Petrogenesis Of Basalts Research Articles

Geochemistry and petrology of Holocene lavas in the Bárðardalur region, N-Iceland. Part I: Geochemical constraints on source provenance

Because of extensive volcanic production in Iceland during the Holocene, many of the early postglacial large-volume fissure lavas cannot be unambiguously traced to their eruptive craters solely by observations in the field. For example, the Bárðarbunga volcanic system has been suggested as a likely source of the large Holocene lava flows found in Bárðardalur valley, but this idea mainly relies on petrographic observations. We conducted a chemical and isotopic study of the lavas in Bárðardalur. For comparative purposes, we also targeted basement rocks of the Bárðarbunga central volcano, as well as several eruptive units in the region north of Vatnajökull. Based on a comparison of chemical and radiogenic isotope data of lavas from the Bárðardalur region and the eruptive units north of Vatnajökull and the Bárðarbunga central volcano, it appears most likely that the lavas of Bárðardalur valley belong to the Bárðarbunga volcanic system. These new data, and a compiled dataset for other selected volcanic systems of the NRZ, shed light on possible limitations when assigning erupted material to its source volcano by means of chemical composition. Furthermore, this study demonstrates that our understanding of the relative importance of the different processes at play during the petrogenesis of Icelandic basalts is likely to be greatly improved by multi-parameter datasets for geologically well-characterized eruptive units.

Petrogenesis of pillow basalts in West Junggar, NW China: Constraints from geochronology, geochemistry, and Sr–Nd–Pb isotopes

West Junggar is located at the southwest margin of the Central Asian Orogenic Belt and includes Silurian pillow basalts of the Mayilashan Formation. The petrogenesis and tectonic setting of these pillow basalts are important for the understanding of the tectonic evolution and metallogeny of the West Junggar area. This paper presents geochronological, geochemical, and whole‐rock Sr–Nd–Pb isotope data from the pillow basalts of the Mayilashan Formation. Zircon LA‐ICP‐MS U–Pb dating of a pillow basalt, which is in conformable contact with the chert, suggests that they were erupted at 437.2 ± 2.2 Ma marking the timing of generation of these rocks as Middle Silurian. Geochemically, all the pillow basalts bear the signature of ocean island basalt (OIB), and are characterized by alkaline affinity with high concentrations of TiO2 (3.28–4.12 wt.%), LILE and LREE enrichment and HREE depletion ((La/Yb)N = 5.5–7.3), with very weak Eu anomalies (Eu/Eu* = 0.96–1.06), and no obvious Nb, Ta, or Ti negative anomalies. Their Sr–Nd–Pb isotopic compositions ((87Sr/86Sr)I = 0.7037–0.7051, εNd(t) = 1.9–2.9, 206Pb/204Pbi = 17.74–18.22, 207Pb/204Pbi = 15.48–15.52, and 208Pb/204Pbi = 36.49–37.86) show Dupal‐like isotopic signature of ophiolites in the southern Paleo‐Asian Ocean. These characteristics indicate that the magmas were derived from a deep OIB reservoir, that is, a depleted but slightly heterogeneous asthenospheric mantle source with ~5–15% partial melting of garnet and spinel lherzolite. Our obtained results, in conjunction with previous published data, allow us to suggest that the alkaline pillow basalts formed in a seamount within an intraoceanic setting, where a larger number of seamounts with different ages occurred in the Paleo‐Asian Ocean.

Geochemistry of late miocene-pleistocene basalts in the Phu Quy island area (East Vietnam Sea): Implication for mantle source feature and melt generation

Geochemistry of late miocene-pleistocene basalts in the Phu Quy island area (East Vietnam Sea): Implication for mantle source feature and melt generation

Petrogenesis of Early Cretaceous basaltic lavas from the North China Craton: Implications for cratonic destruction

