MINERALOGY, GEOCHEMISTRY, AND PETROGENESIS OF SOME LAMPROPHYRES FROM SOUTHWEST SINAI, EGYPT

The calc–alkaline, synorogenic granitoids of the Wadi Nasib–Wadi Suwiq area are dissected by minor dyke intrusions. Two types of lamprophyre dyke rocks, namely, an older spessartite and a younger camptonite, are recorded. They follow the NW, N–S, and NE trends. Pyroxene of the spessartites is augite of subalkaline–alkaline character, while that of the camptonites is ferroaugite, of alkaline to peralkaline affinity. Amphibole of the spessartites is calcic (pargasite), while that of the camptonites are pargasite and kaersutite. The mica of the camptonites is Mg-rich (phlogopite), analogous to the anorogenic alkaline series. Plagioclase ranges from bytownite to labradorite (An_74-50) in the spessartites and labradorite to andesine (An53-31) in the camptonites. The whole rock chemistry confirms that spessartites have subalkaline (tholeiitic) character, whereas the camptonites show alkaline affinity. The camptonites show a higher degree of REE fractionation and negative Eu anomalies compared with the spessartites. The presence of hydrous phases as well as phlogopite may suggest a primitive melt which could be produced from metasomatized mantle (phlogopite-bearing peridotite). For the spessartites, it is concluded that it was mostly generated from fossilized subducted oceanic lithosphere. The alkaline nature of pyroxene and phlogopite, the whole rock chemistry, and the negative Eu anomalies of the camptonites, all together suggest crustal contamination of an ascending silicic melt. The latter was emplaced along pre-existing fractures in a within-plate continental environment.