Geochemically enriched magmas at Etinde volcano, Cameroon Volcanic Line: new insights into enigmatic intraplate provinces

Abstract

Intraplate magmatic provinces occur globally, across the oceans and the continents. The processes which drive magmatism at different intraplate locations are debated, as their intraplate nature means their origins are not explicable by plate tectonic theory. Conventionally, voluminous intraplate magmatic provinces are linked to a mantle plume model, where melting occurs as anomalously hot material is brought up from the deep Earth via convection. This model predicts observable characteristics, such as time-progressive activity as a plate moves over a fixed mantle ‘hotspot’, elevated mantle potential temperatures, and geochemical evidence for a primordial (deep) mantle source. Though the plume model explains magmatic activity well in some locations, such as at Hawaii and Réunion, numerous enigmatic intraplate settings across the world lack evidence of association with plumes. Many of the proposed alternative models are overly specific to a given setting and so lack testability.

The Cameroon Volcanic Line (CVL) is an intraplate magmatic province in West Africa which spans oceanic and continental lithosphere and has a strikingly linear morphology. It lacks age-progression and primordial geochemical signatures and, despite moderately high magma production rates sustained over ca, 65 m.y., evidence for high mantle potential temperature is lacking. Furthermore, a nearidentical magmatic province is located in NE Brazil on the precisely conjugate South Atlantic margin. Magmatic activity in the two provinces started at about the same time (40-50 m.y. after continental separation) and continues to the present day in the CVL and until very recently in NE Brazil. Most alternative models for the CVL fail to reconcile all observed features, ignore the NE Brazil province, and lack applicability to other intraplate magmatic provinces. More recently, however, the CVL and NE Brazil have been used as the case study for a new model for intraplate magmatism which links these provinces to a shallow, enriched asthenospheric source. This model has applicability to other enigmatic intraplate magmatic settings worldwide.

In order to evaluate this new model, this PhD study aims to better understand geochemical enrichment at the CVL. The small volcano Etinde stands out as being composed of the most geochemically enriched igneous rocks of the CVL, with incompatible trace element concentrations up to 1000 times greater than in primitive mantle, and high concentrations of volatile elements such as sulfur and chlorine. Its lavas are highly silica-undersaturated, spanning compositions from olivine melanephelinite to leucite-schorlomite nephelinite. Magmas of this composition cannot be generated by any realistic degree of melting of normal mantle, and must result from partial melting of a geochemically enriched source. As such, in aiming to understand geochemical enrichment at the CVL as a whole, Etinde is the focus of the present study and serves as a window through which the nature of enrichment in the CVL source region can be observed.

Several geochemical investigations are reported in this thesis, each aimed at building upon the limited existing understanding of geochemical enrichment in the CVL source region. For a broad suite of Etinde samples, groupings are identified based on their petrography and geochemistry. Mafic samples fall into distinct groups; the feldsparfree mafic nephelinites with rare, resorbed olivine and feldspar-bearing basanites with abundant euhedral olivine. Intermediate nephelinites form three groups, each with a distinct modal abundance of haüyne. Lastly, there is a group of felsic nephelinites that contain schorlomite and phases such as nosean, leucite and melilite. Enigmatic trace element behaviour in the suite is resolved by the measurement of new perovskitemelt partition coefficients (KDs), specifically tailored to the Etinde system. These KDs span a wide range from ~ 0.02 for aluminium to ~ 116 for thorium, with KDs > 10 recorded for many of the light rare earth elements (LREEs). Whole rock geochemistry is used to better constrain enrichment in the melt source region. Targeted, in-situ chemical analyses of olivine and titanaugite further build on these geochemical insights and textural evidence in olivine-poor nephelinite demonstrates the clear role of carbonate enrichment in magma genesis. The measurement of eight robust new 40Ar/39Ar ages shows that Etinde magmas form two distinct age clusters, nephelinite magmas are older at ~ 0.55 Ma and likely represent the initial, small-degree melts of an enriched source. This source was then diluted in the formation of the much larger neighbouring Mount Cameroon volcano, where basanite magmas erupted from both Mount Cameroon and a reactivated Etinde plumbing system after a gap of ~ 0.4 Ma.

Finally, an ambitious final study presents the first sulfur-isotope values for the CVL, aimed at constraining the origin of enrichment. The δ³⁴S range at Etinde, after accounting for magmatic processes, is demonstrated to be + 3.7‰ to + 6.3‰, significantly higher than the range for typical asthenosphere. This indicates that sulfur, which here serves as a proxy for wider enrichment, is of a recycled origin, delivered to the mantle from the Earth’s surface in the past. The work presented in this thesis facilitates critical evaluation of the origin of magmatism at this enigmatic intraplate province, bringing us closer to the ultimate goal of a full understanding of intraplate magmatic systems and their complex origins.