Mantle-derived Igneous Rocks Research Articles

Data mining for geochemical signatures of volcanic-type uranium mineralization, Duolun-Guyuan prospect, North China

Application of advanced data mining methods to various types of geochemical data is able to fingerprint valid signatures of mineralization, thus unveiling ore genesis and discovering new minerals. But individual studies that apply data mining methods to both local- and regional-scale, both sediment and whole-rock multi-element geochemical data sets are relatively scarce. Here, we applied data mining methods, including multivariate statistical analysis (principal component analysis), spatial analysis (trend surface analysis), unsupervised machine learning algorithm (K-means clustering), supervised algorithms (random forest and deep neural network) to both regional sediment geochemical and local lithogeochemical data from the Duolun-Guyuan prospect, in order to determine the geochemical signatures of volcanic-type uranium mineralization through characterizing: (1) representative element associations; (2) axial zonation of primary haloes; (3) element distribution patterns; and (4) crustal structures (via deep learning-based predictive hafnium (Hf) isotopic mapping). Results of principal component analysis and random forest show that samples from known ore districts (e.g., Zhangmajing and Daguanchang) exhibit a distinct combination of major ore-forming elements (U and Mo), chalcophile elements (Ag, Hg, Pb, Sb and As), rare and rare earth elements (Be, Li, La, Nb and Y), tungsten (W), bismuth (Bi), and rock-forming elements (SiO2, K2O, Na2O and Al2O3), differing from samples of both the mineralized and barren areas. The axial zonation of primary haloes in Daguanchang is comprised of supra-ore haloes (rare earth elements, Th, Nb, Zr, Hf, Ga and Rb), near-ore haloes (U, Mo, Pb, Zn, Cd and Sb), and sub-ore haloes (Li, Be, Sc, V, Cu, Sr, Cs, Ba, W and Bi). Moreover, trend surface analysis shows that in the study area, the spatial distribution pattern of the supra-, near-, and sub-ore elements forms a northwesterly alignment, with the supra-ore elements concentrated in the southeast, the sub-ore elements in the northwest, and the near-ore elements in between. Finally, deep learning-based predictive hafnium (Hf) isotopic mapping reveals that the Duolun-Guyuan prospect is dominated by negative mean zircon εHf(t) values ranging from −17 to 0, except for some local areas in the west and southwest of Duolun and the north of Weichang. The above results may indicate critical signatures of volcanic-type U mineralization, consisting of meta- or pera-luminous, alkaline rhyolite resulted from crustal reworking, surrounding mantle-derived igneous rocks, proximal heat source, accompanying epithermal deposits (e.g., Ag, Au, etc.), and anomalous concentrations of U, Mo and relevant elements particularly Th, W, Bi, Ag and Sb etc. Our study will effectively provide new exploration geochemical indicators of volcanic-type U deposit.

Calcium isotope compositions of subduction-related leucite-bearing rocks: Implications for the calcium isotope heterogeneity of the mantle and carbonate recycling in convergent margins

Large Ca isotope variations recorded in mantle xenoliths and mantle-derived igneous rocks have been widely used to decode the origin of mantle heterogeneity, especially to trace recycled sedimentary carbonates. However, it is still unclear whether and how the subduction of sedimentary carbonate causes significant Ca isotope fractionation in the mantle in correspondence of convergent margins. In this study, we present Ca isotope data for sixteen silica-undersaturated ultrapotassic leucite-bearing rocks and their clinopyroxene (Cpx) phenocrysts from the “Colli Albani” volcano (Italy), and for six carbonate-bearing sediments from nearby Apennine Chain. These subduction-related volcanic rocks have been widely accepted as deriving from a mantle source that involves recycled carbonated sediments indicated by their enriched incompatible trace elements and radiogenic isotopes, and olivine geochemistry. The δ44/40Ca values (normalized to NIST SRM 915a) of leucite-bearing rocks are uniform (from 0.64 to 0.79 ‰, with one exception of 0.86 ‰) with an average of 0.72 ± 0.03 ‰, 2SE, N = 16, which is 0.12 ± 0.03 ‰ (2SE) and 0.22 ± 0.03 ‰ (2SE) lower than MORBs (0.84 ± 0.02 ‰, 2SE, N = 25) and BSE (0.94 ± 0.01 ‰, 2SE, N = 14) respectively. In contrast, the carbonate-bearing sediments show variable and notably low δ44/40Ca values, ranging from 0.44 to 0.77 ‰. The δ44/40Ca of Cpx (0.70 to 0.76 ‰) is indistinguishable from their host rocks (0.73 to 0.76 ‰). In addition, the lack of correlations between δ44/40Ca and SiO2, CaO, Ni and P2O5, and the Ca isotopic simulation result indicate that Ca isotope fractionation caused by fractional crystallization is insignificant. Given that the Ca isotope fractionation is limited during the melting of spinel wehrlite-like mantle source (within ∼0.04 to 0.11 ‰), the slightly low δ44/40Ca values of the Italian leucite-bearing rocks require 10 % to 20 % addition of carbonated sediments in their mantle source. This indicates that recycled carbonated sediments can cause slight δ44/40Ca heterogeneity (∼0.1 to 0.2) of the mantle wedge in local/regional scales.

