جستجوی مقالات مرتبط با کلیدواژه « گلپایگان » در نشریات گروه « زمین شناسی »

The Mohr circle is a well-known method for two-dimensional strain analysis. Due to the formation of the boudins during two stages of the progressive deformation (D1 and D4) in cherty marbles, from north Golpaygan metamorphic complex, they have been analyzed for strain in two-dimension. This analysis is based on estimating of the initial length of the boudins before deformation, and measuring the current length of the boudins, after deformation. Calculation of the longitudinal parameters of the strain and drawing Mohr circles show that the strain ratio varies between 1.06 to 2.35 for D1 stage, and between 1.10 to 2.84 for D4 stage. The mean shear angle for the D1 stage is 22 ° and for the D4 stage is 24 °.

Modeling of geological structures plays an important role in understanding the geometry of the structures and their relationships. In recent years, digital modeling using computers has attracted the attention of engineers as well as researchers in basic sciences. "MATLAB" is one of the softwares that is used with its extensive facilities for data analysis and modeling in various sciences, including structural geology. In this paper, possible models of interference patterns of the three generations of folds in metamorphic rocks of the Golpayegan area were reconstructed using the script written in MATLAB software. The required data include the attitudes of the mean axis and the mean axial planes of three consecutive generations of folds which were obtained during field measurements. Field studies show that the first and second generation folds are almost coaxial, however the third generation folds have a different axial trend. The results of modeling indicate that four classical fold interference patterns may be formed in horizontal (map view) and vertical sections in Golpayegan metamorphic rocks. Modeled fold interference patterns are closely consistent with the natural fold interference patterns observed in the outcrop and regional scales. Using the modeling, it can be determined that the different interference patterns of the folds in the northern Golpayegan metamorphic rocks are related to superposition of the related fold generations.

The Aderba leucogranite in the Golpayegan metamorphic core complex (GMC)¡ a part of Sanandaj-Sirjan zone¡ host lentiform small (2*4 cm)(Type 1) and large (7*14 cm)(Type 2)tourmaline nodules. In terms of mineralogical features¡ the core of these two types tourmaline nodules is different. The Type 1 composed of small blue-green tourmaline¡ quartz¡ K-feldspar (microcline) and apatite while the Type 2 is characterized by tourmaline coarse crystals accompanied by quartz. Based on major and trace elements data the tourmalines under discussion are classified as alkaline¡ schorl (Type 1) and schorl-dravite (Type 2). The mean REE values displays a negative slope and a negative (Type 1) and positive (Type 2) Eu anomalies. The overall petrographic observations and geochemical results indicate that the Type 1 is likely influenced by two mechanisms of liquid immiscibility in the evolved melt followed by biotite breakdown in the final stages of tourmaline crystallization to complete consumption of B (closed system). For nodules¡ Type 2¡ breakdown of biotite in equilibrium with the external fluid (open system) is proposed.

A NW-SE trending ductile shear zone has been generated in the metamorphic rocks of the southwest Golpaygan. Different pellitic and psammitic schists, meta-carbonates and igneous rocks were strongly deformed in this ductile shear zone and produced mylonites and ultra-mylonites. Structural analysis indicates three stages of foliations in the metamorphic rocks. Geometry and kinematics of the fabrics in Nowgan shear zone are divided into two northeastern and southwestern parts (limbs of Nowqan antiform). Mylonitic foliation moderately to steeply dip towards northeast in the northeastern part but dips to the southwest in the southwestern part. Mineral and stretching lineation, are shallowly to moderately plunging to the east-southeast in the northeastern part of the shear zone and, to the west-northwest in the southwestern part. The microstructural indicators of shear sense cleared that the northeastern part dextrally displaced along strike with normal component and the southwestern part sinisterly displaced with reverse component at the present situation. The fabrics evidence clear that this ductile shear zone were originally right-lateral strike–slip shear zone and during its structural evolution it was rotated around its strike during later folding stage. Structural analysis of the surrounded rocks of the shear zone indicates three superposed foliations. The mylonitic foliation in the shear zone and the axial plane foliations of the second stage folding are sub-parallel. Plunge directions of the second stage folds axes and the mineral/stretching lineation are also sub-parallel. Therefore, the initiation and development of the shear zone were synchronous with the second stage folding event.

Two main metamorphic events have occurred within Muteh- Golpayegan metamorphic core complexes (including eastern and western complexes) in the Sanandaj-Sirjan zone. Some grain-scale deformations were also happened in relation to these metamorphisms which have produced various deformation microstructures. Microfaults are among the microstructures that were formed during brittle conditions. Bulging recrystallization of quartz and rarely feldspar grains, and bookshelf structure of fragmented feldspar porphyroclasts indicate various deformation and formation of shear zones under low-temperature conditions in the eastern complex. Polygonal granoblastic texture of polycrystalline quartz porphyroclasts documents post-mylonitization annealing at medium- grade conditions in shear zones of eastern complex. Chessboard pattern quartz subgrains propose high-grade metamorphic conditions during the first deformation. However, their undulatory extinction reveals low-grade conditions for the second deformation. Polygonal granoblastic texture of chevron folded feldspar grains also accurately supports these conditions during the two mentioned deformations. In general, considering deformation microstructures and conditions, three stages of recrystallization are respectively recognizable in the Muteh-Golpayegan metamorphic complexes including: 1- high-grade static recrystallization subsequent to early metamorphic event, 2- dynamic recrystallization and related mylonitization under low-grade to locally medium-grade conditions, 3-post-mylonitization static recrystallization in medium-grade conditions. The north Golpayegan intrusive bodies can be considered as the heat source for annealing in the western complex but the cause of annealing is not evident in the eastern complex.

