A Spatiotemporal Change in Deep Sources for Cenozoic Volcanic Rocks in the Eastern Koryak Highlands

A spatiotemporal analysis of the trace-element ratios and Sr, Nd, and Pb isotopic composition of Cenozoic post-accretionary volcanic rocks in the eastern Koryak Highlands (Northeastern Russia) has been carried out. The Early Paleogene volcanic complex is shown to be represented by moderate-Ti aluminous tholeiites with elevated contents of high field strength elements (HFSE) (except for Ta and Nb), which makes them similar to E-MORBs. The low (La/Yb)(n) and high Zr/Nb (25-35) ratios indicate the contribution of a depleted MORB-type component. At the same time, the basalts have low Nb/Ta, Ce/Pb, and Nb/La ratios at a high K/Nb ratio, which indicates the presence of a subduction component in the source. The low Zr/Hf and (Dy/Yb)(n) ratios indicate melting of a garnet-free protolith. The second (Miocene) stage is represented by subalkaline basaltic flows and dacitic extrusions and dikes forming a bimodal series. The basalts are characterized by low concentrations of large ion lithophile elements (LILE), enrichment in HFSE, and a fractionated REE pattern, with (La/Yb)(n) ratios varying from typical E-MORB values to those of within-plate tholeiites. The Nb/Ta, Ce/Pb, La/Ta, and Hf/Th ratios are close to those of within-plate tholeiites. The (Dy/Yb)(n) and (La/Yb)(pm)-Yb-pm relations indicate the formation of Miocene basalts through selective melting of garnet peridotite with varying garnet contents in the source. Dacitic dikes, compared to the effusive facies, differ in the calc-alkaline differentiation trends and absolute concentrations of some elements. Early Quaternary alkaline olivine basalts and basanites of the Navarin area have high HFSE, LILE, and REE contents and are compositionally close to within-plate volcanic rocks of oceanic islands and continental rifts. The data points of these rocks in the discriminant diagrams are confined to the field of the low-degree melting of a garnet-rich within-plate source. The initial Sr, Nd, and Pb isotopic ratios indicate the derivation of Cenozoic basaltic rocks in the eastern part of the Koryak Highlands from depleted deep-seated sources. The similarity of isotope ratios in the Miocene and Early Quaternary basalts testifies that the source mantle was isotopically homogenized by local convection.

Automated summary

The spatiotemporal analysis of Cenozoic volcanic rocks in the eastern Koryak Highlands in Northeastern Russia reveals a complex composition and origin, including Early Paleogene volcanic rocks influenced by E-MORBs, a bimodal series of subalkaline rocks in the Miocene, and alkaline rocks in the Early Quaternary. The formation of these rocks involves melting of garnet-free protolith with selective garnet peridotite melting, contributions from subduction components, and isotopic homogenization of the source mantle.