Thermobarometry of metamorphic rocks in the Coast Mtns of BC and AK

1. Thermobarometry of metamorphic rocks in the Coast Mtns of BC and AK • D. Pearson, geos 596e

2. Outline • Importance of P-T-t paths of rocks metamorphosed by or surrounding plutons • Problems/difficulties with thermobarometric methods • Brief overview of Coast shear zone • P-T-t data from western British Columbia and SE Alaska • My preliminary research results

3. Importance of metapelite thermobarometry • Often difficult to get reliable P-T data for plutonic rocks (Mihai’s talk to follow) • Pelitic compositions generally conducive to forming mineral assemblages that occupy a small area in P-T space • Commonly has garnet, which makes our lives easier (slow diffusion rates, etc.) • Can often construct a multi-point sequence to obtain a story about deformation history of rock and surrounding region

4. Importance of metapelite thermobarometry (cont.) • In addition to geochem, geochron, etc., we need thermobarometry to understand deformation, plutonism, and exhumation in arcs • Thermobarometry is particularly useful in the Coast Mountains Batholith because it’s complicated (terrane accretion, Coast shear zone, etc.) • We need to get a handle on deformation and exhumation to understand the evolution of the world’s largest batholith

5. Problems/difficulties with thermobarometric methods • Good P-T-t rocks not always present • Don’t always retain info about the event(s) we want to know about • Often difficult to assess whether equilibrium was attained/maintained • To obtain P-T paths of adjacent plutonic rocks, you must assume they share a history (usually reasonable assumption) • Overall, they offer a very good way of obtaining large scale tectonic information

6. Brief overview of Coast shear zone • 2-11 km thick ductilely deformed zone >1200 km long • NW-striking, dips moderately to steeply to NE • Rocks within the CSZ are mainly Eocene tonalitic sills and orthogneisses, with some metapelites

7. Overview of Coast shear zone (cont.) • Kinematic indicators within the CSZ preserve polyphase deformation history • 1st-top to the southwest (reverse) - ~61-56 Ma • 2nd- locally top to the northeast (normal) - concurrent with East Side Detachment? • Possible overprinted earlier strike-slip motion?

8. After Rusmore et al. (2005) Shitty Coloring job by Alex N

9. Thermobarometry in the Coast Mtns • Most common barometer - GASP • Most common thermometer - grt-bt • Additional constraints by retrograde net transfer reactions • Most metapelites from Gravina Belt (marine basinal strata) or Yukon-Tanana (shelf strata) • Ages mostly inferred from U-Pb ages in adjacent plutons • 3 metamorphic episodes are recognized across arc: >101 Ma, 90-80 Ma, and 75-60 Ma

10. >101 Ma: 2-7.5 kbar, 325°C Northern Coast Mtns, SE AK • >101 Ma regional metamorphism near Juneau likely affected majority of western block • Oldest recognized metamorphism • Likely affected most of western belt • Himmelberg et al. (1995)

11. 2-7.5 kbar, 325°C Northern Coast Mtns, SE AK 77+17 Ma (Grt Sm-Nd) ~94 Ma: 8.2+1 kbar, 515-650°C 80+9 Ma (Grt Sm-Nd): 8.5-11.3 kbar, 570-700°C • Regional >101 Ma met. overprinted locally W of CSZ • ~90-80 Ma contact and regional met records increasing pressures toward CSZ • Stowell and Crawford, 2000; Taylor et al., 1996, 1997; Lindline, 1999, in Stowell and Crawford, 2000; Himmelberg et al., 1991, 1994; Stowell and Goldberg, 1997 ~80 Ma: 6+1 kbar, 600+50°C ~90 Ma: 6+1 kbar ~90 Ma: 4-4.5 kbar 0 100 km