AbstractThe North China Craton (NCC) is believed to be the best example of cratonic destruction. However, the processes leading to cratonic destruction remain unclear, largely due to a lack of knowledge of the nature of the Mesozoic NCC lithospheric mantle. Here we report new petrological and geochemical data for Early Cretaceous NCC basalts, which provide insights into the nature of the underlying lithospheric mantle. The Early Cretaceous basalts (all tholeiites) show a limited variation in geochemical composition. In contrast, olivine‐hosted melt inclusions from these basalts display a wide range in compositional variation and include both alkalic and tholeiitic basaltic compositions. This result provides the direct evidence of the contribution of silica‐undersaturated alkali basaltic melts in the petrogenesis of the Early Cretaceous NCC basalts. In addition, the compositions of olivine phenocrysts and reconstructed primary melts indicate that the Early Cretaceous basalts are derived from a mixed peridotite and refertilized peridotite source. The Pb isotopic compositions of melt inclusions in high fugacity of oxygen (fo) olivines combined with trace element characteristics of these basalts reveal that heterogeneous lithospheric mantle sources for Early Cretaceous basalts were metasomatized by carbonate‐bearing eclogite‐derived melts. The Pb isotopic variations of the melt inclusions and clinopyroxene and plagioclase phenocrysts demonstrate that the mantle‐derived magmas were variably contaminated by lower continental crust. We propose that multiple subduction events during the Phanerozoic, combined with mantle‐plume activity, likely play a vital role in the generation of the Early Cretaceous voluminous magmatism and cratonic destruction.

Petrogenesis of Cenozoic basalts in central-eastern China: Constraints from Re–Os and PGE geochemistry

We present Re–Os and PGE (platinum group elements), together with major–, trace–element and Sr–Nd isotope data, to further constrain the petrogenesis for the Cenozoic basalts in central-eastern China. The basalts include low-Ca alkali, high-Ca alkali and tholeiitic varieties. Most low-Ca alkali basalts have unradiogenic 187Os/188Os (mostly <0.14), flat chondrite-normalized PGE patterns and depleted Sr–Nd isotopic compositions, indicating that they probably originated from a depleted mantle source with insignificant shallow-level crustal contamination. Minor high-Ca alkali/tholeiitic basalts have low PGE concentrations, particularly Os (down to 0.0056ppb), and highly suprachondritic initial 187Os/188Os ratios (up to >0.4), indicating involvements of sulfide/PGE-alloy segregation–AFC (assimilation–fractionation–contamination) processes. Most tholeiites and high-Ca alkali basalts have relatively high Os concentrations, highly variable and suprachondritic Os isotopic compositions, high Pd/Ir ratios and enriched Sr–Nd isotopic compositions. Modeling results reveal that the Os isotopic variations in these basalts cannot be ascribed to crustal contamination during ascent but mainly reflect source characteristics. Combined with their low CaO, low Mg#, high FC3MS values (FeOT/CaO–3×MgO/SiO2, all in wt.%), and high Fe/Mn signatures, a large portion of pyroxenite is inferred as an enriched component in the mantle sources to account for their highly suprachondritic Os isotopic compositions. The formation of the pyroxenites was probably due to the subducted sediment-bearing Pacific oceanic crust or the recycled lower continental crust resulted from the collision between the Yangtze and North China blocks.

Petrogenesis of Neoarchean metavolcanic rocks in Changyukou, Northwestern Hebei: Implications for the transition stage from a compressional to an extensional regime for the North China Craton

The 2.5Ga metavolcanic rocks in Changyukou, Northwestern Hebei, can be classified into three groups based on major and trace elements: high-Mg basalts, tholeiitic basalts, and the calc-alkaline series (basaltic andesites-andesites and dacites-rhyolites).Both high-Mg basalts and tholeiitic basalts have negative anomalies of Nb, Zr, Ti and Heavy Rare Earth Elements (HREE) as well as enrichments of Sr, K, Pb, Ba and Light Rare Earth Elements (LREE) and show typical subduction zone affinities. The petrogenesis of high-Mg basalts can be ascribed to high-degree partial melting of an enriched mantle source in the spinel stability field that was previously enriched in Large Ion Lithophile Elements (LILE) and LREE by slab-derived hydrous fluids/melts/supercritical fluids, as well as the subsequent magma mixing processes of different sources at different source depths, with little or no influence of polybaric fractional crystallization. The flat HREE of tholeiitic basalts indicates they may also originate from the spinel stability field, but from obviously shallower depths than the source of high-Mg basalts. They may form at a later stage of the subduction process when rapid slab rollback leads to extension and seafloor spreading in the upper plate.We obtain the compositions of the Archean lower crust of the North China Craton based on the Archean Wutai-Jining section by compiling the average tonalite–trondhjemite–granodiorite (TTG) components, average mafic granulite components, and average sedimentary rock components. The modeling results show that the generation of high-Al basalts, basaltic andesites and andesites can be attributed to assimilation by high-Mg basalts (primary basalts) of relatively high-Al2O3 thickened lower crust and the subsequent crystallization of prevailing mafic mineral phases, while Al2O3-rich plagioclase crystallization is suppressed under high-pressure and nearly water-saturated conditions. Dacites and rhyolites may be the result of further fractional crystallization of basaltic andesites (high-Al basalts) and andesites. Mixing of magmas at various stages along the fractionation course of basaltic andesites (high-Al basalts) toward rhyolites promotes the trend of the calc-alkaline series.To reconcile the 2.55 to 2.5Ga TTGs derived from overthickened crust, the 2.51 to 2.50Ga calc-alkaline volcanic rocks derived from thickened crust, tholeiitic basalts representing low pressure and an extensional tectonic setting, 2493Ma leucosyenogranites derived from overthickened crust, 2437Ma biotite-monzogranites derived from slightly thinner crust than leucosyenogranites but still thickened, as well as the clockwise hybrid ITD and IBC P–T paths of the HP granulites and widespread extension and rifting setting within the NCC from 2300Ma, we propose a model of an evolving subduction process. Among them, the composition of the 2.5Ga Changyukou volcanic rocks and potassic granites as well as the clockwise hybrid ITD and IBC P–T paths of the HP granulites may reveal that the tectonic setting in Northwest Hebei was in a transition stage from a subduction-related compressional regime to an extensional regime related to plate rollback.

