Whole-rock Values Research Articles

Coupled U–Pb and Lu–Hf zircon data of Late Devonian magmatism from Bajo de La Leona (Deseado Massif, Argentina): Tracking the southernmost exposure of the Devonian magmatic arc of Patagonia

In Bajo de La Leona, located in the eastern Deseado Massif, pre-Permian basement rocks of the Río Deseado Complex crop out. They consist of high- to medium-grade metamorphic rocks intruded by two Paleozoic granitoids, namely the Mina de La Leona Granite and the Laguna Larga Granite. The latter is the main goal of this work. It is a calc-alkaline to high-K calc-alkaline, ferroan, calcic to alkali-calcic, and mainly peraluminous monzoleucogranite which plots within the I&S-type and volcanic-arc fields of the tectonic discrimination diagrams. Its trace elements composition (e.g., Sr/Y, LaN/YbN, Sm/Yb, Eu/Eu*, Dy/Dy*, Dy/Yb) suggests plagioclase ± clinopyroxene fractionation in a shallow source at low pressures. Assimilation could have played a role in magma differentiation, thus the magmatic development of the Laguna Larga Granite is likely a consequence of assimilation-fractional crystallization (AFC) processes. Coupled LA-ICP-MS U–Pb and Lu–Hf zircon data for the studied granite, as well as Sm–Nd whole-rock data, are here presented for the first time. A Famennian U–Pb zircon crystallization age of 361.9 ± 1.7 Ma was obtained, together with εHf(t) zircon values from -15 to +6 with TDM-Hf model ages between 2.0 and 0.9 Ga. A thin crust, as suggested by low values of thickness proxies such as Sr/Y (1.06–1.83) and LaN/YbN (3.97–4.96), would allow emplacement of juvenile mantle-derived material, giving rise to the suprachondritic εHf(t) values. A mantle-derived source is also suggested by a suprachondritic εNd(t) whole-rock value of +2.34 (TDM-Nd = 0.93 Ga). On the other hand, the subchondritic εHf(t) values could be related to wall rock assimilation. In a regional context, the obtained results for the Laguna Larga Granite allow to correlate it with coeval rocks from northern Patagonia (e.g., San Martín de los Andes batholith, Paleozoic Central Patagonian igneous-metamorphic belt). Therefore, the Devonian magmatic arc might have extended from the North Patagonian Andes and the western North Patagonian Massif to the eastern Deseado Massif along a NNW-SSE-striking belt, confirming previous proposals.

Uranium in groundwaters: Insights from the Leinster granite, SE Ireland

Uranium concentrations in groundwaters from >100 private wells in the Caledonian Leinster Granite Batholith of SE Ireland range from a few μg/l to >300 μg/l. Approximately 10% of the wells exhibit uranium concentrations that exceed the current WHO provisional guideline of 30 μg/l and a further 10% exceed the original WHO guideline value of 15 μg/l. New whole-rock geochemical analyses and laboratory batch leaching experiments confirm the granite as the ultimate source of the groundwater uranium, but carbonate-bearing glacial deposits that overlie the granite play an important role in facilitating the generation of soluble and mobile anionic uranyl carbonate complexes. Whilst the granite has average uranium concentrations broadly similar to other Irish and British Caledonian granites, it is enriched locally in uranium, with whole-rock values up to c. 30 mg/kg. Uraniferous zircon, apatite, monazite and associations with secondary Fe-bearing oxide minerals were detected in drillcore whole-rock samples that had the highest uranium concentrations, but uraninite was not detected. Broad positive correlations between groundwater uranium concentrations and bicarbonate alkalinity and electrical conductivity are found in both the sampled groundwaters and in laboratory leachates of the granite and surficial glacial deposits. Leachates from the granite had approximately an order of magnitude higher uranium concentrations compared with those from the overlying limestone-bearing glacial tills. (234U/238U) ratios for the groundwaters and the leachates always exceed unity, indicating preferential leaching of loosely bound radiogenic 234U from mineral surfaces or grain boundaries into the groundwaters, rather than wholescale dissolution of the primary uraniferous minerals in the granite. Strontium isotopes ratios in the groundwaters constrain the balance between limestone and granite-derived Sr, and by extension the sources of dissolved Ca. These data indicate that calcite dissolution is the main source of dissolved calcium, highlighting the role of weathering of overlying limestone-bearing glacial tills during recharge in determining the geochemistry of the groundwaters. Geochemical modelling indicates that in the oxidising groundwaters of the study area U6+ is dominant, in the form of UO2(CO3)3−4 at the high pH values (7.49–7.69) of waters from wells overlain by limestone-bearing till. By contrast, in a small number of wells overlain by granite-bearing till, modelling indicates that the dominant species is UO2(CO3)2−2 reflecting their lower pH (6.22–6.30). Overall, carbonate species play a major role in uranium solubility and mobility in these granite-hosted groundwaters.

