Fractionation Of Clinopyroxene Research Articles

Nephelinites from Burko volcano (Tanzania) record the phase relations among perovskite, magnetite, titanite and andradite in evolved alkaline and silica-undersaturated systems

Nephelinitic rocks from Burko volcano in the Northern Tanzanian Divergence Zone of the East African Rift System represent transitional compositions between primitive and evolved nephelinites which exhibit two distinct phase assemblages, allowing to constrain the magmatic history of such particular rock types. Detailed petrography, mineral compositions and whole rock geochemistry were used to reconstruct the crystallization conditions and the petrological evolution of the Burko rocks and to compare them to the nearby volcanoes of Oldoinyo Lengai and Sadiman. Burko samples report a characteristic mineralogy of intermediate nephelinites (Mg# of 60–40) which comprise nepheline-diopside-magnetite-perovskite assemblages. They evolved mainly via fractionation of clinopyroxene, apatite, magnetite, and perovskite/titanite to peralkaline nephelinites (Mg# of 40–25) comprising nepheline-aegirine-augite-titanite-andradite±K-feldspar assemblages. The presence of unexposed primitive olivine nephelinites is, however, indicated by rare forsterite antecrysts.Reworking of crystal mushes and/or magma mixing are evident from different xenocrysts, antecrysts and pyroxenitic–ijolitic inclusions, precluding simple fractional crystallization modelling. The evolution from diopside-bearing nephelinites towards aegirine-augite-bearing ones was accompanied by a decrease in temperature (from ~1100 to ~900 °C) and an increase of log(aSiO2) towards ~−0.5 and of fO2 (∆FMQ ~2.5 to ~3). Especially in the peralkaline nephelinites, late-stage enrichment of Sr, Ba and halogens is documented by Sr-and F-rich apatite, barytolamprophyllite, celsian, götzenite, and eudialyte. In comparison, evolved and mostly peralkaline nephelinites from the nearby Oldoinyo Lengai and Sadiman volcanoes contain similar mineral assemblages, indicating comparable formation processes, but slightly different melt evolution trajectories. The variations in nephelinite composition and phase assemblages are linked to a) slightly different parental melt compositions related to variable amounts of amphibole, mica and carbonate in the molten mantle veins and b) different crystallization conditions, especially redox conditions, during cooling.

Mesozoic lithospheric mantle evolution in South China: Evidence from Ar-Ar dating, geochemistry and Sr-Nd-Pb isotopes of lamprophyres

Understanding the Mesozoic lithospheric mantle beneath South China is challenging, primarily because mantle-derived magmatic rocks are rare, whereas granitic magmatism is far more abundant. In this study, we present new 40Ar/39Ar geochronology, major and trace elemental composition, and Sr-Nd-Pb isotopic data for lamprophyres from northern and central Guangxi Province, South China. Our results reveal two intrusion phases: an earlier phase at 207.7 ± 1.2 Ma, and a later phase between 95.6 ± 0.6 Ma and 102.3 ± 0.7 Ma. The earlier phase, represented by the Danyang lamprophyres, is marked by low Cr (330–355 ppm) and Ni (241–258 ppm) contents and strongly fractionated REE patterns with (La/Yb)n ratios of 57.5 to 62.8. The later phase, represented by the Leidong, Nongchang, and Longchang lamprophyres, shows higher Cr (490–781 ppm) and Ni (237–377 ppm) contents, with less fractionated REE patterns and (La/Yb)n ratios of 20.2 to 27.8. Despite their temporal differences, the lamprophyres share similar isotopic characteristics, including high initial 87Sr/86Sr (0.707940–0.721310), negative εNd(t) (−9.19 to −5.31), and radiogenic Pb isotopic compositions (206Pb/204Pb: 18.200–18.331, 207Pb/204Pb: 15.62–15.72, 208Pb/204Pb: 38.53–38.84). These features suggest derivation from low-degree partial melting of a phlogopite-bearing, refractory peridotite lithospheric mantle, modified by olivine and clinopyroxene fractionation. This refractory mantle was metasomatically enriched by Proterozoic subduction processes beneath the Yangtze Craton. By synthesizing published Sr-Nd-Pb isotopic data, we propose that the lithospheric mantle beneath South China shifted from enriched to depleted mantle between ~220 Ma and ~ 100 Ma, possibly due to extension associated with the subduction and rollback of the Paleo-Pacific Plate.

1.88 Ga granitoids at Sorsakoski, Central Finland: A-type magmatism within the Raahe-Ladoga suture zone

Four quartz monzonite – granite intrusions forming the Sorsakoski granite lithodeme, are found within the Raahe-Ladoga suture zone in Central Finland. The prevailing potassium feldspar megacrystic quartz monzonites and granites form a bimodal association with diorites and gabbros. The granitoids are mainly calc-alkaline, ferroan, per- to metaluminous, and have high Zr and REE contents. Dominant mafic minerals are biotite and hornblende, clinopyroxene and orthopyroxene are locally present. The mafic units display effects of fractionation of clinopyroxene. In our interpretation, these intrusions were emplaced during regional late stages of deformation and post-crystallisation deformation partitioned into major shear zones leaving bulk of the intrusions relatively undeformed. Based on one new (1876 ± 6 Ma) and one pre-existing (1882 ± 5 Ma) U-Pb zircon age determination, the crystallisation age of the granitoids can be assumed at ca. 1880 Ma. Based on mineralogy, petrography, geochemistry, bimodal nature of magmatism and age, we correlate these intrusions to the A-type rocks of the previously described Saarijärvi suite. This shows that the syn-orogenic A-type magmatism extended eastwards beyond the Central Finland Granitoid Complex.

