Assimilation Of Rocks Research Articles

Source contamination, crustal assimilation, and magmatic recycling during three flare-up events in the Cretaceous Peruvian Coastal Batholith: An example from the Ica-Pisco plutons

Source contamination, crustal assimilation and magmatic recycling of older and coeval magmatic materials can potentially have dramatic effects on magma petrology, emplacement and thermal histories, as well as magma addition rates and crustal growth. To establish sources and magnitudes of assimilation and recycling processes in the Ica-Pisco area of the Peruvian Coastal Batholith (PCB), we use data at different scales collected from 125 field localities, 124 thin sections, 10 new U-Pb zircon ages, and 117 elemental and 49 isotopic whole rock analyses.Our new U-Pb zircon ages for the northern Arequipa segment of the PCB document three magmatic flare-up events over 80 m.y. (ca. 136-56 Ma), younging toward the northeast: pre-PCB gabbros at ~136 Ma; the Quilmana volcanics and the Linga, Pampahuasi, and Tiabaya plutons as part of a flare-up at ~120-80 Ma; and a final Incahuasi flare-up event at ~70-56 Ma. Zircon geochronology of xenocrysts and antecrysts provides a record of recycling from isotopically evolved materials, as well as from earlier magmatic events.Field observations reveal widespread evidence for mass transfer processes of older volcanic and plutonic units into younger plutons over a range of scales: (1) host block subsidence and disaggregation at the kilometer-scale during cauldron subsidence; (2) widespread stoping at the meter-scale; and (3) mechanical and chemical disaggregation of xenoliths at the centimeter-scale. Observations at these scales indicate that generally 1–10% of host rock assimilation and 1–5% of magmatic recycling occurred. The high alkalinity in the Linga, Pampahuasi, and Incahuasi plutons is associated with contamination from high-K materials at the magma source and minor crustal contamination at emplacement levels. The Sr/Y ratio and REE whole rock geochemistry indicate that the youngest inboard plutons have a deeper magma source, suggesting a thicker lower crust with greater possibility for assimilation during magma ascent. Sr, Nd, and Pb isotopes indicate a significant contribution from mantle melting, a moderate contribution from melting of subducted Pacific Ocean sediments, and minor assimilation from lower crustal Precambrian and Paleozoic gneisses during magma ascent and emplacement. Assimilation and fractional crystallization (AFC) calculations with Sri and Sr suggest that the Ica-Pisco plutons are composed of at most 25% recycled Pacific Ocean sediments. In summary, the composition of the Ica-Pisco plutons appears to be derived from about 65 to 70% mantle melts and 30–35% isotopically evolved material that is composed of Pacific Ocean sediments, arc and basement crust. Geochemical variations from older to younger plutons show an initial decrease in the mantle component, a general increase in the isotopically evolved component, and an initial increase and later decrease in the volcanic and plutonic host rock components.

The Huckleberry Ridge Tuff, Yellowstone: evacuation of multiple magmatic systems in a complex episodic eruption

AbstractThe 2·08 Ma, ∼2500 km3 Huckleberry Ridge Tuff (HRT) eruption, Yellowstone, generated two fall deposits and three ignimbrite members (A, B, C), accompanying a ∼95 x 65 km caldera collapse. Field data imply that the pre-A fall deposits took weeks to be erupted, then breaks of weeks to months occurred between members A and B, and years to decades between B and C. We present compositional and isotopic data from single silicic clasts (pumice or fiamme) in the three ignimbrite members, plus new data from co-eruptive mafic components to reconstruct the nature and evacuation history of the HRT crustal magmatic complex. Geochemical data, building on field characteristics, are used to group nine silicic clast types into seven compositional suites (A1-A3; B1; C1-C3) within their respective members A, B and C. Isotopic data are then added to define four magmatic systems that were tapped simultaneously and/or sequentially during the eruption. Systems 1 and 2 fed the initial fall deposits and then vented throughout member A, accompanied by trace amounts of mafic magma. In member A, volumetrically dominant system 1 is represented by a rhyolite suite (A1: 73·0–77·7 wt % SiO2, 450–1680 ppm Ba) plus a distinct low-silica rhyolite suite (A2: 69·2–71·6 wt % SiO2, >2500 ppm Ba). System 2 yielded only a low-Ba, high-silica rhyolite suite (A3: 76·7–77·4 wt % SiO2, ≤250 ppm Ba). Glass compositions in pumices from systems 1 and 2 show clustering, indicative of the same multiple melt-dominant bodies identified in the initial fall deposits and earliest ignimbrite. Member B samples define suite B1 (70·7–77·4 wt % SiO2, 540–3040 ppm Ba) derived from magmatic system 1 (but not 2) that had undergone mixing and reorganisation during the A: B time break, accompanying mafic magma inputs. Mafic scoriae erupted in upper member B cover similar compositions to the member A clasts, but extend over a much broader compositional range. Member C clast compositions reflect major changes during the B: C time break, including rejuvenation of magmatic system 2 (last seen in member A) as suite C3 (75·3–77·2 wt % SiO2, 100–410 ppm Ba), plus the appearance of two new suites with strong crustal signatures. Suite C2 is another rhyolite (74·7–77·6 wt % SiO2, with Ba decreasing with silica from 2840 to 470 ppm) that defines magmatic system 3. Suite C2 also shows clustered glass compositions, suggesting that multiple melt-dominant bodies were a repetitive feature of the HRT magmatic complex. Suite C1, in contrast, is dacite to rhyolite (65·6–75·0 wt % SiO2, with Ba increasing with silica from 750 to 1710 ppm) that defines magmatic system 4. Compositions from magmatic systems 1 and 2 dominantly reflect fractional crystallization, but include partial melting of cumulates related to earlier intrusions of the same mafic magmas as those syn-eruptively vented. Country rock assimilation was limited to minor amounts of a more radiogenic (with respect to Sr) evolved contaminant. In contrast, systems 3 and 4 show similar strongly crustal isotopic compositions (despite their differences in elemental composition) consistent with assimilation of Archean rocks via partial melts derived from cumulates associated with contrasting mafic lineages. System 3 links to the same HRT mafic compositions co-erupted in members A and B. In contrast, system 4 links to olivine tholeiite compositions erupted in the Yellowstone area before, sparsely during, and following the HRT itself. All four magmatic systems were housed beneath the HRT caldera area. Systems 1 and 2 were hosted in Archean crust that had been modified by Cretaceous/Eocene magmatism, whereas systems 3 and 4 were hosted within crust that retained Archean isotopic characteristics. The extreme compositional diversity in the HRT highlights the spatial and temporal complexities that can be associated with large-volume silicic magmatism.

