Geology and petrology of intrusive rocks: Eastern Najmabad, Gonabad


Department of Geology, Ferdowsi University of Mashhad, Iran


The study area is located in 15 Km southeast of Gonabad. Regional metamorphism (Jurassic-Cretaceous orogeny) has turned shale and sandstone of Jurassic into slate and quartzite. Slates are metamorphased into different schists around the contact of granodiorite. Biotite granodiorite porphyry batholith intruded Jurassic slate and quartzite with east-west trending. Based on mineralogy and low values of magnetic susceptibility [(5 to 11) × 10-5 SI], therefore, it is classified as belonging to the ilmenite-series of reduced type granitoids (S-type granitoid). Chemically, biotite granodiorite is met-aluminous, and show enrichment in LILE = Rb, Ba, Zr, Th, Hf, K and REE = Ce, Sm and depletion in Sr, P and Ti. Biotite granodiorite is characterized by medium light rare earth element (LREE) enrichment and less low heavy REE (HREE). Based on REE content and low (La/Yb)n = 7-11.5, magma originated from continental crust. Hornblende monzonite porphyry, hornblende biotite monzonite porphyry and biotite monzonite porphyry which are younger than granodiorite are identified. Based on mineralogy and high values of magnetic susceptibility [(>500) × 10-5 SI], therefore it is classified as belonging to the magnetite-series of oxidant type granitoids (I-type granitoids). Monzonite porphyries are characterized by medium light rare earth element (LREE) enrichment and high low heavy REE (HREE). Based on REE content and (La/Yb)n= 9-64, magma did not originate from continental crust. Alteration zones associated with granodiorite are sericite, propylitic, silicified, argillic and tourmaline. Based on geology, geochemistry, alteration zoning and magnetic susceptibility, biotite granodiorite has potential for W-Au-Sn mineralization.


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