Degrees Of Partial Melting Research Articles

The off-axis plume–ridge interaction model: Confirmation from the mineral chemistry of Cretaceous basalts of the Ontong Java Plateau

The Ontong Java Plateau (OJP) is the largest known igneous province on Earth. The proposal that its origins involved off-axis plume–ridge interactions remains highly controversial. Here we present new in-situ major (EPMA), trace element (LA-ICP-MS) and Sr isotopic (LA-MC-ICP-MS) characterization of clinopyroxene and plagioclase phases present in OJP tholeiitic basalts from Sites 1183, 1186, 1185, 1187. Site 1187 and Upper Site 1185 have yielded olivine tholeiites containing olivine, Ca-rich augite, Ca-deficient pigeonite, and plagioclase, inferred to be derived from higher degrees of partial melting of dominantly peridotitic mantle and reflecting relatively low crystallization temperatures. Sites 1183 and 1186 and Lower Site 1185 yielded tholeiitic basalts consisting mainly of plagioclase and Ca-rich augite without olivine, interpreted to originate from lower degrees of partial melting of pyroxenite- and peridotite-derived melts with relatively high clinopyroxene and plagioclase crystallization temperatures. Site 1187 and Upper Site 1185 plagioclase, clinopyroxene, and equilibrium melts have much lower REE contents but slightly higher in-situ Sr isotopic ratios compared to those phases at Sites 1183 and 1186 as well as Lower Site 1185. Sites 1186 and 1187 plagioclases and clinopyroxenes display normal zoned textures which are attributed to the process of fractional crystallization and extensive magma mixing beneath the heterogeneous mantle source. In-situ Sr isotopic disequilibrium between plagioclase phenocrysts and plagioclase clusters further indicates an enriched compositional heterogeneity. Due to the strong tensile forces expected at the spreading ridge, the OJP off-axis mantle plume is inferred to be drawn towards the ridge axis, leading to shifts in mineral compositions, degrees of partial melting, and crystallization temperatures inferred for the volcanics of Sites 1183, 1186, 1185, and 1187 as a function of distance from the spreading ridge. The enriched invariability of in-situ Sr isotopic compositions is likely due to the relatively long half-life of the radiogenic elements. These mineral chemistries are consistent with off-axis ridge–plume interaction.

Effects of variable slab components and tectonics on magma composition in the intra-oceanic Kermadec Arc-Backarc system

The Kermadec Arc-Backarc system (25–39°S) in the southwest Pacific is an important natural laboratory to examine the controls on arc magma compositions, because the physical parameters that are thought to influence subduction-related magmatism (the thickness and composition of the upper plate, mantle depletion and partial melting, and the slab components) change along the arc. Furthermore, the presence of back-arc and rear-arc volcanism allow insight into how mantle composition varies with distance to the trench. We present new major and trace element and Pb–Sr–Nd–Hf isotope data for 101 lava samples from 10 volcanoes along the Kermadec Arc, and combine these with published data for volcanic arc front, rear-arc, and back-arc lavas. The Kermadec Arc-Backarc magmas experienced similar fractional crystallization paths, and those erupted onto oceanic crust are not affected by crustal assimilation. The degree of partial melting varies along and across the arc, partly due to variations in the thickness of the lithosphere of the upper plate, but also due to variations in slab fluid input. We distinguish four different arc segments varying from 100 to 600 km wide, which erupt lavas with distinct compositions due to differences in along-arc slab input. All Kermadec arc and rear-arc lavas were derived from mantle sources to which the sediment melt and a fluid from subducted Pacific oceanic crust or the Hikurangi Seamounts had been added. Increasing sediment melt input into the mantle wedge southwards is reflected in increasing 87Sr/86Sr and Th/Nd, and decreasing 143Nd/144Nd southwards towards New Zealand. Some volcanoes from the central Kermadec Arc show with unusually radiogenic Pb isotope compositions that are probably derived from subducted alkaline rocks from seamounts built on the Hikurangi Plateau. The geochemical segmentation of the Kermadec Arc indicates that slab-derived components can be relatively homogeneous on large scales, but small features like seamounts cause complex additions to the slab component.

Niobium, Critical Metal, and Progeny of the Mantle

Abstract Niobium is a critical metal in high demand because of technological advances and the supply risk created by the fact that over 90% of its production is by a single country (Brazil). In this paper, we review the geology of the deposits that are currently being mined and other potentially economic deposits as well as develop models for their genesis. With the exception of the Lovozero deposit (Russia), which is hosted by a layered silica-undersaturated alkaline igneous complex, all the deposits that are currently being mined for niobium are hosted by carbonatites, and most of the deposits with economic potential are also hosted by these rocks. Niobium owes its concentration in carbonatites and alkaline silicate rocks to its highly incompatible nature and the small degree of partial melting of the mantle required to generate the corresponding magmas. The primary control on the concentration of niobium to economic levels in alkaline silicate magmas is fractional crystallization, partly prior to but mainly after emplacement. In the case of silica-undersaturated magmas, the final residue saturates in minerals like eudialyte and loparite to form niobium-rich horizons in the layered complexes that crystallize from these magmas. The final residue, in the case of silica-saturated magmas, crystallizes the pegmatites that are the hosts to the economic niobium mineralization, which commonly takes the form of pyrochlore. In contrast, carbonatitic magmas undergo little to no fractional crystallization prior to emplacement. Moreover, fractional crystallization on emplacement has minimal impact on the concentration of niobium to economic levels. Instead, we propose that the metasomatic interaction of the carbonatitic magmas with their hosts to form rocks like phlogopitite (glimmerite) consumes much of the magma, leaving behind a phoscoritic residue from which pyrochlore crystallizes in amounts sufficient to form economic deposits. Although many niobium deposits display evidence of intense hydrothermal alteration, during which there can be major changes in the niobium mineralogy, the extremely low solubility of niobium in aqueous fluids at elevated temperature precludes significant mobilization and, thus, enrichment of the metal by hydrothermal fluids. However, weathering of carbonatite-hosted niobium deposits leads to supergene enrichment (due largely to the dissolution of the carbonate minerals) that can double the niobium grade and make subeconomic deposits economic. Pyrochlore is the principal niobium mineral in these laterite-hosted deposits, although its composition differs considerably from that in the primary mineralization. This paper evaluates the processes that appear to be responsible for the genesis of niobium ores and provides a framework that we hope will guide future in-depth studies of niobium deposits and lead to more effective strategies for their successful exploration and exploitation.

