REGIONAL FEATURES OF PRIMARY ALKALINE MAGMAS OF THE ATLANTIC OCEAN

Abstract

In the framework of the GIS project on the geochemistry of Atlantic intraplate magmatism, primary high-magnesian melts were identified there and subdivided into five types: foidites, picrites, basanite-nephelinites, alkaline olivine basalts, and tholeiite. Their relative proportions were determined for both the Atlantic Ocean as a whole and individual magmatic centers. The compositional ranges and average compositions were calculated. It was shown that alkali rocks are predominant, but tholeiite melts account for about 25%. Among ocean-island volcanic rocks, differentiated varieties clearly dominate over primary melts (80 and 20%, respectively). Variations in the proportions of the distinguished types were applied to prepare a map for the petrochemical typification of Atlantic intraplate magmatism. Seven petrochemical zones were provisionally identified, first demonstrating the lateral petrochemical heterogeneity of intraplate sources of the Atlantic Ocean. In addition to the global heterogeneity, each large center of intraplate magmatism (archipelago or island chain) demonstrates local heterogeneities. The variations in the Na/K, Ti/Na, and Si/Ca ratios reflect significant magma generation depths (in the lower mantle) for intraplate magmatism. It was proposed that variations in the Ti/Na ratio in the high-magnesian melts are controlled by a change in the Na and Ti partition coefficients of pyroxene with increasing magma generation depth. A comparison between evolution of the oceanic and continental alkaline magmatism was conducted.