Trace Element Geochemical Data Research Articles

The Merensky-Bastard interval at Hackney, eastern Bushveld Complex: results of a combined Sr–Nd-isotopic investigation

The Merensky and Bastard reefs of the Bushveld Complex occur within what has been called a transitional macro-unit along the boundary of the Critical and Main zones. The transitional unit is characterised by a geochemical hiatus recording distinct inflections in mineral chemistry and isotopic compositions. Previously these inflections in mineral chemistry and changes in isotopic compositions were attributed mostly to the influx of a magma that was compositionally distinct from the resident magma and that was parental to the Main Zone of the complex. Sr-isotopic variations across this interval have been particularly well-studied, but despite this, little consensus exists regarding the petrogenesis and metallogenesis of this economically important interval. Here we report whole-rock Sr–Nd-isotopic, major- and trace element geochemical and mineral chemical data across the Merensky-Bastard interval as intersected by borehole BH8172 on the farm Hackney in the eastern Bushveld Complex. Variations in whole-rock Cr/MgO values and initial Sr isotopic ratios across the interval are consistent with the results of previous studies that argued for the co-accumulation of minerals from compositionally and isotopically distinct magmas, of Critical and Main Zone lineages, respectively. In our model, a magma of Critical Zone affinity enters the chamber causing erosion along the chamber floor. Orthopyroxene and plagioclase crystallise from the Critical Zone magma to form the Merensky Reef, as suggested by high whole-rock Cr/MgO ratios (> 80) and unradiogenic Sr-isotopic compositions (87Sr/86Sri < 0.7068). A plagioclase-laden magma of Main Zone affinity subsequently intruded the chamber as a basal flow, elevating the resident Critical Zone magma. Plagioclase within the former floated, forming a solid raft onto which the Bastard Reef was deposited, a model that is entirely consistent with density considerations and an upward increase in the An-content of plagioclase as observed in the anorthositic package between the Merensky and Bastard reefs. From a metallogenetic viewpoint, this would imply that the Main Zone could not have been the source of the PGEs within the Merensky Reef.

Biogeochemical responses to global warming during the Paleocene–Eocene Thermal Maximum in the eastern Tethys

The Paleocene–Eocene Thermal Maximum (PETM; ∼56 Ma) represents a rapid and sustained climatic perturbation that lasted for ∼170 kyr and coincided with the emplacement of the North Atlantic Igneous Province (NAIP). Global average temperature increased by 5 to 9 °C, which was likely triggered by a rapid emission of 13C-depleted CO2. Numerous sites from the deep oceans have been studied for the PETM, but subtropical shallow-water eastern Tethys remains poorly understood despite its importance in regulating heat and moisture transport. Prior studies on calcareous nannofossil biostratigraphy and stable isotopes of marine carbonates confirmed the occurrence of the PETM in the Tarim Basin of the eastern Tethys in northwestern China. Here we present new, high–resolution major, trace and rare earth element geochemical data, and clay mineral records at the Kuzigongsu section in the Tarim Basin to assess the biogeochemical responses of the eastern Tethys to climatic forcings across the PETM. Chemical weathering proxies (chemical index of alteration [CIA], chemical index of weathering [CIW], plagioclase index of alteration [PIA], and Rb/Sr) support an enhanced terrestrial input shortly after the PETM onset, possibly due to intensified chemical weathering. Furthermore, clay mineral assemblage is dominated by illite and illite/smectite during the PETM, implying increased physical weathering. The PETM is also characterized by an increase in nutrient-sensitive elements such as P, Ni, and Cu and higher abundance of nutrient–sensitive calcareous nannofossils, suggesting an intensification of marine primary productivity. Meanwhile, the Tarim Basin may have encountered episodic bottom water deoxygenation supported by an increase in the enrichment factor of redox–sensitive elements such as U and V. The integrated geochemical proxy records suggest that the eastern Tethys has encountered profound ecosystem stress imposed by elevated nutrient fluxes and ocean deoxygenation, which may have been amplified by simultaneously intensified chemical and physical weathering during the PETM.

