| Deep upper mantle peridotites derived from depths of > 130-185 km occur in several orogenic belts formed by continental collision. The exhumation mechanisms of such deep rocks are controversial. Processes involving very deep continental subduction/exhumation or a combination of mantle convection and orogenic exhumation in two orogenic cycles have been proposed. The aim of this research is to determine how large km scale rock masses are transported from the deep upper mantle into the continental crust and up to the surface. We will focus on unique peridotites from Otroy, Norway, that contain relicts of, the deep mantle mineral, majoritic-garnet. Our previous work on the Otroy peridotites suggests that these rocks were transported from the deep mantle by Proterozoic mantle upwelling to depths around 100km. Crustal emplacement occurred during Caledonian orogenesis. The simplest viable history involves upwelling of hot-asthenosphere diapirs from the deep upper mantle into cratonic lithosphere. However, the ultimate depth of origin and the timing and PT conditions of melting are uncertain so more complex histories are possible including a) derivation of the peridotites from deep cratonic lithosphere that was entrained and heated by upwelling asthenosphere or b) derivation of the peridotites from older lithosphere that has been transported to the deep upper mantle by subduction and returned to shallow depths by later upwelling. We propose an integrated study of relict mineral microstructures and trace element composition of the Otroy peridotites. Electron microscopy studies on exsolution microstructures and crystal structures will provide new information on the early pressure-temperature evolution. Laser abation ICP-MS data on rare earth elements will be used to constrain the depth of melting. With these new data it will be possible to determine the origin and early evolution of these unique large samples of the deep upper mantle. This research will also provide new insights into processes of cratonic lithosphere growth, upper mantle convection and continental collision and subduction. |
