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TitleShort-duration contact metamorphism of calcareous sedimentary rocks by Neoproterozoic Franklin gabbro sills and dykes on Victoria Island, Canada
AuthorNabelek, P I; Bédard, J H; Hryciuk, M; Hayes, B
SourceJournal of Metamorphic Geology vol. 31, 2013 p. 205-220, https://doi.org/10.1111/jmg.12015
Image
Year2013
Alt SeriesEarth Sciences Sector, Contribution Series 20120192
PublisherWiley-Blackwell
Documentserial
Lang.English
Mediapaper; on-line; digital
File formatpdf
ProvinceNunavut
NTS77F; 77G; 78B; 87E; 87F; 87G; 87H; 88A; 88B; 88D/03; 88D/04; 88D/05; 88D/06
AreaVictoria Island
Lat/Long WENS-120.0000 -108.0000 73.5000 70.0000
Subjectsigneous and metamorphic petrology; metamorphism; metamorphic rocks; sedimentary rocks; metamorphism, contact; igneous rocks; gabbros; Precambrian; Proterozoic
Illustrationslocation maps; photomicrographs; plots; tables
ProgramGEM: Geo-mapping for Energy and Minerals, PGE/Base Metals - Victoria Island (NWT and Nunavut)
Released2012 11 22
AbstractThis contribution addresses contact metamorphism and fluid flow in calcareous rocks of the Neoproterozoic Shaler Supergroup on Victoria Island, Arctic Canada. These processes occurred due to intrusion of gabbroic sills and dykes at c. 720 Ma during the Franklin magmatic event, which was associated with the break-up of Rodinia. The intrusive sheets (sills and dykes) are a few metres to ~50 m thick. Metasedimentary rocks were examined in three locations with very good exposures of vertical dykes feeding horizontal sills, the Northern Feeder Dyke (NFD) complex, the Southern Feeder Dyke (SFD) complex and the Uhuk Massif. In the NFD and SFD complexes, protoliths were limestones and dolostones with minor silicates, and at the Uhuk Massif, the protoliths were silty dolostones. At the time of magma emplacement, these locations were at depths of 1 - 4 km. The widths of contact aureoles are only several decametres wide, commensurate with thicknesses of the dykes and sills. Splays of tremolite mark incipient metamorphism. Highest grade rocks in the NFD and SFD complexes contain the prograde assemblage diopside + phlogopite whereas at Uhuk they contain the assemblage vesuvianite + garnet + diopside. The assemblages are successfully modelled with T - X(CO2)fluid pseudosections that suggest achievement of CO2-rich fluid compositions due to early decarbonation reactions, followed by influx of aqueous fluids after peak metamorphism. Rapid heating of host rocks and short near-peak temperature intervals are demonstrated by the prevalent morphology of diopside as radial splays of acicular crystals that appear to pseudomorph tremolite and by incomplete recrystallization of calcite in marbles. Calcsilicates in the roof of one sill at Uhuk experienced metasomatic influx of Fe that is evidenced by nearly pure andradite rims on grossular garnet. Vesuvianite, which overgrew the grossular portions of garnet, also contains ferric iron. Vesuvianite was partially consumed during retrograde growth of serpentine and andradite. The occurrence of serpentine in high-grade portions of aureoles is consistent with eventual levelling-off of temperatures between 350 and 400 °C, an inference that is supported by modelled conductive heat transfer from the cooling magma sheets. Focused fluid flow near intrusion-wall rock contacts is demonstrated by narrow zones of anomalously low d13C and d18O values of carbonate minerals. Although the up to 5& decrease of both d13C and d18O values from sedimentary values is much smaller than is typical for calcsilicate aureoles around large plutons, it is greater than what could have been achieved by decarbonation alone. The decrease in d13C is attributed to fluidmediated exchange with organic low-13C carbon that is dispersed through the unmetamorphosed rocks and the decrease in d18O is attributed to fluid-mediated isotopic exchange with the gabbroic intrusive
sheets. This study shows that when gabbroic sills and dykes intrude a sedimentary basin, (i) contact aureoles are likely to be narrow, only on the scale of several decametres; (ii) short high-temperature regimes prevent achievement of equilibrium metamorphic textures; and (iii) T–X(CO2)fluid paths in calcareous contact aureoles are likely to be complex, reflecting a transition from prograde decarbonation
reactions to influx of aqueous fluids during cooling.
GEOSCAN ID291830

 
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