Metamorphism of the Mullerneset Formation, St. Jonsfjorden, Svalbard

Abstract

The metamorphism of upper greenschist facies metasediments exposed in the extreme southwestern portion of St. Jonsjorden, Svalbard, is described. The rocks form part of the Miillerneset Formation of the late Precambrian age Kongsvegen Group and constitute a portion of the central-western Spitsbergen Caledonides. Four deformations (DI-D~) and two metamorphic episodes (MI and Mz) have affected the rocks of the Miillerneset area. MI was a prograde event which was initiated prior to the onset of the D1 and continued through this deformation. Pre-DI metamorphism reached biotite grade whereas garnet grade was attained syn-DI. MZ was a lower-middle greenschist facies metamorphism associated with Dz. The results of quantitative geothermometry in the pelitic rocks show that peak MI metamorphic temperatures decrease southwards across the field area from about 540°C to 510°C. Geobarometry and estimates of depth of burial indicate that MI pressures were in the range of 5-7kb. The data are consistent with geothermal gradients in the range of 21 f 4"C/km to 24 f S"C/km. MZ metamorphic conditions are not precisely determinable but temperatures and pressures were probably less than those attained during MI. It is suggested that the rocks of central-western Spitsbergen were originally deposited in an aulacogen before the initiation of Caledonian diastrophism.