Variable Crustal Contamination Research Articles

Magmatic and sedimentological arguments for an Ediacaran active margin in the Bayankhongor Zone in western Mongolia, Central Asian Orogenic Belt

Geochronological and geochemical investigation of the magmatic and sedimentary rocks from the south-eastern tip of the Bayankhongor Zone (central Mongolia) constrains the Neoproterozoic evolution of the northern margin of the Baidrag Block. There, ultramafic to felsic igneous rocks of the Khan-Uul Massif intruded the volcano-sedimentary sequence of the Ulziit Gol Unit. U–Pb zircon ages show that both the plutonic and volcanic rocks were coeval products of the same Ediacaran (598–564 Ma) magmatism. Most of the samples have low contents of high field strength elements typical of arc-related magmas; however, some mafic rock samples display N-MORB-like chemistry. Magmatic rocks in the Khan-Uul Massif and Ulziit Gol Unit yielded exclusively positive initial εHf in zircon values (+4.9 to +13.8), implying derivation from depleted-mantle sources. Furthermore, heterogeneity of the whole-rock initial εNd values (–4.0 to +2.1) documents that fractional crystallization was accompanied by variable crustal contamination. The detrital zircon age patterns of the sandstone and tuffites in the Ulziit Gol Unit indicate that the sequence was filled dominantly by Ediacaran magmatic detritus derived from the Khan-Uul Massif and its volcanic equivalents, while the older cratonic material from the Baidrag basement represented only a subordinate component. Combined magmatic and sedimentary records imply that both the Khan-Uul Massif and the Ulziit Gol Unit may have formed in the same back-arc basin environment. This challenges the previous view that the entire Bayankhongor Zone was ophiolitic in nature. Distribution of coeval Ediacaran supra-subduction systems on the scale of the Mongolian Collage indicates the closure of multiple oceanic basins rimming the Siberian Continent. Following Rodinia break-up, this peri-Siberian realm was presumably formed by sub-parallel continental ribbons and oceanic basins. It is proposed that the amalgamation of these blocks and closure of intervening oceans reflected the Ediacaran advancing mode of the Palaeo-Pacific subduction.

Geochemical Signatures of Mafic Volcanic Rocks in Modern Oceanic Settings and Implications for Archean Mafic Magmatism

Abstract Greenstone belts are dominated by mafic volcanic rocks with geochemical characteristics that indicate a range of possible geodynamic influences. Many analogies with modern tectonic settings have been suggested. Increasing exploration of the modern oceans and comprehensive sampling of volcanic rocks from the sea floor are now providing unique opportunities to characterize different melt sources and petrogenesis that can be more closely compared to greenstone belts. In this study, we have compiled high-quality geochemical analyses of more than 2,850 unique samples of submarine mafic volcanic rocks (<60 wt % SiO2) from a wide range of settings, including mid-ocean ridges, ridge-hotspot intersections, intraoceanic arc and back-arc spreading centers, and ocean islands. The compiled data show significant geochemical variability spanning the full range of compositions of basalts found in greenstone belts. This diversity is interpreted to be due to variable crustal thickness, dry melting versus wet melting conditions, mantle mixing, and contamination. In particular, different melting conditions have been linked to mantle heterogeneity, complex mantle flow regimes, and short-lived tectonic domains, such as those associated with diffuse spreading, overlapping spreading centers, and triple junctions. These are well documented in the microplate mosaics of the Western Pacific. Systematic differences in mafic volcanic rock compositions in modern oceanic settings are revealed by a combination of principal components analysis and unsupervised hierarchical clustering of the compiled data. Mafic volcanic rocks from most arc-back arc systems have strongly depleted mantle signatures and well-known subduction-related chemistry such as large ion lithophile element (LILE) enrichment in combination with strong negative Nb-Ta anomalies and low heavy rare earth elements (HREEs). This contrasts with mafic volcanic rocks in Archean greenstone belts, which show no, or at least weaker, subduction-related chemistry, a less depleted mantle, less wet melting, and variable crustal contamination. The differences are interpreted to be the result of the lower mantle temperatures, thinner crust, and subduction-related processes of present-day settings. However, mafic rocks that are geochemically identical to those in Archean greenstone belts occur in many modern back-arc basins, including the Lau basin, East Scotia ridge, Bransfield Strait, and Manus basin, which are characterized by fertile mantle sources, high heat flow, and complex spreading regimes typical of small-scale microplate mosaics. These types of settings are recognized as favorable for volcanogenic massive sulfide (VMS) deposits in modern and ancient greenstone belts, and therefore the particular geochemical signatures of the mafic volcanic rocks are potentially important for area selection in base metal exploration.

Investigating the Influence of Crustal Contamination on the Stillwater Complex, Montana Using Sr, Nd, and Pb Isotopes