Petrogenesis of mantle peridotites in Neo‐Tethyan ophiolites from the Eastern Himalaya and Indo‐Myanmar Orogenic Belt in the geo‐tectonic framework of Southeast Asia

The Neo‐Tethyan ophiolites of the Tuting‐Tidding Suture Zone (TTSZ), Eastern Himalaya (viz., Tidding Ophiolite Complex and Mayodia Ophiolite Complex) and Indo‐Myanmar Orogenic Belt (IMOB, i.e., Nagaland Ophiolite Complex and Manipur Ophiolite Complex) which lie along the southern extension of the Indus‐Tsangpo Suture Zone have been collectively investigated through the mantle‐derived peridotite sequence. The peridotites (harzburgite and dunites) in the Eastern Himalaya ophiolites are refractory, constrained by the high Fo olivine content (~95), high Cr# [Cr/(Cr + Al)] (0.90–0.99), as well as by the parental melt composition of Cr‐spinels (Al2O3 melt = 3.05–7.55 wt%, FeO/MgO = 0.69–6.46). They have slightly “U‐shaped” chondrite‐normalized Rare Earth Elements (REE) patterns with slight enrichment in Light Rare Earth Elements (LREE) relative to the patterns expected for residues of partial melting, thereby indicating a reaction with the LREE‐enriched melt. These peridotites represent residual portions of a depleted/enriched mantle that underwent partial melting up to 23% in the nascent forearc of an intra‐oceanic subduction zone, and later metasomatized by high‐temperature silicate melts and low‐temperature hydrous fluids. They are composed of a very refractory olivine‐spinel assemblage (Fo: 91.68–96.44; Cr#: 0.90–0.99), corroborating a boninitic parentage, with influence from melt‐rock interactions. In the case of the IMOB ophiolites, a wide range of chemical compositions is observed in the mantle sequence. Lherzolites display low Cr# (0.12–0.26) and TiO2 (<0.11) associated with high Mg# [Mg/(Mg + Fet)] (0.69–0.76) in the Cr‐spinels present in them. They represent the residual product of a fertile mantle that underwent low‐degree partial melting (2%–10%) in a divergent mid‐ocean ridge (MOR) tectonic setting. On the other hand, the harzburgites and dunites of the IMOB have high Cr# (0.84–0.90) and low TiO2 (<0.06 wt%) Cr‐spinels and exhibit slightly U‐shaped REE distributions indicating their derivation from a highly depleted mantle source, which experienced higher degree partial melting (17%–24%) similar to those of supra‐subduction zone (SSZ) harzburgites and dunites of the TTSZ peridotites. The occurrence of both MOR and SSZ types of melting regimes indicates that the peridotites in the IMOB ophiolites formed at two major different stages of the pre‐subduction and subduction events, respectively. However, the peridotites of Eastern Himalaya ophiolites were formed only during subduction tectonics. Thus, we argue that the mantle peridotites in the ophiolites of northeast India evolved from lherzolite through clinopyroxene‐harzburgite and harzburgite then to highly refractory dunite, supporting multistage melting and melt‐rock reaction processes during their generation.

Zircon inheritance, sources of Devonian granitic magmas and crustal structure in central Victoria

In central Victoria, inherited zircon in Devonian igneous rocks and detrital zircon in metasedimentary country rocks and an amphibolite-facies xenolith show that Mesoproterozoic parts of the underlying Selwyn Block cannot be the source for all the silicic magmas. Zircon inheritance in S-type samples reveals significant thermal events at 525–425 Ma and 1200–1100 Ma. Both S- and I-type samples have prominent zircon age peaks at 420–410 Ma, which record high-grade metamorphism of the deep crust during the terminal phases of the Benambran and Bindian orogenies. All I-type rocks have 650–500 Ma peaks, suggesting derivation from an arc-related metavolcanic source in the upper Selwyn Block. Protoliths of the greenschist-facies Ordovician metasediments and the amphibolite-facies Cambrian metasedimentary xenolith were deposited in distal backarc settings. Most inherited zircon cores are metamorphic, and the strongest zircon inheritance occurs in hornblende-bearing I-type rocks, highlighting their largely crustal origin. Zircon populations at ca 1400 Ma, thought to signal sediment derivation from East Antarctica and Rodinia-Nuna, are mostly absent in I-type samples and some S-types. The ca 1400 Ma signal probably applies to the upper, metasedimentary Selwyn Block, so Devonian S-type magmas were sourced mainly in the deeper sections. Zircon inheritance in the Devonian igneous rocks was not influenced by the exposed metasedimentary country rocks. Two samples from one of the smaller plutons have contrasting patterns of zircon inheritance, suggesting relatively small-scale source heterogeneity. Many rounded and corroded cores in zircon crystals yield the same ages as the crystallisation dates for the rocks, and thus are antecrysts. Higher whole-rock Zr contents generally correlate with higher proportions of inherited zircon, and differentiation does not affect this relationship. The degree of partial melting of a magma source and the efficiency of crystal entrainment are critical in governing zircon inheritance.

