Petrography, Geochemistry and Tectonic setting of Tertiary volcanic rocks in the Asfich area (Southwest of Sarbisheh, Southern Khorasan)

The magmatic activities of Lut block started from the middle of Jurassic (165-162 Ma) with the intrusion of Kalate Ahani, Shahkoh and Sorkh kooh intrusive masses and reached its peak in the Tertiary. Tertiary volcanic and semi-volcanic rocks cover more than half of the Lot block with a thickness of about 2000 meters, which were formed as a result of subduction before the collision of the Arabian and Asian plates (Camp; Griffis, 1982; Tirrul et al., 1983; Berberian et al., 1982). The studied area with geographic coordinates 59º31′14″ - 59º36′05″ east longitude, 32º32′28″ -32º34′29″ north latitude, is located 40 km southwest of Sarbisheh and includes a thick succession of Tertiary volcanic and pyroclastic rocks that are covered by young Quaternary sediments in some places. This area is located in the 1:100,000 Sarbisheh geological map prepared by Nazari and Salamati (1999). According to the map prepared by Pang et al. (2012) for parts of Sistan zone and Lut block, the studied area is located on the eastern edge of Lut block and on the border of two structural states of Lut block and Sistan zone (Figure 1). Since the Tertiary volcanic rocks in the mentioned region, despite their wide expansion and having large reserves of perlite and clay minerals, have not been subjected to detailed lithology and geochemistry studies, they have been selected as the subject of this research.

Regional Geology:

In this area, there are extensive outcrops of Tertiary volcanic rocks, including pyroxene-andesite, andesite-trachyandesite, dacite, rhyodacite, rhyolite (perlite) and related pyroclastic rocks such as tuff, ignimbrite, and agglomerate, which are on serpentinized peridotites in the east of Fal village to the south of Asfich, and gabbro belongs to the Cretaceous ophiolites of southeast Birjand. In terms of age, the ophiolitic units are related to the late Cretaceous. Tertiary volcanic units are related to the Eocene and Oligocene (Pang et al., 2013).

In order to carry out this research, first of all, library studies including the collection and review of geological and topographical maps and previous studies have been carried out. In the next step, during 10 days, field investigations, separation of different rock units and sampling were done, and then 90 thin sections of the rocks of the area were prepared and their mineralogical and textural characteristics were examined by a Leitz type polarized microscope. In the next step, according to the diversity and geographical spread of different rock units, 9 unaltered or less altered samples were selected and coded (LF200) for chemical analysis by ICP-ES for major elements and ICP-MS for trace elements. Acme laboratory in Canada and 2 samples have been sent to Kansaran Binaloud laboratory. Finally, GCDKit, Excel (@2007), Corel and Minpet software were used to draw diagrams. In order to obtain Fe2O3 and FeO values that are closer to the real values, Minpet software was used according to the Irvine and Baragar method (Irvine and Baragar, 1971).

Petrography:

Tertiary volcanic rocks of the region include pyroxene andesite, andesite, dacite, rhyodacite, rhyolite (perlite), tuff, breccia and agglomerate. The common texture of these rocks is porphyritic with microgranular or microlithic, glomeroporphyritic, flow and cavity texture. Pearlites have a pearlitic texture. Euhedral and subhedral phenocrysts of plagioclase with oligoclase-andesine composition are the main constituents of these rocks and have rounded or bay sides. Some plagioclase phenocrysts show a sieve texture. Plagioclase microlites are the main component of the matrix. Clinopyroxene (augite) and hornblende are present in small amounts. In addition, bay-sided phenocrysts of sanidine and quartz are observed in rhyolites.

Geochemistry:

The results of chemical analysis of volcanic rocks of Asfich area are presented in Table 1. In the diagram of total alkali versus silica, presented by Cox et al. (1979), the samples are in the range of andesite, dacite and rhyolite (Figure 8A). Due to the presence of variation in some samples, charts based on immobile elements have been used for the geochemical nomenclature of rocks. In this regard, in the Nb/Y vs. Zr/TiO2 diagram presented by Winchester and Floyd (1977), the samples are in the range of andesite, trachyandesite, dacite, rhyodacite and rhyolite and show a subalkaline nature (Figure 8B). In the Na2O+K2O-MgO-FeO* triangle diagram (AFM diagram), which is used to identify magmatic series and their transformations, and subalkaline series is divided into two separate tholeiitic and calc-alkaline series and presented by Irvine and Baragar (1971), the samples are in the calc-alkaline range (Figure 8C).

Tectonic setting and origin:

It is possible that the Th/Yb ratio for the samples is higher than the mantle, and this compositional change is attributed to subduction-related processes (Helvaci et al., 2009). Arc magmas are mainly formed as a result of partial melting in the subduction-related mantle wedge, due to the addition of metasomatic components released from the subducting oceanic lithosphere. Metasomatic fluids may include hydrous fluid (supercritical) or primary melts from sediments or basaltic crust subducted into the mantle wedge, which causes the mantle solidus to decrease and magma production (Figure 11) (Harangi et al, 2007; Hoang et al, 2001). Depletion in elements P, Ti, Ta and Nb and enrichment in U, K, Sr, Zr, Rb and Th and enrichment of LREE compared to HREE indicate the formation of these rocks in the active continental margin regime. which are mainly formed as a result of partial melting in the mantle wedge, due to the addition of metasomatic components released from the subducting lithosphere. Geochemical evidence such as Nb/Y versus Rb/Y shows that contamination is one of the most important phenomena in magma evolution in the area. According to the diagram of Dy/Yb versus La/Yb and Dy/Yb versus Dy, it is possible to imagine the origin of partial melting of lherzolite spinel mantle and the range of phlogopite-bearing spinel lherzolite facies for the magma that forms the rocks of the region.