Volcanic Series Research Articles

Why are S-type plutonic rocks more chemically variable than S-type volcanic rocks and S-types more variable than I-types?

S-type granitic magmas form essentially through partial melting of crustal metasedimentary rocks, so this eliminates many possibilities for the origins of chemical variations in their parental magmas. Global, whole-rock, major-oxide data show that S-type volcanic series have far less chemical variability (analytical scatter) than S-type plutonic rocks. On the other hand, published data show far smaller contrasts between metaluminous ‘I-type’ silicic plutonic and volcanic rocks. There are several possible explanations for these features, but sampling bias and analytical errors can be discounted. Likewise, the differences cannot be explained by plutonic S-types being crystal cumulates or products of wall-rock assimilation, nor can they be due to volcanic suites being products of magma mixing or having compositions controlled by a different differentiation mechanism to that which might control S-type plutonic compositions. Variations in major-oxide concentrations suggest that the most probable explanation is that S-type plutonic magmas are derived from somewhat more aluminous metasedimentary source rocks, with higher ratios of metapelites to metagreywackes. If so, there are complementary implications that plutonic S-type magmas may be formed at lower temperatures, that they may be more hydrous than the S-type volcanic magmas, and that the differences in emplacement mechanism reflect this. The weaker contrasts between ‘I-type’ volcanic and plutonic rocks may be due to their crustal magma sources being more homogeneous and the melting reactions stoichiometrically similar, regardless of source-rock type.

Geochemical data of the Ediacaran-Cambrian magmatism of Jbel Belkheit and Jbel Bou-Acila as evidence for the Hercynian Cycle Rift-to-Drift events in the Moroccan Central Hercynian Massif

In the eastern sector of the Moroccan Central Hercynian Massif, Ediacaran and Cambrian magmatic rocks outcrop in the anticlines of Jbel Belkheit and Jbel Bou-Acila. These were emplaced between Pan-African deformed-sequences and Lower Palaeozoic levels in two successive volcanic series, separated by confirmed Lower Cambrian limestones: a) a lower series of Ediacaran age composed of felsic and intermediate volcanic rocks (rhyodacites and andesites). The rhyodacites are clearly calc-alkaline in character, comparable to rocks generated in an active margin orogenic context, whereas the andesites are transitional between calc-alkaline and tholeiitic, reflecting a change in the geotectonic context at the end of the Neoproterozoic; and b) an upper series of Cambrian age, composed of anorogenic tholeiitic basalts with massive flows in the bottom and pillow lavas in the top of the sequence. Although structural, petrographic and geochemical data rule out any genetic link between the felsic and the intermediate Ediacaran rocks, a geochemical link between the andesites and Cambrian basalts can be suggested. Both facies are thought to have been generated in an extensional context, although some of the orogenic features of their protolith are still present.In this sense, the Ediacaran andesites would be precursors of the Late Neoproterozoic – Early Cambrian rift rocks, leading to the eventual formation of Cambrian basalts. The latter facies were already emplaced in a clear extensional geodynamic context, and were probably generated by the same source that evolved during progressive crustal extension and thinning. This is confirmed by the evolution of Cambrian basalt morphostructures and geochemical characteristics from: i) massive flow basalts with a geochemical signature of intraplate tholeiites of E-MORB character; to ii) pillow lavas of oceanic tholeiites with a N-MORB character. This transition, therefore, marks the beginning of an oceanization process, which has never been described elsewhere in the Moroccan Cambrian.

THE RELEVANCE OF THE STUDY OF THE PRIBREZHNY VOLCANIC COMPLEX IN THE CONTEXT OF THE HISTORY INVESTIGATIONS OF SOUTH KAMCHATKA