The origin of Cenozoic continental basalts in east-central China: Constrained by linking Pb isotopes to other geochemical variables

Cenozoic continental basalts in east-central China are characterized by OIB-like trace element patterns with more depleted to less enriched Sr–Nd isotope compositions. Such geochemical signatures are attributable to variable contributions to their mantle sources from crustal components in the oceanic subduction zone. A combined study of basalt Pb isotope variations with other geochemical variables indicates that four mantle and crustal components were involved in the basalt petrogenesis. Model calculations verify the geochemical transfer from the subducted crustal components to the mantle sources. The depleted MORB mantle component is indicated by the depleted Sr–Nd isotope compositions of basalts. Relatively high 206Pb/204Pb and low Δ8/4 ratios are ascribed to contributions from the igneous oceanic crust with high U/Pb and low Th/U ratios, low 206Pb/204Pb and high Δ8/4 ratios are ascribed to the lower continental crust, and high 206Pb/204Pb and high Δ8/4 ratios are linked to the seafloor sediment. This generates different compositions of mantle sources for these OIB-like continental basalts. The basalts with the most depleted Sr–Nd isotope compositions show Pb isotope compositions distinct from Pacific MORB but similar to Indian MORB, suggesting the occurrence of Indian type asthenospheric mantle beneath the continental lithosphere of eastern China. The depleted MORB mantle would be metasomatized by the three crustal components at the slab-mantle interface in oceanic subduction channel, generating the mantle sources that are enriched in melt-mobile incompatible trace elements and their pertinent radiogenic isotopes. Nevertheless, the crustal components were not directly incorporated in the forms of crustal rocks into the mantle sources, but underwent partial melting to produce the felsic melts that predominate the composition of those trace elements and their pertinent radiogenic isotopes in the basalts. As such, the depleted MORB mantle component was contributed to the mantle sources in the form of peridotite, dominating the composition of major elements in the basalts. In is inferred that the westward subduction of Pacific slab beneath the North China Craton would lead not only to replacement of the ancient refractory lithospheric mantle by the depleted MORB mantle but also to incorporation of the crustal components into the mantle sources of continental basalts. Therefore, a series of processes from subduction, anatexis and reaction to storage and heating (SARSH) would be operated for the origin of continental basalts.

Effect of melt/mantle interactions on MORB chemistry at the easternmost Southwest Indian Ridge (61°–67°E)

AbstractThe easternmost part of the Southwest Indian Ridge (61°–67°E) is an end‐member of the global ridge system in terms of very low magma supply. As such, it is a good laboratory to investigate the effect of melt/mantle interactions on the composition of erupted basalts: for a given volume of erupted basaltic melt, the volume of reacted mantle is potentially greater than at more magmatically robust ridges. We analyzed major, trace element and isotopic compositions in three groups of rocks: plagioclase‐bearing ultramafic and gabbroic rocks dredged in nearly amagmatic spreading corridors; basalts from the sparse volcanic cover of these corridors (“ultramafic seafloor basalts”); and basalts dredged from the intervening, more volcanically active domains (“volcanic seafloor basalts”). Ultramafic seafloor basalts have significantly lower CaO and Al2O3 contents at a given MgO than most volcanic seafloor basalts. We propose that both types of basalts are derived from similar parental melts, but that the ultramafic seafloor basalts are more affected by reactions between these parent melts and the mantle rocks in the lithosphere below the ridge. We infer that these reactions occur in the walls of conduits that allow the aggregated melts extracted from the melting mantle to rise through the axial lithosphere and to the eruption sites. The principal effect of these reactions is to enrich the asthenospheric melts in MgO through olivine dissolution. This effect is not expected to be as noticeable, but could still play a role in basalt petrogenesis at more magmatic regions of the global slow‐spreading MOR system.