Opening of the Algerian Basin: Petrological, geochemical and geochronological constraints from the Yaddene Complex (Lesser Kabylia, Northeastern Algeria)

The Yaddene complex, located in Lesser Kabylia (northeastern Algeria), is composed of Tertiary mafic and ultramafic rocks that outcrop within syn-rift sedimentary rocks of the Algerian marginal basin, in the Maghrebide belt of North Africa. Petrographic observations show that the Yaddene complex is composed mainly of two distinct lithologies: (1) a layered quartz-bearing gabbro at the bottom, which consists mainly of plagioclase (An93), clinopyroxene (altered to actinolite) and rare interstitial quartz; and (2) layered plagioclase-bearing lherzolites composed of olivine (Fo86-88) orthopyroxene (En86–87), clinopyroxene (diopside and augite), rare plagioclase (anorthite) and amphibole of pargasitic composition.The trace element concentrations in pyroxenes and olivine of the Yadenne complex show a depletion in light rare earth elements (LREE) similar to basalts of the Mid-Ocean Ridge (N-MORB), which reflects the depletion of the mantle source. In addition, high #Cr in spinel reflects a high degree of partial melting of the mantle source. On the other hand, slight enrichment in Large Ion Lithophile Elements (Cs, Rb, Ba), weak but significant negative Nb, Ta, Zr, Hf and Ti anomalies, and positive Pb spikes are observed in both whole-rocks and minerals. These patterns are best explained by interaction between parental mantle and fluids and/or siliceous melts derived from a subducting oceanic slab. The tectonic discrimination diagrams of minerals and whole rocks indicate that the most likely geodynamic setting for emplacement of the Yaddene Complex is a back arc basin environment.In situ U-Pb dating of euhedral zircons and titanite from a quartz-bearing gabbro yields an age of 18.97 ± 0.12 Ma (2σ), which is interpreted as the crystallization age of magmatic zircons and the gabbroic magma. Zircons from a plagioclase-bearing lherzolite gave a consistent age of 19.05 ± 0.39 Ma (2σ), which is attributed to zircon growth during metamorphic/metasomatic processes coeval to gabbroic magma emplacement. The mafic/ultramafic rocks of the Yaddene complex are intercalated within fine-grained sediments of the syn-rift “Oligo-Miocene Kabyle” and the obtained early Burdigalian ages are interpreted as the early stages of an extensional regime that ultimately led to the opening of the oceanic Algerian Basin.

Geochemistry of meta‐mafic and meta‐tonalite‐trondhjemite intrusives from Jaintia and Karbi Anglong hills of Shillong Plateau, North East India: Implications on the evolution of the Proterozoic Shillong Basin

We present the whole‐rock and mineral chemical data of the Khasi meta‐mafic/basic‐trondhjemite intrusives (KMI) and meta‐tonalite‐trondhjemite (MTT) that coevally emplaced the Proterozoic Shillong Group (SG) of Shillong Plateau, northeast (NE) India and discuss their petrogenetic‐tectonic origin. The calc‐alkaline to tholeiite KMI are compositionally transitional between hornblende‐gabbro; hornblende‐diorite and minor‐hornblendite. They show Ta–Nb troughs and strong decoupling large‐ion lithophile/high field strength elements (HFSE) signature consistent with subduction zone‐related basic magmas. The calcic to calc‐alkaline, low‐Al2O3, and I‐type metaluminous Fe‐hedenbergite‐trondhjemite (KMT) exhibits similar trace and rare earth elements (REE) distribution to the KMI. The MTT on the other hand are depleted in trace elements and REE, and correspond with the I‐type granitoids, and are comparable to Proterozoic to Phanerozoic tonalite‐trondhjemite‐granodiorites. Gentle negative sloping of REE in KMI ([La/Lu]CN = 4.04 to 6.35) with moderate LREE, suggests partial melting or, fractional crystallization from a transitional source between garnet and spinel lherzolite. Although almost a flat pattern ([La/Lu]CN = 3.70 to 4.05) for MTT entails a high degree of partial melting from sources similar to Archaean Mafic Composite or Archaean Tholeiite. Trace elemental modellings ([Sr/Y] vs. Y and [La/Yb]N vs. YbN) for MTT infer low pressure (<15 kbar) and high degree partial melting of >50% in equilibrium with 10% garnet +90% amphibole residue. A relatively flat REE pattern with some elevated HREE of MTT compared to KMI suggests for depleted source. Bimodal plutonism and the prevalence of meta‐dacite‐rhyolite in SG invokes for tonalite‐trondhjemite‐dacite setting of a continental arc or, active continental margin tectonic setting. Emplacement of KMI initiated in a temporary relax‐extensional phase and MTT in a compressional regime. This is possibly caused by an over‐thickened crust later subjected to orogenic collapse, exhumation‐metamorphism, and decompressional melting. This event might have taken place between 1.0 to 0.5 Ga during the Pan‐African Orogenic crustal growth, linked to the amalgamation of Gondwana.

