Magmatic Groups Research Articles

Accessories in Kaiserstuhl carbonatites and related rocks as accurate and faithful recorders of whole rock age and isotopic composition

The accessories perovskite, pyrochlore, zirconolite, calzirtite and melanite from carbonatites and carbonate-rich foidites from the Kaiserstuhl are variously suited for the in situ determination of their U–Pb ages and Sr, Nd- and Hf-isotope ratios by LA-ICP-MS. The 143Nd/144Nd ratios may be determined precisely in all five phases, the 176Hf/177Hf ratios only in calzirtite and the 87Sr/86Sr ratios in perovskites and pyrochlores. The carbonatites and carbonate-rich foidites belong to one of the three magmatic groups that Schleicher et al. (1990) distinguished in the Kaiserstuhl on the basis of their Sr, Nd and Pb isotope ratios. Tephrites, phonolites and essexites (nepheline monzogabbros) form the second and limburgites (nepheline basanites) and olivine nephelinites the third. Our 87Sr/86Sr isotope data from the accessories overlap with the carbonatite and olivine nephelinite fields defined by Schleicher et al. (1990) but exhibit a much narrower range. These and the εNd and εHf values plot along the mantle array in the field of oceanic island basalts relatively close to mid-ocean ridge basalts. Previously reported K–Ar, Ar–Ar and fission track ages for the Kaiserstuhl lie between 16.2 and 17.8 Ma. They stem entirely from the geologically older tephrites, phonolites and essexites. No ages existed so far for the geologically younger carbonatites and carbonate-rich foidites except for one apatite fission track age (15.8 Ma). We obtained precise U–Pb ages for zirconolites and calzirtites of 15.66, respectively 15.5 Ma (± 0.1 2σ) and for pyrochlores of 15.35 ± 0.24 Ma. Only the perovskites from the Badberg soevite yielded a U–P concordia age of 14.56 ± 0.86 Ma while the perovskites from bergalites (haüyne melilitites) only gave 206Pb/238U and 208Pb/232Th ages of 15.26 ± 0.21, respectively, 15.28 ± 0.48 Ma. The main Kaiserstuhl rock types were emplaced over a time span of 1.6 Ma almost 1 million years before the carbonatites and carbonate-rich foidites. These were emplaced within only 0.32 Ma.

Ore Petrography, Mineralogical and Chemical Composition of New Iron Meteorite (Al-Sherqat Meteorite) from Iraq

The meteorite with a single total mass of 630 gm as a visible meteorite has fallen on 22 March 2021, at 10:00 a.m. in Al-Sherqat subdistrict within Salah Al-Din, northern Iraq; and therefore, was named Al-Sherqat meteorite by the authors. It is characterized by a uniform structure of coherent and medium degree of malleability. It is of a well-crystalline structure and not homogeneous in composition. The Al-Sherqat meteorite is composed of metallic phases of 7.6 gm/cm3 density exhibiting an oriented intergrowth of kamacite (α-FeNi) with taenite showing a Widmanstätten pattern on an etched polished section with the finest octahedrite kamacite bandwidth of less than 0.2 mm. It is composed of Fe (86.9 wt%), Ni (9.63 wt%), P (1.31 wt%), S (0.628 wt%), Ti (0.623 wt%), Co (0.446 wt%), Mo (0.146 wt%), Cr (0.103 wt%), Cu (0.141 wt%), V (300 ppm), Nb (220 ppm), W (53 ppm), Ag (50 ppm), Pb (30 ppm), Zn (20 ppm), Sb (16 ppm), Sn (10 ppm) and As (3 ppm). Al-Sherqat meteorite was structurally classified as an iron meteorite belongs to the plessitic group (Opl)) with octahedrite finest bands (less than 0.2 mm) of the kamacite lamellae. Kamacite platelets in Al-Sherqat meteorite are almost not a continuous plate network. Chemically, it belongs to the IIC type of magmatic group based on the amount of nickel (9.63%), where IIC is typically octahedrites has 9.3 – 11.5% Ni. The presence of kamacite, taenite, schreibersite, daubréelites, pentlandite, chromite, and wusite in Al-Sherqat meteorite are in accordance with IIC group of the iron meteorites. Al-Sherqat meteorite belongs to M-type considering a metallic core fragmented by impact asteroid. The most probable source of this meteorite is the core of an asteroid that melted early in its history.

