GEOLOGY, ALTERATION, AGE DATING AND PETROGENESIS OF INTRUSIVE BODIES IN HALAK ABAD PROSPECT AREA, NE IRAN

The Halak Abad prospect occurs in the northeastern part of Central Iran zone (Sabzevar structural zone). In this investigation, geochemical evolution, age and source of part of northeastern Iran magmatic arc (intrusive bodies) in Halak Abad area in the Khorasan Razavi province has been studied. The exposed rocks consist of volcanic rocks with andesite and dacite nature, limestone, plutonic rocks mostly diorite, quartz diorite, monzodiorite, quartz monzonite, granodiorite and granite and sedimentary rocks such as limestone, sandstone and conglomerate. Magnetic susceptibility of intrusive rocks is >100 × 10-5 SI, so they belong to the magnetite-series (oxidized). This magmatism is mainly low-K (tholeiite series) and meta-aluminous. The amounts of Zr, Th, Nb and Ti show depletion compared to N-MORB. Trace elements behavior shows a nearly flat pattern. Age of granodiorite body based on U-Pb zircon dating is 99.7±1.8 Ma (Mid-Cretaceous) and 87Sr/86Sr initial ratio is 0.7047. The geochemical signature and 87Sr/86Sr initial ratio in the area suggest volcanic arc magmatism in subduction zone. This magmatism has characteristic such as high Na2O (3-7 %), low K2O (0.12-1 %), high CaO (4-5.7%), low Rb (1-20 ppm), low total REE (<40 ppm), high Ba/Nb, Sm/Yb<2, (La/Yb)N<5, 87Sr/86Sr initial ratio < 0.7045 and eNd: +4.5 show differences with normal granitoids in subduction zones. Geochemical and petrological characteristics indicate melting in relatively low pressure (shallow depth). The lines of evidence demonstrate that formation of this granitoid needs a suprasubduction zone.