The present paper contains a review of publications, geological maps and reports on the study of the Pribrezhny volcanic complex of South Kamchatka. The Pribrezhny volcanic complex is comprised of a number of closeset volcanic massifs stretching chain-like along the narrow coastline of the Beregovoy ridge from the Avacha Bay to the Vestnik Bay. The relevance of the work is related to the necessity of revising the geodynamic history of formation of the study area which stems from emergence of some fragmentary, contradictory information about magmatic complexes of South Kamchatka. The paper presents the data on magmatic rocks penetrated in the coastal cliffs of South Kamchatka. Consideration is being given to their formation conditions in the context of a once-popular theory of geosynclines and in that of plate tectonics. Based on the review, several open research problems are formulated concerning the undetermined propagation limits of the Pribrezhny volcanic complex and unknown time intervals of its formation, and the lack of mineralogical and isotopic-geochemical studies of magmatic rocks. A separate research problem is the formation of transitional zone between the continental margin of Kamchatka and the Kuril Island Arc. The paper shows the relevance of the study of the Pribrezhny volcanic complex of South Kamchatka and proposes possible solutions to the research problems. A detailed mapping of the junction zone of Pribrezhny volcanic complex of South Kamchatka and the Kronotsky terrain near the Malko-Petropavlovsk zone of transverse dislocations will make it possible to refine the age and kinematics of their relationship. Determination of rare elements and strontium, neodymium and lead isotopes will provide a possibility to clarify the nature of magmatism as compared to heterochronous volcanic series of South Kamchatka. The data obtained will be needed to study magmogeneration conditions for acid volcanism of South Kamchatka and monogenic volcanism of the Malko-Petropavlovsk zone of transverse dislocations. The paleomagnetic studies, along with dating of rock absolute age, will help to reconstruct the paleolatitude of formation of the Pribrezhny volcanic complex and to restore the geodynamic evolution of formation of South Kamchatka.

Volcanism straddling the Miocene–Pliocene boundary on Patmos and Chiliomodi islands (southeastern Aegean Sea): insights from new 40Ar ∕ 39Ar ages

Abstract. The island of Patmos, in the eastern Aegean Sea, consists almost entirely of late Miocene to Pliocene volcanic rocks. The magmatism in the Aegean is governed by subduction of the African plate below the Eurasian plate, back-arc extension, slab rollback, slab edge processes and westward extrusion of central Anatolia to the west along the Northern Anatolian Fault into the Aegean domain. The evolution of the Aegean basin is that of a back-arc setting, with a southerly trend in the locus of both convergent tectonics and back-arc stretching, allowing intermittent upwelling of arc, lithospheric and asthenospheric magmas. Here, we present new 40Ar/39Ar age data for Patmos and the nearby small island of Chiliomodi to place this volcanism in a new high-resolution geochronological framework. High-resolution geochronology provides a key to understanding the mechanisms of both the tectonic and magmatic processes that cause the extrusion of magma locally and sheds light on the tectonic evolution of the larger region of the back-arc basin as a whole. The volcanic series on Patmos is alkalic, consistent with a back-arc extensional setting, and ranges from trachybasalt to phonolites, trachytes and rhyolites, with SiO2 ranging from 51.6 wt % to 80.5 wt %, K2O ranging from 2 wt % to 11.8 wt % and extrusion ages ranging from 6.59 ± 0.04 (0.14) Ma to 5.17 ± 0.02 (0.11) Ma. Volcanism on Patmos and adjacent Chiliomodi can be understood as a combination of mantle and crustal tectonic processes including the influence of transform faults and rotational crustal forces that also caused the widening of the southern Aegean basin due to two opposite rotational poles in the east and west and rollback of the subducting slab south of Crete.

Clays as indicators of paleoclimate and source rocks in The Chu-Sarysu Basin (Kazakhstan)

Newly formed smectite and palygorskite and their association are good proxies of a subtropical climate alternating dry and warm/ humid seasons during the late Cretaceous during the formation of the Chu-Syrasu basin. The association of fine-grain clays, smectite and fibres (palygorskite) and the occurrence locally of grains of albite, and natrolite, indicate they formed from water, slightly alkali-rich, and enriched in silica and magnesium. These clays may result partly from the alteration of volcanic rocks (glass) either in situ in case of volcanic emissions during sedimentation or close as smectite are euhedral and palygorskite well preserved. The flood plain may have been submitted during the hot season to drying, favouring the formation of brines which interacted with volcanic glass. Evaporation processes could have thus triggered the oversaturation with respect to smectite and palygorskite.
 Besides, muscovite as coarse grain particles, illite and chloritized biotites attest to a second source compatible with the coarse grain microcline and quartz, which can derive from granites. Source rocks could be, therefore, dual, with acid plutonic series (peraluminous granites probably) releasing coarse-grained detrital phyllosilicates (muscovite and biotite-chlorite) transported together with quartz and feldspars by rivers, and volcanic series, altered into newly formed clays (smectite and palygorskite).

