Combined bulk-rock Lu-Hf and Sm-Nd isotopic study of Archean granitoids and mafic rocks from Sangmelima terranes (Ntem Complex, south Cameroon): Geodynamic implications

Abstract

Hafnium and neodymium isotopes provide robust tool tracers for tracking the mechanisms of crustal generation and differentiation, from mantle extraction to the late geological processes undergone by the studied rocks. Though these two isotope systems commonly behave similarly and define a positive correlation between 143Nd/144Nd and 176Hf/177Hf referred to as the “terrestrial array”, they may differ in terrains having a complex geological history, such as Archean cratons. This study presents the first combined whole-rock Hf and Nd isotopic analyses of Archean mafic and felsic igneous rocks from the Sangmelima terranes in the Ntem Complex (located in the Northwest of Congo Craton (NW CC) in southern Cameroon). It mainly consists of Mesoarchean charnockites and tonalite-trondhjemite-granodiorite (TTG) suites, greenstone belts, mafic enclaves or dykes, and potassic granitoids. Mesoarchean charnockite and TTG suites have subchondritic initial 176Hf/177Hf and 143Nd/144Nd ratios with a mean initial (suffix (i)) ε176Hf(i) and ɛ143Nd(i) of –3.30 ± 1.40 and –3.85 ± 0.65 (2SD) respectively, implying the involvement of Eo- to Paleoarchean components in their petrogenesis. The bulk rock initial Hf and Nd isotope compositions of the gabbro and gabbronorites are suprachondritic (ε176Hf(i) = +3.4 to + 8.2; ε143Nd(i) = +0.9 to + 1.6), indicate that they were derived from a depleted mantle source. Coupled with pre-existing data from the literature, these new Hf-Nd isotopic data indicate that two episodes of mantle-derived mafic magmatism in the Ntem Complex occurred during the Mesoarchean period (i.e., at ∼ 3.1 and 2.86 billion years (Ga)). Neoarchean dolerite dykes show broadly chondritic to supra-chondritic ε176Hf(i) and ε143Nd(i) values of –0.28 to + 4.53 and TCHUR ages of 2.74 – 2.76 Ga, suggesting the derivation of doleritic magma from a depleted mantle source with either none or limited crustal residence time. Considering other Archean domains of the NW CC, there were multi-stage magmatism events and crustal growth events occurred at ∼3.75 – 3.31 Ga, ∼3.26 – 3.0 Ga, ∼2.92 – 2.85 Ga, and ∼2.75 – 2.72 Ga from Eoarchean to Neoarchean in the NW Congo Craton.