Iron and Nickel Isotopes in IID and IVB Iron Meteorites: Evidence for Admixture of an SN II Component and Implications for the Initial Abundance of 60Fe

Abstract We measured Fe and Ni isotopes in 13 iron meteorites from magmatic groups IID and IVB. Resolvable deficits (≈−0.14) are observed in ε 60Ni in both groups. Small deficits (≈−0.08) are also evident in ε 56Fe but not in the most neutron-rich isotope of Fe (58Fe). Relative to terrestrial material, the observed ε iFe values are consistent with the presence of a small excesses of material in the parent bodies of IID and IVB irons that was produced in a type II supernova. The ε 60Ni and ε 56Fe values are uncorrelated in both groups. This is consistent with a nucleosynthetic origin of the ε 56Fe deficits, whereas the ε 60Ni deficits are best explained as radiogenic in nature due to the former presence of live 60Fe (t 1/2 = 2.62 Ma) in the early solar system. The 60Ni deficits correspond to 60Fe/56Fe ratios of ≈3 × 10−7 at the time of core formation on the IID and IVB parent bodies. These data, in conjunction with previously published 182Hf–182W core formation ages, are used to estimate a solar system initial 60Fe/56Fe = (6.4 ± 2.0) × 10−7 for the formation region of carbonaceous chondrites.

The Determination of Volcanic Characteristics Based on Deuterium and Oxygen-18 Isotope Compositions: A Case Study at Dieng Plateau, Central Java

DOI:10.17014/ijog.7.2.201-213To find out volcanic characteristics, a geochemical approach can be used, one of which is through the measurement of deuterium and oxygen-18 isotopes from condensate and crater water samples. In this study, Dieng Plateau in Central Java was chosen, because it has a wide spread of fumarole fields and crater water. In addition, research in the Dieng Plateau is very useful in the management of geological-based tourism and geopark development in the future. Results of isotope analysis in Candradimuka Crater show the average number of isotope deuterium is δD – 500/00 and isotope oxygen-18 is δ18O –30/00 which produce the value of the mixing fraction of f 18O = 47.11%. This figure is the highest isotope number compared to other craters on the Dieng Plateau. The high value of the isotope-18 mixing fraction is supported by a strong plume from three vents in Candradimuka. Meanwhile, the results of crater water isotope tests obtained the highest results in Sikidang Crater with the value of the deuterium isotope is δD = – 10.300/00 and the isotope oxygen-18 is δ18O = 6.570/00 which are in the metamorphic water area from the mixing of magmatic processes with surface meteoric water. Based on the deuterium isotope approach with oxygen-18 crater water and fumarole condensate supported by subsurface temperature data, most of the Dieng Plateau has magmatic-hydrothermal characteristics, except Candradimuka Crater which belongs to the magmatic group.

Geochemical characteristics of a pre-Middle Jurassic oceanic crust fragment from the Central Pontides in northern Turkey: Geodynamic implications on intra-oceanic subduction initiation

The Central Pontides (northern Turkey) is one of the key localities to understand the geodynamic evolution of the Palaeo- and Neotethyan oceans. It consists of the pre-Jurassic basement units, the Early Jurassic and the Early Cretaceous accretionary complexes, the widespread Middle Jurassic continental arc magmatics and the Late Jurassic to Tertiary cover units. The Early Cretaceous accretionary complex is represented by the Central Pontide Structural Complex and includes the Middle Jurassic oceanic units, which were metamorphosed during the Early Cretaceous. Apart from these oceanic units, a few metaophiolite and serpentinite fragments have been recognized within the basement units, which may represent the remnants of an older ocean. The pre-Middle Jurassic Devrekani Metaophiolite is the largest oceanic fragment and tectonically intercalated within/between the Devrekani Metamorphics and the Çangaldağ Metamorphic Complex. It is mainly composed of harzburgites, dunites with chromite veins and metagabbros, and cut by metabasaltic andesites and metadacites. Petrographically, the gabbro consists mainly of plagioclase and clinopyroxene, and displays phaneritic/porphyritic texture. In contrast, the metabasaltic andesite includes plagioclase and mica phenocrysts within a fine-grained groundmass. Also, the metadacite is composed predominantly of quartz, plagioclase, and mica minerals. Two different magmatic groups belonging to completely different tectono-magmatic settings have been geochemically determined based on the immobile trace element systematics. The metadacites and metabasaltic andesites are akin to continental arc magmatics and characterized by negative Nb and Ta anomalies and depleted HFSE relative to Th and La contents. However, the metagabbro samples display the geochemical signatures of boninitic rocks and characterized by highly depletion in HFSEs and REEs relative to N-MORB. The Devrekani Metaophiolite in the Central Pontides may represent another remnant of pre-Middle Jurassic oceanic crust generation and can be north-eastward continuation of the Permian-aged Almacık complex and the Boğazköy Metaophiolite fragment in the western Sakarya Composite Terrane. It may have been cut by intrusions of the extensive Middle Jurassic continental arc magmatism after its imbrication within the basement unit. The presence of pre-Middle Jurassic oceanic units may indicate that the Paleozoic ocean may have survived as the Jurassic Intra-Pontide Ocean between the Scythian Platform and Sakarya Composite Terrane during the Mesozoic time. Thus, the Intra-Pontide Suture may normally include the Palaeozoic and Mesozoic remnants of the long-lived northward subducting Tethyan ocean.

