Pre-caldera activity

Pre-caldera stage of Nemrut volcanism extends from 1.0 Ma to 80 ka. This construction period can be divided into five sub-stages (Table 2.1, Fig. 2.5: I, II, III, IV, V); two initial sub-stages (I, II) are represented by lava flows and in the third one (III) peripheral doming is associated with lava flows. Lava flows dominated the forth sub-stage (IV), and the final stage (V) corresponds to the pyroclastic activity which lead to the formation of caldera.

Pre-caldera products of Nemrut volcanism, prior to the caldera forming eruptions, are dominantly composed of silica oversaturated trachytes and rhyolites (Çubukçu, 2008). Nevertheless, there are scarce outcrops of basaltic trachyandesites (mugearites) and metaluminous trachytes (Çubukçu, 2008).

It has been suggested that the oldest volcanic products of Nemrut volcanism were fissure basalts (e.g. Özpeker, 1973a; Güner, 1984; Atasoy et al., 1988; Karaoğlu et al., 2005; Özdemir et al., 2006) located in Bitlis Valley, ~45 kilometers south of the Nemrut volcano (Çubukçu, 2008). On the contrary Ercan et al. (1990) proposed a fissural basaltic origin different than Nemrut for these lava flows. Two different ages were obtained from these basalts: <2.5 Ma (Ercan et al. (1990) and 0.79 Ma (Atasoy et al., 1988). Both Çubukçu (2008) and Ercan et al. (1990) suggested that these relatively older Bitlis valley basalts should not be included into the Nemrut volcanic system. These flows most probably belong to an earlier different system.

Sub-stage I (~1.0 Ma – 500 ka)

Nemrut volcanism has been initiated with lava flows represented by metaluminous felsic rocks exposed on the southeastern flanks of the volcano, and continued with, the oldest known peralkaline silicic (Çubukçu, 2008) lavas represented by the samples of Atasoy et al. (1998) and Pearce et al. (1990) taken from the western caldera wall. Lower contacts of these lava flows were not observed in our field studies; consequently this age signifies the lowest limit of temporal space of Nemrut volcanic history (Çubukçu, 2008). These lava flows are about 300 m above the western base of the main cone, it would be viable to state that the volcanism has initiated prior to this activity.

Sub-stage II (500 – 300 ka)

During sub-stages II and III, volcanic activity seems to be intensified and the central cone was formed (Çubukçu, 2008). The lava flows of the second sub-stage exhibit stratigraphical, thus temporal evolution from trachytic to rhyolitic compositions (Çubukçu, 2008).

Sub-stage III (300 – 200 ka)

The third sub-stage continued to produce rhyolitic and trachytic lava flows.

However, peripheral doming marked the third sub-stage, forming Kirkor domes, Yumurtadağ, Fakı, Kalekirana hills and Kale Hills (Figs. 2.1 and 2.6a, b) at the southern flank and Mazik and Girekol domes at the lower western flank of the volcano (Fig. 2.1 and 2.7). The geochemical data and dating are lacking, but stratigraphically domes forming Kayalı, Tavşan and Arizin hills at the northern side of the volcano belong to the same sub-stage. All of these domes are partially covered by the later pyroclastic units.

Kirkor domes (Fig. 2.6a) culminates at 2478 m (western peak) and 2442 m (eastern peak). Domes are comenditic in composition (Çubukçu, 2008) and dated at 242 ± 15 ka (Atasoy et al. 1988) Lavas originating from Kirkor complex have flowed 3 km to the southwest (Figs. 2.1, 2.5 and 2.6a) and formed steep plateaus of ~ 30 m height.

Kale Hills west of the Çekmece village and Kalekirana hill (Figs. 2.1 and 2.6b) are comenditic in composition. These domes are aligned on a fault and as it will be discussed later, the same fault probably separates the caldera into two blocks.

Mazik and Girekol domes are at the western flank of the volcano and southeastern end of Muş basin (Figs. 1.6, 1.7, 2.1 and 2.7). Mazik dome is a trachytic lava dome, covered with ignimbrites, that culminates at 1680 m, rising 370 m from Muş basin.

Figure 2.6. Peripheral domes at the southern flank of Nemrut volcano. a) Kirkor domes and associated lava flows (Comenditic Trachytes) as viewed from south, b) Aligned Kale Hills and Kalekirana hill (from east).

Figure 2.7. Mazik and Girekol domes at east of Nemrut volcano. Landsat image (6:3:1) superimposed on DEM. Projection: UTM, European Datum 1950.