Petrogenesis of submarine volcanic arc rocks from Andaman subduction zone, Northeast Indian Ocean: Constraints from slab components and mantle wedge characteristics

The present study provides new petrological and geochemical data of the dredged rocks from submarine volcanoes along the Andaman arc and describes the petrogenetic evolution of the arc system in terms of mantle wedge characteristics, nature and quantitative input of subducted slab components, and fractionation processes of precursor magma. The studied rocks include basaltic andesite, andesite, dacite and rhyolite. These volcanic rocks exhibit LILE, LREE enrichments and HFSE depletion, corroborating their generation through subduction processes. High abundances of Th/Nd, La/Sm(N), LREE/HFSE than LILE/HFSE, LILE/LREE suggest a substantial contribution of sediments from the subducting slab over slab-dehydrated aqueous fluids to the mantle wedge. The 87Sr/86Sr-Ba/La mixing model suggests 0.6–0.8% addition of slab fluid (90:10 AOC: sediment fluid) to account for the fluid signature, whereas the 143Nd/144Nd-La/Sm(N) mixing model envisages ∼3–4% addition of sediment melt to the mantle source, reconciling the sediment signature in Andaman submarine volcanic rocks. The presence of N-MORB type mantle is attributed to the absence and/or inefficient convection of asthenospheric material from the Andaman back-arc basin to the mantle wedge. This ineffective convection can be equated with the flat subduction of the Indian Plate, caused by the convergence of the aseismic Ninety East Ridge. The non-modal batch-melting model suggests that 13–24% partial melting of the spinel lherzolite mantle beneath the Andaman submarine volcanic arc formed the parent magma. The crystallization model invokes up to 60–70% of fractionation of olivine, plagioclase, clinopyroxene, orthopyroxene, sanidine and magnetite in all the rock types with subordinate proportions of amphibole, biotite, apatite, ilmenite, and sanidine in rhyolites. The basaltic andesites, andesites and dacites do not show upper crustal input, while rhyolites indicate crustal contamination from an upper crust and/or arc crust.

Petrogenesis and tectonic implications of late Permian andesites from Rongzharinian, northern Qiangtang, Tibet

At convergent margins, high-magnesian andesitic rocks (HMAs) can offer vital information about the formation of the continental crust and the interactions between the deep crust and mantle. This study focuses on new identified HMAs from the Rongzharinian area, northern Qiangtang Terrane (NQT). We report the major, trace elemental, Sr-Nd-Hf isotopic data, and zircon U-Pb age of volcanic rocks with the aim of understanding their petrogenesis and tectonic setting. A concordant age of 257.7 ± 1.5 Ma for the Rongzharinian high-Mg andesites is determined by zircon U-Pb dating using LA-ICP-MS techniques. The Rongzharinian HMAs have high MgO contents (5.88–7.39 wt%), Mg# values (63–68), and Cr (143–185 ppm) and Ni (48–66 ppm) concentrations, close to primitive HMAs. They underwent variable degrees of fractionation of amphibole and minor clinopyroxene and negligible crustal contamination during magma evolution. REE modeling results show that the Rongzharinian HMAs can be generated by 4 %–7% melting of hydrous spinel lherzolite at relatively shallow depths. Geochemical data indicate that the Rongzharinian HMAs were dominantly derived from the mixed mantle wedge source composed of N-MORB-like and E-MORB-like components. Based on the regional tectonic evolution, we propose that the Late Permian rollback of the Paleo-Tethyan slab may induce upwelling of the asthenosphere mantle, resulting in melting of the mantle source and the generation of the Rongzharinian high-Mg andesites.

Petrology and Geochemistry of Highly Differentiated Tholeiitic Magmas: Granophyres in the Messejana–Plasencia Great Dyke (Central Iberia)

The Messejana–Plasencia great dyke (MPGD) is a Late Triassic tholeiitic gabbro intrusion related to the Central Atlantic Magmatic Province. Its large outcrop extent (~530 km), combined with its prolongation below the Duero basin (additional 100 km), makes it one of the world’s largest dykes known. We have studied felsic granophyric bodies appearing in its northernmost segment at different scales, from mm-sized (interstitial micrographic pockets) to felsic dykes of up to 10 m thick and 1.5 km long, intruding within the gabbros. Significant differences exist in the mineral and whole-rock composition of gabbros and granophyres, including the Sr–Nd isotopic ratios. The chemical variation in the gabbros is coherent with fractionation of olivine, clinopyroxene and plagioclase at depth. However, the presence of a compositional gap between gabbros and granophyres (absence of intermediate compositions) and the formation of these late-stage intergranular felsic melts within the gabbro mesostasis suggest that they could be derived by liquid immiscibility. The Sr–Nd isotopic heterogeneity in the MPGD gabbros and the presence of zircons with Variscan ages (~286 Ma), inherited from granulitic rocks, indicate that the mafic magmas experienced some degree of lower crust assimilation during fractionation close to the Moho depth. On the contrary, the scarce xenocrystic Variscan zircon crystals found in a granophyric dyke within the MPGD gabbro display similar textures and ages (~299 Ma) to those of the country rock granites and point to contamination at a different crustal level.