Iron Isotopic Compositions of Troilite (FeS) Inclusions from Iron Meteorites

Abstract We report non-mass-dependent Fe isotopic data for troilite (FeS) inclusions from 10 iron meteorites, representing both non-magmatic (IAB) and magmatic groups (IIAB, IIIAB, IVA). No resolvable variations are present in the most neutron-rich isotope (58Fe), but small deficits (≈−0.1 ε) in 56Fe were observed in several inclusions. With the exception of several Ca–Al-rich inclusions in primitive meteorites, these are the first reported non-mass-dependent variations in Fe isotopes for material formed in the early solar system. Nucleosynthetic variations in Ni isotopes were previously reported in these same samples. The effects in Fe isotopes are not correlated with those in Ni, which suggests that the origins of the isotopic variations are decoupled from one another. The 56Fe deficits may represent incomplete mixing of the precursor dust in the protoplanetary disk. Alternatively, a parent body process (e.g., irradiation by galactic cosmic rays) may have modified the Fe isotopic compositions of some inclusions, which initially had homogeneous Fe isotopic compositions.

Formation of non-magmatic iron-meteorite group IIE

Instrumental neutron-activation (INAA) data for metal in 22 nonmagmatic IIE meteorites show narrow ranges in Ir and other refractory siderophiles; the Ir range is a factor of 2.6, a factor of ∼2 smaller than in nonmagmatic IAB-MG, and orders of magnitude smaller than in the large magmatic groups. Siderophile data show no evidence of fractional crystallization. IIE irons can be split into two sets, a larger main-set and a set of 6 Cu- (or FeS) rich irons. Elemental concentrations in metal from veins in H5 chondrite Portales Valley fall within the IIE range with the exceptions of Co (high) and Ga (low).H-group-chondrite and Au-normalized IIE abundances for siderophiles show that IIE irons are ∼30% higher than H in refractory siderophiles Re, Ir and W and are about 30% lower than H chondrites in the volatiles Ga and Sb, inconsistent with proposals that IIE irons formed from H chondrites. The IIE fractionations contrast with those in L chondrites which are about 15% lower than H in the three refractory elements and 40% higher than H in volatiles indicating that IIE irons did not form from H chondrites but from a more reduced and siderophile-rich kind of ordinary chondrite (“HH” chondrites). Most O-isotope data support a close relationship between IIE irons and H or HH chondrites; lower Δ17O in primitive (chondritic) silicates support an HH classification. Literature isotopic data for Ru and Mo also show that IIE metal formed from an ordinary chondrite parent; it appears that the silicates and metal were formed by melting of a single asteroid. There is no evidence for radiogenic (26Al) heating; this, the rapid cooling recorded in the sizes of parental gamma crystal in the metal and the absence of fractional crystallization strongly support the hypothesis that IIE melting was the result of impacts.To summarize, the weight of the evidence favors the conclusion that IIE meteorites were formed by one or more impacts on an HH asteroid. The target probably had a composition like the chondritic materials in Netschaevo, but was unequilibrated and had much higher porosity.

Zircon Geochemical and Geochronological Constraints on Contaminated and Enriched Mantle Sources beneath the Arabian Shield, Saudi Arabia

Arabian Shield granitic zircon geochemistry provides insight into the petrogenetic processes involved in generating one of the planet’s largest tracts of juvenile Neoproterozoic crust. New zircon geochemistry supports previous U-Pb and whole-rock data that defined four magmatic groups: (1) ∼870–675 Ma island arc and synorogenic I-type granitoids (IA+Syn), (2) ∼640–585 Ma I- and A-type granitoids from the Nabitah and Halaban Suture (NHSG), (3) ∼610–600 Ma postorogenic perthitic (hypersolvus) A-type granitoids (POPG), and (4) <600 Ma anorogenic aegirine–bearing perthitic (hypersolvus) A-type granitoids (AAPG). The low Nb (∼1–300 ppm) and intrasuite rare earth element variation in IA+Syn and NHSG zircons indicates that these suites are derivatives of contaminated mantle followed by fractionation. AAPG suites, however, have higher Nb content (∼10–400 ppm) and are derived from limited crust-enriched mantle interaction. Each of the IA, Syn, and NHSG suites have discrete granite subsuites distinguished using zircon morphology and geochemistry whose U-Pb ages in each case form three groups. The IA subgroups are ∼867, ∼847, and ∼829 Ma; the Syn subgroups are ∼730, 716, and 696 Ma; and the NHSG subgroups are ∼636, ∼610, and ∼594 Ma. This apparent subevent repetition suggests some form of magmatic pulsing in the Arabian Shield. It is suggested that IA+Syn suites reflect typical volcanic arc granite settings and incremental subduction/accretion of eastward-migrating oceanic fragments of the East African Orogen. The appearance of ∼636 Ma A-type magmatism within suture zones (NHSG) is possibly derived from a long-lived (∼50 m.yr.) melting, assimilation, storage, and homogenization (MASH) zone resulting from an ∼640 Ma slab tear. These A-types are distinguished from more-enriched anorogenic (<600 Ma) A-types, possibly associated with lithospheric delamination.

