The origin of late archaean granitoids in the Sukumaland greenstone belt of Northern Tanzania: geochemical and isotopic constraints

Authors

M. A. H. Maboko, Department of Geology, University of Dar es Salaam P.O. Box 35052, Dar es Salaam, TanzaniaFollow
S. Manya Manya, Department of Geology, University of Dar es Salaam P.O. Box 35052, Dar es Salaam, TanzaniaFollow
M. Mwache Mwache, Department of Geology, University of Dar es Salaam P.O. Box 35052, Dar es Salaam, TanzaniaFollow
R. B. Pedersen, Department of Geology, University of Bergen Allegt. 41, N-5007 Bergen, NorwayFollow
P. Torssander, 3 Department of Geology and Geochemistry Stockholm University S-106 91 Stockholm, SwedenFollow

Section

Physical Sciences

Abstract

Granitoids intruding the late Archaean sequences of the Sukumaland Greenstone Belt of northern Tanzania belong to two distinct geochemical suites. Suite 1 is characterised by Na2O/K2O > 1 (1.04 – 4.67), high Sr/Y (56 – 204) and Ba/Rb ratios (6.1 – 27.1) and low Rb/Sr ratios (0.08 - 0.25). The rocks are enriched in Sr (405 – 1264 ppm) and depleted in Yb (0.17 – 0.93 ppm) and Rb (56 – 132 ppm). On chondrite-normalised REE diagrams, the rocks display highly fractionated patterns characterised by relative LREE enrichment ((La/Yb)N = 23 – 128 and (Gd/Yb)N = 3.10 – 8.54) and lower concentrations of the HREE (YbN = 0.80 – 4.45). On primitive mantle-normalised spidergrams, Nb and Ti, together with P and Y are depleted relative to adjacent elements. The major and trace element characteristics of Suite 1 are comparable to those of typical Archaean TTG suites and High Silica Adakites (HSA). Suite 2 granitoids are characterised by Na2O/K2O < 1, low Sr/Y (2.80 – 41.7) and Ba/Rb (0.40 – 8.91) ratios and high Rb/Sr (0.30 – 6.27) ratios. Suite 2 is also characterised by low Sr (53 - 326 ppm) and high Rb (40 - 365 ppm) and Yb (0.44 – 1.36 ppm) contents. Compared to Suite 1, Suite 2 rocks display less fractionated REE patterns ((La/Yb)N = 15 – 86 and (Gd/Yb)N = 1.73 – 6.74) and are characterised by higher concentrations of the HREE (YbN = 2.1 – 6.5). On primitive mantle-normalised spidergrams, Suite 2 samples, like those of Suite 1, show relative depletion in Th, Nb and Ti, together with P and Y relative to adjacent elements. Sm-Nd mean crustal residence ages for both suites are indistinguishable and range between 2470 and 2720 Ma with a mean of 2610  35 Ma (2 SE), similar to the emplacement age of 2620  40 Ma. The granitoids are interpreted to have formed by partial melting at the base of a late Archaean thickened sub-arc basaltic crust. Melting to form the Suite 1 granitoids occurred in the eclogite stability field whereas Suite 2 formed by melting at shallower depth in the garnet amphibolite stability field.

Recommended Citation

Maboko, M. A. H.; Manya, S. Manya; Mwache, M. Mwache; Pedersen, R. B.; and Torssander, P. (2006) "The origin of late archaean granitoids in the Sukumaland greenstone belt of Northern Tanzania: geochemical and isotopic constraints," Tanzania Journal of Science: Vol. 32: Iss. 1, Article 7.
Available at:https://doi.org/10.65085/2507-7961.1946