Evaluating the roles of melt-rock interaction and partial degassing on the CO2/Ba ratios of MORB: Implications for the CO2 budget in the Earth’s depleted upper mantle

Abstract Carbon content in the Earth's depleted upper mantle has been estimated in previous studies using CO2/Ba ratios of CO2 undersaturated depleted mid-ocean ridge basalt (D-MORB) glasses and melt inclusions. However, CO2/Ba ratios in CO2 undersaturated MORB may not necessarily record those of the mantle source, as they may be affected by (1) assimilation of Ba-rich plagioclase-bearing rocks in the oceanic crust and (2) CO2 degassing through partial degassing and mixing. In this study, we evaluate these effects on the CO2/Ba ratios as well as other volatile to refractory trace element ratios (H2O/Ce, F/Nd, Cl/K, and S/Dy) in D-MORBs using the compositions of olivine-hosted melt inclusions and glasses from the Siqueiros and Garrett transform faults. The Siqueiros and Garrett melt inclusions are CO2 undersaturated and highly depleted in incompatible trace elements, and their average CO2/Ba ratios show relatively large ranges of 90 ± 34 and 144 ± 53 respectively. A subset of melt inclusions in lavas from both transform faults show potential signatures of contamination by plagioclase-rich rocks, such as correlations between major elements contents (e.g., FeO, Al2O3, and MgO), and trace element ratios (e.g., Sr/Nd). We find that (1) assimilation fractional crystallization (AFC) of gabbro into D-MORB and (2) mixing between partial melts of gabbro and D-MORB can reproduce the observed range in Sr/Nd ratios as well as the general trends between major elements. However, we find that these processes had limited effects on the CO2/Ba ratio of the melt inclusions and it is unlikely that they can account for the observed range in the CO2/Ba ratio. On the other hand, while a partial degassing and mixing model can generate melts with large range of CO2/Ba ratios (as proposed by Matthews et al. (2017)), it cannot reproduce the Pearson correlation coefficients between CO2/trace element and 1/trace element ratios observed in the Siqueiros and Garrett melt inclusions. Instead, when analytical uncertainties on the elemental concentrations are considered, a model without partial degassing can adequately reproduce the majority of the observed range in CO2/Ba ratio and Pearson correlation coefficients. Hence, we postulate that the Siqueiros and Garrett melt inclusions are undegassed and use their average CO2/Ba ratios to estimate the Siqueiros and Garrett mantle source CO2 contents (21 ± 2 ppm and 33 ± 6 ppm respectively). We also evaluate the effects of shallow level crustal processes on H2O/Ce, F/Nd, Cl/K, and S/Dy ratios, and after which we filter those effects, we estimate the H2O, F, Cl and S contents in the mantle sources of the Siqueiros (40 ± 8 ppm, 8 ± 1 ppm, 0.22 ± 0.04 ppm, and 113 ± 3 ppm) and Garrett (51 ± 9 ppm, 6 ± 1 ppm, 0.27 ± 0.07 ppm, and 128 ± 7 ppm) melt inclusions.

Strongly Peraluminous Granites across the Archean–Proterozoic Transition

AbstractStrongly peraluminous granites (SPGs) form through the partial melting of metasedimentary rocks and therefore represent archives of the influence of assimilation of sedimentary rocks on the petrology and chemistry of igneous rocks. With the aim of understanding how variations in sedimentary rock characteristics across the Archean–Proterozoic transition might have influenced the igneous rock record, we compiled and compared whole-rock chemistry, mineral chemistry, and isotope data from Archean and Paleo- to Mesoproterozoic SPGs. This time period was chosen as the Archean–Proterozoic transition broadly coincides with the stabilization of continents, the rise of subaerial weathering, and the Great Oxidation Event (GOE), all of which left an imprint on the sedimentary rock record. Our compilation of SPGs is founded on a detailed literature review of the regional geology, geochronology, and inferred origins of the SPGs, which suggest derivation from metasedimentary source material. Although Archean and Proterozoic SPGs are similar in terms of mineralogy or major-element composition owing to their compositions as near-minimum melts in the peraluminous haplogranite system, we discuss several features of their mineral and whole-rock chemistry. First, we review a previous analysis of Archean and Proterozoic SPGs biotite and whole-rock compositions indicating that Archean SPGs, on average, are more reduced than Proterozoic SPGs. This observation suggests that Proterozoic SPGs were derived from metasedimentary sources that on average had more oxidized bulk redox states relative to their Archean counterparts, which could reflect an increase in atmospheric O2 levels and more oxidized sedimentary source rocks after the GOE. Second, based on an analysis of Al2O3/TiO2 whole-rock ratios and zircon saturation temperatures, we conclude that Archean and Proterozoic SPGs formed through partial melting of metasedimentary rocks over a similar range of melting temperatures, with both ‘high-’ and ‘low-’temperature SPGs being observed across all ages. This observation suggests that the thermo-tectonic processes resulting in the heating and melting of metasedimentary rocks (e.g. crustal thickening or underplating of mafic magmas) occurred during generation of both the Archean and Proterozoic SPGs. Third, bulk-rock CaO/Na2O, Rb/Sr, and Rb/Ba ratios indicate that Archean and Proterozoic SPGs were derived from partial melting of both clay-rich (i.e. pelites) and clay-poor (i.e. greywackes) source regions that are locality specific, but not defined by age. This observation, although based on a relatively limited dataset, indicates that the source regions of Archean and Proterozoic SPGs were similar in terms of sediment maturity (i.e. clay component). Last, existing oxygen isotope data for quartz, zircon, and whole-rocks from Proterozoic SPGs show higher values than those of Archean SPGs, suggesting that bulk sedimentary 18O/16O ratios increased across the Archean–Proterozoic boundary. The existing geochemical datasets for Archean and Proterozoic SPGs, however, are limited in size and further work on these rocks is required. Future work must include detailed field studies, petrology, geochronology, and constraints on sedimentary source ages to fully interpret the chemistry of this uniquely useful suite of granites.