Early Neoproterozoic rift-related magmatism in the Anti-Atlas margin of the West African craton, Morocco

The Moroccan Anti-Atlas contains the relics of a Tonian(?)-early Cryogenian rift developed along the northern margin of the West African craton. A distinct tholeiitic volcanic suite of lava flows, sills and dikes is encased in the Jbel Lkest Group (Kerdous inlier), the Taghdout Group (Zenaga inlier), the U1–U4 units of the Bleida-Tachdamt Group (Bouazzer Elgraara inlier), and the Lower Sedimentary Ensemble and Middle Volcanic Unit of the Tizi n’Taghatine Group (Siroua, Zenaga and Bouazzer Elgraara inliers). Two magmatic groups are distinguished that originated from a spinel–garnet transition zone ranging from zero garnet content to moderately enriched residual garnet content in the source, and partial melting degrees decreasing from 10% to 3%. Incompatible element patterns indicate contribution of both asthenospheric and lithospheric components suggesting an initial rifting geotectonic context, with lithosphere thinning and asthenosphere upwelling, similar to other complex systems of rifts that dissected Rodinia during the Early Neoproterozoic.

Origin of the mafic dykes in Nagarparker area of Pakistan

The Nagarparker area of Pakistan comprises igneous and metamorphic rocks dissected by coarse-grained gabbro dykes and medium- to fine-grained dolerites at Karai, Dhedvero, and the central part of the area. The gabbro dykes contain plagioclase (An79), diopside, diopsidic-augite, and olivine with minor biotite, magnetite, ilmenite, titanite, and ilmenite-titanite composite grains. The dolerites consist of plagioclase, titaniferous augite, augite, hornblende, biotite, titanite, apatite, zircon, titanomagnetite, rutile, and ilmenite. Based on geochemistry, the gabbro dykes (group I) and dolerites (group II) are classified as subalkaline whereas group III dolerites as alkaline. In spidergram, Nb troughs and Sr crests are depicted in the group I and group II dykes whereas the group III dykes illustrate positive Nb and negative Sr with relatively smooth trace element pattern. The mineralogical composition, discriminant quadratic analysis, and trace element variations distinguish the mafic dykes of the study area into two magmatic groups. The first group comprises group I and group II dykes which depict island-arc type whereas the group III dykes resemble OIB-type compositions similar to Seychelles islands and the Tavidar region of Indian Rajasthan. The tentative timing of their emplacement may be Neoproterozoic and Cretaceous (?), respectively.

The Mount Manengouba, a complex volcano of the Cameroon Line: Volcanic history, petrological and geochemical features

The volcanic story of Mount Manengouba is related to four chronological stages: (1) forming of the early Manengouba shield volcano between 1.55 and 0.94Ma, (2) building of the Eboga strato-cone between 0.94 and 0.89Ma, (3) caldera collapse and silicic extrusions of the Elengoum Complex between 0.89 and 0.70Ma, and (4) intra-caldera and flank activity between 0.45 and 0.11Ma. The volume of the volcano is calculated at 320km3±5%. The volcanic rocks are attributed to two magmatic outputs. The first and main magma generation produced the shield volcano, the strato-cone, and the syn- to post-caldera extrusions, displaying a complete series from basanites to trachytes (magmatic Group 1). The second magma generation is limited to the late and flank activity evolving from basanites to trachy-phonolite (magmatic Group 2). Both magmatic groups belong to the under-saturated alkaline sodic series. Petrological calculations locate the magmatic reservoir between 37 and 39km in the upper mantle for the Group 1 lavas, and between 42 and 44km for the Group 2 lavas. Trachytes were generated in a secondary crustal reservoir. Magmatic series evolve with medium to low pressure fractional crystallization of olivine, pyroxene, oxides, feldspar, and apatite. Significant crustal assimilation is evidenced in trachytes. The magma of Group 1 was generated with 3–6% of partial melting of a moderately enriched source containing 3–7% of garnet. Melting took place in the spinel to garnet transition zone located at 70–90km and around 25kb. The magma of Group 2 resulted from a slightly higher partial melting from a less garnet-rich source that indicates uprising of the melting column in the upper part of transition zone. Sr, Nd, and Pb isotope data of the Manengouba rocks and neighboring lavas are analyzed and compared with those of the mafic lavas of the CVL. Three source components are distinguished: a depleted component originated from the asthenospheric swell, a radiogenic component linked to the contaminated lithosphere of the Neoproterozoic mobile belt, and an enriched component or the lithosphere possibly related to pre-rifting magmatic processes.

