Earliest Precambrian ultramafic-mafic volcanic rocks: Ancient oceanic crust or relic terrestrial maria?

Abstract

Data on aspects of Archean shield geology are consistent with, although not necessarily indicative of, impact of extraterrestrial matter, possibly contemporaneous with impacts that affected the Moon about 4.1 to 3.8 × 109 yr ago. The isotopically oldest gneisses of several shield areas abound in xenoliths of ultramafic-mafic volcanic rocks, minor silicic volcanic rocks, and derived and (or) chemical sedimentary rocks. No confident oldest age limits were set on these xenoliths, which antedate the development of the true greenstone belts in these areas and are thought to represent relics of a once-widespread volcanic crust. This crust differed from modern oceanic crust in the following ways: (1) peridotitic, high-Mg, and silicic volcanic rocks were more abundant in the Archean crust than in the modern crust; (2) sea-floor spreading and crustal overturn in early Archean time should have resulted in the rapid development of large continents at that stage, contrary to observations; and (3) the apparent retention of a coherent parallel Archean tectonic pattern on Gondwanaland reconstructions renders large-scale plate movements, and thus sea-floor spreading, unlikely. Had the early Archean ultramafic-mafic crust been genetically related to impact, the recurrence of chemically similar volcanic rocks within greenstone belts up to about 2.6 × 109 yr ago may signify repetition and gradual waning of mantle activity initially triggered by such events. Evidence bearing on the impact theory may be gained by a search for shock metamorphic effects in the oldest agmatites and in fragments and detrital grains incorporated in the earliest sedimentary intercalations of volcanic sequences.