Petrogenesis of antecryst-bearing arc basalts from the Trans-Mexican Volcanic Belt: Insights into along-arc variations in magma-mush ponding depths, H2O contents, and surface heat flux

The Trans-Mexican Volcanic Belt (TMVB) is known for the chemical diversity in its erupted products. We have analyzed the olivine, pyroxene, and plagioclase mineral chemistry of 30 geochemically well-characterized mafic eruptives from Isla Maria at the western end of the arc to Palma Sola in the east. The mineral major oxide data indicate the dominance of open system processes such as antecryst uptake, and the scarcity of mineral-mineral and mineral-melt equilibria suggests that apart from forming microlites, erupted melts do not significantly crystallize during ascent. A combination of plagioclase antecryst chemistry and MELTS thermodynamic modeling of H2O-saturated isobaric fractional crystallization was employed to develop a pressure sensor aimed at determining the ponding depths of the co-genetic magmas from which the erupted plagioclase crystal assemblage originates. We show that the depth of magma-mush reservoirs increase eastward along the TMVB. We suggest that magma ponding is triggered by degassing-induced crystallization during magma ascent, and that the pressure sensor can also be regarded as a degassing sensor, with more hydrous melts beginning to degas at greater depths. Modeled initial magma H2O contents at the Moho range from ~4 to ~9 wt%. Magma-mush ponding depth variations fully explain the observed westward increase of average surface heat flux along the TMVB, supporting a new model of mafic arc magma ascent, where rapidly rising, initially aphyric melts pick up their antecrystic crystal cargo from a restricted crustal depth range, in which small unerupted batches of previously risen co-genetic magmas typically stall and solidify. This implies that, globally, mafic arc magmas may be used to constrain the depths of degassing and mush zone formation, as well as the amount of H2O in the primary melts.

Magma source feature and eruption age of volcanic rocks in the Tram Tau district, Tu Le Basin

Magma source feature and eruption age of volcanic rocks in the Tram Tau district, Tu Le Basin

Petrogenesis and origin of modern Ethiopian rift basalts: Constraints from isotope and trace element geochemistry

The source of continental rift-related basalts and their relation to rifting processes is a continuous matter of debate. We present major and trace element and Sr, Nd, Hf and Pb isotope data for axial rift basalts from eight volcanic centres (Ayelu, Hertali, Dofan, Fantale, Kone, Bosetti and Gedemsa, from NE to SW) in Afar and Main Ethiopian Rift (MER) to assess their source regions and their genetic relationships. These lavas have geochemical characteristics, i.e., a peak at Ba, Nb and troughs at K and Rb in primitive mantle-normalised multielement diagrams, which are consistent with predominant melting of an amphibole-bearing lithospheric mantle. However, the isotopic compositions for these lavas are heterogeneous (87Sr/86Sr=0.70354–0.70431, 143Nd/144Nd=0.51280–0.51294, 176Hf/177Hf=0.28301–0.28315, 206Pb/204Pb=18.48–19.31, 207Pb/204Pb=15.53–15.62, 208Pb/204Pb=38.61–39.06) and require various mantle reservoirs with distinctive isotopic signatures. The range of isotopic compositions requires the involvement of three distinct source components from the asthenospheric and veined lithospheric mantle. Progressive rifting leads to lithosperic thinning and upwelling of hot asthenospheric mantle, which induces melting of the veined lithospheric mantle. The trace element characteristics of the lavas are dominated by the vein material, which has a higher trace element content than the surrounding mantle. The isotopic composition of the vein material, however, is not very different from the ambient mantle, giving rise of apparent uncoupling of trace element and isotope constraints for the melt source. The uprising basaltic liquids in part inherit a lithospheric trace element signature, while their isotopic compositions are mostly unaffected due to short residence times within the lithosphere in context with progressive rifting and lithospheric thinning. Thus, the geochemical and isotope data are consistent with a multi-component source prevailing beneath the Afar and MER areas in which the basalts are generated during progressive rifting and, thus, passive upwelling of a mantle source.

