Source characteristics of the basement rocks from the Sulu and Celebes Basins (Western Pacific): chemical and isotopic evidence |
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Authors: | P Spadea Massimo D’Antonio Matthew F Thirlwall |
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Institution: | (1) Dipartimento di Georisorse e Territorio, Università di Udine, Via Cotonificio, 114, I-33100 Udine, Italy, IT;(2) Dipartimento di Geofisica e Vulcanologia, Università di Napoli, Largo S. Marcellino, 10, I-80138 Napoli, Italy, IT;(3) Department of Geology, Royal Holloway University of London, Egham, Surrey, TW20 OEX, United Kingdom, GB |
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Abstract: | New Sr- Nd- and Pb-isotopic and trace element data are presented on basalts from the Sulu and Celebes Basins, and the submerged
Cagayan Ridge Arc (Western Pacific), recently sampled during Ocean Drilling Program Leg 124. Drilling has shown that the Sulu
Basin developed about 18 Ma ago as a backarc basin, associated with the now submerged Cagayan Ridge Arc, whereas the Celebes
Basin was generated about 43 Ma ago, contemporaneous with a general plate reorganisation in the Western Pacific, subsequently
developing as an open ocean receiving pelagic sediments until the middle Miocene. In both basins, a late middle Miocene collision
phase and the onset of volcanic activity on adjacent arcs in the late Miocene are recorded. Covariations between 87Sr/86Sr and 143Nd/144Nd show that the seafloor basalts from both the Sulu and Celebes Basins are isotopically similar to depleted Indian mid-ocean
ridge basalts (MORB), and distinct from East Pacific Rise MORB, defining a single negative correlation. The Cagayan Arc volcanics
are different, in that they have distinctly lower ɛNd(T) for a given ɛSr(T), compared to Sulu and Celebes basalts. In the 207Pb/204Pb and 208Pb/204Pb versus 206Pb/204Pb diagrams, the Celebes, Sulu and Cagayan rocks all plot distinctly above the Northern Hemisphere Reference Line, with high
Δ7/4 Pb (5.3–9.3) and D8/4 Pb (46.3–68.1) values. They define a single trend of radiogenic lead enrichment from Celebes through
Sulu to Cagayan Ridge, within the Indian Ocean MORB data field. The data suggest that the overall chemical and isotopic features
of the Sulu, Cagayan and Celebes rocks may be explained by partial melting of a depleted asthenospheric N-MORB-type (“normal”)
mantle source with isotopic characteristics similar to those of the Indian Ocean MORB source. This asthenospheric source was
slightly heterogeneous, giving rise to the Sr-Nd isotopic differences between the Celebes and Sulu basalts, and the Cagayan
Ridge volcanics. In addition, a probably slab-derived component enriched in LILE and LREE is required to generate the elemental
characteristics and low Nd(T) of the Cagayan Ridge island arc tholeiitic and calcalkaline lavas, and to contribute to a small extent in the backarc
basalts of the Sulu Sea. The results of this study confirm and extend the widespread Indian Ocean MORB signature in the Western
Pacific region. This signature could have been inherited by the Indian Ocean mantle itself during the rupture of Gondwanaland,
when fragments of this mantle could have migrated towards the present position of the Celebes, Sulu and Cagayan sources.
Received: 23 May 1995/Accepted: 12 October 1995 |
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