PETROLOGY, GEOCHEMISTRY, AND SM-ND AGE OF THE PRE-LATE HERCYNIAN PALEOOCEANIC COMPLEX OF THE DZIRULA SALIENT, TRANSCAUCASIAN MASSIF

Metaophiolites were examined in the tectonic melange of the Chorchana- Utslevi zone, along with the accompanying calc-alkaline gabbroids, dio rites, quartz diorites, and metasediments of the pre-late Hercynian co mplex in the crystalline basement of the Dzirula salient, Transcaucasi an Massif. The metaophiolites correspond to fragments of the paleoocea nic lithosphere of the spreading-zone type. The ultramafic constituent s are clinopyroxene-bearing spinel harzburgite of the oceanic type, wh ich is restite after extraction of tholeiite basalts. The protolith of the metabasics (amphibolites and epidote amphibolites T = 500-450 deg rees C, P = 4-3.5 kbar) compositionally approximated tholeiites of the N- and T-MORB types. The whale rock Sm-Nd isotopic dat of the metabas ites correspond to an age of T = 810 +/- 100 Ma, epsilon(Ndinit) = 7.3 7 +/- 0.55. The pre-Late Hercynian gneiss-migmatite complex underwent extensive metamorphism and palingenesis under amphibolite-facies condi tions. The process was accompanied by the emplacement of numerous subv olcanic mafic intrusions of the calc-alkaline series. The youngest cal c-alkaline gabbroids have the following isotopic-geochemical character istics: T = 607 +/- 78 Ma, epsilon(Ndinit) = 1.2, (La/Sm)(n) = 2.30 +/ - 0.60, (Yb)(MORB) = 0.48 +/- 0.17. The palingenetic diorite and quart z diorite of the gneiss-migmatite complex show strong geochemical simi larities with the gabbroids. The gneiss-migmatite complex was dated at the Late Proterozoic and identified as a type of primitive (immature) continental crust. The rack association examined in the Transcaucasia n Massif was produced during the Pan-African (Baikalian) tectonic cycl e, with the massif itself being the northern periphery of the Arabian- Nubian Shield in Late Proterozoic time. The metasediments from in the tectonic melange of the Chorchana-Utslevi zone originated during the e arly stages of development of the northern Phanerozoic passive margin of Gondwana.