12. 90-89 Ma: 10.2 kbar Western thrusting and contact metamorphism 90-89 Ma: 6.3 kbar • Additional ~90 Ma contact met to S • Also, near Prince Rupert ~90 Ma regional metamorphism associated with thrust-inverted metamorphic sequence • Both contact and regional met record increasing pressures toward the CSZ • Stowell and Crawford, 2000; Crawford et al., 1987; ~90 Ma: 8.1 + 1 kbar, ~600°C ~90 Ma: 5.6+0.5 kbar, 560+20°C

13. ~60 Ma: 8.2 kbar, 620-710°C 62+4 Ma, 59+12 Ma: grt Sm-Nd ages Northern Coast Mtns, SE AK 2-7.5 kbar, 325°C 8.2+1 kbar, 515-650°C ~60 Ma: 6.4 kbar, 600-705°C 8.5-11.3 kbar, 570-700°C • Metamorphism and plutonism disappears on W side of CSZ after ~80 Ma • From ~75-58 Ma (?) plutonism causes high T, lower P met within and to E of CSZ (Central Gneiss) • Himmelberg et al., 1991; Stowell, 1989; Stowell and Crawford, 2000; Stowell and Goldberg, 1997 6+1 kbar, 600+50°C 6+1 kbar 4-4.5 kbar 0 100 km

14. Central Gneiss Complex and CSZ 65-57 Ma: 5.6+0.4 kbar, 710+30°C 6.4 kbar, 525 ºC 8.1 + 1 kbar, ~600°C • From ~75-58 Ma (?) plutonism causes high T, lower P met within and to E of CSZ (Central Gneiss) • Klepeis et al. (1998) - Eastern CSZ near Sitklan Passage • Ky+staur consistently overprinted by sillimanite and/or cordierite indicate decompression • Klepeis et al., 1998; Crawford et al., 1987, Rusmore et al., 2005; Crawford et al., 1979 70-59 Ma: 5.4+1.5 kbar, 726+30 ºC 5.6+0.5 kbar, 560+20°C 70-59 Ma: 7.4+1.5 kbar, 750+50 ºC 70-59 Ma: 6.3+0.8 kbar, 680+30 ºC

15. My preliminary results 5.6+0.4 kbar, 710+30°C 6.4 kbar, 525 ºC 8.1 + 1 kbar, ~600°C • Mostly sillimanite-bearing metapelites (Yukon-Tanana), with relict kyanite in some samples • In CSZ? • For barometry (GASP), used Holland and Powell (1995) data set calibrations • For thermometry (grt-bt), took average of 7 different calibrations • GJP-59 has no Al2SiO5 • Numbers here assume it is nearby (I don’t know whether this is a reasonable assumption) 5.4+1.5 kbar, 726+30 ºC 5.6+0.5 kbar, 560+20°C 7.4+1.5 kbar, 750+50 ºC GJP-59: 3.89+0.12 kbar, 680+10oC 6.3+0.8 kbar, 680+30 ºC GJP-49: 5.03+0.29 kbar, 660+30oC GJP-48: 4.73+0.24 kbar, 640+10oC

16. 2-7.5 kbar, 325°C 8.2 kbar, 620-710°C 5.6+0.4 kbar, 710+30°C 8.2+1 kbar, 515-650°C 8.5-11.3 kbar, 570-700°C 6.4 kbar, 600-705°C 6+1 kbar, 600+50°C 6+1 kbar 4-4.5 kbar 6.3 kbar 10.2 kbar 8.1 + 1 kbar, ~600°C 6.4 kbar, 525 ºC 5.6+0.5 kbar, 560+20°C 7.4+1.5 kbar, 750+50 ºC ~70-60 Ma GJP-59: 3.89+0.12 kbar, 680+10oC 5.4+1.5 kbar, 726+30 ºC ~90 Ma 6.3+0.8 kbar, 680+30 ºC GJP-49: 5.03+0.29 kbar, 660+30oC >101 Ma GJP-48: 4.73+0.24 kbar, 640+10oC