modern research physics

---

Intrusions between Arak and Borujerd in northern Sanandaj Sirjan zone are present different field and lithology. Three different groups: (a) the Astaneh intrusion, (b) the Borujerd complex, and (c), Tavandasht-Gosheh intrusions can be identified. First attempt to date these intrusions began with K-Ar method and obtained ages were about 60Ma based on studies on Alvand in Hamadan. Further Rb-Sr dating, published older ages and revealed two phases 60Ma and -120Ma for Borujerd complex and 100Ma for Astaneh intrusion. Following dating based on zircon, established the age of 170Ma for all unites. In order to cover gaps in data, we have undertaken MS U-Pb zircon geochronological studies for selected granitoids. Results confirm the presence of granitoids with 170Ma in Astaneh and Borujerd complex. But, for first time, the young age of 34.8Ma initiated for Tavandasht intrusions. It could be concluded that emplacements of plutons in this part of Sanandaj Sirjan began in Mid Jurassic (Bathonian stage) and last phase was on Late Eocene (Priabonian stage). However, based on the presence of field and geochemical differences between intrusions, more study needs to find the interval phases. Emplacement of these intrusions are related to calk alkaline subduction of Arabian plate with Central Iran plate, and obtained data, could assist on timing of offered tectonomagmatic models.

---

The Alut granitoid complex is located in the northwestern part of the Sanandaj–Sirjan zone. This complex comprises three main units i.e. monzogranite to granodiorite bodies (SiO2 = 65–77 wt %) which is widespread throughout the area, a quartz-diorite to tonalite unit (SiO2 = 52–63 wt %) exposed as two stocks and mylonitic granitoid scattered as separate outcrops through the area. NE-SW trending microquartz-diorite and NW-SE trending aplite dykes are also present in this complex. The quartz-diorite to tonalite unit has relatively high CaO,FeO, MgO, Al2O3 and low Rb/Sr, Sr/Y, and (La/Yb)N, which preclude an origin of variably fractionated mantle melts and favour a mafic lower crustal source. Dehydration melting of biotite-bearing metasedimentary sources at relatively low pressures is proposed for the origin of monzogranite to granodiorite unit. Geochemically this complex is metaluminous to slightly peraluminous, typical of I-type granites. It belongs to low K (and high K for some monzogranite samples) calc-alkaline series and displays the geochemical characteristics typical of volcanic arc plutons related to an active continental margin area (e.g. significant Nb, Ti, P and Sr depletion). Based on collected geochemical data, the Alut granitoids originated by partial melting of crustal protoliths having different compositions in a deformed active margin.

---

The granitoidic masses from northeast of the Golpayegan area were intruded in a metamorphic complex of Barrovian type and the some area caused contact metamorphism in the adjacent rocks. These masses in the QAP triangular diagram consist of tonalite, granodiorite, granite, alkali feldspar granite, monzogranite, quartz diorite, quartz monzonite and quartz monzodiorite. The aplitic and pegmatitic dykes can also be observed on the rim of these masses. The textures of granitoids are mainly based on primary textures [1)granitic texture, 2)poikilitic texture, 3)granophyric texture] and secondary textures[1)perthitic intergrowths, 2)myrmekite, 3)chloritization 4)sericitiza-tion, 5)kaolinitization]. Fabric and mineralogical study of granitoids show that the presence of k-feldspar mega crystals and rounded zircons could probably indicate the characteristics of S-type granitoids or granitoids with crustal origin. But presence of granophyric texture in some of tonalites shows that these masses are plagiogranite type which is considered as I-type granitoids or the granitoids with mantle origin. Presence of mylonitic texture in some other granitoids which have outcrops in the northeast Golpayegan (north of Varzaneh and Sfajerd villages) show that these masses were formed during an orogenic phase (Laramid). Based on studies conducted on conodont fossils obtained from crystallized calc schists and acrytarge in slates of the area which are introduced for the first time in Sanandaj-Sirjan Zone, the age of the metamorphic rocks, that previously were documented as Precambrian, was reviewed and show that they belongs to Paleozoic. Thus, the age of granitoids from the north of Golpayegan was considered younger than late Jurassic and older than Oligocene. In this paper, based on field observations and paleontological studies, the age of granitoids from the north of Golpayegan emplacement were documented for the first time. Thus, throgh studing the texture and age of the granitoidic masses from the Golpayegan area, it could be concluded that some of these masses (granophyric granitoids or I-type) were formed during Jurassic and initial Cretaceous and synchronous with subduction and before continental collision, and some other masses (mylonitic granitoids) were formed during continental collision and synchronous with the end of deformations at the end of orogenic events. Some others (often S-type granitoids), finally, were formed in late Cretaceous-Paleocene after collision of the Arabian plate with the Iranian plate at the end of Cretaceous.

---

Major cations, Fe, Mg and Al, in biotite are sensitive to magmatic processes and biotite chemistry could be used for petrogenetic investigation of granitioids. In this research composition of biotite from Boroujerd granitoids in Sanandaj-Sirjan Zone has been investigated in order to study its geochemical nature and petrogenesis. Separated biotites were analyzed by WDXRF and biotites in polished thin section were analyzed by EPMA. Samples were selected from Older granites (120 Ma) and Younger granites (60-70 Ma) after intensive petrographic examinations. Data from both techniques are in good agreement and discriminate two generation of granites clearly. Based on Mg, Mn, Fe3+, Fe2+, Ti and Al biotites in Younger granite incorporated more Mg than those in Older granites. The main differences between biotites in two generation of granites are amount of total Al and Fe/Fe+Mg ratio. Biotites in granodiorite and quartz diorite of Younger age are annite-phlogopite with 2.48-2.74 Al per formula unit, and Fe/Fe+Mg ratio of 0.375-0.600. Biotites from Older granites are annite-sidrophyllite with 2.76-3.69 and Fe/Fe+Mg ratio of 0.597-0.720. Composition of biotites indicates that two generation of granites formed under different oxygen fugacity and both are calcalkaline and shows I and S characteristics respectively.