Temporal variations in the incompatible trace element systematics of
Archean volcanic rocks: Implications for tectonic processes in the
early Earth
ABSTRACT
Temporal variations in the incompatible trace element geochemistry of volcanic rocks in Archean greenstone
belts have major implications for the style of tectonics that operated in the early Earth, and if and when plate
tectonic processes occurred in the Archean, which are still subjects of substantial debate. Comparing the
geochemistry of Archean volcanic rocks with that of Phanerozoic arc volcanic rocks has the potential to shed
light on these questions. Geochemical data from 8,249 Eoarchean to Neoarchean volcanic rocks and 20,099
Phanerozoic arc volcanic rocks were compiled from the literature to address the above questions through the
application of temporal incompatible trace element ratio variations, N-MORB-normalised trace element dia
grams,and tectonic setting discrimination diagrams. The sampled rocks range in composition from ultramafic
through basaltic and andesitic to dacitic/rhyolitic. Most of the incorporated Archean volcanic rocks were deemed
to have been unaffected by significant alteration or crustal contamination in the literature and, therefore, to
reflect their provenance, a feature that was corroborated by this study. Most of these Archean volcanic rocks plot
in the plate margin, oceanic arc and continental arc fields in classification and tectonic setting discrimination
diagrams, with the remainder plotting in the alkaline arc, mid-ocean ridge and oceanic island fields. Comparison
between N-MORB-normalised trace element diagrams of volcanic rocks from well-studied Archean greenstone
belts in Greenland, Canada, South Africa, China, Australia, India, Brazil and Finland and volcanic rocks from
well-studied modern arcs demonstrates that their trace element patterns are remarkably similar. This indicates
that the former formed in arc-related settings by modern-style plate tectonic processes that operated throughout
the Archean. The Pb and Nb anomalies of most Archean volcanic rocks are fully consistent with an arc-related
setting. The temporal variations in the incompatible trace element ratios of Archean volcanic rocks, coupled
with their lithological associations, demonstrate that intra-oceanic arc magmatism was prominent in the
Eoarchean before a shift in these ratios in the Paleoarchean signified the beginning of Andean-style continental
arc magmatism between 3500 and 3200 Ma. Modern-style plate tectonic processes were a far more important
contributor to Archean crustal growth and evolution than sagduction-driven vertical tectonic processes.
Keywords:
Archean volcanic rocks
Archean tectonics
Temporal variation
Trace element systematics
Modern arc volcanic rocks