3.45 Ga granitic gneisses from the Yangtze Craton, South China: Implications for Early Archean crustal growth

Zircon U–Pb–Lu–Hf–O isotopic compositions of two granitic gneisses from the Kongling Terrain in the Yangtze Craton, South China were determined by SIMS, LA-ICP-MS and LA-MC-ICP-MS. Whole rocks of the two samples were analyzed for major and trace element compositions. The SIMS and LA-ICP-MS data reveal similar five zircon age groups of 3.4, 3.3, 2.9, 2.7, and 2.0Ga for both gneisses. Three groups (magmatic Group A, metamorphic Group B, and overgrowth Group C) of the 3.4Ga zircons were identified based on their CL images. These three groups have indistinguishable ages and Th/U ratios. Groups A and B show identical 176Hf/177Hf (t), although Group C was too thin to be analyzed by LA-ICP-MS. Taken together, zircons from the two samples with 98–102% age concordance give weighted average SIMS ages of 3434.3±9.6Ma (2σ, MSWD=13, n=8) for Group A, 3446.0±8.8Ma (2σ, MSWD=10.7, n=15) for Group B, and 3479±26Ma (2σ, MSWD=0.49, n=2) for Group C. Groups A and B together yield an upper intercept age of 3457±14Ma (2σ, MSWD=0.85, n=23). The LA-ICP-MS data yield weighted average ages of 3442±19Ma (2σ, MSWD=0.17, n=7) for Group A and 3435±11Ma (2σ, MSWD=0.44, n=16) for Group B. They yield an upper intercept age of 3443±13Ma (2σ, MSWD=0.63, n=23). These SIMS and LA-ICP-MS ages are consistent. We propose that the above SIMS and LA-ICP-MS ages of Groups A and B are the best estimates of the granitic magmatism and the subsequent metamorphism. The metamorphism must have occurred after the granitic magmatism within a few tens of million years, as constrained by their age errors. Accordingly, these two granitic gneisses represent the oldest rocks currently known in South China. They predate the previously reported 3300-Ma-old trondhjemitic gneiss from the Kongling Terrain by 150Ma.The 3.4Ga zircons show near chondritic ɛHf (t) (−0.7±1.0, 2σ, MSWD=1.14, n=8), which is below the coeval value of the depleted mantle. This suggests that the granitic magma contained materials of pre-existing continental crust. Their higher-than-mantle δ18O values (6.1–6.4‰) imply that such materials must have been interacted with surface water. Crust formation ages (TDM2) of the 3.4Ga zircons vary from 3.9 to 3.6Ga with a weighted average of 3703±27Ma (2σ, MSWD=1.05, n=7). Our results support previous studies that the Yangtze Craton may have contained the continental crust as old as 3.8Ga.Among the younger age groups, the 3.3Ga zircons exhibit 176Hf/177Hf (t) and δ18O values similar to the 3.4Ga zircons, suggesting that they were altered from the 3.4Ga zircons. The 2.9 and 2.7Ga zircons in both samples are rare and magmatic. Their 176Hf/177Hf (t) ratios are distinct from the 3.4Ga zircons, indicating different sources. These two age groups are consistent with the 2.9Ga trondhjemitic–tonalitic–granodioritic and the 2.7Ga A-type granitic magmatism in the Kongling Terrain. The 2.0Ga metamorphic zircons, regardless of being concordant or discordant, have 176Hf/177Hf (t) ratios overlapping those of the 2.7Ga zircons, suggesting a common source. In contrast, δ18O of the 2.0Ga zircons is strongly variable and positively correlated with age concordance. The low δ18O (down to 3.1‰) requires interaction with hydrothermal fluid. These results suggest that at least some of the 2.0Ga zircons were likely to have been altered from the 2.7Ga zircons by hydrothermal fluid.