U–Pb zircon dating of Paleozoic volcanic rocks from the Rheno-Hercynian Zone: new age constraints for the Steinkopf formation, Lahn-Dill area, Germany

Small and volumetrically subordinate outcrops of alkali-rhyolitic to trachytic metavolcanic rocks are widespread in the Lahn-Dill area of the Rheinisches Schiefergebirge, Germany. These occurrences are either related to late Early Devonian or to Middle Devonian episodes of volcanic activity. An unambiguous age assignment for the time of eruption could not be established so far. Most volcanic rocks of the study area are not suitable for application of U–Pb geochronology. A notable exception is the Steinkopf rock suite in the SW Lahn-Dill area, but a straightforward interpretation of the LA-ICP-MS U–Pb data is hampered by complex zircon populations recording various processes including crystallization from newly formed melt, inheritance, assimilation of country rocks, variable degrees of Pb-loss and fluid-assisted Zr-mobility. A plausible evaluation of U–Pb zircon data of two metatrachytes and a metavolcaniclastic rock document a coherent age group at ca. 390–384 Ma and a probable eruption age at ca. 385 Ma, suggesting a relationship to Middle Devonian (Givetian) igneous processes. Zircon-bearing veinlets in a metatrachyte indicate post-eruptive infiltration of external hydrothermal fluids and/or internal Zr-redistribution in Late Devonian to Early Carboniferous time (364 ± 10 Ma), but this process is of rather limited importance.

Generation of high-Mg diorites and associated iron mineralization within an intracontinental setting: Insights from ore-barren and ore-bearing intrusions in the eastern North China Craton

High-Mg andesitic rocks (HMAs) are commonly generated through slab-mantle interactions in arc settings. In this study, however, we report the high-Mg diorites in the Zibo and Laiwu areas of the eastern North China Craton (NCC), which were particularly produced during lithospheric thinning within an intracontinental setting. Some of the rocks generated large-scale Fe skarn deposits (e.g., the Zibo quartz monzonite and the Laiwu Kuangshan monzonite), whereas the others are ore-barren (e.g., the Zibo gabbroic diorite and the Laiwu Jinniushan and Jiaoyu monzodiorites), providing an excellent case to reveal how the non-arc HMAs were formed and the crucial factors controlling the Fe mineralization. All the rocks are characterized by high Mg# values (51–84) at andesitic compositions (52.5–65.0 wt%) and show arc-like geochemistry (e.g., enriched in LREEs and LILEs and depleted in HREEs and HFSEs), of which the strongly mineralized intrusions are more felsic with higher Mg# values (69–84) and stronger fractionation between LREEs and HREEs. Zircon U-Pb dating indicates that these rocks were emplaced at the early Cretaceous (130–132 Ma), coeval with the peak thinning of the lithosphere in the eastern NCC. All the rocks have transitional Sr-Nd-Pb isotopic compositions (0.704726 < (87Sr/86Sr)i < 0.707507, −16.0 < εNd(t) < −5.4, 16.334 < (206Pb/204Pb)i < 17.629, 15.205 < (207Pb/204Pb)i < 15.447, 36.306 < (208Pb/204Pb)i < 37.754) between the local lithospheric mantle and the ancient crust. Combined with their high Mg# and arc-like geochemistry, they were most likely derived from an anciently metasomatized and recently hydrated subcontinental lithospheric mantle (SCLM) with various crustal contamination. Magma mixing modelling suggests that in the Zibo area little crustal materials were involved into the gabbroic diorite, but ~20% middle-upper crustal materials were assimilated into the monzodiorite and quartz monzonite during magma ascent. In the Laiwu area, lower crustal materials (at least ~30%) were significantly involved into the Jinniushan, Jiaoyu and Kuangshan intrusions through magma mixing at the base of the lower crust. Sedimentary rocks (mainly carbonates interbedded with coal-bearing shales, sandstones and evaporitic rocks) were particularly assimilated into the strongly mineralized intrusions in both the Zibo and Laiwu areas. Such assimilation probably contributed to promote fluid exsolution and metal extraction from the melts, since the strongly mineralized intrusions have the lowest H2O contents (2.4–3.2 wt% H2O), the highest Mg# values and the lowest oxygen fugacities (2.5 < ∆FMQ < 3.1). It is thus considered that the assimilation of sedimentary rocks is crucial for generating ore-forming fluids. P-T calculations indicate that all the high-Mg diorites were emplaced shallowly (mainly T < 760 °C and P < 140 MPa). In combination with the co-development of bimodal volcanics, metamorphic core complexes and rift basins in the eastern NCC during the early Cretaceous, extreme lithospheric extension should have occurred in the study region. Such intracontinental extension was likely caused by the asthenospheric upwelling induced by the stagnant Paleo-Pacific Plate in the transition zone beneath the eastern Asian continent. The geodynamic regime is different from those of the typical arc settings.

