Geology, geochemical typification and petrogenic model of formation of Middle Paleozoic metabasites of the Khrami crystalline massif (Georgia)

The Khrami crystalline massif is located in the northern part of the Lesser Caucasus, in the Black Sea - Transcaucasian terrane. The massif outcrops Middle Paleozoic metabasites, which crosscut the Precambrian gneiss-migmatite complex and, in turn, are cut by Late Variscan granites. These metabasites have not experienced Precambrian prograde HT/LP (720-770°C, P<1.5 kbar) regional metamorphism, although retrograde LT/LP (T≈430-510⁰C, P≈0.6-1 kbar) metamorphism, associated with the Sudetian orogeny has been recorded. According to the presented geological data, the age of metabasites is within the Cambrian and Upper Paleozoic. Considering the analogy between the metabasites spread in the Dzirula crystalline massif, which is exposed in the same terrane, and the metabasites of the Khrami massif, the age of the latter is most likely Middle Paleozoic (Shengelia et al., 2022). The metabasites of the Khrami massif are represented by veins (1-60 m) and stock-shaped bodies (80-800 m) of fine-grained ophitic gabbro, gabbro-diabases and diabases of various thicknesses. They are cut by numerous granite veins and penetrated by thin quartz-feldspar injections. The paragenesis of the high-temperature magmatic stage - Cpx+Pl_78-84 has been preserved in metabasites in some places; Further, under the conditions of greenschist facies, the paragenesis Ab+Act(Tr)+Chl+Ep±Qz develops. According to the petrogenic diagrams Na₂O+K₂O – SiO₂, the metabasites of the Khrami massif belong to the formations of the subalkaline series (Irvine and Baragar, 1971), correspond to basalts and andesite-basalts (Le bas et al., 1986) and basalts and picrites (Cox et al., 1979). This is confirmed by the data of diagrams Zr/Ti-Nb/Y (Pearce, 1996) and Zr/TiO₂ – Yb/Y (Winchester, Floyd, 1977). According to the Na₂O+K₂O–FeO*-MgO (Irvine and Baragar, 1971), a great part of the metabasites is of tholeiitic composition, and only a small part is of calc-alkaline composition. On the diagram Fe*-SiO₂ (Frost et al., 2008) the dots denoting metabasites are completely disposed in the magnesian field. According to the TiO₂ - Zr/(P₂O₅*10⁴) diagram (Winchester, Floyd, 1976), the metabasites correspond to tholeiite basalts. According to the diagram V-Ti/1000 (Shervias, 1982), the metabasites belong to the MORB genetic formation, and according to the diagram Cr-Y (Pearce, 1982), they belong mainly to the VAB, and also to the MORB. According to the ratio MnO-TiO₂/10-P₂O₅ (Mullen, 1983), dots of mafic rocks are located in the island-arc tholeiitic field. Thus, the Middle Paleozoic metabasites of the Khrami crystalline massif are represented by shallow subvolcanic magmatites predominantly of andesite-basalt and tholeiite-basalt groups of the tholeiitic series. They correspond to the MORB and VAB genetic groups.