Decoupled whole-rock and zircon Hf isotopes in young evolved post-collisional lavas from Dayingshan (SE Tibet): Evidence for open-system magmatic processes

Young zircons from volcanic rocks provide ideal samples for evaluating whole-rock and zircon Hf isotope equilibrium. We report zircon and whole-rock Hf isotopic compositions for Holocene post-collisional lavas (trachyandesites and trachytes) from Dayingshan volcano, SE Tibetan Plateau. Individual zircon εHf values differ from the host whole-rock value by up to 7.4ε units, and average zircon εHf values in individual samples are about 3ε units lower than the corresponding whole-rock εHf values, indicating discrepancy of zircon and whole-rock Hf isotopes. δ18O values for Dayingshan zircons vary from 6.1 to 7.8‰ and are negatively correlated with zircon εHf values. The decoupling of zircon/whole-rock Hf isotopic compositions is attributed to open-system magmatic processes rather than disequilibrium partial melting, as indicated by the negative correlation of zircon Hf-O isotopic compositions, as well as the correlations between whole-rock SiO2 content and Nd-Sr-Hf isotope ratios. The decoupling of zircon/whole-rock Hf isotopic compositions in Dayingshan lavas suggest that care must be taken when using individual zircon εHf values to study crust-mantle evolution. This Hf isotope decoupling might be more common than previously thought in continental post-collisional volcanic rocks where open-system magmatic processes are common.

Iron and magnesium isotope geochemistry in hydrothermal uranium ore systems: Insights from the Bong Deposit, Canada

The basement-hosted Bong uranium deposit in the Kiggavik region is associated with a broad hydrothermal alteration halo characterized by an inner illite-rich zone and an outer chlorite-rich zone. The uranium-bearing zones characterized by intense illite alteration yield high δ56Fe and δ26Mg values relative to the host rocks, with average whole-rock values of δ56Fe = 0.58 ± 0.15‰ and δ26Mg = 0.61 ± 0.29‰, and clay-size fraction values of δ56Fe = 0.51 ± 0.20‰ and δ26Mg = 0.82 ± 0.09‰. These isotopic values correlate with a decrease in average Fe2+ concentrations (0.47 versus 2.30 mol% Fe2+ in least-altered hosts).The high δ26Mg values reflect intense illite alteration, which preferentially incorporated heavier magnesium isotopes during alteration of both the ore zones and an upper hematite-altered horizon. Although the clay alteration with high δ56Fe and δ26Mg values coincides with the main uranium-bearing zone, the δ56Fe signatures reflect multiple stages of alteration and mineralization. The high δ56Fe values and lower Fe2+ contents of the uranium-bearing zones support leaching of Fe2+(aq) enriched in lighter iron isotopes. Sequential leach δ56Fe experiments also indicate that formation of fine-grained iron oxides/hydroxides with positive δ56Fe values (+0.23 to +0.60‰) in late chemical fronts enhanced the primary positive δ56Fe signatures recorded in ore zones. Near surface samples with similar isotopic signatures provide clues to buried mineralization and support structural disruption of the systems after primary mineralization. Distal geochemical expressions of this system may include enrichment in iron contents with negative δ56Fe values and/or negative δ26Mg whole-rock or clay-size fractions values, reflecting precipitation of the isotopically light iron and magnesium leached during hydrothermal alteration. Although more costly to process, sequential leach δ26Mg or δ56Fe analyses produced greater variance in the ore zones relative to background values and removed anomalous values from carbonate minerals. Isotopic analyses of the clay-size fraction produced results comparable to the final digestion in the sequential leach process but this method in particular requires greater understanding the mineralogical controls on the isotopic signatures.

Fluid-rock reactions of the Triassic Taiyangshan porphyry Cu-Mo deposit (West Qinling, China) constrained by QEMSCAN and iron isotope

The multiphase hydrothermal superposition phenomena on porphyry deposits, the mineral complexity and characteristics of the alteration-associated assemblages limit a further determination for the magmatic-hydrothermal evolution in porphyry deposits. The Taiyangshan porphyry Cu-Mo deposit in the northern margin of the Triassic West Qinling Orogenic Belt exhibits a multi-stage, alteration-related quartz sulfide veins that recorded element migration processes during fluid-rock reactions. In this study, we investigated hydrothermal veins in the Taiyangshan deposit focusing on vein compositions and structures as well as the evolution of ore formation, using quantitative evaluation of minerals by scanning electron microscopy (QEMSCAN) and iron isotope analyses. The formation of the early K-feldspar-biotite-quartz-magnetite-apatite veins is closely associated with the early potassic alteration, and during the fluid-rock reactions, Si, S, Fe, and K migrated in, while Mg, Ca and Na migrated out. K-feldspar-chlorite-pyrite-chalcopyrite-quartz veins correlated to the propylitic alteration, dominantly caused the migration of Si, Fe, S, Ca Mg, Al and K. Quartz-sericite-pyrite veins corresponding to the phyllic alteration processes that occurred in a relatively late stage, during the fluid-rock reactions of which, Si, Fe, S, and K migrated into the system. The δ56Fe whole-rock values of altered porphyries range from 0.08‰ to 0.31‰, and δ56Fe values of pyrite from quartz veins range from 0.30‰ to 0.57‰. Iron isotope compositions of both pyrites of the ore-bearing wall rocks and the quartz-sulfide veins of the Taiyangshan Cu-Mo deposit are generally uniform, which indicate the same source origin between the ore-bearing porphyry and the iron isotope compositions of the metal sulfide in the quartz veins, and additionally highlight that the ore-forming source is closely related to the ore-bearing porphyry. Magnetite precipitated in the early stage of potassic alteration, resulting in the enrichment of the light Fe isotope in the early fluids. Crystallization and precipitation of pyrite mainly occurred during processes of propylitic and phyllic alteration, leading to an absorption of the light Fe isotope, and elsewhere, an enrichment of the heavy Fe isotope in the fluids. During the fluid-rock reactions of propylitic and phyllic alteration, a large amount of S migrated into the veins, thereby promoting the fractionation of Fe isotope. Moreover, the development of the ore-forming fluids enriched with heavy Fe isotope from early potassic- to late phyllic alteration, are indicative of isotopic progressions from the early lithostatic pressure to the late hydrostatic pressure. Our research on the Taiyangshan Cu-Mo porphyry deposit helps to explain the evolutionary processes of the magmatic–hydrothermal system, and improves our understanding of the porphyry metallogenic evolution model.