Petrochemistry and Sr-Nd isotopes of post-collisional Neoproterozoic (ca. 950 Ma) amphibolite dykes of continental flood basalt affinity from the Simdega area: Implications for the geodynamic evolution of the Chhotanagpur Gneissic Complex, Eastern India

Metamorphosed mafic dykes are significant stratigraphic markers which also represent a shift from extensional to compressional geodynamic processes. We present petrology, bulk-rock geochemistry, and Sr-Nd isotope data on nineteen E-W to ENE-WSW trending mafic dykes metamorphosed under amphibolite facies in the Simdega area in the south western domain of the Chhotanagpur Gneissic Complex (CGC). The CGC is widely regarded as an eastern extension of the Central Indian Tectonic Zone (CITZ) which separates the Northern Indian and the Southern Indian blocks. Calcic amphiboles and plagioclases are the dominant minerals whereas clinopyroxene (diopside), biotite, chlorite, quartz, apatite, titanite, ilmenite and magnetite are present in minor amounts. Relict igneous textures are also present in some of the samples. Conventional thermobarometry and pseudosection modelling gave consistent peak temperatures (619 ± 13 °C) and pressures (5.8 ± 0.1 kbar) corresponding to the amphibolite facies. Major oxide geochemistry of the studied samples show differentiation trends and display arc-tholeiitic, high Fe-tholeiitic, high Mg-tholeiitic and calc-alkaline characters. Based on the LREE and HREE fractionation (La N /Yb N , La N /Sm N and Gd N /Yb N ) the studied samples can be classified into two broad groups: Group-I and Group II. In terms of the 87 Sr/ 86 Sr initial and 143 Nd/ 144 Nd initial contents, both the Group I and II samples are confined to an array of continental flood basalts and EM-1 and EM-2 reservoirs. The T DM (depleted mantle) model age of ca. 1.9 Ga suggest the source enrichment of the dykes during the Paleoproterozoic. Trace element ratios of both groups of the amphibolite dykes reveal that the (i) crustal contamination to be minimum, (ii) subduction-related fluid enrichment influenced their source regions, and (iii) mixed arc-, OIB-, and oceanic plateau- type geochemical signatures to be their characteristic features. Petrogenetic modelling involving REE suggest variable degrees of partial melting ranging from 5 to 10% for most of the samples of G-I and 1–3% for G-II dykes from a mixed source comprising predominantly of spinel lherzolite and minor amounts of garnet lherzolite. A three-stage model is proposed for the evolution of the CGC amphibolite dykes which involves: (i) sub-continental lithopheric mantle (SCLM) source enrichment due to the fluids derived from the subduction zone prior to ca. 1.9 Ga, (ii) emplacement of the mafic dykes in a post-collisional intra-continental extensional tectonic setting due interaction between the mantle plume and SCLM during the Tonian period at ca. 0.95 Ga and (iii) subsequent amphibolite facies thermal event at 0.92–0.88 Ga, associated with the amalgamation of the Rodinia supercontinent, that metamorphosed the mafic dykes as well as the much of the south-western part of CGC. Our findings support models proposing a northward subduction of the southern Indian block beneath the northern Indian block along the CITZ as well as bring out the role of a mantle plume in the generation of the Precambrian mafic dykes in the CGC. • Emplacement in a post-collisional intra-continental extensional tectonic setting • Trace elements show arc- as well as plume-derived components. • Nd isotope model ages highlight ancient (1.9 Ga) subduction event in their genesis. • Amphibolite grade metamorphism linked to Rodinia amalgamation

A Highly Depleted and Subduction‐Modified Mantle Beneath the Slow‐Spreading Mohns Ridge

AbstractThe Mohns Ridge is a very slow‐spreading ridge that, together with the Knipovich Ridge, marks the boundary between the North American and Eurasian plates in the Norwegian‐Greenland Sea. In this study, we report the major and trace element composition of spatially associated basalts and peridotites from a gabbro‐peridotite complex ∼20 km west of the Mohns Ridge rift flank. Formation of the ∼4–5 Myr crustal section involved accretion of normal mid‐ocean ridge basalts with Na‐content suggesting derivation from a depleted mantle source. This is consistent with the degree of partial melting estimated for clinopyroxene poor harzburgites using the Cr‐number of spinel (14%–18%) and rare earth element modeling of orthopyroxene (16%–24%) and reconstructed whole‐rock composition (14%–20%). If all the melting took place beneath the paleo‐Mohns Ridge, a crustal thickness of ∼7–8 km is expected, which is nearly double the observed thickness. Orthopyroxene trace elements are not consistent with typical fractional melting expected for mid‐ocean ridges but rather resemble that seen in supra‐subduction zone peridotites. The geochemistry of both the basalts and the peridotites suggests that a water‐rich slab flux in the past has influenced the mantle source. In turn, this caused hydrous melting which increased the depletion of the pyroxene components, leading to a highly depleted mantle that is now underlying much of the Arctic Mid‐Ocean Ridges and represents the source for the spreading related magmatism.

Oligocene calc-alkaline lamprophyres and K-rich association in the eastern Iranian ranges: Products of low-degree melting of subduction-modified lithospheric mantle in post-orogenic setting

Late- to post-orogenic lamprophyric and K-rich associations in the eastern Iranian ranges may be able to reveal some obscure issues about the petrogenesis and the role of tectonics in the formation of this rare group of igneous rocks. In this paper, we investigate new zircon U–Pb ages and elemental and Sr–Nd isotope characteristics of coexisting various rock suites as the post-tectonic products of the Paleogene orogeny . Lamprophyres are found in three occurrence modes in the eastern Iranian ranges. The first group is composed of olivine minette/shonkinite bodies in the Lar Complex, which is expressed as a post-tectonic volcano with Calderon subsidence that occurs currently as a ring-dike association and related explosion breccia . The second group consists of lamprophyric lavas of mainly mafic phonolite composition in Hormak. The last group is appinite/porphyrite occurring as stock/sub-volcanic bodies in Malek Siah Kuh. These three magmatic suites including lamprophyre and associated magmatic rocks are genetically, spatially and temporally related to one common K-rich mantle-derived magma . Zircon U–Pb dating of representative monzonite/syenite samples of the Lar Complex yielded ages of 31.43 ± 0.3 and 31.60 ± 0.40 Ma, respectively. Whole-rock geochemical and Sr–Nd isotope investigations demonstrated similar rare earth element (REE) patterns, with distinct enrichment in large lithophile elements (LILEs) and depletion in Ta–Nb–Ti, and limited variations in εNd (t) (+0.50 − +4.80) and 87 Sr/ 86 Sr (0.7043–0.7066) values. These characteristics, together with modeling of batch melting suggest that they were comagmatic and derived from a metasomatized lithospheric mantle source modified by a precursory subduction episode, through a low degree of partial melting (2–5%) at high-pressure conditions at depths >70 km with garnet in the residue. The Harirud fault activity possibly controlled the path, station and rapidity of magma ascent to the crustal levels, and, consequently, the water pressure of crystallization of the K-rich magmas and diversity of magmatic products. • The lithospheric mantle in the eastern Iranian ranges is highly heterogeneous. • Lamprophyres and K-rich association were products of post-orogenic magmatism. • The Harirud fault has crucial role in generation, transport and evolution of K-rich magmas.