Abstract The presence of pegmatoid bodies in the Stillwater Complex is poorly understood, but they have been suggested to have resulted from the presence of fluids in the complex. To better understand the origin of the pegmatoids and to trace the possible influence of country-rock-derived fluid in the Stillwater Complex, bulk rock Rb-Sr, Sm-Nd, and Pb-Pb isotopes for samples from the Archean Stillwater Complex and its metamorphic aureole are reported. Pegmatoid bodies are compared to spatially associated host rock and the underlying hornfels facies country rocks. Evidence of resetting of radiogenic isotopes during regional metamorphism at 1700 Ma is not observed, and the initial radiogenic isotopic ratios in Stillwater Complex rocks overlap those of the underlying hornfels. Despite the isotopic similarity of the country rock to the Stillwater Complex, the intrusion is modestly isotopically heterogeneous. In Stillwater samples, the average εNd,2710Ma = −1.1 ± 6.9, 206Pb/204Pb2710 Ma = 15.24 ± 2.26, and 87Sr/86Sr2710Ma = 0.703043 ± 0.002747 (1σ). The similarity between country rock and intrusive rock isotopic compositions at Stillwater contrasts with the data reported for the Bushveld Complex, South Africa, where the country rock is isotopically distinct from the intrusion. The variability in radiogenic isotope signatures in Stillwater rocks show a noisy but decreasing influence of country rock up through the Lower Banded series interpreted to reflect variable crustal contamination, in part from <1.0 wt % country rock fluids released during intrusion of the Stillwater Complex. The influence of crustal fluid contamination as compared to more traditional crustal assimilation models or simple magmatic heterogeneity suggests that hydrothermal fluids modified the isotopic compositions of more fluid-mobile elements and can explain aspects of isotopic heterogeneity in layered intrusions.

PGE geochemical and Os-S-Sr-Nd isotopic constraints on the genesis of the Shitoukengde magmatic sulfide deposit in the East Kunlun Orogenic Belt, NW China

Numerous early Paleozoic mafic–ultramafic intrusions that host important Ni-Cu sulfide mineralization occur in the northern part of the Qinghai-Tibet Plateau, NW China. The sulfide-bearing Shitoukengde Ni-Cu deposit, which is hosted in a ∼424 Ma mafic–ultramafic intrusion, is one of the most represent magmatic sulfide deposits in the East Kunlun Orogenic Belt. The intrusion is composed of a large gabbroic body, cut by lherzolite, olivine websterite, websterite, and minor dunite. Disseminated and net-textured sulfide mineralization occurs as lenses in the center and towards the bottom of the ultramafic bodies. The sulfide mineralization has variable sulfur contents of 0.3 to 13.6 wt% with Ni grade of 0.41 wt% on average, and low platinum-group element (PGE) concentrations (<8.2 ppb Ru and <11.9 ppb Pd). Pd and Ru tenors of bulk sulfides in the deposit are characterized by a positive correlation, indicating that PGE tenor variations are mainly controlled by varying R-factors (magma/sulfide mass ratio). The estimated PGE concentrations in the Shitoukengde magma are about three orders of magnitude lower than those in continental tholeiites. The significant PGE depletions in the Shitoukengde deposit may be due to sulfide removal in the magma conduit at depth. Our new sulfur isotope data reported range in δ34S values from −5.0 to 4.9‰ for the sulfide minerals of the Shitoukengde deposit, clearly distinct from those of mantle-derived sulfur (0 ± 2‰) and immediate country rocks (1.1 to 2.0‰). This, together with the modeling for S isotope variation as a function of R-factor, suggest that the current δ34S range is attributed to the addition of external sulfides with anomalous δ34S into the magma at depth, followed by progressive modification to crustal signature by uncontaminated pulses of magma. The ultramafic rocks exhibit low εNd(t) values (−4.71 to 2.12) and high (87Sr/86Sr)i ratios (0.7073 to 0.7130), which are more enriched relative to the isotopic compositions of depleted mantle. Mixing calculations indicate that about 15 to 20 wt% of lower crustal contamination plus variable upper crustal contamination is required to explain the variations of Sr-Nd isotopes in the Shitoukengde intrusion. Significant crustal contamination is also supported by the mineralization at Shitoukengde having high (187Os/188Os)i (0.24 to 4.51) and γOs(t) values (0.24 to 2534) that are comparable to or higher than those of the important Ni-Cu deposits worldwide. Current results provide evidence that assimilation of crustal sulfur at deep crust and siliceous material with variable crustal contamination during magma ascent and emplacement played critical roles in the attainment of sulfide saturation in the parental magma and the formation of the Shitoukengde deposit.

Geochemical Signatures of Felsic Volcanic Rocks in Modern Oceanic Settings and Implications for Archean Greenstone Belts

Abstract Felsic volcanic rocks are abundant in ancient greenstone belts and important host rocks for volcanogenic massive sulfide (VMS) deposits. About half of all VMS deposits are hosted by dacite or rhyolite, an association that reflects anomalous heat flow during rifting, partial melting of basaltic crust, and fractional crystallization in high-level magma chambers. For over 30 years, geochemical signatures of these rocks (e.g., F classification of Archean rhyolites) have been widely used to identify possible hosts for VMS deposits in ancient greenstone belts. However, comparisons with modern oceanic settings have been limited, owing to a lack of samples of felsic volcanic rocks from the sea floor. This is changing with increasing exploration of the oceans. In this study, we have compiled high-quality geochemical analyses of more than 2,200 unique samples of submarine felsic volcanic rocks (&amp;gt;60 wt % SiO2) from a wide range of settings, including mid-ocean ridges, ridge-hot-spot intersections, intraoceanic arc and back-arc spreading centers, and ocean islands. The compiled data show significant geochemical diversity spanning the full range of compositions of rhyolites found in ancient greenstone belts. This diversity is interpreted to reflect variations in crustal thickness, the presence or absence of slab-derived fluids (dry melting versus wet melting), and mantle anomalies. Highly variable melting conditions are thought to be related to short-lived microplate domains, such as those caused by diffuse spreading and multiple overlapping spreading centers. Systematic differences in the compositions of felsic volcanic rocks in the modern oceanic settings are revealed by a combination of principal components analysis, unsupervised hierarchical clustering, and supervised random forest classification of the compiled data. Dacites and rhyolites from midocean ridge settings have moderately depleted mantle signatures, whereas rocks from ridge-hot-spot intersections and ocean islands reflect enriched mantle sources. Felsic volcanic rocks from arc-back-arc systems have strongly depleted mantle signatures and well-known subduction-related chemistry (strong large ion lithophile element enrichment in combination with strong negative Nb-Ta anomalies and low heavy rare earth elements [HREEs]). This contrasts with felsic volcanic rocks in Archean greenstone belts, which show high field strength element and HREE enrichment (so-called FIIIb-type) due to a less depleted mantle, a lack of wet melting, and variable crustal contamination. The differences between modern and ancient volcanic rocks are interpreted to reflect the lower mantle temperatures, thinner crust, and subduction-related processes in present-day settings. We suggest that the abundance of FIIIb-type felsic volcanic rocks in Archean greenstone belts is related to buoyant microplate domains with thickened oceanic crust that were better preserved on emerging Archean cratons, whereas in post-Archean tectonic settings most of these rocks are subducted.