Mineralogical and Isotopic Characteristics of Sodic-Calcic Alteration in the Highland Valley Copper District, British Columbia, Canada: Implications for Fluid Sources in Porphyry Cu Systems

AbstractThe Highland Valley Copper porphyry Cu (±Mo) district is hosted in the Late Triassic Guichon Creek batholith in the Canadian Cordillera. Fracture-controlled sodic-calcic alteration is important because it forms a large footprint (34 km2) outside of the porphyry Cu centers. This alteration consists of epidote ± actinolite ± tourmaline veins with halos of K-feldspar–destructive albite (1–20 XAn) ± fine-grained white mica ± epidote. The distribution of sodic-calcic alteration is strongly influenced by near-orthogonal NE- and SE-trending fracture sets and by proximity to granodiorite stocks and porphyry dikes. Multiple stages of sodic-calcic alteration occurred in the district, which both pre- and postdate Cu mineralization at the porphyry centers.The mineral assemblages and chemical composition of alteration minerals suggest that the fluid that caused sodic-calcic alteration in the Guichon Creek batholith was Cl bearing, at near-neutral pH, and oxidized, and had high activities of Na, Ca, and Mg relative to propylitic and fresh-rock assemblages. The metasomatic exchange of K for Na, localized removal of Fe and Cu, and a paucity of secondary quartz suggest that the fluid was thermally prograding in response to magmatic heating. Calculated δ18Ofluid and δDfluid values of mineral pairs in isotopic equilibrium from the sodic-calcic veins and alteration range from 4 to 8‰ and −20 to −9‰, respectively, which contrasts with the whole-rock values for least altered magmatic host rocks (δ18O = 6.4–9.4‰ and δD = −99 to −75‰). The whole-rock values are suggested to reflect residual magma values after D loss by magma degassing, while the range of hydrothermal minerals requires a mixed-fluid origin with a contribution of magmatic water and an external water source. The O-H isotope results favor seawater as the source but could also reflect the ingress of Late Triassic meteoric water. The 87Sr/86Srinital values of strongly Na-Ca–altered rocks range from 0.703416 to 0.703508, which is only slightly higher than the values of fresh and potassic-altered rocks. Modeling of those data suggests the Sr is derived predominantly from a magmatic source, but the system may contain up to 3% seawater Sr. Supporting evidence for a seawater-derived fluid entrained in the porphyry Cu systems comes from boron isotope data. The calculated tourmaline δ11Bfluid values from the sodic-calcic domains reach 18.3‰, which is consistent with a seawater-derived fluid source. Lower tourmaline δ11Bfluid values from the other alteration facies (4–10‰) suggest mixing between magmatic and seawater-derived fluids in and around the porphyry centers. These results imply that seawater-derived fluids can infiltrate batholiths and porphyry systems at deep levels (4–5 km) in the crust. Sodic ± calcic alteration may be more common in rocks peripheral to porphyry Cu systems hosted in island-arc terranes and submarine rocks than currently recognized.

Neodymium isotopes track sources of rare earth elements in acidic mine waters

Acid mine drainage (AMD) has been proposed as a potential source of strategic rare earth elements (REEs) due to its high concentrations (>1000 µg/L) of dissolved REEs, as well as the middle rare earth element (MREE) enriched pattern exhibited by both coal mine- and metal mine-derived AMD, in which some of the most energy-critical REEs are significantly concentrated. However, the source of REEs and the origin of the MREE enrichment observed in AMD discharges is poorly understood; suggestions include dissolution of MREE-enriched surface coatings, fractionation by colloids, solid-liquid exchange reactions, incongruent dissolution of pyrite, and selective leaching of MREE-enriched phases. Filtration experiments on acidic to circumneutral AMD (unfiltered to <0.002 µm) indicate that REEs are in a dissolved state. Thus, colloids are not a significant carrier of REEs in these samples and are not responsible for the observed MREE-enriched patterns. The similarity of MREE-enriched patterns exhibited by AMD to those of laboratory sulfuric acid (simulated AMD) leachates of corresponding overburden samples suggest that these lithologies are a major contributor of REEs to AMD. Isotopes of strontium (87Sr/86Sr) and neodymium (143Nd/144Nd) in AMD discharges are distinct from whole-rock values of associated lithologies, but fall within the range of values of the leachates. Whole rocks, leachates, and AMD define a trend on a Sm-Nd isochron diagram consistent with the ∼310 Ma depositional age of the coal. These data suggest that MREE enrichment of AMD results from preferential leaching of a readily dissolvable, MREE-enriched mineral phase, most likely apatite or a related phosphate phase, that formed during or shortly after deposition of the coal and associated units.