Nature of Paleozoic Basement of the Catalan Coastal Ranges (Spain) and Tectonic Setting of the Priorat DOQ Wine Terroir: Evidence from Volcanic and Sedimentary Rocks

The Paleozoic volcano-sedimentary rocks within the structural basement of the horst-and-graben system of the Catalan Coastal Ranges (CCR) are composed of black shale, volcaniclastic sediments, lava flows, sills and lithocrystalloclastic tuffs. Paleozoic sediments are depleted in high-field strength elements (HFSE) such as Nb, Ta, Zr, Hf and Ti, suggesting their accumulation within the Andean-type active continental margin environment. Volcanic rocks within the Paleozoic CCR sequence belong to shoshonitic and high-K volcanic series and are enriched in Cs, Rb and Ba and depleted in HFSE, which is consistent with their derivation from metasomatized (possibly through deep recycling of pelagic sediments) subduction-related mantle source. The presence of sills (sill-sediment complex) suggests that Paleozoic basement of the CCR was formed within the rifted active continental margin or an arc-back-arc basin system akin to the modern Western Pacific subduction configuration. This complex volcanic terroir hosts world-class wines of the Priorat DOQ region. The presence of framboidal pyrite and magnetite, siderite, sphalerite xenotime, (La–Ce–Nd)-monazite, zircon and baddeleyite, as well as cuprite, tenorite and cupriferous and native silver in volcanic-derived black shale (and consequently in the world-famous “llicorella” soil overlying it) may have had dramatic effects on wine quality and sensory characteristics. These mineral features, together with strong enrichment of Priorat shale in Au, Ag and, to a lesser extent, Pt could have pronounced effects on (1) rates and specific types of chemical reactions; (2) plant metabolism; (3) response to nutrient components and (4) determination of grape flavor. Volcanic wine terroirs, such as the Priorat DOQ region, are special geologic environments for wine growth, capable of producing unique wine aromatic and gustatory characteristics.

The Kataevo Island Arc System of the Paleoasian Ocean (Transbaikalia): Composition, Age, Paleomagnetism, and Formation Geodynamic Settings

Abstract—Based on new data on the geology, composition, U–Pb isotopic age, and paleomagnetism of the metavolcanic rocks of the Kataevo Formation, we consider the geodynamic conditions of their formation and alteration. The Kataevo Formation metavolcanic rocks belong to the K–Na-high-alumina andesite–andesibasalt–basalt volcanic series. Results for U–Pb analysis of magmatic zircon (SHRIMP II, 8 spots) from a metaandesibasalt sample of the stratotype section on Ungo River yielded and age of 852 ± 9 Ma. Isotope systems for Sm–Nd yield a positive εNd(852) = +9.29, which indicates a juvenile magmatic source, close to depleted mantle (DM), with a Neoproterozoic protolith TNd(DM) model age. The content of the less mobile HFSE and REE (ppm) is consistently low for Nb (8–15), Ti (7074–12,410), Ta (0.32–0.93), Eu (1.80–2.29), Се (50–79), Y (21–25), Yb (2.1–2.8), Rb (10–24) and elevated for Sr (1000–1500), Zr (170–270), La (25–41), and Ba (600–800). All studied parameters place the metavolcanic rocks close to the contemporary Kurile–Kamchatka type of developed island arcs. Paleomagnetic analysis of the section of metavolcanic rocks shows a complete remagnetization ca. 120 Ma. This is synchronous with manifestations of intraplate basaltoid magmatism in the studied region, the most typical example of which is the Lower Cretaceous Khilok Formation.