Magma mixing for the genesis of Neoproterozoic Mopanshan granitoids in the western Yangtze Block, South China

Crust- and/or mantle-derived igneous rocks in the Neoproterozoic continental magmatic arc along the western Yangtze Block (South China) have been extensively studied, but the crust-mantle interaction that might have been involved in the formation of these rocks was poorly understood. In this paper, we present an integrated study on petrology, whole-rock compositions, in-situ plagioclase Sr and zircon U-Pb-Hf isotopes for granodiorites and monzogranites from the Neoproterozoic Mopanshan pluton in the western Yangtze Block. Our objective is to provide vital constraint on the magma mixing process in the formation of these rocks. The new LA-ICP-MS zircon U-Pb dating results reveal that both granodiorites and monzogranites of the Mopanshan pluton were coeval and formed at ca. 820 Ma. They are calc-alkaline to high-K calc-alkaline (SiO2 = 66.06–70.30 wt%, K2O = 2.24–3.34 wt%) and metaluminous to slightly peraluminous (A/CNK = 0.99–1.10) rocks, and characterized by middle-upper crustal trace element patterns (e.g., enrichment in Rb, K, Th, U, and Pb, and depletion in Nb, Ta, Sr, and Ti). The magma mixing process is supported by following ample evidence, including (1) the disequilibrium mineral textures and an abruptly increased An values (up to 60) as well as significant decrease of (87Sr/86Sr)i ratios from core to rim in plagioclase crystals, (2) higher Mg# values (45–50 > 40) than those of experimental melts from basalts at the same silica contents, (3) positive and negative whole-rock εNd(t) (–0.72 to + 1.01) values as well as variable whole-rock (87Sr/86Sr)i (0.702778–0.705404) and zircon εHf(t) (+1.76 to + 7.72) values. The special geochemical characteristics and element modeling support that the Mopanshan pluton was generated by magma mixing of ancient crust-derived melts and relatively mafic melts from metasomatized mantle source. The adakitic signatures observed in the Mopanshan pluton were attributed to subsequent hornblende-dominant intra-crustal fractional crystallization, rather than derived from a thickened lower crust source. In combination with regional geology and our compilation for Nd-Hf isotopes of Neoproterozoic igneous rocks, the Mopanshan pluton is magmatic response of intensive crust-mantle interaction induced by underplating of voluminous mantle-derived magma in a back-arc extension setting. This research highlights that the crust-mantle interaction is significant for the petrological and geochemical diversity of Neoproterozoic granitoids from the western Yangtze Block.

Melting of hydrous pyroxenites with alkali amphiboles in the continental mantle: 1. Melting relations and major element compositions of melts

Melting experiments on ultramafic rocks rich in the hydrous minerals phlogopite or phlogopite + K-richterite, some including 5% of accessory phases, have been conducted at 15 and 50 kbar. The assemblages represent probable source components that contribute to melts in cratonic regions, but whose melt compositions are poorly known. A main series of starting compositions based on MARID xenoliths consisted of a third each of clinopyroxene (CPX), phlogopite (PHL) and K-richterite (KR) with or without 5% ilmenite, rutile or apatite. Additional experiments were run without KR and with higher proportions of accessory phases. Melt traps were used at near-solidus temperatures to facilitate accurate analysis of well-quenched melts, for which reversal experiments demonstrate equilibrium.Results show that KR melts rapidly and completely within 50 °C of the solidus, so that melts reflect the composition of the amphibole and its melting reaction. Melts have high SiO2 and especially K2O but low CaO and Al2O3 relative to basaltic melts produced from peridotites at similar pressures. They have no counterparts amongst natural rocks, but most closely resemble leucite lamproites at 15 kbar. KR and PHL melt incongruently to form olivine (OL) and CPX at 15 kbar, promoting SiO2 contents of the melt, whereas orthopyroxene OPX is increasingly stable at lower lithosphere pressures, leading to an increase in MgO and decrease in SiO2 in melts, which resemble olivine lamproites. Melts of mica pyroxenites without KR are richer in CaO and Al2O3 and do not resemble lamproites. These experiments show that low CaO and Al2O3 in igneous rocks is not necessarily a sign of a depleted peridotite source. Accessory phases produce melts exceptionally rich in P2O5 or TiO2 depending on the phases present and are unlike any melts seen at the Earth’s surface, but may be important agents of metasomatism seen in xenoliths. The addition of the 5% accessory phases ilmenite, rutile or apatite result in melting temperatures a few ten of degrees lower; at least two of these appear essential to explain the compositions of many alkaline igneous rocks on cratons.Melting temperatures for CPX + PHL + KR mixtures are close to cratonic geotherms at depths > 130 km: minor perturbations of the stable geotherm at >150 km will rapidly lead to 20% melting. Melts of hydrous pyroxenites with a variety of accessory phases will be common initial melts at depth, but will change if reaction with wall-rocks occurs, leading to volcanism that contains chemical components of peridotite even though the temperature in the source region remains well below the melting point of peridotite. At higher temperatures, extensive melting of peridotite will dilute the initial alkaline melts: this is recognizable as alkaline components in basalts and, in extreme cases, alkali picrites. Hydrous pyroxenites are, therefore, components of most mantle-derived igneous rocks: basaltic rocks should not be oversimplified as being purely melts of peridotite or of mixtures of peridotite and dry pyroxenite without hydrous phases.

