QE209 : Petrology, geochemistry and geodynamic model of Soltan Maydan Basaltic Complex (SMBC), north of Shahrood
Thesis > Central Library of Shahrood University > Geosciences > MSc > 2014
Abstarct: Soltan Maidan Basaltic Complex (SMBC) crops out with a NE–SW trend in the eastern part of Alborz mountain range in northwest to northeast of Shahrood. This complex consists of basaltic lava flows, basaltic agglomerates, tuffs and some thin sedimentary intercalations that accumulated during the magmatic activity. In this study, with regard to the extent and thickness of the basaltic complex, three main and proper outcrop of this complex in the near of villages of Nekarman (Kaludar valley), Galeh-e-Nowkhareqan (Cheshmeh-Seyed valley) and Khoshyeilaq were selected and scrutinized. Soltan Maidan Basaltic Complex lies conformably on the Ghelli Formation and is covered with disconformity by Padeha Formation. Careful measurements show that thickness of this complex is 436, 1286 and at least 880 meters in the Kaludar valley, Cheshmeh-Seyed valley and east of the Khoshyeilaq village, respectively. Generally in the geological maps and various sources the age of Soltan Maidan Basaltic Complex has been attributed to the Silurian and also its formation environment has been considered as submarine. But the study of fossils found in the shale intercalations of this complex show late Ordovician to early late Silurian age for it. Also, various evidence such as presence of green shale-siltstone interlaxyer’s with marine palynomorphs and brachiopods, pillow lavas, columnar joints and thin interlaxyers of red sandstones in different parts of this complex show that it was accumulated in both subaerial and submarine environments. Petrographic studies show that Soltan Maidan basalts are relatively uniform in mineralogy composition and they generally have been undergone slight to severe alteration degrees. In these rocks, labradorite plagioclase and augite clinopyroxene are major minerals, and Fe-Ti oxides and sometimes olivine form accessory minerals. Also chlorite, epidote, calcite, quartz, sphene and Fe-Ti oxides are the most important secondary minerals and chloritization is the most predominant alteration in this complex. The most important textures in the Soltan Maidan basaltic rocks include porphyritic with microlitic and/or microlitic-glassy groundmass, microlitic, seriate, trachytic, ophitic, sub-ophitic, intersertal, amygdaloidal and glomeroporphyritic. Mineralogical and textural properties of studied rocks show emplacement of magma in magma chamber/chambers and develop of them through fractional crystallization before the final ascent. Microprobe results indicate liberate calcium and import sodium to composition of most plagioclases and in spite of remains a few amount of labradorite (An56to An64), plagioclases generally have undergone albitization process (An1-2). Despite the altered plagioclase, clinopyroxene generally are not altered. Thermobarometry results on the clinopyroxene indicate that they have crystallized in temperatures between about 1100oC to 1200oC, pressures lower than 6 kbar and show that they have crystallized in magma chamber/chambers located at depths lower than 23 kilometers. Geochemical evidence indicate transitional to alkaline nature of the primary magma, genetic relationship between all rocks of this complex via fractional crystallization and no or weak role of magmatic contamination in magma evolution. Also different geochemical evidence and diagrams show generation of primary magma by about 10-20 percent partial melting of an enriched garnet peridotitic mantle source and depths of about 100 to 110 kilometers in an extensional intra-continental rift setting. Elemental and isotopic ratios in Soltan Maidan Basalts indicate that they generated from anasthenospheric mantle source with some depletion and then enriched as a result of the mixing and interaction with the sub-continental lithospheric mantle (SCLM). So that stops mantle plume beneath the lithosphere and interactions between plume and sub-continental lithospheric mantle has provided the appropriate conditions for the source mixing. In addition to Soltan Maidan basalts, the lower-middle Palaeozoic basic magmatism in the study area is seen with smaller volume as dyke, sill and lava in the Ghelli, Padeha and Khoshyeilaq formations. Basic rocks in the Ghelli Formation and Devonian rock units have transitional-alkaline and alkaline nature, respectively, and similar to Soltan Maidan basalts were formed in intra-continental rift setting. Magmatic nature and tectonic setting of basaltic rocks in the studied area with the presence of many disconformities and absence of any angular unconformity in the Ordovician to Devonian rock units, indicating the persistence of a long-term extensional intra-continental (rifting) condition and upright tectonic movements (vertical) in this part of the Alborz. Also, evidence indicates emplacement of a shallow granitic pluton, simultaneous with Soltan Maidan Basaltic Complex in the study area. Coarse fragments of this granite can be seen in the conglomerate intercalations of this complex. Evidence suggests that short period between crystallization of granitic magma in deep to exhumation, erosion and the company of coarse fragments in the conglomerate intercalations is related to emplacement of granitic pluton in the relatively low depths and also severe uplift in the study area at this time. Since petrographic evidence and the results of thermobarometry on the Soltan Maidan basalts suggest hold of primary magma within the magma chamber/chambers located at depths less than about 23 km in the crust, probably hold large amounts of hot mantle magma in the shallow depths, and the transfer of latent heat of crystallization, leading to melting of crustal materials and the formation of granitic magma in these depths (bimodal magmatism). Soltan Maidan basalt properties such as the nature, thickness and tectonic setting of eruption are very similar to the flood basalts and large igneous provinces (LIPs). It appears that due to the high volume of magmatism in the study area, intra-continental rift setting, as well as relationship between magmatic activity with presence of mantle plumes and also near the eruption location to the birth place of Paleo-Tethys oceanic crust, this magmatic activity has played an effecting role in breakup and separation of continental lithosphere. Also, many similarities between Soltan Maidan Basaltic Complex and LIPs can be indicative of the fact that this huge basaltic complex may be part of a LIP that mainly is gone as a result of factors such as erosion, continental breakup and/or subsequent subduction.
#Soltan Maidan Basaltic Complex #Lower Paleozoic #Rifting #Paleotethys #Shahrood.
Keeping place: Central Library of Shahrood University
Keeping place: Central Library of Shahrood University