Volcano–Plutonic Complex of the Tumrok Range (Eastern Kamchatka): An Example of the Ural-Alaskan Type Intrusion and Related Volcanic Series

Zoned plutons, composed of dunites, pyroxenites, and gabbroic rocks, have been referred to as the Ural-Alaskan type complexes (UA-complexes) and occur in numerous paleo-arc settings worldwide. Many of these complexes are source rocks for economic placers of platinum-group metals. Thus, it is important to understand how UA-complexes form and the origin and behavior of platinum-group elements (PGEs). It is widely assumed that the UA-complexes result from differentiation of supra-subduction high-Ca high-Mg sub-alkaline magmas. However, there is a lack of direct evidence for the existence and differentiation of such magmas, mainly because cases of UA-complexes being spatially and temporally linked to co-genetic volcanics are unknown. We studied an UA-complex from the Tumrok range (Eastern Kamchatka) where a dunite-clinopyroxenite-gabbro assemblage is spatially and temporary related to high-Ca volcanics (i.e., picrites and basalts). Based on the mineral and chemical composition of the rocks, mineral chemistry, and composition of melt inclusions hosted within rock-forming minerals, we conclude that the intrusive assemblage and the volcanics are co-genetic and share the same parental magma of ankaramitic composition. Furthermore, the compositions of the plutonic rocks are typical of UA-complexes worldwide. Finally, the rocks studied exhibit a full differentiation sequence from olivine-only liquidus in picrites and dunites to eutectic crystallization of diopside or hornblende, plagioclase, and K-Na feldspar in plagio-wehrlites and gabbroic rocks. All these results make the considered volcano–plutonic complex a promising case for petrological studies and modelling of UA-complex formation.

Genetic pattern of the Albian volcanic rocks in the Ziruco area, Northern Tibet: Implications for A-type granites

Genetic pattern of a continuous volcanic series of andesites, dacites, and rhyolites from the Ziruco area, northern Tibet was debated in this paper to evaluate the later evolution of A-type granites. We present new zircon UPb ages, whole-rock geochemical, zircon Hf isotopic, and whole-rock SrNd isotopic data for these magmatic rocks. Our analysis shows that the zircon UPb ages of the andesites, dacites, and rhyolites are ca. 105.4 Ma, ca.105.3 Ma, and ca.106.8 Ma, respectively. The andesites have high Nb contents (16.3–24.9 ppm) and Nb/U ratios (22.7–32.4), and chondrite normalized REE and primitive-mantle normalized trace element patterns similar to Nb-rich basalts. They were most likely derived from the mantle that had been metasomatized by melts derived from subducted slab and sediment, and then underwent fractional crystallization. The dacites and rhyolites have high Ga/Al ratios (3.05–3.70 and 2.93–3.43, respectively), REE (226–256 and 195–285 ppm, respectively), Nb (30.0–34.4 and 20.3–24.4 ppm, respectively) and Zr contents (443–513 and 147–163 ppm, respectively), and zircon saturation temperatures (856–886 °C and 823–837 °C, respectively), which are similar to A-type granites. The dacites were most likely formed by partial melting of underplated basaltic lower crust with ilmenite and zircon, followed by slight fractional crystallization. The rhyolites were most likely formed by fractional crystallization of the dacite magmas. The minerals of fractional crystallization probably include feldspar, apatite, zircon, and titanite. This continuous volcanic series in Ziruco area may be related to break-off of the Meso-Tethyan slab in northern Tibet during the late Early Cretaceous. Our studies indicate that fractional crystallization is important later evolutionary process for A-type granites from the crustal source area, and it mainly controls the geochemical characteristics of significant negative Ba, Sr, P, Ti, and Eu anomalies of A-type granites from the crustal source area.