250 Ma metagranitoid from Drangovo Village: a new discovery of Permo-Triassic magmatism in the Eastern Rhodopes, Bulgaria

This short communication reports a 251.4 ± 6.8 Ma age of a Permo-Triassic metagranitoid (augen gneiss) in the Bulgarian part of the Eastern Rhodopes. The rock is intruded by the early Eocene Drangovo pluton and represents part of the upper metamorphic unit of the Kessebir dome. The analyzed sample has slightly peraluminous (ASI = 1.11) granitic composition with SiO2 = 70.6 wt.%. It is enriched in LILE and LREE and depleted in HREE, with a deep Eu (Eu/Eu* = 0.49) anomaly consistent with garnet and plagioclase fractionation. The large number of xenocrystic zircons, along with the low (780 °C) crystallization temperature and petrochemical data, suggests significant assimilation of basement rocks by the granitic magma. The rock has a subduction-related signature.

Petrogenesis of plagiogranites from the Troodos Ophiolite Complex, Cyprus

Small volumes of felsic melt, commonly known as oceanic plagiogranites, appear as melt pockets or dikes within the gabbroic section and the sheeted dikes root zone of the oceanic crust. Plagiogranites from the Troodos Ophiolite Complex on Cyprus are among the best exposures of felsic rocks that are embedded in a complete section of obducted oceanic lithosphere. Nevertheless, their exact petrogenesis is still a matter of debate, largely due to limited high-quality trace element and radiogenic isotope data. Previously proposed models for Troodos plagiogranites have included both low-pressure dehydration melting and fractional crystallisation at deeper levels of the oceanic crust. To evaluate both models, oceanic plagiogranites from the Troodos Ophiolite Complex were analysed in this study for their major and trace elements, and for the first time also for Hf–Nd–Sr isotope compositions. The trace element measurements also include for the first time high-precision measurements of high-field-strength element (HFSE) abundances that now permit to better unravel the petrogenesis of the Troodos plagiogranites and their possible mantle sources. In general, the Troodos plagiogranites exhibit a narrow range of Nb/Ta and Zr/Hf that overlap the compositions of mid-ocean ridge basalts (MORB). In line with earlier studies, three compositional groups can be identified: two groups formed by either fractional crystallisation or combined fractional crystallisation and wall rock assimilation, and one group derived from partial melting of slightly altered oceanic crust. The majority of the Troodos plagiogranites (Main Group) are the product of extensive fractional crystallisation of ambient arc-tholeiitic mafic melts. A second group of plagiogranites (Spilia Group) is generated by fractional crystallisation of boninitic precursor melts and the assimilation of arc-tholeiitic crustal material. Variable HFSE concentrations and diagnostic Hf–Nd isotope signatures that are unique to both suites allow discriminating between the two parental melts and fractionation processes. A small group of plagiogranites (Zoopigi Group) is interpreted to derive from partial melting in the conductive layer of active magma chamber lenses (AML). Elevated Nb/Ta, Zr/Hf, and light rare-earth element (LREE) enrichments in these rocks are in support of this model. Collectively, our data suggest that low-pressure fractional crystallisation (also in combination with assimilation of wall rocks) might be the predominant process controlling the formation of felsic rocks on Cyprus, whereas dehydration melting appears to be less important. If compared to Archean tonalitic–trondhjemitic–granodioritic suites (TTGs), compositions of plagiogranites from Cyprus mirror shallow-level processes in thin oceanic crust, which is illustrated by their narrow, MORB-like range of HFSE ratios and their distinct enrichment in heavy rare-earth elements (HREE) that distinguishes them from the Archean TTGs.

The Cambrian peraluminous Santa Luzia granite suite in the Araguaia Belt, central Brazil: Evidence for closure of the Clymene Ocean based on zircon and monazite U[sbnd]Pb data

The Araguaia Belt (AB) in northcentral Brazil formed as the result of the amalgamation of the Amazonian and São Francisco-Congo cratons along with the Parnaíba Block during late Neoproterozoic times. Ediacaran/Cambrian late- to post-orogenic magmatism took place at ∼540 Ma following this continental collision at 550-540 Ma. Such magmatism is represented in the southern portion of the Araguaia Belt by the peraluminous Santa Luzia and Barrolândia granites, as well as by the Presidente Kennedy Granodiorite in the central portion of the AB. Inherited zircon cores with distinct rims are abundant in the Santa Luzia Granite. This pluton yields UPb MC-LA-ICP-MS zircon and monazite ages of 538 ± 1 Ma and 534 ± 5 Ma, respectively. In addition, UPb zircon and monazite ages of 536 ± 6 Ma and 524 ± 6 Ma, respectively, are obtained for the Barrolândia Granite. The data presented in this work and in the literature allow to group the studied Santa Luzia and Barrolândia granites, together with Presidente Kennedy Granodiorite, in a single suite, named Santa Luzia Suite, maintaining the Santa Luzia pluton as the type-area. Rocks related to this Cambrian magmatism are essentially peraluminous, magnesian to slightly ferroan, and calc-alkaline in character. Such rocks probably originated from magmas formed by partial melting of meta-igneous sources that were subsequently contaminated via assimilation of metasedimentary rocks. In the AB, both the collision related to the 550-540 Ma Brasiliano/Pan-African orogeny, and the intimately associated ∼540 Ma late- to post-orogenic Santa Luzia Suite magmatism record the closure of the Clymene Ocean as well as the late stages of compressional tectonics related to west Gondwana amalgamation. The latter was followed by exhumation related to collapse of the Araguaia orogen.

