Contrasting styles of magmatism and rifting in the High Arctic LIP, Sverdrup Basin, Canadian Arctic

<p>Located along the Canadian polar continental margin, the Sverdrup Basin is an elongated, intracontinental sedimentary basin that originated during Carboniferous-Early Permian rifting. Starting in the Early Cretaceous, volcanic complexes (VC) were emplaced throughout the basin, which are associated with the High Arctic Large Igneous Province (HALIP). Geochronological and geochemical data on HALIP rocks exposed on Axel Heiberg Island and northern Ellesmere Island suggest several discrete stages of emplacement; (1) voluminous mafic intrusive activity of tholeiitic character accompanied by minor extrusive volcanism at <em>ca</em>. 125-110 Ma (VC1a<strong>); </strong>the eruption of tholeiitic flood basalts on Axel Heiberg Island at <em>ca</em>. 100-90 Ma (VC1b); the emplacement of mildly alkaline lava flows, sills and dykes on Ellesmere Island at <em>ca</em>. 100-90 Ma (VC2);<strong> </strong>and the eruption of a suite of alkaline lava flows from central volcanoes at <em>ca</em>. 85-75 Ma (VC3). Each magmatic episode is characterized by a distinctive eruptive style and coherent geochemical signature regardless of the mode of emplacement. In this context, onshore manifestations of the HALIP can be viewed as time-markers in the evolution of the adjacent polar continental margin.</p><p>We use digital plate tectonic models, constructed via the <em>GPlates</em> software, to explore the parallel development of the Sverdrup Basin and proto-Arctic Ocean (Amerasia Basin) during the Early Cretaceous, and the transition from a sedimentary to volcanic Sverdrup Basin. Plate reconstructions of the Amerasia Basin at <em>ca</em>. 125 Ma suggest two zones of extension; one within the Canada Basin, which may include seafloor spreading, (Zone 1, more distal to the Sverdrup Basin) and the second further northwards in the Alpha-Mendeleev Ridge and Makarov Basin domains offshore northern Ellesmere Island (Zone 2, proximal to the northeastern portion of the Sverdrup Basin). The potential for enhanced melting caused by mantle flow (possibly related to the arrival of a mantle plume) towards the Sverdrup Basin depocentre could explain widespread magmatism of tholeiitic character from <em>ca</em>. 125-90 Ma (VC1). The transition to mildly alkaline (VC2) and alkaline magmatism (VC3) at <em>ca</em>. 100 Ma may have signaled the end of extension in Zone 1. The persistence of localized extension in Zone 2 could explain the shift in magmatic style and compositional diversity of igneous rocks emplaced at intrusive complexes (VC2) vs constructional volcanic edifices (VC3). In addition, greater depth to Moho along the northeastern Sverdrup Basin may have contributed to restricted mantle flow in Zone 2. We propose that the spatio-temporal evolution of HALIP magmatism in the Sverdrup Basin during the Cretaceous relates to (1) different styles of tectonic extension (distal vs proximal, protracted vs discrete, widespread vs narrow, seafloor spreading vs hyper-extensional rifting), and (2) the presence of hot, thin lithosphere close to the basin depocentre vs cold and thick lithosphere in the northeastern part of the basin.</p>

Petrology of peridotite host basaltic lavas of northern Ngaoundéré (Adamawa plateau, Cameroon, Central Africa)

Small volcanoes and flows of Cainozoic basaltic lavas, containing numerous mantle peridotite xenoliths, outcrop at northern Ngaoundéré in Adamawa plateau. They are composed of arena of decimeter to meter in size of bowls and blocs of dark matrix, showing crystals of olivine, clinopyroxene and oxides. All lavas present microlitic porphyritic texture with euhedral to subhedral crystals of the same phases drowned in the matrix of the same minerals plus plagioclase microlites.Microprobe analyses show that olivine phenocrysts are relatively Fo-rich (80.9-84.3 %) compared to microphenocrysts and microcrysts (Fo71.1-75.9 %). Olivine xenocrysts are highly magnesian (83.9-89.8 %). Clinopyroxene are diopside and augite. Oxides crystals are Ti-magnetite and plagioclase are labradorite and bytownite.ICP-AES and ICP-MS whole rocks analyses show that the host peridotite basaltic lavas of northern Ngaoundéré are undersaturated basanites of typical alkaline lava series. They seem not contaminated by crustal materials. They are the results of low partial melting rate of the garnet mantle source located at more than 80 km depth. The eruptions of northern Ngaoundéré lavas have been facilitated by Pan African cracks and they have sampled the subcontinental lithospheric mantle as xenoliths at different pressures and depths on their way to the surface.

