Geochemistry of the Pliocene basalts erupted along the Malatya-Ovacik fault zone (MOFZ), eastern Anatolia, Turkey: Implications for source characteristics and partial melting processes
Introduction
The Neotectonic evolution of Turkey reflects the interaction between the Eurasian, African, Arabian, and Turkish plates (Şengör and Yılmaz, 1981; Şengör et al., 1985; Dewey et al., 1986; Fig. 1a). The Turkish plate is bounded to the north by the North Anatolian Fault Zone (NAFZ), which separates it from Eurasia, and to the south and west by the active plate margin formed by the Hellenic and Cyprus trenches, along which the African plate is subducting northward. The eastern boundary of the Turkish plate is a complex left-lateral strike-slip fault zone, adjoining both the African and Arabian plates (Westeway, 2003). The Turkish–Arabian plate boundary follows the left-lateral East Anatolian Fault Zone (EAFZ), which splays southwestward in SE Turkey into the Turkish–Africa and Africa–Arabian plate boundaries, the latter being known as the Dead Sea Fault Zone (DSFZ). Malatya-Ovacık Fault Zone (MOFZ) is a 240 km long left-lateral strike-slip fault zone in eastern Turkey (Westeway and Arger, 2001). Although Koçyiğit and Beyhan (1998) have suggested that MOFZ is active and takes part of the present relative motion between the Turkish and Arabian plates, Westeway and Arger (2001) and Westeway (2003) suggested that MOFZ slipped <30 km during Early Pliocene (<5 to <3 Ma) and it was the main strand of the boundary between the Turkish and Arabian plates. It has been suggested that the MOFZ intersected the NAFZ across the Erzincan Basin, which was the site of the triple junction between the Arabia, Eurasian, and Turkish plates between <5 and 3 Ma (Westeway, 2003).
In this paper, we attempt to determine the geochemical characteristics and genesis of Pliocene basalts erupted along the MOFZ in eastern Anatolia. We will compare the chemistry of basalts taken from different lava flows in relation to their stratigraphical positions, in order to recognize similarities and differences, which may serve as genetic fingerprints. The geochemical data collected in the present study will also shed some light on the process of differentiation, which controlled the variation of chemistry of these rocks. In addition, based on stratigraphical heights of different lava flows, the regional trend of magmatic evolution in this flood basalt province will also be deduced.
Section snippets
Geological setting
The study area is located at the Arguvan region in the central-eastern Anatolia (north of Malatya, Fig. 1a), whereas Pliocene volcanic rocks occur as lavas of basaltic character. Rock units in this area range from Lower Miocene to Pliocene (Alpaslan and Terzioğlu, 1996; Ekici, 2003; Fig. 1b). Arguvan basalt outcropping as lava flows, which is the main interest of this study, outcrops as the youngest lithological unit in study area (Fig. 1b). Based on existing geological studies, three
Analytical techniques and sampling
Eleven samples were collected from sampling site. Four sets of samples were taken from four different lava flows. These flows were vertically separated from one another by 1–2 m. From each lava flow, three samples were taken except the uppermost lava flow, which is sampled from bottom and top. To minimize chemical variation between samples due to secondary alteration, samples were taken only from areas of flows that were hard, massive and unfractured, and therefore showed the least evidence of
Classification and petrography
All the lava flows were classified as basalt, using the total alkalis–SiO2 diagram of Le Maitre (1989). The division between the alkaline and subalkaline fields defined by Irvine and Baragar (1971) has also been plotted onto this diagram (dashed line). All the samples fall in the subalkaline field (Fig. 2a). AFM diagram of Irvine and Baragar (1971) exhibits the tholeiitic character of the Arguvan basalt indicated by normative compositions (Fig. 2b and Table 1). According to normative mineral
Chemical characteristics
All the Arguvan basalt rocks are tholeiitic basalts with normative olivine and hyperstene. They are relatively primitive with MgO contents ranging from 8.05 to 9.69 wt% and Mg numbers varying from 59.86% to 63.48% where Mg# [atomic Mg/(Mg+Fe2+)]×100, assuming Fe3+/Fe2+=0.20 recommended by Middlemost (1989). The major element contents of all the flows of the Arguvan basalt (Table 1) fall within a restricted range (Fig. 4), particularly Fe2O3 with typical values of about 12%. Most trace-element
Discussion
Basalts of all the four lava flows can be evaluated on the basis of their chemical signatures in order to determine their tectonic provenance. Knowledge of the type of volcanic activity in a region over time is thus essential to the reconstruction of tectonic evolution of study area. Varying concentrations of trace elements, partial melting degrees and depths have resulted from tectonic activity on the MOFZ during Pliocene time. All the four lava flows are olivine and hyperstene-normative
Fractional crystallization
Under low pressures (∼1 kbar), the anhydrous fractional crystallization of the assemblage olivine–clinopyroxene–plagioclase does not exhibit significant La/Sm ratio (Viereck et al., 1989). On the other hand, the high La/Sm ratios of tholeiitic basalts that differentiated under higher pressures, e.g. 10 kbar, are frequently explained by the fractional removal of high proportions of clinopyroxene since DLaCpx/liquid<DSmCpx/liquid (Halliday et al., 1995). La/SmN increases only from 1.90 to 2.62.
Partial melting
The ratio of an incompatible element during melting, such as P relative to Al is expected to decrease systematically with increasing melting, assuming that alumina is controlled by garnet or spinel throughout the melting interval (Furman, 1995). P2O5/Al2O3 vs. SiO2 diagram of the Arguvan basalt exhibits an increase in P2O5/Al2O3 ratios from the first flow to upper flows and indicates that the first flow has lower values of P2O5/Al2O3 ratio than the upper flows (Fig. 8d). It has been shown above
Crustal assimilation
As the Arguvan basalt is interpreted to have erupted through continental crust, magmatic differentiation involving crustal assimilation is a possibility. It has been commonly believed that the continental tholeiites have distinctive negative Nb and Ta anomalies (with respect to normalized trace-element patterns (Cox and Hawkesworth, 1985)). In addition, continental crust has been characterized by highly fractionated and enriched LREE, flat HREE, a positive Pb but negative anomalies at Nb–Ta (
Magma sources
To evaluate the different components involved in magmatism related to MOFZ, it can be used to incompatible element ratios. The element ratios used are relatively insensitive to moderate to large degrees of partial melting and to fractional crystallization processes, and thus should approximately represent source compositions. In the plot of Zr/Yb–Nb/Yb reflecting the similar incompatibilities of Zr and Nb in mantle melts, the increasing Zr and Nb reflects increasing contributions from enriched
Conclusions
The Arguvan basalt lavas are dominated by relatively evolved olivine and hyperstene-normative tholeiitic basalts. Even the most primitive lavas are unlikely to represent primary mantle-derived melts. Although these basalts have relatively monotonous major element compositions, samples belonging to the first flow have lower TiO2 and K2O wt% than the upper flows, and also lower concentrations of HFSEs and heavy REEs. They exhibit a narrow range in terms of trace-element enrichment (La/SmN ratios
Acknowledgments
This research has been supported by grants from Cumhuriyet University Research Funds. We specially thank Dr. Hüseyin Yılmaz (Cumhuriyet University) (Mersin University) for collecting samples during the field study and to Fevzi Öner (Mersin University) for his support in framing the manuscript.
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