Plumbing System Architecture and Differentiation Processes of the Nyiragongo Volcano, DR Congo

ABSTRACT The Nyiragongo volcano is one of the most alkali-rich volcanic centres on the planet (Na2O + K2O generally >10 wt.%, agpaitic index up to 1.34), characterized by a semi-permanently active lava lake which hosts silica-undersaturated (SiO2 < 40 wt.%), low viscosity lavas. To improve our understanding of this unique magmatic system, we present a set of 291 samples, acquired during new field excursions between 2017 and 2021. The major and trace element composition of all samples was measured, revealing a lithological range extending from primitive picrites (Mg# 82) erupted from parasitic cones to a variety of highly evolved nephelinites, leucitites, and melilitites erupted from the main edifice as recently as 2002, 2016, and 2021. We measured major and trace element compositions from the full spectrum of minerals present in all sampled lithologies. From these we calculated that the main magma reservoirs feeding Nyiragongo are at approximately 9–15 and 21–33 km depth, in agreement with recent seismic observations. Fractional crystallization modelling using observed mineral compositions and proportions was performed to quantitatively link the lithologies to specific residual liquid fractions assuming evolution from an olivine-melilite parental melt. Our modelling indicates that fractionation and cumulate formation in deep chambers reduces the remaining melt fraction to ~60%, after which melts are injected into upper, liquid dominated magma chambers where fractionation and accumulation of clinopyroxene, melilite, and feldspathoids dominates. Characterisation of mineral textures and geochemistry reveals high crystal mobility in a repeatedly recharging plumbing system split between liquid-dominated, evolved magma chambers and more solid-dominated, primitive mushes, decreasing in liquid fraction with depth.

The Petrology and Geochemistry of REE-Enriched, Alkaline Volcanic Rocks of Ambitle Island, Feni Island Group, Papua New Guinea

Ambitle in the Feni Island Group is located within the NW trending Tabar–Lihir–Tanga–Feni (TLTF) volcanic island chain, Melanesian Arc, northeastern Papua New Guinea. The TLTF chain is renowned for its alkaline magmatism, geothermal activity, copper–gold mineralization, and world-class gold mining. Although its geochemical patterns indicate island arc signatures (i.e., high LILE and depleted HFSE), TLTF volcanism is not directly related to the older Melanesian Arc subduction system. However, it may have been influenced by source mantle metasomatism linked to the older subduction. The purpose of this study is to (1) present and interpret the petrographic, mineralogical, and geochemical data from Feni within the context of the tectonic evolution of the TLTF and (2) propose a geodynamic, petrogenetic model for the Feni volcanic rocks. The key methodologies used in this study are field mapping and sampling, petrographic analysis using the optical microscope, whole-rock geochemical analysis via XRF and ICP MS, and mineralogical analysis using an electron microprobe. The main rock types sampled in this study include feldspathoid-bearing basalt, trachybasalt, phonotephrite, trachyandesite, and trachydacite. Minerals identified include forsteritic olivine, diopside, augite, labradorite, andesine, anorthitic plagioclase, nepheline, and leucite in the primitive mafic suites, whereas the more evolved intermediate and felsic hypabyssal suites contain amphibole, albite, orthoclase, biotite, and either rare quartz or feldspathoids. Amphibole composition is primarily magnesiohastingsite with minor pargasite formed under polybaric conditions. Accessory minerals include apatite, titanite, and Ti-magnetite. We propose that limestone assimilation followed by fractional crystallization are plausible dominant processes in the geochemical evolution of the Ambitle volcanics. Clinopyroxene fractionation is dominant in the mafic volcanics whereas hornblende fractionation is a major petrologic process within the intermediate suites proven by the enrichment of LREE and depletions in MREE and HREE. Feni magmas are also highly enriched in REEs relative to neighboring arcs. This study is globally significant as alkaline magmas are important sources of Cu, Au, and REE as critical elements for green energy and modern technology.

