Amphibole Analyses Research Articles

Constraints of upper crust magmatic processes on the formation of Saindak porphyry Cu deposit: Insights from mineral geochemistry

Upper crustal magmatic processes play a crucial role in linking the magmatic and hydrothermal systems of porphyry deposits, ultimately controlling the generation of ore-forming fluids. Understanding these processes within a magma chamber is essential for deciphering the formation of porphyry deposits. The Saindak deposit, a large porphyry Cu system in the western Chagai belt, features coeval ore-forming and barren magma pulses, providing key insights into the characterization of porphyry systems. In this study, detailed petrographic observations and geochemical analyses of amphibole, plagioclase, and apatite were conducted to unravel the magmatic evolution at Saindak. Based on textural relationships and crystallization sequences, three distinct populations of amphibole in the ore-forming magma were identified. These amphiboles record the entire fractional crystallization process of magma that initially emplaced at a depth of ∼5 km to the connected magma system at ∼2 km. The study of amphibole and apatite reveals that the fertile magma underwent large-scale fluid exsolution in an open system. The simultaneous and rapid depletion of Cu, Cl, S, and H2O in the magma indicates that Cl and S were highly partitioned into the exsolved fluid phase. These elements formed complexes, which effectively concentrated metals in the fluid. Conversely, the barren magma was characterized by either low metal content or limited fluid exsolution within the magma chamber. Overall, we reconstruct the upper crustal magmatic processes at Saindak and conclude that efficient extraction of Cu through fluid exsolution is the key to porphyry deposit formation.

Effectiveness of Ti-in-amphibole thermometry and performance of different thermometers across lower continental crust up to UHT metamorphism

Metabasites are important constituents of deep crustal sections and are the favored rock type for studying lower crustal amphibolite to granulite transitions. However, metapelites may develop a larger number of temperature-sensitive mineral assemblages and are particular useful when extreme ultrahigh temperature (UHT) conditions are envisaged. A recent calibration of the Ti-in-amphibole thermometer by Liao et al. (2021) was supposed to make thermometry on metabasites quick and easy to apply. However, their calibration is based on experiments which were not originally designed to investigate in detail the temperature dependence of Ti in amphibole. In addition, a possible effect of aTiO2 and/or pressure on the Ti content of amphibole was not fully taken into account. This resulted in a calibration uncertainty of ± 70 °C (2σ), much higher than that of other single-mineral thermometers. In this study we firstly test the newly calibrated Ti-in-amphibole thermometer across the mid to lower crustal section of the Ivrea–Verbano Zone (IVZ; NW Italy) and compare the performance of different thermometric techniques across the sequence. Ti-in-amphibole thermometry records increasing peak temperatures from amphibolite (600–700 °C), transition (750–800 °C) and granulite (850–950 °C) zones. Titanium content of amphibole may be modified by retrograde fluid influx returning temperatures c. 200–300 °C lower than in non-altered domains. The comparison reveals that Zr-in-rutile thermometer in pelitic granulites seems to be more prone to post-peak resetting than the Ti-in-amphibole thermometry in nearby mafic rocks. This behavior is also confirmed by amphibole analyses from other UHT localities, where the performance of Ti-in-amphibole thermometry is comparable with that of Al-in-orthopyroxene in pelitic granulites. However, Ti-in-amphibole temperatures are underestimated in rutile-bearing samples and this limitation is not solely restricted to rocks containing high H2O contents as previously thought. Derived constraints on the diffusivity of Ti through amphibole demonstrate the robustness of the Ti-in-amphibole thermometer to later thermal disturbances. However, ad-hoc experiments are still necessary to improve the accuracy and precision of calibration and to extend its applicability. This advance will make mafic granulites routine targets for studies devoted to understanding the regional extent of UHT metamorphism.

