Compositional layering in salt

Category Tectonic & Seismotectonic
Group GSI.IR
Location 21th symposium on geosciences
Holding Date 24 May 2008

      The salt of Qom Kuh is layered on all scales form mm to tens of meters. Such layers appear on planar outcrops as bands of halite of different grain size with impurities of different colors (red marl, white anhydrite etc.) and concentrations. Particular layers can range in composition from 100% transparent halite through all possible mixtures with other materials, usually marl, shales and sandstones.
   Talbot (1979, 1998) considered compositional layering in extruded salt to be inherited from the primary sedimentary bedding (emphasized to different degrees by recrystallization and flow refining). The salt exposed in an extrusion is strongly deformed after having traveled some kilometers along the source layer and up the diapir to exposure.
   Inclusions in the salt of Qom Kuh vary in size from
   As is common in namakiers elsewhere (Talbot, 1998). Pebbles of quartz or chert derived from underlying soils occur in the basal facies of impure distal salt on the W-side of Qom Kuh. Inclusions of amygdoidal hornblende andesite lavas ranging in size from cm to tens of meters are dispersed on the main dome and the southern and eastern slopes of Qom Kuh.
   The largest volcanic inclusions, tens of meters across and a few meters thick, are stranded in a row just short of the terminus of the southern namakier (Ganser, 1960; Talbot, 1998). These volcanic inclusions are highly jointed, presumably because they represent one or more continuous lava flows interlayered with the primary salt which were fragmented as they were carried long distances in the flowing salt. The salt in most outcrops on Qom Kuh display megacrysts (big crystals grains or) of clear halite in a finer grained impure matrix. Megacrysts can grow as phenocrysts during primary crystallization of porphyritic igneous rocks (or as megacrysts in evaporates), or as porphyroblasts (that grow by blastesis during strain) in a deforming augen gneiss. They can also survive as porphyroclasts in strongly deformed rocks (e.g. mylonites) that have undergone grain-size decrease by dynamic recrystallization.
   Some megacrysts of halite on Qom Kuh are the same color as their matrix; others are honey colored in matrixes of other colors (red to pink or black to grey). Salt megacrysts differ widely in size and shape, ranging from circular (a=b) to strongly elliptical (a>>b),. Eye shaped augen are distinguishable on the eastern slopes, and in SE part of southern namakier.
   In some of the impure magenta coloured salt (e.g. on the northern slopes) of Qom Kuh, dispersed megacryst of halite (5 mm across) are noticeably purer than their finer grained marl- or clay-rich surroundings. These megacrysts may be porphyroblasts in which halite had sufficient crystallization energy for their faces to have pushed impurities in front of them as they grew. Most of the other megacrysts on Qom Kuh are assumed to be remnant porphyroclasts that have survived a general decrease in grain size during flow of the rock salt that contains them (Talbot, 1999;personal communication).Crystal rotation during grain flow (Park&Means,1997) may control the shape of the megacrysts.

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