Vesta and extensively melted asteroids: Why HED meteorites are probably not from Vesta

Most researchers hold that the HED clan of differentiated meteorites originated on Vesta largely based on the assumption that nearly all V-type asteroids with basaltic reflection spectra are fragments spalled off Vesta. Although it is a reasonable working hypothesis that most of the V-type asteroids in the Vesta family originated on Vesta, the spectra are not unique enough to confirm this; a sizable fraction may have been produced during the destruction of a differentiated asteroid in the same large region of dynamic space. Observations of asteroids in the inner Asteroid Belt show that more than half of the V-type asteroids do not belong to the Vesta dynamic family.Iron-meteorite evidence shows that at least 26 asteroids experienced extensive melting and would have generated basalts and other differentiated stony meteorites. Most iron meteorites show high degrees of elemental fractionations that lead to the conclusion that they experienced fractional crystallization; it is probable that all these bodies generated basalts. There are 9 of these “magmatic” iron-meteorite groups and test criteria mainly based on extreme fractionations indicate that an additional 17 disrupted asteroids hosted fractionally crystallized cores and thus that ⩾26 asteroids experienced extensive melting; this estimate is much lower than previous estimates that included nonmagmatic irons.Within expected planetary heterogeneities the O-isotopic composition of HEDs is the same as that in oxides from IIIAB irons, the largest magmatic group of iron meteorites. εCr54 values are also very similar in IIIABs and HEDs. The O- and Cr-isotopic ties are much stronger than the spectral tie thus the working hypothesis should be that HEDs are from the IIIAB parent asteroid.Remote elemental analysis could confirm that HEDs are not from Vesta. If future remote analysis measures K contents ⩾0.6 mg/g this will indicate that HEDs did not originate on Vesta.

Zircon U–Pb and pyrite Re–Os age constraints on pyrite mineralization in the Yinjiagou deposit, China

We report new zircon U–Pb and pyrite Re–Os geochronological studies of the Yinjiagou poly-metallic deposit, sited along the southern margin of the North China Craton (SMNCC). In this deposit, pyrite, the most important economic mineral, is intergrown/associated with Mo, Cu, Au, Pb, Zn, and Ag. Prior to our new work, the age of chalcopyrite–pyrite mineralization was known only from its spatial relationship with molybdenite mineralization and with intrusions of known ages. The U–Pb and Re–Os isotope systems provide an excellent means of dating the mineralization itself and additionally place constraints on the ore genesis and metal source. Zircons separated from the quartz–chalcopyrite–pyrite veins include both detrital and magmatic groups. The magmatic zircons confine the maximum age of chalcopyrite–pyrite mineralization to 142.0 ± 1.5 Ma. The Re–Os results yield an age of 141.1 ± 1.1 Ma, which represents the age of the chalcopyrite–pyrite mineralization quite well. The common Os contents are notably low (0.5–20.1 ppt) in all samples. In contrast, the Re contents vary considerably (3.0–199.2 ppb), most likely depending on intensive boiling, which resulted in an increase of Re within the pyrite. This study demonstrates that the main chalcopyrite–pyrite mineralization occurred late in the magmatic history and was linked to a deeper intrusion involving dominant mantle-derived materials. This mineralization event might be related to the Early Cretaceous lithospheric destruction and thinning of the SMNCC.

Petrochemistry and tectonic setting of mafic volcanic rocks in the Chon Daen–Wang Pong area, Phetchabun, Thailand

AbstractThe mafic volcanic rocks and hypabyssal rocks in the Chon Dean‐Wang Pong area are possibly the southern extension of the western Loei Volcanic Sub‐belt, Northeast Thailand. They are least‐altered, and might have been formed in Permian–Triassic times. The rocks are commonly porphyritic, with different amounts of plagioclase, clinopyroxene, orthopyroxene, amphibole, Fe–Ti oxide, unknown mafic mineral, and apatite phenocrysts or microphenocrysts, and are uncommonly seriate textured. The groundmass mainly shows an intergranular texture, with occasionally hyalophitic, intersertal and ophitic–subophitic textures. The groundmass constituents have the same minerals as the phenocrysts or microphenocrysts and may contain altered glass. The groundmass plagioclase laths may show a preferred orientation. Chemically, the studied rock samples can be separated into three magmatic groups: Group I, Group II, and Group III. These magmatic groups are different in values for Ti/Zr ratios. The averaged Ti/Zr values for Group I, Group II, and Group III rocks are 83 ± 6, 46 ± 12, and 29 ± 5, respectively. In addition, the Group I rocks have higher P/Zr, but lower Zr/Nb relative to Group II and Group III rocks. The Group I and Group II rocks comprise tholeiitic andesite–basalt and microdiorite–microgabbro, while the Group III rocks are calc‐alkalic andesite and microdiorite. According to the magmatic affinities and the negative Nb anomalies on normal mid‐oceanic ridge basalt (N‐MORB) normalized multi‐element plot, arc‐related lavas are persuasive. The similarity between the studied lavas and the Quaternary lavas from the northern Kyukyu Arc, in terms of chondrite‐normalized rare earth element (REE) patterns and N‐MORB normalized multi‐element patterns, leads to a conclusion that the mafic volcanic rocks and hypabyssal rocks in the Chon Daen–Wang Pong area have been formed in a volcanic arc environment.

