Fractionation of a shallow silicic magmas body through interaction of collapse of volcanic structure and roof boundary layer documented in the Wadi Dib ring complex, Eastern Desert of Egypt

Abstract

<p>Silicic magmas categorized "petrogeny's residua system” of Bowen and Tuttle, 1949) are highly polymerized and high in viscosity (> 10^5 Pas depending on water content and other parameters), which is one of the physical properties of magmas that could inhibit magmatic fractionation by making crystal-melt separation difficult (Pitcher, 1997). Diversities of volcanic and intrusive rock suite showing a consistent chemical trend, such as from andesitic to rhyolite in an arc environment and from trachyte to rhyolite in a continental region have been reported from many localities. Some of these cases involve various extent of crustal assimilation through introduction and mingling of silicic melt from partially molten crustal rocks. However, there are many cases, particularly from continental regions, in which extensive fractional crystallization with negligible material input is shown to have played a major role in magmatic fractionation from unequivocal geochemical evidence. One mechanism to cause extensive fractionation of silicic magmas is segregation of fractionated melt from highly crystalline crystal-melt system so-called crystal mush. This mechanism requires compositional convection or a pressure gradient in the interstitial melt. The latter may be attributed to compaction driven by deformation of the melt-crystal system, such as gravity-driven compaction and convection of the mush as a whole. However, actual mechanisms and controlling factors for their operation are still unclear. We address this issue by examining an alkaline ring complex in the continental region, where extensive fractional crystallization without crustal assimilation took place to form diverse rocks from trachybasalt to rhyolite. The Wadi Dib ring complex (WDRC), Eastern Desert in Egypt, consists of multiple circular rings of volcanic and plutonic units. The plutonic rings show zoning progressively more fractionated inwards from the syenite periphery to the central granitic core through the intermediate zone of quartz syenite. The progressive fractionation from the margin to the center, pyrometamorphism in the country rocks neighboring the ring complex and their enclaves only in the periphery of the outer ring, pyrometamorphism in the overlying volcanic unit and the occurrence of their enclaves only in the inner ring, systematic grain size reduction from the outer ring to the granitic core, and high-temperature shear deformation in the outer ring closer to the inner ring suggest that the ring complex formed at a very shallow crustal level under effective and progressive cooling from the surface accompanying localized brittle and ductile deformation. The significant fractionation of acidic rocks of the WDRC is attributed to the development of roof mush zone, which was later collapsed by surpassing strength of the overlying crust and roof mush to induce fractionation of the upper zone of a magma body followed by its intrusion into the shallow-level.</p>