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

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.