Magmatic evolution of the Cerro Maricunga gold porphyry-epithermal system, Maricunga belt, N-Chile

The Cerro Maricunga gold project is located in the Maricunga gold belt in northern Chile, within the remnant edifice of the Tertiary Ojo de Maricunga volcano. The gold mineralization is hosted within a 23‒15 Ma old complex system of interwoven porphyry intrusions, flows, dikes and diverse breccia types, all forming part of a large diatreme system. In particular, phreatomagmatic and phreatic processes contributed to the formation of the synmineral diatreme complex. The rocks are calc-alkaline (daci)andesites and very rare dacites of the medium-K series with a high-K tendency and a conspicuous “adakitic” signature. Rare-earth element patterns reflect formation during a period of distinctly thickened continental crust with retention of heavy rare-earth elements by a garnet-bearing residue at a depth of ≥45 km. U-Pb zircon ages show a wide spectrum from Proterozoic, over Permian and Triassic ages to Paleocene, Oligocene, early and mid-Miocene ages. The Phanerozoic zircon ages indicate the assimilation of country rocks, whereas the Proterozoic age reflects the presence of a lower crustal Neoproterozoic basement that is also suggested by εNd values of −3.9 to −3.1 for bulk rock, i.e. mantle depletion model ages around 1 Ga. The last magmatic activity at the Ojo de Maricunga volcano was at ∼14.9 Ma. Stable isotope data on hydrothermal barite and supergene acanthite and gypsum (δ34Sacanthite: −0.6 to −0.4; δ34Ssulfate: 0.2 to 4.6; δ18Osulfate: 9.2 to 13.8) indicate an igneous origin of sulfur and arid conditions during mineral formation. The emplacement of the diatreme complex took place during the adjustment of the Chilean flat slab setting in a time of increased crustal thickness. Although the rocks at Ojo de Maricunga show a pronounced adakite-like signature, they formed in a common MASH zone or a deep hot zone within the lower crust. Gold mineralization took place at ∼17.9 Ma (Re-Os on molybdenite), syngenetic with diatreme formation and emplacement of hydrous magma. The thickening of continental crust was more advanced in the northern part of the Maricunga belt in the early to mid-Miocene than previously assumed, and then shifted south where several younger gold porphyries formed.

Spatial distribution of sulfur isotope ratios associated with the Eagle Ni-Cu sulfide deposit in Upper Peninsula, Michigan: Implications on country-rock contamination in a dynamic magma conduit

The Eagle Ni-Cu magmatic sulfide deposit in the Upper Peninsula of Michigan is hosted in mafic-ultramafic rocks associated with the 1.1 Ga Midcontinent Rift event. The deposit consists of massive, semi-massive and disseminated sulfide textural zones located in two closely spaced funnel-shaped intrusive bodies, called the Eagle and Eagle East intrusions. The intrusions are surrounded by metamorphosed supracrustal Paleoproterozoic rocks of the Michigamme Formation and at depth by a Neoarchean migmatitic banded gneiss. δ34S values were obtained from sulfide minerals separated by micro-drilling from different parts of the intrusions and the surrounding rocks. The δ34S values from the intrusions cluster within a small range between 1 and 3‰ relative to V-CDT regardless of the mode of occurrence, textural type, mineral type and location of the sample. However, disseminated sulfide minerals in the metasedimentary country rocks of the Michigamme Formation show a substantially greater range in δ34S values between 4 and 35‰ with a median of 7.7‰ while disseminated sulfides in the basement granite-gneiss show values ranging from −9 to 13‰ with a median of 2.5‰. The narrow range of δ34S values within the intrusions despite wide variabilities in the surrounding country rocks can be explained by the incorporation of crustal sulfur by a mantle-derived magma, followed by a homogeneous mixing of the immiscible sulfide liquid, with a silicate liquid to sulfide liquid ratio (R-factor) of 200 or more. The assimilation of crustal rocks and the mixing of the resultant immiscible sulfide liquid must have been facilitated by the rapid movement of magma along a narrow conduit system.

The oldest (~ 1.9 Ga) metadolerites of the southern Siberian craton: age, petrogenesis, and tectonic setting

Geological, geochronological, and isotope-geochemical studies of the metadolerites of the Angaul complex, widespread in the Urik-Iya graben of the southern Siberian craton, were carried out. The metadolerites forming separate conformal bodies (sills) among the metasandstones of the Ingash Formation were studied in detail. U-Pb zircon (SHRIMP) dating of metadolerites yielded an age of 1913 ± 24 Ma, and U-Pb baddeleyite (ID-TIMS) dating of these rocks yielded an age of 1914.0 ± 1.7 Ma. Thus, the date of 1914 ± 2 Ma can be taken as the most precise age estimate for the studied rocks. The metadolerites of the Angaul complex correspond in chemical composition to the normal-alkaline tholeiitic basalts. Metadolerites are differentiated rocks with mg# of 36 to 58. They show fractionated REE patterns: (La/Yb)n = 1.2-3.5. All metadolerites, independently of their mg# value, have low contents of Nb (1.6-10.2 ppm) and show well-pronounced negative Nb-Ta anomalies in multielement patterns (Nb/Nb* = 0.19-0.54). The metadolerites are characterized by positive εNd(T) values ranging from 0.4 to 5.2, which correlate well with their SiO2 content and mg# value. The isotope-geochemical parameters of the metadolerites of the Angaul complex indicate that fractional crystallization, along with the assimilation of the host rocks (AFC), might have been the main process during the formation of the most differentiated metadolerites. The geochemical characteristics of metadolerites with the maximum mg# values of 57-58 and εNd(T) = 5.2 suggest that the parental mantle source of the metadolerites resulted from mixing of predominant depleted mantle material with the subcontinental-lithosphere material. Intrusion of the dolerites of the Angaul complex, as well as the deposition of the sedimentary strata of the Ingash Group, took place at the Paleoproterozoic stage of intracontinental extension caused by the collapse of the orogen resulted from the collision of the Biryusa block with the Tunguska superterrane in the southern Siberian craton.