The highly siderophile elements and Re–Os isotope geochemistry of Variscan lamproites from the Bohemian Massif: implications for regionally dependent metasomatism of orogenic mantle

Orogenic lamproites represent a group of peralkaline, ultrapotassic and perpotassic mantle-derived igneous rocks that hold the potential to sample components with extreme compositions from highly heterogeneous orogenic mantle. In our pilot study, we present highly siderophile element (HSE) and Re–Os isotope systematics of Variscan orogenic lamproites sampled in the territories of the Czech Republic, Austria and Poland, i.e., from the termination of the Moldanubian and Saxo-Thuringian zones of the Bohemian Massif. Orogenic lamproites of the Bohemian Massif are distinguished by variably high contents of SiO2, high Mg# and predominant mineral associations of K-rich amphibole and Fe-rich microcline. The HSE show (i) consistently very low contents in all investigated orogenic lamproites compared to the estimated concentrations in majority of mid-ocean ridge basalts, hotspot-related volcanic rocks (e.g., ocean island basalts, continental flood basalts, komatiites, some intraplate alkaline volcanic rocks such as kimberlites and anorogenic lamproites) and arc lavas, and (ii) marked differences in relative and absolute HSE abundances between the samples from the Moldanubian and Saxo-Thuringian Zone. Such a regional dependence in HSE from mantle-derived melts is exceptional. Orogenic lamproites have highly variable and high initial suprachondritic 187Os/188Os values (up to 0.631) compared with rather chondritic to subchondritic Os isotope values of the young lithospheric mantle below the Bohemian Massif. The highly radiogenic Os isotope component in orogenic lamproites may be derived from preferential melting of metasomatised vein assemblages sitting in depleted peridotite mantle. This process appears to be valid generally in the petrogenesis of orogenic lamproites both from the Bohemian Massif and from the Mediterranean area. As a specific feature of the orogenic lamproites from the Bohemian Massif, originally ultra-depleted mantle component correlative with remnants of the Rheic Ocean lithosphere in the Moldanubian Zone was metasomatised by a mixture of evolved and juvenile material, whereas the lithospheric mantle in the Saxo-Thuringian Zone was enriched through the subduction of evolved crustal material with highly radiogenic Sr isotope signature. As a result, this led to observed unique regionally dependent coupled HSE, Rb–Sr and Re–Os isotope systematics.

Is exhumation of UHP terranes limited to low latitudes?

How the ultrahigh-pressure (UHP) terranes are exhumed to shallow levels but preserving intact relics of the UHP phase assemblages is among the most interesting but challenging topics in geosciences. We investigate all the paleolatitudes where the UHP terranes were exhumed. Our results show that all the UHP terranes in continental collision zones or oceanic accretionary wedges were exhumed within low latitudes (0°–30°), and the average paleolatitude for exhumations of the investigated 43 UHP terranes is ˜5.1° N. In contrast, those UHP xenoliths in mantle-derived igneous rocks could be brought to surface at higher paleolatitudes. Furthermore, the pattern of frequency for the UHP terranes exhumed at convergent boundaries is consistent with that of interglacial stages throughout the Earth history, indicating that the UHP exhumation is controlled by the climate and thus suggesting that the exhumed UHP terranes may be useful paleoclimate indicators.

NO EXCESSIVE CRUSTAL GROWTH IN THE CENTRAL ASIAN OROGENIC BELT: FURTHER EVIDENCE FROM FIELD RELATIONSHIPS AND ISOTOPIC DATA

Abstract We provide new field observations and isotopic data for key areas of the Central Asian Orogenic Belt (CAOB), reiterating our previous assessment that no excessive crustal growth occurred during its ca. 800 Ma long orogenic evolution. Many Precambrian blocks (microcontinents) identified in the belt are exotic and are most likely derived from the northern margin of Gondwana, including the Tarim craton. Ocean opening in the Palaeo-Asian Ocean, arc formation and accretionary processes began in the latest Mesoproterozoic along the southern margin of the Siberian craton and continued into the Neoproterozoic, giving rise to tectono-metamorphic terranes distinct from the exotic microcontinents in that they include tectonically mixed ancient crust as well as juvenile, mantle-derived igneous rocks. Several previous assessments of crustal growth based on the distribution of oceanic and island arc complexes and on Nd isotopic data for post-accretion igneous rocks are questionable, and we show that such data, in combination with the occurrence of old zircon xenocrysts, frequently signify tectonic mixing of juvenile and ancient crustal components. The only truly juvenile terranes, including oceanic crust and intra-oceanic arcs, seem to occur in northeastern Kazakhstan, in the Altai-Sayan region of Siberia and in the Lake and Trans-Altai zones of Mongolia. The largest area of pre-CAOB continental crust forms a broad belt from northwestern Kazakhstan via the Kyrgyz North and Middle Tianshan to the Yili Block and Chinese Central Tianshan in NW China. Most arcs in the CAOB formed on older continental crust, or with substantial addition of old crustal material via sediment recycling, similar to the situation in the present Southwest Pacific in southern Indonesia, and we suspect that the volume of old material in the lower crust of the CAOB is considerable but largely unaccounted for because of lack of geophysical data. Comparing the lithospheric mantle domains as revealed by Os model ages, with ancient crust at least Mesoproterozoic in age and predating formation of the CAOB significantly reduces the volume of new juvenile crust generated during the orogeny. We conclude that the volume of truly juvenile crustal material in the CAOB is about 20%, similar to that in other accretionary orogens through Earth history, and considering the ca. 800 Ma history of the belt this is not anomalous.

