3 Известия НАН РА, Науки о Земле, 2018, 71, № 1, 3-27 ON THE PETROLOGY AND GEOCHEMISTRY OF JURASSIC MAGMATICS OF THE SOMKHETI SEGMENT OF SOMKHETO- KARABAGH TECTONIC ZONE (NORTHERN ARMENIA) © 2018 Gh.L. Galoyan *, R.L. Melkonyan *, L.S. Atayan*, S.-L. Chung **, R.H. Khorenyan*, Y.-H. Lee ***, S.V. Amiraghyan* *Institute of Geological Sciences NAS RA; 24a M. Baghramian Ave., Yerevan 0019, Republic of Armenia **Department of Geosciences, National Taiwan University, Roosevelt Rd., Taipei 106, Taiwan ***Department of Earth and Environmental Sciences, National Chung Cheng University, 168 University Rd., Chia-Yi 62102, Taiwan E-mail: ghazar.galoyan@gmail.com Received by the editor Поступила в редакцию 01.12.2017г. In the present work the most important attention was paid to the Jurassic magmatic formations of the Somkheti segment of Somkheto-Karabagh (SK) tectonic zone. The results of the petrologic-geochemical studies and U-Pb isotope dating in the magmatic rocks are presented, which have a decisive role in the context of further comparisons. Magmatism took place here in the Middle and Late Jurassic epochs; volcanism has had mainly submarine nature. The studied rocks are subalkaline (partly tholeiitic, mostly calc-alkaline) and are characterized by enrichment of LILE, partly LREE and depletion of HFSE compared to N-MORB. Expressed negative anomalies of Nb, Ta, and Ti indicate the subduction origin of magmatism. Obtained petrologic-geochemical data indicate the formation of basic-medium rocks from the subduction fluid metasomatized spinel-lherzolite mantle and the importance of the fractional crystallization during the formation of acid rocks. According to U-Pb dating, plutonic rocks have been formed in two main Middle and Late Jurassic intervals: ~ 165 Ma (plagiogranite formation) and ~ 155 Ma (tonalitic formation). As an alternative to the prevailing tectonic models, it is suggested that the Paleotethyan lithosphere was subducting to the south beneath the SK zone during Jurassic. Keywords: Lesser Caucasus, Somkheto-Karabagh, Paleotethys, Jurassic magma- tism, subduction Introduction In the Lesser Caucasus Jurassic magmatic formations are widely spread in the Somkheto-Karabakh (SK) tectonic zone and in the Spitak-Kapan (Galoyan et al., 2013) zone (including Tsaghkunyats metamorphic massif and Kapan block). The study of magmatic rocks of the SK zone with the use of modern methods and analytical equipments plays an important role in the context of both local and regional issues including the petrogenetic, geochronological and geodynamic studies. Comparison of Juassic subduction magmatism manifes- tations in the southeastern (Karabagh) and northwestern (in Armenia: Lori and Tavush provinces) segments of SK tectonic zone has a special significance in the paleogeodynamic reconstructions of the Lesser (or South) Caucasus region. Meanwhile comparing the same type and coeval igneous manifestations of SK belt through southwestern Georgia to the Pontides (Turkey) and/or towards southeast (in Iran) has an important regional significance in the field of