Low–Ti Continental Tholeiite Origin of Magmas With Calc‐Alkaline Signature in Transcurrent Settings: The Mississippian Matachel Volcanic Field (SW Iberian Massif)

AbstractIn the Mississippian Matachel small volcanic field of the Ossa‐Morena Zone (southern Iberian Massif) outpoured basic‐intermediate lavas exhibit geochemical characteristics of Low‐Ti continental tholeiites and calc‐alkaline lavas. Low‐Ti continental tholeiites integrate two contrasting groups of rocks: basalts (Mg#: 54 to 70; Ti/Zr: 61–79; LaN/LuN: 1.6–2.9; εNdi: +4.0–+6.6; “Group #1”), and basalts and basaltic andesites (Mg#: 43 to 66; Ti/Zr: 36–58; LaN/LuN: 2.5–5.9; εNdi: −0.2–+3.5; “Group #2”). Primitive Group #1 tholeiitic magmas were generated by partial melting of a garnet‐free lherzolite from an enriched lithospheric mantle, near the lithosphere‐asthenosphere thermal boundary layer (with a very limited asthenosphere melting input). Progressive interaction of these magmas with crustal alkali igneous rocks resulted in the formation of the petrological evolutionary trends observed, to a larger extent in the case of Group #2 Low‐Ti tholeiites. Further assimilation of amphibole‐rich calc‐alkaline metaigneous rocks might have originated the basalts and basaltic andesites with calc‐alkaline signature (Mg#: 33 to 56; Ti/Zr: 25–78; LaN/LuN: 2.0–5.6; εNdi: +2.8–+4.8). These exhibit a “Cordilleran‐type” evolutionary trend, though are unrelated to plate convergence. The magmas with calc‐alkaline signature attest to a closed‐system differentiation process controlled by the fractionation of plagioclase, clinopyroxene, magnetite and ilmenite. It is proposed that Mississippian lithospheric‐scale intra‐continental wrenching, unrelated to coeval mantle plume upwelling, reworked complexly docked mantle domains and triggered mantle melting. Enduring mid‐upper crustal processes (magma storage in mid‐crustal chambers and crustal assimilation) likely shaped the latest petrologic and geochemical aspects of the Matachel Low‐Ti tholeiites and related rocks with calc‐alkaline signature.

Unravelling the pre-eruptive conditions of the rhyolitic Šumovit Greben lava dome from clinopyroxene-dominant glomeroporphyritic clots

Detailed analyses of mineral composition and whole-rock geochemical data helped to unravel the volcanic plumbing system beneath the rhyolitic Šumovit Greben lava dome, the westernmost member of the Kožuf-Voras volcanic system (N. Macedonia). It is characterized by high SiO2 content (> 70 wt%) coupled with low MgO (< 1 wt%) and Sr (< 500 ppm) suggesting fractionation of clinopyroxene and plagioclase at depth forming a crystal mush and a crystal-poor rhyolitic lens by fractional crystallization and melt extraction on top of it. The crystal mush is composed of mainly clinopyroxene, biotite and plagioclase, whereas sanidine and plagioclase are the most abundant phenocrysts of the rhyolitic lens. The main dome forming event occurred at ca. 2.9 Ma, which sampled the crystal-poor rhyolitic lens. After a short quiescence time, an explosive eruption occurred depositing a massive lapilli tuff layer northwest of the lava dome, and an extrusion of a small-volume lava flow on the northern side of the lava dome at ca. 2.8 Ma. This latter sampled also the crystal mush, as it contains abundant glomeroporphyritic clots of clinopyroxene ± plagioclase ± biotite. The clinopyroxene phenocrysts are chemically homogeneous, their crystallization temperature is ca. 900 °C representing the crystal mush, whereas the plagioclase and the sanidine crystallized at a lower temperature (ca. 790 °C) representing the rhyolitic lens. Noble gas isotopic composition of the clinopyroxene indicate no mantle-derived fluids (< 0.5%) having an R/Ra of ca. 0.04 Ra. The rejuvenation of the system probably occurred due to implementation of mafic magma at depth leading to a heat transfer and partial melting of the cumulate. This led to crystallization of Ba-rich rims of the sanidine and An- and Sr-rich rims of the plagioclase. The crystal mush zone beneath Šumovit Greben might be connected to the nearby, more mafic volcanic centers, and the eruption of Šumovit Greben could have been the start of the last cycle in the lifetime of the Kožuf-Voras volcanic system.

Generation of tholeiitic and calc-alkaline arc magmas and its implications for continental growth

Arc magmas, the building blocks of the Phanerozoic continents, are compositionally diverse. The most prominent compositional diversity is that some arcs show early Fe enrichment while some others show the opposite trend. What causes this Fe differentiation diversity is important for understanding continent formation but remains highly debated. Here, we use pressure-sensitive geochemical proxies, combined with experimental results, to evaluate the petrological mechanism(s) for the different Fe trends in arc magmas that traverse through crusts of various thicknesses. We show that magnetite starts to fractionate only at MgO ≤ 4 wt% for nearly all arc magmas and thus has no bearing on the early Fe depletion in calc-alkaline arc magmas. The Fe enrichment at MgO > 4 wt% for tholeiitic arc magmas is largely caused by clinopyroxene fractionation. The culprits for the Fe depletion at MgO > 4 wt% in calc-alkaline arc magmas change from pyroxene- to garnet-dominated (±amphibole) as the arc crust thickens. The similarity between the upper continental crust and the calc-alkaline arc magmas suggests that calc-alkaline arc magmas are an essential component for the continental crust. The high La/Yb and Dy/Yb estimates for the average upper continental crust imply that garnet fractionation is an intrinsic feature of the upper continental crust. Thus, crustal thickness, that is differentiation pressure, exerts a first-order control on arc magma compositions and high-pressure intracrustal differentiation in thickened crust is necessary for making the Phanerozoic continental crust.

