The roots of the magma plumbing system beneath a cold-thick oceanic lithosphere: magma fractionation and mixing along the South West Indian Ridge

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Fossil submarine hydrothermalism in metabasalts from the Gudon (Bressanone) amphibolite (Southalpine basement, Eastern Alps, NE Italy)

Relict blocks of massive metabasalts occur within foliated epidote amphibolites, which are interlayered in the Paleozoic metapelitic-metapsammitic sequences of the Southalpine basement near Bressanone, Eastern Alps. The original magmatic clinopyroxene–plagioclase-opaque association in metabasalts is variably overprinted by a static alteration, which encompasses a wide spectrum of mineral assemblages. A high-temperature alteration stage is recorded by topotactic overgrowth of Fe-Ti-rich clinopyroxene on primary Fe-poor clinopyroxene, precipitation of datolite and Fe-Ti-rich clinopyroxene (± calcite, chlorite and Co–Ni sulphides) in amygdales, and formation of albite-enriched patches in plagioclase. In places, the primary pyroxenes are partially replaced by kaersutite ± Ti-biotite. In the most altered metabasalts, a probably lower-temperature alteration assemblage, made of Fe-rich and Ti-poor clinopyroxene, calcite, Ti-garnet and datolite, replaces the cores of the magmatic pyroxenes. Late veinlets made of Fe-rich and Ti-poor clinopyroxene (with minor datolite, garnet and sulphides) are accompanied by further alteration of magmatic clinopyroxenes into chlorite + smectite ± talc, further albitisation of plagioclase, filling of amygdales by datolite ± calcite ± chlorite ± Ca-rich garnet, and precipitation of disseminated sulphides. The alteration sequence recorded in the metabasalts predates the Variscan regional metamorphism recorded by the host amphibolites and is interpreted as being related to circulation of hydrothermal fluids in a submarine continental basin with active magmatism. The lithostratigraphic and structural analogies between the Paleozoic basements of the Southalpine (e.g. Gudon) and Austroalpine (Northern Graywacke Zone) domains of the Eastern Alps, as well as the presence of hydrothermally altered mafic layers in both, suggest that magmatism and associated submarine hydrothermalism in these two domains were related to similar extensional processes affecting the pre-Variscan continental crust, which were possibly synchronous (middle Paleozoic) at the regional scale

Structural Evolution and Metasomatism of Subducted Metaophiolites in the Northwestern Alps

A subduction complex of the northwestern Alps consists of serpentinites, eclogitic metagabbros, flysch-like metasediments, meta-ophicarbonates, and gneissic slices. Unlike other subduction complexes, it contains unusual hybridized rocks described here for the first time in the northwestern Alps. They are preserved as patches interstitial in the metagabbro and as layers within metagabbros and serpentinites. The hybridized rocks are made of high modal zoisite/clinozoisite + white mica pseudomorphs of lawsonite, garnet, and amphibole associated with an Alpine eclogite-facies fabric. While these eclogitic metagabbros are chemically comparable to oceanic oxide gabbros from the ultraslow Southwest Indian Ridge, the layers are extremely enriched in Al2O3 and CaO and depleted in TiO2, MgO, and SiO2 relative to metagabbros. Patches have a geochemical signature that is intermediate between that of layers and metagabbros. Trace element compositions of hybridized rocks suggest a contribution from a fluid derived from a mixed source made of sediments and serpentinites. Except for Ba, Rb, and K, layers are comparable to the global subducting sediments, indicating a sedimentary contribution, whereas the enrichment in Cr indicates a serpentinite contribution. Metasediment dehydration and chemical exchange of Ca and Sr have resulted in significant lawsonite crystallization in the subduction zone, as reflected by the ubiquitous presence of lawsonite pseudomorphs. In light of the unique textures and geochemical signature of the lawsonite pseudomorph-bearing hybridized rocks, an origin by fluid-rock interaction and Ca-metasomatism in the subduction environment is inferred and considered in the Western Alps context

