Journal of Economic Geology (Aug 2017)
Geochemical studies, magmatic evolution, microstructures and replacement mechanisms in Jebale-Barez granitoid Complex (East and Southeast Jiroft)
Abstract
Introduction The Jebale-Barez Plutonic Complex (JBPC) is composed of many intrusive bodies and is located in the southeastern province of Kerman on the longitude of the 57◦ 45 ' east to 58◦ 00' and Northern latitudes 28◦ 30' to 29◦ 00'. The petrologic composition is composed of granodiorite, quartzdiorite, granite, alkali-granite, and trace amounts of tonalite with dominant granodiorite composition. Previously, the JBPC was separated into three plutonic phases by Ghorbani (2014). The first plutonic phase is the main body of the complex with composition of quartz-diorite to granodiorite. After differentiation of magma in the magmatic chamber, the porphyritic and not fully consolidated magmas have intruded into the main body. Their compositions were dominantly granodiorite and granite that are defined as the second plutonic phase. Finally, the last phase was started by an intrusion of the holo- leucogranite into the previous bodies. This plutonic activity was pursued by the minor Quaternary basaltic volcanism that shows metamorphic haloes in the contacts. They are dominantly porphyric leucogranites. However, some bodies show dendritic texture that may imply the existence of silicic fluids in the latest crystallization stages. Materials and methods In this article different analysis methods were used. For example, we used a total of two hundred samples of the various granitoids that were selected for common thin section study. Forty four representative samples from the different granitic rocks were selected for whole rock chemical analyses. The analyses of both major and trace elements were performed at the Department of Earth Sciences, the University of Perugia, Italy. The analysis for all major elements was carried out by an X-ray fluorescence spectrometry (XRF) using a tube completed with a Rn and W anode under conditions with acceleration voltage of 40-45 kV and electric current ranging from I=30-35 mA. After calcination of powdered samples and full matrix correction, the sum of all major oxides was equal to about 100 wt.%. The concentration of trace elements in the selected samples has been performed by Inductively Coupled Plasma-Mass Spectrometry (ICP-MS). The uncertainty is <10% for trace element contents higher than 2 ppm (except for Pb, <15%) and <15% for all the other trace elements. Results The microstructures observed in thin sections in this study were grouped into three types: (i) magmatic microstructures; (ii) submagmatic microstructures and (iii) mylonitic microstructures. Magmatic and submagmatic microstructures occurred simultaneously with the emplacement of granitoid complex and mylonitic microstructures that occurred after emplacement of granitoid complex. The magma nature of these rocks is sub-alkaline-(calc-alkaline), which fall into calc-alkaline series with high potassium in SiO2-K2O plots. The geochemical variation diagrams of major oxides, the continuous spectrum of rock compositions has been carried out which indicates the crystallization of magmatic differentiation and extensive appendices. Field observations, petrographic and geochemical studies suggest that the rocks in this area have type I and CAG subsections. Studying the geochemical diagrams of the rocks in the studied area indicates that these rocks have been formed in active continental margin tectononic settings. It seems that the Jebale-Barez granitoid Complex is located within a shear zone. Magma has been percolated through Mijan caldera and emplacement Forms of Sill along the shear zone during various periods and the structural setting of granitoid complex in the Jebale-Barez is extensional-shear fractures which are the product of transpression tectonic regime. Discussion The JBPC is calc-alkaline, high-K, subalkaline, and mostly metaluminous except granite and alkali-granite units which are slightly peraluminous and I type in character. These geochemical properties of the studied granitoids suggest subduction-related arc magmatism. The systematic variation for the major elements implies involvement of fractional crystallization in the evolution of JBPC. The trends are consistent with the fractionation of plagioclase feldspar and ferromagnesian minerals as indicated by decreasing MgO, CaO, FeOt and TiO2 with increasing SiO2 despie the content of (K2O+Na2O). It generally increases with increasing SiO2 for intermediate compositions (67 wt% SiO2 ≤) and then decreases for more felsic granitic rocks, indicating that sodic feldspar was a major fractionating phase for alkali-granite and granite suit (Rasouli, 2015). Overall REE abundances slightly decrease with increasing SiO2 consistent with plagioclase fractionation. The distribution of voluminous volcanic rocks in the studied area implies that the JBPC could be a part of the mature magmatic arc. The field petrography and geochemical studies indicated that the JPBC originated from both crustal and mantle derived magmas: The increase in temperature and excess fluid pressure caused by subduction trigged melting of mantle edge and formation of basaltic magma and its ascending and introducing into the crust was followed by partial melting (Rasouli, 2015). The juxtaposed series of mafic-felsic pulses formed a mixed magma. Finally this magma is emplaced at broad, shallow magma chamber (9-12 km), where the differentiation took place by fractional crystallization and produced a wide variety of rocks form quartz-diorite to alkali granite. In such shallow magma reservoirs, the emplacement of magma took place as sill (Fridrich et al, 1991). Combining field observations and petrofabric studies displayed a deep caldera as a feeder zone for Eocene volcanic rocks (Rasouli, 2015). The JBPC is located in a shear zone and multiple magmatic pulses were injected as sills. The magmatic fabrics show active tectonic controls on magmatism during and after magma emplacement. The transpressional tectonic regime is well compatible with our data. References Fridrich, C.J., Smith, R.P., DeWitt, E. and McKee, E.H., 1991. Structural, eruptive, and intrusive evolution of the Grizzly Peak caldera, Sawatch Range, Colorado. Geological Society of America Bulletin, 103(2): 1160–1177. Ghorbani, M., 2014. Geology of Iran. Aryan Zamin, Tehran, 479 pp. Rasouli, J., 2015. Petrology and geochemistry of Jabal Barez granitoid batholith with a view to the zoning alteration and copper mineralization (North East Jiroft). Ph.D. Thesis, Shahid Beheshti University, Tehran, Iran, 369 pp.
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