Melt Migration and Depletion-Regeneration Processes in Upper Mantle of Continental and Ocean Rift Zones

Abstract

Tectonized harzburgites constituting the ultramafic basement of inter-arc and back-arc basins and forming the lower part of corresponding ophiolites provide textural and mineralogical evidences that some veins and other local mineral segregations resulted from crystallization from infiltrated basicultrabasic melt and the reaction of that melt with harzburgites. Veins of dunite are found by reaction. Veins of pyroxenite and gabbro may be formed by disequilibrium crystallization from infiltrated melts during the general cooling. These vein assemblages occur in many ophiolites including Upper Precambrian and Palaeozoic ophiolites of the Urals and southern Siberia.

The evolution of the upper mantle beneath Baikal Rift Zone (BRZ) in the Cenozoic was estimated based on the studies of a great number of mantle xenoliths from the basaltic lavas and tuffs of three stages of volcanic activity (about 30 m.y., 18-9 m.y. and 5-2 m. y.). The xenoliths of the first stage are composed of Mg-and Cr-rich minerals whereas those of the second and the third stages are enriched in Al, Fe, Ti, Na. Magma segregation was deepest at the early stage, shifted upwards at the second stage and went down again at the third stage. These changes may be correlated with the compositions of lavas and the scale of volcanic activity. The geotherms plotted using garnet-bearing assemblages show that the temperatures for the same depth levels increased by about 100-150°C from the first to the last stage. We suppose that the heating and alteration of mantle was started by the intrusion of the picritic magma at the depth of 100-120 km. Green pyroxenites crys-tallized from picritic melts while black pyroxenites and amphibole-phlogopite cumulites were formed from basaltic liquids. A direct evidence of infiltration of such melts is glass-bearing veins in the mantle lherzolites. The composition of these glasses from the Cenozoic mantle xenoliths of BRZ includes olivine-rich to felsic differentiated liquids.

The composition of rocks and minerals show the same similarities and differences between the processes in oceanic and continental upper mantle. The main differences can be related to composition of migrating melt and great depth of processes in continental upper mantle. In both cases the interactions of mantle peridotites with basic-ultrabasic melts and evolution of melts due to those reactions from picritic to basaltic liquids were followed by mantle diapiric rise. The texture-temperature relationships and distribu-tion of different mantle peridotites reflect the local zoning in the mantle formed as a result of mantle diapirism. Such a process of melt migration and compositional change in the mantle has been referred to as “mantle metasomatism” (Manzies and Hawkesworth, 1987) or “paratexis” (Dobret-sov, 1980). It might result in progressive depletion of the mantle as shown in the ophiolite harzburgite mantle or in the regeneration of the mantle at the regressive stage. A good example of the regeneration of the mantle is in BRZ.