Neoproterozoic volcanism at Um Shilman-Um Dubr area, Southeast Aswan, Egypt: Geology, geochemistry and tectonic environment of the Dokhan Volcanic Formation

摘要

The volcanic history of the Neoproterozoic belt in the Eastern Desert of Egypt encompasses two major magmatic episodes. An earlier episode (950-750 Ma) produced the Shadli metavolcanic assemblages, and a younger episode (680-550 Ma) produced the Dokhan volcanic rocks. The Dokhan volcanics (DV) at Um Shilman-Um Dubr, situated in the Southeast Aswan of Egypt, range in composition from basalt-an-desite-dacite to rhyodacite-rhyolite. The latter unit is made up largely of ash flow tuffs and ignimbrites that are locally interstratified with basalt and andesite lava flows. Field, petrological and petrochemical investigations reveal that the rocks, which were previously mapped as island arc metavolcanics, are composed mainly of a thick sequence of volcaniclastics and flows that were subjected to low-pressure metamorphism. Geochemically, the Dokhan volcanics (DV) form a continuum in composition with a wide range of SiO_2 (47.94-76.94 wt. percent), CaO (0.14-8.85 wt. percent), Sr (80-905 ppm), Zr (72-340 ppm), U (1.4-13.1 ppm), Th (3.4-16.8 ppm) and are moderately enriched incompatible elements, including REE. The fractionation index (FeO*/MgO) increases gradually from the basalts through basaltic andesites to rhyolites (0.74, 0.92 and 1.59 respectively). The high Zr/Y ratio in the investigated volcanics characterizes rocks that evolved in continental arc setting and suggests involvement of subcontinental lithosphere in magma genesis. The relatively well-defined geochemical trends obtained for the Dokhan volcanic suite, along with the progressive increase in Eu-anomalies toward more-evolved compositions, combined with results of the geochemical modelling, support the idea that a fractional crystallization process has played a major role during the evolution of the Dokhan volcanic magma series. The enrichment of LILE (Sr, K, Rb and Ba) and the relative depletion of HFSE (Zr, P Y and Ti) seem to be inherited from the mantle source. It is a typical calc-alkaline orogenic suites and exhibits mineralogical-geochemical traits of arc-related volcanism. A subduction-related tectonic setting for the emplacement of the investigated suite is indicated by the petrological and geochemical evidences. The geochemical study suggests that fractional crystallization played an important role in the formation of Um Shilman-Um Dubr Dokhan volcanics. A mantle-derived basaltic magma fractionated, with amphibole 12.4 percent, plagioclase 20.2 percent and magnetite 4.9 percent dominating the fractionating assemblage, to produce the more felsic varieties, as suggested by major fractionation modelling.