Investigation of replacement and flow direction of granitic body at Golpaygan region using anisotropy of magnetic susceptibility (AMS)

Document Type : Research

Authors

1 M.Sc. Graduated, Department of Geophysics, Research and Science Branch, Islamic Azad University, Tehran, Iran

2 Ph.D. in Geophysics, Geological Survey & Mineral Exploration of Iran, Tehran, Iran

3 Assistant Professor, Department of Physics, Razi University, Kermanshah, Iran

Abstract

The Golpaygan massive granite in the northern part of Golpaygan city is a part of Sanandaj – Sirjan zone. This granitic body has been intruded in surrounding metamorphosed schists of Paleozoic age. The Paleocene age (58 Ma) with K/Ar method has been assigned for this granite. Basalts, Porphyritic tuffs and Cataclastics volcanic rocks are the main rocks of this formation. The major minerals of the granite are Quartz, acidic to intermediate plagioclase (oligoclase, andesite) and orthoclases which occasionally show pertitic texture. The micaceous minerals include biotite, muscovite and sericite. In order to study AMS of Golpayegan granite, 171 cores with 10 cm length and 2.5 cm diameter were collected with drilling portable machine. The dip and azimuth of the cores were measured with magnetic compass. Each core was cut to 22 mm  length in the paleomagnetic laboratory of geological survey of Iran. Bulk samples were also collected in order to examine rocks petrogically and mineralogically. The polished thin sections show the following metallic minerals: Rutile and Anatase, Oxides and oxyhydroxides, Hematite, Pyrite and Ilmenite. Anisotropy of magnetic susceptibility (AMS) is defined as a second order tensor. Due to symmetry of nondiagonal components, only diagonal ones K33, K22, K11 remain which are named as Kmax, Kmin and Kint. Lineation intensity values show alignment of magnetic dipole moments of the specimens. This parameter is maximum for sites 8, 12 and 17. The dip and direction of lineation parameter of the above sites are 261.5/44, 38.2/79 and 22/17 respectively. The dip value of site 12 , i.e. 79, may indicate place of the source of Golpayegan granite. The direction of lineation in sites 17 and 8 are opposite to each other which may indicate the existence of a fault in this area. The map of foliation parameter shows that in the central part of the study area, the dip of foliation has much more value than the surrounding area. The shape factor values are negative in the north east and center of the granite body which indicate prolate shape of magnetic susceptibility ellipsoid while in the other parts it is positive which means it is oblate. The AMS results also reveal that the study area can be divided in to two parts which have separate convergence directions. The diffusion directions may indicate mushroom type of the granite intrusion at two phases. The main mushroom type granitic body has intruded at the first phase and then in the second phase, another granitic body with the same pattern is injected into the main body. The AMS directions of the granite show northwest- southeast and northeast- southwest trends while at the center, they show on east-west trend. On the basis of the interpretation of total granitic body directions, we propose the existing of a probable fault with north-south trend at the center of the granite. The intensity of anisotropy of magnetic susceptibility at the western side of this fault is high in comparison to that of the eastern side. The occurrence of this fault can also be proved by petrological investigation and other studies. The susceptibility-temperature analysis of the granite rocks shows that magnetite and hematite are the main magnetic carriers which may indicate I or A type origin of this massive granite.

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