Geochronology, petrogenesis, and tectonic implications of Permian felsic rocks of the Central Great Xing’an Range, NE China

The Hegenshan–Heihe Suture (HHS) is an important suture zone between the Xing’an and Songliao–Xilinhot blocks in the eastern Central Asian Orogenic Belt (CAOB). Many studies have examined the northern and southern parts of this suture zone, but the middle segment remains under-researched and controversial. This paper presents new research into Permian felsic rocks along the middle segment of the HHS, including alkaline granites, rhyolites, and granodiorites. Zircon U–Pb dating of these rocks yielded precise Permian ages of 300.4 ± 2.5, 292.8 ± 2.7, and 293.1 ± 2.2 Ma. The alkaline granites and the rhyolites are A-type rocks that were derived from lower crustal protoliths, with the melting of these protoliths caused by asthenospheric upwelling associated with lithospheric delamination. In comparison, the granodiorites are I-type granites were generated from a mantle-derived magmatic source. The petrogenesis of these granites also involved the assimilation of sedimentary rocks and fractional crystallization. This Permian magmatism occurred during post-collisional extension following the amalgamation of the Xing’an and Songliao–Xilinhot blocks.

Mg–Sr–Nd isotopic constraints on the genesis of the giant Jinchuan Ni–Cu–(PGE) sulfide deposit, NW China

Magmatic sulfide deposits are principal resources of nickel and platinum-group elements (PGE) in the world. These deposits form as the result of the concentration of immiscible sulfide-liquid droplets segregated from mafic or ultramafic magma. Sulfide saturation in the magmas is commonly linked to contamination by the continental crust. The Jinchuan Ni–Cu–(PGE) deposit is the largest single magmatic sulfide deposit in the world. It is hosted by a small mafic–ultramafic intrusion in the North China Craton. The Jinchuan mafic–ultramafic intrusion intruded Paleoproterozoic marbles, gneisses, and granitic migmatites. This paper reports the magnesium, strontium and neodymium isotopic compositions of the intrusion and the country rocks. Olivine, pyroxene and whole rocks of the lherzolites from the Jinchuan intrusion have δ26Mg values ranging from −0.40‰ to −0.26‰, with a weighted mean value of −0.31 ± 0.06‰ (2SD), which is slightly lighter than expected in a mantle-derived magma. The marble country rocks have relatively light δ26Mg values varying from −2.00‰ to −1.17‰, with a weighted mean value of −1.53 ± 0.02‰ (2SD). The variations are attributed to the interaction between basaltic magma and marble country rocks as the principal mechanism which generated the low Mg isotopic signatures in the Jinchuan deposit. The (87Sr/86Sr)i and εNd values of the lherzolites vary from 0.706615 to 0.714612 and from −8.29 to −6.13, respectively. The marble country rocks have relatively high (87Sr/86Sr)i ratios (0.713662 to 0.740069) and low εNd values (−25.43 to −12.41). The negative Nb anomalies in all samples from the intrusion indicate that the parental magma underwent crustal contamination. Mg–Sr–Nd isotopic mixing calculations indicate that the Jinchuan intrusion underwent 10–20% bulk contamination involving the lower continental crust, followed by <10% bulk contamination by marbles. Our petrological and geochemical investigation of the Jinchuan intrusion provides evidence that a combination of the two processes led to the formation of the ore deposit. The assimilation of lower crustal rocks reduced the sulfur solubility; the assimilation of marble country rocks made the magma more oxidized and hence decreased the sulfur solubility further. The results from this study show that Mg isotopes are useful in the study of carbonate assimilation by mafic magma, and marble assimilation by basaltic magma is an important mechanism to produce sulfide saturation in the genesis of Ni–Cu–PGE sulfide deposits.

Coupled δ18O-δ17O and 87Sr/86Sr isotope compositions suggest a radiogenic and 18O-enriched magma source for Neapolitan volcanoes (Southern Italy)

