Petrology of the Dizi series Bathonian Intrusions

Author: Irakli Javakhishvili
Keywords: Svaneti, Geology, Dizi Series, magmatite, pyroxenite, gabro, diorite, syenite, granite
Annotation:

The Dizi series of Devonian-Triasic age, located in the extreme north of the Black Sea-Central Transcaucasian terrane, is exposed on the Southern Slope of the Greater Caucasus, Svaneti (Georgia). It is built up of 1800-2000 m thick mainly terrigenous sediments: sandstones, gritstones, argillites, organic carbon-rich shales, silicites, tuff sandstones, volcanics, limestone and marble lenses, graphitized phyllites, phyllitic shales and various hornfels altered in the most low-temperature greenschist facies conditions. In the series Middle Jurassic (Bathonian) intrusions, represented by pyroxenite, gabbro, gabbro-diorite, diorite, syenite-diorite, monzonite-diorite, quartz diorite and granite, occur. Among the ten mapped exposures of these intrusions the multiphase Kirari, Abakuri and the Dizi intrusions are the largest. Under the influence of these intrusive bodies the Dizi series underwent contact (thermal) metamorphism. K-Ar age dating of the Dizi series magmatites established 176-165 Ma [11] confirming the formation of intrusives during the Bathonian orogeny. Despite the fact that these magmatic rocks were the object of interest of many researchers, many petrological and mineralogical questions require further specification. Actually, the studies in respect of their geochemistry haven’t been performed so far. Important for petrogenetic modeling trace and rare earth elements content and their distribution regularities have to be defined as well. Integrated field, petrological and geochemical studies of abovementioned intrusions were undertaken including accurate mapping of the intrusions boundaries, standard petrography of more than 150 thin sections, identification of the intrusions effects on host-rocks and whole-rock XRF analyses on selected from intrusions 33 representative samples (pyroxenite, diorite, syenite, monzonite and granite). According to the total alkali vs silica (TAS) classification diagram [44] the larger part of the Dizi intrusions are classified as syenites. Diorites and monzonites are in limited amounts established mainly in the peripheral part of the syenite intrusions. On the K2O / SiO2 diagram [35] the majority of samples from the Kirari, Abakuri and Dizi area intrusions, plot in the field of banakites and high-K andesites, rarely – in the field high-K dacites and shoshonite field. Basic rocks from Kirari and Abakuri intrusions plot in the field of basalts and low-K basalts. In the P2O5% / Zr ppm diagram [36], the main part of figurative points (more than 80%) of intrusions plot in the field of tholeiitic basalt. On the Na2O + K2O%/SiO2 diagram [40] half of the points disposed within alkaline and the rest in sub-alkaline field. The rocks of basic composition are less alkaline. Rock alkalinity increases with the acidity of the rocks. Figurative points of the Kirari intrusive rock samples are evenly arranged in the alkaline and sub-alkaline fields while for Abakuri figurative points the deviation to the sub-alkalinity is observed. In AFM diagram [40] all the points of diorites, syenites and monzonites from the Dizi village area are plotted in the calc-alkaline field. The Kirari intrusive formations are of more calc-alkaline in composition, though tholeiitic formations are observed as well. The Abakuri intrusive formations also are of tholeiitic composition. According to trace and rare earth element concentrations discrimination diagrams for the tectonic interpretations of granitic rocks [45] are created. As a result, syenite-diorite intrusions of the Dizi series belong to the granitoids of continental, volcanic arc and syn-collisional type.

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