The behavior of Fe and S isotopes in porphyry copper systems: Constraints from the Tongshankou Cu-Mo deposit, Eastern China

Iron isotopes have been widely applied to trace mineralization processes of magmatic-hydrothermal ore systems, yet the application of Fe isotopes in porphyry Cu deposits (PCDs) is still restricted by the lack of understanding of its behaviors during hydrothermal alteration and ore formation processes. In this study, Fe isotope systematics in whole-rocks and Fe-bearing minerals coupled with S isotope compositions of Fe-sulfides are investigated for the early Cretaceous Tongshankou porphyry Cu-Mo deposit from Eastern China. The results show that altered ore-hosting granodiorite porphyries display a small δ56Fe variation (0.04‰ to 0.17‰) with an average of 0.10 ± 0.08‰ (2SD, n = 10), indistinguishable from the values for global igneous rocks. The δ56Fe of magnetite (0.22‰) and biotite (-0.08‰ to 0.12‰) from the altered porphyries are also within the δ56Fe ranges of those from global igneous rocks. These results indicate that hydrothermal fluids have negligible effect on Fe isotope compositions of the porphyry host rocks, likely due to the insignificant contributions of fluid-derived iron relative to the total Fe budget of the host rocks.The Fe-sulfides in the porphyry system record geochemical and iron isotopic characteristics of the ore-forming fluids. Specifically, chalcopyrites show large Fe isotope variations with δ56Fe ranging from −0.60‰ to 0.61‰. The calculated Fe isotope compositions of ore-forming fluids based on the chalcopyrite δ56Fe are thus not uniform and fall in the range from −0.69 ± 0.18‰ to 0.52 ± 0.18‰ (2SD). On the other hand, pyrites exhibit a δ56Fe variation between −0.48‰ and 0.40‰ and show distinct Fe isotopic characteristics for disseminated and vein-type pyrite. The δ56Fe for disseminated pyrites are consistently higher than whole-rock values, ranging from 0.14‰ to 0.40‰, likely reflecting equilibrium Fe isotopic exchange between pyrite and dissolved Fe in fluid. By contrast, δ56Fe for pyrites in veins are highly variable (−0.48‰ to 0.34‰) and show an inverse correlation with the chalcopyrite δ56Fe. These observations, in concert with S isotope data, have been interpreted to reflect pyrite formation through the FeS pathway, whereas the wide range in δ56Fe for pyrite may record isotopic change from initial FeS-fluid equilibrium towards pyrite-fluid equilibrium due to different extent of reaction upon precipitation. Additionally, when compared to sulfides formed by non-magmatic, low-temperature processes, the sulfides in our study exhibit a relatively narrow range of δ56Fe and δ34S, implying a high-temperature magmatic origin for both elements. Overall, our study demonstrates that the coupling of Fe and S isotopes could be a useful tool for tracking the processes of sulfide mineralization in porphyry-related deposits.

Lithium- and oxygen-isotope compositions of chondrule constituents in the Allende meteorite

We report in situ ion-microprobe analyses of Li- and O-isotope compositions for olivine, low-Ca pyroxene, high-Ca pyroxene, and chondrule mesostasis/plagioclase in nine chondrules from the Allende CV3 chondrite. Based on their mineralogy and O-isotope compositions, we infer that the chondrule mesostasis/plagioclase and ferroan olivine rims were extensively modified or formed during metasomatic alteration and metamorphism on the Allende parent asteroid. We excluded these minerals in order to determine the correlations between Li and both O and the chemical compositions of olivines and low-Ca pyroxenes in the chondrules and their igneous rims. Based on the O-isotope composition of the olivines, nine chondrules were divided into three groups. Average Δ17O of olivines (Fo>65) in group 1 and 2 chondrules are −5.3 ± 0.4 and −6.2 ± 0.4‰, respectively. Group 3 chondrules are characterized by the presence of 16O-rich relict grains and the Δ17O of their olivines range from −23.7 to −6.2‰. In group 1 olivines, as Fa content increases, variation of δ7Li becomes smaller and δ7Li approaches the whole-rock value (2.4‰; Seitz et al., 2012), suggesting nearly complete Li-isotope equilibration. In group 2 and 3 olivines, variation of δ7Li is limited even with a significant range of Fa content. We conclude that Li-isotope compositions of olivine in group 1 chondrules were modified not by an asteroidal process but by an igneous-rim formation process, thus chondrule olivines retained Li-isotope compositions acquired in the protosolar nebula. In olivines of the group 3 chondrule PO-8, we observed a correlation between O and Li isotopes: In relict 16O-rich olivine grains with Δ17O of ∼−25 to −20‰, δ7Li ranges from −23 to −3‰; in olivine grains with Δ17O > −20‰, δ7Li is nearly constant (−8 ± 4‰). Based on the Li-isotope composition of low-Ca pyroxenes, which formed from melt during the crystallization of host chondrules and igneous rims, the existence of a gaseous reservoir with a δ7Li ∼ −11‰ is inferred.

The age and tectonic significance of the Warraweena Volcanics and related rocks, southern Thomson Orogen