Coupled U–Pb and Lu–Hf zircon data of Late Devonian magmatism from Bajo de La Leona (Deseado Massif, Argentina): Tracking the southernmost exposure of the Devonian magmatic arc of Patagonia

In Bajo de La Leona, located in the eastern Deseado Massif, pre-Permian basement rocks of the Río Deseado Complex crop out. They consist of high- to medium-grade metamorphic rocks intruded by two Paleozoic granitoids, namely the Mina de La Leona Granite and the Laguna Larga Granite. The latter is the main goal of this work. It is a calc-alkaline to high-K calc-alkaline, ferroan, calcic to alkali-calcic, and mainly peraluminous monzoleucogranite which plots within the I&S-type and volcanic-arc fields of the tectonic discrimination diagrams. Its trace elements composition (e.g., Sr/Y, LaN/YbN, Sm/Yb, Eu/Eu*, Dy/Dy*, Dy/Yb) suggests plagioclase ± clinopyroxene fractionation in a shallow source at low pressures. Assimilation could have played a role in magma differentiation, thus the magmatic development of the Laguna Larga Granite is likely a consequence of assimilation-fractional crystallization (AFC) processes. Coupled LA-ICP-MS U–Pb and Lu–Hf zircon data for the studied granite, as well as Sm–Nd whole-rock data, are here presented for the first time. A Famennian U–Pb zircon crystallization age of 361.9 ± 1.7 Ma was obtained, together with εHf(t) zircon values from -15 to +6 with TDM-Hf model ages between 2.0 and 0.9 Ga. A thin crust, as suggested by low values of thickness proxies such as Sr/Y (1.06–1.83) and LaN/YbN (3.97–4.96), would allow emplacement of juvenile mantle-derived material, giving rise to the suprachondritic εHf(t) values. A mantle-derived source is also suggested by a suprachondritic εNd(t) whole-rock value of +2.34 (TDM-Nd = 0.93 Ga). On the other hand, the subchondritic εHf(t) values could be related to wall rock assimilation. In a regional context, the obtained results for the Laguna Larga Granite allow to correlate it with coeval rocks from northern Patagonia (e.g., San Martín de los Andes batholith, Paleozoic Central Patagonian igneous-metamorphic belt). Therefore, the Devonian magmatic arc might have extended from the North Patagonian Andes and the western North Patagonian Massif to the eastern Deseado Massif along a NNW-SSE-striking belt, confirming previous proposals.

Geochronology and geochemistry of the Miocene Kadıkalesi monzonite (Bodrum Peninsula, western Anatolia): Implications for its relation with the South Aegean Volcanic Arc

The Late Miocene igneous rocks of the Bodrum magmatic complex (SW Anatolia, Turkey), are composed of small monzonitic intrusions and trachyandesite-trachydacite volcanic rocks. The Kadıkalesi monzonite (KM) forms a small stock (<1 km2) made of plagioclase, biotite, hornblende, clinopyroxene, and quartz, and geochemically belongs to high-K calc-alkaline to shoshonitic, metaluminous series. The zircon separates from the KM yielded 9.5 ± 0.15 Ma core and 8.6 ± 0.04 Ma rim ages, revealing that emplacement of the monzonite continued up to ~8 Ma. The geochemical and mineralogical evidence suggests that the KM was derived by extensive fractionation of plagioclase, alkali feldspar, apatite, and clinopyroxene and/or Fe-Ti oxides and mixing of differently evolved melts. The monzonite has 760–866 Ma depleted mantle Nd Model ages which are considerably younger than the Cycladic and Menderes Massif plutons, as well as the other Miocene magmatic rocks from western Anatolia.On the basis of geochronological and geochemical features, we propose that the KM was fractionated from mafic melts derived from partial melting of spinel-facies (shallow depth) and crustally-contaminated (metasomatized/orogenic) asthenospheric mantle. Asthenospheric input were likely supplied from the slab window between the Aegean and Cyprian slabs, and likely contaminated by subducted sediments from the Aegean slab. It is also suggested that the current geodynamic setting of the Aegean region is also applicable to the Middle Miocene, meaning that the magmatic activity of the SAVA likely commenced at ~10 Ma, earlier than previously considered.