The origin of large variations in stable and radiogenic isotope compositions of magmas erupted from the Neapolitan volcanoes, including Somma-Vesuvius and Campi Flegrei (Southern Italy), has always been contentious. Indeed, the role and relative importance of sediment subduction versus crustal assimilation to explain the chemical and isotopic variations of the erupted magmas remain unclear. Isotopic disequilibrium between minerals and their host indicate that bulk rock analyses are incapable of constraining the isotopic composition of the source. Therefore, we use isotopic (87Sr/86Sr, 18O/16O and 17O/16O) data on separated minerals (feldspar, clinopyroxene and olivine phenocrysts) from pyroclastic successions and lava flows of the Neapolitan volcanic area (Phlegrean Volcanic District and Somma-Vesuvius complex) to better constrain magmatic oxygen and strontium isotope ratios. Magmatic values recalculated from δ18O of olivine and clinopyroxene phenocrysts range from typical mantle values of 5.2‰ to almost 9‰ relative to SMOW. These compositions are very different from those of typical mantle sources. In order to assess the degree of magma evolution from which these minerals formed, the Mg# of clinopyroxene and olivine were converted into host melt Mg#, resulting in the range 44 to 76. Simple assimilation of silicic crustal rocks is difficult to reconcile with the mafic nature of these estimated host magma compositions. This indicates that some mafic, mantle-derived magmas, having unusually heavy oxygen isotope (up to ~9‰) and high Sr isotope compositions (0.7050 to 0.7085), must exist. Crustal assimilation of carbonates can be excluded by the lack of a link between isotopes and major and trace element signatures. Assimilation of either Hercynian-like crust or altered pyroclastic rocks, however, cannot be ruled out completely. However, assimilation of partial melts from a Hercynian-like crust would have to be ~12% and ~21% to explain the heavy oxygen isotope values of Campi Flegrei and Somma-Vesuvius, respectively. Such degrees of assimilation are unlikely since the magmas are either too mafic or too alkaline to be consistent with such high proportion of crustal components. Other, less mafic Campi Flegrei, Somma-Vesuvius and Ischia magmas with more typical mantle oxygen isotopes, have been possibly generated in a mantle source affected by minor contamination by pelagic and carbonate sediments and subsequent assimilation of Hercynian-like crust.Sr-O mixing models indicate that such magmas were derived from a mantle source that was contaminated by up to 10% of a 1:1 mixture of pelagic and carbonate sediments. These findings together demonstrate the highly complex and varied magma sources in the Campania volcanic district. Triple oxygen isotope variations (Δ17O), based on measurements of both 18O/16O and 17O/16O ratios in potential assimilants are in agreement with these conclusions.

Petrogenesis of the Rio Blanco epithermal Au-Ag mineralization in the Cordillera Occidental of southwestern Ecuador: Assessment from host rocks petrochemistry and ore constituents isotopic (O, S, H, and Pb) compositions

The petrogenesis of the Rio Blanco Au-Ag mineralization in the Cordillera Occidental (Western Cordillera), southwest Ecuador, was assessed using magmatic host rocks and ore constituents geochemical and isotopic (S, O, H, Pb and Sr) composition. The magmatic rocks spatially associated with the Rio Blanco Au-Ag mineralization are of arc affinity and mostly include lava flows of andesitic composition, as well as plutons and pyroclastic falls that range from dioritic (andesitic) to granodioritic (dacitic) compositions. These arc rocks as evidenced by their initial Sr isotope compositions (87Sr/86Sri: 0.70443–070694), which are symptomatic of crustal contamination, are of two contrasting magmatic suites (transitional and calc-alkaline), and typically display enrichment in LREE with respect to HREE, with resulting moderately elevated La/Yb ratios of 5.75–8.79.Unlike parental melts of most Miocene intrusions associated with porphyry-related mineralization from the Cordillera Occidental of Ecuador, which differentiated at a deep crustal level, the Rio Blanco parental magma, which was generated in a relatively thin crustal setting (as suggested by Sr/Y ratios) differentiated in the upper crust (as supported by the relatively low Sr/Y) at <0.4 GPa, where significant plagioclase fractionation took place as evidenced by their consistent negative Eu anomaly (Eu/Eu* ranges from 0.065 to 0.102), as well as the positive correlation between Sr/Y and Eu/Eu*. Important plagioclase fractionation was accompanied by minor amphibole and pyroxene fractionation as evidenced by their sporadic occurrence as phenocrysts in Rio Blanco magmatic rocks under investigation.Magmatic rocks spatially associated with the Rio Blanco Au-Ag mineralization also show a significant radiogenic signature, which was acquired through assimilation of various basement rocks within the upper crust, thus leading to heterogeneous and wide Pb isotopic composition ranges (206Pb/204Pbi = 18.605–19.174; 207Pb/204Pbi = 15.621–15701; 208Pb/204Pbi = 38.442–39.043). Sulfides and electrum are radiogenic as well (206Pb/204Pb ratios are between 19.002 and 19.143, whereas 207Pb/204Pb and 208Pb/204Pb ratios span from 15.644 to 15.697 and from 38.827 to 38.998, respectively) and overlap the compositions of the highly radiogenic magmatic rocks, indicating Pb and by inference, the other metals such as Au and Ag are of magmatic origin like S, which δ34S (V-CDT) values range between −3 and 4.9‰. Unlike S and metals of magmatic origin, the degased (as indicted by δD (V-SMOW) values range between −92.54 and −78.89‰) mineralizing H2O is a mixture between magmatic and meteoric fluids as supported by their δ18O (V-SMOW) values ranging from −0.959–4.21‰.

Geochemical and Petrological Studies on the Early Carboniferous Sidingheishan Mafic–Ultramafic Iintrusion in the Southern Margin of the Central Asian Orogenic Belt, NW China

