Polymetamorphic sapphirine rocks of the Irumide age in Lower Luangwa Valley, Zambia

摘要

Mg-Al-rich rocks of the Southern Irumide Belt in Zambia contain mineral assemblages consisting of sapphirine-enstatite-minor spinel and later retrogressive anthophyllite-chlorite. The whole-rock compositions have high mg# 0.88 and 0.89 and indicate a sedimentary or volcanosedimentary protolith. The monazite-dominated pattern of normalized REE abundances is steep, with high LaN/YbN values of 96.1 and 67.4 and a negative Eu anomaly (Eu/Eu* = 0.58). The occurrence is an example of sapphirine formed near the low-temperature limit of its stability field. Sapphirine-enstatite geothermometry indicates (minimal) equilibration temperatures of 680-780 deg C. This estimate is in agreement with relatively low Al_2O_3 contents in enstatite near 4.4 wt. percent. Chemical dating of monazite gave an Irumide age of metamorphic crystallization of 1044 +- 16 Ma. A minor fraction of monazite shows a superimposed partial recrystallization at approx 520 Ma, during the Pan-African event. This younger recrystallization probably correlates with partial retrogression of enstatite and sapphirine to the assemblage anthophyllite-chlorite. Comparison with available data indicates that the studied sapphirine rocks, except their lower SiO_2 content, are compositionally related to other magnesian-aluminous rocks such as kyanite-cordierite-anthophyllite/gedrite rocks, which also occur in the Palaeoproterozoic Muva Supergroup, or kyanite-quartz-talc schists in the Rufunsa Metavolcanic Formation.