GEOCHEMICAL AND MINERALOGICAL EVIDENCE FOR THE PROVENANCE OF MIXED VOLCANOGENIC/TERRIGENOUS HEMIPELAGIC SEDIMENTS IN THE PLIOCENE- PLEISTOCENE WOODLARK BACKARC RIFT BASIN, SOUTHWEST PACIFIC: OCEAN DRILLING PROGRAM LEG 180
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Date
2002Author
Robertson, Alastair H F
Sharp, Timothy R
Metadata
Abstract
Middle Miocene to Holocene fine-grained argillaceous sediments
(clays, claystones/muds, and mudstones), which volumetrically dominated
the sediment recovery in the Woodlark Basin during Leg 180,
were chemically analyzed for major elements, trace elements, and some
rare earth elements by X-ray fluorescence. Selected samples also underwent
X-ray diffraction (XRD) analysis for mineral determination. The
results shed light on sediment provenance when combined with shipboard
sediment descriptions, smear slide study, and XRD. The oldest
sediments recovered (Site 1108) of middle-late Miocene age include
volcanogenic muds with distinctive high MgO and K2O, indicative of a
relatively basic calc-alkaline source related to an inferred Miocene
forearc succession. The forearc basement, composed of diabase and basalt,
was locally exposed (Site 1109) and eroded in the late Miocene
(<5.4-9.93 Ma), giving rise to fluvial conglomerates (Sites 1109, 1115,
and 1118). Chemically distinctive fine-grained claystones and siltstones
(with relatively high Ti, low K) are compatible with derivation from
tropically weathered basic igneous rocks, correlated with the Paleogene
Papuan ophiolite. Overlying latest Miocene-Pleistocene fine-grained sediments throughout the Woodlark Basin were partly derived from
calc-alkaline volcanic sources. However, relatively high abundances of
Al2O3 and related element oxides (K2O and Na2O) and trace elements
(e.g., Rb and Y) reflect an additional terrigenous input throughout the
basin, correlated with pelitic metamorphic rocks exposed on Papua
New Guinea and adjacent areas. In addition, sporadic high abundances
of Cr and Ni, some other trace metals, and related minerals (talc, crysotile,
and chlorite) reflect input from an ophiolitic terrain dominated by
ultramafic rocks, correlated with the Paleogene Papuan ophiolite. The
source areas possibly included serpentinized ultramafic ophiolitic rocks
exposed in the Papua New Guinea interior highlands. Chemical evidence
further indicates that fine-grained terrigenous sediment reached
the Woodlark Basin throughout its entire late Miocene-Holocene history.
Distinctive high-K volcanogenic muds rich in tephra and volcanic
ash layers that appear at <2.3 Ma (Sites 1109 and 1115) are indicative of
high-K calc-alkaline volcanic centers, possibly located in the Dawson
Strait, Moresby Strait, or Dobu Seamount area. Chemical diagenesis of
fine-grained sediments within the Woodlark Basin is reflected in clay
neomorphism and localized formation of minerals including dolomite,
ankerite, and zeolite but has had little effect on the bulk chemical composition
of most samples.