Age, geochemistry, and origin of the mid-Proterozoic Häme mafic dyke swarm, southern Finland

We have reappraised the age and composition of the mid-Proterozoic Häme dyke swarm in southern Finland. The dominant trend of the dykes of this swarm is NW to WNW. Petrographic observations and geochemical data indicate uniform, tholeiitic low- Mg parental magmas for all of the dykes. Nevertheless, the variability in incompatible trace element ratios, such as Zr/Y and La/Nb, provides evidence of changing mantle melting conditions and variable crustal contamination. Our ID-TIMS 207Pb/206Pb ages for four low-Zr/Y-type dykes indicate emplacement at 1639 ± 3 Ma, whereas the most reliable previously published ages suggest emplacement of the high-Zr/Y-type dykes at 1642 ± 2 Ma. We propose that the Häme dyke swarm, and possibly also the other mid- Proterozoic mafic dyke swarms in southern Finland, records a progressive decrease in Zr/Y values due to magma generation under developing areas of thinned lithosphere. We consider that the formation of mafic magmas was most probably associated with the upwelling of hot convective mantle in an extensional setting possibly related to the nearby Gothian orogeny. The generation of tholeiitic magmas below continental lithosphere was probably promoted by the elevated mantle temperature underneath the Nuna supercontinent. We speculate that the origin of most of the relatively small mid-Proterozoic mafic dyke swarms, anorthosites, rapakivi granites, and associated rocks found across Nuna was similarly triggered by extensional plate tectonics and the convection of anomalous hot upper mantle below the supercontinent.

The Signature of Metasomatized Subcontinental Lithospheric Mantle in the Basaltic Magmatism of the Payenia Volcanic Province, Argentina

AbstractThe Payenia region of Argentina (34.5–38°S) is a large Pliocene‐Quaternary volcanic province of basaltic compositions in the Andean Cordillera foothills representing the northernmost extent of back‐arc volcanism in the Andean Southern Volcanic Zone (SVZ). Although the chemical diversity of the Payenia basalts has been characterized previously, the processes and sources responsible for such variation remain controversial. Here, we report new whole‐rock major and trace element concentrations, Sr‐, Nd‐, Hf‐, and Pb‐isotope ratios and high‐precision olivine oxygen‐isotope ratios in a suite of 35 alkaline basalts from Payenia. These lavas have major and trace elements that define a compositional range from arc‐influenced to intraplate signature. Variable crustal contamination and/or recent slab‐derived inputs inadequately account for elemental and isotopic systematics and spatial compositional variations of Payenia lavas. We present a simple forward model indicating that early metasomatism and subsequent melting of the metasomatized subcontinental lithospheric mantle (SCLM) has significantly contributed to the Payenia lava compositional range. Isotopic ingrowth calculations of radiogenic Sr, Nd, Hf, and Pb suggest that the SCLM metasomatism occurred at 50–150 Ma, consistent with the timing of the breakup of Gondwana and the development of the proto‐Pacific Andean arc. Variations in δ18Oolivine values from modeled melts indicate that the metasomatism and melting within the SCLM can fractionate oxygen isotopes even when the metasomatizing melt has MORB‐like δ18O values, providing a different explanation for the low‐δ18O signatures observed in continental arc settings.

The geochemical evolution of the Logan Igneous Suite, Ontario, Canada: new insights from the Logan Basin and implications for the genesis of the Mesoproterozoic Midcontinent Rift System

Abstract The 1.1 Ga Midcontinent Rift System (MRS) of North America comprises a series of Mesoproterozoic flood basalts and intrusive rocks emplaced in the Lake Superior region. The mafic rocks preserved on the NW flank of Lake Superior offer insights into the early development of the rift. New geochemical data collected from intrusive rocks in the Logan Basin, coupled with improved constraints on timing relationships between units, allow for a better understanding of the geochemical evolution of intrusive rocks therein. The extensive dataset suggests many previously unrecognized relationships between MRS intrusive rocks, indicating multiple, distinct mantle-source characteristics with highly variable crustal contamination histories, implying a complicated magma plumbing system. The data presented here suggest that five geochemically distinct mantle source regions were involved in the emplacement of the Logan Igneous Suite, each with its own distinct contamination history and perhaps different degrees of partial melting. The geochemical variations could suggest either a progressive, relative depletion in the mantle source over time or heterogeneity of the source region. However, based on the model for MRS magmatism presented here, we suggest that units related to the Logan Igneous Suite were tapping a heterogeneous mantle source that varied over time.

