Writers : Eskandary,Amir;Amini, Sadraddin;Farahkhah, Nasrin
Refference : The first international applied geological congress; Department of geology Islamic azad university- Mashhad
Publishing Year : 2010

Abstract :

Textural analysis techniques were used to quantify key aspects of the crystal population, including crystal size distributions (CSD), crystal shape (S) and spatial distribution patterns (SDP) and to discuss physical processes ongoing in the magma chamber. CSDs of plagioclases were determined from micro diorite-quartz diorites of 120 km southeast of Birjand. The CSDs lie in 3 similar groups. All CSDs are curved, concave up and coincident but differing in their maximum crystal size, amount of curvature and concavity. Shape of crystals changed from tabular crystals (with I, L>2) to more equant forms (with I, L<2) with change of CSDs. The SDP of plagioclase crystals as seen in the R value against porosity diagram, are clustered and the trends of data in this diagram consistent with overgrowth trend due to textural coarsening and or asymmetric growth. A dynamic model is proposed for the origin and observed evolution of textures in these rocks. Crystallization of plagioclases started following emplacement of dacitic magma at a depth of at least 5km. Nucleation was probably heterogeneous that causes early clustering of crystals. A steep, straight CSD developed by nucleation and growth. This process was interrupted by the injection of mafic magma into the chamber, or convective overturn of hotter magma. The magma temperature rose until it was buffered, initially by plagioclase solution and later by crystallization. Under these conditions textural coarsening (Ostwald ripening) of plagioclase following CN (Communicating Neighbors) model occurred: crystals smaller than critical radius (or size) dissolved to feed larger crystals. The CSD became less steep and extended to larger crystal size. A repetition of this cycle has generated the observed family of CSDs but the step number of cycle was different for each of three groups of CSDs. During these periods, Shape and SDP of crystals are changed also. Results complement petrographic evidence and geochemical modeling of magma mixing in studied rocks.

Subject List : Magmatic Rocks, Textural measurements