『Abstract
We preset new major element, trace element ad Nd-isotope data
for 30 alluvial sediments collected from 25 rivers in Queensland,
E Australia. Samples were chosen to represent drainage from the
region's most important lithologies, including Tertiary intraplate
volcanic rocks, a Cretaceous igneous province (and sedimentary
rocks derived thereof) as well as Proterozoic blocks. In most
chemical and isotopic aspects, the alluvial sediments represent
binary or ternary mixing relationships, with absolute abundances
implied to reflect the proportion of lithologies in the catchments.
When averaged, the studied sediments differ from other proxies
of upper continental crust (UCC) mainly in their relative middle
rare earth element enrichment (including an elevated Sm/Nd ratio),
higher relative Eu abundance and higher Nb/Ta ratio. These features
are inherited from eroded Tertiary intraplate basalts, which commonly
from topographic highs in the studied region.
Despite the high degree of weathering strong to excellent coherence
between similarly incompatible elements is found for all samples.
From this coherence, we suggest revisions of the following upper
crustal element ratios: Y/Ho = 26.2, Yb/Tm = 6.37, Th/W = 7.14,
Th/Tl = 24 and Zr/Hf = 36.9. Lithium, Rb, Cs and Be contents do
not seem depleted relative to UCC, which may reflect paucity of
K-feldspar in the eroded catchments. Nickel, Cr, Pb, Cu and Zn
concentrations are elevated in polluted rivers surrounding the
state capital. River sediments in the Proterozoic Georgetown Inlier
are elevated in Pb, Cu and Zn but this could be a natural phenomenon
reflecting abundant sulphide mineralisation in the area.
Except for relative Sr concentrations, which broadly anticorrelate
with mean annual rainfall in catchments, there is no obvious relationship
between the extent of weathering and climate types, which range
from arid to tropical. The most likely explanation for this observation
is that the weathering profiles in many catchments are several
Myr old, established during the much wetter Miocene period.
The studied sediment compositions (excluding those from the Proterozoic
catchments) are used to propose a new trace element normalisation
termed MUQ (MUd from Queensland), which serves as an alternative
to UCC proxies derived from sedimentary rocks.』
1. Introduction
2. Regional geology and samples
3. Analytical details
4. Results
4.1. Major elements
4.2. Trace elements
4.2.1. Conservative trace elements (REE, Y, Th, Tl, Sc and Co)
4.2.2. Elements affected by mineral sorting (Zr, Hf, Ti, Nb,
Ta, Cr and Sn)
4.2.3. Elements affected by pollution (Pb, Cu, Zn)
4.2.4. Elements affected by loss during weathering (Li, Rb,
Cs, Be, Sr, Ba, V, W, U)
4.2.5. Other elements (Ni and Ga)
4.3. Nd-isotopes
5. Discussion
5.1. Coherent geochemical UCC features of the studied sediments
5.2. Suggested revisions to existing crustal estimates
5.3. The MUQ normalisation
5.4. The uniformity of weathering indices and implications for
provenance studies
Acknowledgments
References