D. Gagnevin

Open-system processes in the genesis of silica-oversaturated alkaline rocks of the Rallier-du-Baty Peninsula, Kerguelen Archipelago (Indian Ocean)

Abstract The Rallier-du-Baty Peninsula forms the southwestern part of the Kerguelen Archipelago (Indian Ocean), whose magmatic activity is related to the long-lived 115-Ma Kerguelen plume. The peninsula is mostly made of alkaline rocks constituting two well-defined ring complexes. This paper focuses on the northern ring complex, which is not yet known. Recent field studies have revealed seven discrete syenitic ring dykes ranging in age from 6.2 to 4.9 Ma, and two later volcanic systems. 40Ar/39Ar dating of a trachytic ignimbrite linked to the Dome Carva volcano complex yields an age of 26±3 Ka. This represents the last major eruptive event on the Kerguelen Archipelago. The volcanism is bimodal with trachybasalts and trachyandesites constituting the mafic lavas and trachytes and rhyolites constituting the felsic lavas. The volume of erupted felsic magma is by far the larger, and is represented by abundant pyroclastic deposits and lava flows. Boulders of plutonic rocks are found to the northwest of Dome Carva, and represent intermediate rocks (i.e. monzogabbros and monzonites) that are not present at the surface. Basic rocks are mostly trachybasalts and trachyandesites, while true basalts are scarce. Their mineralogy consists chiefly of plagioclase, olivine, diopside and oxides. Sieve-textured plagioclase is common, as well as corroded olivine and diopside phenocrysts. Peralkaline commenditic trachytes are the most abundant type of acid volcanic rocks. They consist of abundant sanidine, augite and magnetite phenocrysts and interstitial quartz, aegerinic pyroxenes and Na-amphiboles. Ring dykes of quartz-poor alkali feldspar syenites display the same mineralogy, except hornblende is common and replaces diopside. Hornblende is particularly abundant in intermediate monzogabbros. Major and trace element variations of volcanic rocks emphasise the predominant role of fractional crystallisation with a general decrease of MgO, CaO, P2O5, TiO2, FeO, Ba, Sr and Ni from basic to felsic rocks. However, the scattering of the data from the basic rocks indicates that other processes have operated. The overall evolution from trachyte to rhyolite is in agreement with the fractionation of sanidine as the major control. An increase of incompatible elements from trachyte to rhyolite is observed. The felsic lavas display an increase of 87Sr/86Sr(i) without any significant variations in the Nd isotopic composition. The genesis of the basic rocks is complex and reflects concomitant processes of fractional crystallisation, mixing between different basic magmas and probable assimilation of Ba-rich oceanic crust. Major and trace element modelling confirms the possibility of producing the trachytes through continuous differentiation from a basaltic alkaline parent. Discrepancies observed for some trace elements can be explained by the crystallisation of amphibole at an intermediate stage of magma evolution. The overall evolution from trachyte to rhyolite is thought to be controlled by crystal fractionation. High 87Sr/86Sr(i) of the trachytes is interpreted to reflect interaction with an ocean-derived component, probably during assimilation of hydrothermally altered oceanic crust. Boulders of amphibole-bearing monzonites and monzogabbros found to the northwest of Dome Carva are thought to represent intermediate magma composition that formed at depths but did not erupt.

Geological and geophysical evidence for a mafic igneous origin of the Porcupine Arch, offshore Ireland

Sedimentary basins west of Ireland contain a number of deep seismic structures that have been variously interpreted as fault blocks, serpentinite extrusions or igneous complexes. The Porcupine Arch (PA) is a deep-level (>11 km) domal 50 km wide seismic feature associated with a prominent free-air gravity anomaly high and high P-wave velocities. Detailed seismic mapping of igneous sill complexes in the Porcupine Basin suggests a possible connection with the PA. The sills form a thick (>5 km) interconnected network extending from the PA into the flanking post-rift Cretaceous stratigraphy, suggesting that the PA may be the top of a large (ultra)mafic intrusion that fed the sills. An intrusive origin for the PA is in agreement with geophysical modelling (gravity and Vp), the seismic character of the Porcupine Arch structure and the primitive bulk composition of the Porcupine sills, and is consistent with documented regional Cenozoic uplift in the basin with the development of shallow water and the occurrence of Paleocene–Eocene deltaic depositional systems. Similar mafic–ultramafic intrusive complexes have been documented elsewhere on the northeastern Atlantic margin, including the Rockall Trough. These findings emphasize that higher heat flow in the early Cenozoic may have prevailed over the northern part of the Porcupine Basin.

Sand supply to the Lake Albert Basin (Uganda) during the Miocene‐Pliocene: A multiproxy provenance approach

A multi-proxy provenance approach (heavy mineral analysis, U-Pb zircon geochronology and Pb isotopic analysis of K-feldspar) has constrained sediment supply within the Upper Nile drainage system in the Miocene - Pliocene. Provenance data from sandstones were obtained from three exploration wells, two situated on the north-eastern margin and one on the eastern flank of the Lake Albert Basin, NW Uganda. Data suggest that high- to low-grade metamorphic rocks and granitoids have variably supplied the heavy mineral assemblages around the Lake Albert Basin during the Miocene-Pliocene, with contributions from the isotopically-heterogeneous Archean Cratons (including the local Ugandan Craton, Tanzanian and Congo Cratons) and the Pan-African rocks (the Mozambique Belt) with possible contributions from the Neoproterozoic and Paleoproterozoic rocks. These data also highlight clear differences between supply to the eastern basin margin, compared with the northeast, which is reconcilable with current models for Miocene-Pliocene drainage in the region. Supply to northeastern Lake Albert during the Miocene-Pliocene appears to have been through a proto-Albert Nile (draining from NE to SW) and from a proto-Victoria Nile or similarly oriented palaeo-river systems draining from the east. In contrast, the eastern flanks of the basin were likely supplied via the palaeo-Nkusi river, tapping local hinterland sources and more distal basement to the far-east (Mozambique Belt). This study highlights the importance of utilizing a multi-proxy approach in provenance analysis as no one signal is capable of distinguishing the different source lands and constraining the evolving drainage patterns.