Mantle sources for Central Atlantic Magmatic Province basalts from Hf isotopes

The Central Atlantic Magmatic Province (CAMP) was one of the most voluminous LIP events in Earth history and likely triggered the end-Triassic mass extinction. The tectonic and mantle processes that produced such significant magmatic emplacement are thus of great interest. To further explore the origins of CAMP, we present new 176Hf/177Hf isotope data for a broad geographic sampling of CAMP dikes, sills, and basalt flows. We find that basaltic intrusions from the Carolinas in Eastern North America trend along a shallower slope than the terrestrial array on a diagram of 176Hf/177Hf vs. 143Nd/144Nd. This trend may reflect the presence of variable quantities of sediment-derived material in the mantle source region. This is consistent with previous suggestions that the asthenosphere beneath CAMP has been partially metasomatised by fluids derived from subducted sediments, as well as with isotopic trends observed in other LIP, such as Karoo [Jourdan et al., 2007, Jour. Petrology,doi:10.1093/petrology/egm010]. Distinct from the Carolina trend, we further observe that high-TiO2 basalts from Amazonia exhibit unusually radiogenic 176Hf/177Hf for a given 208Pb/206Pb ratio. The high-TiO2 basalts, which trend towards EM1-type compositions, may be asthenospheric melts that have experienced the addition of melts from local subcontinental lithospheric mantle (SCLM). Similarly high-TiO2 CAMP rocks from Sierra Leone may likewise have incorporated enriched lithospheric melts of lamproite-like composition in the source region [Callegaro et al., JPet, accepted; GSA Abstract #302853, 2017]. Low-TiO2 basalts from the same region in Brazil and of similar age to the high-TiO2 basalts lack the observed radiogenic 176Hf/177Hf ratios. This suggests that the melt source region beneath Brazil was heterogeneous, containing variable material with relatively radiogenic 176Hf/177Hf ratios, perhaps due to the greater age of subcontinental lithosphere and the presence of garnet. It remains unclear, however, whether the hypothesized SCLM source represents lithospheric domains which are still intact, or if this material reentered the convecting mantle by delamination prior to melting

Upper-mantle provinces at the Rodrigues Triple Junction, Indian Ocean

Across the Rodrigues Triple Junction (RTJ, 25\ub030\u2019S, 70\ub0E), marked changes in virtually all aspects of crustal accretion occur, as this junction connects three ridges with contrasting spreading rates and morphotectonic segmentations: the ultra-slow-spreading Southwest Indian Ridge (SWIR, half-rate: ~0.7 cm/yr), and the intermediatespreading Central (CIR, half-rate: ~2.4 cm/yr) and Southeast Indian (SEIR, half rate: ~2.9 cm/yr) ridges. Near the junction, some of the deepest parts of the global mid-ocean ridge system lie within the easternmost SWIR, resulting from the presence of an unusually cold underlying mantle. In contrast, the CIR and SEIR limbs near the RTJ are characterizered by shallower and more subdued axial topographic variations, reflecting warmer mantle temperatures. Significant subaxial mantle flow converging on the RTJ is thus expected as an inevitable consequence of gradients in upper mantle temperature around the junction. Here, we present new major, trace element and Sr-Nd-Hf-Pb isotope data for Mid-Ocean Ridge Basalts (MORB) collected during several INDEX cruises on- and off-axis along the CIR and SEIR in the vicinity of the RTJ. The northern extremity of the first segment of the CIR exhibits off-axis a wide diversity of MORB ranging from ultra-depleted to N-MORB. High MgO lavas (MgO>9 wt.%) more primitive and depleted in incompatible elements compared to typical MORB erupt in this area. These latters may reflect 're-melting' of pre-existing ancient SEIR mantle due to the episodic southward propagation of the CIR at the junction. Also present near the RTJ is a distinctive chemical and thermal mantle boundary between a SWIR and SEIR/CIR upper-mantle provinces. This boundary probably occurs over less than 150 kilometers along the SWIR close to the RTJ. This mantle boundary probably marks the episodic northeastward SWIR mantle migration when the CIR is receding

Propagation of a melting anomaly along the ultraslow Southwest Indian Ridge between 46 degrees E and 52 degrees 20'E: interaction with the Crozet hotspot ?