Petrogenesis of Early Cretaceous alkaline basalts in the West Qinling: Constraints from olivine chemistry

Olivine crystals in alkaline basalts preserve important information about the Oxygen fugacity and thermal condition of the mantle‐derived magma. This paper presents new olivine mineral chemistry and whole‐rock geochemistry for Early‐Cretaceous alkali basalts in the Ganjia area, northeastern margin of the Tibetan Plateau. The olivine phenocrysts of Ganjia basalts have low Ni (74–2261 ppm) and high Ca contents (1217–4443 ppm), similar to the olivines in MORB, suggesting that they were crystallized from peridotite‐derived basaltic magma. Al‐in‐olivine/spinel thermometer results indicate crystallization temperatures of olivine around 1202 ± 46°C, corresponding to a mantle potential temperature of 1279 ± 47°C, with low oxygen fugacity of FMQ −1 to FMQ 0, suggesting a reduced mantle source. Whole‐rock geochemical compositions of the Ganjia basalts were characterized by low SiO2 contents (45.39%–48.70 wt%), high alkali contents (3.88–5.11 wt%), positive Nb and Ta anomalies, and enrichment in LREEs, HFSEs and LILEs, similar to those of OIB rocks, with high ratios of (Dy/Yb)N (1.38–1.54) and (La/Yb)N (5.37–7.42) ratios, indicating garnet residue. The above results suggest that the Early Cretaceous intraplate alkaline basalts in West Qinling were formed by a low degree of partial melting of peridotite and that no high‐temperature mantle plume existed. The Qinling Orogenic Belt may have been affected by the subduction of the Palaeo‐Pacific. We propose that the Ganjia basalts originated from the interaction between the upwelling asthenosphere and a lithospheric source.

Architectural and Compositional Diversity of Early Earth Ocean Floor Evidenced by the Paleoarchean Nondweni Greenstone Belt, South Africa

Abstract The nature of the early Archean ocean floor remains a topic of important debate. There are relatively few well-preserved occurrences worldwide where such terrains may be studied in detail because of structural dismemberment, metamorphic overprinting and pervasive early stage hydrothermal alteration to recent weathering. The 3.41-Ga dominantly mafic formations of the Nondweni Greenstone Belt (NGB) covering 270 km2 in the south-eastern Kaapvaal Craton comprise submarine volcanics that exhibit a wide range of textural features, including pillows, chill zones and brecciated flow tops, and various spinifex textures, including the rare platy pyroxene type, cumulate layers, and tuffs. Channelized subaqueous lava lakes that underwent fractionation are capped by thick spinifex-textured units and pillows. Early stage seafloor alteration is regionally variable, ranging from intense to minimal, with preservation of original mineralogy in many areas. Mafic volcanic rocks of the NGB contrast with those of the Barberton Greenstone Belt both in the style of volcanism and in the associated compositional range of komatiitic basalt to basalt with a complete absence of high-Mg komatiites. Olivine-phyric rocks, or derivatives thereof, are largely absent and pyroxene is the main controlling phase with orthopyroxene in the most primitive komatiitic basalts and clinopyroxene in the evolved lava lake sequences. The abundance of orthopyroxene typifies the long-standing silica-enriched character of the Kaapvaal Craton. Three exceptionally well-preserved and well-exposed sequences were studied utilizing hand-drilled samples and deep coring providing unprecedented stratigraphic and textural detail and field controls for more than 400 samples. A unifying feature of the mafic volcanics of the NGB is the range of compositions and ratios of incompatible elements most clearly illustrated by a series of high- and low-Ti compositional lineages reflecting differing sources or degrees of mantle partial melting. Sharp boundaries between high- and low-Ti flow successions indicate sudden changes in the melting regimes or the interaction of flow sequences from different volcanic centres. Th/Nb ranges from 0.1 to 0.2 and reveals crustal contamination of primitive lavas. The primary magma that gave rise to the most primitive komatiitic basalts with 19.5% MgO was derived from partial melting of a mantle plume source in the garnet stability field. Trace element modeling shows that the sequences studied in detail have been modified by fractionation and crustal contamination with the most likely contaminant being the Ancient Gneiss Complex (3.43–3.66 Ga), which is extensively exposed in Eswatini and probably underlies the Paleoarchean terrains in the southern Kaapvaal Craton. The geotectonic setting was likely that of a submerged felsic crustal platform as enclaves within an oceanic plateau.