The anisotropy of geomaterial granite

Abstract. Granites appear to be isotropic, which qualifies them as suitable crystalline host rocks for nuclear waste repository sites. However, despite their optical appearance, granites show a primary structural anisotropy (Bouchez, 1997) that evolved during emplacement and crystallization of the melt. The major processes involved are magmatic flow and oriented crystal growth (Müller et al., 2011). Hypothetically, it is expected that different tectonic environments, i.e. different orientations of the stress tensor, cause significant differences in the primary anisotropy, which is expressed by the crystallographic preferred orientation (CPO) of the rock-forming minerals. It is likely that primary anisotropic petrophysical properties control the orientation of post-magmatic structural features like extensional fractures and thus shape potential fluid pathways. We present the first results of a systematic study of felsic plutonites, i.e. the GAME project (Gefüge, Textur- und Anisotropie-Messungen von potenziell für die Endlagerung geeigneten Graniten zur Charakterisierung möglicher Fluidwegsamkeiten). The samples of syn-Variscan felsic plutons from two sites (Erzgebirge and Fichtelgebirge) represent different tectonic settings during intrusion: extension and compression. Furthermore, they depict different stages of fractionation of the peraluminous granite suites. The CPOs were analysed using the neutron time-of-flight (ToF) texture diffractometer SKAT (Keppler et al., 2014; Ullemeyer et al., 1998) and EBSD (electron backscatter diffraction). Using scanning electron microscope (SEM) automated mineral liberation analysis (Schulz et al., 2020), modal mineral compositions are quantified. This enables us to model primary or “intrinsic” petrophysical properties for these granites based on the elastic stiffness tensor of the individual rock-forming minerals (Mainprice et al., 2011). Main- and trace-element geochemical data (ICP-AES and ICP-MS) allow for a characterization of the different magmatic settings of the samples. All granites show distinct preferred orientations of rock-forming minerals. The quartz textures, for example, exhibit similar CPOs, with point maxima of the positive rhombs in combination with small circles to crossed-girdle c-axis distributions. However, the orientation with respect to the geographic reference system strongly varies. We will discuss the CPOs in relation to the stress tensor orientation during emplacement of the felsic plutons and compare the primary anisotropy with the post-magmatic fracture patterns of the particular granites.

Marble trace element and C-O isotope geochemistry of the Paleoproterozoic Jingshan Group, North China: Insights into BIF formation during the Lomagundi-Jatuli Event

The North China Craton, one of the oldest continental blocks in the world, comprises Paleoproterozoic metamorphic sedimentary formations that record the effects of multiple major events in early Earth history, including the Great Oxidation Event (GOE) and its consequent Lomagundi-Jatuli carbon isotope event (LJE). The lack of focused research on the geological record of LJE-equivalent rocks of the Jiaodong Peninsula, Shandong Province, eastern China, has hindered our understanding of the Precambrian atmospheric oxygen evolution recorded by rocks of the North China Craton. This paper considers carbon and oxygen isotope and trace element geochemical data recovered from marble deposits of the Paleoproterozoic Jingshan Group (2.20–1.90 Ga) of the Jiaolai Basin. The investigated strata display a pronounced positive δ13CV-PDB excursion greater than 4.2 ‰. δ13CV-PDB and δ18OV-SMOW values range from −3.7 ‰ to 5.4 ‰ and 4.5 ‰ to 18.7 ‰, respectively. Positive co-variance of carbon and oxygen isotope values of the analyzed Jingshan Group samples is interpreted to reflect the effects of metamorphism and water–rock reactions. Marble deposits of the Jingshan Group experienced recrystallization, metamorphism, and silicification resulting in strong decarbonation. High amphibolite facies metamorphism may have reduced δ13CV-PDB by 3 ‰ to 5 ‰. Thus, estimated original sedimentary δ13CV-PDB values of as great as +8.4 ‰ suggest the existence of a strong positive anomaly that may reflect the global Lomagundi-Jatuli event. Rare earth and other trace element geochemical results of the present study suggest that banded iron formation (BIF) deposits associated with the Jingshan Group marble formed in a redox stratified ocean in the Jiaolai Basin dominated by ferruginous conditions. Moreover, our results suggest that positive carbon isotope anomaly associated with the LJE is largely constrained to sediment deposited in shallow and coastal marine environments.

Barrovian metamorphism in nominally lowermost Alpujarride Complex units: tectonic implications for the development of the orogenic wedge in the Western Mediterranean

ABSTRACT The sedimentary successions of the Permian-Triassic rifting related to the Alpine Tethys in the easternmost Lower Alpujarride units (Internal Betic Zone, south Spain) host shallow-intrusive basic bodies of Mesozoic age. These gabbroic bodies experienced low-grade Alpine metamorphic overprint, singularly recorded in three outcrops nearby Alicante city in southeast Spain: Callosa and Orihuela Ranges and Nueva Tabarca Island. Overall, the metabasic bodies still preserve magmatic ophitic/subophitic texture and relicts of magmatic augite and ferropargasite-ferroedenite-edenite and kaersutite. In contrast, metamorphic minerals include magnesioriebeckite, actinolite, albite, stilpnomelane, phengite, and chlorite. Major and trace element geochemical data suggest crystallization of the igneous protoliths from subalkaline (tholeiitic) to alkaline within-plate (continental) basaltic magmas slightly enriched in incompatible elements. These magmas with E-MORB geochemical affinity very likely intruded at shallow crustal levels in an extensional-synrift-geodynamic setting related to the breakoff of Pangea in the region. Pressure-Temperature (P-T) conditions estimated by average P-T method and pseudosection calculations suggest metamorphic peak at ca. 6 kbar at 340°C consistent with burial metamorphism down to ca. 18 km depth with intermediate thermal gradients of ca. 18°C/km. These conditions are different to the typical subduction-related blueschist facies conditions of the Lower Alpujarride units and suggest, instead, a collisional (Barrovian) event. This can be explained by a late-stage accretion of the studied units after the main subduction-related event, pointing to the dispersion of the corresponding Lower Alpujarride sections along the margin in different palaeogeographic positions before deformation and movement to their actual structural position. The metamorphic correlation of the studied units with the accreted passive margin of South Iberia (Subbetic zone) is also discussed.