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Cratonic keels controlled the emplacement of the Central Atlantic Magmatic Province (CAMP)

Large Igneous Provinces (LIPs) are exceptionally voluminous magmatic events frequently related to continental break-up, global climate changes and mass extinctions. One interesting aspect of many LIPs is their spatial proximity to cratons, begging the question of a potential control of thick lithosphere on their emplacement. In this study, we investigate the relationship between the emplacement of the Central Atlantic Magmatic Province (CAMP) and the thick lithospheric mantle of the Precambrian cratons that formed the central portion of Pangea and are currently located on the continents surrounding the Central Atlantic Ocean. CAMP outcrops are frequently located over the margins of the thick cratonic keels, as imaged by recent tomographic studies, suggesting a role of lithosphere architecture in controlling magma genesis and emplacement. Here we focus on CAMP dykes and sills from the Hank, Hodh, and Kaarta basins in North-Western Africa (NW-Africa, Mali and Mauritania) emplaced at the edge of the Reguibat and Leo-Man Shields. The investigated intrusive rocks show compositions similar to most CAMP magmas, in particular those of the Tiourjdal geochemical group, limited to NW-Africa, and of the Prevalent group, occurring all over the CAMP. Geochemical modelling of CAMP basalts requires a Depleted MORB Mantle (DMM) source enriched by recycled continental crust (1-4%) and melting beneath a lithosphere of ca. 80 km in thickness. On the contrary, melting under a significantly thicker lithosphere (>110 km) does not produce magmas with compositions similar to those of CAMP basalts. This suggests that CAMP magmatism was likely favoured by decompression-induced partial melting of the upwelling asthenospheric mantle along the steep lithospheric boundaries of stable cratons. The architecture of the pre-existing lithosphere (i.e., the presence of stable thick cratonic keels juxtaposed to relatively thinner lithosphere) appears to have been a critical factor for localizing mantle upwelling and partial melting during extensive magmatic events such as in the CAMP

Upper and lower crust recycling in the source of CAMP basaltic dykes from southeastern North America

The densest dykes swarm of the Central Atlantic magmatic province (CAMP) occur in southeastern North America (SENA) and were intruded between 202 and 195 Ma during Pangea break-up. New combined geochemical data (major and trace elements, Sr–Nd–Pb–Os isotopes) constrain the mantle source of these magmatic bodies and their evolution path. While Sr–Nd isotopic compositions for SENA rocks (87Sr/86Sr200Ma 0.70438–0.70880 and 143Nd/144Nd200Ma 0.51251–0.51204) fall within the low-Ti CAMP field, Pb–Pb isotopes (206Pb/204Pb200Ma 17.46–18.85, 207Pb/204Pb200Ma 15.54–15.65, 208Pb/204Pb200Ma 37.47–38.76) are peculiar to this area of the CAMP and cover a considerable span of compositions, especially in 206Pb/204Pb200Ma. Given the generally unradiogenic Os isotopic compositions (187Os/188Os200Ma 0.127–0.144) observed and the lack of correlation between these and other geochemical markers, crustal contamination during the evolution of SENA dykes must have been limited (less than 10%). Thus the isotopic variation is interpreted to reside primarily within the mantle source.These observations, coupled with typical continental signatures in trace elements (positive anomaly in Pb and negative anomalies in Ti and Nb), require another means of conveying a continental flavor to these magmas, which is here hypothesized to be the shallow recycling within the upper mantle of subducted lower and upper crustal materials. Pseudo-ternary mixing models show that a maximum of 10% recycled crust is enough to explain their trace element patterns as well as their isotopic heterogeneity. Looking at the larger picture of the origin of the CAMP, the thermal contribution of a mantle plume cannot be ruled out due to the relatively high mantle potential temperatures (1430–1480 °C) calculated for high-Fo SENA olivines. Nevertheless, our results suggest that the chemical involvement of a mantle plume is negligible (less than 5%) if either a C- or an EM-flavored plume is considered. Rather, the possibility of a PREMA-flavored mantle plume, enriched by 5–20% recycled crustal material, remains a possible, though less plausible, source for these tholeiites

Geology of Lanzarote's northern region (Canary Island, Spain)

ABSTRACTThis work presents a detailed volcano-geological map of the Northern region of Lanzarote (Canary Islands, Spain). This map is a synthesis of gathered and interpreted field data and geological maps. We have integrated information obtained from: (a) detailed geological field surveys, (b) high-resolution digital elevation models (DTMs), (c) aerial orthophotographs, (d) morphometric analysis of eruptive deposits and volcanic structure (i.e. lava tubes), and (e) integrated with data from previous publications (IGME – Instituto Geológico y Minero de España). This map provides a detailed view of the volcanic diversity of the region and an overview of the lava tube system of La Corona, both of which may be used as references for future research work