ПРОСТРАНСТВЕННО-ВРЕМЕННАЯ СМЕНА ГЛУБИННЫХ ИСТОЧНИКОВ КАЙНОЗОЙСКИХ ВУЛКАНИЧЕСКИХ ПОРОД ВОСТОКА КОРЯКСКОГО НАГОРЬЯ

Abstract

A spatiotemporal analysis of the trace element ratios and the isotopic composition of Sr, Nd and Pb in Cenozoic post-accretionary volcanic rocks in the east of the Koryak Highlands (Northeastern Russia) has been carried out. The Early Paleogene volcanic complex is shown to be represented by moderately Ti-rich aluminous tholeiites with elevated contents of high field strength elements (HFSE) (except for Ta and Nb), which makes them similar to E-MORBs. Low (La/Yb)n and high Zr/Nb (25–35) ratios characterize one of the source components as depleted and similar to MORBs. At the same time, low ratios of Nb/Ta, Ce/Pb, Nb/La with a high ratio of K/Nb were determined in basalts, which indicates the presence of a subduction component in the source. Low Zr/Hf and (Dy/Yb)n ratios indicate melting of the garnet-free substratum. The second (Miocene) stage is represented by the subalkaline basalt flows and dacite extrusions and dikes forming a bimodal series. Basalts are characterized by low concentrations of large ion lithophile elements (LILE), enrichment in HFSE, and fractionated distribution of REE, with (La/Yb)n ratios varying from the values characteristic of E-MORBs to the values corresponding to intraplate tholeiites. The Nb/Ta, Ce/Pb, La/Ta, and Hf/Th ratios tend to exhibit compositions of intraplate tholeiites. The (Dy/Yb)n and (La/Yb)pm - Ybpm ratios indicate the formation of Miocene basalts during the selective melting of garnet peridotite with varying garnet contents in the source. Dikes of dacites, in comparison with effusive facies, differ in calc-alkaline differentiation trends and other absolute concentrations of a number of elements. Early Quaternary alkaline olivine basalts and basanites of the Navarin area are high in HFSE, LILE, and REE and are compositionally similar to intraplate volcanics of oceanic islands and continental rifts. Data points of these rocks plotted on discriminant diagrams are confined to the field of the garnet-rich intraplate source with low degrees of partial melting. The initial isotopic ratios of Sr, Nd, and Pb in the Cenozoic basaltoids characterize the deep sources of Cenozoic rocks in the eastern part of the Koryak Highlands as depleted. The similarity of isotope ratios in the basalts of the Miocene and Early Quaternary stages of volcanism suggests that the mantle region was isotopically homogenized as a result of local convection.