Precise Pb-Pb baddeleyite ages and petrogenesis of the NW-SE trending Paleoproterozoic mafic dyke swarm from the Bundelkhand craton, India: Implications to the enriched sub-continental lithospheric mantle

Based on a similar structural trend, the NW-SE trending mafic dykes from the Bundelkhand craton were assigned an emplacement age of 1979 ± 8 Ma. Many geochemistry and paleomagnetic studies on these dykes have proposed the presence of more than one mafic dyke event within the NW-SE trending mafic dyke swarm. Still, until now, this suggestion was not supported by precise geochronology. This study presents TE-TIMS Pb-Pb baddeleyite ages of 2001.5±1.3 Ma, 2005.0±0.8 Ma, and 2004.5±0.7 Ma from the NW-SE trending dykes of Ajaygarh dyke swarm located towards the southeast margin of the Bundelkhand craton. The mean age of 2004.3±3.5 Ma is inferred as the emplacement age of these dykes. This dyke event is hitherto unknown and the oldest in the Bundelkhand craton. Further, we present the major and trace element geochemistry data on 34 samples from NW-SE trending dykes. These results suggest the presence of three geochemically distinct groups of dykes from NW-SE trending dykes. Further, the geochemical modeling of studied dykes suggests their derivation from subcontinental lithospheric mantle sources enriched by subduction-derived fluids at different depths and times. These dykes were emplaced as part of a sporadically spread global crustal extension event.

Trace element variations of pyrite in orogenic gold deposits: Constraints from big data and machine learning

Orogenic gold deposits represent key global gold resources, yet its internal differences are poorly constrained. Pyrite is ubiquitous in orogenic gold ores, and its trace element geochemistry has significant implications for the ore-forming processes. However, the diffusion regularity and controlling factors of pyrite trace element geochemistry of orogenic gold deposits lack effective constraints. To address these issues, pyrite trace element geochemical data (4092 sets of data) from 67 orogenic gold deposits were compiled for big- data and machine learning analyses. The principal component analysis (PCA) divides the pyrite data into four groups according to trace element correlations: I) Au-As; II) Sb-Cu; III) Ag-Pb-Bi; and IV) Co-Ni-Te-Se-Mo. This division is likely ascribed to the element occurrence in pyrite. Partial least squares-discriminant analysis (PLS-DA) and Random Forests classification were used to identify pyrite trace elemental variations among orogenic deposits of different mineralization ages, wallrocks, and tectono-metallogenic settings. The results show that the influence on pyrite geochemistry from temporal variation is not obvious, and only Triassic and Jurassic orogenic-type auriferous pyrite shows enrichments of Cu, Pb, and Sb, which may reflect the multiple mineralization events in these deposits as Pb commonly occurs as cation in the fluids, while Sb usually occurs in the melt. The spatial difference is more distinct with less overlapping of confidence circles in PLS-DA and higher accuracy in Random Forests, which may have been associated with fluid source variation influenced by the geodynamic setting (mantle model, supposed by distinctive geodynamic variation from metallogenic belts in China) and source strata (metamorphic model, supposed by Archean deposits mostly hosted in green-schist phase besides China). Pyrite in orogenic gold ores hosted in different wallrocks can be differentiated by the different As, Pb, and Cu contents, as the contents of these elements vary widely in the wallrocks and participate in pyrite lattice through fluid-rock reactions. Random Forests regression reveals that pyrite As-Sb contents in pyrite show distinct negative correlations with ore-forming temperature, but Se content shows a non-linear influence. This suggests that the pyrite As-Sb-Se contents in orogenic gold ores can be used as a geothermometer. Furthermore, we suggest that the pyrite Co-Ni contents are not good indicators for the ore-forming temperature, as the low salinity fluid in orogenic gold systems makes Co-Ni difficult to form chloride complexes which is sensitive to temperature variation.

Compositions and ages of Early Cretaceous volcanic and plutonic rocks in central Tibet: Insights into the magmatic and uplift response to slab breakoff

AbstractWe present new zircon U-Pb ages and Hf isotope compositions as well as whole-rock major- and trace-element geochemical and Sr-Nd isotopic data for silicic plutonic and volcanic rocks from the Duolong area of central Tibet. Combined with existing data, our new data indicate that these plutonic and volcanic rocks were formed in two stages ca. 120 Ma and ca. 110 Ma, respectively, in a postcollisional extensional setting that was triggered by slab breakoff. The similar geochemical compositions of granitoids and rhyolites, combined with their close spatial and temporal relationships, suggest that they were both derived from juvenile crustal material within a single magmatic system. We propose that the two inferred crustal melting events in the Duolong area were caused by two episodes of deep mantle activity triggered by the transition of the plate subduction angle from steep to shallow in response to the ascent of buoyant continental lithosphere during slab breakoff. Furthermore, rapid surface uplift during the late Early Cretaceous caused by slab breakoff made an important contribution to the formation of the proto–Tibetan Plateau. This study provides new insights into postcollisional tectonomagmatism and plateau uplift in central Tibet triggered by slab breakoff. We propose more generally that tectonic uplift during postcollisional processes (i.e., slab breakoff and lithospheric delamination) is a major contributor to plateau uplift in collision zones.

