Exploring Causal Relationships for Geoheritage Interpretation — Variable Effects of Cenozoic Volcanism in Central European Sedimentary Tablelands

Modern conceptual approach to geointerpretation and geoeducation emphasizes the holistic understanding of the environment and attends to linkages between various abiotic, biotic, and cultural components. In this paper, we highlight multiple relationships between Cenozoic volcanism and host sedimentary rocks, mainly sandstones of Cretaceous age, which can be explored in the context of geotourism and geoeducation in several Central European geoparks (Bohemian Paradise UNESCO Global Geopark, Land of Extinct Volcanoes Aspiring Geopark, Ralsko National Geopark) and their surroundings. These include the effects of magmatism on sandstones, with further consequences for landform development at different spatial scales, the origin of mineral resources, underpinning of biological diversity, and specific land use contrasts. Existing interpretation provisions are reviewed, and a three-tiered framework to show these different linkages is proposed. It is argued that different, but complementary themes can be addressed at the landscape, landform, and individual outcrop (geosite) level.

Lithospheric mantle refertilization by DMM-derived melts beneath the Cameroon Volcanic Line—a case study of the Befang xenolith suite (Oku Volcanic Group, Cameroon)

The origin and evolution of subcontinental lithospheric mantle (SCLM) are important issues of Earth’s chemical and physical evolution. Here, we report detailed textural and chemical analyses on a mantle xenolith suite from Befang (Oku Volcanic Group, Cameroon Volcanic Line), which represents a major tectono-magmatic structure of the African plate. The samples are sourced from spinel-facies mantle and are dominated by lherzolites. Their texture is cataclastic to porphyroclastic, and foliation defined by grain-size variation and alignment of spinel occurs in part of peridotites. Spinel is interstitial and has amoeboidal shape. Clinopyroxene REE patterns are similar to those of Depleted MORB Mantle (DMM) except LREEs, which vary from depleted to enriched. The A-type olivine fabric occurs in the subset of one harzburgite and 7 lherzolites studied by EBSD. Orthopyroxene shows deformation consistent with olivine. The fabric of LREE-enriched clinopyroxene is equivalent to those of orthopyroxene and olivine, whereas spinel and LREE-depleted clinopyroxene are oriented independently of host rock fabric. The textural, chemical and thermobarometric constraints indicate that the Befang mantle section was refertilised by MORB-like melt at pressures of 1.0–1.4 GPa and temperatures slightly above 1200–1275 °C. The olivine-orthopyroxene framework and LREE-enriched clinopyroxene preserve the protolith fabric. In contrast, the LREE-depleted clinopyroxene, showing discordant deformation relative to the olivine-orthopyroxene protolith framework, and amoeboidal spinel crystallized from the infiltrating melt. The major element and REEs composition of minerals forming the Befang peridotites indicate subsequent reequilibration at temperatures 930–1000 °C. This was followed by the formation of websterite veins in the lithospheric mantle, which can be linked to Cenozoic volcanism in the Cameroon Volcanic Line that also brought the xenoliths to the surface. This study therefore supports the origin of fertile SCLM via refertilization rather than by extraction of small melt fractions, and further emphasizes the involvement of depleted melts in this process.

Evolution of lithospheric mantle beneath mobile belt between two cratons: An example from the Oku Massif, Cameroon Volcanic Line (W Africa)

