REE and Nd isotope geochemistry, petrogenesis and volcanic evolution of contaminated komatiites at Kambalda, Western Australia

Abstract

The 2.7 Ga-old volcanic sequence at Kambalda comprises a lower formation of massive and pillowed tholeiitic basalts (Lunnon basalt); a middle formation (Kambalda komatiite) of channel- and sheet-flow facies komatiitic peridotites with thin, intercalated sedimetns (Silver Lake member) and thin, massive, aphyric and differentiated spinifex-textured komatiites (Tripod Hill member); upper formations of ocellar; pillowed and massive komatiitic basalts (Devon Consuls basalt) and thin, massive and pillowed komatiitic basalts and thick layered sills/flows (Paringa basalt). The komatiites represent a regressive lava sequence, reflecting decreasing flow rates and increasing magma viscosities with time. These mantle-derived lithologies are intruded and overlain by crustally-derived lithologies: felsic and intermediate dikes and plutons (e.g. Kambalda granodiorite) and felsic and intermediate lavas, breccias, tuffs and epiclastic sediments (Black Flag group). Whole-rock major-element, trace-element and Nd-isotopic compositions of the mafic and ultramafic lavas vary systematically with stratigraphic location. REE have been mobile in certain strongly carbonated komatiites, but alteration cannot explain the systematic variations. Lunnon basalts are characterized by low MgO (5–8%), moderately low [La/Sm]n (0.7–0.9) and moderately high ϵNd values (+2.1 to +3.7); they are uncontaminated, slightly fractionated, low-degree partial melts of a mixture of depleted and undepleted mantle, and were probably derived from the head of a mantle plume. Spinifex-textured lavas in the channel-flow facies of the Silver Lake peridotite are characterized by very high MgO (16–31%), low [La/Sm]n (0.4–0.7) and variably high ϵNd values (+1.8 to +5.4); they are variably fractionated, only slightly contaminated, high-degree partial melts of highly depleted mantle, and were probably derived from the tail of a mantle plume. The parental komatiite is inferred to have contained ⩾30% MgO, [La/Sm]n ⩽ 0.5 and ϵNd ⩾ +5. Pyroxene-spinifex-textured and porphyritic-pyroxene lavas in sheet-flow facies are characterized by moderated MgO (21 to 12%), moderately low [La/Sm]n (0.6–1.0) and a moderately high ϵNd value (+2.3); they were contaminated by 2–5% of granitic crust and/or interflow sediment ± basalt and fractionated during eruption and emplacement. Aphyric and olivine-spinifex-textured lavas in the Tripod Hill komatiite have variably high MgO (15–32%, generally <26% MgO), moderately low [La/Sm]n (0.6–0.8) and moderately high ϵNd values (+2.7 to +4.4); they were contaminated by 2–5% of granitic crust and fractionated during ascent. Devon Consuls and Paringa basalts are characterized by low to moderate MgO (4–16% and 12–16%, respectively), high to very high [La/Sm]n (1.3–1.4 and 2.5–2.8) and moderate to very low ϵNd values (+3.3 to +1.2 and −1.2 to −2.4); they were contaminated by 5–7% and 20–30% of granitic crust, respectively, and fractionated during ascent. Thus, the degree contamination-accelerated fractional crystallization increased with time (Lunnon basalt→Silver Lake peridotite→Tripod Hill komatiite→Devon Consuls basalt→Paringa basalt), as magma conduits became heated, as erupted rates declined, and the magmas cooled and became more viscous. This produced the observed regression in lava facies upwards through the sequence. Age dates and thermal constraints indicate that this was only indirectly related to crustal underplating by the source magma (plume), but crustal heating eventually culminated in wholesale melting of the crust, intrusion of felsic plutons and eruption of felsic-intermediate volcanics. The degree of contamination also varied with location and time within individual flow units. The channel-flow facies of the Silver Lake peridotite, which thermally-eroded the most footwall rocks, contains some of the least-contaminated lavas, whereas the adjacent sheet-flow facies, which overlies uneroded sediment, containes some very contaminated komatiites and rare sediment xenomelts. The contaminants in the sheet-flow facies must be derived from an earlier stage of turbulent flow and thermal erosion in a turbulently-flowing sheet flow upstream from Kambalda, which evolved with increasing distance and time into a turbulently-flowing channel-flow facies flanked by a laminarly-flowing sheet-flow facies at Kambalda. Both facies crystallized and accumulated significant amounts of olivine during emplacement, forming thick lower cumulate zones, but eventually ponded and fractionated, producingk highly differentiated spinifex-textured zones. The sheet-flow facies crystallized relatively early, preserving the contaminants, whereas the channel-flow facies was replenished by more primitive magmas and crystallized later, after eruption rates declined and the conduits and lava formed chilled margins.