Mineral chemistry, thermobarometry and petrogenesis of Band-e-Hezarchah dike swarms (southeast of Shahrood)


1 Department of Petrology, School of Geosciences, Shahrood University of Technology, Shahrood, Iran

2 دانشگاه صنعتی شاهرود - دانشکده علوم زمین

3 State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Science, Beijing, China and State Key Laboratory of Continental Dynamics, Northwest University, Xian, China

4 شاهرود- بلوار دانشگاه- دانشگاه صنعتی شاهرود- دانشکده علوم زمین


The Band-e-Hezarchah dyke swarms is located in the northern margin of Central Iran structural zone. These diabase dyke swarms have crosscut the late Neoproterozoic – early Cambrian granitoids pluton, metapelites, meta psammites and gneissic rocks and late Triassic – early Jurassic conglomerates, sandstones and shales (equal to Shemshak formation in Alborz zone) but they haven't cut the younger rocks such as lower Cretaceous (Neocomian) limestones. Plagioclase, pyroxene and opaque minerals are the main rock-forming minerals. Plagioclase is mostly albite, oligoclase, andesine and labradorite with An=1.3-50.5, Or=0.6-6.1 and Ab=48.3-98.18. Pyroxene with Wo=36.7-44.4, En=37.3-47.3 and Fs=11.8-21.2, is augite. Amphibole and chlorite are main secondary minerals. Amphibole is mostly magnesio hornblende, with Si = 6.5-6.8 a.p.f.u., XMg=0.65- 0.71 and A-site =0.17-0.39. Chlorite with Si=4-5.53a.p.f.u. and XFe=0.84-0.91 is mostly daphnite and pseudothuringite. Temperatures and pressures obtained from clinopyroxene thermobarometry are in the range of 1100-1190°C and 2-7.2 kbar respectively. Based on the results of bulk rock trace and rare earth elements, these rocks have alkaline to subalkaline nature and their magma forming originated by 8-10% partial melting of a spinel lherzolitic source. These rocks have formed in an initial back arc basin setting related to subduction of Neo- Tethys oceanic lithosphere beneath the Central Iran microcontinent during Middle Jurassic. This magmatic event is a consequent of middle Cimmerian orogeny.