Garnet-micaschists from Haji Qara high, north of Golpayegan, are part of the metamorphic rocks of the Sanandaj-Sirjan Zone (SSZ). Lepidoporphyroblastic and porphyropoikiloblastic are the main textures and the mineral assemblages consist dominantly of Fe-biotite, muscovite and ferriphengite, metamorphic Fe-Mg chlorites (I-type and IIb), garnet and quartz. Staurolite, andalusite, plagioclase carbon, Fe-oxides and tourmaline are present as accessories. Petrographic evidences as well as mica chemistry along with other paragenesis and thermometric estimations indicate a path change from amphibolites facies to lower pressure and higher temperature gradient suggesting the presence of a local pluton which led to garnet porphyroblast, biotite and muscovite (overprinted in different orientation) and andalusite crystallization. The occurrence of chlorite, ferriphengite and Fe-oxides demonstrate a retrograde metamorphism during uplift and decompressional cooling path in the area.

Extensive mylonitic fabrics are observed in the north Golpayegan metamorphic rocks in the Sanandaj-Sirjan zone. Earlier researchers did not present any evidence for the relative timing of mylonitic fabrics development. In this paper, the relation between folding and metamorphism events and formation of shear zones in the north Golpayegan are documented. The evidence is systematically presented at the microscopic, outcrop (mesoscopic) and map scales in order to study the relative timing of mylonitization event. The evidence at the microscopic scale indicates that the mylonitization occurred after the first stage amphibolite facies high-grade metamorphism, synchronous with the second stage greenschist facies retrograde metamorphism. The evidence at the microscopic and outcrop scales are compatible with each other and show the mylonitization event during the second stage (D2) folding, coeval with the greenschist facies metamorphism. Transposition of mylonitic foliations on the limbs of the second-generation microscopic and mesoscopic folds is documented. At the map scale, concordance of stretching lineations with second-generation fold axes, and folding of mylonitic foliations during the third stage of deformation also indicate mylonitization event during second stage (D2) deformation. Through a correlation of the isotopic ages, an Early Paleocene age is proposed for the mylonitization during D2 event in the north Golpayegan.

The North Esfajerd ductile shear zone is exposed in NE Golpaygan in the Sanandaj-Sirjan zone. The shear sense indicators are observed in the both outcrop and microscopic scales in this shear zone. These indicators are representing a NW striking dextral shear. The dynamic analysis and outcrop- scale indicators of extension direction exhibit a NW extension sub-parallel to the dextral shear and a compression perpendicular to it. The relative simultaneity and parallelism between the North Esfajerd ductile shear zone and North Varzaneh thrust shear zone propose the partitioning of strain components in a transpressional deformation. Regarding the geochronologic data and the angular unconformity between the middle Cretaceous and Eocene rock units, the transpressional deformation and its related mylonitization occurred during the Laramide orogeny in late Cretaceous-Paleocene. The gently dipping mylonitic foliations with dextral shear imply an incompatibility between geometry and kinematics in the North Esfajerd ductile shear zone. However, the kinematic and dynamic characteristics of the North Esfajerd ductile shear zone are accommodated with lateral extrusion of material in a dextral domain. Parallelism between the trends of dominant stretching lineations and the second generation large scale fold axes document that the major mylonitization in the North Esfajerd shear zone occurred during the second generation deformation. These mylonitic fabrics were folded due to the third generation deformation. Two generations of crenulation cleavages, respectively in relation to two refolding events, can be recognized in this shear zone. One of the refolding events with type III interference patterns (coaxial refolding) occurred during the second stage deformation in the late Cretaceous- Paleocene interval, caused the formation of the North Esfajerd shear zone, and can be observed in an outcrop scale. The other has emerged during the third stage of deformation, probably in the post Paleocene-pre Miocene interval, folded the North Esfajerd shear zone and formed the type II (boomerang shape) interference pattern in a map scale.

Structural and morphotectonical evidences of the Golpayegan Region show that there are two active faults systems parallel and normal to the Main Zagros thrust. The Shazand fault has 190 km length and is a major NW-SE-trending fault that stretched from northwest Shazand town toward southwestern Isfahan. Around Golpayegan town, the Shazand fault and its branch, Hosseinabad fault make an active area. These faults have reverse and right-lateral motions. The Shazand fault cuts and rotate the Quaternary alluvial fans, displaced Golpayegan and Khomein rivers about 4 km and uplift the Golpayegan depression. Increase of vertical erosion rate in recent years show that tectonically, the Golpayegan Region is active.