PROMOTING ACCESS TO AFRICAN RESEARCH

Tanzania Journal of Science

The AJOL site is currently undergoing a major upgrade, and there will temporarily be some restrictions to the available functionality.
-- Users will not be able to register or log in during this period.
-- Full text (PDF) downloads of Open Access journal articles will be available as always.
-- Full text (PDF) downloads of subscription based journal articles will NOT be available
We apologise for any inconvenience caused. Please check back soon, as we will revert to usual policy as soon as possible.





The emplacement age of gabbroic rocks and associated granitoids of the Liganga-Msanyo complex, south eastern Tanzania.

MAH Maboko

Abstract




The Msanyo Gabbro and the associated Mdando granitoids, which intrude Paleoproterozoic high-grade metamorphic rocks of the Ubendian Belt in southern Tanzania, yield imprecise Sm-Nd whole rock isochron ages of 1608±134 Ma (ε (Nd) = 2.1) and 1642±100 Ma (ε (Nd) = 1.6), respectively. These ages are indistinguishable at the 95% confidence level, indicating that the two rock suites were intruded at the same time. A more precise estimate of the time of emplacement is provided by a 5-point mineral isochron age of 1505±42 obtained from one of the gabbro samples. Calculated initial ε (Nd) values and mean crustal residence ages are similar for both rock suites and range from –0.7 to 1.5 and 1.7 to 1.9 Ga. Respectively. Initial Sr isotope ratios lie between 0.7032 and 0.7039 and these values are similar to those obtained in the mantle and the lower continental crust. The rather evolved Nd isotope systematics, combined with the relatively non-radiogenic Sr isotopic signature, suggest that both suites cystallised from mantle-derived magma that had assimilated a significant amount of lower continental crustal material. Subsequent to their emplacement, the rocks experienced localised shearing and sericitisation during the Ukingan tectonic event. A Rb-Sr plagioclase-whole rock age dates this event at 1137±50 Ma.



Tanzania Journal of Science Volume 27A (Special Issue) 2001, pp. 1-14




http://dx.doi.org/10.4314/tjs.v27i2.18342
AJOL African Journals Online