SUBDUCTION-RELATED JURASSIC ANDESITES IN THE NORTHERN GREAT CAUCASUS

During the Jurassic the major tectonic units of the Great Caucasus (Be chasyn, Front Range, Main Range and Southern Slope zone) were affected by intensive magmatic activity. Magmatism within the Bechasyn zone, t he northernmost unit, which represents the southern part of the Varisc an-consolidated Skythian platform is considered here. With the beginni ng of the Early Jurassic this zone was reactivated by subsidence, acco mpanied by the deposition of epicontinental shallow water sediments. T he Lower Jurassic portion of this sedimentary pile was intruded by num erous sills which display a clear temporal and spatial evolution. The older basic rocks are lower in the profile than the younger, more acid ic rocks. A set of 75 samples, representing all exposed sills and thei r feeder-dikes, was analyzed for major and 21 trace elements. All samp les appear more or less affected by alteration under lower greenschist facies conditions. However, these alterations essentially took place on local scales and did not affect the overall chemistry. According to their main element composition the rocks constitute a calc-alkaline s eries ranging from basaltic - andesitic to rhyolitic. Most of the samp les are andesites. Chemically, these andesites closely resemble modern orogenic andesites occurring at convergent plate margins. Altogether, the field evidence and the chemical and mineralogical data obtained s how the investigated rocks to be comagmatic and derived from basalt - andesitic initial melts by magmatic fractionation processes. Tholeiiti c melts have to be considered as parental magmas, which according to t he trace element characteristics of the basalt-andesitic rocks, were g enerated from an enriched peridotitic mantle source. Sr-87/Sr-86 isoto pe ratios and delta(18)O values confirm the mantle origin of this rock series. The observed compositional evolution can be explained as a re sult of olivine and clinopyroxene fractionation of the tholeiitic melt s followed by amphibole and plagioclase separation. Ar-40/Ar-39 measur ements on biotite and plagioclase phenocrysts separated from these roc ks vary between 190 and 180 Ma and thereby place the magmatic activity in the late Early Jurassic, in good agreement with the stratigraphic observations. Genetically, the calc-alkaline rocks are related to a su bduction zone of the Andean type. Their chemical and isotopic composit ions and their age setting corroborate the plate tectonic models for t he evolution of the Caucasus orogenic belt during the Jurassic.