Mantle processes in the sub-continental lithosphere: the case study of the rifted sp-lherzolites from Zabargad (Red Sea)
Piccardo, Giovanni B.; Rampone, Elisabetta; Vannucci, Riccardο; Shimizu, Nοbumichi; Ottolini, Luisa; Bottazzi, Pierο
European Journal of Mineralogy Volume 5 Number 6 (1993), p. 1039 - 1056
published: Dec 1, 1993
manuscript accepted: Jul 26, 1993
manuscript received: Dec 29, 1992
ArtNo. ESP147050506016, Price: 29.00 €
Abstract This paper presents new petrological and geochemical data on peridotites and associated pyroxenites from the Zabargad Island (Northern Red Sea), and, particularly, new data on REE bulk rock composition of pyroxenites, and trace element composition of amphiboles (determined by ion probe) from both peridotites and pyroxenites. Results are discussed in the context of field, structural-textural, petrological, geochemical and isotopic knowledge on the Zabargad peridotite. The Zabargad peridotite consists of variably depleted mantle ultramafics. The dominant rock type is a four-phase, granular, spinel(sp)-lherzolite characterized by: i) relative abundance of clinopyroxene(cpx) (15-20% by volume), ii) high contents of fusible components in the constituent minerals, iii) high bulk-rock REE concentrations with Cl-normalized pattern almost flat from H- to MREE, at more than 2 times Cl, and a slight negative LREE fractionation. Other lithologies consist of anhydrous Al-diopside(Di) pyroxenite layers and hydrous (Ti-pargasitebearing) Cr-Di pyroxenite dykes. Field, petrological and geochemical evidence suggests the following sequence of events developed at mantle depth: 1) Near-solidus evolution of lherzolites and the formation of anhydrous Al-Di pyroxenite layers. The compositional features of the anhydrous Al-Di pyroxenites indicate that they crystallized from tholeiitic melts as garnetfree and garnet-bearing clinopyroxenites, at T higher than 1200°C. 2) Static recrystallization at lower T (and probably P) conditions, most probably around 1000°C. Lherzolites and pyroxenites recrystallized to granular, equilibrium sp-bearing (gnt- and pl-free) assemblages. 3) Widespread kaersutitic amphibole crystallization in equilibrium with the sp-bearing granular association, probably related to infiltration of H2O-rich mantle fluids. 4) Decompression: tectonite-mylonite fabrics were formed along localized shear zones and plagioclase(pl)-bearing assemblages developed in both massive and deformed rocks. During decompression, Zabargad peridotites were intruded by hydrous melts which generated amphibole-bearing, Cr-Di websterite dykes, characterized by dominant subhedral pyroxenes with interstitial Ti-pargasite and Ti-phlogopite. Both clinopyroxenes and Ti-pargasites are strongly LREE-enriched, suggesting that these rocks crystallized from alkaline melts. Intrusion was most probably accompanied by circulation of hydrous fluids which produced widespread cryptic and local modal metasomatism (i.e. crystallization of Ti-pargasite + Tiphlogopite +/- apatite and opaques) in the surrounding lherzolite. 5) Dynamic recrystallization along localized shear zones which produced amphibole(Mg-hornblende)-peridotites after lherzolites and amphibolites after pyroxenites. This late stage developed at lowered P and T conditions, under progressive fluid/rock interaction during uplift: fluids could be either related to the previous alkaline magmatism or originated from (or equilibrated with) the continental crust. The textural, petrological, geochemical and isotopic data indicate that the Zabargad lherzolites represent pre-rift lithospheric, sub-continental mantle which was accreted from the asthenosphere to the Arabian-Nubian lithosphere prior to, or coeval with, the Pan-African evolution. After early accretion and cooling to the local geothermal gradient of the conductive lithosphere, the Zabargad peridotites, as an integral part of the Arabian-Nubian sub-continental, lithospheric mantle, underwent a sub-solidus, non-adiabatic upwelling and sea-floor emplacement during the early rifting stage in the northern sector of the Red Sea system. This evolution is indicative of a rifting mechanism dominated by passive and asymmetric extension of the lithosphere. The P-T path recorded by the Zabargad ultramafics is consistent with the thermal history expected for the foot-wall rocks of a lithosphere-scale dipping extensional detachment zone.