Mineral chemistry and P–T conditions of the winchite-bearing metabasic rocks in the NE edge of the Menderes Massif (Western Türkiye)

The Menderes massif is a large metamorphic crystalline complex located in western Turkey. The massif consists of late Neoproterozoic basement (core) rocks and Palaeozoic - Cenozoic cover units that have undergone by high- and low-grade polyphase metamorphisms. Palaeozoic to Mesozoic cover units cropped out in the NE edge of the massif (east of Sivaslı) are overlaid tectonically by upper Cretaceous meta-ophiolitic rocks. The meta-ophiolitic unit comprises blocks mainly of metabasalt, metagabbro, metadiorite and metaultramafite within an intensively sheared serpentinite matrix. It also includes blocks of epidote-actinolite-schist and tremolite-actinolite-schist, which originated from basic rocks, as well as chlorite schist blocks, which originated from ultramafic rocks. Sodic-calcic amphiboles recorded in the samples of metabasalt block taken from the marble-metabasalt block boundary, at the tectonic contact where the meta-ophiolitic rocks overlap the underlying marble sequence. Sodic-calcic amphiboles were classified into winchite and ferri-winchite with relatively homogeneous TSi (7.35–8.01 a.p.f.u.), and XMg (0.69–0.80) values. P-T conditions were estimated to be around 300–400 °C and 5–6 kb based on the mineral chemical analyses of the sodic-calcic amphiboles. According to these values, the NE edge of the Menderes Massif must have undergone metamorphism under a medium-pressure (MP) greenschist facies.

Recognizing crystal accumulation in plutonic system: Evidence from mafic microgranular enclaves in Early Silurian Qingshan granodiorite, North Qilian Orogenic Belt, NW China

Granitoids are the primary constituents of the continental crust; recognizing and quantifying the effects of crystal accumulation in evolved magmas is a challenging task. Here we present whole-rock chemical, Sr-Nd isotopic, zircon U-Pb geochronological, and Hf isotopic data as well as EPMA and in-situ LA-ICP-MS analyses of amphiboles from Qingshan granodiorite and mafic microgranular enclaves (MMEs) from the North Qilian Orogenic Belt (NQOB). The MMEs and granodiorite have identical crystallisation ages (ca. 434 Ma), the same mineral assemblage, but different mineral modal proportions as the MMEs comprise more amphibole. The MMEs and granodiorite have indistinguishable and depleted zircon Hf (εHf (t) values of 6.1–8.3, and 6.4–9.1, respectively) and whole-rock Sr-Nd isotopic compositions, i.e. (87Sr/86Sr)i values of 0.70514–0.70913 and 0.70207–0.70552, respectively; and εNd(t) values of 0.27–2.23 and 0.21–2.39, respectively. The occurrence of idiomorphic amphibole grains, low MREEs and HREEs concentration, low Mg# values of equilibrium melts (AEMs), and decreasing Zr/Hf ratios indicate that Qingshan MMEs represent cumulates formed at a thermal boundary layer between the ascending conduits and the wall rocks. The Qingshan granodiorite was formed by partial melting of subducted oceanic crust with crustal materials during southward subduction of the North Qilian Ocean, followed by further amphibole-dominated fractionation by compaction without crystal deformation. We propose a model of southward subduction with the back-arc basin for the tectonic evolution of the western NQOB. Both the subduction- and collision-related granitoids of the NQOB have played a vital role in continental crust growth.

Water contents and hydrogen isotope compositions of amphibole in aillikites from the Tarim large igneous province, NW China: Insight into Earth’s deep water cycle

Water is known to play a crucial role in the generation of many large igneous provinces (LIP) worldwide, but the amount and origin of the water in their sources is still under debate. To address this question, this paper presents in situ major-, trace-element, and Sr isotope data coupled with bulk-mineral O-H-He isotope analyses of amphibole in a suite of aillikites from the Tarim LIP (NW China). The cores of zoned macrocrysts and microcrysts display partially overlapping compositions ranging between edenite and pargasite (75−83 versus 75−80 Mg#), which suggest a common origin from an evolving magma. The rims (Mg# = 75−80) of both macrocrysts and microcrysts are very similar to their cores for many elements, except for higher Sr and Ba contents. All the amphibole zones show similar primitive mantle−normalized trace element patterns, suggesting that they crystallized at different stages during magmatic evolution. This interpretation is confirmed by the homogenous Sr isotope compositions (87Sr/86Sr(i) = 0.70298−0.70394) of these amphiboles, which overlap with those of magmatic apatites and perovskites in these aillikites. The hydrogen isotope compositions (δD = −120‰ to −140‰) of the amphiboles are significantly lower than average mantle values. Given the low water contents (<0.66 wt%) of these minerals, the low H isotope signatures of the amphiboles might be caused by variable H2O loss during magma ascent. However, open-system Rayleigh fractionation modeling suggests that the hydrogen isotope compositions of these amphibole phenocrysts cannot be fully reproduced by crystallization following magmatic degassing. These low δD values require incorporation of recycled altered oceanic crust containing hydrous components in the plume source of these aillikites, which is consistent with the previously published moderately radiogenic He isotope ratios of olivine separates and bulk-rock Os and Pb isotope data for these same samples. We conclude that water in these magmas was derived from a plume source containing recycled water-bearing oceanic crust.

