3-3-Geochemical data from the Neogene volcanic rocks
The eastern Anatolian lavas’ modal and normative compositions show a
broad spectrum from nepheline normative basalts to quartz normative
felsic lavas (Yılmaz et al. 1987A.B; 1998; Pearce et al. 1990; Keskin
2003;). The southern volcanic centers produced large quantities of basic
and intermediate lavas. The northern volcanoes extruded voluminous
intermediate volcanic edifices in which pyroclastic rocks dominate,
particularly in the northeast of the Erzurum city (Fig 2).
Based on the geochemistry of the Eastern Anatolian Neogene lavas, the
previous studies (e.g., Yılmaz et al. 1987A; B 1988; Yılmaz 1990; Pearce
et al. 1990; Notsu et al. 1995; Keskin et al. 1998; 2003; 2006;
2007;2012; Lustrino et al. 2012; Özdemir and Güleç 2014; Oyan et al.
2016 A; 2016 B; Lebedev et al. 2016 A and B; Kaygusuz et al. 2018; Açlan
et al. 2020; Üner 2021) reached the following conclusions on the magma
compositions.
1-The magmas were derived from the heterogeneous mantle source and
retained their compositions during the transit with minor modifications.
2- Magma chemistry of the alkaline, mildly alkaline, and calc-alkaline
lavas displays distinct enrichment by the subduction components
(enriched in LILE and LREE and depleted in HFSE) (e.g., Pearce et al.
1990; Jean et al. 2010; Lebedev et al. 2018 A and B. Kaygusuz et al.
2018; Üner 2021 and the references therein) and also boninitic
enrichment (Üner 2021). In accordance with this conclusion, the seismic
images obtained by various analytical methods demonstrate a remnant
subducted oceanic lithosphere that is broken into pieces underneath
Eastern Anatolia (Piromallo and Morelli 2003; Piromallo and Regard 2006;
Facenna et al. 2006; Gans et al., 2009; Warren et al., 2013) (for the
discussion on the time, place, and role of the subducting events in the
development of the surrounding orogenic belts, see the two accompanying
papers by Yılmaz et al. in this volume). The enriched mantle source
generated a wide compositional range in the consecutive magma batches.
3- The Sr 86/ Sr 87 ratios of lavas of the eastern Anatolian volcanic
provinces are on an average of 0.704, which varies in a narrow range
commonly between 0.703 and 0.705. The Nd 143 Nd/144 Nd ratio ranges
between 0.5127 and 0.5128 (e.g., Kaygusuz 2018 and the references
therein). The isotope values plot in the mantle array (Kaygusuz et al.
2018). The isotope ratios together with the magma composition compare
favorably with the OIB lavas (Üner 2021; Yang et al 2019; McNabb et al.
in 2018).
4- The petrochemical characteristics indicate that the eastern Anatolian
magmas were derived from an ophiolitic host and underwent significant
fractionation in the magma chamber and during the transit.
5-Upper crustal contribution into the magmas is negligible (Kaygusuz et
al., 2018; Üner 2021).
The petrochemical data derived from the Eastern Anatolian magmas may be
used to test the validity of the hypothesis that assumes the presence of
a thick continental crust under eastern Anatolia. Substantial chemical
modifications would be expected in the magma composition if they passed
through the thick and hot continental crust. These may be listed as
follow; 1-higher Sr and Nd isotope values, 2-increasing amount of
continental crustal components, 3-large volumes of magmas of continental
crust origin (granitic magmas), 4-mixing and mingling of magmas of
diverse compositions.
The petrochemical properties of the lavas from the entire eastern
Anatolian region show boninitic or metasomatic enrichment but do not
confirm these alternatives and thus do not support presence of a thick
continental crust.