Geotechnical Characterization of the Subsurface Using Seismic and Electrical Resistivity Techniques in Mikinduri Meru County, Kenya

Abstract Geotechnical study of the Mikinduri area was conducted with the aim to establish the causes of landslides and ground subsidence during the heavy rainfall. Sounding curves, travel time curves, 2D geoelectric sections, and 2D seismic models revealed 3-5 lithologic units to a depth of 30-250m. The study established thick loose alluvium deposits mixed with clay to a depth of 0.3-15m on an impermeable sub-basement as the possible major trigger of the landslides witnessed during rainy seasons. The thick alluvium deposits are due to erosion. They have been transferred by the location of the Mikinduri area, which lies in the lower regions of the Nyambene domes. Geologically, the domes were formed during Mount Kenya volcanic series and parasitic activities between the early Miocene and late Pleistocene eras. Resistivity modeling revealed deep geologic structures, including possible intrusions extending from 50m deep and faulted regions to a depth of 60-150m in the central and northeast area of study. The faulted area is highly saturated, as revealed by the low resistivity values. The delineated top black cotton soils and alluvium deposit are 40-50% clay and have poor foundation bearing capabilities. In contrast, compacted gravel, silt, and compact basaltic rock basement have good foundation capabilities. Keywords: Geotechnical, characterization, subsurface, seismic, electrical resistivity, foundation, geoelectric layer, Mikinduri Meru, Kenya.

FRACTIONATION AND ASSIMILATION DURING THE FORMATION OF THE DEVONIAN ANTIDROMIC IGNEOUS SERIES OF THE SISIM AREA IN THE MINUSINSK TROUGH: GEOCHEMICAL AND Sr-Nd ISOTOPIC EVIDENCE

In the Minusinsk trough, being the large fragment of the Devonian Altai-Sayan rift system, the antidromous series along with the homodromous volcanic series are widespread, sialic rocks with SiO2 content over 65 wt. % being predominant. Abundance of high-silica rocks in these series implies an extensive involvement of crustal igneous sources. However, in contrast to the homodromic series, for which the fractionation of basaltic magmas to trachyte compositions and assimilation processes during the formation of rhyolites are reconstructed, the rocks of the antidromic series are poorly studied in terms of their geochemical and isotope characteristics, which makes it difficult to assess the mechanisms of their formation. This article reports geological, petrographic, geochemical and Sr-Nd isotope characteristics of the igneous series of the Sisim area of the Minusinsk trough, its Early Devonian age is reliably identified by previous researchers, e.g. U-Pb dating by zircon. The X-ray fluorescence and mass spectrometric analyses were used to estimate the material composition. This series is composed of moderately alkaline basalts, dolerites, basaltic trachyandesites, trachyandesites, trachytes, trachydacites, rhyodacites and rhyolites. The intermediate and sialic rocks occur in the lower part of the section, prevailing over mafic rocks. As SiO2 increases in the rocks of the Sisim antidromous series, similarly to the rocks of the homodromous series, the contents of incoherent elements are dispersed, and there is a trend of decreasing εNd and increasing εSr values. This evidence points to a disruption in fractionation resulting from the appearance of anatectic melts, which are close in composition to the upper continental crust. Thus, regardless of the sequence of formation of mafic, intermediate, and sialic rocks and their volume ratios in the homodromous and antidromous series, a combination of fractional crystallization of basaltic magmas and their assimilation of silicate crustal substrates is reconstructed for them.

The Bafoussam volcanic series: origin and evolution of the volcanism along the Cameroon volcanic line

The Bafoussam volcanic series: origin and evolution of the volcanism along the Cameroon volcanic line

The Lower Cretaceous Tuburan Limestone of Cebu Island, Philippines: Microfacies, micropalaeontology, biostratigraphy, and palaeogeographic perspectives