Early Cretaceous mafic dykes from the Chhota Nagpur Gneissic Terrane, eastern India: Evidence of multiple magma pulses for the main stage of the Greater Kerguelen mantle plume

Early Cretaceous NNW- to WNW-trending dolerite dykes of the eastern Indian Shield, collectively termed the Raniganj-Koderma swarm, are studied for their emplacement ages and petrogenetic history, and assessed for a possible linkage with the Greater Kerguelen plume. 40Ar/39Ar dates of four dolerite dykes from different locations in the Chhota Nagpur Gneissic Terrane, indicate three pulses of emplacement ca. 118–116 Ma, ca. 112–111 Ma, and ca. 109 Ma. Geochemistry of 19 samples analysed herein (and an additional 12 samples from literature) shows sub-alkaline high-Mg tholeiitic basaltic andesite compositions. All the dyke sets belonging to the Raniganj-Koderma swarm with sub-sets WNW-trending ca. 118 Ma, NNW-trending ca. 116 Ma Salma, and a N-S trending ca. 109 Ma dyke showing similar chemistry. However, NW-trending dykes emplaced ca. 112–111 Ma have slightly different geochemical characteristics. Key trace element geochemistry data, particularly Nb/Y, Zr/Y, Nb/Yb, Ti/Yb, Th/Nb and Th/Yb ratios, indicate derivation from mantle melts generated from interaction of a plume with a spreading ridge (producing OIB – E-MORB melts) with a minor role for interaction with an enriched lithospheric mantle that was metasomatized during an earlier (pre-Mesozoic time) subduction event. Spatiotemporal distribution of the studied dolerite dykes connects them with the second plume head phase of the Greater Kerguelen mantle plume.

Metavolcanic formations in the Paraautochthonous Triassic successions of the Bükk Mts, NE Hungary

The Bükk Mts. in NE Hungary exposes Paleozoic and Mesozoic successions containing volcanic formations both in the oceanic crust-derived Szarvaskő Unit and in the continental crust-derived Paraautochthonous Unit. The rocks of this latter unit were subject of multiple metamorphic, also metasomatic alterations and deformation events obscuring and overprinting original petrographic and geochemical characteristics and producing a complex structure in which stratigraphic relationships are not always possible to be identified. This situation was leading to various stratigraphic hypotheses. This study aims to provide a basis for distinguishing metavolcanic formations using trace element geochemical data combined with quantitative mineralogical data based on XRD and EPMA. Our data were obtained from a wide range of samples collected in the Bükk Mts. Mineralogy and major element geochemistry reflect regional Alpine metamorphism and local alteration processes but high field strength elements remained relatively stable during most of these processes except local HFSE enrichment. Zr/TiO2, Nb/Y and Nb/Ta ratios were effectively used for classifying the rocks into three formations: Bagolyhegy Metarhyolite, Szentistvánhegy Metavolcanics and Szinva Metabasalt. Bagolyhegy Metarhyolite is a unique volcanic formation formed from highly differentiated and HFSE-depleted magma, probably in a single volcanic centre of uncertain age. Szentistvánhegy Metavolcanics comprises heterogeneous rocks of a calc-alkaline arc-type suite with wide distribution in a Ladinian chronostratigraphic horizon. Szinva Metabasalt represents within-plate-type alkaline lava flows and adjacent volcaniclastic, mostly peperitic rocks embedded in Carnian platform and basin facies limestone formations. Like the sedimentary formations of the Bükk Mts, rocks of magmatic origin can be correlated with the formations of the South Alpine and Dinaric successions with the significant difference that abundant metavolcanics are not accompanied by intrusive bodies.

Mineralogical and geochemical characteristics of near-vent metalliferous sediments: Implications for hydrothermal processes along the southern Mid-Atlantic ridge (12°S–28°S)

