characteristic of the Dashwoods subzone. -- Metasedimentary rocks are separated into two main units: metapsammites to the west and metapelites to the east. Mafic-ultramatic inclusions lie within shear zones in the metapelites and are interpreted to represent parts of a disrupted ophiolite suite. All rocks within the shear zones (both metapelite and mafic-ultramatic) are extensively retrogressed to greenschist facies mineral assemblages but evidence of a higher grade, amphibolite facies metamorphism is evident in rocks that have not been completely retrogressed. The mafic-ultramafic rocks are interpreted to have undergone the full range of metamorphism and deformation that has affected the metasedimentary rocks. Geothermobarometry calculations suggest that the higher grade metamorphic event may have reached temperatures in the order of 800°C with pressures in the order of 5 to 7 kbars. -- The metasedimentary rocks are correlated with the Fleur de Lys metasediments found in northeastern Newfoundland on the basis of similar stratigraphic position along the eastern edge of the miogeocline of eastern North America. An upper age of deposition of the metasediments is approximately middle Ordovician as constrained by the age of the granitoid rocks. Silurian mafic bodies in the Dashwoods subzone have not been affected by high grade metamorphism or high degree of deformation and the Dashwoods subzone had probably stabilized by late Ordovician to Silurian time.

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