The occurrence of orbitolinid-bearing shallow water limestone blocks (Tuburan Limestone) incorporated into the volcanic series from Cebu Island, Central Philippines, has been known since the 1950's. Taxonomic studies including solid biostratigraphic data however are lacking or not substantiated. Herein we report the occurrence of Mesorbitolina texana (Roemer), transitional forms between M. texana and M. subconcava (Leymerie), Mesorbitolina birmanica (Sahni), Palaeodictyoconus actinostoma Arnaud-Vanneau & Schroeder, Neorbitolinopsis conica (Matsumaru), Paracoskinolina sp. and other benthic foraminifera (Akcaya, Praechrysalidina, Vercorsella) indicating a latest Aptian age. The previously accepted Late Albian age for the Tuburan Limestone based on Neorbitolinopsis conulus (Douvillé) is rejected herein and suggested as a misidentification with the recently revised Aptian - Lower Albian Neorbitolinopsis conica (Matsumaru). The general poverty of Lower Cretaceous dictyoconids in the Tuburan Limestone is interpreted as being caused by the lack of suitable extensive lagoonal facies that is generally typical for carbonate platforms of passive continental margins. The observed microfacies types instead refer to external platform margin settings with corals, stromatoporoids, sclerosponges (e.g., Acanthochaetetes), and planktic foraminifera. The recovered (micro) fauna from Cebu Island shows striking similarities to assemblages reported from Western and Mid-Pacific guyots but with indicated younger ages (up to the late Albian) based on data that - in our opinion - do not stand up to close scrutiny. Finally, a model is proposed interpreting the Tuburan Limestone from Cebu Island as remnants of a former carbonate cover of a guyot that originated as a volcanic island in the Western-Central Pacific, and later became incorporated into an accretionary wedge/mélange zone due to subduction-collisional processes.

New 40Ar/39Ar age of the full vector Upper Mammoth geomagnetic polarity transition recorded in the Pu’u Kualakauila volcanic sequence, Hawaii

Paleomagnetic measurements, coupled with 40Ar/39Ar dating, are improving our understanding of the geodynamo by providing detailed terrestrial lava records of the short-term behavior of the Earth's magnetic field. As part of an investigation of the Wai'anae Volcano, Oahu, and the short-term behavior of the geomagnetic field, we sampled a long volcanic section located on the volcano's collapsed flank at a locality known as Pu’u Kaulakauila. Prior paleomagnetic investigations of the Kamaile'unu Volcanic Series (i.e. Herrero-Bervera and Coe, 1999, Herrero-Bervera and Valet, 1999, 2005) revealed transitional directions. The fresh lava flows, easy access, and close geographical proximity to KAr dated flows made this ~215-m thick sequence of flows an excellent candidate for detailed paleomagnetic analysis. At least eight samples, collected from each of 47 successive flow sites, were stepwise demagnetized by both alternating field and thermal methods. Magnetostratigraphic results indicate the existence of four excursions or aborted reversals occurring at approximately ~36, ~75, ~130, and ~ 151 m above flow#1. The mean directions of magnetization of the entire section sampled indicate a reversed polarity, with ∼10 m of the section characterized by truly excursional/Cryptochron directions (~7 lava flows). Paleointensity (P.I.) determinations indicate a steady decreasing of the PI values from 75 μ-T to ~12.2 μ-T reaching a minimum of 7.1 μ-T. These very low PI values are within the transitional/excursional part of the record. The corresponding VGPs are located on the western part of Australia. 40Ar/39Ar incremental heating experiments on groundmass from transitional flow sites at different stratigraphic levels yield a weighted mean plateau age of 3.201 ± 0.041 Ma, which, combined with the overall reversed polarity and two older polarity reversals, strongly suggests that the transitional lavas correspond to the Upper Mammoth polarity transition.

Petrogenesis of Cretaceous felsic volcanic rocks in Zhejiang province, China: insights from zircon U-Pb age and Lu-Hf isotopes

ABSTRACT Late Mesozoic felsic volcanic rocks are mostly distributed in the Zhejiang, Fujian, and Guangdong provinces, southeastern China, with an outcrop area of >100,000 km2. Although many studies have been published, however, the petrogenesis and tectonic background of these felsic volcanic rocks have remained controversial. Here, we present nineteen new SHRIMP U-Pb zircon ages and Lu-Hf isotopes of the felsic volcanic rocks from southeastern Zhejiang Province. The studied samples have zircon U-Pb ages range from 134 to 89 Ma and become younger towards the coastal zone. A group of zircons with U-Pb ages of 120–110 Ma have been documented, which argue against the proposed magmatic quiescence of 120–110 Ma in the study area. Together with all the published data, our zircon U-Pb mapping has shown that the lower volcanic series (140–120 Ma) are all distributed in the inland of Zhejiang Province, whereas the upper volcanic series (120–90 Ma) are mostly exposed in the coastal zone. Furthermore, nine felsic volcanic samples from Yandangshan area show negative correlation between the zircon ɛHf(t) values and the volcanic ages, which suggests the increased juvenile components in the source region with time. Our zircon Lu-Hf isotope mapping does not show a clearly trend towards the coastal zone. The variation of zircon Lu-Hf isotopes among felsic volcanic rocks were most likely attribute to the different degrees of juvenile contribution. Based on our new results, a episodic roll-back of the Palaeo-Pacific plate model can well explain the petrogenesis of the felsic volcanic rocks. There is no evidence supporting the scenario that a large volume of felsic magma erupted at a short interval of 3–10 Ma. Therefore, at least in Zhejiang Province, the Cretaceous felsic volcanic rocks cannot be considered as a part of a Silicic Large Igneous Province.

