ALBERT

Description: The Early Palaeogene magmatic rocks of North and Silhouette Islands in the Seychelles contain clues to the Cenozoic geodynamic puzzle of the Indian Ocean, but have so far lacked precise geochronological data and palaeomagnetic constraints. New 40Ar/39Ar and U–Pb dates demonstrate that these rocks were emplaced during magnetochron C28n; however, 40Ar/39Ar and palaeomagnetic data from Silhouette indicate that this complex experienced a protracted period of cooling. The Seychelles palaeomagnetic pole (57.55°S and 114.22°E; A9512.3°, N=14) corresponds to poles of similar ages from the Deccan Traps after being corrected for a clockwise rotation of 29.4°±12.9°. This implies that Seychelles acted as an independent microplate between the Indian and African plates during and possibly after C27r time, confirming recent results based on kinematic studies. Our reconstruction confirms that the eruption of the Deccan Traps, which affected both India and the Seychelles and triggered continental break-up, can be linked to the present active Reunion hotspot, which is being sourced as a deep plume from the Plume Generation Zone.Supplementary material: Experimental data are available at http://www.geolsoc.org.uk/SUP18482.

Description: The anorthositic members of the Mealy Mountains Intrusive Suite (MMIS; Labrador, Canada) are host to 0·5–5 m diameter pegmatitic, pod-like segregations, originally described as graphic granite pods. U–Pb zircon geochronology confirms that the pods are coeval with the 1650–1630 Ma emplacement age range for the MMIS, yielding ages of 1654 ± 8 to 1628 ± 3·5 Ma. Petrographic and geochemical analysis of five pods from anorthositic rocks of the MMIS reveals that the pods have a diverse compositional range from monzodiorite to granite, varying from Fe-rich and Si-poor, to Fe-poor and Si-rich compositions. Fe-rich, Si-poor pods in the MMIS and other massifs (e.g. Laramie Anorthosite Complex) tend to be hosted by olivine-bearing anorthosites, whereas Si-rich, Fe-poor pods are hosted by pyroxene-bearing anorthosites. Each pod shows a range of graphic, myrmekitic and symplectitic textures, along with distinctive mineral assemblages (e.g. apatite and zircon) and highly enriched trace-element compositions. Evolved mineral assemblages, high concentrations of Fe, Ti and P (and in some cases SiO 2 ), and 10–1000 x chondrite enrichment in light rare earth elements, U, Th and Rb indicate that many of the pods are highly fractionated. The array of textural intergrowths provides clues about the final stages of crystallization in the pods and, by extension, the anorthosites. Macroscopic quartz–K-feldspar graphic intergrowths indicate high-viscosity, fluid-bearing and significantly undercooled magmatic conditions, whereas microscopic myrmekitic (plagioclase–quartz) and symplectitic (plagioclase–orthopyroxene) intergrowths on primary grain boundaries indicate replacement of phases in the presence of reactive fluids. In assessing the nature of these pegmatitic pods based on field, petrographic and geochemical evidence, we conclude that they represent the fluid-bearing, late-stage crystallization products of a residual liquid in the massif anorthosite system. The Fe and Si compositional variations observed in these late-stage pods can be linked to a fundamental olivine–pyroxene dichotomy observed in most Proterozoic anorthosite massifs, suggesting that pulses of magma experience variable contamination (in amount and/or composition) leading to varying differentiation paths. A range of lithologies (monzonites, monzonorites, ferrodiorites and jotunites) observed in similar pod-like structures, as well as dykes and plutons, has been observed in other Proterozoic anorthosite massifs and all have, at one time or another, been interpreted as the residual liquids of anorthosite crystallization. Our observation of in situ pods with similar compositions to all of the aforementioned lithologies, and displaying textures indicative of late-stage crystallization, supports the notion that all of these associated lithologies can be interpreted as comagmatic with, but variably contaminated and isolated residual liquids of, anorthosite crystallization. However, using isotopic evidence we cannot support the notion that the far larger granitic plutons associated with Proterozoic anorthosites are also residual liquids of anorthositic magma fractionation.

Description: The magmatic activity (0–16 Ma) in Iceland is linked to a deep mantle plume that has been active for the past 62 My. Icelandic and northeast Atlantic basalts contain variable proportions of two enriched components, interpreted as recycled oceanic crust supplied by the plume, and subcontinental lithospheric mantle derived from...

Description: The Doros Complex of north-western Namibia is a relatively small (~17 km 2 in area; ≥500 m thick), lopolithic layered mafic intrusion (LMI) within the early Cretaceous Paraná-Etendeka Large Igneous Province. It is hosted in Neoproterozoic granitoids and metasedimentary rocks of the Southern Kaoko Zone of the Damara Orogen. It is undeformed and unmetamorphosed and therefore represents a good candidate for the comprehensive characterisation and investigation of a layered intrusion. Detailed mapping and petrographic analysis in this study show that the intrusion consists of a stacked series of semi-concordant gabbroic layers, dipping shallowly in towards the centre. The fundamental mineralogy throughout the layered sequence is plagioclase + calcic clinopyroxene + oxy-exsolved Fe-Ti oxides ± olivine. Orthopyroxene is completely absent in the Doros Complex. The stratigraphy broadly comprises a partially-exposed, fine-grained gabbroic sill (An 46-21 ; clinopyroxene Mg# 80 to 59), overlain by a thick sequence of massive, olivine-cumulate melagabbros (Fo 75-70 ; An 73-23 ; clinopyroxene Mg# 80 to 76), followed by a massive, plagioclase-cumulate gabbro layer (Fo 61-54 ; An 75-51 ; Mg# 79 to 73), and topped by a series of interlayered variable, strongly foliated, plagioclase-, olivine- or magnetite-cumulate gabbro layers (Fo 63-60 ; An 72-24 ; Mg# 79 to 69). The foliated gabbros are also host to pegmatitic and monomineralic enclaves. The basal contact displays a glassy, micro-porphyritic chilled margin (An 2-0 ; Mg# 80 to 78). A syenitic (bostonite) phase (An 27-01 ; Mg# 66 to 3) – a late-stage differentiate – occurs as cross-cutting dykes. The intrusion is also cut by dolerite dykes, from a later event. Fold and foliation structures within the Damaran rocks likely facilitated magma emplacement. The layering in the intrusion is defined by variations in the modal proportions of the major minerals, and in the mineral and rock textures. The fine-scale rhythmic or cyclic layering characteristic of the larger LMIs is not evident. Despite the small size and simple mineralogy of the intrusion, this study reveals unexpectedly high degrees of complexity in the textures, mineralogy, compositions, styles of layering, internal layer variations and contact relations in the Doros Complex. We document systematic stratigraphic changes in modal abundances and mineral compositions through the intrusion that are consistent with the fractional crystallisation of a basaltic system, but also reveal compositional breaks and subtle reversals. Together with evidence of intrusive internal contacts and grain-size layering, these are consistent with emplacement as multiple magma pulses.

Description: Earth’s residual geoid is dominated by a degree-2 mode, with elevated regions above large low shear-wave velocity provinces on the core–mantle boundary beneath Africa and the Pacific. The edges of these deep mantle bodies, when projected radially to the Earth’s surface, correlate with the reconstructed positions of large igneous provinces...