P.1.l.030 - Sexual dimorphic effect of the invalidation of GHS-R on GH ultradian secretion and meal pattern

Many monogenetic vents display systematic temporal–compositional variations over the course of eruption. Previous studies have proposed that these trends may reflect variable degrees of crustal assimilation, or melting and mixing of heterogeneous mantle source(s). Discrimination between these two endmember hypotheses is critical for understanding the plumbing systems of monogenetic volcanoes, which pose a significant volcanic hazard in many areas. In this study, we examine the Papoose Canyon (PC) monogenetic vent in the Big Pine Volcanic Field (BPVF), which had been well characterized for temporal–compositional variations in erupted basalts. We present new major and trace element and Sr–Nd–Pb-O isotopic data from the PC “crystal cargo” (phenocrysts and xenoliths). Comparison of “crystal cargo” and host basalt provides new constraints on the history of magma storage, fractionation, and crustal contamination that are obscured in the bulk basalts due to pre- and syn-eruptive magma mixing processes.The abundances of phenocrysts and ultramafic xenoliths in the PC sequence decrease up-section. Olivine and clinopyroxene phenocrysts span a wide range of Mg# (77–89). The majority of phenocrysts are more evolved than olivine or clinopyroxene in equilibrium with their host basalts (Mg#=68–71, equilibrium Fo≈85–89). In addition, the ultramafic xenoliths display cumulate textures. Olivine and clinopyroxene from ultramafic xenoliths have Mg# (73–87) similar to the phenocrysts, and lower than typical mantle peridotites. Sr–Nd–Pb isotope compositions of the xenoliths are similar to early PC basalts. Finally, many clinopyroxene phenocrysts and clinopyroxene in xenoliths have trace element abundances in equilibrium with melts that are more enriched than the erupted basalts. These features suggest that the phenocrysts and xenoliths derive from melt that is more fractionated and enriched than erupted PC basalts. Pressure constraints suggest phenocrysts and ultramafic xenoliths crystallized at ∼5–7 kbar, corresponding to mid-crust depths. Correlations between HFSE depletion and Sr–Nd–Pb isotopic compositions, high δ18O values in olivines, and radiogenic Os isotopic compositions in whole rocks also suggest incorporation of a crustally contaminated component.We propose that the phenocrysts and ultramafic xenoliths derive from melts that ponded and fractionated and assimilated continental crust, possibly in mid-crustal sills. These melts were drained and mixed with more primitive melts as the eruption began, and the temporal–compositional trends and decreasing crystal phase abundances reflect gradual deflation and exhaustion of these sills as the eruption progressed. The isotopic variations in the PC sequence span much of the compositional range observed in the BPVF. Evidence for variable crustal contamination of PC basalts suggests that much of the isotopic variation observed in the BPVF may also reflect crustal contamination rather than mantle source heterogeneity as previously proposed. In addition, evidence of pre-eruptive magma ponding and fractionation, if applicable to other monogenetic vents, may have significant implications for monitoring and hazard assessment of monogenetic volcano fields.

Initial breakup of supercontinent Rodinia as recorded by ca 860–840 Ma bimodal volcanism along the southeastern margin of the Yangtze Block, South China

It is considered that mantle plumes play an important role in the breakup of supercontinents, but continental rifting and associated bimodal volcanism often predate mantle-plume magmatism and the major stage of supercontinent breakup. Therefore, the petrogenesis of those bimodal volcanic rocks is crucial for understanding the causal relationship between magmatism and supercontinent breakup. In this paper, new SIMS U–Pb zircon ages, Nd isotopic data, and whole-rock major and trace element data are reported for Huangshan and Meiling bimodal volcanic rocks that crop out along the southeastern margin of the Yangtze Block. The Huangshan and Meiling bimodal volcanic rocks were formed respectively at 860±9Ma and 840±5Ma, about 35 to 15 Mys earlier than the recorded Neoproterozoic plume event in South China. The Huangshan basalts have arc-like geochemical signatures with enrichment of water (pre-eruption magma water content of ∼3.6%) and fluid-mobile elements, and depletion of high-field-strength elements (HFSE), as well as homogeneous Nd isotopic compositions (εNd(t)=+4.4 to +5.3). Although such geochemical characteristics suggest a hydrous asthenospheric mantle source for the Huangshan basalts, their incompatible trace elements plot within an intraplate tectonic setting, offset from true arc basalts. In contrast, the younger Meiling basalts have intraplate type (OIB-like) geochemical and homogeneous Nd isotopic compositions (εNd(t)=+4.3 to +4.7), implying a normal asthenospheric mantle source. Geochemical analyses indicate that the Huangshan basaltic rocks underwent olivine→olivine+amphibole+clinopyroxene+magnetite and then plagioclase fractional crystallization during magma ascent. Although mafic and felsic end-members have distinctive chemical compositions, the two end-members share similar Nd isotopic compositions. This implies that the felsic volcanic rocks were derived from basaltic rocks through fractional crystallization. The assemblage of fractional crystallization minerals may be amphibole-dominated and then plagioclase, coincident with fractioned mineral assemblage of the basaltic rocks. In contrast, Meiling felsic rocks have varied εNd(t) values that are correlated negatively with silica contents. Geochemical analyses indicate that the precursor magma underwent fractional crystallization of amphibole and plagioclase with minor titanite. This implies that Meiling felsic rocks were produced by partial melting of hydrous juvenile basaltic rocks, possibly ca 860Ma underplated basaltic rocks, and then followed by variable crustal contamination. The petrogenesis of the 860–840Ma bimodal volcanic rocks suggests that multi-stage rift-related anorogenic magmatism occurred before ca 825Ma mantle-plume magmatism in South China. Wet upwelling, probably within or on top of the mantle transition zone, may have been the main cause of the hydration of the shallow asthenospheric mantle, which drove the mantle partial melting that generated these bimodal volcanic rocks. We conclude that mantle hydration was the trigger for early intracontinental rift-related magmatism, probably corresponding to the initial break-up of the Rodinia supercontinent.

