Geochronology and Genesis of the Young (Pliocene) Granitoids of the Greater Caucasus: Dzhimara Multiphase Massif of the Kazbek Neovolcanic Area

This paper reports an integrated petrological, geochronological, and isotopic geochemical study of the Pliocene Dzhimara granitoid massif (Greater Caucasus) located in the immediate vicinity of Quaternary Kazbek Volcano. Based on the obtained results, it was suggested that the massif has a multiphase origin, and temporal variations in the chemical composition of its granitoids and their possible sources were determined. Two petrographic types of granitoids, biotite-amphibole and amphibole, were distinguished among the studied rocks of the Dzhimara Massif belonging to the calc-alkaline and K-Na subalkaline petrochemical series. The latter are granodiorites, and the biotite-amphibole granitoids are represented by calc-alkaline granodiorites and quartz diorites and subalkaline quartz diorites. Geochemically, the granitoids of the Dzhimara Massif are of a "mixed" type, showing signatures of S-, I-, A-, and even M-type rocks. Their chemical characteristics suggest a mantle-crustal origin, which is explained by the formation of their parental magmas in a complex geodynamic environment of continental collision associated with a mantle "hot field" regime. The granitoids of the Dzhimara Massif show wide variations in Sr and Nd isotopic compositions. In the Sr-Nd isotope diagram, their compositions are approximated by a line approaching the mixing curve between the "Common" depleted mantle, which is considered as a potential source of intra-plate basalts, and crustal reservoirs. It was suggested that the mantle source (referred here as "Caucasus") that contributed to the petrogenesis of the granitoids of the Dzhimara Massif and most other youngest magmatic complexes of the region showed the following isotopic characteristics: Sr-87/Sr-86-0.7041 +/- 0.0001 and CNd- + 4.1 +/- 0.1 at Sm-147/(144) Nd = 0.105-0.114. The Middle-Late Pliocene K-Ar ages (3.3-1.9 Ma) obtained for the Dzhimara Massif are close to the ages of granitoids from other Pliocene "neointrusions" of the Greater Caucasus. Based on the geochronological and petrological data, the Dzhimara Massif is formed during four intrusive phases: (1) amphibole granodiorites (3.75-3.65 Ma), (2) Middle Pliocene amphibole-biotite granodiorites and quartz diorites (similar to 3.35 Ma), (3) Late Pliocene amphibole-biotite granodiorites and quartz diorites (similar to 2.5 Ma), and (4) K-Na subalkaline biotite amphibole quartz diorites (similar to 2.0 Ma). The close spatial association of the Pliocene multiphase Dzhimara Massif and the Quaternary Kazbek volcanic center suggests the existence of a long-lived magmatic system developing in two stages: intrusive and volcanic. Approximately 1.5 Ma after the formation of the Dzhimara Massif (at ca. 400-500 ka), the activity of a deep magma chamber in this area of the Greater Caucasus resumed (possibly with some shift to the east).