Magmatic-textured zircon from medium- to high-K calc-alkaline Warraweena Volcanics (WV) in two drill holes have yielded concordant U–Pb dates of 417 ± 3.5 and 414 ± 4.0 Ma and are interpreted as maximum emplacement ages. The Warraweena volcanics were previously considered to be either Neoproterozoic or Macquarie arc equivalents. Whole-rock εNdt values of these volcanics are +4.5 and +4.8. Along strike of the drill holes, Devonian zircon U–Pb ages (411 ± 5.5 Ma) were obtained from coherent S-type rhyolite flows that have highly negative εNdt values (–7.9 and –7.8). These are a component of the Oxley volcanics. The ages of the Warraweena and Oxley volcanics are identical within uncertainty.The Oxley volcanics (OV) are interbedded with predominantly fine- to medium-grained metasedimentary and so imply a Lower Devonian deposition age for these host rocks. Based on their geophysical characteristics, the metasediments are widely distributed. These metasedimentary rocks yield a wide range of maximum depositional ages, from Early Devonian to earliest Ordovician–latest Cambrian, similar to the Cobar Basin. The absence of complex fabric development typical of Ordovician supracrustal rocks in the region, and conformity with the OV where observable suggest the widespread sedimentation was synchronous with rift-related volcanism in the Early Devonian.Regionally, the WV is temporally, geochemically and isotopically (εNd values) similar to the calc-alkaline Louth Volcanics located over 100 km to the southwest of the WV. Louth Volcanics define a complexly folded belt in geophysical data. Other potentially correlative Early Devonian igneous rocks occur in the nearby Cobar Superbasin and elsewhere in the eastern Lachlan Orogen and are considered to represent the products of a post-orogenic, nascent continental back-arc rift system.

The dynamics of the southern Okinawa Trough magmatic system: New insights from the microanalysis of the An contents, trace element concentrations and Sr isotopic compositions of plagioclase hosted in basalts and silicic rocks

The Okinawa Trough (OT) is a back-arc, initial marginal sea basin located behind the Ryukyu arc–trench system. Analysis of the plagioclase phenocrysts from two basalts and two silicic rocks dredged from different locations in the southern Okinawa Trough (SOT) provides a detailed record of the nature and composition of and changes in the magma within the magmatic system. Plagioclase phenocrysts from the basalt record 87Sr/86Sr values of 0.704015–0.704360 and 0.704686–0.704863, respectively, which overlap with previously reported whole-rock values and suggest that the parent magma of the plagioclase represents upwelling mantle materials influenced by a subducted component. Plagioclase-melt thermobarometry records the crystallization of plagioclase phenocrysts in the basalt at the bottom of the lower crust near the Moho (depths of ~20 ± 7.5 km and ~25 ± 7.5 km). The temporal record of the Sr and Ba contents in the melt recorded by these crystals, which were calculated using partition coefficients and the Sr isotopic compositions of the plagioclases from the basalts, demonstrates the existence of two distinct mafic magmas within the SOT due to the presence of different subducted components (i.e., the subduction of the Gagua Ridge).Significant textural variations and wide ranges in the An35-89 and trace element concentrations and in situ Sr isotopic compositions (0.705748–0.707520) are observed in plagioclase crystals from silicic rocks in the SOT. The barometry results for the plagioclases from silicic rocks yield a wide range of crystallization depths ranging from 6.6 to 17.8 km. The Sr isotopic compositions and trace element concentrations of high-An80 plagioclases in basalt and silicic rocks indicate that there is no direct relationship between these two magma chambers. The negative correlation between the An contents and the Sr isotopic compositions in the plagioclases from silicic rocks indicates that more upper continental crust materials joined the magma source as its evolution proceeded. The presence of core-sieved plagioclase in the silicic rock indicates that decompression and/or magma mixing occurred. Thus, we propose a multilayer magma chamber system to explain the complex plagioclase crystals in silicic rocks. Two potential mush zones were recognized; their generation may have been controlled by the adiabatic decompression melting of lower crustal materials resulting from crustal extension or the post-collisional extension of the Northern Taiwan Mountain Belt. The two potential mush zones at greater depths are relatively basic, whereas the shallow magma reservoirs, which are the host magmas of the plagioclase, are relatively silicic.

Mineralogy, fluid inclusions and C–H–O–S–Pb isotopes of the Palaeocene Longgen Pb-Zn deposit in the western Nyainqentanglha belt, Tibet

The medium-sized Longgen Pb-Zn deposit deposit (proven Pb + Zn resources of 0.13 Mt, 3.21% Pb and 2.43% Zn) is located in the western Nyainqentanglha belt in southern Tibet. The orebodies occurring as stratiform, lenticular and vein types are hosted in the skarn or marble along the contacts between the limestones and the granite porphyries. Alteration zonations are observed from granite porphyry to limestone, the proximal garnet is reddish-brown and becomes more pale brown and green with distance. The values of grossular gradually increase, while the andradite end members gradually decrease with distance (And(92.5−99.5)Gro(0.5−7.5)Pra(0.0−0.1) to And(9.0−36.5)Gro(63.4−90.9)Pra(0.1−0.5)). Similarly, the values of diopside (Di9.2–30.4Hd65.9–89.9 Jo0.6−3.7 to Di65.2–77.6Hd21–33.2Jo1.3−1.7) gradually increased close to the limestone. Retrograde metasomatic minerals overprint the prograde skarn and are dominantly composed of epidote, actinolite, ilvaite, and chlorite. The ore minerals consist mainly of galena and sphalerite, followed by magnetite, pyrite, pyrrhotite, chalcopyrite, molybdenite and bornite.At least five stages have been recognized, and from stage I to stage V, the δ18Ofluids values are 7.7% to 8.3%, −6.0% to −4.4%, −8.8% to −6.4%, −8.7% to −3.3%, and −13.2%, respectively. The δD values from stages I to V are −141.1% to −129.4%, −136.5% to −124.2%, −129.5% to −106.4%, −126.6% to −101.4%, and −106.8%, respectively. The δD and δ18Ofluids indicate that the ore-forming fluids gradually evolved from magmatic to meteoric in origin. The main types of fluid inclusions for the Longgen deposit are liquid-rich inclusions. Laser Raman analysis indicates that the fluid inclusions within the studied minerals are dominated by H2O, although some contain CO2 gas. Given the fluid inclusion data, the ore-forming fluids of stages I and II were most likely high-temperature (460–480 and 340–360 °C) and low-moderate salinity (7.9–15.0 and 2.4–12.6 wt% NaCl eq.) fluids, which were formed in a magmatic–hydrothermal system in stage I and as a mixture of magmatic and meteoric waters in stage II. In the subsequent stages (III and IV), the fluids changed to moderate temperature (230–270 and 160–200 °C) and low-moderate salinity (0.2–14.8 and 0.9–10.6 wt% NaCl eq.) mixed fluids of magmatic and meteoric waters. In the last stage V, the fluids changed to low-temperature (130–160 °C) and low-salinity (0.2–5.6 wt% NaCl eq.) meteoric water.The δ13C values in calcite lie within a narrow range of −6.1% to −4.4%, similar to that for the magmatic source, which indicates that the carbon was sourced from the granite porphyry. Sulfur (δ34S = 0.4–5%, estimated δ34Sfluids = 2.77%) and lead isotopic compositions of sulfides (206Pb/204Pb = 18.549–18.776, 207Pb/204Pb = 15.685–15.839 and 208Pb/204Pb = 38.94–39.592, similar to the whole-rock values for the granite porphyry) indicate that the ore-forming materials were derived mainly from the granite porphyry, which originated from the Nyainqentanglha Group. Consequently, taking the data together suggests that the Longgen Pb–Zn deposit is a typical skarn deposit related to Palaeocene magmatic-hydrothermal systems, and that fluid cooling caused by fluid mixing is the most crucial mechanism for Pb-Zn deposition.