In this study, we present mineralogical and geochemical analyses of 50 unconsolidated submarine surface sediments collected from separate locations within six active hydrothermal fields (Zouyu, Taij, Caifan, Deyin, Xunmei, and Tongguan) along the southern Mid-Atlantic Ridge (SMAR) at 12°S–28°S. The samples consisted of 33 metalliferous sediments (MS) and 17 non-metalliferous sediments (non-MS). The study area was divided into a northern segment that contained the Zouyu, Taij, Caifan, and Deyin hydrothermal fields and a southern segment containing the Xunmei and Tongguan hydrothermal fields. The presence of chalcopyrite, octahedral pyrite, and cubic pyrite without streaks on the surface of the sediments reflected a stable high temperature mineralization environment with a small temperature gradient in the Xunmei hydrothermal field. Cubic pyrite with growth striations co-existing with pentagonal dodecahedon-form pyrite indicated an unstable ore-forming environment with large temperature gradient in the early stage and a more stable ore-forming environment in the late stage in the Tongguan hydrothermal field. Major and trace element geochemistry data indicated that the most important material source for the abiogenic component of the sediments along SMAR 12°S–28°S was hydrothermal materials, the second most important source was lithogenic components, and the third source was elements scavenged from seawater. The Xunmei and Tongguan hydrothermal fields may have recorded phase separation, a process that would have resulted in the formation of two separate hydrothermal fluids, thereby forming two types of sulfide-rich MS with rare earth element (REE) patterns with positive and absent Eu anomalies, respectively. The Si-Fe-Mn oxide-rich MS had positive Eu anomalies, which also indicated that these oxides in sediments were directly precipitated from initial hydrothermal fluids at high temperature. The chlorite-rich MS exhibited nearly flat REE patterns similar to the volcanic material-rich samples and mid-ocean ridge basalt (MORB), which reflected the origin of the hydrothermal alteration of volcanic materials. In the northern segment, most of the MS were derived from plume fall-out Fe oxide particles. In addition, the chlorite-rich MS was likely derived from hydrothermal alteration. In the southern segment, the hydrothermal materials in the MS were primarily derived from the collapse of sulfide-bearing chimneys and Si-Fe-Mn oxide debris directly precipitated from hydrothermal fluids.

Destabilization of Long‐Lived Hadean Protocrust and the Onset of Pervasive Hydrous Melting at 3.8 Ga

AbstractThe nature of Earth's earliest crust and crustal processes remain unresolved questions in Precambrian geology. While some hypotheses suggest that plate tectonics began in the Hadean, others suggest that the Hadean was characterized by long‐lived protocrust and an absence of significant plate tectonic processes. Recently proposed trace‐element proxies for the tectono‐magmatic settings in which zircons formed are a relatively novel tool to understand crustal processes in the past. Here, we present high‐spatial resolution zircon trace and rare earth element geochemical data along with Hf and O isotope data of a new location with Hadean materials, 4.1–3.3 Ga detrital zircons from the 3.31 Ga Green Sandstone Bed, Barberton Greenstone Belt. Together, the hafnium isotope and trace element geochemistry of the detrital zircons record a major transition in crustal processes. Zircons older than 3.8 Ga show evidence for isolated, long‐lived protocrust derived by reworking of relatively undepleted mantle sources with limited remelting of surface‐altered material. After 3.8 Ga, Hf isotopic evidence for this protocrust is muted while relatively juvenile source components for the zircon's parental magmas and flux‐like melting signatures become more prominent. This shift mirrors changes in Hf isotopes and trace element geochemistry in other Archean terranes between ∼3.8 and 3.6 Ga and supports the notion that the global onset of pervasive crustal instability and recycling—A possible sign for mobile‐lid tectonics—Occurred in that time period.

From initiation to termination: The evolution of the Ediacaran Volyn large igneous province (SW East European Craton) constrained by comparative geochemistry of proximal tuffs versus lavas and zircon geochronology

The Volyn Series, representing a large intra-plate igneous province, is the result of an Ediacaran lithospheric-scale extension related to the breakup of the Rodinia/Pannotia supercontinent. Due to its subsequent burial and thus limited subsurface access, the precise determination of the igneous activity’s initiation and termination, as well as the reconstruction of its geochemical evolution throughout time still remains a challenge. In its western part (the study area) abundantly cored deep boreholes provide an advantage in accessing rock samples. Among them Kaplonosy IG 1, representing the most complete section with the greatest thickness, preserves a record of initiation and termination of volcanic activity. Tuff samples, collected from the top the Volyn Series, were used for U-Pb zircon dating focussed on autocrysts but from the bottom also on antecrysts. The initiation of effusive activity was dated at 580 ± 10 Ma, and its termination at 547 ± 6 Ma. These volcanics have been affected by later alteration processes (hydrothermal and K- metasomatism), although trace element geochemical data hold information on the primary magmas. The data plot along a common geochemical trend, defined by the trace element contents of the lava samples. Detailed geochronological zonation of the vertical section (in this and previous studies) and its good geochemical coverage, allow for the reconstruction of the igneous system’s geochemical evolution through time. Three main phases were recognized. The initial phase, composed of picrite lavas and tuffs (c. 580–567 Ma), is characterised by extremely low concentrations of Zr and TiO2, but elevated Cr and Ni contents. The main phase (c. 567–551 Ma) is characterised by a large number and volume of subalkaline basalt to andesite flows and tuff beds. The eruptions in the terminal phase (c. 551–547 Ma) indicate a significant decrease in volume, longer hiatuses between eruptive episodes and an increase in the silicic components as well as in the most evolved compositions. The ages measured indicate that, during the break-up of Rodinia/Pannotia, the rifting and separation of Baltica from Amazonia occurred later than the separation of Laurentia from Baltica and from Amazonia.

