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摘要
Dyke emplacement in the course of Parana-Etendeka volcanism in northwestern Namibia has been considered as a short-lived event related to a specific magma source at approximately 135-130 Ma. New geochemical, whole rock Sr-Nd-Pb isotope data and Ar-40/Ar-39 ages reveal that at least three geochemically and isotopically different tholeiitic dyke generations, with intrusion ages of 135.2 +/- 0.7, 124.1 +/- 0.8 Ma and 113.0 +/- 0.5 Ma, can be distinguished in the Mowe Bay area, Skeleton Coast of northwestern Namibia. Distinct mantle source components were identified in the petrogenesis of the various dyke generations. Magma composition of the two older dyke suites, both of which were emplaced in a tectonic setting dominated by E-W extension, evolved from a within plate to an enriched mid-ocean ridge basalt type. Contamination by continental crust and/or lithospheric mantel is suggested by a high U-238/Pb-204-value (=mu(2)) of 9.97, which is typical of the Kalahari Craton. The third dyke generation, which intruded in a tectonic regime dominated by SE-NW extension, corresponds to high-Ti ocean island-type basalt with unradiogenic isotopic compositions derived from a mantle source with a mu(2)-value of 9.35. The formation of the older dykes is attributed to the presence of a thermal anomaly in the upper sublithospheric mantle. This anomaly was most likely caused by the peripheral part of a mantle plume that impinged at the base of the lithosphere and caused erosion of the subcontinental mantle lithosphere and melting above the plume head. The initial break-up of SW Gondwana and the formation of early oceanic crust were most likely due to passive rather than active rifting. Continued plume upwelling facilitated by progressive thinning of SW-Gondwana crust led to the formation of the younger, c. 113 Ma old dykes, which are chemically and isotopically identical to coeval rocks from the northeastern portion of the Walvis Ridge and thus are interpreted as onshore expressions of the Tristan-Gough plume head at that time. The difference in the dominant extension directions of the older and the younger dyke generations can be explained by rotation induced by the Aptian/Albian opening of the Equatorial Central Atlantic, accompanied by a substantial increase in the South Atlantic spreading rate.
- 出版日期2016-12-9
