The derivation of potassic and ultrapotassic alkaline volcanic rocks from an orogenic lithospheric mantle source: The case of the kalecik district, Ankara, central Anatolia, Turkey

The Kalecik region (Central Anatolia, Turkey) contains alkaline volcanic rocks that can be classed as tephrites, phonolites, basanites and trachybasalts. Most of these rocks carry analcime and all indicators demonstrate that analcime is secondary. This tranformation can be explained by the analcimization of leucite. This alteration process will in particular increase the contents of Na₂O and H₂O and decrease the K₂O content of the whole-rock. Thus, the composition of the analcime-bearing mafic volcanic rocks can be defined as ultrapotassic on the basis of geochemical parameters of original rocks (K₂O/Na₂O > 2, K₂O > 3 wt%,MgO > 3 wt% (Foley et al. 1987)). Their compositions match those of Roman-type ultrapotassic rocks. Some of the samples have just potassic characteristics (K₂O > Na₂O). The variable Mg# (47- 66) of the rocks indicates that they have undergone different degrees of fractional crystallization that likely involved clinopyroxene ± olivine in the magma chambers. The estimated crystallization pressure of clinopyroxenes was determined from electron microprobe analyses, and are in the range of 4.2 - 5.8 kbar for the basaltic rocks and 3.7- 8.5 kbar for the trachybasaltic lavas. The wide range of pressure estimates for the trachybasaltic unit may be explained by multistage crystallization of clinopyroxene in magma chambers located at different depths. After assessing the effects of fractional crystallization, the geochemical signatures (low Zr/Ba, Nb/La and high La/Ta) can be used to indicate that the mantle source of the Kalecik potassic and ultrapotassic alkaline rocks is lithospheric mantle. The samples contain a range of geochemical features (enrichments of large ion lithophile elements (LILE) and light rare earth elements (LREE) relative to high field strength elements (HFSE) and heavy rare earth elements (HREE)), negative Nb, Ti anomalies and a positive Pb anomaly, low Zr/Ba, Nb/La and high K/P, La/Ta, Ba/Nb, Th/Yb) that are characteristics of orogenic volcanism, and this suggests that a subduction-modified subcontinental lithospheric mantle source was responsible for the origin of the Kalecik potassic and ultrapotassic alkaline volcanic rocks.