Petrogenesis of early Jurassic basalts in southern Jiangxi Province, South China: Implications for the thermal state of the Mesozoic mantle beneath South China

Early Jurassic bimodal volcanic and intrusive rocks in southern South China show distinct associations and distribution patterns in comparison with those of the Middle Jurassic and Cretaceous rocks in the area. It is widely accepted that these rocks formed in an extensional setting, although the timing of the onset and the tectonic driver for extension are debated. Here, we present systematic LA–ICP–MS zircon U–Pb ages, whole-rock geochemistry and Sr–Nd isotope data for bimodal volcanic rocks from the Changpu Formation in the Changpu–Baimianshi and Dongkeng–Linjiang basins in southern Jiangxi Province, South China. Zircon U–Pb ages indicate that the bimodal volcanic rocks erupted at ca. 190Ma, contemporaneous with the Fankeng basalts (~183Ma). A compilation of geochronological results demonstrates that basin-scale basaltic eruptions occurred during the Early Jurassic within a relatively short interval (<5Ma). These Early Jurassic basalts have tholeiitic compositions and OIB-like trace element distribution patterns. Geochemical analyses show that the basalts were derived from depleted asthenospheric mantle, dominated by a volatile-free peridotite source. The calculated primary melt compositions suggest that the basalts formed at 1.9–2.1GPa, with melting temperatures of 1378°C–1405°C and a mantle potential temperature (TP) ranging from 1383°C to 1407°C. The temperature range is somewhat hotter than normal mid-ocean-basalt (MORB) mantle but similar to an intra-plate continental mantle setting, such as the Basin and Range Province in western North America. This study provides an important constraint on the Early Jurassic mantle thermal state beneath South China.

Petrogenesis of high-CaO lavas from Mauna Kea, Hawaii: Constraints from trace element abundances

The role of a mafic component in the petrogenesis of Oceanic Island Basalts (OIBs) is highly debated. As the best studied OIB, Hawaiian lavas provide critical insights into OIB genesis. At a given MgO content, the CaO content in the melt has been used to distinguish between partial melts of peridotite and garnet pyroxenite/eclogite. However, calculations using the BATCH program show that CaO contents in volatile-free melts saturated with all four phases, garnet, clinopyroxene, orthopyroxene and olivine, are controlled by both degrees of partial melting and source compositions, and low melt CaO content is not diagnostic of partial melts from garnet pyroxenite/eclogite. This is an important consideration in understanding the origin of high-CaO lavas recovered from the Hawaii Scientific Drilling Project (HSDP). Detailed geochemical and isotopic studies have been focused on the HSDP high- and low-SiO2 group lavas, and high-CaO lavas were not well studied because they were not included in the original reference suite samples. Here, we report trace element abundances obtained on a suite of high-CaO glasses and compared the trace element abundances of high-CaO lavas to those in high- and low-SiO2 lavas. When normalized to the average composition of low-SiO2 lavas, high-CaO lavas form a U-shaped trace element pattern, enriched in both the most incompatible (Nb, Th) and the least incompatible (Sc, V) elements. This compositional distinction is best explained if high-CaO parental magma represents a mixture of a low degree partial melt of the low-SiO2 mantle source with a high degree (>80%) partial melt derived from a mafic cumulate component. This mafic cumulate must be clinopyroxene-rich, and it could be delaminated mafic cumulate formed under arcs during continent formation, lower continental crust, recycled lower oceanic crust, or high pressure cumulates from a magma chamber.

Empirical Approach for Estimating Reference Material Heterogeneity and Sample Minimum Test Portion Mass for "Nuggety" Precious Metals (Au, Pd, Ir, Pt, Ru).