Preserved ancient oceanic lithosphere within the Buem structural unit at the eastern margin of the West African Craton

This work presents new major and trace element geochemical data and the first SmNd and LuHf isotopic data for the mafic suite of the Buem structural unit (BSU) in Ghana in order to infer its origin and geodynamic setting, relationship to the Pharusian suture zone and significance in the Dahomeyide belt. The BSU, which occurs about 50 km to the west of the Pharusian suture zone, comprises well-preserved, weakly metamorphosed, mafic volcanic and mafic-ultramafic plutonic rocks. Two lava types can be distinguished: Type I and Type II lavas that are spatially separated and geochemically distinct. The Type I lavas are subalkaline and show trace element patterns akin to mid-ocean ridge basalt (MORB). Their εNd800Ma values of +5.3 to +6.2 and εHf800Ma values of +11.1 to +17.2 are indications of juvenile magma derived from a depleted mantle source in the spinel stability field. The Type II lavas have alkaline signatures with trace element patterns that resemble ocean island basalts. They formed by smaller degrees of disequilibrium partial melting at depth in which garnet likely played a role as a residual phase having εHf800Ma and εNd800Ma values of +3.2 to +5.1 and + 4.1 to +6.3 respectively. The gabbros show similar trace elements concentration and isotopic ratios as the BSU lavas, an indication of a similar mantle source and environment of formation. Because the geochemical features of the BSU mafic suites are comparable to the MORB-type high-pressure rocks of the Pharusian suture zone, both represent the remnant of the Pharusian oceanic crust, formed during the breakup of Rodinia. The BSU corresponds to the true oceanic crust of the West Gondwana Orogen continental margin-type ophiolite, sharing similar lithological and geochemical features to other continental margin-type ophiolites in the world.

Lithogeochemical and isotopic characterization of Devonian molybdenite mineralization in the Pabineau Falls Granite, northeastern New Brunswick, Canada

An unusual molybdenite occurrence in northeastern New Brunswick, referred to herein as the Pabineau Lake Mo (PLM), is situated to the south of Bathurst. This occurrence consists of high-grade molybdenite-bearing granites that occur within the large Pabineau Falls Granite (PFG). To help evaluate the evolution of this occurrence, a large composite sample containing disseminated, very coarse molybdenite (up to 5.02 wt% MoS2) was collected from mineralized granitic blocks excavated from a blasted trench. Although the host rocks are assumed to be PFG, being the only defined intrusive body in the area, major- and trace-element geochemical data together with U-Pb geochronological results suggest that the molybdenite is hosted by an intrusion distinct from the PFG. The radiogenic Sr-Nd-Hf-Pb isotopic data of the host intrusion are consistent with Gander Zone granites of the Appalachians orogenic system.The PLM's granitic host rock is characterized by a high silica content (78.93 wt% SiO2), an enrichment in incompatible elements, and a high molybdenum content. The high initial 87Sr/86Sr of 0.71268, negative εNd of −1.28, and high Pb isotope ratios together with trace- and major-element concentrations indicate a moderately to highly radiogenic Sr source and derivation from a significant amount of supracrustal materials, with a contribution from the upper mantle; a source similar to that proposed for the Gander Zone Siluro-Devonian granitic bodies. A positive εHf (+1.34) also suggests involvement of sedimentary components. This is consistent with elevated Cr (20 ppm) and Ni (20 ppm). The involvement of a pelagic sedimentary component is invoked to explain the enrichment in elements commonly associated with mafic compositions.A new precise, LA ICP-MS zircon crystallization age of 390±1 Ma makes the PLM host granite Middle Devonian, which argues against a direct relationship with the older PFG (397.2±1.9 Ma). The new age opens the possibility of an as yet undelineated, mineralized intrusion within the Gander Zone, which has perhaps been missed during previous investigations due to the extensive glacio-fluvial cover and very limited bedrock exposures. Alternatively, the younger age could be attributed to very slow cooling and/or intra-pluton fractionation, resulting in a younger much more fractionated phase of the PFG. However, there is no geochemical evidence for such a prolonged continuous fractional crystallization process.Notably, recent geophysical surveys on the Bathurst Mining Camp indicate that the PFG is substantively larger than its surface expression as it continues to the east beneath the Carboniferous unconformity.Comparing ages of PLM and PFG with the nearby Nicholas-Denys Granodiorite (381±4 Ma) and Antinouri Lake Granite (372±4 Ma), it appears that magmatic activity in the region occurred every 10 Myr over a span of about 30 Myr.Collectively, this evidence suggests the potential of significant buried intrusions within this tectonic belt of the Gander Zone. At the PLM, this might include exposure of an otherwise hidden intrusion that is younger and much more fractionated than the PFG. More detailed geophysical and geochemical analyses would be required to revise the current geological map and, most importantly, trace unmapped units at depth to possible additional mineralized sequences.

