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Petrological and in situ SHRIMP geochronological constraints on the tectonometamorphic evolution of the Committee Bay belt, Rae Province, Nunavut [查看] R.G.Berman、M.Sanborn-Barriea、N.Rayner、C.Carson、H.A.Sandeman、T.Skulski

Petrology and in situ SHRIMP monazite geochronology provide new constraints on the polymetamorphic and polydeformational history of the Committee Bay belt, Rae Province, Nunavut, Canada. Rare ca. 2.58 Ga monazite inclusions in garnet porphyroblasts in the eastern part of the belt are interpreted to reflect regional contact metamorphism (M1) associated with voluminous 2.61–2.58 Ga plutonism. Monazite also occurs as ca. 2.35 Ga inclusions in andalusite in the central supracrustal belt, but is absent from the east. These data complement previous studies to the west, and suggest that the Committee Bay belt represents a ca. 2.35 Ga tilted crustal section that was tectonically thickened±imbricated during the Arrowsmith orogeny (D1–M2). Penetrative reworking of the belt occurred during a diachronous D2–M3 tectonometamorphic event which involved northwest-vergent thickening to depths of ∼5 kbar across most of the belt. Although D2 folds and fabrics across the belt are geometrically correlative, monazite is ~20 Myr older in the western migmatite domain (ca. 1.86 Ga) than in the western supracrustal belt (ca.1.84 Ga). This temporal difference is attributed to burial of more radiogenic lithologies in the migmatite domain. Tectonic thickening is attributed to ca. 1.87 Ga collision of Meta Incognita microcontinent with the southeastern flank of the Rae Province. The eastern supracrustal belt exposes similar amphibolitefacies (M3) northeast-striking, northwest-vergent D2 structures that are dated at 1.815 Ga. This younger reworkingmayreflect the influence of a major northward projecting promontory of the Superior Province which continued to affect the softened hinterland of the Rae Province during terminal collision at ca.1.82 Ga. The central Committee Bay belt records a younger, ca. 1.79 Ga structural overprinting (D3–M4) which involved dextral shearing and differential burial to temperatures above monazite growth in a gentle synformal structure formed during the final stages of amalgamation of Laurentia.

Permian to Cretaceous polymetamorphic evolution of the Stewart River region, Yukon-Tanana terrane, Yukon, CanadaP-T evolution linked with in situ SHRIMP monazite geochronology [查看] R.G.BERMAN、J.J.RYAN、S.P.GORDEY、M.VILLENEUVE

Textural relationships and the trace element chemistry of accessory minerals and garnet can provide the linkage between in situ SHRIMP ages and quantitative pressure–temperature data that is required to decipher complex polymetamorphic and polydeformational histories. Application of these methods to lower amphibolite facies rocks of the Stewart River area, Yukon (Canada) yields robust new constraints on the tectonic evolution of central Yukon Tanana Terrane (YTT). A TIMS U/Pb titanite age of 365–350 Ma is interpreted to date low-P metamorphism (M1) and D1 deformation associated with arc plutonism above an east-dipping subduction zone. Monazite inclusions in garnet porphyroblasts record a transition from low to high pressure (9 kbar and 600 C) at c. 239 Ma. These data help to establish a c.260–240 Ma tectonometamorphic event (M2–D2) reflecting intra-arc thickening during west-dipping subduction of Slide Mountain Ocean. Another transition from low- to high-P (M3–D3; 7.8 kbar and 595 C), dated by c. 195–187 Ma monazite, is interpreted to reflect the change from regional contact metamorphism during arc plutonism to internal duplication of YTT during initial collision of YTT with the North American craton. The Mt Burnham (north-eastern) region records a different history because of its proximity to later plutons and its late exhumation via extensional faulting. Monazite growth at 146 Ma dates 9 kbar metamorphism (M4), interpreted to reflect a previously unrecognized period of plutonism associated with auriferous quartz veins in the Klondike region. Monazite growth at 114–107 Ma reflects low-P (<4.6 kbar) contact metamorphism (M5) accompanying regional plutonism and extension.

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