COMPARATIVE SYSTEMATICS OF BASALTOIDS FROM THE EXTENSION ZONES OF THE ASIAN CONTINENTAL MARGIN AND THE OCEANIC ISLANDS OF THE PACIFIC OCEAN: ISOTOPIC AND SEISMIC TOMOGRAPHIC PERSPECTIVES

Abstract

Comparison of the Sr-Nd-Pb isotopic characteristics of the Cenozoic extension zone basalts (MEZB) of the Eurasian margin and the Cretaceous-Cenozoic OIB of the southern Pacific isotopic and thermal anomaly (SOPITA) demonstrated their close resemblance. Both rock suites belong to WPB-type volcanics showing enrichment in EM components and wide occurrence of EMI compositions, which are considered as indicators of a lower mantle source (Hart, 1988). The origin of SOPITA OIB is explained by the activity (beginning at a boundary of 130 Ma) of the lower mantle Pacific superplume, which corresponds to a low-velocity anomaly (Larson, 1991; Fukao et al., 1994; etc.). The synthesis of global seismic tomographic images (Vasco and Johnson, 1998) demonstrated that this superanomaly is connected with a system of lateral circum-Pacific low-velocity anomalies of various levels. This allows us to suppose that the lower mantle material of the Pacific superplume extended horizontally at several levels (1270-1470 and 35-200-350 km) giving rise to lateral plumes. Consequently, the Pacific superplume system with connected lateral plumes was probably the general cause of the WPB-type volcanism of the Cretaceous-Holocene in islands of the Darwin Rise and Polynesia and the Cenozoic in extension zones at the Eurasian margin. Since the superplume developed from the D″ layer of the lower mantle toward near-surface layers, the lower mantle perioceanic lateral plume at a depth of 35-200 km is the youngest, which explains the latest, Cenozoic, occurrence of alkaline volcanism in the Eurasian margin in contrast to the intraoceanic areas, where the major stem of the Middle Cretaceous superplume projected. The near-surface localization (according to seismic tomography) of the lower mantle lateral plume at the western periphery of the Pacific Ocean explains the WPB composition of MEZB and, in general, the DUPAL mantle signature of this region. Another common characteristic of OIB and MEZB is the lateral mosaicism in the distribution of basalts with different isotopic signatures. It is probably related to the activity of numerous individual plumes of higher orders branching out of the superplume or a heterogeneous lateral plume. The differences between the OIB and MEZB compositions (contributions from DMM and HIMU in OIB and subduction comportent in MEZB) are related to different environments of their accumulation: intraoceanic in the former case and continental margin (with preliminary suprasubduction metasomatism) in the latter case.