Monazite Behaviour and Time-scale of Metamorphic Processes along a Low-pressure/High-temperature Field Gradient (Ryoke Belt, SW Japan)

Low-pressure/high-temperature metamorphic rocks exposed in the western part of the Ryoke belt (Iwakuni–Yanai area, SW Japan) include a section with increasing temperature conditions from ∼425 to 880°C. We use this setting to explore the evolution of monazite grain size, texture and composition, and variations in the whole-rock composition of 11 metapelite, metapsammite or metachert samples collected along the metamorphic field gradient. Monazite grain size increases with rising metamorphic grade, regardless of the whole-rock composition. From low- to high-grade conditions we infer: (1) the initial nucleation of monazite aggregates after allanite (∼425°C); (2) monazite coarsening and coalescence driven by incipient monazite recycling; that is, dissolution of small grains to grow larger ones by Ostwald ripening (500–600°C); (3) a first major recycling stage enhanced by fluid liberation owing to muscovite breakdown (600–630°C); (4) a second recycling stage assisted by an increase in the proportion of anatectic melt owing to biotite breakdown (> 850°C). A succession of four compositional domains is recognized in monazite. We emphasize the usefulness of comparing their Ce/ThMnz, Ce/YMnz and Th/UMnz molar ratios with those derived from whole-rock analyses to constrain the origin of each domain. Domain I, with variable ratios, reflects the progressive transfer of Th ± U from allanite to monazite at low-grade conditions. Domain II, with Ce/ThMnz matching the whole-rock values, indicates growth under rock(decimetre)-scale equilibrium conditions. Domains II and III, with Th/UMnz and Ce/YMnz departing from the whole-rock values, record the competition with zircon (for U) and garnet (for Y) during growth at peak P–T conditions. Domain IV points to Y supply by garnet resorption during retrograde chloritization (< 550°C). In the highest-grade sample, zircon grains included in garnet or cordierite show metamorphic rims with sillimanite and Si-rich inclusions. These rims formed at suprasolidus conditions (650–880°C) and yield 206Pb/238U ages of 103–97 Ma (± 5 Ma), which bracket the timing of high-temperature metamorphism. Monazite dating by electron microprobe and laser ablation inductively coupled plasma mass spectrometry reveals two age groups. For domains I–III, some relatively old 206Pb/238U ages (99–95 ± 3–5 Ma) represent minimum estimates for the timing of prograde to peak metamorphism, whereas the similar oldest 206Pb/238U age for domain IV (93 ± 7 Ma) points to rapid cooling after the temperature peak. A more dominant population of younger Th–U–Pb and 206Pb/238U dates is ascribed to age resetting by heterogeneous annealing of the monazite crystal lattice. The youngest concordant 206Pb/238U age of 86 ± 4 Ma is correlated with the end of intermediate-temperature conditions (< 200–300°C) below which monazite, in metamorphic rocks as well as in the surrounding granitoids, eventually behaved as a closed isotopic system. This contribution is dedicated to the memory of Professor Kazuhiro Suzuki, for his contribution to geochronology and Japanese geology.