Crustal Growth Identified by High-δ18O Zircon and Olivine: A Perspective from Ultramafic Arc Cumulates in Southern Tibet

Abstract In recent studies of crustal growth using global zircon Hf–O isotopic datasets, high-δ18O zircons are typically attributed to intra-crustal reworking during which very little juvenile mantle-derived magmas were added to the crust. Although arc magmas may originate from a high-δ18O mantle wedge, it has been difficult to decipher the contribution of high-δ18O mantle to zircon-saturated felsic magma due to superimposed intra-crustal processes. We address this issue by combining the data from high-δ18O zircon-bearing ultramafic cumulates and coeval lavas from a Cretaceous magmatic arc in southern Tibet. The cumulates mainly consist of different proportions of cumulus olivine and intercumulus amphibole. Amphibole analyses show a transition from increasing to decreasing Zr with increasing SiO2 (50–74 wt.%) contents in the intercumulus melts, indicating zircon saturation in late-stage interstitial melts. The εNd(t) values (2.4 ± 1.4) of the apatite grains crystallized before and after zircon remain almost constant. Interstitial zircons have δ18O (6.1–7.2‰) values similar to the earliest crystallized olivine (δ18O = 6.3–7.1‰) in the cumulates. The coeval lavas may represent the intercumulus melts extracted from amphibole-rich cumulates at different depths. Both the lavas and cumulates were ultimately derived from high-δ18O arc mantle modified by small amounts (<12%) of subducted sediments, and crystallized zircon during intra-crustal magma evolution without involving crustal contamination or melting. These high-δ18O zircons therefore are not products of crustal reworking, but record crustal growth during their crystallization (110 ± 2 Ma). Our study shows that the combination of zircon and olivine oxygen isotopes for ultramafic to felsic rocks is more effective than zircon data alone in evaluating the role of crustal growth vs. reworking in an arc system. The implication is that global zircon-based crustal evolution models that attribute all high-δ18O zircons to crustal reworking may conceal recent crustal growth.

Magmatic evolution in an open and polybaric system feeding the Miocene calcalkaline lava domes of Cixerri (SW Sardinia, Italy)

The lava domes of the Cixerri half-graben (SW Sardinia) are part of the subduction-related igneous activity that developed in Sardinia during the Upper Eocene-Middle Miocene (38–12 Ma), with calcalkaline and high-K calcalkaline affinity. The investigated rocks are porphyritic basaltic andesites and andesites, with amphibole, plagioclase and minor clinopyroxene phenocrysts included in a groundmass composed of the same phases together with feldspar, quartz and opaque oxides. 40Ar/39Ar analyses of plagioclase and amphibole separates from an andesite indicate a crystallization age of 21.31 ± 0.05 Ma. The high abundance of amphibole in the Cixerri rocks, unlike andesites from the neighboring Sardinian districts, could be explained with the high calculated oxygen fugacity (NNO+1–NNO+2) and H2O content (up to 9 wt%) in the Cixerri magmas. Bulk-rock major and trace element variations, and mass balance calculations are consistent with a magma evolution mainly driven by fractional crystallization of amphibole and plagioclase, which occurred in a polybaric plumbing system, as highlighted by the calculated pressure of amphibole crystallization (2–4 kbar and 6–9 kbar). Bulk rock isotope variations (87Sr/86Sri = 0.70701–0.70786 and 143Nd/144Ndi = 0.512328–0.512436) indicate that the magma evolution took place in open-system conditions with a low degree of crustal assimilation. The HREE flat patterns point to a magma source in the spinel stability field located in a mantle wedge depleted in incompatible elements and metasomatized by slab and sediment derived fluids and melts.

