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The origin and evolution of magmas parental to Proterozoic massif-type anorthosites are still matter of debate. Oxygen isotope data of anorthosites of the Kunene Intrusive Complex, NW Namibia, combined with LA-ICPMS analyses of the anorthositic mineral assemblages provide new insights into the magma source and the magmatic processes like fractional crystallization and crustal contamination.
The Kunene Intrusive Complex comprises two anorthosite bodies, a pale, mainly pyroxene-bearing anorthosite suite, intruded by olivine-bearing anorthosites. The anorthosite massif intruded the high-grade metamorphic Epupa Complex during the Mesoproterozoic (1.38 Ga) and experienced no metamorphic overprint after its emplacement. New and published (Drppel et al., 2007) δ18O values of plagioclase separates range between 6.0-7.5‰ for the pyroxene-bearing and 5.6-6.0‰ for olivine-bearing anorthosites, in accordance with a mantle-origin of the parental melts. Oxygen isotope whole-rock analyses of ortho- and paragneisses of the Epupa Complex yield higher δ18O values of 7.2-11.2‰. Crustal contamination may be responsible for the shift of the δ18O of the pyroxene-bearing lithologies towards higher values and also for the formation of pyroxene instead of olivine. Chondrite normalized REE patterns of plagioclase generally display variable enrichment of light REE (LaN/NdN: 2.1-5.3) and positive europium anomalies (Eu/Eu*: 5-26). Trace element profiles across plagioclase of both anorthosite varieties display different core rim zonation patterns. Negative correlation of incompatible trace elements like V with LREE in plagioclase of both anorthosite varieties points to simultaneous fractionation of magnetite and ilmenite, whereas the decreasing concentration of compatible Sr across plagioclase profiles is controlled by plagioclase fractionation. Increasing concentration of incompatible Zr from the core towards the rim of plagioclase in a pyroxene-bearing sample points to extensive assimilation of crustal material during plagioclase crystallisation.
According to the trace element patterns of plagioclase and there oxygen isotopic composition, older pyroxene-bearing anorthosites derived by simultaneous fractional crystallization of plagioclase, pyroxene, magnetite, and ilmenite and assimilation of crustal material before and during crystallization of plagioclase. The younger olivine-bearing anorthosites, on the other hand, mainly evolved by fractional crystallization of plagioclase, olivine, magnetite and ilmenite in an initially uncontaminated mantle-derived magma that underwent episodic replenishing with undifferentiated mantle melts.
Drppel et al., 2007, Precambr. Res., 156, 1-31.
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