Quantification of precious metal content is important for studies of ore deposits, basalt petrogenesis, and precious metal geology, mineralization, mining, and processing. However, accurate determination of metal concentrations can be compromised by microheterogeneity commonly referred to as the "nugget effect", i.e., spatially significant variations in the distribution of precious metal minerals at the scale of instrumental analytical beam footprints. There are few studies focused on the spatial distribution of such minerals and its detrimental effects on quantification of the existing suite of relevant reference materials (RM). In order to assess the nugget effect in RM, pressed powder pellets of MASS-1, MASS-3, WMS-1a, WMS-1, and KPT-1 (dominantly sulfides) as well as CHR-Pt+ and CHR-Bkg (chromite-bearing) were mapped with micro-XRF. The number of verified nuggets observed was used to recalculate an effective concentration of precious metals for the analytical aliquot, allowing for an empirical estimate of a minimum mass test portion. MASS-1, MASS-3, and WMS-1a did not contain any nuggets; therefore, a convenient small test portion could be used here (<0.1 g), while CHR-Pt+ would require 0.125 g and WMS-1 would need 23 g to be representative. For CHR-Bkg and KPT-1, the minimum test portion mass would have to be ∼80 and ∼342 g, respectively. Minimum test portions masses may have to be greater still in order to provide detectable analytical signals. Procedures for counteracting the detrimental manifestations of microheterogeneity are presented. It is imperative that both RM and pristine samples are treated in exactly the same way in the laboratory, lest powders having an unknown nugget status (in effect all field samples for analysis) can not be documented to be representing a safe minimum mass basis.

Geochemistry of Paraná-Etendeka basalts from Misiones, Argentina: Some new insights into the petrogenesis of high-Ti continental flood basalts

The Early Cretaceous (∼135–131 Ma) Paraná-Etendeka continental flood basalts, preserved in bulk in the Paraná basin of southern Brazil and vicinity, have been divided into low-Ti and high-Ti types that govern the southern and northern halves of the basin, respectively. We have examined a new sample set from the southern margin of the northern high-Ti segment of Paraná basalts in Misiones, northeastern Argentina. These basalts are strongly to moderately enriched in TiO2 (2–4 wt.%), have relatively high Ti/Y (300–500), low MgO (3.5–6.5 wt.%), and high Fe (FeO(tot) 12–14 wt.%) and belong to the Pitanga and Paranapanema magma types of Peate et al. (1992). Nd and Sr isotope compositions are quite unvarying with εNd (at 133 Ma) values of −4.6 to −3.6 and initial 87Sr/86Sr of 0.7054–0.7059 and show no variation with fractionation. Compared to high-Ti lavas in the central and northern parts of the Paraná high-Ti basalt segment, the lavas from Misiones are similar to those in the northeastern magin of the basin but less radiogenic in initial Nd isotope composition than those in the central part. This variation probably reflects mixed EM1-EM2 source components in the sublithospheric mantle. A polybaric melt model of a sublithospheric mantle source at the garnet lherzolite-spinel lherzolite transition is compatible with the observed Ti budget of the Pitanga and Paranapanema lavas, regardless of the Nd isotope composition of their purported source.

Petrogenesis and tectonic implications of the iron-rich tholeiitic basalts in the Hutuo Group of the Wutai Mountains, Central Trans-North China Orogen

The Hutuo Group, located in the Wutai Mountains area, is characteristic of Paleoproterozoic strata in the North China Craton, and plays an important role in establishing evolutionary processes in the Trans-North China Orogen. We present petrological observations along with geochemical and Nd isotopic data for basalts in the Qingshicun and Hebiancun Formations of the Hutuo Group. These basalts are enriched in TFeO (11.97−18.01wt.%), consistent with occurrences of iron-rich basalt world-wide. They also display relatively high MgO (4.95−12.25wt.%), Ni (79−121ppm) and Cr (37−101ppm) and low in SiO2 (41.37−51.95wt.%). Therefore, the iron-rich character of the Hutuo Group basalts originated with their parental magma rather than via shallow crustal fractionation at low oxygen fugacity, as inferred for many high iron basalts. The Hutuo Group basalts exhibit weak to moderate differentiation of light REE over heavy REE ((La/Lu)cn=2.61−6.79) with minor or slight Eu anomalies (Eu/Eu*=0.77−1.41), and also show the Nb and Ti troughs without obvious Zr and Hf anomalies. The ɛNd(t) values of the basalts range from −1.23 to +0.81 with TDM of 2489−2664 Ma. The basalts most plausibly originated from the fertile subcontinental lithosphere mantle and, in the case of the Qingshicun Formation basalts, also underwent the crustal contamination. Basalts in the Hutuo Group also have the high Zr/Hf ratios, and present high Ti/V values of 24−55 and Zr/Y ratios ranging from 4.6 to 13.6, suggesting that they were emplaced in a within-plate setting. Considering the sedimentary associations and detrital fragments in the Hutuo and Gantaohe Groups, and bimodal magmatism along the Trans-North China Orogen, we propose that 2.2−2.0Ga geological events within the orogen were rift-related, but did not result from subduction or island arc processes.