New Data on the Rock and Mineral Composition of Kharchinsky and Zarechny Volcanoes (Central Kamchatka depression): Heterogeneity of the Mantle Source and Peculiarities of Magma Evolution in Crust

The Kharchinsky and Zarechny volcanoes and the Kharchinsky Lake zone of monogenetic cones are unique eruptive centers of magnesian lavas located above the northern margin of the Pacific Plate subducting beneath Kamchatka. This work presents new geochemical data on the composition of rocks (55 samples) and minerals (over 900 analyses of olivine, pyroxenes, amphibole, and plagioclase) of these centers analyzed by XRF and LA-ICP-MS (rocks) and electron microprobe (minerals). Most of the studied rocks are represented by magnesian (Mg# = 60–75 mol. %), medium-K basalts and basaltic andesites. Moderate-magnesian (Mg# = = 52–59 mol. %) basaltic andesites are present among the monogenic cones of the Kharchinsky Lake. The rare rock varieties include high-K basalts-basaltic andesites of dikes in the center of the Kharchinsky volcano and magnesian andesites (Mg# = 58–61 mol. %) of the extrusions of Zarechnу volcano. The distribution of trace element contents in these samples demonstrates the enrichment of large-ion lithophile elements, light REEs and depletion of high field strength elements and heavy REEs typical of arc rocks. High-K basalts and basaltic andesites show anomalous enrichment in Ba (1000 ppm), Th (3.8 ppm), U (1.8 ppm), Sr (800 ppm, Sr/Y 50) and light REE (La 20 ppm); their compositions are close to those of low-Si adakites. Basalts and basaltic andesites contain high-Mg olivine phenocrysts (up to Fo92.6) and clinopyroxene (Mg# up to 91 mol. %). The rocks show petrographic and geochemical signs of fractional crystallization along with the processes of mineral accumulation and magma mixing. Some of the olivine phenocrysts show high NiO contents (up to 5000 ppm) and elevated Fe/Mn ratio (up to 80), interpreted as evidence of participation of the pyroxenite source in the magma generation processes. The use of Ca/Fe and Ni/Mg ratios allowed us to distinguish the composition fields and evolution trends of olivines associated with different sources – peridotite and pyroxenite, formed by the reaction of mantle wedge peridotites and high-Si melts of the subducted oceanic crust. The new data are consistent with other evidence of melting of the subducted Pacific plate edge beneath the northern part of the Central Kamchatka depression at the Kurile-Kamchatka and Aleutian subduction zone junction and testify to significant heterogeneity of the mantle in this area.

Reactive interaction between migmatite-related melt and mafic rocks: clues from the Variscan lower crust of Palmi (southwestern Calabria, Italy)

Abstract. In the Variscan lower–intermediate crust exposed in the Palmi area (southwestern Calabria, Italy), amphibolites occur as foliated, decimeter-thick layers within migmatitic paragneiss and as a decametric main body adjacent to the migmatites. The main body is mostly fine-grained and weakly to moderately foliated; unfoliated medium-grained portions rarely occur. Amphibolites are mainly composed of plagioclase (An80−91) frequently developing triple junctions, amphibole consisting of cummingtonite rimmed by hornblende and variable amounts of biotite. Minor quartz is present in amphibolite layers within paragneiss. Accessory allanite occurs in amphibolite layers within migmatites and in foliated, fine-grained portions from the main body. This study mainly aims to achieve information about the effects triggered by the migration of migmatite-related melts into associated mafic rocks and its role in the re-distribution of major and trace elements out of the anatectic source. On the basis of whole-rock major- and trace-element compositions, the protolith of amphibolite is recognized as of cumulus origin, likely derived from basic melt emplaced in the framework of the late-Variscan lithospheric extension. The rocks experienced high-temperature subsolidus re-equilibration (∼800 ∘C) in conjunction with the development of amphibole. The origin of amphibole is attributed to a coupled dissolution–precipitation process related to the reaction between migrating SiO2-rich hydrous melt and precursor orthopyroxene (± plagioclase). Reactive melt migration also caused the crystallization of biotite ± allanite ± quartz ± plagioclase. SiO2-rich hydrous melt had REE (rare earth element) compositions similar to late-Variscan peraluminous granites and could have been derived by partial melting of metasediments akin to neighboring migmatitic paragneisses. Both whole-rock and amphibole analyses reveal a decrease in Mg# (Mg/(Mg+Fe2+)) from amphibolite layers within paragneiss to fine- and medium-grained rocks of the main body. Hornblende shows an increase in SiO2 and a decrease in Al2O3 and K2O with increasing Mg#. Amphibolites interlayered with paragneiss have higher K2O, Rb, Ba, Th, U and Zr relative to those from the main body. Furthermore, amphibole from amphibolites interlayered with paragneiss is distinct for relatively high Rb, Ba, MREE (middle rare earth element) and HREE (heavy rare earth element) concentrations. Within the main body, foliated, fine-grained rock has both the whole rock and amphibole enriched in Rb and Ba and high Zr bulk-rock contents. Whole-rock and mineral chemistry heterogeneity most likely reflects (i) variation of the composition of the melt during its reactive migration, in response to dissolution of pre-existing minerals and crystallization of new phases, and (ii) variable modification of the original compositions. Foliated and fine-grained amphibolites record the strongest modification, thereby suggesting that they represent permeable pathway enabling effective interaction of the reacting melt with precursor minerals and nucleation of new mineral phases.

