Composition and thermal evolution of the lithospheric mantle beneath the Harrat Uwayrid,eastern flank of the Red Sea rift (Saudi Arabia)

摘要

Harrat Uwayrid is one of several volcanic fields that developed along the western margin of the Arabian plate during opening of the Red Sea rift.Volcanic activity started in the late Miocene and Pliocene with the eruption of alkaline flood basalts.After a period of extensional faulting,numerous cinder cones formed in the central part of the volcanic field during the Quaternary.Xenoliths of both mantle and crustal origin are abundant.The mantle-derived suite is dominated by spinel lherzolite with X_(Mg) of olivine and X_(Cr) of spinel ranging from 0.868 to 0.911 and from 0.074 to 0.636,respectively.In addition to lherzolite,various types of pyroxenites and wehrlites occur.Most xenoliths contain primary and/or secondary amphibole (pargasite/kaersutite),indicating at least two enrichment events,both related to the formation of the Red Sea.Chemical zonation profiles of mineral grains constrain the thermal evolution of the xenoliths and their source regions in the shallow upper mantle.Ca contents in olivine are constant in the inner parts of the grains,but increase in the outermost zones (<35 mu m).X_(Mg) values of spinel grains adjacent to olivine often show a rimward increase in their outer zones (< 150 urn).Both types of zonation patterns are interpreted to result from heating by the host magma during transport.Zonation patterns of Al and Ca/(1 -Na) in pyroxene grains record different thermal evolutions,such as (i) simple cooling,whereby a 'near-equilibrium' state was rarely achieved,(ii) cooling followed by heating,and (iii) simple heating.Diffusion modelling on measured zoning profiles suggests that cooling lasted for no more than ~ 10 Myr.Temperature estimates for pyroxene core and rim compositions,calculated by using various pyroxene geothermometers range from ~ 800 to 1140 °C (at an assumed pressure of 1.5 GPa).In conjunction with upper pressure limits derived from the composition of spinel,these values imply relatively high temperatures for the shallow upper mantle.This is corroborated by P~T estimates for spinel lherzolites obtained by combination of the two-pyroxene thermometer based on the solvus between enstatite and diopside and the Ca-in-olivine geothermobarometer.While the high- T state that preceded cooling was probably a regional phenomenon,renewed heating was of local nature,most probably related to advective heat transport by ascending and stagnating magmas.