Papanin Ridge and Ojin Rise Seamounts (Northwest Pacific): Dual Hotspot Tracks Formed by the Shatsky Plume

AbstractAlthough previous findings support an origin of the Shatsky Rise igneous plateau (Northwest Pacific) through interaction of a mantle plume with a mid‐ocean ridge triple junction, the evidence for the involvement of a mantle plume is equivocal. The identification of an intraplate hotspot track emanating from the plateau could solve this controversy. Here we present major and trace element geochemical data from two different bathymetric features that emanate from the youngest end of Shatsky Rise: Papanin Ridge and the Ojin Rise Seamount province. Combining our results with plate tectonic reconstructions, we conclude that Papanin Ridge represents a hotspot track formed by plume‐ridge interaction. Whereas the southwestern part was formed along the path of the retreating Pacific‐Farallon‐Izanagi triple junction, the northeastern part was built by preferential drainage into its Pacific‐Farallon branch. In contrast, the Ojin Rise Seamounts formed as a true intraplate hotspot track of the Shatsky plume tail. Our wide‐ranging study reveals systematic spatial geochemical variations, consistent with a lithospheric thickness control on magma composition derived from melting a heterogeneous plume source. The recognition of two hotspot tracks and in particular of the Ojin Rise Seamounts as an intraplate hotspot track that is directly linked to Shatsky plateau volcanism both in terms of geochemistry and plate tectonic reconstructions confirms the long‐disputed involvement of a mantle plume for the formation of Shatsky Rise.

Exploring volcanic-intrusive connections and chemical differentiation of high silica magmas in the Early Cretaceous Yanbei caldera complex hosting a giant tin deposit, Southeast China

Study of the origin and chemical differentiation of silicic magmas can provide important insights into crustal evolution and rare metal metallogeny. Tin mineralization always tends to form in relatively reduced, highly fractionated granite systems. Recognition of distinctions between fertile and barren magmas is of enormous benefit to mineral exploration. The Yanbei caldera complex (YCC), is a typical inland volcanic-intrusive complex that also hosts a giant porphyry-type tin ore deposit in Southeast China. To study the origin and chemical differentiation of the YCC and constrain its relationship with tin mineralization, a comprehensive petrological, whole-rock major and trace element geochemical data, along with zircon UPb ages, trace element and Hf isotopic data of the Yanbei caldera complex (YCC) and the Xiaoji granite (XG) near the YCC are carried out. LA-ICP-MS zircon UPb dating indicates that the generation and evolution of the YCC and the XG took place in a short time (4 M.y.) of between 142.4 and 138.4 Ma. Distinct zircon Hf isotopic compositions of the volcanic and intrusive units from the YCC suggest that they were derived from different magma sources. The volcanic rocks and the XG have consistent and low zircon Hf isotopic compositions (εHf(t) = −14.9 ~ −9.0), implying that they were almost exclusively derived from melting of Paleoproterozoic crustal rocks. But the magma source of the intrusive units (the granite porphyry, GP and the biotite granite, BG) (εHf(t) = −6.0 ~ −0.8) contains a significant mantle-derived component input. The whole-rock and zircon compositions suggest that the compositionally zoned volcano of the YCC can be interpreted in terms of the “crystal mush model”. The rhyolite has the geochemical characteristics of highly evolved magmas which undergo crystal fractionation, while the dacite displays a complementary geochemical signature implying that it represents the residual crystal mush after extraction of the rhyolitic melts. The XG displays similar chronological, compositional, and isotopic features to the rhyolite, suggesting that it is the intrusive equivalent of the rhyolite. The intrusive units (GP and BG) of the YCC are highly-evolved granites with elevated tin contents, responsible for tin mineralization. By comparison of fertile and barren systems in the YCC, we suggest that simple crystal fractionation for the high-evolved/extracted granitic melt does not necessarily lead to tin mineralization. A tin-bearing and volatile-rich melt originated under high-temperature partial melting of crustal source induced by underplating/input of a hot mantle magma is an essential precondition for some magmatic-hydrothermal tin mineralization systems.

Formation processes and tectonic implications of mantle peridotites of the Yushigou ophiolite in the North Qilian Orogenic Belt, NW China

The upper-mantle section of the Yushigou ophiolite in the North Qilian Orogenic Belt consists mainly of moderately depleted harzburgite. New mineral and whole-rock major- and trace-element geochemical data show that these rocks could not have formed by simple partial melting of more enriched peridotites. The harzburgites contain high-Mg olivine (Fo = 91.0–92.2) and orthopyroxene (Mg# 91.4–92.6) accompanied by sparse high-Cr magnesium chromite (Cr# = 54.1–67.6) and clinopyroxene with low Al2O3 (1.19–2.11 wt%) and TiO2 (<0.02 wt%) contents. Most of the analysed samples exhibit depleted, U-shaped, chondrite-normalized rare earth element patterns, whereas primitive mantle-normalized PGE display flat patterns from Os to Pt with significant depleted in Pd. We propose that these peridotites underwent a multi-stage melting process beginning with 20%–25% partial melting of a mantle source beneath a mid-ocean ridge environment followed by second-stage partial melting and related refertilization in a supra-subduction zone tectonic setting, experience an additional 5%–10% melting, yielding a total of 25%–35% melting in the two episodes. TiO2 enrichment (up to 0.09 wt%) of spinels in some of the harzburgites point to interaction between an already depleted mantle and various proportions of silicate melts with LREE-enriched, boninitic compositions in a forearc setting. Both the ReOs isotope and the PGE characteristics of the Yushigou harzburgites were controlled mainly by ancient depletion events. All the samples have low Re concentrations (0.006–0.228 ppb) and subchondritic Os isotopic compositions (0.11216–0.11979). The Re-depletion model ages (1.5–1.2 Ga) of the Yushigou harzburgites may record a period of Mesoproterozoic intracontinental rifting and formation of the original ocean basin in the North Qilian Orogenic Belt.