Exceptionally high whole-rock δ18O values in intra-caldera rhyolites from Northeast Iceland

ABSTRACTThe Icelandic crust is characterized by low δ18O values that originate from pervasive high-temperature hydrothermal alteration by18O-depleted meteoric waters. Igneous rocks in Iceland with δ18O values significantly higher than unaltered oceanic crust (~5.7‰) are therefore rare. Here we report on rhyolitic intra-caldera samples from a cluster of Neogene central volcanoes in Borgarfjörður Eystri, Northeast Iceland, that show whole-rock δ18O values between +2.9 and +17.6‰ (n= 6), placing them among the highest δ18O values thus far recorded for Iceland. Extra-caldera rhyolite samples from the region, in turn, show δ18O whole-rock values between +3.7 and +7.8‰ (n= 6), consistent with the range of previously reported Icelandic rhyolites. Feldspar in the intra-caldera samples (n= 4) show δ18O values between +4.9 and +18.7‰, whereas pyroxene (n= 4) shows overall low δ18O values of +4.0 to +4.2‰, consistent with regional rhyolite values. In combination with the evidence from mineralogy and rock H2O contents, the high whole-rock δ18O values of the intra-caldera rhyolites appear to be the result of pervasive isotopic exchange during subsolidus hydrothermal alteration with18O-enriched water. This alteration conceivably occurred in a near-surface hot spring environment at the distal end of an intra-caldera hydrothermal system, and was probably fed by waters that had already undergone significant isotope exchange with the country rock. Alternatively,18O-enriched alteration fluids may have been produced during evaporation and boiling of standing water in former caldera lakes, which then interacted with the intra-caldera rock suites. Irrespective of the exact exchange processes involved, a previously unrecognized and highly localized δ18O-enriched rock composition exists on Iceland and thus probably within the Icelandic crust too.

Fluid and metal sources of the Wenquan porphyry molybdenum deposit, Western Qinling, NW China

The Wenquan porphyry moybdenium deposit, Western Qinling, NW China, with a resource of 247 million tonnes at 0.048% Mo, formed during Triassic collision between South China and North China blocks. Ore fluids at Wenquan vary widely in composition from single-phase, low-salinity aqueous to low-salinity vapor, and hypersaline inclusions. Fluid inclusion assemblages in a quartz-molybdenite vein contain >60vol% vapor, are equant to negative-crystal shaped, and form clusters or distinct inclusion trails. They yield homogenization temperatures of 285–295°C, with an estimated trapping temperature of 425°C, suggesting a paleodepth of about 5km calculated at a pressure correction of 100–150MPa. Hydrothermal K-feldspar from early stockwork veins related to potassic alteration have calculated δ18Ofluid values of −1.9‰ to +1.9‰. Hydrothermal sericite from an overprinting phyllic alteration associated with late quartz-pyrite veins has calculated δDfluid values between −68 and −60‰, and δ18Ofluid values from −3.7 to +1.4‰. These isotopic data suggest that both early- and late-stage fluids are dominated by magmatic fluids, with influx of meteoric water during the late stage. Exsolution of volatiles from magma in a late-stage open system, results in more variable δD values than an earlier closed-system.Molybdenite and pyrite have δ34S values ranging from 1.1‰ to 6.6‰, indicating that sulfur at Wenquan had a magmatic source. A linear relationship between δ34S values of sulfides separated from early potassic alteration and late phyllic alteration could reflect incorporation of isotopically heavy evaporate sulfate into source magmas from underlying Devonian sedimentary rocks during late alteration. The δ56Fe whole-rock values of altered porphyries range from 0.08‰ to 0.26‰, similar to δ56Fe values of 0.15‰–0.32‰ for pyrite from quartz veins related to both the potassic and phyllic alteration assemblages. The δ56Fe values of pyrite are positively correlated to, but in general slightly lighter than, those of altered porphyries, indicating similar metal sources. Moreover, both altered porphyries and the pyrite are progressively enriched in heavy Fe isotopes from biotite, through potassic to late phyllic alteration, consistent with isotopic evolution from an early lithostatic load to a late hydrostatic load.The Triassic intrusive rocks and fluids responsible for mineralization were derived from a deeply-sourced hybrid mantle-crustal magma crystallizing at a paleodepth of 5km under lithostatic load. External Late Triassic meteoric or Devonian formational fluids became a part of the hydrothermal system during the post-fracturing final stage of ore formation under a hydrostatic regime. The formation of the Wenquan magmatic-hydrothermal systems spans the ductile-brittle transition based on a normal thermal gradient as constrained by the temperature range of 550–300°C. The early, high-temperature potassic alteration developed under a ductile regime, whereas the later, low-temperature phyllic alteration correlates to a brittle environment, where hydraulic rock fracturing enhances permeability and fluid circulation. Most sulfur and metals were precipitated from vapor-rich fluids resulting from cooling and expansion of a single-phase fluid exsolved from a hybrid magma.

KREEP basalt petrogenesis: Insights from 15434,181

AbstractReturned lunar KREEP basalts originated through impact processes or endogenous melting of the lunar interior. Various methods have been used to distinguish between these two origins, with varying degrees of success. Apollo 15 KREEP basalts are generally considered to be endogenous melts of the lunar interior. For example, sample 15434,181 is reported to have formed by a two‐stage cooling process, with large orthopyroxene (Opx) phenocrysts forming first and eventually cocrystalizing with smaller plagioclase crystals. However, major and trace element analyses of Opx and plagioclase coupled with calculated equilibrium liquids are inconsistent with the large orthopyroxenes being a phenocryst phase. Equilibrium liquid rare earth element (REE) profiles are enriched relative to the whole rock (WR) composition, inconsistent with Opx being an early crystallizing phase, and these are distinct from the plagioclase REE equilibrium liquids. Fractional crystallization modeling using the Opx equilibrium liquids as a parental composition cannot reproduce the WR values even with crystallization of late‐stage phosphates and zircon. This work concludes that instead of being a phenocryst phase, the large Opx crystals are actually xenocrysts that were subsequently affected by pyroxene overgrowths that formed intergrowths with cocrystallizing plagioclase.