The Magmatic Evolution and the Regional Context of the 1835 AD Osorno Volcano Products (41°06’S, Southern Chile)

Abstract Osorno volcano (41°06’S, 72°20’W) is a composite stratovolcano of the Central Southern Volcanic Zone of the Chilean Andes. It is the southernmost member of a NE–SW trending alignment of volcanic edifices including La Picada and Puntiagudo volcanoes and the Cordón Cenizos chain. According to contemporary descriptions recorded by Charles Darwin in 1835, two eruptive events occurred: the first during January–February, and the second during November–December 1835 and January 1836. The volcano erupted basaltic andesite lavas and tephra fall deposits (52.4 to 52.9 SiO2 wt. %), which contain phenocrysts of olivine, plagioclase, clinopyroxene, and spinel. The compositions of these phenocryst phases, together with those of olivine-hosted melt inclusions, allowed us to constrain intensive parameters for the pre-eruptive magmas. These varied from 1060°C to 1140°C, with an oxygen fugacity buffer of ~ΔQFM +1.1, dissolved water concentrations of up to 5.6 wt. % (average of ~4.2 wt. %) and maximum pressures equivalent to ~7-km depth. Textural relations, such as crystal accumulations and clots, zoning in crystals and other indications of disequilibrium, lead us to infer the involvement of a crystal mush, rich in individual crystals and clots of crystals, which underwent a degree of disaggregation and entrainment into the transiting magma prior to eruption. Comparison of trace element abundances, including rare earth elements, fluid-mobile elements, and relatively fluid-immobile elements, combined with 87Sr/86Sr and 143Nd/144Nd isotope ratios, allows us to consider variations in slab-derived fluid input and the minor role of crustal contamination on the Osorno eruptive products and those from neighboring volcanic systems. Our results suggest both a greater contribution from slab-derived fluid and a higher degree of partial melting in the systems supplying stratovolcanoes (Osorno, Calbuco, and La Picada) relative to those supplying small eruptive centers built over the major regional Liquiñe-Ofqui Fault Zone.

A snapshot of the transition from monogenetic volcanoes to composite volcanoes: case study on the Wulanhada Volcanic Field (northern China)

Abstract. The transition processes from monogenetic volcanoes to composite volcanoes are poorly understood. The Late Pleistocene to Holocene intraplate monogenetic Wulanhada Volcanic Field (WVF) in northern China provides a snapshot of such a transition. Here we present petrographic observations, mineral chemistry, bulk rock major and trace element data, thermobarometry, and a partial melting model for the WVF to evaluate the lithology and partial melting degree of the mantle source, the crystallization conditions, and pre-eruptive magmatic processes occurring within the magma plumbing system. The far-field effect of India–Eurasia collision resulted in a relatively high degree (10 %–20 %) of partial melting of a carbonate-bearing eclogite (∼ 3 wt % carbonate; Gt/Cpx ≈ 2 : 8, where Gt denotes garnet and Cpx denotes clinopyroxene) followed by interaction with ambient peridotite. The primary melts ascended to the depth of the Moho (∼ 33–36 km depth), crystallized olivine, clinopyroxene and plagioclase at the temperature of 1100–1160 ∘C with the melt water contents of 1.1 wt %–2.3 wt %. Part of the primary melt interacted with the lithospheric mantle during ascent, resulting in an increase in the MgO contents and a decrease in the alkaline contents. The modified magma was subsequently directly emplaced into the middle crust (∼ 23–26 km depth) and crystallized olivine, clinopyroxene and plagioclase at the temperature of 1100–1160 ∘C. The primary melts from the same mantle sources migrated upward to the two-level magma reservoirs to form minerals with complex textures (including reverse and oscillatory zoning and sieve texture). Magma erupted along the NE–SW-striking basement fault and the NW–SE-striking Wulanhada–Gaowusu fault in response to the combined effects of regional tectonic stress and magma replenishment. The crustal magma reservoir in the WVF may represent a snapshot of the transition from monogenetic volcanoes to composite volcanoes. It is possible to form a composite volcano with large magma volumes and complex compositions if the magma is continuously supplied from the source and experiences assimilation and fractional crystallization processes in the magma plumbing system at crustal depth.

Age and origin of the West Kimberley lamproites, Western Australia

New high precision 40Ar/39Ar ages are presented which more tightly constrain the eruption ages of the West Kimberley lamproites of northwestern Western Australia. The data show a southward progression in ages of the lamproites over a distance of ∼135 km, from ∼22.5 Ma in the northernmost lamproites of the Ellendale Field to ∼19 Ma in the Noonkanbah Field in the south, excluding the Walgidee Hill lamproite. The large (∼490 ha) zoned Walgidee Hills lamproite at the northern edge of the Noonkanbah Field is the youngest of the West Kimberley lamproites and represents a later pulse of lamproite magmatism at 17.4 Ma, >1 Ma after the main lamproite magmatism.The southward progression of lamproite ages extends across a wide swath from the Proterozoic King Leopold Orogen at the southern margin of the Kimberley Craton to the Phanerozoic Fitzroy Trough in the Canning Basin. The magmatism does not appear to be the result of the migration of a mantle plume, but more likely reflects small degrees of partial melting of old, thick, metasomatised lithosphere extending from the craton margin beneath the basin, with melting triggered by regional uplift and/or small-scale mantle convection at the craton margin. Emplacement of the lamproites likely exploited ancient trans-lithospheric structures associated with the Proterozoic orogen at the craton margin and structures associated with the formation of the Canning Basin in the Phanerozoic.