Garnet major and trace element evidence of the alteration and mineralizing processes associated with genesis of the Qiaomaishan skarn deposit, Xuancheng ore district, eastern China

The Qiaomaishan deposit is a skarn-type Cu–W deposit located within the Xuancheng ore district of the Middle–Lower Yangtze River Metallogenic Belt of eastern China. The mineralization within the deposit is predominantly located along the contact between a porphyritic granodiorite intrusion and surrounding Carboniferous limestone units. The deposit is associated with a well-developed and zoned skarn system that includes both endoskarn and exoskarn zones. This study presents new garnet major, trace, and rare earth element (REE) geochemical data for these skarn zones, discusses the factors (e.g., crystal chemistry, oxygen fugacity and water/rock interaction) that control the incorporation of trace elements into garnet, and constrains the formation of skarn within the Qiaomaishan deposit. Grossular (Adr21–70Grs29–78; type 1) garnet is generally located within the endoskarn and is light REE (LREE) enriched but heavy REE (HREE) depleted with weakly negative Eu anomalies and higher total REE contents than the exoskarn garnet in the study area. Type 1a grossular garnet is further split from type 1b grossular garnet by the presence of positive Pr and Nd anomalies and high LREE concentrations. Exoskarn garnet has pure andradite compositions and is split into type 2 and 3 garnets, where type 2 andradite garnet has variably positive Eu anomalies and variable total REE contents that are indicative of formation under transitional or oscillating conditions (e.g., pH, oxygen fugacity). In comparison, the type 3 andradite garnet has low total REE contents and positive Eu anomalies. Endoskarn garnet Al2O3 and total REE contents positively correlate, indicating that REE is incorporated into this garnet through coupled YAG-type substitution. The differences between the total REE contents of the type 2 exoskarn and endoskarn garnets may reflect host rock buffering as well as variations in the crystal chemistry of the two garnets present within the different parts of the skarn system (grossular and andradite). In comparison, the differences in the total REE contents of the two types of exoskarn garnet may reflect variations in water–rock ratios and crystal growth rates. The different Eu anomalies and U contents present within the different types of garnets record variations in the pH and oxygen fugacity of the hydrothermal fluid over time. These changes may have promoted the crystallization of magnetite and scheelite and the formation of the tungsten mineralization within the deposit. This study demonstrates the usefulness of garnet in tracking variations in mineralizing conditions and processes and confirms the validity of these minerals as vectors or indicator minerals that can be used in combination with traditional exploration approaches to determine whether a system is mineralized or not during the early stages of exploration.

Zircon geochronology and geochemistry of Quaternary rhyolite domes of the Coso volcanic field, Inyo County, California

The Quaternary Coso volcanic field (CVF) is a compositionally bimodal volcanic field located within a releasing bend along the eastern range-front Sierra Nevada fault zone in California's southern Owens Valley. The erupted products of CVF silicic magmatism since ~1 Ma comprise 38 high-silica rhyolite domes, with the volumetric majority (~99%) of rhyolite emplaced within the past ~300 ka. The CVF hosts an economically important geothermal field driven by heat associated with a shallow (~5 km) igneous intrusion. The CVF is potentially an immature analog to the nearby Long Valley system, which culminated in generation and eruption of the voluminous and widespread Bishop Tuff. As such, the CVF represents a considerable volcanic hazard, making a detailed understanding of the eruptive history and pre-eruptive conditions of the system critically important. We present uranium-series isochron dates from zircon ± allanite crystal surfaces and zircon trace element geochemical data on the youngest 17 rhyolite domes at Coso, which represent ~60% (by volume) of the silicic magma erupted by the system. These data suggest: (1) a shorter emplacement duration than previously recognized for these domes, with a duration of 20 ± 5 ka; (2) 4 shorter-duration eruption pulses within this interval, all of which occur during the marine isotope stage (MIS) 5 interglacial period; (3) an uptick in the volume of CVF magma erupted between ~200 ka and ~ 78 ka relative to that emplaced over the lifetime of the system; (4) near-coeval eruption of geochemically distinct magma in close geographic proximity, either sourced from different portions of the same magma system at depth or from discrete, uncommunicating bodies; (5) ambiguity with respect to whether or not CVF magmatism is time-predictable, as previously suggested, or erupted as a series of punctuated episodes; (6) no rhyolite volcanism in the past ~78 kyr.