Age and petrogenesis of Ni-Cu-(PGE) sulfide-bearing gabbroic intrusions in the Lausitz Block, northern Bohemian Massif (Germany/Czech Republic)

The Lausitz Block, located in the northernmost part of the Bohemian Massif, hosts a large number of dike- to stock-shaped gabbroic intrusions that mainly comprise brown hornblende-poor (Group I; i.e. olivine gabbronorite, olivine gabbro, gabbro and diorite) and subordinately brown hornblende-rich lithologies (Group II; i.e. olivine-hornblende gabbro and hornblende gabbro). Several of these intrusions host small-scaled magmatic Ni-Cu-(PGE) sulfide accumulations. The intrusions are part of interconnected mafic–(ultramafic) plumbing systems that intruded Cadomian granodiorites of Lausitz Block in the Middle to Late Devonian during the early stages of the Variscan Orogeny. The previously inferred Devonian age of the intrusions is refined by biotite Ar-Ar dating that yield ages between 389.1 ± 3.9 Ma and 372.2 ± 3.7 Ma (2σ). Group I and Group II lithologies differ in their mineralogical and geochemical composition. Compared to the Group I lithologies those of Group II are characterized by higher modal contents of primary brown hornblende, Fe-Ti oxides and apatite, by Ti- and Al-enriched clinopyroxene and by lower contents of SiO2 and increased contents of TiO2, P2O5, LILE, HFSE and LREE. The differences suggest at least two different magmatic series where Group I rocks are linked to tholeiitic basaltic magmas with low to moderate Ti and volatile contents, whereas Group II rocks are derived from Ti- and volatile-enriched alkaline basaltic magmas. The magmas experienced clinopyroxene fractionation during their crustal ascent and storage, but were only minor affected by crustal contamination (< 5%) according to Sr-Nd-Pb isotope systematics. The tholeiitic and alkaline basaltic magmatism could be explained by a tectonic environment between a subduction and intraplate setting. Group II rocks have an intraplate chemical signature and their parental magmas are clearly originated from a deeper-seated mantle source with melting of garnet peridotite, likely associated with an asthenospheric upwelling. By contrast, precursor magmas of Group I rocks are originated from partial melting of a less-enriched source, presumably spinel peridotite, from a subduction-related environment. Trace elements systematics suggest that both groups at least partly resulted from an interaction (mixing) of the alkaline and tholeiitic magmas. Partial melting estimates for both groups vary between 5 and 20%. The proposed tectonic setting could have been related to a subduction slab retreat within the framework of the Variscan orogeny, which led to an upwelling of the asthenospheric mantle. Cu/Zr ratios <1 of gabbroic rocks from several intrusions suggest a previous segregation of magmatic sulfides in other sections of the magmatic plumbing system and give rise for a vertical and lateral Ni-Cu exploration potential.

Petrogenesis of Butajira-Kibet Quaternary basaltic rocks; Central Main Ethiopian Rift

Major and trace element data are presented for Quaternary basalts from Butajira-Kibet area, close to the western escarpment of the Central Main Ethiopian Rift to investigate the petrogenetic processes involved and the nature of mantle source compositions. The data indicate that the basalts are mildly alkaline characterized by moderate to high contents of MgO (6.85-9.00 wt. %) and TiO2 (2.40-2.92 wt. %). In the primitive mantle-normalized multi-element diagram, they exhibit troughs at Th, U, Rb and K and peaks at Ba, Nb, Ta, La, Pb and Sr with positive Eu/Eu*(1.01-1.17). Incompatible element ratios e.g., Ba/Nb (8.92-16.25), Rb/Nb (0.42-0.70), K/Nb (233.00-340.00) and Nb/U (47.97-61.05) suggest that the basalts are unaffected by crustal contamination. The acquired data do not evidence fractionation of plagioclase and clinopyroxene. On the other hand, relationships between major elements and their ratios (e.g., Na2O and Ca2O/Al2O3) and trace element ratios (e.g., Zr/Y and (La/Sm)N) indicate partial melting of a clinopyroxene - rich mantle at medium pressure. Chondrite - normalized REE diagram is characterized by relatively flat HREE (TbN/YbN=1.73-2.13) and significantly enriched HREE (higher than 10-times chondritic values) patterns. LREE/MREE versus MREE/HREE modelling of non-modal batch melting indicates that the basalts are formed by 1.5-3% partial melting of spinel peridotite mantle source with minor contribution from garnet-bearing peridotite. Primitive mantle-normalized multi-element diagram and trace element ratios, such as Nb/Zr, Nb/U, Ba/La, Rb/Zr and Ba/Nb, suggest that the studied basalts are not affected by carbonatite and/or hydrous phase metasomatism. Rather, they are closely similar to the OIB source (mainly EM-I) and the SCLM. They are most likely the result of significant interaction of melts from a mantle plume and the SCLM.

Geochemistry and Petrogenesis of Mesoproterozoic Mafic Intrusive Rocks from the Singhora and Chandarpur Groups, Eastern Chhattisgarh Basin, Bastar Craton: Possible Implication for the Time of Sedimentation