Carboniferous Alaskan-type complex along the Sino–Mongolian boundary, southern margin of the Central Asian Orogenic Belt

We present zircon ages and geochemical data for the Hongshishan Carboniferous Alaskan-type mafic–ultramafic complex exposed in the Beishan area along the Sino–Mongolian boundary, southern margin of the Central Asian Orogenic Belt. This complex mainly consists of dunite, harzburgite, lherzolite, wehrlite, and gabbro, which intrudes Early Carboniferous volcanic rocks and reveals a zoned structure. Zircons of a gabbro sample yielded a 206Pb/238U age of 357 ± 4 Ma, reflecting the time of Early Carboniferous magmatism. Zircon ages were also obtained for an andesite (322 ± 3 Ma) and a basaltic andesite (304 ± 2 Ma). High initial Nd isotope whole-rock values suggest that the Hongshishan gabbro [e Nd(t) = +9.6 − +10.2] and basalt [e Nd(t) = +10.0 − +10.8] were derived from a depleted mantle source. Slightly lower e Nd(t) values for the ultramafic rocks [e Nd(t) = +8.5 − +8.7] suggest some interaction of the parental magma with the continental crust. In contrast, the Late Carboniferous Quershan samples in this area represent subduction-related arc volcanic rocks with Adakite-like compositions. The early Carboniferous Hongshishan Alaskan-type complex was interpreted to represent the remnants of a magma chamber that crystallized at the base of a mature island arc, whereas the Quershan island arc volcanic rocks suggest the resurrection of the subduction process after arc-continent collision and uplift of the roots of the arc.

Quantifying crustal thickness over time in magmatic arcs.

We present global and regional correlations between whole-rock values of Sr/Y and La/Yb and crustal thickness for intermediate rocks from modern subduction-related magmatic arcs formed around the Pacific. These correlations bolster earlier ideas that various geochemical parameters can be used to track changes of crustal thickness through time in ancient subduction systems. Inferred crustal thicknesses using our proposed empirical fits are consistent with independent geologic constraints for the Cenozoic evolution of the central Andes, as well as various Mesozoic magmatic arc segments currently exposed in the Coast Mountains, British Columbia, and the Sierra Nevada and Mojave-Transverse Range regions of California. We propose that these geochemical parameters can be used, when averaged over the typical lifetimes and spatial footprints of composite volcanoes and their intrusive equivalents to infer crustal thickness changes over time in ancient orogens.

A syn-collisional model for Early Cretaceous magmatism in the northern and central Lhasa subterranes

Voluminous Early Cretaceous volcanic rocks from the northern and central Lhasa subterranes contain important information on the tectono-magmatic processes. In this contribution, we focus on the Lower Cretaceous volcanic rocks in the Nagqu area, northern Lhasa subterrane, and present their zircon LA-ICP-MS U–Pb ages, in situ Hf isotopic data, whole-rock major and trace element compositions, and Sr–Nd isotopic data. The Nagqu volcanic rocks are high-K calc-alkaline to shoshonitic (K2O: 1.41–8.52wt.%; K2O/Na2O: 0.36–6.65) and feature similar geochemical characteristics (e.g., subparallel distribution of incompatible elements and REEs). Assimilation–fractional crystallization of magmas from identical source (rather than diverse magma sources) was mainly responsible for the formation of the diverse volcanic rock types. High Hf/Sm (>0.7) and high U/Yb whole-rock values and low Y values in zircon grains suggest the involvement of terrigenous components rather than subducted oceanic crust in the magma source. Tectonic discrimination diagrams, sedimentary environment (marine–continental transition), magma compositions (K-rich and terrigenous components), crustal thickening, and spatio-temporal variations in subduction, syn-collisional and post-collisional processes, indicate that the geodynamic setting of the Early Cretaceous magmatism was associated with the collision between the Lhasa and Qiangtang terranes. The high-K characteristics were inherited from the melts derived from the partial melting of lower metasomatized lithospheric mantle (the K-rich layer), which was transported to great depths by the continuously thickening lithosphere, eventually triggering melting.

Hafnium isotope characteristics of late Palaeoproterozoic magmatic rocks from Blekinge, southeast Sweden: possible correlation of small-scale Hf and Nd isotope variations in zircon and whole rocks

The isotopic composition of hafnium has been measured in zircons from seven magmatic rocks from the Blekinge region, southeast Sweden, previously dated by the U–Pb zircon method to 1.74–1.76 Ga, with one rock at 1.81 Ga. The results from all samples show limited variation, with a total range in initial ϵHf between − 1.5 and +3.2, only slightly more than analytical uncertainty ( ± 1.5 epsilon units). However, taken together with previously published initial ϵNd whole-rock values of − 0.3 to +1.0 from the same rock samples, a trend with a positive correlation between average initial ϵHf (zr) for each sample and initial ϵNd (WR) can be discerned. This trend could be taken to indicate mixing between a “mildly depleted” component derived from the lithospheric mantle or lower crust, and a slightly more enriched crustal component, either by mixing of separate magmas or by contamination of a mantle-derived magma with crustal material. Such mixing may have occurred both on a regional scale, as indicated by the overall trend, and locally within each intrusion and each rock sample with small-scale variations in isotope composition of the magma during crystallization, indicated by the variation in initial ϵHf between the zircons in each sample. The overall results support the presence of a “mildly depleted” lithospheric mantle reservoir, both in terms of Nd and Hf isotopes, also beneath southern Fennoscandia in Proterozoic time.