Scientific Quarterly Journal of Geosciences

Scientific Quarterly Journal of Geosciences

An outline of very low-grade Metabasites and its kinds in Saveh region

Author
Farideh Helmi
Geological Survey of Iran, Tehran, Iran
Abstract
Very low-grade metamorphism is a ubiquitous process in the upper portion of the earth's crust. Limit of recrystallization that has occurred in this stage of metamorphism does not cause structural and textural changing in rocks. Depending on the tectonic setting and physico-chemical conditions, various types of low temperature metamorphism are hence produced in both basaltic- andesitic volcanic rocks with different thickness and shapes: Ocean ridge + ocean- floor, burial, subduction zone metamorphism, thermal and hydrothermal metamorphism. Those generally have slightly affected on the chemical composition of rocks. Mineral assemblages of low- T metamorphism of basaltic and andesitic rocks commonly consist of one or two Ca-Al hydrosilicates together with albite, white mica, quartz, chlorite (or smectite), sphene, carbonates, pyrite, hematite and/or magnetite. Zeolite, prehnite- pumpellyite, prehnite- actinolite, pumpellyite- actinolite and blueschist are facies kinds in very low-grade- metamorphism. Zeolite Facies all those assemblages produced under physical conditions in which the following are commonly formed: quartz- analcime, quartz- heulandite, and quartz-laumontite. Low and high T zeolite facies have different assemblages.
    Prehnite- pumpellyite facies metabasites are common in all tectonic settings and characterized by the stable association of Pumpellyite + Epidote Prehnite and the absence of laumontite and actinolite. In many regionally metamorphosed terranes, Pumpellyite commonly coexists with actinolite. The assemblage pumpellyite + actinolite + epidote (+ chlorite + albite + quartz) occurs in deeply buried volcanics with considerable thickness.
    CO2 has an important role in diagenesis and metamorphism. The most significant effects of CO2 on very low-grade metamorphic parageneses include (i) decreasing the dehydration temperatures of Ca-Al and other hydrosilicates; and (ii) stabilizing carbonates at the expense of Ca- Al silicates.
    Phenomena related to alteration or very low- grade metamorphism are visible in the most Tertiary volcanic rocks of Saveh region. Hydrothermal and fumarole alterations, burial and thermal metamorphisms have produced in the temporal order in Saveh.
Keywords
Metabasites, Saveh

Subjects

 
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Scientific Quarterly Journal of Geosciences
Volume 5, 17-18
Autumn & Winter 1996, Vol. 5, No. 17-18
Winter 1996
Pages 40-53