4.2 Article

Petrology of Karoo volcanic rocks in the southern Lebombo monocline, Mozambique

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JOURNAL OF AFRICAN EARTH SCIENCES
卷 52, 期 4-5, 页码 139-151

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PERGAMON-ELSEVIER SCIENCE LTD
DOI: 10.1016/j.jafrearsci.2008.06.002

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Movene Basalts; Mozambique; Sica Beds Rhyolites; Trace-element composition of zircon; Sr-Nd isotopes; Karoo volcanic rocks

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The Karoo volcanic sequence in the southern Lebombo monocline in Mozambique contains different silicic units in the form of pyroclastic rocks, and two different basalt types. The silicic units in the lower part of the Lebombo sequence are formed by a lower unit of dacites and rhyolites (67-80 wt.% SiO(2)) with high Ba (990-2500 ppm), Zr (800-1100 ppm) and Y (130-240 ppm), which are part of the Jozini-Mbuluzi Formation, followed by a second unit, interlayered with the Movene basalts, of high-SiO(2) rhyolites (7678 wt.%; the Sica Beds Formation), with low Sr (19-54 ppm), Zr (340-480 ppm) and Ba (330-850 ppm) plus rare quartz-trachytes (64-66 wt.% SiO(2)), with high Nb and Rb contents (240-250 and 370381 ppm, respectively), and relatively low Zr (450-460 ppm). The mafic rocks found at the top of the sequence are basalts and ferrobasalts belonging to the Movene Formation. The basalts have roughly flat mantle-normalized incompatible element patterns, with abundances of the most incompatible elements not higher than 25 times primitive mantle. The ferrobasalt has TiO(2) similar to 4.7 wt.%, Fe(2)O(3t) = 16 wt.%, and high Y (100 ppm), Zr (420 ppm) and Ba (1000 ppm). The Movene basalts have initial (at 180 Ma) (87)Sr/(86)Sr = 0.7052-0.7054 and (143)Nd/(144)Nd = 0.51232, and the Movene ferrobasalt has even lower (87)Sr/(86)Sr (0.70377) and higher (143)Nd/(144)Nd (0.51259). The silicic rocks show a modest range of initial Sr-((87)Sr/(86)Sr = 0.70470-0.70648) and Nd-((143)Nd/(144)Nd = 0.51223-0.51243) isotope ratios. The less evolved dacites could have been formed after crystal fractionation of oxide-rich gabbroic cumulates from mafic parental magmas, whereas the most silica-rich rhyolites could have been formed after fractional crystallization of feldspars, pyroxenes, oxides, zircon and apatite from a parental dacite magma. The composition of the Movene basalts imply different feeding systems from those of the underlying Sabie River basalts. (C) 2008 Elsevier Ltd. All rights reserved.

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