Petrogenesis and geodynamic evolution of the Kajan Neogene subvolcanic rocks, Nain, Central Iran

Kajan subvolcanic rocks in the Urumieh–Dokhtar magmatic arc (UDMA), Central Iran, form a Late Miocene-Pliocene shallow-level intrusion. These subvolcanics correspond to a variety of intermediate and felsic rocks, comprising quartz diorite, quartz monzodiorite, tonalite and granite. These lithologies are medium-K calc-alkaline, with SiO2 (wt.%) varying from 52% (wt.%) to 75 (wt.%). The major element chemical data also show that MgO, CaO, TiO2, P2O5, MnO, Al2O3 and Fe2O3 define linear trends with negative slopes against SiO2, whilst Na2O and K2O are positively correlated with silica. Contents of incompatible trace elements (e.g. Ba, Rb, Nb, La and Zr) become higher with increasing SiO2, whereas Sr shows an opposite behaviour. Chondrite-normalized multi-element patterns show enrichment in LILE relative to HFSE and troughs in Nb, P and Ti. These observations are typical of subduction related magmas that formed in an active continental margin. The Kajan rocks show a strong affinity with calc-alkaline arc magmas, confirmed by REE fractionation (LaN/YbN=4.5–6.4) with moderate HREE fractionation (SmN/YbN=1.08–1.57). The negative Eu anomaly (Eu/Eu* <1), the low to moderate Sr content (<400ppm) and the Dy/Yb values reflect plagioclase and hornblende (+- clinopyroxene) fractionation from a calc-alkaline melt Whole–rock Sr and Nd isotope analyses show that the 87Sr/86Sr initial ratios vary from 0.704432 to 0.705989, and the 143Nd/144Nd initial ratios go from 0.512722 to 0.512813. All the studied samples have similar Sr-Nd isotopes, indicating an origin from a similar source, with granite samples that has more radiogenic Sr and low radiogenic Nd isotopes, suggesting a minor interaction with upper crust during magma ascent. The Kajan subvolcanic rocks plot within the depleted mantle quadrant of the conventional Sr-Nd isotope diagram, a compositional region corresponding to mantle-derived igneous rocks.

Early Mesozoic Mo mineralization in the Qinling Orogen: An overview

As one of the most important Mo provinces in the world, the Qinling Orogen is host to mainly Late Mesozoic (Late Jurassic–Early Cretaceous) porphyry Mo systems. Recently, additional Early Mesozoic (mainly Triassic plus minor Early Jurassic) Mo deposits have been identified, which occur as fault-controlled quartz veins or as carbonatite veins and as such, they are fundamentally different from the commonly observed porphyry systems. Furthermore, Early Mesozoic porphyry systems are also present but they are different from those of Late Mesozoic age. Thus, Early Mesozoic Mo systems not only offer a new and comprehensive view of the Mo metallogenesis, but also provide unique insight into the tectonic evolution of the Qinling Orogen taking into consideration the role of mineral deposits as ideal probes of the geodynamic evolution.The known Early Mesozoic Mo deposits in Qinling can be classified into three types based on their geological and geochemical features: porphyry, carbonatite, and orogenic systems. Porphyry Mo systems, including porphyry, porphyry-skarn and intrusion-related veins, are spatially and genetically associated with granitic porphyries. They display typical porphyry type alterations and are formed by magmatic water. Carbonatites are considered as mantle-derived igneous rocks, and Mo mineralization is associated with magmatic fluids. Orogenic Mo systems occur as fault-controlled quartz lodes, and the ore-forming fluids are medium-high temperature, CO2-rich metamorphic fluids. Spatially, Early Mesozoic porphyry Mo systems are concentrated in the western and middle part of the Qinling Orogen, whereas carbonatite veins and orogenic Mo systems are located in the northeastern part. Temporally, orogenic Mo deposits were the earliest (mainly between 220 and 250Ma), followed by carbonatite veins (mainly between 205 and 225Ma) and porphyry Mo systems (mainly between 190 and 205Ma). With some exceptions, there is generally a trend of decreasing deposit ages toward the western part of the orogen. Detailed comparison reveals that the Early Mesozoic porphyry Mo systems in the Qinling Orogen are different from the Late Mesozoic ones in terms of their economic importance, spatial distribution, ore-causative intrusions, hydrothermal alteration and the ore-forming fluids.To explain the above geological and geochemical features, the Triassic Qinling is considered as an active continental margin caused by the northward subduction of the Mian–Lue oceanic plate. A transition from continental arc to back-arc basin is suggested for the formation of Triassic orogenic and carbonatite vein Mo deposits, whereas continental arc is host to the Early Mesozoic porphyry Mo systems.

