REE AND ND ISOTOPE GEOCHEMISTRY, PETROGENESIS AND VOLCANIC EVOLUTION OF CONTAMINATED KOMATIITES AT KAMBALDA, WESTERN-AUSTRALIA

The 2.7 Ga-old volcanic sequence at Kambalda comprises a lower formati on of massive and pillowed tholeiitic basalts (Lunnon basalt); a middl e formation (Kambalda komatiite) of channel- and sheet-flow facies kom atiitic peridotites with thin, intercalated sediments (Silver Lake mem ber) and thin, massive, aphyric and differentiated spinifex-textured k omatiites (Tripod Hill member); upper formations of ocellar, pillowed and massive komatiitic basalts (Devon Consuls basalt) and thin, massiv e and pillowed komatiitic basalts and thick layered sills/flows (Parin ga basalt). The komatiites represent a regressive lava sequence, refle cting decreasing flow rates and increasing magma viscosities with time . These mantle-derived lithologies are intruded and overlain by crusta lly-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-e lement, trace-element and Nd-isotopic compositions of the mafic and ul tramafic lavas vary systematically with stratigraphic location. REE ha ve been mobile in certain strongly carbonated komatiites, but alterati on cannot explain the systematic variations. Lunnon basalts are charac terized by low MgO (5-8%), moderately low [La/Sm](n) (0.7-0.9) and mod erately high epsilon(Nd) values (+2.1 to +3.7); they are uncontaminate d, slightly fractionated, low-degree partial melts of a mixture of dep leted and undepleted mantle, and were probably derived from the head o f a mantle plume. Spinifex-textured lavas in the channel-flow facies o f the Silver Lake peridotite are characterized by very high MgO (16-31 %), low [La/Sm](n) (0.4-0.7) and variably high epsilon(Nd) values (+1. 8 to +5.4); they are variably fractionated, only slightly contaminated , high-degree partial melts of highly depleted mantle, and were probab ly derived from the tail of a mantle plume. The parental komatiite is inferred to have contained greater than or equal to 30% MgO, [La/Sm](n ) less than or equal to 0.5 and epsilon(Nd) + 5. Pyroxene-Spinifex-tex tured and porphyritic-pyroxene lavas in sheet-flow facies are characte rized by moderate MgO (21 to 12%), moderately low [La/Sm](n) (0.6-1.0) and a moderately high epsilon(Nd) value (+2.3); they were contaminate d by 2-5% of granitic crust and/or interflow sediment +/- basalt and f ractionated during eruption and emplacement. Aphyric and olivine-spini fex-textured lavas in the Tripod Hill komatiite have variably high MgO (15-32%, generally < 26% MgO), moderately low [La/Sm], (0.6-0.8) and moderately high epsilon(Nd) values (+2.7 to +4.4); they were contamina ted by 2-5% of granitic crust and fractionated during ascent. Devon Co nsuls and Paringa basalts are characterized by low to moderate MgO (4- 16% and 12-16%, respectively), high to very high [La/Sm]. (1.3-1.4 and 2.5-2.8) and moderate to very low epsilon(Nd) values (+3.3 to +1.2 an d -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 of contamination-accelerated fractional crystallization increased wit h time (Lunnon basalt-->Silver Lake peridotite-Tripod Hill komatiite-- >Devon Consuls basalt-->Paringa basalt), as magma conduits became heat ed, as eruption 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 fel sic-intermediate volcanics. The degree of contamination also varied wi th location and time within individual flow units. The channel-flow fa cies of the Silver Lake peridotite, which thermally-eroded the most fo otwall rocks, contains some of the least-contaminated lavas, whereas t he adjacent sheet-flow facies, which overlies uneroded sediment, conta ins 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 dista nce and time into a turbulently-flowing channel-flow facies flanked by a laminarly-flowing sheet-flow facies at Kambalda. Both facies crysta llized and accumulated significant amounts of olivine during emplaceme nt, forming thick lower cumulate zones, but eventually ponded and frac tionated, producing highly differentiated spinifex-textured zones. The sheet-flow facies crystallized relatively early, preserving the conta minants, whereas the channel-flow facies was replenished by more primi tive magmas and crystallized later, after eruption rates declined and the conduits and lava flows formed chilled margins.