<p>Cameroon Volcanic Line (CVL) is located in the western part of equatorial Africa and consists of volcanoes which were active from Eocene to recent, stretching ca. 1700 km from the Atlantic in the SW into the African continent in the NE. The continental part of the CVL is located on the Neoproterozoic Central African Orogenic Belt and is situated between the Congo craton and Sahara/Western Africa craton. Mantle peridotite xenoliths which occur locally in lavas of the CVL come from the spinel facies only, suggesting a relatively shallow lithosphere-asthenosphere boundary (LAB). This is supported by seismic studies, showing the LAB at 90-100 km.</p><p>In order to understand better the evolution of the lithospheric mantle beneath the CVL, we studied xenolith suite (16 xenoliths) from Befang in the Oku Massif (Tedonkenfack et al., submitted). The Befang xenoliths are almost entirely lherzolites which have cataclastic to weakly porphyroclastic texture. Harzburgites and websterites occur subordinately. Spinel is interstitial and has amoeboidal shape. The studied peridotites (14 lherzolites, 1 harzburgite) consist of minerals with almost constant composition (olivine Fo<sub>88.7-90.3</sub>, orthopyroxene Al 0.17-0.19 atoms per formula unit (a pfu), clinopyroxene Al 0.28-0.30 a pfu, spinel Cr# dominantly 0.09-0.11). Spinel of Cr# 0.15 occurs in one of the lherzolites, whereas that occurring in harzburgite has Cr# 0.19. Clinopyroxene REE patterns are similar to those of Depleted MORB Mantle (DMM) except LREEs, which vary from depleted to enriched. The A-type olivine fabric occurs in the EBSD-studied subset of 8 samples (one harzburgite and 7 lherzolites). Orthopyroxene shows deformation consistent with olivine. The fabric of LREE-enriched clinopyroxene is equivalent to those of orthopyroxene and olivine, whereas spinel and LREE-depleted clinopyroxene are oriented independently of the fabric of host rock.</p><p>These data, thermometry, phase relationships and phase equilibria diagrams suggest that the Befang mantle section was refertilised by MORB-like melt at pressures 1.0-1.4 GPa and temperatures slightly above 1200 – 1275 ºC. The olivine-orthopyroxene framework and LREE-enriched clinopyroxene preserve the fabric of protolith. On the other hand, the LREE-depleted clinopyroxene shows discordant orientation relative to olivine-orthopyroxene protolith framework, and amoeboidal spinel crystallized from the melt. The major element and REEs composition of pyroxenes occurring in the Befang peridotites indicate chemical reequilibration at temperatures 930 – 1000 ºC. Trace element modeling shows that websterites can be linked to Cenozoic volcanism. We speculate that they form veins in the lithospheric mantle. Our study therefore supports the origin of fertile SCLM via refertilization rather than by extraction of small melt fractions, and further emphasizes the involvement of depleted melts in this process, which contrasts with the incompatible element-enriched melts typically invoked in within-plate settings.</p><p>This study originated thanks to the project of Polish National Centre of Research NCN 2017/27/B/ST10/00365 to JP. The bilateral Austrian-Polish project WTZ PL 08/2018 enabled extensive microprobe work.</p><p>References:</p><p>Tedonkenfack SST, Puziewicz J, Aulbach S, Ntaflos T., Kaczmarek M-A, Matusiak-Małek M, Kukuła A, Ziobro M: Lithospheric mantle refertilization by DMM-derived melts beneath the Cameroon Volcanic Line – a case study of the Befang xenolith suite (Oku Volcanic Group, Cameroon). Submitted.</p>

The story of post-Variscan lamprophyres of the Bohemian Massif: from ultramafic (Upper Cretaceous–Palaeocene) to alkaline (Eocene–Oligocene) types

Post-Variscan lamprophyres of the Bohemian Massif hold the potential for the understanding of deep-mantle processes beneath the Bohemian Massif in association with mantle metasomatism as a consequence of Variscan subduction and Late Palaeozoic extension in Central Europe and tectonic processes between Variscan blocks. Two principal types of post-Variscan lamprophyres occur in the Bohemian Massif, contrasting in their age and composition: ultramafic lamprophyres of Late Cretaceous to Palaeocene age and alkaline lamprophyres of Mid Eocene to Late Oligocene age. Combination of published and new whole-rock, isotope (Sr-Nd-Pb) and radiometric (K/Ar) data on lamprophyres of both types (including new data from samples from the deep boreholes) significantly contributes to the understanding of the changing tectonomagmatic position of post-Variscan volcanism in the Bohemian Massif. The striking shift in lamprophyre geochemistry is paralleled by a change in their structural position from the initial pre-rift period of volcanism to the developed syn-rift period and the related change in their mantle sources beneath the Bohemian Massif. The Late Cretaceous and Cenozoic volcanism is explained as related to lithospheric flexuring during the Alpine orogeny, resulting in an asthenospheric upwelling, or associated with the lithosphere deformation and perturbation of the thermal boundary layer.Supplementary material at https://doi.org/10.6084/m9.figshare.c.5277861

The Cenozoic volcanic fields in northern Hainan Island and Leizhou Peninsula, South China: eruption history, magma source and dynamic background

