Sedimentary and volcanic rocks overlying basement in the Mount Isa Inlier, Australia, are subdivided into three cover sequences. The upper two sequences range in age from 1800 to 1670 Ma and each consist of rift and overlying sag phases of sedimentation. Fine-grained siliciclastic sedimentary rocks in the upper two sequences define two distinct geochemical groups. Samples from Groups I and II are mainly from the lower and upper rift and upper thermal phases, respectively. One sample from the lower rift phase has affinities with Group II, whereas samples from the lower thermal phase belong to both Groups I and II. Eu/Eu* values are 0.70 to 0.85 for Group I, and 0.55 to 0.70 for Group II. Group I samples are characterized by lower La/Sc and Th/Sc ratios and higher Cr/Th ratios than Group II. Cr, Ni, and Co concentrations are greater in Group I than Group II. Groups I and II also are distinguished on plots of Eu/Eu* vs. La(N)/Sm(N) and Eu/Eu* vs. Gd(N)/Yb(N). Two samples from Group I have elevated LREE/HREE and La(N)/Yb(N) ratios that are attributed to intense weathering as revealed by their high Chemical Index of Weathering values. The geochemical data reflect a combination of local and more distant regional provenances and can be related to the tectonic evolution of the Mount Isa Inlier. Group I samples are attributed to dilution of a mature, post-Archean source by addition of recycled mafic volcanics. Group II samples are typical post-Archean sediments derived from differentiated upper continental crust with no dilution by local sources. The lower rift phase displays great geochemical variability (samples from both groups) that is typical of active tectonic settings such as rifts. The great variability of samples from the lower sag phase (both groups) is surprising but probably reflects the diverse rock types beneath this basin. Stable tectonic conditions during this phase of sedimentation are indicated instead by major element geochemistry that records severe weathering. Similarity of the upper rift phase sample with some samples from the lower sag phase reflects recycling of the latter. Samples from the upper sag phase are remarkably uniform in composition (Group II) and reflect the stable tectonic conditions under which this basin developed.
