First Evidence for Ultrahigh Pressure Metamorphism on Gossa, Western Gneiss Region, Norway
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Ultrahigh pressure (UHP) metamorphism takes place in rock at depths within the coesite stability field. In the Western Gneiss Region (WGR) in Norway, three domains are present where UHP metamorphic rocks occur. In the northern UHP domain, evidence for UHP is concentrated in three distinct areas. In between these areas lies the island of Gossa. This island is often included in the northern UHP domain, but this is based on extrapolation of lithologies around Gossa and no direct evidence exists that these rocks have experienced UHP metamorphic conditions. Some authors dispute the existence of one UHP domain, and explain the UHP outcrops as small UHP thrust nappes within a single larger lower pressure domain. The aim of this research is to constrain the peak metamorphic conditions of the rocks on Gossa, which places this part of the WGR either within or outside the northern UHP domain. Fieldwork was carried out on Gossa during the summer of 2008. One of the lithologies that are found here is orthopyroxene (opx) bearing eclogite, enclosed as lenses within basement gneisses. This lithology is sampled for determination of peak metamorphic conditions by evaluating the Al2O3 content of opx in contact with garnet. Granulites and basement gneisses have been sampled to constrain the retrograde pressure and temperature (PT) path. All samples are analysed by optical microscopy for determination of their metamorphic facies based on their mineral content. Pressure and temperature are calculated for eclogites and granulites by various geothermobarometric methods, using major element mineral chemistry obtained by analysis of the samples by Electron Microprobe. Al2O3 contents in opx of 0.26 and 0.30 wt% are measured in eclogites from two different outcrops. These very low Al2O3 contents indicate UHP conditions, probably within the diamond stability field, with pressures of >37kbar and temperatures of 780-1000ºC. The PT path, based on optical microscopy as well as geothermobarometry, shows isothermal decompression after the peak metamorphic stage until 8-10 kbar. This is followed by cooling without significant change in pressure until ±500ºC. This PT path shows a good correlation with published PT paths of the surrounding area. These results indicate that the northern UHP domain forms one coherent body instead of small scale thrust nappes, and Gossa can be included in this domain.