Northern Hainan Island and Leizhou Peninsula volcanic fields (Leiqiong), the southernmost continental Cenozoic volcanism in China, cover an area of ∼8000 km2 with 177 volcanoes recognized. Far from the subduction areas, volcanoes in this area provide an ideal opportunity to study the geodynamics of intraplate volcanoes. Here, we review the geochronological and geochemical data of the volcanic rocks in Leiqiong volcanic fields and discuss their magma sources and geodynamics on the basis of the geological and geophysical observations. Chronological data (34.78-0.01 Ma) show that the volcanic activities started approximately in Miocene and continued to Quaternary. These basalts show typical geochemical characteristics of oceanic island basalts (OIB), and tomographic images reveal that a mantle plume is situated beneath Hainan Island and extends down to the core-mantle boundary. Thus, we suggest Hainan mantle plume is responsible for the Cenozoic volcanism in Leiqiong volcanic fields and this plume is sourced from the lower mantle with additions of dehydrated slab fragments. These mixed plume materials were brought to the upper mantle and produce solid pyroxenites, which are the major source of Leiqiong magmas. Although there is no documental record of volcanic eruptions in Leiqiong volcanic fields, the volcanic danger cannot be neglected.Supplementary material at https://doi.org/10.6084/m9.figshare.c.5227601

Magnetotelluric investigation of the Precambrian crust and intraplate Cenozoic volcanism in the Gour Oumelalen area, Central Hoggar, South Algeria

SUMMARY The Tuareg Shield was assembled by oceanic closures and horizontal movements along mega-shear zones between approximately 20 terranes during the Pan-African Orogeny (750–550 Ma). Although there is an ongoing debate about its origin, the exhumation of the Tuareg Shield is assumed to be related to Cenozoic intraplate volcanism. The Gour Oumelalen is a key region of the Tuareg Shield and is located in the northeastern part of the Egéré-Aleksod terrane, corresponding to the eastern boundary of the Archean–Palaeoproterozoic microcontinent LATEA (Central Hoggar). The eastern boundary of the study area corresponds to a Neoproterozoic suture zone separating two old microcontinents, LATEA and the Orosirian Stripe. We deployed two magnetotelluric (MT) profiles consisting of 33 broad-band MT stations and combined these with aeromagnetic data, aiming to define the crustal structure in detail. The resistivity cross-sections obtained from the 3-D inversion of full impedance tensor and tipper data from stations along the profiles, confirm the main Precambrian faults, some of which are covered by Quaternary sediments and hence, have not yet been deciphered. The cross-sections also highlight the Cretaceous–Quaternary sedimentary basins represented by low resistivities. The upper crust is typically cratonic with a high electrical resistivity. On the contrary, the lower crust shows a drastic drop in resistivity (<10 Ωm). The most plausible hypothesis is that the study area corresponds to a Cretaceous rifting zone. The Cretaceous magmatic event and its related fluids and mineralization as well as the recent fluids associated with Cenozoic volcanism, are plausible causes of a very conductive lower crust. However, we cannot exclude other reasons such as: (i) a high-temperature and strongly sheared mobile belt or (ii) a contribution of inheritance involving Pan-African events that affected this former suture area.

Lithospheric mantle, asthenosphere, slab and crustal contribution to petrogenesis of Eocene to Miocene volcanic rocks from the west Alborz Magmatic Assemblage, SE Ahar, Iran

Significant uncertainty remains regarding the exact timing and nature of subduction events during the closure of the Tethyan seas in what is now NW Iran. This study thus presents new geochemical compositions and U–Pb ages for a suite of volcanic rocks emplaced during Cenozoic volcanism in the west Alborz Magmatic Assemblage, which is commonly regarded as the back-arc of the Neotethyan magmatism in Central Iran. The subalkali basalts and andesites are dated to 57 ± 1.2 Ma, and are likely derived from a supra-subduction mantle wedge. Later, trachytic A-type rocks erupted from ~42 to 25 Ma during an anorogenic (extensional) stage triggered by slab retreat and associated asthenospheric mantle influx. A-type melts were at least partly concurrent with lithospheric mantle magmatism implied by eruption of subalkali basalts–andesites around 26–24 Ma. Next, Amp-Bt trachybasaltic volcanism with high-Nb basaltic affinity at ~19 Ma likely records slab deepening and slab partial melting, which reacted with the mantle wedge to produce the source material for the high-Nb basalts. Sr–Nd isotopic ratios for SE Ahar mafic as well as A-type rocks imply rather enriched mantle source(s). Some crustal contamination is implied by the presence of inherited zircons dominated by those derived from Neoproterozoic–Cambrian basement rocks and Carboniferous magmatism. Rhyolitic rocks with adakitic affinity probably mark the final volcanism in the study area. The adakitic rocks show crustal signatures such as high K and Th, probably formed as a consequence of higher temperature gradients, at crustal levels, imposed by both slab and mantle partial melts.