The Orekhov–Pavlograd Zone, Ukrainian Shield: Milestones of its evolutionary history and constraints for tectonic models

The Orekhov–Pavlograd Zone (OPZ), which is located in the Ukrainian Shield, represents the reworked western margin of the Archaean Azov Province and separates it from the Archaean Middle Dnieper Province. In the Vasilkovka area, which is located in the northern OPZ, Archaean tonalites and amphibolites and Palaeoproterozoic quartzites, high-Al paragneisses, and mafic granulites were studied. Structural observations allow the OPZ to be interpreted as a steep transpressional ductile shear zone. The tonalites and amphibolites were dated at 3.50 and 2.88Ga, respectively (SIMS zircon ages hereafter). The metasedimentary rocks originated from mature sediments from Archaean continental sources and were deposited in a shallow intracratonic basin. The basaltic protoliths of the mafic granulites were produced by a mantle-plume-related source in a quiescent intraplate setting. Several metamorphic events were recognised (3.5, 3.4, 3.3, 2.85 and 2.1–2.0Ga). The main metamorphic reworking occurred in the Palaeoproterozoic; its onset was dated at 2.08–2.07Ga, and it ended at 2.00–2.01Ga. The earliest Palaeoproterozoic metamorphism occurred at P=ca. 8.5kbar and T=ca. 600°C, then at P=5–7kbar and the peak metamorphic temperature T=830°C, and the final stage was at T=ca. 600°C and P=3.3–4kbar (based on the TWEEQU and THERIAK-DOMINO methods), which suggests a clockwise P–T–t loop. In small shear zones, the Archaean tonalites were converted to garnet–biotite gneisses that inherited the tonalitic geochemistry and Palaeoarchaean magmatic zircons. Subduction-related igneous rocks and accretionary prism sediments are absent in both the Vasilkovka area and the entire AP, which makes subduction-related models questionable. An alternative tectonic model for the OPZ could be an intracratonic (intraplate) ductile shear zone that developed due to far-field stresses due to the tectonic juxtaposition of the Sarmatia and Volgo-Uralia continental blocks, which were parts of the East European Craton. Regional metamorphism and the coeval emplacement of mantle-derived igneous rocks and crust-derived granites are suggested to have been produced by magmatic underplating at ca. 2.02–2.10Ga. This model needs to be developed further.

Reply to Comment on “U–Pb baddeleyite ages and geochemistry of dolerite dykes in the Bas-Drâa inlier of the Anti-Atlas of Morocco: Newly identified 1380 Ma event in the West African Craton” by André Michard and Dominique Gasquet

Baddeleyite (ZrO2) is today recognized as the key mineral for dating the emplacement of mantle-derived igneous rocks, primarily gabbro and dolerite dykes and sills. The comment by Michard and Gasquet calls for a discussion on the significance of baddeleyite U–Pb TIMS dates of two dykes in the Bas-Drâa inlier, dated at 1381±8 and 1384±6Ma, presented in El Bahat et al. (2013-in this issue). They suggest that the U–Pb systematics were “seriously perturbed in the Bas-Drâa region” with the main argument being a lack of age matches between the ca. 1385Ma dykes in Bas-Drâa and the 2040, 1650 and 885Ma dykes in adjacent inliers to the east. To infer an older age for these dykes, as suggested by Michard and Gasquet, and explain the ca. 1385Ma TIMS results would require two separate events of perturbation, in the Mesoproterozoic and a subsequent Variscan event. However, such an alternative is improbable from basic theoretical reasoning using U–Pb isotopic systematics and our current understanding of the behaviour of baddeleyite in metamorphic systems. In addition, we present new LA-ICPMS U–Pb age data of baddeleyite from one of the dated dykes from Bas-Drâa. The new LA-ICPMS U–Pb age of 1416±7Ma is slightly but significantly older than our TIMS 1384±6Ma upper intercept age, nevertheless confirming this dyke could not correspond to any of the other mafic intrusions dated in the inliers to the east. The LA-ICPMS result also contributes to a deeper understanding on the effects of secondary processes in baddeleyite in general, and suggests a modified methodological approach for samples with complex geological histories.

Noble gas and halogen constraints on regionally extensive mid-crustal Na–Ca metasomatism, the Proterozoic Eastern Mount Isa Block, Australia

