Polymetamorphic fluid-rock interaction of the Tudor Gabbro and adjacent marble, Ontario

Multivectorial paleointensity determination from the Cordova Gabbro, southern Ontario

“…TU and CO lie in the low-metamorphic-grade central Elsevir (Fig. 1a), where silicate geothermometers indicate reheating to 500^550 ‡C ( [20], ¢gure 6), in agreement with calcite-dolomite and calcite-graphite thermometry on marbles in contact with the Tudor Gabbro [21,22]. However, an 40 Ar/ 39 Ar hornblende age (Ar closure temperature of V500 ‡C), from Madoc (just east of CO) is 1006 Ma and has been reset ( [14], ¢gure 3).…”

“…The Biscotasing VADM has been cited as evidence for a rapid increase of geomagnetic ¢eld intensity caused by nucleation of Earth's inner core [36]. However, the Preissac result is based on only three specimens and has a large dispersion, whereas the much lower VADM of (3.90 þ 0.97)U10 22 A m 2 for the younger Sudbury Norite is based on 52 specimens and has low dispersion. Large £uctuations in VADMs are known from the Phanerozoic as well.…”

“…However, an 40 Ar/ 39 Ar hornblende age (Ar closure temperature of V500 ‡C), from Madoc (just east of CO) is 1006 Ma and has been reset ( [14], ¢gure 3). Our interpretation is that the Tudor Gabbro was never reheated above 500 ‡C [21,22] and has preserved its primary TRM [2] whereas the Cordova was reheated above 500 ‡C and later, around 1000 Ma, passed through 500 ‡C during post-metamorphic uplift and cooling.…”

“…The VADM from V1100 Ma Keweenawan rocks [34] is 2^6 times higher than VADMs recorded between V850 and 1240 Ma ( [2], this study). There is an even larger contrast between the VADM of (18.3 þ 5.5)U10 22 A m 2 for the 2167 Ma Biscotasing dikes [35,36] and nine VADMs from rocks 2000^2800 Ma in age, three VADMs from V2000 Ma rocks of the Hamersley Basin, Australia [37], and the VADM of the 1850 Ma Sudbury Norite [38]. The Biscotasing VADM has been cited as evidence for a rapid increase of geomagnetic ¢eld intensity caused by nucleation of Earth's inner core [36].…”

“…The time-averaged A and B paleo¢elds, combined with mean A and B inclinations of 360 ‡ and 35 ‡, give virtual axial dipole moments (VADMs) of (3.12 þ 0.36) and (1.82 þ 0.38)U10 22 A m 2 , respectively (Tables 1 and 2). These are about 35% and 20% of the mean dipole moments of 8.8U10 22 Fig.…”