Zircon and molybdenite geochronology and geochemistry of the Kalmakyr porphyry Cu–Au deposit, Almalyk district, Uzbekistan: Implications for mineralization processes

Copper, gold and molybdenum mineralization of the Kalmakyr porphyry deposit in Uzbek Tien Shan occurs as stockworks, veinlets and disseminations in the phyllic and K-silicate alteration zones developed predominantly in a middle to late Carboniferous intrusive complex composed of monzonite and granodiorite porphyry. Zircon U–Pb dating yielded an age of 327.2±5.6Ma for the ore-hosting monzonite and an age of 313.6±2.8Ma for the ore-bearing granodiorite porphyry. Re–Os dating of seven molybdenite samples from stockwork and veinlet ores yielded model ages from 313.2 to 306.3Ma, with two well-constrained isochron ages of 307.6±2.5Ma (five stockwork ores) and 309.1±2.2Ma (five stockwork ores and two veinlet ores), respectively. These results indicate that Cu–Au mineralization post-dated the emplacement of the monzonite, started right after the emplacement of the granodiorite porphyry, and lasted for ca. 7Ma afterward. The geochronological and geochemical data suggest that the Kalmakyr deposit was formed in a late Carboniferous mature magmatic arc setting, probably related to the latest subduction process of the Turkestan Ocean beneath the Middle Tien Shan. The εHf(t) values of zircon grains from the monzonite vary from +11 to +1.7, with an average of +5.1, and those of zircon grains from the granodiorite porphyry range from +5.7 to −1.8, with an average of +2.4. These data indicate that the magma of both monzonite and granodiorite porphyry was derived from partial melting of a thickened lower crust with input of mantle components and variable crustal contamination, and that there was more mantle contribution to the formation of the monzonite than the granodiorite porphyry. The high rhenium concentrations of molybdenite (98–899ppm) also indicate major mantle contribution of rhenium and by inference ore metals. The relatively high EuN/EuN∗ values (average 0.68), Ce4+/Ce3 values (average 890) and Ce/Nd values (average 36.8) for zircon grains from the granodiorite porphyry than those from the monzonite (average EuN/EuN∗=0.33, average Ce4+/Ce3=624, average Ce/Nd=3.9) suggest that the magma for the syn-mineralization granodiorite porphyry has higher oxygen fugacity than that for the pre-mineralization monzonite. Based on these data, it is proposed that while the monzonite was emplaced, the oxygen fugacity and volatile contents in the magma were relatively low, and ore metals might disperse in the intrusive rock, whereas when the granodiorite porphyry was emplaced, the oxygen fugacity and volatile contents in the magma were increased, favoring copper and gold enrichment in the magmatic fluids. The Kalmakyr deposit formed from a long-lived magmatic-hydrothermal system connected with fertile magmatic sources in relation to the subduction of the Turkestan Ocean beneath the Middle Tien Shan.

Origin of temporal compositional trends in monogenetic vent eruptions: Insights from the crystal cargo in the Papoose Canyon sequence, Big Pine Volcanic Field, CA

Many monogenetic vents display systematic temporal–compositional variations over the course of eruption. Previous studies have proposed that these trends may reflect variable degrees of crustal assimilation, or melting and mixing of heterogeneous mantle source(s). Discrimination between these two endmember hypotheses is critical for understanding the plumbing systems of monogenetic volcanoes, which pose a significant volcanic hazard in many areas. In this study, we examine the Papoose Canyon (PC) monogenetic vent in the Big Pine Volcanic Field (BPVF), which had been well characterized for temporal–compositional variations in erupted basalts. We present new major and trace element and Sr–Nd–Pb-O isotopic data from the PC “crystal cargo” (phenocrysts and xenoliths). Comparison of “crystal cargo” and host basalt provides new constraints on the history of magma storage, fractionation, and crustal contamination that are obscured in the bulk basalts due to pre- and syn-eruptive magma mixing processes.The abundances of phenocrysts and ultramafic xenoliths in the PC sequence decrease up-section. Olivine and clinopyroxene phenocrysts span a wide range of Mg# (77–89). The majority of phenocrysts are more evolved than olivine or clinopyroxene in equilibrium with their host basalts (Mg#=68–71, equilibrium Fo≈85–89). In addition, the ultramafic xenoliths display cumulate textures. Olivine and clinopyroxene from ultramafic xenoliths have Mg# (73–87) similar to the phenocrysts, and lower than typical mantle peridotites. Sr–Nd–Pb isotope compositions of the xenoliths are similar to early PC basalts. Finally, many clinopyroxene phenocrysts and clinopyroxene in xenoliths have trace element abundances in equilibrium with melts that are more enriched than the erupted basalts. These features suggest that the phenocrysts and xenoliths derive from melt that is more fractionated and enriched than erupted PC basalts. Pressure constraints suggest phenocrysts and ultramafic xenoliths crystallized at ∼5–7 kbar, corresponding to mid-crust depths. Correlations between HFSE depletion and Sr–Nd–Pb isotopic compositions, high δ18O values in olivines, and radiogenic Os isotopic compositions in whole rocks also suggest incorporation of a crustally contaminated component.We propose that the phenocrysts and ultramafic xenoliths derive from melts that ponded and fractionated and assimilated continental crust, possibly in mid-crustal sills. These melts were drained and mixed with more primitive melts as the eruption began, and the temporal–compositional trends and decreasing crystal phase abundances reflect gradual deflation and exhaustion of these sills as the eruption progressed. The isotopic variations in the PC sequence span much of the compositional range observed in the BPVF. Evidence for variable crustal contamination of PC basalts suggests that much of the isotopic variation observed in the BPVF may also reflect crustal contamination rather than mantle source heterogeneity as previously proposed. In addition, evidence of pre-eruptive magma ponding and fractionation, if applicable to other monogenetic vents, may have significant implications for monitoring and hazard assessment of monogenetic volcano fields.

