T204 Ultrahigh-Pressure Metamorphism: Observations from Nano through Outcrop to Plate-Tectonic Scales II: In Honor of J.G. Liou, 2011 MSA Roebling Medalist (Note: This session has moved to Wednesday, 8 a.m.–noon, Room 101FG)

Tuesday, 11 October 2011
1:30 PM - 5:30 PM
Room M101AB

ECLOGITE TO MYLONITE: PETROLOGIC STUDY OF THE SALTA SHEAR ZONE, WESTERN GNEISS REGION, NORWAY

Paper number: T204-6
Program: Technical Sessions
Time: 2:45 PM – 3:00 PM

Authors

Abstract

Elucidating the relationship between eclogite pods and quartzofeldspathic host gneiss is an important part of understanding exhumation conditions and mechanisms of the Western Gneiss Region (Norway) UHP terrane. Garnet and plagioclase compositions of felsic gneiss (+/- garnet) and associated mafic boudins from a discrete shear zone in the southern UHP domain in the WGR are consistent with retrogression from higher P/T to amphibolite facies. The plagioclase is highly recrystallized with straight grain boundaries, triple junctions, an average grain size of ~70 um, and shows distinct reverse zoning from core (An25) to rim (An35). The felsic gneiss hosts boudins and layers of eclogite and garnet amphibolite ranging in size from cm- to several meter scale. Garnets from all samples lack strong zoning. The eclogite pod (gt + omp + pl + qz + white mica + rt) has an average garnet composition of Alm41Py46Sps12Gr1. Palisade quartz inclusions in garnet are present and have been interpreted as pseudomorphs after coesite. Garnet in the garnet amphibolite (hbl + bi + gt + pl + qz + zo + ap) has an average composition of Alm42Py28Sps29Gr1. The garnets are surrounded by a hbl + pl symplectite. A small (6 cm thick) garnet amphibolite layer (hbl + bi + gt + pl + qz + ap + rt) contains abundant small garnets with plagioclase rich coronas and an average garnet composition of Alm47Py24Sps28Gr1.

The mineral assemblages, compositions, and textures are consistent with retrogression of the shear zone during decompression from (U)HP conditions to amphibolite facies. This interpretation is consistent with temperature and strain estimates from quartz microstructures. Quartz in the felsic gneiss shows dominant prism <a> glide. The presence of two chemically distinct garnet populations could be ascribed to re-equilibration during decompression or crystallization from different bulk compositions. Ongoing integrated petrologic and structural investigation is underway to better understand the deformation conditions of this mylonite and associated mafic phases.