Alkaline rocks of the Bobaomby volcanic field point to a petrogenetic link between Comoros and northern Madagascar lithosphere

The Bobaomby volcanic field (10–11 Ma) is the northernmost volcanic area of Madagascar, and is a monogenetic volcanic field comprising outcrops of lava flows, dykes, scoria cones, tuff rings and plugs, widely scattered over an area of roughly 500 km2. The volcanic rocks range in composition from nephelinite, basanite and tephrite, through tephritic phonolite, to F- and Cl-rich peralkaline phonolite (MgO from 13 to 0.01 wt%), and the serial affinity varies from sodic to potassic. A few mica-amphibole-rich lamprophyric dykes have tephritic composition and ultrapotassic affinity. The mafic lavas host intrusive xenoliths with evident cumulate features (wehrlites, composite olivine gabbros s.l., amphibole clinopyroxenites and “kaersutitites”), as well as various types of mantle-derived xenoliths and xenocrysts in the most primitive rocks. The very wide compositional variations of the observed phases (olivine, clinopyroxene, amphibole, oxides, feldspars, feldspathoids, apatite, titanite, aenigmatite and other accessories) in lavas, dykes and cognate xenoliths are fully consistent with the variable degree of differentiation of the host lavas/dykes, and pointing out to limited open-system or polybaric crystallization. The mafic lavas have marked enrichment in incompatible elements and light rare-earth element (LREE) (e.g., Lan/Ybn = 19–27), whereas concave REE patterns are found in the peralkaline phonolites, as a result of removal of accessory titanite starting from tephritic phonolite magmas. The gabbroic/ultramafic xenoliths are interpreted as crustal cumulates of basanitic and tephritic magmas. Several liquid lines of descent in the basanites and tephrites are evident from the trace-element distribution, and from the differing geochemistry of the evolved rocks. The isotopic compositions reach extreme values (e.g., 206Pb/204Pb = 20.065 in the ultrapotassic lamprophyre) when compared to the rest of the Cenozoic/Recent Madagascan volcanic rocks, but similar to those of the Comoros archipelago, suggesting analogies of mantle sources and enrichment processes in the lithosphere of this volcanic archipelago. The origin of the Bobaomby mafic rocks is compatible from a derivation from low degree partial melting of an incompatible element-enriched peridotite source (possibly located in the lowermost lithospheric mantle) rich in volatile-rich phases (pargasite, locally also phlogopite and possibly carbonates), matching the sources of other Cenozoic volcanic areas throughout Madagascar, and perhaps Comoros.

Petrogenesis of peridotites in the Dargai Complex ophiolite, Indus Suture Zone, Northern Pakistan: Implications for two stages of melting, depletion, and enrichment of the Neo-Tethyan mantle

The upper mantle section of the Dargai ophiolite along the Indus Suture Zone, also known as the Main Mantle Thrust of northern Pakistan, consists mainly of clinopyroxene-bearing (Cpx)-harzburgites, depleted harzburgites, and dunites. Here we document a two-stage origin for these ultramafic rocks using new mineral and whole-rock geochemistry, integrated with Re -Os isotopic data of these upper mantle peridotites. Chromian spinel in the Cpx-harzburgites have significantly lower Cr# values (15.2–20.7) than the depleted harzburgites (38.4–75.2) and dunites (75.0–81.9). The Cpx-harzburgites are characterized by high average Al 2 O 3 (2.04 wt%), CaO (2.26 wt%), ∑REE (24.3 ppb), and 187 Os/ 188 Os values ranging between 0.12495 and 0.12997. In contrast, the depleted harzburgites and dunites are characterized by lower average Al 2 O 3 (0.51 wt% and 0.14 wt%, respectively), CaO (0.68 wt% and 0.19 wt%, respectively), and ∑REE (3.41 ppb and 0.92 ppb, respectively). The 187 Os/ 188 Os of the depleted harzburgites range between 0.11699 and 0.12612, values that are less radiogenic than those of the Cpx-harzburgites. The mantle-normalized trace element patterns of the Cpx-harzburgites reflect low degrees of partial melting (~10–15%), whereas those of the depleted harzburgites and dunites record somewhat higher degrees of partial melting (~15–23% additional melting of the Cpx-harzburgites). The low degree of melting, coupled with low degrees of melt extraction, therefore point to a spreading center at amid-ocean ridge (MOR) or in a distal forearc basin. In contrast, the depleted harzburgites and dunites formed during the second stage of melting and related refertilization, and were clearly linked to a supra-subduction zone (SSZ) setting. The Re -Os isotopic compositions of the Dargai Complex peridotites provide model ages of ca. 250 and ca. 450 Ma recording protracted tectonic events in close association with the geodynamic evolution of the Neo-Tethyan, Rheic, and Proto-Tethyan oceans. The extremely heterogeneous Os isotopic compositions of this part of the Neo-Tethyan upper mantle reflect multiple melt production and melt extraction processes throughout its tectonic evolution. • Peridotites in the Dargai Complex are formed by episodic melt-rock reaction. • The Dargai Complex peridotites experienced two stage of depletion and enrichment. • Peridotites were formed in the Indus Suture Zone after low-high degrees partial melting. • Re-Os isotopic composition infer derivation from heterogeneously depleted mantle.

Zircon evolution from migmatite to crustally-derived granite: A case study of late Neoarchean migmatite in the Yishan area, western Shandong, North China Craton