Fluorine-rich mafic lower crust in the southern Rocky Mountains: The role of pre-enrichment in generating fluorine-rich silicic magmas and porphyry Mo deposits

AbstractFluorine-rich granites and rhyolites occur throughout the southern Rocky Mountains, but the origin of F-enrichment has remained unclear. We test if F-enrichment could be inherited from ancient mafic lower crust by: (1) measuring amphibole compositions, including F and Cl contents, of lower crustal mafic granulite xenoliths from northern Colorado to determine if they are unusually enriched in halogens; (2) analyzing whole-rock elemental and Sr, Nd, and Pb isotopic compositions for upper crustal Cretaceous to Oligocene igneous rocks in Colorado to evaluate their sources; and (3) comparing batch melting models of mafic lower crustal source rocks to melt F and Cl abundances derived from biotite data from the F-rich silicic Never Summer batholith. This approach allows us to better determine if the mafic lower crust was pre-enriched in F, if it is concentrated enough to generate F-rich anatectic melts, and if geochemical data support an ancient lower crustal origin for the F-rich rocks in the southern Rocky Mountains.Electron microprobe analyses of amphibole in lower crustal mafic granulite xenoliths show they contain 0.56–1.38 wt% F and 0.45–0.73 wt% Cl. Titanium in calcium amphibole thermometry indicates that the amphiboles equilibrated at high to ultrahigh temperature conditions (805 to 940 °C), and semiquantitative amphibole thermobarometry indicates the amphiboles equilibrated at 0.5 to 1.0 GPa prior to entrainment in magmas during the Devonian. Mass balance calculations, based on these new measurements, indicate parts of the mafic lower crust in Colorado are at least 3.5 times more enriched in F than average mafic lower crust. Intrusions coeval with the Laramide Orogeny (75 to 38 Ma) pre-date F-rich magmatism in Colorado and have Sr and Nd isotopic compositions consistent with mafic lower crust ± mantle sources, but many of these intrusions contain elevated Sr/Y ratios (>40) that suggest amphibole was a stable phase during magma generation. The F-rich igneous rocks from the Never Summer igneous complex and Colorado Mineral Belt also have Sr and Nd isotopic compositions that overlap with the lower crustal mafic granulite xenoliths, but they have lower Sr/Y, higher Nb and Y abundances, and distinctly less radiogenic 206Pb/204Pbi compositions than preceding Laramide magmatism. Batch melt modeling indicates low-degree partial melts derived from rocks similar to the mafic lower crustal xenoliths we analyzed can yield silicic melts with >2000 ppm F, similar to estimated F melt concentrations for silicic melts that are interpreted to be parental to evolved leucogranites.We suggest that F-rich silicic melts in the southern Rocky Mountains were sourced from garnet-free mafic lower crust, and that fluid-absent breakdown of amphibole in ultrahigh temperature metamorphic rocks was a key process in their generation. Based on the composition of high-F amphibole measured from lower crustal xenoliths, the temperature of amphibole breakdown and melt generation for these F-enriched source rocks is likely >100 °C higher than similar lower crust with low or average F abundances. As such, these source rocks only melted during periods of unusually high heat flow into the lower crust, such as during an influx of mantle-derived magmas related to rifting or the post-Laramide ignimbrite flare-up in the region. These data have direct implications for the genesis of porphyry Mo mineralization, because they indicate that pre-enrichment of F in the deep crust could be a necessary condition for later anatexis and generation of F-rich magmas.

