Journal of Economic Geology (Dec 2022)
Petrology and geochemistry of high temperature I type granitoids in Nusha region, Mazandaran province
Abstract
The study area is located about 30 km south of Ramsar, in the central Alborz zone. In addition to the Nusha granitoids (with an age of about 56 million years), the outcrops in this area, mainly include Paleozoic and Mesozoic rock units. Petrographically, the Nusha granitoids have diorite, syenite, monzonite, monzodiorite, granodiorite and quartz monzonite compositions. Moreover, mineralogically, feldspar is the principal mineral, and the texture superiority in them belongs to the granular type. In terms of magmatic series these rocks are metaluminous and range from high K calcalkaline to shoshonitic. The geochemical characteristics of the major and rare elements, as well as the petrographic ones indicate that these granitoids are I type granites, and at the same time they belong to high temperature ones based on the behavior of Ba, Ce and Y elements. Enrichment in LILE and LREE and low concentrations of heavy rare earth elements HREE and high field strength elements HFSE, together with Nb and Ti negative anomaly in the spider diagrams are signs of magmas related to the subduction zone. The high temperature nature and characteristics such as Y/Nb, Rb/Sr and Rb/Ba ratios show that the Nusha granitoids have the geochemical properties of both crustal and mantle origin materials with different ratios. Based on tectonomagmatic discrimination diagrams and trace element compositions, these granitoids belong to an active continental margin environment. The parental magma has originated from melting of an enriched mantle source and contaminated with continental crust during ascent. Introduction The study area is part of the Alborz-Azerbaijan magmatic belt. Many of the intrusive masses present in this area are high potassium calc-alkaline to shoshonite and are of I type granitoids (Aghazadeh, 2009; Aghazadeh et al., 2013; Nabatian et al., 2014; Taki, 2011). In this research study, we seek to determine the nature of the granitoids of the Nusha region by using geochemical characteristics and determine their origin and tectonomagmic setting. Exposed rock units in the study area, in addition to intrusive igneous rocks, include sedimentary carbonate and detrital rocks belonging to the Mobarak (Carboniferous), Dorood (Lower Permian), Ruteh (Upper Permian), Nesen (Upper Permian), Elika (Lower-Middle Triassic), Shemshak (Upper Triassic-Lower Jurassic) Formations and Cretaceous sedimentary and volcanic rocks. In this area, the granitoid intrusive masses have northwest-southeast trends and have intruded during the Eocene (56±2 million years ago) (Axen et al., 2001). The outcrop of the Nusha granitoids starts from the western slope of Sehezar Valley and extends north-westward to the south-west of Nusha. The granitoid unit is separated into two masses by a dextral fault. The southern border of this mass is completely faulted. This has resulted that the Upper Paleozoic assemblage has been thrusted onto the granitoids. Its northern border is also mainly faulted. The only normal contact present in the western part with Lower and Middle Jurassic sediments. Research method After sampling of intrusive rocks and petrographic studies of the study area, 10 samples of rocks related to intrusive masses were sent to the Zarazma company in Iran for chemical analysis and 7 samples were sent to the Actlabs company in Canada. In the laboratory of these companies, the ICP-OES method is used to measure the major elements and some minor elements, and the ICP-MS method is used to evaluate the abundance of rare and trace elements. In this research study, since iron is reported unseparated, the Irvine and Baragar method (1971) was used to calculate divalent and trivalent iron. Results and discussion The granitoids of the study area are petrographically composed of diorite, syenite, monzonite monzodiorite, granodiorite and quartz monzonite. Mineralogically, feldspar is the principal mineral and the granular is superior texture. Based on several geochemical characteristics such as aluminum saturation [the molar (Al2O3/(CaO+Na2O+K2O)) or ASI] and agapiitic [A.I.=molar (Na+K)/Al] indices, Na2O/K2O ratio, range of SiO2 content, Na2O weight percentages in acidic terms, average values of Na2O, Zr, Y, Ce and Rb/Sr, and diagrams of ANK-ACNK, normative corundum and P2O5 versus Rb, (A/CNK-Fe2O3+FeO), Th-Rb and P2O5-SiO2 as well as petrographic features like, petrographical composition ranges and the nature of enclaves and ferromagnesian minerals in the studied samples all confirm the I type nature of the Nusha granitoids. At the same time, the variations of Ba, Ce and Y elements in the Nusha granitoids are such that they first increase and then decrease with increase of silica content. Thus, they are high temperature I type granite and must have been originated from the melting of mafic rocks of crust or evolved mantle. These rocks have high K calc-alkaline to shoshonite magmatic series nature. REE patterns of all the studied samples are parallel and similar (so they have a common origin) and relatively highly enriched (than primitive mantle) and have no Eu anomaly (due to the participation of feldspar in the magma during partial melting of the source rock or lack of differentiation of this mineral during the fractional crystallization of the parental magma). Like many other active continental margin calc-alkaline rocks, these rocks have negative slope on the LREE side and are flat on the HREE side. Enrichment in LILE and LREE and low concentration of HREE and HFSE, along with negative anomaly of Nb and Ti in the spider diagrams are indicators of magmas related to the subduction zone. The very distinct Nb-Ta trough in the arc systems spider diagrams is due to crustal contamination or retention of these elements in the source during partial melting. Positive Pb-K anomalies and overall enrichment of LILE are also indicators of crustal contamination. The extreme U and Th enrichment in the spider diagrams indicate addition of pelagic sediments or altered oceanic crust in the melting process. The high Th/Ta and relatively low Nb/Th ratios indicate formation of magma in an arc environment, and the tectonomagmatic discrimination diagrams show continental arc one as well and according to their age, like many Cenozoic igneous rocks in the Alborz, Western Alborz-Azerbaijan and South Caucasus must have resulted from subduction of the Neotethys oceanic crust. In the Nb versus Rb/Zr diagram, the Nusha granitoids are in the range of normal to mature continental arcs. On the Sm/Yb-La/Sm and Rb-Sr diagrams, the continental crust is about 45 km and shows enriched mantle at the parental magma generation source. The wide range of variations in Y/Nb ratios indicates that the study area granitoids have the geochemical characteristics of both crustal and mantle origin materials. According to the Rb/Sr versus Rb/Ba diagram, the mantle to crust materials ratios are between 20 and 50%. The low Tb/Yb ratio and the multiple concentration of rare earth elements compared to the primitive mantle in the Nusha granitoids indicate a mantle source with the composition of spinel bearing peridotite without garnet origin.
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