Rb–Sr biotite and whole-rock data from the Kapuskasing uplift and their bearing on the cooling and exhumation history

Abstract

Rb–Sr isotopic ratios were measured on biotite and whole rocks from a suite of 21 granitoid rocks from the southern Kapuskasing uplift. Results of 18 of the whole-rock analyses fall on an isochron with an age of 2.677 ± 0.057 Ga and Sri = 0.70080. Biotite model ages range from 2.50 to 1.93 Ga in a general spatial relationship of decreasing age with depth in the structural section as calibrated with the igneous hornblende barometer for tonalitic rocks and garnet–pyroxene barometers for granulites. Near the Michipicoten belt in the west, biotite ages of ~ 2 Ga reflect disturbances also recorded by paleomagnetic and whole-rock Rb–Sr systems. To the east, in a 70 km long northwest–southeast transect toward the Ivanhoe Lake fault zone, ages decrease from 2.50 to 1.95 Ga. In a southern transect ages are in the range 2.30–2.38 Ga to within 15 km of the fault, reflecting consistent erosion levels corresponding to ~ 0.53 MPa, but decrease to 1.93 Ga to the east, in concert with paleopressures in the 0.7–1.0 GPa range.A plateau of low Rb–Sr biotite dates is not evident, suggesting that the Kapuskasing uplift event exposed a frozen-in cooling profile, rather than setting the Rb–Sr clock by rapid cooling of the rocks from above their closure temperature. The youngest biotite date of 1.93 Ga therefore provides a maximum age for uplift, consistent with paleomagnetic data from Archean rocks and Early Proterozoic dykes, but in conflict with some K–Ar and 40Ar/39Ar biotite age data.

Lithospheric scale gravitational flow: the impact of body forces on orogenic processes from Archaean to Phanerozoic

“…The evidence of Archaean high-grade terranes such as the Pikwitonei and Kapuskasing domains in the Superior Province, the granulite facies migmatite from the Limpopo belt, and the granulite gneisses of the Karelian craton in central Finland have sometimes been advanced as an argument that regional crustal thicknesses on the order of 60-80 km were created by Archaean orogenies. These rocks, however, record pressures lower than 1100 MPa (c. ,40 km) and have been exposed to the surface following Proterozoic exhumation (Percival & Peterman 1994;Kamber et al 1995Kamber et al , 1996Korsman et al 1999), not during Archaean orogenesis involving crustal-scale thrusts. Crustal-scale gravitational flow is also suggested by the maintenance of a sub-aerial to shallow subaqueous surface topography during the accumulation of volcano-sedimentary sequences (greenstones), some up to 20 km thick, above mainly felsic basement (Arndt 1997;De Witt & Ashwal 1997).…”

Petrogenetic significance of orthopyroxene‐free garnet + clinopyroxene + plagioclase ± quartz‐bearing metabasites with respect to the amphibolite and granulite facies

“…The lower end of this range is barely above the solidus (Fig. 3), most likely indicating down‐temperature Fe‐Mg resetting during the slow cooling (Percival & Peterman, 1994) of the Kapuskasing rocks. Thermobarometric evidence in support of the higher temperatures comes from the locally developed Opx + Pl symplectites around garnet, which yield postpeak P–T conditions of about 7 kbar, 830 °C based on Grt‐Opx Al‐solubility thermobarometry corrected for late Fe‐Mg exchange (Pattison et al ., 2003).…”

Magnetic properties of rocks of the Kapuskasing uplift (Ontario, Canada) and origin of long-wavelength magnetic anomalies

“…However, uplift must have continued for at least another 400 Ma, through the time of intrusion of the 2040 Ma Kapuskasing dikes (Percival & West1994). Decreasing biotite Rb/Sr ages along a 70-km transect, from 2500 Ma for shallower rocks to the west to 1950 Ma near the ILCZ, confirms the long duration of uplift (Percival & Peterman 1994).…”

“…The gneisses were then intruded, deformed and recrystallized during a long residence in the deep crust, as documented by 2696-2584 Ma metamorphic zircon ages and 2600-2493 Ma titanite ages (Krogh & Moser 1994;Percival & West 1994). Intruding granitoids have an Rb-Sr whole-rock isochron age of 2677 Ma (Percival & Peterman 1994) and U/Pb zircon ages of 2690-2660 Ma (Krogh & Moser 1994). Computed metamorphic temperatures of 750-790 • C in the 50-km-wide zone of garnet-clinopyroxene granulite gneisses (Mäder et al 1994) exceed the Curie temperatures of both magnetite and hematite, so that the natural remanent magnetization (NRM) of the KSZ rocks must be postmetamorphic.…”