Nb-Ta systematics of Kohistan and Gangdese arc lower crust: Implications for continental crust formation

• Kohistan and Gangdese (garnet) hornblendites and/or garnet gabbros are of cumulate origins. • Amphiboles in these lower crust cumulates have high Nb/Ta but low Nb and Ta concentrations. • Amphibole fractionation will not change Nb/Ta ratio of arc magmas significantly. • Rutile likely plays an important role in Nb/Ta fractionation in continental crust formation. Earth’s continental crust has a lower Nb/Ta ratio than that of its building blocks –basaltic melts from the mantle. This Nb/Ta mass imbalance implies the existence of a complementary high Nb/Ta reservoir in deep crust. Amphibole-rich arc cumulates have been proposed to be one such candidate due to the preferential partitioning of Nb over Ta into amphibole. To test this proposal, we carried out in-situ analyses of amphibole and rutile in the lower crustal rocks from Kohistan arc and Gangdese arc. Petrological observations and geochemical results suggest that these rocks in both Kohistan arc and Gangdese arc are of magmatic origins and formed by crystal accumulation, i.e. of cumulate. The Kohistan garnet-bearing hornblendite cumulates were formed at 952–1009 ℃; the garnet-free hornblendite cumulates in Kohistan arc and Gangdese arc were formed at 941–992 ℃ and 955–1027 ℃, respectively. We find that, although the amphibole of the Kohistan and Gangdese cumulates have high Nb/Ta ratios (average 18.2 and 17.0, respectively), their Nb and Ta concentrations are far lower than mantle-derived volcanics (Nb < 1.1 ppm, Ta < 0.11 ppm). We show that, with such low concentrations of Nb and Ta, amphibole fractionation will not change arc magma Nb/Ta ratios significantly. Instead, the rutile in Kohistan cumulates has high Nb/Ta ratios and high Nb-Ta concentrations. The calculation shows that rutile may play a more important role in Nb/Ta fractionation during arc magma differentiation in thickened arc crust. Our study provides a mineralogical perspective to understand the formation of continental crust, and improves our understanding how Nb or Ta becomes enriched in arc setting.

Crystal mush rejuvenation induced by heat and water transfer: Evidence from amphibole analyses in the Jialuhe Composite Pluton, East Kunlun Orogen, northern Tibet Plateau

Crystal mush rejuvenation is capable of magma differentiation, evolution, migration, and mixing and magma mixing may act as a trigger for crystal mush rejuvenation. The Jialuhe Composite Pluton in the eastern part of East Kunlun Orogen, northern Tibet Plateau, is a product of magma mixing that contains abundant mafic microgranular enclaves (MMEs) that often form dike–like clusters in the central parts of the pluton that is composed of porphyritic amphibole granodiorite containing medium– to coarse–grained amphibole crystals and fine–grained biotite, whereas the external zone consists of biotite granodiorite with dominant biotite accompanied by a small fraction of amphibole. Amphibole aggregate bands developed within the MMEs consist of densely packed amphibole megacrysts. We selected amphiboles from the MMEs, host rocks, and amphibole aggregate bands to study mush rejuvenation triggered by magma mingling. The amphibole megacrysts are euhedral and show zoning from cores to rims with decreasing in Si, Mg, and Na, and increasing in K, Al, Fe, and Ca. We have classified these amphibole megacrysts into three types according to their different rare earth element concentration zoning from cores to rims, which reflect recharge of dioritic magma and the final chemical equilibrium in the melt. The physicochemical crystallization conditions of the three types of amphibole megacrysts in the amphibole aggregate bands indicated that the temperature and oxygen fugacity decreased, while the pressure and water content in melt increased during amphibole crystallization. These results combined with petrography, suggest that injection of dioritic magma gave rise to the MMEs and transferred heat and water to the host granitic mush. This transference resulted in coarsening of amphiboles in the host mush, especially within the granodiorite patches encaged within the MMEs, where the coarsened amphiboles joined to forming the aggregate bands. Therefore, these bands are a good indicator of crystal mush rejuvenation caused by the combination of heat and introduction of water, but the rejuvenation process was limited and local. Recharge of mafic magma in transcrustal felsic mush systems could not rejuvenate the entire felsic magma reservoir, but only parts near the mafic magma recharge channel.

