QE32 : The study of mafic magmatism in the baxse of Shemshak Formation in the eastern Alborz zone
Thesis > Central Library of Shahrood University > Geosciences > MSc > 2010
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Abstarct: In the baxse of Shemshak Formation in Alborz zone, basic igneous rocks have been
reported as lavas. However, precise investigations of geological outcrops in different
parts of eastern Alborz show that these rocks in Gheshlagh (Khosh Yeilagh area),
Tazare, Talo and Kalate Rodbar (in north of Damghan) regions are intrusive bodies as
same as sill, dike and small stocks and in Cheshmeh Ali region are lavas. baxsed on,
field and petrography observation, Gheshlagh basic intrusion as a fractionated sill (with
about 100 meters thickness) intruded into the baxse of Shemshak Formation and has a
complete fractional sequence of olivine gabbro, gabbro, diorite to monzonite from up to
the bottom. Talo basic pluton, intruded in the lower part of the Shemshak Formation as
same as small bodies and continued as sill or dikes parallel or cutting the shale and
sandstone laxyers respectively. The existence of a fractional sequence of olivine gabbro
to diorite has been proved in field and petrography observation on the igneous rocks.
Petrography characteristics of Gheshlagh and Talo region are same. Plagioclase,
clinopyroxene and hornblende are the most abundant minerals in these rocks and typical
textures are accumulated, granular, microgranular and ophitic. The presence of
phlogopite, apatite and primary and secondary sphene in these rocks suggest high
contents of TiO2 and P2O5 and alkaline nature of parental magma. Igneous rocks in
Tazare and Kalate Rodbar region, has been observed in the form of sill and diabasic
dike, or gabbroic intrusion in shale and sandstone laxyers of Shemshak Formation. These
rocks have intergranular and ophitic texture and contain plagioclase and clinopyroxene
phenocrysts and apatite microcryst. Basic rocks in Cheshmeh Ali region are alkali
olivine basalts and have been strongly altrated. These rocks are composed of
plagioclase, pyroxene and olivine in microcrystal matrix and show porphyry, intersertal
and gelomeroporphyry texture. The samples of Cheshmeh Ali, Tazare and Kalate
Rodbar regions have no petrographic variations, therefore, variations diagrams of these
regions do not show special trends in constant to Gheshlagh and Talo regions, where
petrographic varieties are abundant and differential trends can be seen. Identification
diagrams of magmatic series and tectonic setting discrimination diagrams have been
proved an alkaline nature and intracontinental setting for these rocks. Enrichment in
LREE and high incompatible and depletion in HREE are obvious in chondrite and
primitive mantle normalized spider diagrams. Parallel trends of REE variations and Pbpositive
anomaly associated with patterns of compatible and incompatible variation
diagrams, indicating an unique source for these igneous rocks and the main role of
fractional crystallization in the evolution of magma, although, minor amounts of
assimilation and contamination of magma by crustal rocks occurred also. These
signatures indicate that this alkaline magma has been formed by low degree partial
melting (10-15 %) of an enriched sub-continental lithospheric garnet-lerhzolite mantle
in an extensional intracontinental environment. This tectonic setting corresponds with
the extensional regimes governed on fluvial-deltaic basins of upper Triassic-lower
Jurassic passive continental margins in Alborz.
Keywords:
#Mafic magmatism #Shemshak Formation #eastern Alborz. Link
Keeping place: Central Library of Shahrood University
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Keeping place: Central Library of Shahrood University
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