Late Cretaceous intraplate silicic volcanic rocks from the Lake Chad region: An extension of the Cameroon volcanic line?

Abstract

Silicic volcanic rocks at Hadjer el Khamis, near Lake Chad, are considered to be an extension of the Cameroon volcanic line (CVL) but their petrogenetic association is uncertain. The silicic rocks are divided into peraluminous and peralkaline groups with both rock types chemically similar to within‐plate granitoids. In situ U/Pb zircon dating yielded a mean 206Pb/238U age of 74.4 ± 1.3 Ma indicating the magmas erupted ∼10 million years before the next oldest CVL rocks (i.e., ∼66 Ma). The Sr isotopes (i.e., ISr = 0.7021–0.7037) show a relatively wide range but the Nd isotopes (i.e., 143Nd/144Ndi = 0.51268–0.51271) are uniform and indicate that the rocks were derived from a moderately depleted mantle source. Thermodynamic modeling shows that the silicic rocks likely formed by fractional crystallization of a mafic parental magma but that the peraluminous rocks were affected by low temperature alteration processes. The silicic rocks are more isotopically similar to Late Cretaceous basalts identified within the Late Cretaceous basins (i.e., 143Nd/144Ndi = 0.51245–0.51285) of Chad than the uncontaminated CVL rocks (i.e., 143Nd/144Ndi = 0.51270–0.51300). The age and isotopic compositions suggest the silicic volcanic rocks of the Lake Chad region are related to Late Cretaceous extensional volcanism in the Termit basin. It is unlikely that the silicic volcanic rocks are petrogenetically related to the CVL but it is possible that magmatism was structurally controlled by suture zones that formed during the opening of the Central Atlantic Ocean and/or the Pan‐African Orogeny.

Upper Mantle Earth Structure in Africa From Full‐Wave Ambient Noise Tomography

“…While some amount of lateral and/or vertical smearing is expected, particularly in the middle of the Sahara (supporting information Figures S8, S9, and S15), these regions of shallow upper mantle low velocities throughout North Africa may be connected to each other and/or connected to a deep upper mantle or MTZ source. Similar to the plume melt channelization concept explored by Ebinger and Sleep (), we note that several of the volcanic domes in this region lie close to or along old rift zones and shear zones (e.g., Njome & de Wit, ; Shellnutt et al, ), which may connect the anomalies at upper mantle depths. The low upper mantle anomalies throughout North Africa and the CVL are coincident with regions of surface uplift, which agrees with prior studies that suggest uplift due to mantle dynamics (supporting information Figure S25; Burke & Gunnell, ; Forte et al, ; Müller et al, ; Roberts & White, ).…”

The Mount Cameroon southwest flank eruptions: Geochemical constraints on the subsurface magma plumbing system

“…Many other proposed models Anderson, 1995, 1998;Meyers et al, 1998;King and Ritsema, 2000) for the CVL formation rely on convection driven by various forces. Shellnutt et al (2016), put forward shear zone reactivation, leading to 'leaking' of mantle-derived material through pre-existing lithospheric structures. Geochemical and isotopic investigations (Aka et al, 2004;Halliday et al, 1988;Halliday et al, 1990;Lee et al, 1994) suggest the involvement of a depleted (DM) and a high U/Pb (HIMU) or FOZO mantle…”

Petrogenesis of the Early Paleogene North Island Syenite Complex, Seychelles

“…However, zircon geochronology suggests that the rocks of North Island are slightly (~0.2 Ma to ~2.0 Ma) younger than the rocks of Silhouette and that they may be petrogenetically distinct magma systems (Ganerød et al, 2011;Shellnutt et al, 2017). Furthermore, the metaluminous nature of the syenitic rocks contrasts with the notion that they evolved at relatively low pressure as this tends to generate peralkaline compositions (C. D. Frost & Frost, 2011;Macdonald et al, 2011;Macdonald, 2012;Shellnutt et al, 2011Shellnutt et al, , 2016White et al, 2009). It is possible that contamination by crustal material (melt, fluids) may be responsible for the metaluminous nature of the syenites, but evidence for contamination is limited to non-existent (Owen- Smith et al, 2013).…”