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

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 char-nockites 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)) epsilon 176Hf(i) and epsilon 143Nd(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 (epsilon 176Hf(i) = +3.4 to + 8.2; epsilon 143Nd(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 epsilon 176Hf(i) and epsilon 143Nd(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.

Automated summary

Hafnium and neodymium isotopes are powerful tools for tracking the mechanisms of crustal generation and differentiation. In this study, the Hf and Nd isotopic analyses of Archean mafic and felsic igneous rocks from the Sangmelima terranes in southern Cameroon were conducted. The results show a subchondritic initial isotopic ratio for Mesoarchean charnockite and TTG suites, indicating the involvement of Eo-to Paleoarchean components in their petrogenesis. The study also reveals two episodes of mantle-derived mafic magmatism in the Ntem Complex during the Mesoarchean period.