A significant tin deposit has been identified in the northeast part of the Late Proterozoic Arabian Shield. Cassiterite-topaz-quartz greisens are distributed over 16 km2 in flat-topped cupolas of a highly evolved, zinnwaldite-bearing alkali-feldspar granite which composes part of the silsilah ring complex (lat 26"06'N, long 4240'E). The 587 m.y. old alkali-feldspar granite is overlain by a cara...

The central pluton within the Neoproterozoic Katharina Ring Complex (area of Gebel Mussa, traditionally believed to be the biblical Mt. Sinai) shows a vertical compositional zoning: syenogranite makes up the bulk of the pluton and grades upwards to alkalifeldspar granites. The latters form two horizontal subzones, an albite–alkali feldspar (Ab–Afs) granite and an uppermost perthite granite. The...

105 Abstract: Origin of A-type alkali feldspar granites is currently the subject of a world-wide debate. Contrasting hypotheses have been proposed, which range from an entirely crustal origin to an almost complete mantle derivation. A-type alkali feldspar granites belong to either unimodal granite (rhyolite)-dominated association, or bimodal gabbro (basalt)-granite (rhyolite) suite. It is argue...

gabal el-zeit block at the western coast of the gulf of suez region in egypt consists of two varieties of precambrian granites, namely monzogranite and a peripheral zone of alkali granite. both granitic varieties display moderate (60.1-68.5 wt%) and high silica (73.36-76.16 wt%) contents with high na2o+k2o (7.21-8.61 wt%), respectively. the investigated monzogranite is calc-alkaline metaluminou...

Petrography and physicomechanical properties of alkali granites, alkali quartz syenite, and nepheline syenite from Ambela, NW Pakistan, have been investigated. Whereas the alkali quartz syenite and most of the alkali granites are megaporphyritic, the nepheline syenite and some of the alkali granites are microporphyritic. Their phenocryst shape and size and abundance of groundmass are also diffe...

Synneusis, the drifting together and systematic attachment of crystals in a melt, is involved in three of the known origins for mantled feldspars: (1) overgrowth, (2) exsolution, and (3) filling of skeletal plagioclase. Quartz latite dikes of the Chocolate Mountains, California, contain plagioclase crystals in parallel and twinned synneusis. Sanidine mantles consisting of segments, each crystal...