The Sidingheishan mafic–ultramafic intrusion is located in the eastern part of the Northern Tianshan Mountain, along the southern margin of the Central Asian Orogenic Belt in northern Xinjiang autonomous region of China. The Sidingheishan intrusion is mainly composed of wehrlite, olivine websterite, olivine gabbro, gabbro and hornblende gabbro. At least two pulses of magma were involved in the formation of the intrusion. The first pulse of magma produced an olivine‐free unit and the second pulse produced an olivine–bearing unit. The magmas intruded the Devonian granites and granodiorites. An age of 351.4±5.8 Ma (Early Carboniferous) for the Sidingheishan intrusion has been determined by U‐Pb SHRIMP analysis of zircon grains separated from the olivine gabbro unit. A U‐Pb age of 359.2±6.4 Ma from the gabbro unit has been obtained by LA‐ICP‐MS. Olivine of the Sidingheishan intrusion reaches 82.52 mole% Fo and 1414 ppm Ni. On the basis of olivine‐liquid equilibria, it has been calculated that the MgO and FeO included in the parental magma of a wehrlite sample were approximately 10.43wt% and 13.14wt%, respectively. The Sidingheishan intrusive rocks are characterized by moderate enrichments in Th and Sm, slight enrichments in light REE, and depletions in Nb, Ta, Zr and Hf. The εNd (t) values in the rock units vary from +6.70 to +9.64, and initial 87Sr/86Sr ratios range between 0.7035 and 0.7042. Initial 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb values fall in the ranges of 17.23–17.91, 15.45–15.54 and 37.54–38.09 respectively. These characteristics are collectively similar to the Heishan intrusion and the Early Carboniferous subduction related volcanic rocks in the Santanghu Basin, North Tianshan and Beishan area. The low (La/Gd)PM values between 0.26 and 1.77 indicate that the magma of the Sidingheishan intrusion was most likely derived from a depleted spinel–peridotite mantle. (Th/Nb)PM ratios from 0.59 to 20.25 indicate contamination of the parental magma in the upper crust. Crystallization modeling methods suggest that the parental magma of the Sidingheishan intrusion was generated by flush melting of the asthenosphere and subsequently there was about 10vol% contamination from a granitic melt. This was followed by about 5vol% assimilation of upper crustal rocks. Thus, the high‐Mg basaltic parental magma of Sidingheishan intrusion is interpreted to have formed from partial melting of the asthenosphere during the break‐off of a subducted slab.

Geochemical and thermodynamical modeling of magmatic sources and processes for the Xiarihamu sulfide deposit in the eastern Kunlun Orogen, western China

Magmatic sulfide deposits in subduction-collision settings are genetically related to oxidized and hydrous magmas, where the degree of partial melting, fractional crystallization, and sulfide saturation are all expected to differ significantly from those in dry and reduced magmas associated with most giant magmatic sulfide deposits in intracratonic settings. This study provides petrological and geochemical data as well as multiple modeling approaches for the post-collisional Xiarihamu magmatic sulfide deposit of the eastern Kunlun Orogen to investigate melting mechanism and magmatic differentiation in an oxidized and hydrous magmatic system. Petrographical observations and whole-rock geochemistry suggest that the olivine orthopyroxenite, orthopyroxenite, websterite and gabbronorite in the Xiarihamu complex were cumulates. Pressures estimated from clinopyroxene and hornblende compositions and oxygen fugacities from hornblende and biotite compositions further suggest that these cumulates crystallized under moderate- to high-pressure, oxidized conditions. The Xiarihamu parental magma was high-MgO basalt as evidenced by multiple modeling methods. This parental magma was generated from a metasomatized mantle peridotite source in the spinel stability field with moderate melting degrees and then experienced a fractional crystallization process with a first iron-enriched and then silica-enriched trend under moderate-high pressures to form the observed crystallization sequence and mineral compositions. Minor amounts of wall rock assimilation during magmatic evolution were supported by Sr-Nd isotopic modeling. Sulfide saturation previously proposed by crustal sulfur addition at Xiarihamu was not straightforward but was likely caused by an oxygen fugacity decrease triggered by magmatic differentiation.

Spatial and temporal variability in Marquesas Islands volcanism revealed by 3He/4He and the composition of olivine-hosted melt inclusions

The Marquesas Islands are a volcanic chain having an oblique orientation to Pacific plate motion. They represent the surface manifestation of a mantle hotspot having a low melt flux and a large intrinsic chemical and isotopic heterogeneity. Here we provide new helium isotope data for olivine separated from 23 basalts and one gabbroic xenolith, plus whole rock major and trace element data for the same lavas. We also carried out trace element analyses of 41 olivine-hosted melt inclusions (OHMIs) from 15 of the basalts, and 40Ar/39Ar age determinations for basalt groundmass from the islands of Fatu Hiva and Hiva Oa. 3He/4He shows a large range of values from <5 RA up to 14.5 RA. Elevated 3He/4He ratios >9 RA are found at Nuku Hiva and Hiva Oa near the center of the archipelago, in agreement with previous findings (Castillo et al., 2007). The new ages for the Hiva Oa lavas indicate that 3He/4He increases from ~8 RA to 14.5 RA during the mid-late shield stage of volcanism (2.6–2.0Ma), followed by a decrease during the post-shield stage (<1.8Ma) after collapse of the Temetiu caldera. Lavas from Fatu Hiva, Ua Huka, Ua Pou, Tahuata and Motane have 3He/4He<8.3 RA, with values typically between 5 and 7 RA. Incompatible element ratios observed in the OHMIs show more variability than what is observed in the lavas because they represent a less extensive aggregation of melts derived from small scale chemical heterogeneities within the Marquesas mantle plume. The Cl/K ratio of OHMIs ranges from 0.02 to 0.4, potentially due to both mantle source heterogeneity and assimilation of brine or seawater-altered rocks at crustal levels. The geographic distribution of 3He/4He suggests the mantle plume is concentrically zoned in He isotope composition, with high 3He/4He material concentrated near its central axis that is sourced from hot, primitive material entrained near the core-mantle boundary region.

Loparite-(Ce) from the Khibiny Alkaline Pluton, Kola Peninsula, Russia

Data on the occurrence, morphology, anatomy, composition, and formation conditions of loparite-(Ce) in the Khibiny alkaline pluton are given. Loparite-(Ce), (Na,Ce,Sr)(Ce,Th)(Ti,Nb)2O6, resulted from metasomatic alteration and assimilation of metamorphic host rocks at the contact with foyaite as well as foyaite on the contact with foidolite. This alteration was the highest in pegmatite, and albitite developed there. A decrease in temperature resulted in enrichment of the perovskite and tausonite endmembers in loparite-(Ce) owing to a decrease in the loparite and lueshite endmembers. La and Ce sharply predominate among rare earth elements in the composition of loparite-(Ce).