Inception and Evolution of La Corona Lava Tube System (Lanzarote, Canary Islands, Spain)

AbstractGrowing interest in studying large terrestrial lava tubes is motivated in part by their analogy with their extra‐terrestrial counterparts. However, on Earth, the formation of such structures is still poorly understood. Here, the lava tube system of La Corona (Lanzarote, Canary Islands, Spain) is studied to identify how pre‐existing stratigraphy can govern a lava tube's evolution. Combining terrestrial laser scanner technology with field observations and geochemical analyses of the pre‐existing lava enabled us to reconstruct the three‐dimensional geometry of the lava tube system, the paleo‐surface trough which it developed, and the volcanic series into which it carved its path. We show that a pyroclastic layer played a key role in the development of the lava tube. The layer—Derived from late Quaternary Strombolian activity—Is traceable along almost the full length of the tube path and defines the paleo‐topography. The excavation process mostly happens because of the mechanical strength of the substrate, that controls the widening of the growing lava tube. Other influential parameters controlling erosion include slope variations of the paleo‐surface (i.e., knickpoints), and the lava physical properties. Since weak layers such as regolith are a common feature of extra‐terrestrial lava flows, the processes seen at La Corona to the may be highly relevant to the development of planetary lava tube systems.

Petrogenesis of Lava from Christmas Island, Northeast Indian Ocean: Implications for the Nature of Recycled Components in Non-Plume Intraplate Settings

Lava samples from the Christmas Island Seamount Province (CHRISP) record an extreme range in enriched mantle (EM) type Sr-Nd-Pb-Hf isotope signatures. Here we report osmium isotope data obtained on four samples from the youngest, Pliocene petit-spot phase (Upper Volcanic Series, UVS; ~4.4 Ma), and four samples from the earlier, Eocene (Lower Volcanic Series, LVS; ~40 Ma) shield building phase of Christmas Island. Osmium concentrations are low (5–82 ppt) with initial Os isotopic values (187Os/188Osi) ranging from (0.1230–0.1679). Along with additional new geochemical data (major and trace elements, Sr-Nd-Pb isotopes, olivine δ18O values), we demonstrate the following: (1) The UVS is consistent with melting of shallow Indian mid-ocean ridge basalt (MORB) mantle enriched with both lower continental crust (LCC) and subcontinental lithospheric mantle (SCLM) components; and (2) The LVS is consistent with recycling of SCLM components related to Gondwana break-up. The SCLM component has FOZO or HIMU like characteristics. One of the LVS samples has less radiogenic Os (γOs –3.4) and provides evidence for the presence of ancient SCLM in the source. The geochemistry of the Christmas Island lava series supports the idea that continental breakup causes shallow recycling of lithospheric and lower crustal components into the ambient MORB mantle.

Geochemistry of Magmatic Rocks of the Syama Belt, Southern Mali, West African Craton

Within southern Mali, the Syama belt constitutes a linear major structure-oriented N-S, which host several gold deposits (e.g., Syama and Tabakoroni) and prospect areas (e.g. Tellem). The Syama Belt is formed by magmatic rocks (basalts, lamprophyres, andesites, dacites and microgranites); sedimentary rocks (shales) and volcano-sedimentary rocks (pyroclastics). The magmatic rocks are divided into two main volcanic series: tholeiitic affinity rocks (basalts and lamprophyres) and calc-alkaline affinity (andesites) that are the most evolved. The field relationships between rocks of these two series suggest that the calc-alkaline series are younger the tholeiitic series. These tholeiitic series present the Mid-Ocean Ridge Basalt (MORB) affinity whereas the calc-alkaline series would be linked to an island arc-type. This coexistence is not an isolated case within the West African Craton (WAC). Otherwise, the Syama belt has all the characteristics of other belts, within which a number of gold deposits are developed, in the WAC.

