Geochemical variations of the Eocene volcanic rocksfrom Kahak area (south of Qom): Evidence for different different conditions of mantle melting in the Urumieh-Dokhtar magmatic arc

The volcanic outputs of the Paleogene are the most conspicuous magmatic products in Iran and the Urmieh-Dokhtar magmatic belt (UDMB) and as the main manifestations of this magmatism (Verdel et al., 2011). They are  part of the southern Eurasian active continental margin formed by the Neotethyan subduction beneath the Central Iranian microcontinent. Geochemically, the magmatism of UDMB is mostly calc-alkaline (especially in the Eocene), although, towards the Oligocene and younger times, alkaline magmatism is also observed (Verdel et al., 2011). The magma origin, partial melting conditions, homogeneity/heterogeneity of mantle sources, evolutionary processes, and tectonic setting of the magma formation are some debatable issues. The purpose of the present study is to investigate field relations, geochemical diversity, and mantle source characteristics of the volcanic units exposed in the south of Qom (Kahak area).

The bulk rock major and trace elements contents were obtained by inductively coupled plasma-optical emission spectrometry (ICP-OES) and inductively coupled plasma-mass spectrometry (ICP-MS), respectively, at Zarazma laboratory in Tehran, Iran. The sample powders were melted using lithium metaborate, dissolved and the final solution were analyzed by ICP-OES. To obtain rare earth and trace element contents, the sample powders were dissolved using multi-acid procedure and then the solution has been analyzed by ICP-MS. The detection limit for rare earth and trace elements is between 0.01 to 1 ppm.

Field Evidence and Petrography:

The Eocene eruptive products in Kahak region are shown by broadly exposed lava flows and pyroclastics alternated with volcaniclastic and carbonaceous sediments. They are somewhere overlain by clastic sediments of the Lower Red Formation (Early Oligocene) allowing us to deduce their relative age. The thickness of lava units varies from <10 to several tens of meters. The basic lavas, the subject of this study, are dark gray to brownish-colored and aphyric to porphyritic. They show various textures of hyalopilitic, hypocrystalline, intersertal to intergranular, and ophitic. Some of the samples contain plagioclase- to clinopyroxene-phyric, in which the size of phenocrysts may reach up to 1 cm. Plagioclase, clinopyroxene, and Fe-Ti oxides are the common phenocrystic to microphenocrystic phases. Plagioclase is the most abundant phase (up to 40 vol.%). Clinopyroxene (5-10 vol.%) is the common ferromagnesian phase in most of the samples occurring as phenocrystic or interstitial phase. Olivine is rarely observed (<5 vol.%), and when present, it is almost altered to secondary products such as chlorite, serpentine, and iddingsite. Fe-Ti oxides are <0.5 to 0.2 mm-sized and form <5% of the mode.

Geochemistry:

The SiO2 content in the study samples ranges from 50.5 to 53 wt.% and in Zr/TiO2 versus Nb/Y diagram, they fall in subalkaline basalt field. The TiO2 amount varies from 0.42 to 1.92 wt.%. Also, the CaO, FeOt, and Na2O+K2O show relatively wide variation from 2.3 to 13.8, 4.9 to 12.7, and 3.2 to 9.3 wt.%, respectively.Mg# [(MgO/MgO+FeOT)*100] ranges from 34 to 52.6. Based on variation diagrams, normalized rare earth elements (REE), and multi-element patterns, the samples can be divided into four distinct groups (Fig. 1A). Group 1 rocks have a lower LREE/HREE ratio than the others and are characterized by (La/Yb)N values of 2.5-4.7. Group 2 rocks are Eu-depleted in which the (La/Yb)N ratio ranges from 3.7 to 4.6. Group 3 rocks display steeper REE patterns with (La/Yb)N ratio of 8.7-9.7. In Group 4 rocks, HREE display lower concentration, and the patterns are relatively steeper [(La/Yb)N= 8.4-10.4]. In the normalized multi-element diagrams, all the samples display relative depletion of high field strength elements (HFSE) such as Nb, Ta, Ti, Zr, Y, and Hf with respect to large ion lithophile elements (LILE) (i.e. Rb, K, Sr, and Ba). Despite the overall similarity of multi-element diagrams, there are some geochemical distinctions between them, for example, Group 1 and Group 4 rocks show Sr enrichment or a more pronounced P negative anomaly than the others.

Variations in the major and trace element contents and the different REE and multi-element patterns are all indicating of geochemical distinction between the studied volcanic rocks (Groups 1 to 4) of the Kahak area. In the major and trace element versus MgO variation diagrams, the scattered plots are also inconsistent with cogenetic relationships among different rock groups. The varied REE values or variously-sloped REE patterns can be attributed to heterogeneous mantle sources or different partial melting conditions. In the studied samples, the Nb/La ratio of 0.20 to 0.58 suggests a lithospheric mantle origin. To model the mantle source and partial melting condition, Sm/Yb versus Sm plot has been used, by which, it is inferred that:All the samples of the Kahak region were derived from a spinel-garnet-lherzolite mantle source with spinel: garnet ratio of 50:50. Geochemical differences between the samples under study are more probably the result of different degrees of partial melting. Accordingly, the primitive magma of Group 3 rocks was derived from the lower degree of partial melting (<10%), while those of the Group 1 rocks resulted from a higher degree of partial melting (10 to 20%). The Groups 2 and 4 rocks fall between these two ranges.Geochemical differences of the Kahak volcanic rocks is most likely the result of partial melting conditions rather than distinct mantle sources. Fig. 1. A) Chondrite-normalized REE patterns of the Kahak volcanic rocks; B) The proposed tectonic model for the Eocene magmatism of UDMB. Relative depletion of HFSE (such as Zr, Ti, Nb, and Ta) in normalized multi-element diagrams is commonly attributed to subduction zone geochemical signature. Also, in the tectonic discrimination diagrams (e.g. Th-Hf/3-Ta diagram), all samples plot in the field of calc-alkaline basalts of arc environment. Therefore, based on our geochemical data, the volcanic rocks of the Kahak region related to arc magmatism. The structural evidences like normal faulting and sedimentary basins with thickened Eocene volcanic-volcanoclastic associations point to an extensional environment. It is probable that the Neotethyan slab-rollback was responsible for the intra-arc (or back-arc) extensional environment in which asthenospheric upwelling led to partial melting of the metasomatized lithospheric mantle and voluminous Eocene magmatism of UDMB (Fig. 1B).