The most striking feature of the Mazik dome is the NE-SW-trending breach related to the two faults on the top of the dome. Together with the concentric faults at southern flank of the dome (Figs. 2.1 and 2.7), it resembles an interrupted tumescence of a rigid body. Girekol dome is smaller, its total height is 200 m, and it culminates at 1480 m. A lava flow originates from comenditic Girekol dome (Figs. 2.1 and 2.7), and at the northern side, there are (at least) two hot springs (Fig. 2.7).

Atasoy et al. (1988) stratigraphically places the İncekaya volcanism concurrent with the post-caldera volcanism of Mt. Nemrut. On the contrary by petrological means, Çubukçu (2008) suggest that İncekaya volcanism is generated in the third evolutionary sub-stage, during the pre-caldera period. Although there are no dated samples representing İncekaya volcanism, fine-grained base-surge deposits observed at İnce cape north of Lake Van are overlain by caldera forming Nemrut Ignimbrites. Consequently, İncekaya volcanism was most probably generated prior to the collapse of the caldera.

Sub-stage IV (200 – circa 80 ka)

Initial period of this stage is represented by lava flows of basaltic and mugearitic composition. One of these flows can be observed on Kerkorumıksi hill (Figs. 2.1 and 2.28). The other one crops out through Kale hills to Benekli village at south (Fig. 2.1 and 2.28). This flow observed as small outcrops covered with ignimbrites (Fig. 2.8a). Later period generated pantellerite, comenditic trachyte and comendite type lava flows. First group of these flows occur at elevations higher than ~2100 m (Figs. 2.1 and 2.5) and gave the final shape to the pre-caldera volcanic edifice. A comendite flow, which is observed to change direction after bumping onto Kelakirana and Kerkorumıksi domes, is older than 158 ± 4 ka (i.e. Çubukçu, 2008).

Çubukçu (2008) concluded that the timing of peripheral doming, in at least the well-exposed southern area, could be confined to ages older than 160 ka. The second group of the fourth sub-stage lava flows is distributed on the lower flanks at the periphery of the edifice, they are observed near the peripheral lava domes at north of the caldera and on the shoreline of Lake Van (Figs. 2.1 and 2.5).

Figure 2.8. Mugearite outcrops covered with ignimbrites at southeast of Kale hills.

Tuğ and Yasintepe ignimbrites

There are at least two pyroclastic activities within the third and probably fourth sub-stages. Although these pyroclastic units are covered by the products of later activities, they were observed in the drill holes of UNOCAL and TPAO (Atasoy et al., 1988 and references therein) and alternate with lava flows of third and fourth sub-stages.

In our field studies, we evidenced two ignimbrite units that are differing from caldera forming ignimbrite series; they were not mentioned in previous studies.

The first ignimbrite unit was observed south of Tatvan town (Fig. 2.1) at about 17

km distance from the caldera. Ignimbrite unit outcrops along the E-W directed valley between Dalda village and Tuğ district, close to Tuğ district and named after it. Tuğ ignimbrite unit is white-yellow in color and bears white pumice fragments up to 3-4 cm. The ignimbrite is often overlain by resedimented layers and lacustrine sediments, or at some locations, by plinian units. These ignimbrite deposits are subjected to intense weathering. On the northern flank of the valley, Nemrut ignimbrites uncomformably overlie the Tuğ ignimbrites.

The second ignimbrite unit is observed at Yasin hill west of Ahlat town. Yasin hill is at 6.5 km north of Taşharman village, and 17 km NE of Nemrut caldera (Figs. 2.1 and 2.9a). This unit is named after this location as Yasintepe ignimbrite. Large pumices (Fig. 2.9a; mean of 5 max. pumice sizes: 14.9 cm) and white color of the unit (Fig. 2.9b) are the most distinctive characteristics of the Yasintepe ignimbrite.

The ignimbrite outcrops along Yasin hill, southwestern and northeastern side of the exposure is covered with later Nemrut ignimbrites. Other ignimbrite units do not bear coarse pumices as Yasintepe ignimbrite at a distance of 17 km from the caldera. For the origin of Yasintepe ignimbrite, possibility of a source different than Nemrut volcano should not be disregarded.

Figure 2.9. a) Outcrop of Yasintepe ignimbrite at Yasin hill, b) Sample ignimbrite blocks mined for commercial purposes. Color changes in Nemrut ignimbrite from black to reddish brown.

Yasintepe ignimbrite displays white color, differently from all other ignimbrite units in the vicinity of Nemrut volcano.