Mantle exhumation at magma-poor passive continental margins. Part I. 3D architecture and metasomatic evolution of a fossil exhumed mantle domain (Urdach lherzolite, north-western Pyrenees, France)

In two companion papers, we report the detailed geological and mineralogical study of two emblematic serpentinized ultramafic bodies of the western North Pyrenean Zone (NPZ), the Urdach massif (this paper) and the Saraillé massif (paper 2). The peridotites have been exhumed to lower crustal levels during the Cretaceous rifting period in the future NPZ. They are associated with Mesozoic pre-rift metamorphic sediments and small units of thinned Paleozoic basement that were deformed during the mantle exhumation event. Based on detailed geological cross-sections and microprobe mineralogical analyses, we describe the lithology of the two major extensional fault zones that accommodated: (i) the progressive exhumation of the lherzolites along the Cretaceous basin axis; (ii) the lateral extraction of the continental crust beneath the rift shoulders and; (iii) the decoupling of the pre-rift cover along the Upper Triassic (Keuper) evaporites and clays, allowing its gliding and conservation in the basin center. These two fault zones are the (lower) crust-mantle detachment and the (upper) cover décollement located respectively at the crust-mantle boundary and at the base of the detached pre-rift cover. The Urdach peridotites were exposed to the seafloor during the Late Albian and underwent local pervasive carbonation and crystallization of calcite in a network of orthogonal veins (ophicalcites). The carbonated serpentinized peridotites were partly covered by debris-flows carrying fragments of both the ultramafics and Paleozoic crustal rocks now forming the polymictic Urdach breccia. The mantle rocks are involved in a Pyrenean overturned fold together with thin units of crustal mylonites. Continent-derived and mantle-derived fluids that circulated along the Urdach crust-mantle detachment led to the crystallization of abundant metasomatic rocks containing quartz, calcite, Cr-rich chlorites, Cr-rich white micas and pyrite. Two samples of metasomatized material from the crust-mantle detachment yielded in situ zircon U/Pb ages of 112.9 ± 1.6 Ma and 109.4 ± 1.2 Ma, thus confirming the Late Albian age of the metasomatic event. The cover décollement is a 30-m thick fault zone which also includes metasomatic rocks of greenschist facies, such as serpentine-calcite association and listvenites, indicating large-scale fluid-rock interactions implying both ultramafic and continental material. The lowermost pre-rift cover is generally missing along the cover décollement due to tectonic disruption during mantle exhumation and continental crust elision. Locally, metasomatized and strongly tectonized Triassic remnants are found as witnesses of the sole at the base of the detached pre-rift cover. We also report the discovery of a spherulitic alkaline lava flow emplaced over the exhumed mantle. These data collectively allow to propose a reconstruction of the architecture and fluid-rock interaction history of the distal domain of the upper Cretaceous northern Iberia margin now inverted in the NPZ.

Subvolcanic neck of Cabugi Peak, state of Rio Grande do Norte, Brazil, and origin of its landform

This article reports geologic, petrographic, and geomorphological observations of the mafic alkaline subvolcanic neck of the Cabugi Peak, located in the State of Rio Grande do Norte, Brazil. The massif is 370 m high and has 0.4 km³ of total volume. It is constituted mainly by Caicó orthogneiss. The neck is exposed on the top of the massif forming a conical morphologic protrusion with relative height of 160 m and diameter of 500 m. The volume of the mafic alkaline rock is 0.056 km³ occupying 14% of the whole massif. The general form of the massif is strongly convex with the MCI (Macro Concavity Index) of -2.3. The neck is constituted by olivine-rich alkaline micro-gabbro in the centre and alkaline dolerite at the contact zone. There are well-developed cooling columnar joints with typical diameter of 60 cm. They are steep at the centre of the neck and sub-horizontal at the contact zone. On the foothill surface, called Sertaneja surface, no outcrops of mafic alkaline lava, other eruptive deposits, or volcanic rock debris have been observed. These observations allow a conclusion that the volcanic edifice and eruptive deposits of the late Oligocene were completely removed by later uplift and consequent regional denudation and that the present-day surface is significantly lower than that of the eruption time. The outcrops of the Cabugi Peak exhibit the underground geologic structure of the late Oligocene volcano. The original volcano form is not preserved anymore and the present morphologic elevation is attributed to differential erosion of the subvolcanic neck. According to the volcanological definition, the present-day morphology of the Cabugi Peak is not classified as an extinct volcano.