The petrogenesis of the Neoarchean Kukuluma Intrusive Complex, NW Tanzania

Abstract

The Kukuluma Intrusive Complex (KIC) is a late Archean igneous complex, dominated by monzonite and diorite with subordinated granodiorite. The monzonite and the diorite suites have low silica content (SiO2 ≤ 62 wt%), moderate Mg# (Mg#average = 49), high Sr/Y (Sr/Yaverage = 79) and high La/Yb (La/Ybaverage = 56) ratios, and strongly fractionated (Lan/Ybn = 9–69) REE patterns. Their moderate Ni (Niaverage = 50 ppm), Cr (Craverage = 85 ppm), variable Cr/Ni ratio (0.65–3.56) and low TiO2 (TiO2average = 0.5 wt%) indicate little to no interaction with the peridotitic mantle. For most major elements (Al2O3, FeOt, Na2O, TiO2 and P2O5) the monzonite and the diorite suites display subparallel trends for the same SiO2 content indicating they represent distinct melts. Intrusions belonging to the diorite suite have high Na2O (Na2Oaverage = 4.2 wt%), Dy/Ybn (Dy/Ybn-average = 1.6), a positive Sr anomaly and uncorrelated Nb/La and Zr/Sm ratios suggesting derivation from partial melting of garnet-bearing amphibolite. Intrusions belonging to the monzonite suite have higher Na2O (Na2Oaverage = 5.61 wt%), Dy/Ybn (Dy/Ybn-average = 2.21), a negative Sr anomaly and correlated Nb/La and Zr/Sm ratios consistent with derivation from partial melting of eclogite with residual rutile. Small variations in the Th/U ratio and near chondritic/MORB average values (Th/Umonzonite = 3.65; Th/Udiorite = 2.92) are inconsistent with a subducting slab signature, and it is proposed that the monzonite and the diorite suites of the KIC formed by partial melting of garnet-bearing, lower mafic crust of an oceanic plateau. The granodiorite suite has lower Mg# (Mg#average = 41), moderately fractionated REE, low Sr/Y (Sr/Yaverage = 20), La/Yb (La/Ybaverage = 15), Dy/Ybn (DyYbn-average = 1.24) and small negative Eu anomalies suggesting derivation from partial melting of amphibolite and plagioclase fractionation. Near-MORB Th/U (Th/Uaverage = 2.92) and Zr/Sm (Zr/Smaverage = 30.21) ratios are consistent with intracrustal melting of amphibolite.Archean rocks with an “adakitic” geochemical signature have been used to argue in favour of a plate tectonics regime in the Archean. The results presented here suggest that tectonic models for the Tanzania Craton, which invoke a subduction-related setting for all greenstone belts may need revision.