ABSTRACT A number of Mesoproterozoic mafic intrusions transect the Chhattisgarh basin, Bastar craton, and they are mostly exposed in the Singhora and Chandarpur Group of rocks in the eastern part of the basin. These mafic intrusive rocks are studied for their petrological and geochemical characteristics to understand nature and genesis and likely implication on the time of sedimentation. They chiefly consist of clinopyroxene and plagioclase and show ophitic texture. Geochemically, they are sub-alkaline basalt to basaltic andesite and strongly supports cogenetic nature of mafic intrusives from both the Groups. The fractionation of olivine, clinopyroxene, plagioclase and Cr-spinel primarily controlled geochemical variations. Negative Nb-Ta-Ti anomalies suggest a possible role of crustal components in the evolution of the melt; however, the absence of crustal signature in most primitive high-Mg containing samples rules out any significant role of crustal contamination. Incompatible trace elements modelling suggests their derivation from a mantle melt, originated from the transition zone of garnet-spinel stability field. Further, geochemical analysis supports an SZLM (subduction zone-modified lithospheric mantle) source for these mafic intrusions. Low CaO/MgO (1.20 – 1.64), intermediate FeO/MnO (45.71 – 71.80), positive PX# (1.10 - 5.89), and high values of FC3MS (0.58 – 1.35) and FCKANTMS (0.59 – 0.93) explicitly support derivation of the primary melt from a olivine-poor and pyroxene-rich (pyroxenite type) source. The observed geochemical characteristics and published age data on these mafic intrusions indicate that sediments of both the stratigraphic units are older than ca. 1.42 Ga. Further, based on the relationship with the neighbouring ca. 1.85 Ga Sonakhan mafic dyke swarm of the Bastar craton, the age of sedimentation in the Chhattisgarh basin has been proposed to be in between ca. 1.85 Ga and 1.42 Ga.

Petrological evidence of an anhydrous carbonatitic peridotite source for the Xiaogulihe ultrapotassic volcanic rocks, northeastern China

This study presents new olivine and whole-rock geochemical data for the Xiaogulihe ultrapotassic volcanic rocks of northeastern China. These data provide insights into the petrogenesis of the rocks of the Erkeshan–Wudalianchi–Keluo–Xiaogulihe (EWKX) potassic–ultrapotassic volcanic belt. The petrography and whole-rock geochemical compositions of the Xiaogulihe volcanic rocks are indicative of formation from magmas that primarily underwent olivine + clinopyroxene fractionation with insignificant crustal contamination and post-magmatic alteration. The high-Fo (87–90) olivine within the Xiaogulihe volcanic rocks has high Ni (2050–3800 ppm) and Mn (1190–1750 ppm) contents, high 10000Zn/Fe values (12–15), low Ca (1310–1800 ppm) contents, and low Fe/Mn (56–72) and Mn/Zn (10–14) values. Their high Mn contents and low Fe/Mn values together with their low FC3MS (<0.65; FC3MS = FeO/CaO – 3MgO/SiO2), low FCKANTMS (<0.37; FCKANTMS = ln(FeO/CaO) − 0.08ln(K2O/Al2O3) − 0.052ln(TiO2/Na2O) − 0.036ln(Na2O/K2O) ln(Na2O/TiO2) − 0.062(ln(MgO/SiO2))3–0.641(ln(MgO/SiO2))2–1.871ln(MgO/SiO2) − 1.473) and low CaO–Al2O3–Fe2O3Total values are indicative of derivation from a mantle source containing refractory peridotite. The Xiaogulihe olivines have slightly higher Zn/Fe and lower Mn/Zn values than expected for olivines formed from typical peridotite-derived melts, suggesting that the source for these lavas was previously metasomatized by interaction with Zn-rich sedimentary carbonatitic melts. The low magmatic water contents of the Xiaogulihe lavas (<1 wt% determined by use of the Ca-in-olivine geohygrometer) is also indicative of derivation from an anhydrous mantle source. These data suggest that the Xiaogulihe ultrapotassic lavas represent partial melts of anhydrous refractory subcontinental lithospheric mantle peridotite. The K-rich nature of these volcanic rocks is indicative of derivation from the K-rich reservoir of the mantle, which is closely associated with carbonatitic metasomatism, indicating the role of this process in the generation of the K-rich EWKX volcanics.

Inferences on the magmatic plumbing system at Stromboli volcano (Italy) from trace element geochemistry of matrix glasses and minerals in different types of explosive eruptions

Stromboli (Italy) is a basaltic volcano characterized by persistent, mild strombolian activity, occasionally interrupted by lava effusion and more violent explosive events, named major explosions and paroxysms depending on their intensity and magnitude. The normal activity is fed by a shallow and degassed highly porphyritic (HP) shoshonitic basalt carrying about 50 vol.% crystals settled in a shoshonitic glassy matrix (K2O > 3.8 wt.%). The more energetic explosions erupt a deep, volatile-rich, low-porphyritic (LP) magma with < 10 vol.% crystals in a shoshonitic basaltic glassy matrix (K2O < 2.4 wt.%). Products with intermediate glass composition are also found in the more violent explosive events. In this study, we present a new data set of major and trace element contents in matrix glasses and minerals performed in products from different types of explosive activity that occurred at Stromboli between 1998 and 2020. This large data set is used to put constraints on the evolution and architecture of the intermediate plumbing system, where the transformation from LP to HP occurs. Results indicate that, compared to paroxysms, the glassy matrices of the LP pumices from major explosions are richer in incompatible trace elements (and K2O wt.%) due to < 15 wt.% fractionation of clinopyroxene and olivine. This points to a chemical zoning of the deep reservoir and suggests that major explosions are fed by magmas residing in its upper part. Among the major explosions, the homogeneous intermediate glasses in the products from the 19 July 2020 event originate from the interplay of mixing and crystal fractionation processes. The crystallization of euhedral microphenocrysts of An-rich plagioclase suggests that batches of magma can pond and crystallize for few days (< 11) at the base of the intermediate zone of the plumbing system, at pressure coinciding with the entering of plagioclase into the system (< 100 MPa). As a relevant point for understanding the pre- and syn-eruptive magma dynamics, data indicate a positive correlation between the magnitude of the explosions and the depth of the supply magma.

