This rate, in conjunction with a geothermal gradient, yields the degree of exhumation that took place during cooling:

Transcription

1 DR6136 Appendix Tohver et al. Grenville Asymmetry The technique used for calculating the degree of exhumation follows the approach outlined by Cosca et al., (1991) using the mineral blocking temperatures from Hodges (1991). Given the clockwise Pressure-Temperature paths for collisional orogens, peak metamorphism is expected to post-date the high pressure (collisional) phase. Thus, coupled cooling and exhumation from peak metamorphism are post-collisional. The conditions of this retrograde metamorphism for the North American Grenville have been established by phase equilibria calculations of Anovitz and Essene (1989). Using the different blocking temperatures of various metamorphic thermochronometers (x and y) to establish a cooling rate is straightforward: ΔBlocking Temperature x_y Cooling rate ( C/Ma) = Δx_y This rate, in conjunction with a geothermal gradient, yields the degree of exhumation that took place during cooling: o Cooling( C/Ma) Exhumation (km/ma) = o Geotherm( C/km) In reality, two approaches can be used: either calculating the cooling rate for the short cooling intervals between different minerals or using the integrated cooling rate over the entire range of blocking temperatures. We prefer the second approach, as geographic coverage is not always uniform, meaning that individual analyses or less common minerals can be overweighted. Because we are less interested in exhumation rates per se than in comparing the crustal level (paleodepth) of the sampled region with other regions, we must use a chronological anchor, selected as 1. Ga for reasons outlined in the main body. We use the cooling rate determined above to predict the temperature of our sample at 1. Ga, which is then divided by the geothermal gradient in order to determine the paleodepth at 1. Ga. While a decrease in exhumation rates are expected as isostatic equilibrium is approached, as demonstrated by workers using lower temperature thermochronometers ( Ar/ 39 Ar dating of multidomain K-feldspar, for example), cooling rates are fairly constant over the ca.75-3 interval that our data cover. Some possible sources of error were mentioned in the text. Generally speaking, the paleodepth calculated for a given mineral sample will be too young or too old. A shallow bias will be introduced by mineral ages that are too young: due to daughter product argon or lead loss, or deformation at temperatures below the thermal blocking temperature. Erroneously deep paleodepths will result from ages that are too old (excess argon, igneous age). As mentioned in the text, several older Amazon samples give paleodepths that plot above the modern surface, the result of an exhumation interval that is too long. The fact that these minerals preserve such old ages is testament to their residence in the shallow crust. Space constraints prevented us from citing all of the geochronological references used in assembling such a large body of data. We have appended below a list of all of the references used in Figure 2 or referred to in this appendix that were not included in the body of the paper.

2 DR6136 References Bettencourt, J.S., Onstott, T.C., de Jesus, T., and Teixeira, W., 1996, Tectonic Interpretation of Ar/ 39 Ar s on Country Rocks from the Central Sector of the Rio Negro-Juruena Province, Southwest Amazonian Craton, International Geology Review, v. 38, p Busch, J.P., and van der Pluijm, B.A., 1996, Late orogenic, plastic to brittle extension along the Robertson Lake shear zone: Implications for the style of deep-crustal extension in the Grenville Orogen, Canada: Precambrian Research, v. 77, p Busch, J.P., van der Pluijm, B.A., Hall, C.M., and Essene, E.J., 1996b, Listric normal faulting during postorogenic extension revealed by Ar/ 39 Ar thermochronology near the Robertson Lake shear zone, Grenville Orogen, Canada: Tectonics, v. 15, p Childe, F.C., Doig, R., Gariépy, C., 1993, Monazite as a metamorphic chronometer, south of the Grenville Front, western Quebec: Canadian Journal of Earth Sciences, v.3, p Corfu, F., and Easton, R.M., 1995, U-Pb geochronology of the Mazinaw Terrane, an imbricate segment of the Central Metasedimentary Belt, Grenville Province, Ontario: Canadian Journal of Earth Sciences, v. 32, p Corrigan, D., Culshaw, N.G., and Mortensen, J.K., 1994, Pre-Grenvillian evolution and Grenvillian overprinting of the Parautochthonous Belt in Key Harbor, Ontario - U-Pb and field constraints, Canadian Journal of Earth Sciences, v.31, p

