Phase transformations of continental crust during subduction and exhumation: Western Gneiss Region, Norway
Peterman, Emily M.; Hacker, Bradley R.; Baxter, Ethan F.
European Journal of Mineralogy Volume 21 Number 6 (2009), p. 1097 - 1118
published: Dec 1, 2009
ArtNo. ESP147052106003, Price: 29.00 €
Whether quartzofeldspathic rocks transform to (U)HP minerals during subduction - and back to low-pressure minerals upon exhumation - remains one of the more profound questions pertaining to collisional orogenesis. Garnet-bearing quartzofelds-pathic gneisses from the Western Gneiss Region, Norway provide an opportunity to answer this question. High-precision Sm-Nd garnet geochronology of these gneisses documents garnet growth from 418 to 398 Ma. Garnet zoning in two samples implies growth during subduction-related increase in pressure from 0.5 GPa and 550 °C to 1.7 GPa and 700 °C. Zoning in all other garnets suggests growth over rather narrow P-T ranges from 1.0-1.6 GPa and 725-800 °C during decompression, possibly accompanying melting. If these samples constitute a representative suite, (i) the dearth of (U)HP garnets suggests that most of the quartzofeldspathic gneisses in the Western Gneiss Region did not transform to eclogite-facies parageneses during subduction; (ii) the abundance of garnets grown during decompression indicates that the major period of densification was during exhumation. The widespread metastability of quartzofeldspathic rocks during subduction is substantially different from the findings of previous work and suggests commensurately less subduction of continental crust before the slab is positively buoyant.