Geochemical and Sr–Nd isotopic constraints on the petrogenesis of late Cenozoic basalts from the Abaga area, Inner Mongolia, eastern China

Over the past 30years, the Cenozoic basalts in eastern China have been the subject of many investigations, but their origin remains highly controversial. The Abaga–Dalinuoer volcanic field in Inner Mongolia consists of an approximately 10,000km2 lava plateau and more than 300 volcanic cones and extends northwestward to the adjacent Dariganga lava plateau of Mongolia, forming one of the largest Cenozoic volcanic fields in eastern Asia. In this paper, we concentrate on the Abaga volcanic field that comprises more than 200 monogenetic cones, including scoria cones, lava flows, and maars. Phreatomagmatism has contributed to the generation of several volcanic edifices, and the diameter of the maars craters can reach 3 to 6km, which are among the largest in eastern China. The volcanic rocks in the Abaga area can be divided into alkaline basalts and tholeiites. According to trace element and Sr–Nd isotopic characteristics, the Abaga magmas were primarily derived from a relatively homogeneous garnet peridotite source within the asthenosphere. Variable degrees and depths of partial melting played a key role in the genesis of magma from alkaline basalts to tholeiites. In contrast to the surrounding Cenozoic volcanic fields, such as Datong, Jining, and Hanuoba, in the western block of the North China Craton, the Abaga volcanic region in the southern part of the Central Asian Orogenic Belt is distinguished by the deepest melting and the thickest lithosphere. Negative Ba-, Rb-, K-, and Ti anomalies and relatively high 87Sr/86Sr ratios suggest the presence of residual phlogopite/amphibole during mantle melting. The origin of the Abaga magma most likely required involvement of fluids released from the stagnant Pacific slab in the mantle transition zone.

Petrogenesis of Potassic Basalts from Northeast China: New Constraints from Trace Elements in Olivine

Petrogenesis of Potassic Basalts from Northeast China: New Constraints from Trace Elements in Olivine

Age and petrogenesis of Anisian magnesian alkali basalts and their genetic association with the Kafang stratiform Cu deposit in the Gejiu supergiant tin-polymetallic district, SW China

Gejiu is geographically located near Gejiu city, SW China. It is one of the largest tin-polymetallic districts in the world and contains approximately 3milliontons (Mt) of Sn and smaller quantities of Cu, Pb, and Zn. The deposit primarily yields three different types of ore: skarn-hosted ore, basalt-hosted stratiform ore, and carbonate-hosted stratiform ore. Kafang is one of the primary ore deposits in the Gejiu district and is an unusual occurrence hosted in basaltic rocks. Genetic models of the Kafang deposit suggest that it is related either to Anisian (Lower stage of Middle Triassic) Gejiu basalts or to Cretaceous Gejiu granite. In this study, we performed zircon SIMS U–Pb dating, major and trace element analyses, and Sr–Nd–Pb isotopic analyses for the Gejiu basalts and S isotopic analyses for stratiform Cu ore. Our results and previous studies are used to interpret the petrogenesis of the Gejiu basalts and the origin of the basalt-hosted stratiform Cu deposit. The SIMS zircon U–Pb analyses of the Gejiu basalts yield an age of 244.4Ma. The trace element ratios of the Gejiu basalts are similar to those of ocean island basalt and have positive εNd(t) values (ranging from 0.6 to 2.5) and uniform (87Sr/86Sr)i values (ranging from 0.70424 to 0.70488). These ratios are close to those of the Permian Emeishan flood basalt. Thus, the Gejiu basalts may represent coeval volcanisms within the plate involving remelting of the Emeishan plume head through a stress relaxation process after the main plume event. The Pb and S isotopic compositions of the Gejiu basalts and the stratiform Cu ores indicate that the source of Cu and S is primarily derived from the Gejiu basalts. However, the age of sulfide mineralization (84.2–79.6Ma) and the age of hydrothermal alteration (85.5–81.9Ma) are temporally consistent with the age of the Cretaceous granite emplacement (85.5–83.3Ma). From a petrological and geochemical study, we determine that the Gejiu basalts may have been subjected to pervasive granite-related hydrothermal alteration during the emplacement of granite. These processes increase the K and Mg contents of basalt and probably caused the formation of the Cu ores. Thus, the Kafang stratiform Cu deposit can be considered as a granite-related hydrothermal deposit.

