Origin of the 3500-3300 Ma calc-alkaline rocks in the Pilbara Archaean: isotopic and geochemical constraints from the Shaw Batholith

Abstract

Calc-alkaline plutonic rocks, intruded at ∼3450Ma, comprise a major component of the Shaw Batholith in the Archaean east Pilbara Block, Western Australia. New whole-rock Pb isotopic geochronology confirms the extent of these rocks, but a minor plutonic phase is dated at 3338±52 Ma and represents a second plutonic event of the same age as much of the nearby Mt Edgar Batholith. The Sm-Nd isotopic systematics of the ∼3450Ma rocks imply their derivation from a heterogeneous source, which probably included a slightly older crustal component as well as a depleted mantle component. The 3338±52 Ma pluton includes components derived from crustal sources older than 3600 Ma. The geochemistry and SmNd isotopic systematics of these rocks are consistent with crustal growth in the early Archaean from upper mantle sources as depleted as the modern upper mantle. The Shaw Batholith calc-alkaline suites exhibit very similar chemical trends on variation diagrams to modern calc-alkaline plutonic rocks which can be modelled by a combination of mixing and fractionation. A suite collected from outcrops displaying prominent igneous layering shows distinct geochemical trends which can be modelled by differentiation into a component enriched in ferromagnesian minerals, principally hornblende, and possibly sphene, magnetite and epidote, and into a leucocratic component containing quartz, plagioclase and K-feld-par. These Archaean calc-alkaline plutonic rocks, in common with rocks from many other Archaean calc-alkaline provinces, exhibit very fractionated REE patterns with depleted HREE contents, a feature considered to result from equilibrium with garnet at depth in lower crustal regions. The geochemistry of the Pilbara Archaean calc-alkaline rocks is identical to the subset of modern continental-margin calc-alkaline plutonic rocks with fractionated REE patterns, such as those from the central and eastern Peninsular Ranges Batholith, western USA. The tectonic setting in which the Archaean calc-alkaline rocks formed is still not known. This reflects both uncertainty associated with the petrogenesis and environments of modern calc-alkaline rocks, as well as the limited knowledge of the precise timing and relationships of plutonic, depositional and tectonic events in the Pilbara Archaean.