Nd isotopes and tectonics of 1.9-1.7 Ga crustal genesis

Abstract

The origin of 1.9-1.7 Ga crust in Europe, Greenland and North America can be evaluated using Nd and other isotopic information. Terrains stabilized 1.9-1.7 Ga ago make up 74% of the pre-1.6 Ga continental area, but interpretation of Nd data shows that only ca. 50% of this is new mantle-derived crust, while the other half is recycled or reactivated Archean material. These terrains can be divided into (1) those with > 90% reactivated Archean crust, concentrated in northern Greenland and Canada, whose origin is enigmatic, and (2) those with > 80% newly-differentiated material, containing only a limited Archean contribution, probably in the form of recycled sediments. The new terrains occur principally in a wide zone from Arizona through Colorado, Michigan, South Greenland, Sweden, Finland to the western U.S.S.R., and they border the present southern margin of the Wyoming, Superior, North Atlantic and Kola Archean blocks. The new terrains contain a high proportion of volcanic and plutonic rocks resembling those of present-day island arcs and continental margins, and they aomost certainly represent a major subduction-related mantle-to-crust differentiation during 1.9-1.7 Ga. Igneous activity was complex in pattern from area to area, with sequential accretion of volcano-plutonic belts outwards from Archean continents. The overall crust production rate from the northern continents in the period 1.9-1.7 Ga was ∼ 1.2 km 3/a, which is very slightly greater than the total Phanerozoic island-arc accretion rate. Thus Phanerozoic or only slightly higher rates of subduction can explain the 1.9-1.7 Ga new terrains only if most subduction-related igneous activity in the 1.9-1.7 Ga world was concentrated in the Arizona-Finland zone. Interpretation of limited geological information available from other continents suggests that this was not the case, and that a crust production rate around double the present one prevailed 1.9-1.7 Ga ago. The results document major crustal growth during Proterozoic time.