Fluid inclusions in late-Isan quartz veins associated with regional Na–Ca alteration (albitisation), in the Mary Kathleen Fold Belt and the Cloncurry District of the Eastern Mt Isa Block, have been analysed for naturally occurring and neutron produced isotopes of Ar, Kr and Xe. The noble gases have been extracted using a thermal decrepitation procedure that enables partial deconvolution of the different fluid inclusion types, including variably saline aqueous, liquid carbon-dioxide and mixed aqueous-carbonic varieties. The variably saline (<5–65 wt%) aqueous fluid inclusions dominate and have 40Ar/ 36Ar values of less than 2700 in most of the samples from across the region. These fluid inclusions have extremely variable molar Br/Cl values of 0.3–4 × 10 −3 and I/Cl values of 0.2–35 × 10 −6, and the fluids are interpreted to represent sedimentary formation waters derived from the upper crust that have dissolved variable quantities of halite (or scapolite) to achieve their ultra-high salinity. Fluid inclusions in a sample from the Snake Creek anticline in the Cloncurry District have the highest 40Ar/ 36Ar value of ∼25,000 demonstrating a deep magmatic or metamorphic fluid origin in this case. A magmatic origin is favoured because this fluid is very similar to a fluid involved in Iron-Oxide-Copper-Gold (IOCG) mineralization at Ernest Henry that was also interpreted to have a magmatic origin, and because the aqueous fluid inclusions have Br/Cl of ∼1–2 × 10 −3 and I/Cl of ∼10 × 10 −6 that are similar to mantle-derived igneous rocks. Carbon-dioxide fluid inclusions dominate samples from the Knobby Quarry in the Mary Kathleen Fold Belt and have a maximum 40Ar/ 36Ar value of 6000–7000. These fluid inclusions are estimated to have a 36Ar concentration of 1–4 ppb, that is similar to the range determined for aqueous fluid inclusions in all the other samples. In addition, aqueous and carbonic fluid inclusions in all the samples have similar 129Xe/ 36Ar plus 84Kr/ 36Ar values that are unfractionated, and close to the air—Air Saturated Water (ASW) range. These data are interpreted to indicate an independent and dominantly metamorphic origin for CO 2, and the presence of mixed aqueous-carbonic fluid inclusions are attributed to fluid mixing and/or mingling, rather than fluid unmixing. However, the data do not preclude the presence of a minor magmatic CO 2 component in the samples from the Knobby Quarry. Fluid inclusions in most of the samples from the Mary Kathleen Fold Belt have higher Br/Cl and I/Cl values, higher 36Ar concentrations and lower maximum 40Ar/ 36Ar values than fluid inclusions in samples from the Cloncurry District. This suggests Na–Ca alteration in these different parts of the Eastern Mt Isa Block occurred independently. However, fluid inclusions associated with Na–Ca alteration in the Cloncurry District have a very similar composition to fluid inclusions in IOCG mineralization-stage quartz veins from Ernest Henry. These data are therefore compatible with a genetic relationship between regional Na–Ca alteration and IOCG mineralization in the Cloncurry District. In both cases the ultra-saline hydrothermal fluids had a dominant origin from sedimentary formation water, but are interpreted to contain a magmatic component sourced from the late-Isan Williams-Naraku Batholiths which may have driven fluid convection.

Decoupling of surface and subsurface sutures in the Dabie orogen and a continent-collisional lithospheric-wedging model: Sr-Nd-Pb isotopic evidences of Mesozoic igneous rocks in eastern China

There are significant differences of Nd and Pb isotopic compositions between Mesozoic mafic igneous rocks from the North China Block (NCB) and the South China Block (SCB). Mesozoic mantle-derived igneous rocks from the North China Block have very lowɛNd values (−15 to −21), and206Pb/204Pb ratios ( −10 and206Pb/204Pb > 18.3. The very lowɛNd values (−16 to −20) and206Pb/204Pb ratios (< 17.3) of the early Cretaceous mafic-ultramafic intrusions developed in the north part of the Dabie orogen (NDZ) suggest that the deep lithosphere underneath the NDZ belongs to the NCB but not the SCB. Therefore, although the surface suture between the NCB and SCB is located on the north side of the NDZ, the subsurface suture between the NCB and SCB should be located to the south side of the NDZ. This is consistent with the previous suggestion that the subsurface suture in the Sulu terrane east of the Tanlu fault was the south displacement, but contradictory to northward continental subduction of the SCB. A. continent-collisional lithospheric-wedging model can interpret the decoupling of the surface and subsurface sutures in the Dabie-Sulu orogen. After slab break-off, the continuing convergence of two continental blocks must increase the compression force acting on the suture zone, which might induce the lithosphere splitting of SCB. Thus, the lower crust and lithospheric mantle on the south margin of the NCB can wedge into the north margin of the lithosphere of the SCB along the Dabie-Sulu collision zone. This process caused the overthrust of the mid-upper continental crust with exhumed ultrahigh pressure meta-morphic (UHPM) rocks and underthrust of the deep litho-sphere of the SCB. It could be an important mechanism responsible for the second rapid cooling and uplifting of the UHPM rocks and lithospheric delamination as well as the corresponding magmatism in Jurassic in the Dabie orogen. The southward movement of subsurface suture in the Dabie-Sulu orogen may also provide a tectonic setting in depth for the large-scale Mesozoic magmatic-metallogenesis along the middle-lower reaches of the Yangtse River in eastern China.

An Early Oxidized Mantle

GEOCHEMISTRY Oxygen is the most abundant element in Earth, but its distribution and state (or fugacity) in the mantle and atmosphere have varied over time with important consequences for Earth's geologic and biologic history. For example, degassing of the mantle through melting and volcanism is thought to have affected the composition of the atmosphere. To examine the mantle aspect of this coupled evolutionary process, Canil studied the partitioning of vanadium in mantle minerals and melts and in a suite of mantle-derived igneous rocks stretching back to the Archean. Vanadium can exist in several oxygenation states and partitions differently in melts in response to the local oxygen fugacity and mineralogy. The vanadium data imply that Earth's mantle was just as oxidized in the Archean (about 3.5 billion years ago) as it is today. Previous studies have supported a later increase of atmospheric oxygen abundance; thus, this result challenges proposed mechanisms for producing a gradual and later oxidation of Earth's atmosphere that are based on an early reduced mantle. — BH Earth Planet. Sci. Lett. 195 , 75 (2002).