3 DR6136 Cosca, M.A., Essene, E.J., Kunk, M.J., and Sutter, J.F., 1992, Differential unroofing within the Central Metasedimentary Belt of the Grenville Orogen: Constraints from Ar/ 39 Ar geochronology: Contributions to Mineralogy and Petrology, v. 1, p Cosca, M.A., Essene, E.J., Mezger, K., and van der Pluijm, B.A., 1995, Constraints on the duration of tectonic processes- protracted extension and deep-crustal rotation in the Grenville orogen, Geology, v.23, p Cureton, J.S., van der Pluijm, B.A., and Essene E.J., 1997, Nature of the Elzevir- Mazinaw domain boundary, Grenville Orogen, Ontario, Canadian Journal of Earth Sciences, v. 34, p Culshaw, N.G.; P.H. Reynolds and G. Check, 1991, An Ar/ 39 Ar study of post-tectonic cooling and uplift in the Britt domain of the Grenville Province, Ontario: Earth and Planetary Science Letters, v.5, p Dahl, P.S., Pomfrey, M.E., and Foland, K.A., 4, Slow cooling and apparent tilting of the Adirondacks Lowlands, Grenville province, New York, USA, based upon Ar/ 39 Ar ages: GSA Memoir 197, p de Paulo, V.G., Geraldes, M.C., Vasconcelos, Tohver, E., Teixeira, W., Ar/ 39 Ar s of Paleo- and Mesoproterozoic rocks of SW Amazonian craton, 32nd International Geological Congress, Florence, Italy, August -28, 4. Friedman, R., and Martignole, J., 1995, Mesoproterozoic sedimentation, magmatism and metamorphism in the southern Grenville Province (western Quebec): U-Pb geochronological constraints: Canadian Journal of Earth Sciences, v. 32, p

4 DR6136 Haggart, M.J., Jamieson, R.A., Reynolds, P.H., Krogh, T.E., Beaumont, C., and Culshaw, N.G., 1993, Last gasp of the Grenville orogeny - thermochronology of the Grenville Front Tectonic Zone near Killarney, Ontario, Journal of Geology, v. 1, p Hodges, K.V., 1991, Pressure-Temperature-Time Paths: Annual Review of Earth and Planetary Sciences, v. 19, p , doi:.1146/annurev.ea Ketchum, J.W.F., Heaman, L.M., Krogh, T.E., Culshaw, N.G., Jamieson, R.A., 1998, Timing and thermal influence of late orogenic extension in the lower crust: a U- Pb geochronological study from the southwest Grenville orogen, Canada, Precambrian Research, v. 89, p Martignole, J., and Reynolds, P., 1997, Ar/ 39 Ar thermochronology along the western Québec transect of the Grenville Province, Canada: Journal of Metamorphic Geology, v. 2, p Mezger, K., Rawnsley, C.M., and Bohlen, S.R., 1991, U-Pb garnet, sphene, monazite, and rutile ages - implications for the duration of high-grade metamorphism and cooling histories, Adirondack Mts., New York, Journal of Geology, v.99, p Reynolds, P.H., Culshaw, N.G., Jamieson, R.A., Grant, S.L., and McKenzie, K.J., 1995, Ar/ 39 Ar traverse - Grenville Front Tectonic Zone to Britt domain, Grenville Province, Ontario, Canada: Journal of Metamorphic Geology, v.13, p Streepey, M.M., van der Pluijm, B.A., Essene, E.J., Hall, C.M., and Magloughlin, J.F.,, Late Proterozoic (ca. 93 Ma) extension in eastern Laurentia: Geological Society of America Bulletin, v. 112, p

5 DR6136 Streepey, M.M., Lithgow-Bertelloni, C., van der Pluijm, B.A., Essene, E.J., and Magloughlin, J.F., 4, Exhumation of a collisional orogen: A perspective from the North American Grenville Province, in Tollo, R.P., Corriveau, L., McLelland, J., and Bartholomew, M.J., eds., Proterozoic tectonic evolution of the Grenville orogen in North America: Boulder, Colorado, Geological Society of America Memoir 197, p Tohver, E., van der Pluijm, B.A., Mezger, K., Scandolara, J.E., and Essene, E.J., Two stage tectonic history of the SW Amazon craton in the late Mesoproterozoic: Identifying a cryptic suture zone: Precambrian Research, v.137, p Tuccillo, M.E., Mezger, K., Essene, E.J., and van der Pluijm, B.A., 1992, Thermobarometry, geochronology, and the interpretation of P-T-t data in the Britt domain, Ontario Grenville orogen, Canada, Journal of Petrology, v.33, p

