Metamorphism of the Flin Flon greenstone belt, Manitoba
Date
2020-11-30
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Abstract
The Flin Flon greenstone belt (FFB), part of the Paleoproterozoic Trans-Hudson Orogen, comprises metamorphosed volcanic, plutonic and sedimentary rocks. The FFB is exposed in west-central Manitoba and east-central Saskatchewan north of the approximately EW-trending unconformity with overlying Paleozoic sedimentary strata but continues beneath these strata to the south. Two phases of metamorphism are recorded in the FFB. The first consists of contact metamorphism resulting from the intrusion of plutons between 1.87–1.83 Ga; the second is a regional event interpreted to have been caused by the collision of accreted terranes with the Superior craton at 1.83–1.80 Ga. Regional metamorphic grade in the exposed FFB increases to the north from prehnite-pumpellyite facies to upper amphibolite facies. Nine isograds were defined for metavolcanic rocks (actinolite-in, prehnite and pumpellyite-out, biotite-in, hornblende-in, oligoclase-in, actinolite-out, chlorite-out, garnet-in and epidote-out) and six for metasedimentary rocks (garnet-in, staurolite-in, andalusite-in, sillimanite-in, staurolite-out and migmatite-in). South of the exposed FFB, grade increases from greenschist to amphibolite facies. Five areas of the exposed FFB with different metamorphic characteristics have been defined, from west to east: (1) the Flin Flon area contains metavolcanics ranging from prehnite-pumpellyite to amphibolite facies, in which garnet-in occurs upgrade of epidote-out; (2) the Cranberry-Iskwasum-Elbow Lake area is a large domain of greenschist facies rocks; (3) the File Lake-Reed Lake area contains adjacent domains of metavolcanics and low-K metasediments with well-defined isograd sequences ranging from upper-greenschist to upper-amphibolite facies. In the metavolcanics, the epidote-out isograd occurs upgrade of garnet-in, whereas in metasediments, all of the isograds (garnet-in through migmatite-in) occur in the amphibolite or upper amphibolite facies, and sillimanite is the only Al2SiO5 mineral observed; (4) the Snow Lake area contains a similar sequence of isograds to the File Lake-Reed Lake area, but develops kyanite in altered rocks associated with the ore deposits in the area; and (5) the Niblock Lake-Saw Lake area consists of metasediments and contains a sequence of isograds with andalusite and sillimanite. The observed metamorphic sequences were used to test the predictions of thermodynamically-calculated phase diagrams. The main discrepancies for metabasites are: (1) the wider observed domains of coexisting amphiboles and feldspars compared to the narrow predicted intervals, an effect ascribed primarily to metastable persistence of the lower grade minerals; and (2) the inability of the phase diagrams to predict the development of garnet. The main discrepancy for the low-K metasediments is the incongruence between the observed and predicted occurrence of muscovite and chlorite in the sequence, a problem exacerbated by the difficulty of identifying whether the minerals in the rocks are primary or secondary. Despite these incongruities, the agreement between the observed and predicted phase relations is sufficient to allow assessment of pressure-temperature conditions and metamorphic field gradients of the FFB. In the western FFB (Area 1), the prehnite- and pumpellyite-out isograds occurred at conditions of 250-300°C and 2.3-3.2 kbar, whereas the hornblende-in and actinolite-out isograds developed at 430-460°C and 2.7-4.2 kbar. In the central-eastern FFB (Areas 3 and 4), the hornblende-in and actinolite-out isograds in metabasites developed at 440-450°C and 3.5-4.0 kbar, whereas the sillimanite-in/staurolite-out isograds in metasediments developed at 570-640°C and 4.5-6.5 kbar. In the easternmost FFB (Area 5), the andalusite-sillimanite sequence developed at 550-600°C and 3-3.5 kbar. The resulting metamorphic field gradients range from 100°C/kbar to 150°C/kbar for all areas of the FFB.
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Keywords
metamorphic petrology, metabasite, phase diagram, isograd, mineral assemblage
Citation
Lazzarotto, M. C. (2020). Metamorphism of the Flin Flon greenstone belt, Manitoba (Doctoral thesis, University of Calgary, Calgary, Canada). Retrieved from https://prism.ucalgary.ca.