Relationship between monogenetic magmatism and stratovolcanoes in western Mexico: The role of low-pressure magmatic processes

A large Quaternary monogenetic volcanic field is present in the western part of the Trans-Mexican Volcanic Belt. It is composed by mafic-intermediate scoria cones and silicic domes that are arranged in two NNW–SSE alignments. These mark the north and south borders (Northern Volcanic Chain and Southern Volcanic Chain, SVC) of the San Pedro–Ceboruco graben. The products of this monogenetic volcanic field span a large range of compositions (from basalt to rhyolite) and magma affinities (from sub-alkaline to Na-alkaline), defining different magmatic groups. Mafic and silicic monogenetic centres from the north alignment also coexist with two stratovolcanoes (Ceboruco and Tepetiltic) and sometimes punctuate their flanks. Whole-rock analyses indicate the existence of 4 different types of primitive magmas (Na-alkaline, High-Ti, Low-Ti/SVC and sub-alkaline) which have evolved independently by low-P magmatic processes. Despite the relatively small size and simplicity of the monogenetic magmatism, open-system processes have modified the geochemical and isotope composition of erupted products. The negative correlation between Sr isotope ratios and MgO contents observed for Southern Volcanic Chain and High-Ti groups points to crustal interaction via AFC processes, involving upper granitic crust and mafic lower crust respectively. In contrast, the large variability in Nd-isotopic ratios, combined with low and less variable 87Sr/ 86Sr, shown by the most mafic compositions of the High-Ti group is mostly due to mantle source heterogeneities. Low-Ti and Na-alkaline compositions are only slightly modified by crustal contamination processes and their whole-rock geochemistry reflects the complex nature of the western Mexico sub-arc mantle. It is therefore apparent that a combination of mantle source processes plus crustal assimilation has generated complex geochemical and isotopic characteristics in the western part of the Trans-Mexican Volcanic belt. Despite the presence of monogenetic cones on the flanks of stratovolcanoes, limited magma interaction between monogenetic and polygenetic magmatism has been recognised only at Ceboruco, possibly producing the chemical variability of post-caldera lavas. Indeed, mafic magma feeding High-Ti monogenetic systems might represent the possible mafic end-member which triggered the Ceboruco caldera-forming event. This may have important implications for other explosive systems in which monogenetic magmatism is associated with stratovolcanoes. A geographic/tectonic control is also suggested by the geochemical data. Na-alkaline compositions are only found in the northern part of the Northern Volcanic Chain. Parental magmas of both the High-Ti and Low-Ti monogenetic series, erupted between the Ceboruco and Tepetiltic stratovolcanoes, were modified by lower crust AFC processes possibly favoured by the stress regime. Indeed, the presence of a local left-hand step over along the northern main fault systems between the two stratovolcanoes might inhibit free uprising of monogenetic mafic magmas. The preferential alignment of stratovolcanoes and monogenetic volcanic vents parallel to the northern main fault systems and the possible mixing between High-Ti mafic monogenetic magmas and more evolved Ceboruco magmas suggests that, under the predominance of regional stress, the influence of central volcanic vents on monogenetic magmatism might be more complex than simple control of vent directions and might favours magma mixing processes.

Gem-corundum megacrysts from east Australian basalt fields: trace elements, oxygen isotopes and origins*

Gem corundum, a minor but persistent megacryst in east Australian basalt fields, is mined from some placer concentrations. Laser ablation, inductively coupled plasma mass spectrometry analyses and O isotope determinations on a colour range of corundum from different fields, show that chromophore (Fe, Cr, Ti, V) and genetic indicator (Ga, Mg, δ18O) values can distinguish corundum sources (magmatic, metamorphic and metasomatic) before basalt incorporation. They also characterise corundum groups from different fields. This identified two metamorphic groups, one carrying ruby at Barrington Tops, and a magmatic group distinct from those from other gem fields (lower Fe, northeast Tasmania; higher Fe, Yarrowitch). Ruby-bearing groups show clear provincial characteristics and include lower temperature spinel-facies groups (Barrington, Yarrowitch) and higher temperature garnet-facies groups (Cudgegong–Macquarie River). High Mg/Fe and Ni values in the latter approach those for corundum in diamond, and are a possible diamond indicator. The corundum derived from diverse fold-belt and felsic sources in underlying lithosphere forms a dataset for comparing corundum from other basalt fields.

