Formation process of a shallow mantle source heterogeneity the Walvis Ridge revisited

Details Formation process of a shallow mantle source heterogeneity the Walvis Ridge revisited Formation process of a shallow mantle source heterogeneity the Walvis Ridge revisited

Apr 2003

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2003eaeja.....7253c&link_type=abstract

EGS - AGU - EUG Joint Assembly, Abstracts from the meeting held in Nice, France, 6 - 11 April 2003, abstract #7253

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Oceanic basalts from the Walvis Ridge DSDP Site 525A have unradiogenic 143Nd/144Nd and 206Pb/204Pb isotopic compositions relative to other Tristan plume-related oceanic basalts but are indistinguishable from some flood basalts of the Paraná-Etendeka, thought to represent melts of ancient subcontinental lithospheric mantle (SCLM). Thermal erosion by the Tristan plume and tectonic detachment due to continental breakup have been proposed as mechanisms to disperse ancient SCLM into the asthenospheric mantle during the breakup of Gondwana to form the source of Walvis Ridge DSDP 525A [1] and other unusual oceanic basalts in the South Atlantic and Southwest Indian Ocean. To test these hypotheses a direct spatial and compositional relationship is evaluated between DSDP 525A basalts and SCLM potentially delaminated from the sub-Gondwana lithosphere. The Walvis Ridge is an especially suitable location for such a test because the site of possible erosion of SCLM by the Tristan plume differs from the likely site of tectonic detachment during continental breakup. The plume track and absolute motion of the African plate places the Tristan plume under the >2.5 Ga Congo Craton at ˜124 Ma. In contrast, tectonic detachment of SCLM during continental breakup would have occurred further south from beneath the 0.5--1 Ga Damara Belt, since stretching of the continental lithosphere would displace tectonically detached SCLM in the direction of the relative plate motion between the African and South American plates. Lherzolite xenoliths in a Cretaceous nephelinite plug near Swakopmund in the Damara Belt have depleted trace element patterns, and clinopyroxene separates give initial ɛNd=12.5 and 87Sr/86Sr=0.70242 at the time of continental breakup at ~124 Ma. The nephelinite host gives ɛNd=5.92 and 87Sr/86Sr=0.703965 at the time of emplacement at 76 Ma, which is significantly more depleted than inferred for the Tristan plume. Non-modally metasomatized spinel lherzolites from a second locality within the Damara Belt (Okenyenya) are as equally depleted as the Swakopmund mantle samples. To the extent that these samples constrain the composition of the SCLM of the Damara Belt, it is concluded that tectonic detachment of SCLM during continental breakup would not produce a source for DSDP 525A basalts. Rather thermal erosion of cratonic SCLM by the Tristan plume is required. However, this calls for a return flow of such thermally eroded material to explain its occurrence at the site of plume activity 45 Ma after its thermal erosion by the plume. [1] S.C. Milner &A.P. le Roex, (1996), Earth Planet. Sci. Lett. 141, 277--291.

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