Petrogenesis of variolitic lavas of the Onega structure, Central Karelia

Abstract

Varioles and matrix have been studied in the typical globular rocks—variolites of the Yalguba Range and Suisari Island in the Onega structure, Central Karelia. It was determined that the cores of the varioles are significantly enriched in silica, Na, K, Rb, Cl, and Ba, and have lower K/Na ratio as compared to matrix. In addition, varioles strongly differ from matrix in oxygen isotope composition (δ18Ov-δ18Om varies from 1.6 to 2.6‰). The consideration of possible mechanisms of the formation of the Onega variolites with allowance for available isotopic and geochemical data (major element composition, REE and trace element distribution) demonstrated that the observed geochemical characteristics of matrix and globules of the Yalguba Range and Suisari Island variolites cannot be formed by melt mixing. Such processes as greenstone alteration and crystallization of spherulites during melt overcooling also did not define the isotopic and geochemical peculiarities of the Onega variolites, but played only subordinate role. Except for significant oxygen isotope shift in the variole-matrix system, the obtained data are best consistent with liquation model. However, few available experimental data on the distribution of trace elements and oxygen isotopes during liquid immiscibility make it impossible to reach decisive conclusion concerning liquation genesis of the Onega variolites.