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 Na 2 O/K 2 O > 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 (Yb N = 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 Na 2 O/K 2 O < 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.