The Cenozoic mantle magmatism and motion of lithosphere on the north margin of the Tibetan Plateau

Geodynamic properties and evolution of the lithosphere on the north margin of the Tibetan Plateau are recently hot topics to geoscientists in the world. Have the northern plates been subducting underneath the Plateau? It is still an unsolved problem. One of the keys to solving this problem is to understand the genetic processes of Cenozoic magmas on the north margin of the Tibetan Plateau. However, there is no enough evidence supporting the subduction model. In contrast, a series of evidence indicates that collision-induced huge shearing faults and large-scale crust shortening played a main role in lithosphere motion on the north margin of the Tibetan Plateau. The mantle-derived igneous rocks strictly distribute at the intersections of large strike-slip faults on the north margin of the Plateau. Generation of magmas may be related to local extensional condition induced by strike-slipping faults, which lead to lithosphere gravitational instability and collapse, as well as upwelling of the deep hot material. Heat induced by shearing and carried by upwelling hot material may cause partial melting on H2O-bearing mantle.

Helium and argon isotopic geochemistry of Jinding superlarge Pb-Zn deposit

The study results of He and Ar isotopes from fluid inclusions in pyrites formed during mineralization stage of Jinding superlarge Pb-Zn deposit in west Yunnan, China are reported. The data show that the40Ar/36Ar and3He/4He ratios of fluid inclusions are respectively in the range of 301. 7–385. 7 and 0. 03–0.06Ra, suggesting the oreforming fluid is a kind of air saturated meteoric groundwater. On the basis of research on coupled relationships among He, Ar, S and Pb isotopes, the evolution history of ore-forming fluid of the deposit can be summarized as (i) air saturated meteogenic groundwater infiltrated down and was heated→ (ii) leached S, C and radiogenic He, Ar from the basinal strata → (iii) leached Pb and Zn from mantle-derived igneous rocks located in the bottom of the basin→ (iv) ore-forming fluid ascended and formed the deposit. Due to this process, the isotope signatures of crustal radiogenic He, atmospheric Ar (with partial radiogenic40Ar), crustal S and mantle-derived Pb remained in the ore-forming fluid.

Strontium and neodynium isotope ratios in the Fraser River, British Columbia: a riverine transect across the Cordilleran orogen

The Fraser is the largest undammed river to traverse the North American Cordillera. Its source is in Precambrian/Palaeozoic sedimentary rocks of the Rocky Mountains, at the margin of the North American Craton, from where it flows west across accreted ensimatic terranes, to the young magmatic arc of the Coast Range. 87 Sr 86 Sr and 143 Nd 144 Nd in suspended matter from the Fraser and its tributaries outline a mixing curve between two end members, (a) LILE-rich cratonic material with 87 Sr 86 Sr ∼ 0.77 and ϵ Nd (0) ∼ −23 and an Archaean crustal residence age and (b) material from mantle-derived igneous rocks with 87 Sr 86 Sr ∼ 0.704 and ϵ Nd (0) ∼ +4 and a crustal residence age of ∼ 0.6 Ga. In the eastern, ensialic region, dissolved Sr is less radiogenic than Sr in suspended matter, because of the contribution of dissolved Sr from Rb-poor carbonate rocks. As the Fraser flows west over younger terranes, this relationship inverts, with dissolved Sr more radiogenic. Inversion reflects a difference in the transport behaviour of dissolved and suspended material. Dissolved Sr from upstream, with a radiogenic signature, remains in solution, whereas the suspended load from upstream tends to fall out where the current decreases, to be replaced by local, less radiogenic material. For riverborne fluxes entering the ocean at active margins, compositions of conservative dissolved elements, such as Sr, represent both the interior craton and the outer orogen, while elements carried mainly in suspended form, such as Nd, are biased towards the outer orogen.

The hydrothermal fluid of Archæan lode-gold deposits at different metamorphic grades: compositional constraints from ore and wallrock alteration assemblages

A low-salinity, mixed aqueous-carbonic fluid is common to all Archaean lode-gold deposits throughout the range of mineralising conditions from sub-greenschist to lower-granulite facies temperatures. Alteration assemblages and fluid-inclusion data give constraints on the fluid composition. Fluid XCO2 is 0.1–0.3 in typical greenschist-facies (mesothermal) deposits. At higher temperatures, the assemblages are consistent with formation from a fluid of similar composition, but slightly higher or lower XCO2 cannot be ruled out, and fluid-inclusion data indicate that CH4 may be an important component in ore fluids at these temperatures. Fluid pH is neutral or weakly alkaline at all conditions. A range of relative oxidation states of four orders of magnitude fO2 is indicated at any temperature, with deposits more oxidising relative to QFM at lower temperature. Sulphur contents of the fluids vary from ≈ 10 to 10−3.5m∑S, with a trend towards lower sulphur contents at lower temperatures. The relative concentrations of major cations in solution are similar at all conditions with Na ≫ K ≥ Ca, although Ca may be less abundant at low temperatures. The broad similarities in ore-fluid composition at all temperatures give support to ‘crustal-continuum’ models, in which Archaean lode-gold mineralisation involved either a single fluid moving through the middle and upper crust, or derivation of ore fluids by similar processes at different crustal levels. Many of the compositional differences between high- and lowtemperature ore-fluids may be attributed to evolution of deep-sourced hydrothermal solutions as they rise along structurally-controlled conduits. The constancy of major ore-fluid component concentration (e.g. CO2, Cl, ± K) suggests fluid-buffering and high fluid-rock ratios along fluid pathways. Fluid-buffered conditions can also explain the ore-fluid fO2-temperature relations; with equilibria between oxidised and reduced aqueous carbon or sulphur species controlling the oxidation state. In contrast, the concentrations of components present in lesser abundance in Archaean gold ore-fluids (e.g. S, Ca, H+) were probably controlled either by saturation of one or more mineral phases brought on by decreasing temperature, or were rock-buffered through fluid-rock reactions. Extrapolation to high temperatures of the K, Na and Ca contents of the gold-bearing fluids indicates that their composition is consistent with derivation from, or final equilibration with, rocks of intermediate-granitic composition, thus giving support to isotopic and geological arguments for ore-fluid source regions external to the greenstone belts. The fluid oxidation states are characteristic of a wide range of potential source rocks, including mantle-derived igneous rocks, calc-alkaline granitoids and magmas, and seaflooraltered metabasalts. Strongly oxidised magmatic sources or unusually oxidising source processes (e.g. CO2-streaming during granulitisation of the lower crust) are therefore not required in the genesis of Arch≸an lode-gold deposits.