6 DR6136 Table A1 Sample locations and summary of new Ar- 39 Ar geochronological data. Mean (for paleodepth calculation Individual analyses (Plateau or*total Gas) Sample Latitude Longitude Rock type Mineral RO ' ' Amphibolite hb RO ' ' Schist bt RO ' 62 9.' Amphibolite bt RO ' 62 9.' Amphibolite hb RO ' Amphibolite hb RO-5V ' 62.6' Granitoid hb RO-6A ' ' Amphibolite hb RO-6B ' ' Schist bt RO ' ' Amphibolite bt RO-8A ' ' Amphibolite hb RO-8C ' ' Amphibolite hb RO ' ' Schist mus RO ' ' Amphibolite hb RO-15B ' ' Granitoid hb RO-15Q ' ' Schist mus RO ' ' Amphibolite hb RO ' ' Amphibolite hb RO ' ' Amphibolite hb RO ' 61.1' Amphibolite hb RO ' ' Amphibolite hb RO ' ' Granitoid bt RO ' ' Amphibolite hb RO ' ' Amphibolite hb RO ' Charnockite hb RO ' ' Sheared charnockite hb 1159* 1126* 1192* RO ' ' Sheared charnockite bt et ' 59 7' Tonalite bt et ' 59 8' Granitoid bt et24 15 ' Opxamphibolite hb Garnet amphibolite hb et ' 59 8' et ' 58 15' Granitoid bt 156* 15* 1511* et9 14 3' 59 48' Granitoid hb 13* 1414* 1391* et31 15 ' 59 8' Amphibolite hb et ' 58 14' Granitoid bt

7 DR6136 RO-6B - Bio. Ar/39Ar Step-Heating Spectra for Runs , and RO-7 - Bio. Ar/39Ar Step-Heating Spectra for Runs , and M ± 1.8 Ma 18.7 ± 1.7 Ma 27.6 ± 1.4 Ma Integrated = 19 ± 1 Ma Integrated = 13.1 ± 1.3 Ma Integrated = 23.9 ± 1.1 Ma RO-9 - Musc. Ar/39Ar Step-Heating Spectra for Runs , and J ± 1.6 Ma 969 ± 2 Ma ± 1.6 Ma Integrated = ± 1.3 Ma 85 Integrated = ± 1.5 Ma Integrated = 97.8 ± 1. Ma B RO-15Q - Musc. Ar/39Ar Step-Heating Spectra for Runs , and RO-5 - Hornbl. Ar/39Ar Step-Heating Spectra for Runs , and ± 3 Ma Integrated = ± 1.6 Ma Integrated = ± 1.3 Ma Integrated = ± 1.4 Ma 59 ± 16 Ma ± Ma 3 ± 3 Ma Integrated = 87 ± 9 Ma Integrated = 11 ± Ma Integrated = 84 ± 15 Ma I ± 2 Ma 1321 ± 3 Ma 1329 ± 2 Ma Integrated = ± 1.6 Ma Integrated = ± 1.5 Ma Integrated = ± 1.9 Ma RO-5V - Hornbl. Ar/39Ar Step-Heating Spectra for Runs , and ± 6 Ma Integrated = 994 ± 4 Ma Integrated = 32 ± 3 Ma Integrated = ± 3 Ma G

8 Ar/39Ar Step-Heating Spectra for Runs 993-1, and DR6136 RO-26 - Honbl. Ar/39Ar Step-Heating Spectra for Runs 994-1, and RO-27 - Honbl ± 4 Ma Integrated = 1617 ± 3 Ma Integrated = 1596 ± 3 Ma Integrated = 1636 ± 3 Ma.8.8 RO-35 - Biot Ar/39Ar Step-Heating Spectra for Runs 5-1, 5-2 and ± 2 Ma Integrated = ± 1.3 Ma Integrated = 1614 ± 3 Ma Integrated = ± 1.1 Ma A B RO-36 - Hornbl Ar/39Ar Step-Heating Spectra for Runs 6-1, 6-2 and ± 1.6 Ma ± 1.6 Ma ± 1.6 Ma 1 1 A Integrated = 1159 ± 1 Ma Integrated = ± 1.1 Ma Integrated = ± 1. Ma RO-37 - Hornbl Ar/39Ar Step-Heating Spectra for Runs 8-1, 8-2 and ± 1.5 Ma Integrated = ±.9 Ma Integrated = ± 1.1 Ma Integrated = 1152 ± 1 Ma RO-38 - Hornbl Ar/39Ar Step-Heating Spectra for Runs 9-1, 9-2 and ± 2 Ma ± 1.3 Ma ± 1.2 Ma ± 1.5 Ma Integrated = ± 1.1 Ma Integrated = ± 1.1 Ma Integrated = ± 1.2 Ma Integrated = ± 1. Ma Integrated = ± 1.1 Ma Integrated = ± 1.3 Ma

