Scientific Quarterly Journal of Geosciences

Scientific Quarterly Journal of Geosciences

Geochemistry and Tectonic Setting of Basaltic Lavas in Deh-Sard Area, South of Kerman: Implications for Melting and Enrichment Processes in an “Off-Axis” Environment

Document Type : Original Research Paper

Authors
Najmeh Sadat Mahdavi
Mohsen Arvin
Department of Geology, Faculty of Science, Shahid Bahonar University, Kerman, Iran
Abstract
The upper Jurassic to lower Cretaceous volcano-sedimentary rocks of  Deh-Sard  are  located  in southern part of Sanandaj-Sirjan  zone in the south of Kerman. They consist of intermittent lava flows (mainly basalt, basaltic-andesite and trachy-basaltic-andesite), pyroclastics (various types of tuffs and bereccias) and sedimentary rocks (limestones and shales).
The Deh-Sard basalts composed essentially of plagioclase and clinopyroxene microphenocrysts, set in a matrix of the same mineralogy  associated  with opaque  minerals and apatite as accessory phases. The existence of disequilibrium textures in Deh-Sard basalts reveals the rapid changes in the pressure of rising magma.
The pyroclastic rocks are less common in the area and consist mainly of crystal litic tuffs, in which crystals are mainly plagioclases and lithic fragments mainly  basaltic-andesites and trachy-basalt-andesites. The sedimentary rocks were formed in a  shallow marine environment.    
The Deh-sard basalts shows a selective enrichment of elements with low ionic potential (eg., K, Rb, Sr, Ba,+ Th) and  a depletion in elements  having high ionic potential (eg., Nb, Ce, P, Y, Cr)compared to N-type MORB. Based on the geochemical data, trace and REE element diagrams, the Deh-Sard basalts show characteristics of transitional tholeiitic basalt. MORB and chonderitic normalized trace element patterns are believed to reflect that the Deh-Sard basalts have affinities toward subduction and within-plate basalts. A rift environment formed on transitional continental-oceanic crust “off-axis”  from the spreading center is suggested for the formation of Deh-Sard basaltic rocks. In this case, off-axis magmas do not represent the same range of chemical variation as magmas delivered to the axial system, but they show similarity with seamount-type volcanism. The occurrence of intermittent limestones and lava flows may suggest that Deh-Sard basalts might have formed in a rift environment on transitional continental-oceanic crust “off-axis” from the spreading center.
Keywords
Deh-Sard
transitional basalt
subduction zone
off-axis
within-plate

Subjects

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Scientific Quarterly Journal of Geosciences
Volume 14, Issue 53
Autumn 2004, Vol. 14, No. 53
Autumn 2004
Pages 88-99