Origin and Evolution of the Fatu Kapa Magmatic System (North-Western Lau Back-arc Basin): Insight on the Genesis of High-Silica Lavas

Abstract The South Eastern Futuna Volcanic Zone (SEFVZ; 50 km × 80 km) is located in the NW of the Lau back-arc basin at the northern boundary of the Futuna and YAO microplates. Fatu Kapa is an area (∼15 km × 10 km) of diffuse volcanic activity at the center of the SEFVZ and at the intersection of two volcanic axes with the Futuna fracture zone. At this place, the volcanic, tectonic, and hydrothermal activities are enhanced. The most differentiated volcanic series of the whole Futuna region are found in the Fatu Kapa area. Based on major and trace element data, four groups of volcanic rocks are distinguished in the Fatu Kapa area. The main groups form a well-defined transitional suite from basalts to andesitic and dacitic lavas that display enrichments in Zr, Cl and heavy rare earth elements. The most mafic lavas of this group are extracted from an Indian-type depleted mantle mixed with a component with trace element and isotopic characteristics similar to the nearby Samoa mantle plume. There is no evidence of subduction input. Petrological modelling shows that the mafic melts of the main group (Group 1A) evolve towards trachyandesites through fractional crystallization coupled with assimilation of 10 % of a Cl-rich and Th-poor melt generated by low-degree (≤2 %) partial melting of a hydrothermally altered basaltic crust. Rocks of Groups 1B(a) and 1B(b) were then generated by mixing of a mafic melt from Group 1A with a melt obtained by partial melting of andesitic and hydrothermally altered wallrock in shallow magma chambers. Extremely high Cl concentrations can be explained by assimilation of interstitial K- and Cl-rich brines in the pore water of the hydrothermally altered volcanic rocks before melting of these rocks. This is in good agreement with unusual extension of the Fatu Kapa hydrothermal area and indicates that, in the north of the Lau basin, the microplate boundaries are privileged zones to generate a complex felsic magmatism within areas of widespread hydrothermal alteration. Our study confirms the importance of assimilation and fractional crystallization and mixing processes in generating high-silica lavas in back-arc environments decoupled from subduction.

The Origin of Felsic Intrusions Within the Mantle Section of the Samail Ophiolite: Geochemical Evidence for Three Distinct Mixing and Fractionation Trends

AbstractAn isotopically diverse suite of felsic dikes, sills, and plugs (εNd(t) = −7.8 to +7.8) intrude the uppermost mantle and lower crust in the Samail ophiolite in Oman and the United Arab Emirates (UAE). These features have been interpreted to represent amphibolite and metasediment melts from an underthrust sheet of oceanic lithosphere. As such, the intrusions provide a record of melting of oceanic crust and sediment at depth, with implications for mass transfer from the down‐going plate in young, hot subduction settings. Several studies have used geochemical data to constrain the magmatic sources of the dikes; here we build on this previous work using integrated whole rock major element, trace element and Nd isotopic data from a more geographically extensive suite of dikes. New and existing data suggest the felsic intrusions preserved within mantle peridotites in the Oman portion of the ophiolite were generated by three distinct mixing and fractionation trends: (1) three‐component mixing between sediment melt, amphibolite melt and a mantle component; (2) two component mixing between amphibolite and sediment melts, with little mantle contribution; and (3) fractional crystallization of depleted, mantle derived magmas, likely related to the ophiolite V2 volcanic series. Combined geochemical and pseudosection modeling suggest that amphibolite melting occurred at P ≤ 1.4 GPa (∼40–45 km) and T ≥ 700–750°C. Felsic intrusions in the mantle section in the UAE, including garnet‐andalusite‐cordierite leucogranites, follow similar mixing trends, but crystallized ∼0.9–4.0 Ma after the Oman intrusions.