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Mesozoic ductile shear and paleogene extension along the eastern margin of the central Gneiss Complex, coast Belt, Shames River area, near Terrace, British Columbia

University of British Columbia

Near Terrace, British Columbia, the eastern margin of the Central Gneiss Complex (CGC) is a 3-4 km thick, gently northeast dipping, ductile-brittle shear zone with northeast movement of the upper plate. Along Shames River, deformed amphibolite-facies rocks to the west are juxtaposed against lower greenschist to amphibolite facies units to the east along the steep, east side down, brittle Shames River fault (SRF). Gentle to moderate northwest and northeast dips west of SRF contrast with steep southeast dips to the east. Lineations plunge gently northeast and southwest. West of SRF, the Shames River mylonite zone (SRMZ) separates granitoid rocks below from less deformed granitoid rocks, orthogneiss and metasedimentary rocks above. West of Exstew River, the moderately northeast dipping, ductile Exstew River fault, juxtaposes the SRMZ against metamorphic rocks and granitoids of the CGC. The SRMZ is cut by anastomosing brittle-ductile shear zones. Most kinematic indicators show northeast directed shear. Heterogeneous strain in SRMZ accommodates a minimum upper plate movement of 25 km to the east-northeast. Hornblende geobarometry indicates a structural omission of 13.4 km across SRMZ. East of SRF, amphibolite and greenschist facies supracrustal and plutonic rocks of Lower Permian and older Stikine Assemblage are thrust above greenschist facies volcanic strata correlated with Telkwa Formation of the Lower to Middle Jurassic Hazelton Group. Foliation in late synkinematic, 69 Ma granodiorite which intrudes this thrust package dips steeply southeast. Stikine Assemblage is comprised of lower greenstone, granitoid rocks, volcanic breccia and flows overlain by fusulinid-rich marble. A deformed intrusive rock in Stikine Assemblage has a minimum Pb-Pb date of 317 ± 3 Ma. Hazelton Group contains lower andesitic and upper dacitic to rhyolitic packages comprised of agglomerate, volcanic breccia, tuff, and plagioclase porphyry flows. The earliest recognised metamorphism and deformation in the SRMZ, at upper amphibolite grade, affects 188 ± 8 Ma orthogneiss, and occurred before intrusion of a garnet-biotite granite dated by Woodsworth et al. (1983) at 83.5 Ma. Early fabrics are overprinted by Campanian to Paleocene ductile deformation and a second metamorphism. The second deformation waned during intrusion of three granitic intrusions with concordant U-Pb zircon crystallization dates of 68.7 - 69 Ma. A late to post-kinematic granite dyke in the SRMZ has a U-Pb zircon crystallization date of 60 ± 6 Ma. The second phase of metamorphism began before, and outlasted ductile deformation. The SRF and other high angle normal faults cut 69 Ma granodiorite, but do not significantly offset Eocene (46.2-52.3 Ma) K-Ar biotite cooling isothermal surfaces. The 60 Ma granite is deformed by low angle semi-brittle faulting with upper plate movement to the northeast. A 48 ± 3 Ma synkinematic granite dyke in the footwall of SRMZ was intruded during this deformation, which ended before 46.2 - 46.5 ± 1.6 Ma, the K-Ar biotite cooling dates from SRMZ. The entire region is deformed by post-ductile open, upright, east-northeast plunging folds. K-Ar biotite dates for granitoid rocks range from 51.1 Ma in the upper plate to 46.2 Ma in SRMZ, indicating downward progression of cooling. North-northwest trending brittle faults and lamprophyre dykes cut the SRMZ, and are therefore younger than mid-Eocene. Thermobarometry of pelitic and granitoid rocks indicates increasing metamorphic grade with increasing structural depth. Al-j; in hornblende geobarometry indicates slightly lower pressure of crystallization for the interior than the margin of a granodiorite body east of SRF.In the upper plate of SRMZ, west of SRF, sillimanite-staurolite-garnet schist records ductile deformation and metamorphism at 3.8 ± 1.6 kbar and 570 ± 50°C. The schist is intruded by orthogneiss cut by 68.7 Ma granodiorite. The granodiorite crystallized at 3.4 ± 1 kbar, and was deformed at 2.2 ± 1 kbar at 68.7 Ma. In SRMZ, hornblende in pre-kinematic, 188 ± 8 Ma granodiorite crystallized at 5.5 ± 1 kbar. Deformation and synkinematic metamorphism occurred at 4.9 ± 1 kbar, between 83.5 and before 60 ± 6 Ma. East of SRF, greenschist conditions prevailed, except near the southern margin of the 69 Ma granodiorite body, where amphibolite facies was stable during ductile deformation. A metapelitic sample gives near-peak metamorphic conditions of 4.9 ± 1.6 kbar and 700 ± 50°C, and contact metamorphic conditions of 2.9 ± 1.6 kbar and 610 ± 50°C during intrusion of late synkinematic, 69 Ma granodiorite. P-T-time paths for the upper plate of SRMZ west of Shames River indicate initial rapid, near-isothermal decompression beginning before 69 Ma, continuing to 69 Ma, followed by rapid cooling to 0.9-1.1 kbar, at 51.1 Ma. Paleogene to middle Eocene deformation was probably extensional in nature. It occurred in a vigorous magmatic arc, in response to, and possibly coeval with, crustal thickening.

Text

2010-11-08

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http://hdl.handle.net/2429/29869

Geological Sciences

University of British Columbia

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