Sequential changes in the texture, composition and U-Pb age of zircon from the late Neoarchean migmatites and related rocks in the Yishan area of western Shandong, eastern North China Craton, record the evolution of melt and magma from paleosome to crustally-derived granite. Zircon grains from the paleosome and metatexite have features of recrystallization, with narrow rims; those from leucosome have inherited cores and wide homogenous rims; those from diatexite preserve evidence of a complex evolution involving dissolution and growth, with the euhedral morphology of the rims, and the cross-cutting relationship between cores and rims, consistent with a magmatic origin. With the increase in melt content toward granite, the newly-grown zircon has clearer magmatic zoning and higher Th/U ratios. The low whole-rock Zr saturation temperatures (684–728 ℃) indicate that the diatexites, with abundant residual zircon, might be the product of low-temperature melting. The contemporaneous, crustally-derived granites, however, contain no or much less xenocrystic zircon and have high zircon saturation temperatures (740–868 ℃). Only at the higher temperature was sufficient partial melt produced to start the migration of magma out of the source region to form the granites. Anatectic (magmatic) zircons in diatexite commonly contain xenocrystic (residual) zircons, no matter how the magmatic system evolves further, the xenocrystic zircons in the anatectic (magmatic) zircons will still be retained. On the other hand, the crustlly-derived granites widely distributed in the area contain few residual zircons. Therefore, the crustally-derived granites widely distributed in western Shandong cannot form by further evolution of the diatexites, but require a higher degree of partial melting under higher temperature conditions. SHRIMP U-Pb zircon dating, combined with CL imaging, reveals that zircon crystals from the diatexites mostly are characterized by inherited cores with thick magmatic overgrowths, recording a regional anatectic event at ∼2.51 Ga. Residual source rock enclaves and xenocrystic zircons in the diatexites mostly have ages of ∼2.70 Ga and ∼2.53 Ga, suggesting that the diatexites were derived from Neoarchean continental crust.

Age, geochemistry and mantle source of the Alto Diamantino basalts: Insights on NW Paraná Magmatic Province

The Paraná-Etendeka is one of the largest and most thoroughly investigated large igneous provinces in the world formed in a very narrow time-range (i.e., 1.6–3.0 Ma) between 135 and 132 Ma. The South American portion of this province, i.e. the Paraná Magmatic Province, is mainly characterized by a mafic magmatism of coeval low- and high-Ti magmatic terms. This bimodal activity has been explained by different degrees of melting of a homogenous mantle source or by plume activity and metasomatism of the sub-continental mantle. Despite the large body of literature, the true origin of the mantle source and processes leading to this bimodal magmatism are still debated. In this work, we present the first geochronological, geochemical and isotopic data of basalts from Alto Diamantino (southern Mato Grosso, Brazil). These basalts have tholeiitic affinity and can also be classified in two different suites based on TiO 2 content (L-Ti < 3 wt%; H-Ti > 3 wt%) and Ti/Y (450 ppm), similarly to other volcanics from the Paraná Magmatic Province. Other geochemical parameters as Zr/Y and Nb/Yb ratios range from 3.78 to 8.60 and from 3.82 to 8.31, respectively. The L-Ti and H-Ti series have significant geochemical similarities, including 87 Sr/ 86 Sr ratios, with the Ribeira and Pitanga basalts of the Paraná Magmatic Province, respectively. The 40 Ar/ 39 Ar ages of the two series are statistically equivalent ranging between 134.4 ± 1.6 and 132.9 ± 2.0 Ma. Our data suggest that the Alto Diamantino basalts constitute the northwestern most expression of the Paraná Magmatic Province. Sr and Nd isotope ratios suggest that the original source of the Alto Diamantino basalts was a EM-I metasomatized mantle characterized by small amounts of different components (DMM and EM-II). Positive Ba and lack of Pb anomalies, together with K/Rb ratio, indicate the presence of amphibole in the mantle source of the Alto Diamantino basalts. Th/Nb and TiO 2 /Yb ratios suggests that the compositional difference between the L-Ti and H-Ti basalts could be explained by different degree of melting at different depths of a heterogeneously metasomatized mantle source. The H-Ti group of basalts is compatible with low degrees of partial melting at depth while the L-Ti suite records larger degrees of melting. The metasomatic event occurred after the Mesoproterozoic, possibly during the Neoproterozoic formation of the Gondwana supercontinent. • Alto Diamantino basalts are northernmost part of Paraná Magmatic Province. • Low- and High-Ti basalts have similar geochemical affinity but different evolution. • Magmatic series are coeval between 134.4 ± 1.6 and 132.9 ± 2.0 Ma. • Mantle source was refertilized after Mesoproterozoic subduction by hydrous melts. • L- and H-Ti basalts derived from different melting degree of a heterogeneous source.

Can Post-Subduction Porphyry Cu Magmas Form by Partial Melting of Typical Lower Crustal Amphibole-Rich Cumulates? Petrographic and Experimental Constraints from Samples of the Kohistan and Gangdese Arc Roots

Abstract Sulfide-rich hydrous arc cumulates in the lower crust are considered a likely source for porphyry Cu-forming magmas in post-subduction settings. However, it remains controversial whether or not post-subduction porphyry Cu magmas can be produced by partial melting of lower crustal arc cumulates. Here we performed detailed petrographic and geochemical studies on exhumed lower to middle crustal cumulates from the Gangdese and Kohistan arc sections, and then conducted partial melting experiments of the Gangdese sulfide-rich lithologies at 1.5 GPa and 800°C–1150°C using a piston cylinder press. Both the Gangdese and the Kohistan arc-root samples formed as cumulates in the lower to middle crust, but the Kohistan samples were subsequently brought into the garnet stability field during the India–Eurasia continental collision. Most of the Gangdese samples contain relatively high abundances of magmatic sulfides preserved in the form of Cu-poor monosulfide solid solution. In contrast, most of the Kohistan samples contain very low abundances of Cu-rich sulfides preserved as inclusions in garnet, probably due to intense metamorphic dehydration and up to 2 wt% partial melting in the garnet stability field. The original magmatic sulfides at Kohistan appear to have been removed mainly by metamorphic fluids rather than by partial melts, as the arc-root samples could have experienced only very low degree of partial melting and the sulfur solubility in experimentally produced partial melts is low (≤0.1 wt%) even at oxygen fugacities two log-units above the fayalite-magnetite-quartz buffer. Furthermore, the experimental partial melts contain much less K2O, MgO, Cl and ±S than typical post-subduction porphyry Cu-forming magmas. Hence, we conclude that the role of sulfide-rich arc cumulates in the formation of post-subduction porphyry Cu magmas has been severely overestimated.