Cretaceous subduction-related magmatism and associated porphyry-type Cu–Mo prospects in the Eastern Pontides, Turkey: New constraints from geochronology and geochemistry

This study focuses on the Elbeyli–Ordu, Emeksen–Giresun, Güzelyayla–Trabzon and Ulutaş–Ispir porphyry-type prospects located in the Eastern Pontides, Turkey. Our new LA-ICP-MS U–Pb zircon age data reveal that the Elbeyli–Ordu Mo–Cu mineralization is hosted by a 77.0±1.3Ma-old monzonite/monzodiorite with a shoshonitic character. The Emeksen Mo mineralization, located ~40km southeast of the Elbeyli–Ordu prospect, consists of NW- and NE-striking quartz veins crosscutting a high-K calc-alkaline to shoshonitic granite dated at 78.5±0.8Ma, a granodiorite dated at 78.7±0.5Ma and porphyry granite dated at 77.7±0.5Ma. The Güzelyayla porphyry Cu–Mo prospect consists of a stockwork-type Cu–Mo mineralization crosscutting a calc-alkaline 81.4±1.1Ma-old dacite porphyry and Late Cretaceous calc-alkaline andesite. The Ispir–Ulutaş mineralization is hosted within a highly sericitized 131.1±0.9Ma-old quartz-porphyry that intruded into a 132.9±0.6Ma-old calc-alkaline granite porphyry. Our new U–Pb zircon ages, lithogeochemical and radiogenic isotopic data of the host rocks associated with the porphyry-type prospects in the Eastern Pontides indicate that they formed in an arc-related environment during Cretaceous subduction of the Neotethys Ocean, and the Ispir–Ulutaş prospect is attributed to the main stage of the northward subduction of the Neotethys during the Early Cretaceous. We conclude that the Güzelyayla and Emeksen hydrothermal systems were formed during a transitional compressional to extensional tectonic evolution, whereas the Late Cretaceous Elbeyli hydrothermal system was emplaced during an extensional arc magmatic event. Highly-oxidized, high-K calc-alkaline to shoshonitic magmas at Emeksen and Elbeyli were derived from a metasomatized, heterogeneous and enriched lithospheric mantle, with variable degrees of partial melting of the mantle wedge and variable crustal contamination. Mixing/mingling processes between mafic magmas derived from the lower crust and acidic magmas at upper crustal levels played an important role in the formation of Cu–Mo porphyry-type mineralization in the Eastern Pontides.

Geology, alteration, and lithogeochemistry of the Hood volcanogenic massive sulfide (VMS) deposits, Nunavut, Canada

The Hood volcanogenic massive sulfide (VMS) deposits are hosted by the ~2.68 Ga Amooga Booga volcanic belt (ABVB) in the northwestern Archaen Slave Craton and consist of three deposits (Hood 10, 41, and 41A) and three occurrences (46, 461, and 462). The mineralized zones consist of massive to semi-massive pyrrhotite, pyrite, chalcopyrite, sphalerite, and galena hosted predominantly by felsic volcanic flows within the predominantly mafic ABVB. The mineralized lenses occur at different stratigraphic levels and have textural, alteration, and stratigraphic features consistent with formation via subseafloor replacement. The felsic volcanic rocks in the Hood deposits can be subdivided into groups based on immobile trace element geochemistry. The main felsic types (A and B) are petrographically indistinguishable. Type A has higher high field strength element (HSFE) and rare earth element (REE) contents than type B, suggesting a higher temperature of formation. Type A rocks also have higher Nb/Ta values indicative of a greater mantle input in their genesis compared to type B rocks. Mineralization is more closely associated with type A than type B rocks. The two mafic volcanic rock types previously identified in the ABVB, type I and type II, both occur within the Hood deposits. The type II mafic group is interpreted to be the result of variable crustal contamination of type I magma. The volcanic rocks of the ABVB are interpreted to have formed in a continental margin arc/back-arc setting. The genesis of these magmatic suites involved magmatic underplating and emplacement through pre-existing sialic basement that resulted in crustal melting, mantle-crust mixing, and contamination leading to the aforementioned geochemical features in both mafic and felsic suites. This type of extensional tectonic environment was likely associated with high heat flow and is similar to global VMS environments proximal to extending continental margins (e.g., Sturgeon Lake, Bathurst, and Finlayson Lake). The similarities of the ABVB to other Slave Craton greenstone belts further highlights the overall potential for greenstone-hosted VMS mineralization in the Slave Craton. Chlorite-sericite (+/− quartz) is the dominant hydrothermal alteration assemblage in the Hood deposits and is typical of VMS-style mineralization. Mass change calculations illustrate that elemental changes are typical of VMS environments with gains in Fe2O3, MgO, and base metals associated with chlorite alteration near mineralized zones; K2O gains associated with sericite alteration; and losses of Na2O in both alteration types.