Trace element partitioning in zoned clinopyroxene as a proxy for undercooling: Experimental constraints from trachybasaltic magmas

Sector-zoned clinopyroxene records kinetic effects imposed by variable degrees of magma undercooling, ΔT, and can be utilised to track the dynamics of magmatic systems. The partitioning of trace elements into sectors grown in different crystallographic orientations can be used as a proxy for ΔT. However, an experimental assessment of the relationship between trace element zoning and ΔT has been lacking to date. Here we present trace element data from a series of undercooling crystallisation experiments on a primitive trachybasalt from Mt. Etna (Italy), at conditions of crustal storage (400 MPa, NNO + 2), and ΔT ranging from 23 to 173 °C. Changes in ΔT were modulated by varying both resting and liquidus temperatures, the latter via the melt-H2O content of the experiments. The resting temperature was retained for 24 h to ensure the attainment of near-equilibrium conditions.High-resolution elemental mapping reveals the distribution of trace elements in individual clinopyroxene zones. Increasing ΔT drives a shift from polyhedral morphologies with Al-rich prism and Al-poor hourglass sectors (ΔT = 23–25 °C), to skeletal (ΔT = 75–123 °C) and dendritic (ΔT = 132–173 °C) crystals with Al-rich skeletons and Al-poor overgrowths. Aluminium-rich zones have higher concentrations of rare earth elements (REE) and high field strength elements (HFSE) than Al-poor zones across all investigated ΔT conditions, and overall, Al, REE and HFSE contents increase with ΔT. This indicates that tetrahedral aluminium (TAl) and associated charge-balancing mechanisms govern the incorporation of REE and HFSE within clinopyroxene. Lattice strain parameters for REE in the M2 site indicate the incorporation of light relative to heavy REE in clinopyroxene is controlled by competing effects between the strain-free partition coefficient, D0, and the optimum cation radius, r0. Critically, the middle and heavy REE switch from incompatible to compatible with increasing ΔT. Used to model fractional crystallisation, our data demonstrate that fractionation of clinopyroxene at low ΔT controls pre-eruptive melt evolution. Importantly, this indicates crystallisation of clinopyroxene in the deep portions of Mt. Etna’s plumbing system is not rapid and is unlikely to result in the early formation of dendrites.We develop a parameterisation of ΔT based on REE partitioning between experimental clinopyroxene and coexisting melt, which can be applied to sector-zoned augite crystallising from mafic alkaline magmas, to reconstruct dynamic processes and thermal pathways during magma transport and storage. Applied to sector-zoned clinopyroxene microphenocrysts and groundmass microcrysts from the 1974 eccentric eruption at Mt. Etna, our parameterisation tracks an increase in ΔT with magma ascent and eruption, following recharge of Cr-rich mafic magma at depth. Sector-zoned clinopyroxene can track ΔT variations leading to volcanism at Mt. Etna and could be applied to quantify magma dynamics in other active volcanoes.

IBM-type forearc magmatism in the Qilian Orogen records evolution from a continental to an intra-oceanic arc system in the Proto-Tethyan Ocean

Forearc magmatism in IBM-type intra-oceanic arc systems is crucial for understanding the processes of subduction initiation and subsequent evolution of the arc system. Here we report a forearc volcanic sequence exposed in the Danghe-Nanshan, at the westernmost end of the South Qilian Accretionary Belt of the Qilian Orogen, NW China. Based on petrography and bulk-rock composition, three types of volcanic rocks (boninite, high-Mg basaltic andesite and andesite) and arc-related felsic plutons are identified. These rocks show correlating trends of trace elements and isotopes. The high-Cr spinels and clustered Ti/V ratios indicate that the boninite and high-Mg basaltic andesite magmas were derived from highly refractory peridotite sources in a forearc setting, whereas the andesite and felsic plutons were formed by fractionation of clinopyroxene and plagioclase from primary magma. Notably, slab-derived components have limited involvement in magma generation. Variation of fluid mobile/immobile element contents and Sr-Nd isotopic mixing calculations suggest less than 1% incorporation of subducted slab-derived fluids into the boninite source, whereas ∼1–3% fluids and ∼3–7% partial melts from the slab contributed to the high-Mg basaltic andesite and andesite, respectively. Therefore, an abnormally high mantle potential temperature (>1429 °C) was required for boninite production because there were little aqueous fluids involved. Zircon U-Pb dating reveals these forearc magmas formed at ∼450 Ma. Combined with the regional tectonics and published data in this region, we suggest that these magmas record a tectonic process of arc system from active continental margin to newly formed intra-oceanic subduction zone during trench retreat of the Proto-Tethys Ocean.