HR: 1330h
AN: V32C-1038    [PDF]
TI: Combining SHRIMP, TIMS U-Pb and Ar-Ar to Resolve Magmagenesis, Metamorphism and Fluid Flow in Three Coincident Magmatic Arcs (Yukon-Tanana Terrane, Western Yukon)
AU: * Villeneuve, M
EM: mvillene@nrcan.gc.ca
AF: Geological Survey of Canada, 601 Booth St.,, Ottawa, ON K1A 0E8 Canada
AU: Ryan, J J
EM: jryan@nrcan.gc.ca
AF: Geological Survey of Canada, 101-605 Robson St., Vancouver, BC V6B 5J3 Canada
AU: Gordey, S
EM: sgordey@nrcan.gc.ca
AF: Geological Survey of Canada, 101-605 Robson St., Vancouver, BC V6B 5J3 Canada
AU: Piercey, S
EM: spiercey@laurentian.ca
AF: Mineral Exploration Research Centre and Department of Earth Sciences, Laurentian University, Ramsey Lake Road, Sudbury, ON P3E 2C6 Canada
AB: Multi-method geochronology has been instrumental in resolving the tectonic evolution within the Stewart River portion of the Yukon-Tanana Terrane in west-central Yukon Territory. Two short-lived and distinct arc assemblages of Paleozoic age are now manifest as highly strained, polydeformed metasedimentary, metavolcanic and metaplutonic rocks. These assemblages, as dated by SHRIMP and TIMS show the presence of an early 360-345 Ma arc, marked by 360 Ma augen granites carrying a strong transposition foliation. This same foliation is also manifested in the later arc assemblage as marked by a 263.6$\pm$2.6 Ma, highly deformed felsic volcanic rock interlayered with amphibolite-grade mafic metavolcanics and second suite of augen granites, dated at 259.1$\pm$0.4 Ma. SHRIMP dating of zircon overgrowths on detrital zircons from a metamorphosed and deformed arkosic conglomerate give an age of 259.3$\pm$3.4 Ma, coeval with emplacement of the deformed Permian augen granites. These overgrowths are demarcated by a thin band of inclusions that are trapped between the core and new zircon growth. Furthermore, there is evidence for severe dissolution of zircons in the felsic volcanic rock, but no such effect on the detrital zircons cores, despite having a range of morphology, U content and age. This suggests that fluid-rock interaction is the primary driver for transport and regrowth of new zircon in the conglomerate, likely during a time of peak metamorphism at 259 Ma. However, dating of metamorphic titanite provides evidence of an amphibolite grade metamorphism and deformation concomitant with the earlier arc assemblage. This titanite, formed within a package of mixed amphibolite (mafic volcanic rocks?) and calc-silicates rocks, gives an age of 345-360 Ma. This age shows that the pervasive Permian metamorphism was insufficiently hot enough to reset the titanite U-Pb systematics. These two arc sequences were intruded by weakly to undeformed Triassic-Jurassic granites. Detailed sampling for CO2 laser Ar-Ar geochronology took place on two transects across the area. Samples along both transects (including a sample set co-located with the metamorphic titanite sample) give relatively uniform 180$\pm$7 Ma ages on hornblende, muscovite and biotite. There is no correlation of age with distance from Mesozoic plutonic bodies, showing that it is a regional thermal event. When taken in concert with titanite data, it is apparent that an episodic Jurassic cooling event was likely driven by rapid uplift of the entire block. Furthermore, these ages are in accord with previously reported Ar-Ar ages across the Alaska border within the FortyMile River assemblage.
DE: 1035 Geochronology
DE: 1040 Isotopic composition/chemistry
DE: 1094 Instruments and techniques
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2003 Fall Meeting