Thermobarometry, Geochronology and the Interpretation of P-T-t Data in the Britt Domain, Ontario Grenville Orogen, Canada

Abstract

Textural evidence, thermobarometry, and geochronology were used to constrain the pressuretemperature-time (P-T-t) history of the southern portion of the Britt domain in the Central Gneiss Belt, Ontario Grenville Province. Typical metapelitic assemblages are quartz + plagioclase+ biotite + garnet + kyanite ± alkali feldspar ± sillimanite ± rutile ± ilmenite ± staurolite ± gahnite ± muscovite. Metatonalitic assemblages have quartz + plagioclase + garnet + biotite + hornblende + rutile + ilmenite. Metagabbroic rocks contain plagioclase + garnet + clinopyroxene + biotite + ilmenite ± hornblende ± rutile± quartz. Notable textural features include overgrowths of sillimanite on kyanite and of spinel on staurolite. The spinel overgrowths can be modeled by the breakdown of staurolite via the reaction Fe-staurolite = hercynite +kyanite + quartz+ H 2 O. The decomposition of staurolite to hercynite has a steep dP/dT slope and constrains the late prograde path of a staurolite metapelite. Garnet-Al 2 SiOj-plagioclase-quartz (GASP) barometry applied to metapelitic garnets that preserve calcium zoning reveals a pressure decrease from ~ 11 to 6 kb at an assumed temperature of 700 °C. Garnet-plagioclase-ilmenite-rutile-quartz and garnet-clinopyroxene-plagioclase-quartz barometry is in good agreement with pressures obtained with the GASP barometer. Geochronologic data from garnet, allanite, and monazite in metapelitic rocks give ages that fall into two groups, ~ 1-4 Ga and 1 • 1 Ga, suggesting the presence of at least two metamorphic events in the area. It is most reasonable to assign the 1-4 Ga age to the high-pressure data and the 1-1 Ga age to the lower-pressure data. Collectively the P-T-t data indicate a complex and protracted history rather than a single cycle of burial and uplift for this part of the Grenville Province.

Plastic behavior of magnetite and high strains obtained from magnetic fabrics in the Parry Sound shear zone, Ontario Grenville Province

“…1). On the basis of geochronologic and petrologic work (van Breeman et al 1986, Tuccillo et al 1992, northwest-directed thrusting of the Parry Sound domain over the Britt domain along the Parry Sound shear zone occurred under upper-amphibolite facies conditions at cu. 1120…”

Geochemistry and geochronology of the 1.3 Ga metatonalites from the Central Metasedimentary Belt boundary thrust zone in southern Ontario, Grenville Province, Canada

“…1) contains mostly 1400-1330 Ma rocks along with younger plutonic rocks, and records ca. 1160 Ma and 1120 Ma metamorphic events Tuccillo et al, 1992;Wodicka et al, 2000), leading some to correlate the Parry Sound domain with the CMBbtz (Wodicka et al, 1996;Culshaw et al, 1997).…”

“…In detail, the CMBbtz consists of a series of coherent plutonic thrust sheets enveloped in carbonate-rich tectonites; here we follow Hanmer and McEachern (1992), who defi ned the Redstone, Dysart, and Glamorgan as individual thrust sheets within the 30-km-wide CMBbtz. The northwesternmost thrust sheet, the Redstone, is characterized by coarse-grained, biotite-hornblende tonalitic orthogneiss (1344 +93/-32 Ma; van Breemen and Hanmer, 1986) with rare, relict strongly pleochroic metamorphic ortho pyroxene (Hanmer , 1988). The Redstone thrust sheet is the only thrust sheet within the CMBbtz surrounded by orthogneiss tectonites (Fig.…”

“…However, there are few published constraints on the geochronology and geochemistry of tonalitedominated thrust sheets in the CMBbtz. The geochronology of these rocks is limited to two isotope dilution-thermal ionization mass spectrometry determinations of zircon performed in the 1980s: one poorly constrained from the Redstone thrust sheet (1344 +93/-32 Ma; van Breemen and Hanmer, 1986), and one unpublished date from the Dysart sheet. The unpublished date is described as 1370-1350 Ma by Lumbers et al (1990) and by McNutt and Dickin (2012) as 1337 Ma.…”

Emplacement and deformation of granite pegmatite dykes in a mid-crustal regime of late-orogenic extension, southwest Grenville Province, Ontario, Canada

“…l), where the tectonics of Approximately vertical, c. 990 Ma old pegmatite dykes seem to occur throughout the c. 1.6-1.7 Ga and 1.35-1.45 Ga old Proterozoic gneisses of the Georgian Bay region, central Ontario (Fig. lb;van Breemen et al 1986;Krogh, Culshaw & Ketchum, 1993: Tuccillo et al 1992Bussy et al 1995). The dykes are virtually undeformed at many localities, and known as "postkinematic pegmatites" (Culshaw et al 1994, p. 168).…”