N Shirdashtzadeh; G Torabi; R Samadi
Abstract
Some dark green fragments of amphibolites are found within skarns at the south of Nain Ophiolite at the northeast of Nain city. They are similar to ortho-amphibolites (metamorphosed basic rocks) of this ophiolite in hand specimen, but mineralogically they are composed of amphibole (magnesio-hornblende, ...
Read More
Some dark green fragments of amphibolites are found within skarns at the south of Nain Ophiolite at the northeast of Nain city. They are similar to ortho-amphibolites (metamorphosed basic rocks) of this ophiolite in hand specimen, but mineralogically they are composed of amphibole (magnesio-hornblende, Mg#>0.95), clinopyroxene (diopside, Mg#~0.61), garnet (grossular – andradite, with Grs~63-87 And~12-35), quartz, and minor amount of calcite and wollastonite. Accessory minerals are including chlorite and prehnite, mostly filling the fractures. Field studies, petrography and mineral chemistry indicate that amphibolitic fragments mineralogically differ from the skarns and ortho-amphibolites of this ophiolite; so they can be considered as olistoliths with sedimentary origin (calcic marls of sea floor), turned into amphibolitic rocks (para-amphibolite) during the regional metamorphism at amphibolite - granulite facies and low oxygen fugacity.
R Samadi; N Sh; H Kawabata
Abstract
Plutonic suite of Khajeh Morad at southeast of Mashhad includes granodiorites, which are cut across by younger aplite, granitic pegmatite dykes and monzogranites. Aplites are mineralogically including quartz, feldspar (albite to orthoclase and microcline), muscovite, and accessory minerals of garnet ...
Read More
Plutonic suite of Khajeh Morad at southeast of Mashhad includes granodiorites, which are cut across by younger aplite, granitic pegmatite dykes and monzogranites. Aplites are mineralogically including quartz, feldspar (albite to orthoclase and microcline), muscovite, and accessory minerals of garnet (almandine-spessartine), tourmaline, biotite, and ilmenite. Pegmatites are composed of quartz, feldspar (albite to oligoclase, orthoclase, and microcline), muscovite, and minor amounts of garnet (almandine-spessartine), tourmaline, ilmenite, beryl, and columbite. Based on mineralogical and geochemical evidence, Khajeh Morad pegmatites are related to Li-rare elements (RE-Li) and lithium-cesuim-tantalum (LCT) pegmatite family. According to the field evidence and whole rock geochemistry, origin of garnet-bearing aplite-pegmatite melts could be related to the S-type monzogranites, as their differentiation products at late stages, occurred in a continental collision belts.
GH Abbasi; M Ebrahimi; M Sharifi; N Shirdashtzadeh; J Ahmadian
Abstract
The studied volcanic rocks to the northeast of the Gavkhuni playa lake are composed of rhyolite, trachydacite, trachyandesite and pyroclastic rocks including lithic tuff and volcanic breccia. Rhyolitic rocks have flow structure and hyalophyric texture with glassy to cryptocrystalline ...
Read More
The studied volcanic rocks to the northeast of the Gavkhuni playa lake are composed of rhyolite, trachydacite, trachyandesite and pyroclastic rocks including lithic tuff and volcanic breccia. Rhyolitic rocks have flow structure and hyalophyric texture with glassy to cryptocrystalline matrix in which quartz and feldspar crystal are present. The dominant texture in trachydacitic and trachyandesitic rocks in porphyritic texture in which plagioclase and rarely sanidine phynocrysts are set in a glassy to microcrystalline groundmass containing feldspar microlites. In trachydacites, quartz is present as small crystals and rarely as microphynocrysts. Plagioclase phynocrysts in trachydacitic and trachyandesitic rocks display sieve-texture. Amphibole and biotite are the mafic minerals of the volcanic rocks and they are partialy, at rims, or completely altered to Fe-Ti oxides. All of the studied rocks are high- K calc-alkaline in nature. According to the geochemical data, the volcanic rocks are depleted in high field strength elements such as Nb, Ti and Ta and enriched in large ion lithophile elements like Cs, K, Ba, Rb and Th which are characteristics of subduction related volcanic rocks. Nb negative anomaly is characteristic of continental rocks and indicates the continental crust involvement in the magmatic processes. The rocks are enriched in Cs, Rb, Ba and Pb indicating mantle metasomatism by fluids released from subducting oceanic lithosphere. Therefore, magma contamination and mantle metasomatism have affected the magma from which the volcanic rocks of the Gavkhuni playa lake have were generated. The calc-alkaline magmatism in the study area was associated with the closure of the Neotethyan Ocean.
N Shirdashtzadeh; Gh Torabi; R Samadi; T Meisel; S.N Hussain Bokhari
Abstract
Picrites are seen within the mantle peridotites in the Darreh Deh area (in the east of Nain ophiolitic mélange). Based on petrography and mineral chemistry, these picrites are composed of olivine (chrysolite), clinopyroxene (diopside) and orthopyroxene (enstatite), showing a cumulate texture, ...
Read More
Picrites are seen within the mantle peridotites in the Darreh Deh area (in the east of Nain ophiolitic mélange). Based on petrography and mineral chemistry, these picrites are composed of olivine (chrysolite), clinopyroxene (diopside) and orthopyroxene (enstatite), showing a cumulate texture, and small amounts (less than 10 volume%) of plagioclase and chromian spinels as accessory minerals. Application of clinopyroxene geothermobarometry methods indicates the equilibrium temperatures of ~1040 to 1205 °C and pressure of 8.6 kbar for the clinopyroxene crystallization in the melt. Amphibole and talc crystallization and the results of amphibole thermobarometry (~675-700 °C, 4.8-6.5 kbar) indicates that these rocks have undergone metamorphismunder amphibolite facies in Jurassic, similar to their host mantle peridotites. Whole rock chemistry and chromian spinel composition confirm high partial melting degrees of mantle and progressive ascending melt/wall rock reactions in a suprasubduction zone in which tholeiitic melts were produced. During the production of this melt, picrites with cumulous texture formed within the mantle peridotites by precipitation of olivine, pyroxenes and minor amounts of intercumulous plagioclase crystals from the primitive melt.
