Original paper

Nature of feldspars in felsic plutonic complexes from northeastern Egypt: implications for the evolution of orogenic and anorogenic magmas

Abdel-Rahman, Abdel-Fattah M.

Abstract

Data on feldspars from orogenic to anorogenic complexes in eastern Egypt show the prominence of low albite in their perthitic lamellae. The structural state of feldspars reflects that the granodiorite-adamellite suite contains orthoclase, whereas samples from the leucogranite are represented by low microcline content. K-feldspar from the peralkaline granite is imperfectly ordered low microcline (its NOr values in the range 93.2-98.5% indicate a range of closure temperatures between 400 and 150°C). Chilling of the peralkaline granites roof facies resulted in a relatively highly disordered K-feldspar in this unit. The metasomatized granite contains virtually end-member KAlSi3O8, providing evidence of a closure temperature near 150°C. The prominence of low albite - low microcline suggests the presence of post-magmatic alkaline fluids promoting Al-Si ordering. The intrusion of the peralkaline granites cannot have been accompanied by a "regional" reheating, and its contact effects evidently were localized. The nature of feldspars was controlled in part by various physiochemical conditions prevailed during the crystallization of dry anorogenic, versus wet orogenic magmas. The results could have wide implications for the development of granites in various regions worldwide.

Keywords

egyptalbiteordered microclinehypersolvusclosure temperaturepost-magmatic fluid phaseal-si ordering