Petrography, geochemistry and petrogenesis of Damavand volcano: Comparison of different volcanic generations

1-Introduction Damavand volcano was formed by explosive and non-explosive eruptions on the old eroded rock units (Mesozoic and older) of Central Alborz during the Quaternary period and formed two huge cone (Old and Young Damavand). Davidson et al. (2004) determined the time of Old-Damavand activity from 1800 to 800 thousand years ago by measuring Ar/Ar and U-Th/He methods. According to their report, following subsidence of the Old Damavand cone, the Young Damavand cone has formed by continuing eruptions during 600 to 7.3 thousand years ago (Fig. 1).  Several mechanisms have been proposed to explain Damavand volcano with subduction-like geochemical signatures including: 1) Subduction of Neo-Tethys crust under Iranian continental crust, 2) intracontinental rifting, 3) hot spot, and 4) lithospheric mantle delamination. To better understand the source and tectonic setting, and differences between several eruptions, we investigated field relations of major eruptive units of Damavand volcano, and carried out petrographic and whole-rock geochemical (including a wide range of trace elements) data for several key stratigraphic units. Based on these results, we present here a geodynamic model for post-collision volcanism during Quaternary time for northern Iran. 2-Materials and methods More than 500 non-weathered and unaltered samples were collected from multiple localities covering Damavand volcano to reconnoiter the variability of rock compositions over the major stratigraphic units. We collected 35 samples (~ 2 kg) for X-ray fluorescence (XRF) and ICP-MS analysis. Rock powders were analysed for loss on ignition (LOI) and major elements using X-Ray refractory fluorescence (XRF) at the Geological Survey of Iran laboratory. Following lithium metaborate fusion, the trace elements were analysed using ICP mass spectrometry (MS) analytical protocols at the Applied Research Center of the Geological Survey of Iran laboratory. Analyses of a selection of 35 representative samples from the different layers are presented in Table 1. 3-Results and discussion The Old- and Young-Damavand volcanic rocks are olivine basalt-trachyandesite and trachyandesite, respectively. Plagioclase and alkali feldspar phenocrysts with ferromagnesian mineral inclusions are common in young lavas that have not seen in the old rocks. The young specimens often have clinopyroxene phenocrysts with twining which have not observed in old ones. Orthopyroxene has been observed in some of the old rocks that had not been seen in young specimens. Rocks from the Damavand volcano have SiO2 contents ranging from 46 to 67 wt% (Table 1). On the Zr/TiO2 versus Nb/Y diagram (Winchester and Floyd, 1977), the compositions of Old-Damavand rocks classify as alkali-basalt to trachyandesite, whereas the Young-Damavand rocks are trachyandesite. On the basis of the Peccerillo and Taylor (1976) classification, all rocks of Damavand volcano are assigned to the shoshonitic trend. Primitive mantle-normalized trace element variation diagrams (Sun and McDonough, 1989) further highlight the geochemical similarities between the Old- and Young-Damavand rock suites. All volcanic suites show spider diagrams with prominent enrichments in large ion lithophile elements (LILE), although the LIL/HFS ratios of 1800-800 ka volcanic rocks (exception of olivine basalts) are smaller than the younger suites. Old- and Young-Damavand rocks are all enriched in light REE relative to heavy REE and with a small positive Eu anomaly, in the chondrite normalized of rare earth elements pattern (Sun and McDonough, 1989). These are characteristic for enriched mantle sources and/or continental crustal contamination. As major elements, similar features in trace element abundance patterns for trachyandesitic rocks imply that they are derived from parental magmas which remained uniform through time, and which contaminated with crustal materials. Moreover, trace element chemical characteristics of Damavand trachyandesitic rocks resemble those of high-silica adakitic magmas (Defant and Drummond, 1990), specifically high Sr/Y and high (La/Yb)N. The geochemical characteristics of Damavand volcanic rocks, including high values ​​of LIL elements (e.g. Rb, Ba, K, Sr) and negative anomalies of Nb, Ta and Ti in primary mantle normalized spider diagrams, and the enrichment of LREE relative to HREE in the chondrite normalized of rare earth elements pattern are similar to subduction zones magmas. The high content of U and Pb elements is a feature of upper continental crust (Radnik and Gao, 2003), which are also found in Damavand adakite rocks. Because the average of Ba/Th and Pb/Ce ratioes in Old- and Young-Damavand adakitic rocks are 136.82 and 0.073, respectively and with low Ba/La, therefore, the these volcanic rocks were not formed in subduction zone, either by the melting of the oceanic crust sediments or metasomatized mantle. Obviously, the volcanic rocks of Damavand could not have originated from the melting of the lithospheric mantle about 20 million years after finishing of subduction. The comparative normalized spider diagrams and normalized REE distribution patterns show acceptable similarity between Damavand samples with upper-crust composition. Moreover, the lower crust has a more depleted composition relative to Damavand samples. Investigation of Damavand tectonic setting based on Th versus K2O diagram (Ayuboglu et al., 2012), shows that they have formed in the layered lithosphere. Due to lithosphere delamination and local pressure reduction at this point of Alborz, the mantle had melted and formed the primary magma of Damavand. This new generated hot magma ascent through the lower crust and then emplace in the upper crust, where it assimilate crust rocks, mixed and contaminated with them. It seems the early olivine basaltic magma of Old-Damavand has passed rapidly through the crust with minimal contamination (Figure 12a), however, the Damavand trachyandesitic rocks (adakites) of stayed for a long time in crust and undergo higher contamination. 4-Conclusion - According to field investigations and a newly established geochemistry, the lithology of Old- and Young-Damavand volcanic rocks are olivine basalt to trachyandesite and trachyandesite, respectively, with shushonite affinity, except the olivine basalts that are alkaline. - All volcanic rocks have enrichment of LILE to HFSE and LREE to HREE in normalized multi element diagrams. Moreover, the Damavand volcanic rocks have negative anomalies in Th, Nb, Ce, Pr, Sm, Dy, Yb and positive anomalies in Ba, Eu, U, K, Pb, Sr, Zr, Y in multi elements diagrams. The amount of Sm to Lu elements in Old-Damavand rocks is closer to mantle values, while Young-Damavand rocks are more depleted. - The olivine basalts- trachyandesitic rocks are formed by melting of enriched mantle. The trachyandesitic (Old-and Young-Damavand) adakitic affinities seem to have originated from a similar source of enriched mantle at different depths with different amounts of crust contamination, by locally pressure drop related to lithosphere delamination. The Young-Damavand volcanic rocks probably stopped longer in the upper crust than the older and undergo more contamination with the crust, so it resembled the composition of the upper crust.