Post-accretionary permian granitoids in the Chinese Altai orogen: Geochronology, petrogenesis and tectonic implications

The Altai orogen is an important constituent of the Central Asian Orogenic Belt (CAOB). The main orogenic processes occurred mainly in the early to middle Paleozoic and involved a series of northward subduction and terrane accretion. However, termination of the accretionary and post-accretionary processes remains poorly defined. The Chinese Altai is located in the southern part of this orogeny, which is widely intruded by Permian granitic plutons. These plutons are approximately circular in shape, free of deformation and generally cutting pre-Permian structures, suggesting a post-tectonic formation. We report the results of geochronological and geochemical data from five specific granitic plutons (Buerjin, Xibodu, Daqiaonan, Aweitan, and Adenbluk), which all yielded magmatic zircon U-Pb ages of about 270 Ma. These plutons are composed of high-K calc-alkaline rocks, including K-feldspar megaphyric granite, biotite granite and monzogranite that have metaluminous to weakly LREE-enriched, coupled with negative Eu anomalies. Significant negative anomalies of Ba, Sr, P, and Ti are also observed in the primitive-mantle normalized diagram. They have positive whole-rock ε~Nd~(t) (+1.3 to +7.2) and zircon ε~Hf~(t) values (+5.6 to +12.9), yielding Sm-Nd model ages of ≤0.9 Ga. Therefore, these granitoids are proposed to have been generated by differentiation of mantle-derived magmas with variable crustal contamination. In view of the field occurrence, structural analysis, regional tectonics and geochemical characteristics, these Permian plutons are concluded to be post-accretionary or post-collisional. Asthenospheric upwelling after the collision and amalgamation of the Altai and Junggar blocks could have caused the mantle-derived magmas that evolved to form the granitoids. We note that Permian intrusions are not only widespread but also voluminous in the CAOB. They mostly are post-collisional products, and some of them might have been related to the large igneous province activity in the Tarim Block.

Heterogeneous mantle sources feeding the volcanic activity of Mt. Karacadağ (SE Turkey)

The volcanic activity of Mt. Karacadağ (SE Anatolia) is divided into three major stages: Siverek Stage (∼11–2.7Ma), Karacadağ Stage (∼1.9–1.0Ma) and Ovabağ Stage (0.4–0.01Ma). The magmas are mildly alkaline mafic rocks, mostly basanites, hawaiites and alkali basalts. Detailed geochemical investigation indicates a continuous variation of composition with time, with the oldest products (Siverek Stage) being characterized by average lowest HFSE (Ti, Hf, Zr, Nb, Ta), 143Nd/144Nd, Nb/U, Ta/Yb, Nb/Nb* and the highest ΔQ, La/Nb, Ti/Nb, Zr/Nb, Ba/Nb, Th/Ta, K/La, and the youngest products (Ovabağ Stage) at the opposite end of the trend. The overall incompatible element content of the Karacadağ volcanic rocks resembles closely average HIMU-OIB compositions, with the oldest samples deviating more strongly from typical compositions of “anorogenic” magmas.We interpret these geochemical variations with a process of partial melting of a chemically and mineralogically heterogeneous mantle source rather than with process of variable crustal contamination at shallow depths. During the first stages of mantle melting the volumes with lowest solidus temperature (e.g., the amphibole and phlogopite-rich metasomes, particularly abundant in the Arabia mantle xenolith suite) contributed significantly to the partial melts. “Average” peridotitic matrix is involved in partial melting processes only when these metasomatic volumes start to be exhausted, producing mantle melts with geochemical composition resembling average “anorogenic” mildly alkaline sodic rocks, common in the circum-Mediterranean area.

U–Pb isotopic ages and Hf isotope composition of zircons in Variscan gabbros from central Spain: evidence of variable crustal contamination

Ion microprobe U–Pb analyses of zircons from three gabbroic intrusions from the Spanish Central System (SCS) (Talavera, La Solanilla and Navahermosa) yield Variscan ages (300 to 305 Ma) in agreement with recent studies. Only two zircon crystals from La Solanilla massif gave slightly discordant Paleoproterozoic ages (1,848 and 2,010 Ma). Hf isotope data show a relatively large variation with the juvenile end-members showing ɛHfi values as high as +3.6 to +6.9 and +1.5 to +2.9 in the Navahermosa and Talavera gabbros, respectively. These positive ɛHfi values up to +6.9 might represent the composition of the subcontinental mantle which generates these SCS gabbros. This ɛHfi range is clearly below depleted mantle values suggesting the involvement of enriched mantle components on the origin of these Variscan gabbros, and is consistent with previous whole-rock studies. The presence of zircons with negative ɛHfi values suggest variable, but significant, crustal contamination of the gabbros, mainly by mixing with coeval granite magmas. Inherited Paleoproterozoic zircons of La Solanilla gabbros have similar trace element composition (e.g. Th/U ratios), but more evolved Hf-isotope signatures than associated Variscan zircons. Similar inherited ages have been recorded in zircons from coeval Variscan granitoids from the Central Iberian Zone. Granitic rocks have Nd model ages (TDM) predominantly in the range of 1.4 to 1.6 Ga, suggesting a juvenile addition during the Proterozoic. However, Hf crustal model ages of xenocrystic Proterozoic zircons in La Solanilla gabbro indicate the presence of reworked Archean protoliths (TDM2 model ages of 3.0 to 3.2 Ga) incorporated into the hybridized mafic magma.

