Fractional Crystallization Of Pyroxene Research Articles

Neoproterozoic magmatism in NW Sinai, Egypt: magma source and evolution of collision-related intracrustal anatectic leucogranite

The present study deals with tectonomagmatic evolution of the collision-related leucogranite located near the northwest corner of exposed basement in Sinai, Egypt. The area is composed of: (1) a gabbroic complex; (2) amphibolite; (3) post-orogenic leucogranite; (4) Feiran gneisses. The amphibolite and gabbroic suites, generated in an island arc environment, have a high Fe-tholeiitic affinity and were derived from two independent magmas. On the basis of rare earth element (REE) patterns, the gabbroic melts could have been generated from a garnet- and amphibole-bearing, enriched mantle, and were subsequently modified by fractional crystallization of pyroxene and amphibole with minor plagioclase, whereas the amphibolite melts could be derived from garnet-free depleted mantle. The leucogranite has high Al2O3 content (>13 wt%), alumina saturation index (ASI) mostly >1, and normative corundum, indicating a peraluminous nature. Chondrite-normalized REE patterns for the leucogranite show light REE enrichment (La/SmN=2.7–4.86), general flattening of the heavy REE (Gd/LuN=1.2–2), and negative europium (Eu) anomalies (Eu/Eu*=0.24–0.47). The peraluminous nature and enrichment of the incompatible elements (K, Rb, Ba and Th) in the leucogranite strongly suggest derivation from a crustal source. The most probable source for the leucogranite magmas is represented by the adjacent Feiran gneisses, which could have generated the leucogranite by dehydration melting under water-undersaturated conditions. It appears likely that the restite unmixing model is responsible for the chemical variations within the leucogranite. In accordance with this model, the chemical variation of the leucogranite can be attributed to varying degrees of separation of restitic material from the melt during its emplacement and solidification and fractional crystallization could have played a minor role during magma ascent through the crust.

Petrogenesis of the Dullstroom Formation, Bushveld Magmatic Province, South Africa

The Precambrian Dullstroom Formation of South Africa, which is predominantly composed of basaltic andesites interbedded with subordinate sedimentary and felsic volcanic strata, represents the first phase of an extended period of magmatism that was responsible for the Bushveld Magmatic Province, including the extrusive Rooiberg Group and the intrusive Bushveld Complex. New geochemical and isotopic data for the Dullstroom Formation are presented in an effort to elucidate the petrogenetic processes operative during the initiation of this magmatic episode. The volcanic units of the central portion of the Dullstroom Formation have been subdivided into at least three interbedded compositional groups: low Ti mafic to intermediate units, high-Ti mafic to intermediate units, and high Mg felsic units. High Ti and low Ti volcanic units are similar in some compositional characteristics to basalts of the nearby northern and southern provinces, respectively, of the Mesozoic Karoo continental flood basalts. Isotopic and compositional data for low Ti Dullstroom strata are consistent with bulk assimilation into a melt similar in composition to a southern Karoo basalt of 20% upper continental crust accompanied by 20% fractional crystallization of pyroxene and plagioclase. Isotopic and compositional data for high Ti Dullstroom strata are consistent with magma mixing of 30% northern Karoo K-rich basalt and 70% southern Karoo basalt followed by 20% assimilation of upper continental crust and 20% fractional crystallization of pyroxene and plagioclase. Compositions of high Mg felsic volcanic strata are consistent with 25% assimilation of a mixture of silica-rich sedimentary rock and upper continental crust into a melt similar in composition to low Ti volcanic units with 25% fractional crystallization of pyroxene and plagioclase. However, it has been suggested that compositions of these high Mg felsic strata may also be consistent with interaction of a crustal melt. Assimilation, fractional crystallization, and magma mixing that apparently affected these Dullstroom Formation volcanic strata may have occurred in a series of shallow magma chambers. These data are consistent with the suggestion that Dullstroom Formation volcanic rocks are the result of a mantle plume. Mantle plume origin also is suggested by the large volume of intrusive and extrusive strata associated with this magmatic episode. These data do not support the hypothesis that the Bushveld Complex and the Rooiberg Group formed by impacts of a cluster of comets or asteroids.

Petrological and geochemical characteristics of Cenozoic high-K calc-alkaline volcanism in Konya, Central Anatolia, Turkey

Late Miocene to Pliocene volcanic rocks outcrop west, northwest and southwest of the Konya area in Central Anatolia, Turkey. Volcanic products are lava domes, nuée ardentes and ignimbrite deposits, predominantly andesitic to dasitic in composition, together with rare basalt, basaltic andesite, basaltic trachyandesite and trachyandesite (50.35–69.39% SiO 2). The serie exhibits high-K calc-alkaline affinities. Fractional crystallization of pyroxene, plagioclase and Fe–Ti oxides is the main process in the magmatic evolution of Konya volcanic rocks. Volcanic units exhibit typical high-K calc-alkaline character. Their geochemical characteristics (e.g., enrichments in LIL elements such as K, Rb, Ba, Sr, depletion in HFSE such as Ti, Nb, and high Ba/Nb and Low Nb/Y ratios) are consistent with those of active continental margin regions. High 87Sr/ 86Sr (0.704841–0.707340) and low 143Nd/ 144Nd (0.512390–0.512618) ratios suggest crustal involvement in their petrogenesis. Correlations between 86Sr/ 87Sr isotope with Rb, Rb/Nb, Rb/Ba, and Rb/Sr also emphasize the effect of crustal contamination on the andesitic and dacitic magmas. As a consequence, Konya volcanic rocks are products of assimilation and fractional crystallization (AFC) processes of a magma which seems to be linked to the subduction of the African plate underneath the Anatolian plate during Miocene.