پترولوژی (Apr 2024)
Geochemistry, petrography, and tectono-magmatic setting of Eocene volcanic lavas in the south of Mamoniyeh, Urumieh-Dokhtar magmatic arc, Markazi Province, Iran
Abstract
The study area lies in the south of Mamoniyeh, a part of the Zaviyeh 1:100000 geological map, which covers the middle part of the Urumieh-Dokhtar magmatic arc. The volcanic phases and the intrusive masses constitute the predominant rocks of the area which has been subjected to magmatic and tectonic activities. Thus, owing to the lack of detailed studies on the volcanic rocks of this area, we try to link the tectonic setting and the magmatic evolution of the rocks under study. In addition, magma evolution processes, such as fractional crystallization, crustal contamination, and magma mixing may play an important role in the genesis of these rocks. The present paper presents new petrological and geochemical data on these volcanic rocks, which were formed during the Eocene. The Urumieh Dokhtar Magmatic arc is characterized by a series of volcanic and plutonic rocks that formed during the Late Cretaceous to Early Miocene, approximately 95 to 20 million years ago, and extends for about 2,000 kilometers.The magmatic rocks in the Urumieh Dokhtar Magmatic Arc are lavas, pyroclastic deposits, and plutonic rocks (i.e., granites and diorites) believed to have originated in the course of subduction of the Arabian plate beneath the Eurasian plate. which gave rise to the melting of the mantle. In this area, the Eocene volcanic units have been completely disrupted due to the influx of intrusive masses and high displacement by shear-compressional faulting. Volcanic rocks in the region with over 4 km thick are composed of flows, pyroclastic layers, tuff, and ignimbrite.Regional GeologyThe Urumieh Dokhtar Magmatic arc is characterized by a series of volcanic and plutonic rocks formed during the Late Cretaceous to Early Miocene, approximately 20 to 95 million years ago, extending for about 2,000 kilometers. The magmatic rocks in Urumieh Dokhtar Magmatic Arc include a variety of lithologies, such as lavas, pyroclastic deposits, and plutonic rocks (i.e., granites and diorites). These rocks were formed as a result of the subduction of the Arabian plate beneath the Eurasian plate, which led to the melting of the mantle and the formation of magma. In this area, the Eocene volcanic units have been completely disrupted due to the influx of intrusive masses and high displacement by shear-compressional faulting.Research methodologyPreparation of a 1:20000 geological map of the area, sampling of surface volcanic units, and drilled boreholes were carried out simultaneously. Based on lithological diversity, 50 samples of volcanic rocks were collected, and 15 of those with the least amount of alteration were analyzed by ICP-MS and XRF methods. Using GCDkit software, petrogenesis and the development of the studied volcanic rocks were investigated.PetrographyThe main rock-forming minerals are plagioclase, K-feldspar, quartz, amphibole, biotite, and pyroxene, with small amount of olivine, apatite, zircon, and Fe-Ti oxides. Minor minerals include opaques, quartz, carbonate, epidote, chlorite, sericite, apatite, sphene, and zircon, accompanied by rare tremolite and actinolite. Fine-grained and idiomorphic apatites as well as short prismatic zircon crystals are found as inclusions in plagioclase. and biotite. respectively. The predominant textures are clastic and porphyritic with trachytic and cryptocrystalline. Also. sieve intergranular, intersertal, hyalomicrolitic hyaloporphyritic, and microgranular are common textures.GeochemistryThe amounts of SiO2 and Al2O3 vary from 47 to 73 wt% and from 11.6 to 17.8 wt%, respectively. The total amounts of alkali elements in the investigated samples range from 4.5 to 10.1 wt%. As the different charts of rare elements display the magmatic series of rocks are in the range of basalt-andesite, calc-alkaline series, and dacite-rhyodacite of the calc-alkaline series. The ratios of Nb versus Nb/Zr, Sr/Zr versus Ti/Zr, Rb versus Rb/Sr, and Rb/Sr versus Ti/Zr show the effect of crystal segregation and mixing processes during magma ascent, and the presence of large plagioclase and clinopyroxene crystals also support the hypophysis. In the spider diagram (primitive mantle-normalized multi-element spider) samples show enrichment in large ion lithophile elements (LILE), especially Rb, Ba, K, and Cs and depleted in high field strength elements (HFSE) (i.e., Ti, Yb, and Zr). The positive Pb and K anomaly can be caused by crustal involvement in magmatic processes. On the chondrite-normalized REE diagram, samples show an LREE-enriched and HREE-depleted pattern. The rather flat HREE patterns imply the absence of garnet and /or hornblende in the source of these magmas. The absence of Eu negative anomaly in non-altered volcanic rocks explains the high-water content or oxygen fugacity and an abundance of hornblende, pyroxene, sphene, and garnet may cause a positive Eu anomaly.Discussion and ConclusionElevated LILE, and LREE, and depleted HFSE content of volcanic rocks in the south of Mamoniyeh indicate the metasomatized asthenospheric mantle source in a subduction zone. The presence of non-equilibrium textures such as sieve texture in plagioclase and opacification of amphiboles are probably related to crustal contamination. Zr/Y>3 ratio in most samples corresponds to a continental volcanic arc rather than the oceanic arc. The available geochemical data show calc-alkaline magmatism occurred by 20 to 45% partial melting of a garnet-spinel lherzolite to spinel lherzolite. Trace elements reveal crustal contamination and magma mixing during the parent magma ascent as well as the role of fluids released from the subducting plate. The subduction-related process, including water and volatile materials released from the subducting slab, resulted in mantle wedge metasomatism and partial melting. This genetic model is consistent with other models suggested for the magmatism along the Urumieh-Dokhtar magmatic arc.AcknowledgmentsThe authors are very grateful to the esteemed referees of Petrological Journal for their valuable suggestions for improving the scientific structure of the article.
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