Petrogenesis of basalt–high-Mg andesite–adakite in the Neoarchean Veligallu greenstone terrane: Geochemical evidence for a rifted back-arc crust in the eastern Dharwar craton, India

The ∼2.7Ga Veligallu greenstone terrane in the eastern Dharwar craton is one of the least studied Neoarchean greenstone belts in India. To our knowledge, there is no published dataset that provides a detailed account of the petrogenetic processes, which culminated into the evolution of this greenstone belt. This paper presents a comprehensive account of field, petrography and geochemistry of the basalt–high-Mg andesite–adakitic volcanic rock association in the Veligallu greenstone belt.The basalts are tholeiitic in nature and are characterized by primitive geochemical compositions with ∼49wt% SiO2, ∼9wt% MgO, Mg# 54, Cr∼197ppm, Ni∼122ppm, Y∼27ppm, Yb∼2.7ppm; Zr/Y<3 and Ce/Yb<5. They do not plot on a mixing line between Mesoarchean TTG and N-MORB, therefore, inconsistent with any interaction with the upper continental crust. Accordingly, they are interpreted to have erupted in an oceanic setting. On the basis of Nb/Th ratio the Veligallu basalts are divided onto two groups. The basalts with Nb/Th>8 display systematically flat to light-REE depleted chondrite normalized REE patterns (LaN/YbN=0.76–1.0), in contrast to the samples with Nb/Th<8, which display slight enrichments in light-REE (LaN/YbN=1.0–1.6). The depleted basalts are characterized by primitive mantle normalized Nb concentrations lower than La but equal to Th, in contrast to the enriched basalts, which are characterized by slight depletions of Nb relative to both La and Th. Collectively, as a group, they display Nb/Yb ratio higher than the N-MORB. The enrichment in Nb is attributed to moderate to low degree (∼10%–4%) partial melting of a depleted MORB source. The trace, REE and their interelement ratios in the Veligallu basalts exhibit transitional characteristics between the average N-MORB and the typical Phanerozoic island arc basalts. Overall, the geochemical characteristics of the Veligallu basalts are identical to basalts erupted in the Phanerozoic back-arc basins.The Veligallu high-Mg andesites are orthopyroxene-bearing calc-alkaline andesites and are characterized by moderate SiO2 (∼56wt%), high MgO (∼8wt%), Mg# (61–67), Cr (174–330ppm) and Ni (173–242ppm) contents; low Y (∼13ppm) and Yb (∼1.2ppm) concentrations, relative to typical island arc andesites. They exhibit light-REE enriched (LaN/SmN∼1.7) and heavy-REE depleted (GdN/YbN∼1.9) fractionated REE patterns. On a primitive mantle normalized trace element variation diagram they exhibit negative Nb and Ti, and contrasting positive Zr anomalies relative to the neighboring REE. The La–Yb systematics suggest that these high-Mg andesites were generated by hydrous partial melting of a slab-melt metasomatized mantle wedge in the sub-arc mantle. The felsic volcanic rocks are characterized by high SiO2∼73wt%, Na2O∼4.5wt%, and significantly low concentrations of MgO (∼0.72wt%), Cr∼19ppm, Ni∼23ppm, Y∼5.9, and Yb∼0.44ppm; strongly fractionated chondrite normalized REE patterns (LaN/YbN∼30) with occasionally minor positive Eu anomaly, and primitive mantle normalized negative Nb and Ti, and contrasting positive Zr anomaly. The geochemical attributes of the Veligallu felsic volcanic rocks are analogous to the Cenozoic high silica-type adakites that are produced by partial melting in the sub-arc mantle, and presumably without any significant interaction of the slab-melts with the overlying mantle wedge.The juxtaposed occurrence of basalts with the banded iron formations, and their close spatial association with the high-Mg andesites, in the Veligallu greenstone belt, indicates that the petrogenesis of these volcanic rocks is consistent with their eruption in an intraoceanic setting. The close proximal association of the back-arc type basalts, and the high-Mg andesites and adakites within the Veligallu greenstone belt potentially suggests that this greenstone belt represents a rifted back-arc crust. This event appears to have significantly contributed to the peak of crustal growth activity in the eastern Dharwar craton at 2.7Ga.