Tectonics and heat sources for granulite metamorphism of supracrustal-bearing terranes

There are generally three important processes in the granulite-facies metamorphism of supracrustal rocks: (1) transport from the surface to 15–30+ km, (2) heating to 700–800+ °C, and (3) transport back to the surface and re-exposure at the top of a crust of “normal” thickness (35–40 km). Mantle-derived magmatism, and/or magmatic and/or metasomatic fluid flow, can transport the necessary heat for granulite metamorphism, but many granulite terranes do not contain sufficient volumes of mafic intrusive rocks, or evidence of extensive fluid flow, to readily explain heating by this mechanism. One possible process for the formation of large granulite terranes involves continental collision and crustal doubling by simple-shear thrusting. Heating of the underthrust supracrustal rocks would occur by thermal relaxation, but, if granulite-facies temperatures are to be attained at mid-crustal levels, this mechanism necessarily produces extensive melting of the lower crustal plate, suggesting that granulite terranes formed in this way should contain substantial syn- or post-metamorphic crustally-derived intrusions. In this situation, subsequent uplift and re-exposure at the surface is a natural consequence of erosion and isostatic readjustment. An alternative process which may allow the heating of supracrustal rocks to granulite-facies temperatures is imbricate thrusting in which thin slivers or “flakes” of supracrustal rocks are thrust down to the lowermost crust. This mechanism requires a separate tectonic event, such as a second thrusting event, to transport the granulite-facies rocks back to the surface. Other possible heat sources for supracrustal granulite terranes include preheating of the overthrust crust by arc or pre-orogenic magmatism, and shear heating on thrust planes. Models indicate that pre-heating of the overthrust plate has only minor thermal effects upon the thermal history of the underthrust crust, although shear heating may be significant. Heat sources for granulite metamorphism of the Archean Limpopo Belt of South Africa are considered in terms of possible tectonic mechanisms. The metamorphic grade and presence of abundant supracrustal rocks in this terrane demonstrate that it was once at the Earth's surface, buried to ∼ 8.5 kbar (∼ 28 km) and heated to ∼ 850°C, then re-exposed at the surface. There are no recognizable mantle-derived igneous rocks of suitable age, and no evidence for extensive fluid flow that could have provided a magmatic heat source for the metamorphism. Shear heating is unlikely because the pattern of metamorphic isograds is not related to faults or shear zones. Imbricate thrusting of thin silvers of supracrustal rocks into the lowermost crust to allow heating without magmatism would require a separate tectonic event to transport the granulites to the surface; there is no evidence for such a later event. The most satisfactory and self-consistent model for Limpopo metamorphism involves thermal relaxation of a perturbed temperature profile produced by Tibetan-style continental collision of the Zimbabwe and Kaapvaal Cratons, with consequent crustal doubling to at least 65 km thick, and the generation of crustal melts which advected heat into the zone of granulite metamorphism. There is no evidence for significant pre-collisional heating of either crustal plate; both probably had typical “shield-type” pre-collisional geothermal gradients. Crustally-derived magmas, therefore, must have been produced during thermal and structural equilibration of the Limpopo collision. Abundant granitoid rocks of similar age to the metamorphism occur both as migmatites and as discrete plutons.

Homogenization and lowering of 18O/ 16O in mid-crustal rocks during extension-related magmatism in eastern Nevada

The oxygen isotope systematics of Tertiary volcanic rocks of east-central Nevada and of plutonic and metamorphic rocks of the Ruby Mountains-East Humboldt Range core complex provide complementary evidence for major 18O-depletion and 18O/ 16O homogenization of mid-crustal rocks during metamorphism and magmatism. The δ 18O value of crustal source material for silicic volcanic rocks decreased from between +9 and +11‰ to between +7 and +8‰ over ∼ 5 Ma. Mid-crustal metasedimentary and granitic rocks in the East Humboldt Range have δ 18O values very similar to the volcanic rocks and values are lower and more homogeneous at deeper structural levels. Exchange with deep-seated mantle-derived igneous rocks, or fluids derived therefrom, is the most plausible 18O-depletion mechanism. Intrusion of these mafic magmas promoted crustal melting and fluid migration. Homogenization of 18O/ 16O resulted from migration of high-temperature fluids and melts at mid-crustal levels, and was less effective at higher structural levels where the crust was dominated by less permeable carbonate rocks.