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

• Published in: Lithos 1995, Vol.34 (1), p.127-157
• Main Authors: Lesher, C.M., Arndt, N.T.
• Format: Article
• Language: English
• ISSN: 0024-4937; 1872-6143
• DOI: 10.1016/0024-4937(95)90017-9

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 ϵ Nd 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 ϵ Nd 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 ϵ Nd ⩾ +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 ϵ Nd 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