MINERALOGICAL AND GEOCHEMICAL ASPECTS OF THE INCOMPATIBLE LARGE-IONIC ELEMENTS BEHAVIOR OF THE TRACHYBASALT-ESSEXITE AND TEPHRITE-TESCHENITE COMPLEXES OF THE CAUCASUS SEGMENT OF THE MEDITERRANEAN BELT
M.N. Mamedov * O.Z. Dudauri ** G.Dj. Babayeva * M.G. Toganidze **
*Geology Institute of Azerbaijan National Academy of Sciences,
** State University of Tbilisi, Institute of Geology named after A.I. Janelidze, Tbilisi, Georgia
Minerals-indicators, characterizing the various crystallization stages of alkaline and sub-alkaline basaltoid melts, are carriers and concentrators of the compatible and incompatible elements. In this context, reveal of the mineralogical-geochemical peculiarities of the iron-group elements behavior, alkaline and calc-alkali and also rare earth elements in the evolution of the primary melts of the Cretaceous trachybasalt-essexite and tephrite-teschenite complexes become important. Considered complexes within the northern slope of the Greater Caucasus (in Besachyn and watershed parts), Georgian block (Kursebi, Tsitskhvati, Opurchkheti, Banodja, Khanchiori and etc.) as well as Khojavend trough (Tug, Gishi, Chartaz and etc.) are located among the Jurassic and Cretaceous deposits.
Trachybasalt-essexite complex compared with tephrite-teschenite complex is characterized by lower areal extent.
Only subvolcanic facies of the above-mentioned complex have developed within the Greater Caucasus. They as low-power force and stock-shaped body are confined to the Early Jurassic deposits and according to absolute age determination confirm to intervals of the upper part of the Early Cretaceous and lower part of the Late Cretaceous (80-130 Ma).
Teschenites as essexites have been presented by hypabyssal low-power force and stratal intrusives.
Trachybasalt-essexite complex is characterized by limited areal extent within Georgian blocks. Moreover effusive and subvolcanic facies of this complex have been formed by trachybasalts and trachydolerites in the bulk of Mtavar suite. They correspond to Turonian-Santonian age according to formation time.
As part of the sequential complex the effusive facies rock formed by picrotephrites and tephrites in bulk of the Mtavar suites, have also restrictive evolution. Hypabyssal teschenite intrusives of the Georgian blocks have been separated from its volcanic component. Moreover all intrusives are characterized by differentiated structure. Thereby low-power (0.5-1.0 m), dark grey, black interlayers of picroteschenites are located in the intrusive bottom parts which gradually changed vertically by melano-, meso-and leucocratic teschenites. The light grey interlayers of the teschenite-syenite are located in the apical parts of the intrusives. Finally intrusives have been cut by whitish-grey analcimite and in some cases, nepheline syenites.
In contrast to the previous regions within Khodjavend trough the rocks of the considered complexes have a large areal extent in the effusive facies.
The rocks of the trachybasalt-trachydolerite complex have mainly concentrated in the south-western and north-western trough margins. Subvolcanic facies are located in the intersection nodes of the transverse and longitudinal faults as well as tawing formation.
Tephrite-teschenite complex has confined to the axial zone of the trough with strong lava-pyroclastic formations predominance. Subvolcanic dykes and hypabyssal intrusives are relatively less spread.
In the mineralogical and geochemical ratio the rocks of the trachybasalt-essexite complex enriched by the same incompatible elements as potassium, rubidium, strontium, barium and light lanthanides. K-feldspar, biotite, partially calcium amphibole and plagioclase are the carrier and the concentrator of these elements in the rocks of this complex. The behavior of the mentioned components in the tephrite-teschenite complex has been determined by crystallization differentiation. So, in the early accumulative stage of the crystallization of the carriers and concentrators of potassium, rubidium, barium were the phlogopite megacrystals in the condition of the intermediate focus and hypabyssal intrusive chamber.
In connection with the crystallization limitation in the composition of the melanocratic representative of tephrites, the above-mentioned elements were concentrated in the residual melt. In this regard the largest concentration of these elements has been determined in the composition of the late differentiates of the teschenite intrusives.
Therefore the large lithophilous elements behavior in the teschenite intrusives composition has been caused by crystallization differentiation which occurred in the hypabyssal intrusive chamber.
Short duration of the tephrite melt fractionation process in the intermediate focus was the reason of the weak quantitative difference between petrographic rocks types of the effusive facies.