9 2.6 RO-39C - Hornbl. Ar/39Ar Step-Heating Spectra for Runs 12-1, 12-2 and DR6136 RO-39C - Bio. Ar/39Ar Step-Heating Spectra for Runs 14-1, 14-2 and Integrated = ± 1.2 Ma Integrated = ±.9 Ma Integrated = ± 1.1 Ma.6 RO-1 - Hornbl. Ar/39Ar Step-Heating Spectra for Runs , and ± 6 Ma ± 1.7 Ma 5.5 ± 1.9 Ma Integrated = 5.5 ± 1.8 Ma Integrated = ± 1.5 Ma Integrated =.5 ± 1.8 Ma RO-4A - Bio. Ar/39Ar Step-Heating Spectra for Runs , and I 79.9 ± 1.9 Ma 17. ± 1.4 Ma Integrated = 74.5 ±.9 Ma Integrated = 66.7 ± 1. Ma Integrated = 14.5 ±.8 Ma A RO-3 - Bio. Ar/39Ar Step-Heating Spectra for Runs , and ± 1.5 Ma ± 1.7 Ma Integrated = ± 1.1 Ma Integrated = 97 ± 1.1 Ma Integrated = ± 1.1 Ma RO-4B - Hornbl. Ar/39Ar Step-Heating Spectra for Runs and ± 1.6 Ma ± 1.7 Ma 979 ± 2 Ma 871 ± 5 Ma 888 ± 3 Ma Integrated = ± 1.1 Ma Integrated = 973. ± 1.2 Ma Integrated = 975. ± 1.4 Ma 5 Integrated = 87 ± 4 Ma Integrated = 882 ± 2 Ma 5 A

10 DR6136 RO-6A - Hornbl. Ar/39Ar Step-Heating Spectra for Runs , and Rondônia RO-8A - Hornbl. Ar/39Ar Step-Heating Spectra for Runs 156-1, and A ± 11 Ma 941 ± 15 Ma 941 ± 4 Ma Integrated = 949 ± 7 Ma Integrated = 1198 ± 4 Ma Integrated = 962 ± 5 Ma RO-8C - Hornbl. Ar/39Ar Step-Heating Spectra for Runs , and ± 5 Ma 976 ± 4 Ma 967 ± 3 Ma Integrated = 98 ± 4 Ma Integrated = 978 ± 3 Ma Integrated = 968 ± 3 Ma A Integrated = 952 ± 4 Ma Integrated = 929 ± 3 Ma Integrated = 969 ± 6 Ma RO-14 - Hornbl. Ar/39Ar Step-Heating Spectra for Runs , and ± 11 Ma 1298 ± 9 Ma 1291 ± 13 Ma Integrated = 128 ± 7 Ma Integrated = 1278 ± 6 Ma Integrated = 1322 ± 8 Ma RO-15B - Hornbl. Ar/39Ar Step-Heating Spectra for Runs , and RO-24 - Hornbl. Ar/39Ar Step-Heating Spectra for Runs 988-1, and ± 7 Ma 13 ± 8 Ma Integrated = 1289 ± 4 Ma Integrated = 1273 ± 3 Ma Integrated = ± 1.8 Ma 1558 ± 4 Ma ± 6 Ma ± 6 Ma 1 Integrated = 1535 ± 5 Ma Integrated = 1561 ± 4 Ma Integrated = 1546 ± 6 Ma L

11 Ar/39Ar Step-Heating Spectra for Runs 982-1, and DR6136 RO-18 - Hornbl. Rondônia - Ar/39Ar Step-Heating Spectra for Runs 984-1, and RO-19 - Hornbl ± 6 Ma 1311 ± 6 Ma Integrated = 1324 ± 5 Ma Integrated = 1276 ± 7 Ma Integrated = 15 ± 7 Ma MI74 et3 Biotite #1 and # ± 4 Ma 1324 ± 3 Ma ± 3 Ma Integrated = 1316 ± 2 Ma Integrated = 1318 ± 4 Ma Integrated = 1299 ± 3 Ma K MI74 et 16 Biotite #1 and # ± 2 Ma 89 ± 2 Ma Total Gas = 884 ±2 Ma Total Gas = 888 ±3 Ma Ma Ma Total Gas = 1322 ±2 Ma Total Gas = 1315 ±2 Ma MI74 et24 Hornblende #1 and # MI74 et 1 Hornblende Ma Total Gas = Total Gas = Ma Total Gas = 14 3 Total Gas =

12 DR6136 MI8 Xc et25 Biotite MI74 et9 Hornblende in Ma Total Gas = (white) Total Gas = (black) Total Gas = (white) Total Gas = (black) MI8 et31 a9 Hornblende MI8 et26 Biotite Ma ± 3 Ma in Ma Total gas age = Ma (black) Total gas age = Ma (white) in Ma Ma Total gas age = Ma Total gas age = Ma