BLUESCHISTS AND BLUE AMPHIBOLES: HOW MUCH SUBDUCTION DO THEY NEED?

For more than 40 years the name W. Gary Ernst has been inseparable from the concept of subduction and the blueschists and blue, Al-rich sodic amphiboles that constitute explicit evidence for this process. Nevertheless, there are still occurrences of blueschists and blue amphibole where regional plate-tectonic considerations and the lack of coeval volcanics raise doubts on the existence of former subduction zones. Focussing our analysis on case studies in NW South America and Cuba, we have used finite-difference plus marker-technique numerical modeling (200 x 100 Eulerian points and 500,000 markers) of a conceptual 400 x 200 km subduction zone configuration (see Gerya and Yuen, 2003) to ascertain minimum duration and convergence of subduction for achieving P-T conditions amenable to blueschist formation within subducted oceanic crust under conditions of insignificant shear heating. The initial boundary conditions chosen refer to an intraoceanic setting with analogous thermal structures in both hanging wall and subducting slab. Subduction rates were allowed to vary between 2 and 14 cm/yr. Thermal structures were chosen to correspond to oceanic crust up to 100 Myr of age. With the condition of slab bending at 200 km radius, the subduction angle was not considered critical and kept at 45°. The facies distribution of Peacock (1992) was chosen as P-T template. Our first results indicate that blueschists could form after as little as 300,000 years for young crust and high subduction rates. Even at 2 cm/yr, blueschist P-T conditions are reached between 1.9 and 2.6 Myr, regardless of crustal age. These data translate into a minimum of 38 to 50 km of subducted slab for all subduction rates and ages of 60 Myr or less, and increase to 75 km for ages of 100 Myr and subduction rates of 14 cm/yr. These data indicate that short-lived subduction and/or closure of narrow oceanic basins in tectonically complex areas such as the Caribbean can lead to isolated belts of blueschist that may be difficult to interpret in regional terms.

T.V. Gerya and D.A. Yuen (2003) Earth Plan. Sci. Letters, v. 212, 47-62. S.M. Peacock (1992) J. Geophys. Res., v. 97, p. 17693-17707.