Re–Os isotopic constraints on the genesis of the Limahe Ni–Cu deposit in the Emeishan large igneous province, SW China

There are three types of magmatic sulfide deposits (sulfide-poor PGE deposit, sulfide-rich Ni–Cu deposit and Ni–Cu–PGE deposit) associated with mafic–ultramafic intrusions in the Late-Permian Emeishan large igneous province, SW China. The Limahe deposit represents a sulfide-rich Ni–Cu deposit in the region. Re–Os concentrations and isotopic compositions of the sulfide ores and associated ultramafic rocks in the Limahe intrusion are used to evaluate the relationships between magma evolution and oregenesis. There are two types of sulfide-barren olivine websterites in the Limahe intrusion: one is PGE depleted and the other is PGE undepleted. The PGE undepleted olivine websterites have γOs (260 Ma) between 5 and 8, whereas a PGE depleted olivine websterite sample has a γOs (260 Ma) value of ∼ 30. A disseminated sulfide ore sample has a γOs (260 Ma) value close to 77. Net-textured sulfide ores and sulfide separates from net-textured sulfide ores have the highest γOs (260 Ma) values, varying between 102 and 124. The results can be explained by successive contamination of a mantle-derived magma with the lower and upper crusts during magma ascent. Mixing calculations indicate that the total amount of crustal contamination is ∼ 15%. The concentrations of Os and Re–Os isotopes in the sulfide ores of the Limahe deposit are consistent with the sulfide liquids that segregated from a PGE depleted magma due to previous sulfide segregation. Higher γOs (260 Ma) values in the sulfide ores than the sulfide-barren ultramafic rocks in the Limahe intrusion suggest that crustal contamination played a critical role in sulfide saturation. Significant variations in Os concentrations and Re–Os isotopes in the Limahe intrusion suggest that multiple pulses of magma with variable crustal contamination were involved in the development of the intrusion. The different pulses of magma are likely related to each other by variable crustal contamination and sulfide segregation. Our new results support the hypothesis that the Limahe intrusion may represent a dynamic conduit through which magma successively ascended to higher crustal levels or to the surface.

Provenance and tectonic development of the late Archaean Gawler Craton, Australia; U–Pb zircon, geochemical and Sm–Nd isotopic implications

Integrated Sensitive High Mass Resolution Ion Microprobe (SHRIMP) U–Pb zircon, geochemical and Sm–Nd isotopic data from the late Archaean Gawler Craton, southern Australia, constrain major basin-forming and magmatic processes to the interval 2560–2500 Ma. This terrane represents a convergent margin associated with the second half of a full global Wilson cycle, during which continental fragments formed between 2780 and 2590 Ma amalgamated with younger cratons, perhaps forming the Earth's first supercontinent. Metasedimentary rocks in the central and southern Gawler Craton deposited between 2535 and 2500 Ma, have dominant detrital SHRIMP U–Pb zircon ages, geochemical and ɛ Nd signatures which reflect input from predominantly felsic, juvenile to moderately evolved late Archaean crust. Geochemical patterns in ca. 2560–2500 Ma felsic to intermediate igneous lithologies that in part were produced during on-going basin development, suggest affinities with an arc-like environment, and have a range of ɛ Nd values that indicate variable crustal contamination of primitive arc components. However, approximately coeval komatiites have geochemical and ɛ Nd signatures consistent with a mantle–plume origin. We envisage a tectonothermal regime for development of the late Archaean Gawler Craton that reflects interaction between a convergent margin and mantle–plume, in which sedimentary deposition occurred during active basin development in a back-arc or arc-rift setting. Basin formation and associated magmatism was terminated by regional granulite grade metamorphism and crustal thickening during the ca. 2500–2400 Ma Sleafordian Orogeny, which was proceed by ∼400 Ma of tectonic quiescence. This suggests that the Sleafordian Orogeny was driven by collision between continental components, leading to the formation of a continental interior. While the late Archaean Gawler Craton is younger than prodigious late Archaean metallogenic provinces associated with the first half of a full global Wilson cycle (ca. 2780–2590 Ma), there are important lithological and tectonic similarities to those mineralized systems.

Geology of the Darreh-Zerreshk and Ali-Abad Porphyry Copper Deposits, Central Iran

This paper describes the petrology, mineralogy, alteration, structural characteristics, and geological evolution of the Darreh-Zerreshk and Ali-Abad copper deposits within the central Iranian volcano-plutonic belt, Yazd Province, central Iran. Intrusions in this area, a result of subduction magmatism, range in composition from quartzmonzodiorite to granite, yet copper-molybdenum porphyry—type mineralization is restricted to quartzmonzodioritic to granodioritic plutons. The mineralizing intrusions cut Cretaceous and Eocene strata, and have been dated as Oligocene—Miocene. Strong negative anomalies for Nb, Ta, P, and Ti and enrichment in the LREE, LILE, Ba, Rb, Sr, and Pb for the mineralizing igneous rocks suggest they are of I type and have magmatic arc affinity. REE patterns, major-, and trace-element data suggest that these rocks were derived from partial melting of enriched upper mantle, modified by variable crustal contamination. Hornblende fractionation was an important control on the evolution of copper-bearing plutonic rocks. Structural observations suggest that these deposits were emplaced in tensional pull-apart domains formed between two secondary dextral strike-slip faults of the regional Dehshir-Baft shear system. Rapid exhumation of the porphyry copper systems allowed for the development of only minor supergene enrichment.