21. VOLCANIC BRECCIA FROM DSDP SITE 357: IMPLICATIONS FOR THE COMPOSITION AND ORIGIN OF THE RIO GRANDE RISE
A volcanic breccia (4 m thick) was recovered from middle Eocene sediments at Site 357, Leg 39, Rio Grande Rise, South Atlantic Ocean. The breccia is composed largely of basaltic fragments (~0.5 to 3 cm) and Eocene shallow-water fossils in a clayey matrix. Texture suggests a hyaloclastic origin, and...
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| Format: | Text |
| Language: | English |
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| Online Access: | http://citeseerx.ist.psu.edu/viewdoc/summary?doi=10.1.1.516.7405 http://www.deepseadrilling.org/39/volume/dsdp39_21.pdf |
| Summary: | A volcanic breccia (4 m thick) was recovered from middle Eocene sediments at Site 357, Leg 39, Rio Grande Rise, South Atlantic Ocean. The breccia is composed largely of basaltic fragments (~0.5 to 3 cm) and Eocene shallow-water fossils in a clayey matrix. Texture suggests a hyaloclastic origin, and sorting of components indicates slump deposition. Although most basaltic material is altered (smectities, carbonate), fresh clinopyroxenes (Fs*-i*Wθ45-ji) remain and have compositions similar to clinopyroxene in highly alkalic basaltic rocks, thus identifying the parentage of the altered basalt fragments. The affinity of both hyaloclastites and alkalic oceanic basaltic-material to seamounts and islands suggests that the breccia formed by eruption from an alkali-rich seamount or volcano, probably during Eocene time. Shallow-water fauna and sorting indicate that it subsequently slumped to a pelagic environment. This breccia provides the first quantitative data for the composition of basement material of the Rio Grande Rise, whereby it is composed, at least in part, of volcanic material not typically oceanic crust (tholeiitic). |
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