GEOCHEMISTRY AND TECTONO-MAGMATIC AFFINITY OF MAFIC EXTRUSIVE AND DYKE ROCKS FROM THE OPHIOLITE MELANGE OF THE SW ZAGORJE-MID-TRANSDANUBIAN ZONE (MT. MEDVEDNICA, CROATIA)

Mafic extrusive rocks occasionally intersected by dolerite dykes are the most abundant fragments of oceanic crust in the lower Callovian to upper Valanginian ophiolite melange exposed on the Mt. Medvednica, at the SW tip of the Zagorje-Mid-Transdanubian Shear Zone. The melange includes a chaotic mixture of various extrusive rocks which may be analogues of the oceanic rocks derived from (at least) two different tectonic settings. The extrusive and dyke rocks of this melange are geochemically classified into three magmatic groups: high-Ti group of N-MORB-like with arc signatures (Ti/V = 26.3-38.5), medium- Ti group (Ti/V = 20.2-23.0) and low-Ti group showing untypical LREE depleted [(La/Sm)cn = 0.49-0.62] IAT-like signatures (Ti/V = 13.2-18.0). All three geochemical groups of magmas were successfully monitored by clinopyroxene geochemistry. The magmas have consistent initial Nd-isotopic values (eNd(T=163 Ma) +5.9 to +6.3) and initial 87Sr/86Sr ratios (0.703748-0.704286) suggesting a similar parental source related to the initial stage of intraoceanic subduction of a back-arc basin ridge. It represents the new crust formed from melts derived from LREE extremely depleted MORB-type mantle which was affected by concomitant metasomatism and partial melting in a suprasubduction forearc setting. K-Ar ages on plagioclase separates indicate uppermost Bathonian - lower Oxfordian magmatic activity between 165±3.3 Ma and 160±3.3. A geodynamic model for the Mt. Medvednica ophiolites, part of the Repno oceanic domain of the Meliata-Maliak oceanic system, is here proposed. There are many common geochemical and geological correlation features between the Mt. Medvednica and Szarvasko ophiolites, located in NE Hungary that allow to integrate them in the same oceanic domain, whereas this is not obvious in the case of the Dinaric ophiolites.

Puna (Argentina) and northern Chile Ordovician basic magmatism: A contribution to the tectonic setting

Geochemical characteristics of Ordovician basic volcanic rocks help to define the evolving tectonic setting of the Argentine Puna and northern Chile. Four spatially distinct magmatic groups are defined on geological, petrographical, geochemical and isotopic bases, each associated with particular geodynamic environments. The Tremadoc western group of subalkaline low K tholeiites with arc and modified MORB like signatures represent early stages of a back-arc basin, where spreading was incipient. The Arenig western group, medium K calc-alkaline basalts to andesites have volcanic arc in transition to back-arc signatures. The Tremadoc subalkaline basalts of the eastern group have REE patterns similar to E-MORB and at the same time weak subduction characteristics suggesting a rather mature supra-subduction zone (SSZ) basin. In contrast, the Late Tremadocian-Arenig basalts of the same group have intra-plate signatures, interpreted as magmas that ascended along pull apart regions associated with a transtensional regime. The geochemical patterns were applied to correlate basic sequences of doubtful geological setting. So, basalts from Chile were related to the Tremadocian western group, where they represent a slightly more mature stage of spreading of the basin. Basic rocks from Pocitos and part of Calalaste represent pre-Ordovician records of a back-arc system similar to that of the Tremadoc western group. Clearly similar arc patterns to those of the Arenig western group allow extending the arc environment to the southern Puna. The Tremadocian basalts from the eastern group were related to metabasites from the southern Puna, as part of a back-arc environment at that time.

Rapid accretion and differentiation of iron meteorite parent bodies inferred from 182Hf–182W chronometry and thermal modeling

New high-precision W isotope measurements are presented for 33 iron meteorites from 8 magmatic groups (IC, IIAB, IID, IIIAB, IIIE, IIIF, I VA and IVB), 2 non-magmatic groups (IAB–IIICD and IIE), and one ungrouped iron (Deep Springs). All magmatic irons have ε182W values that are, within errors, equal to, or less radiogenic than, the Solar System initial of −3.47±0.20. A method was developed to correct the measured ε182W values of magmatic iron meteorites for the presence of cosmogenic effects produced during space exposure to galactic cosmic rays. The corrected data provide new constraints on the timing of metal-silicate differentiation in iron meteorite parent bodies, which must have taken place within a few million years (<2 to 6 My) of condensation of calcium–aluminum-rich inclusions (CAIs). Metal-silicate differentiation ages (from 182Hf–182W systematics) were combined with parent body sizes (from metallographic cooling rates) into a model of planetesimal heating by 26Al-decay, to constrain the accretion timescale of iron meteorite parent bodies. Accretion of iron meteorite parent bodies most likely occurred within 1.5 My of the formation of CAIs. The fast accretion times of iron meteorite parent bodies are consistent with dynamical models indicating that these objects may have originated in the terrestrial planet-forming region, where the accretion rates were high. Our W isotopic data for non-magmatic IAB–IIICD and IIE irons provide new constraints for their formation mechanisms. In particular, they support formation of IAB–IIICD iron meteorites by melting during a single collision event dated at 4–7 My after formation of the Solar System.