Trace element composition of amphibole and petrogenesis of hornblendites and plutonic suites of Cretaceous magmatic arcs developed in the Fuegian Andes, southernmost South America

The evolution of continental crust in convergent margins can be explored in southernmost South America (54-56°S). Plutonic rocks of the Fuegian Batholith and the rear-arc satellite Ushuaia Pluton were emplaced within the magmatic arc and the Fuegian fold-and-thrust belt, respectively. They record subduction zone processes in two distinct tectonic settings during the evolution of the Rocas Verdes Basin. We report new U-Pb zircon geochronology, bulk rock chemistry, Sr-Nd isotope data, and EPMA and in-situ LA-ICP-MS analyses of amphibole from ‘hornblendites’ and gabbroic-granitoid suites in order to evaluate the origin and evolution of the magmatic plumbing systems in the upper plate of the subduction zone. Textural relationships and amphibole compositions in hornblendite indicate crystallization at lower crustal depths with pressures of 7-8 kbar in the Fuegian Batholith and of 5-6 kbar in the Ushuaia Pluton. Lower Cretaceous suites of hornblendite and calc-alkaline hornblende-gabbro, diorite and tonalite in the Fuegian Batholith have εNdt values ranging between +2 and +4. They were emplaced within an island arc coeval with mid-oceanic type spreading in the Rocas Verdes back-arc basin. Isotope ratios and amphibole compositions in hornblendite indicate crystallization from primitive and hydrous sub-alkaline basaltic melts with relatively low LREE/HREE and low alkali contents. The Late Cretaceous plutons in the fold-and-thrust belt were emplaced after the tectonic juxtaposition of Rocas Verdes ophiolitic complexes. The Ushuaia Pluton, consisting of clinopyroxene-hornblende cumulates, hornblende-gabbro, diorite and monzodiorite, was emplaced during the waning stage of Late Cretaceous magmatism. In this case hornblendite amphiboles show high contents of alkalis, LREE and incompatible elements with a strong crustal affinity (Th, Ba, Rb). The enriched incompatible trace element patterns indicate their derivation from K-rich transitional magmas formed in supra-subduction settings. Chemical variations in amphibole from hornblendites and spatially related plutonic rocks are evaluated in terms of fluid flux from the subducted slab and partial melting of the sub-arc mantle, ultimately controlled by the thermal state of the subducted slab and convergence rates.

Experimental study of amphibole crystallization from the highly magnesian melt of Shiveluch volcano

The paper reports results of an experimental study of amphibole crystallization from the highly magnesian andesite melt of Shiveluch volcano, Kamchatka. The experiments were carried out in IHPV at 300 MPa and 940–980°С in iron-saturated platinum capsules, using rapid quenching and temperature oscillations (in some experiments). The redox state of iron in the system was measured before and after the experiments using Mössbauer spectroscopy. The maximum size of the experimental amphibole crystals (up to 200 μm) was close to those of natural amphibole phenocrysts in the volcanic rocks of Shiveluch volcano. The experimental data show that the content of octahedrally coordinated Al (Al6) in the amphibole considerably varies with small variations in the intensive parameters (P, T, and fO2) and composition of the melt, and the maximum Al6 concentration can be evaluated only by using a reasonably large dataset of amphibole analyses. A modified 13eCNK method is suggested to calculate the values of Al6 and Fe3+/Fe2+ with regard for the Ti concentration and the probable partial transfer of Mg into site B in high-Mg amphibole. Calculations with this modified technique yield lower Fe3+/Fe2+ and higher Al6 values. Our experimental data show that the temperature of amphibole liquidus crystallization decreases from about 990 to 960°C when the oxygen fugacity drops from NNO + 1.5 to NNO + 0.4. In view of this, the transition from amphibole-bearing to anhydrous mineral assemblage in the magmas of Shiveluch volcano might have been caused by variations of the oxygen fugacity but not water. The application of our geobarometer to amphiboles from Shiveluch volcano (extrusions Krasnaya and Karan) yields the highest pressure estimate of above 1 GPa, corresponding to the P-T conditions of the melting of garnet-bearing amphibolite in the lower crust.

Measurement of elongate mineral particles: What we should measure and how do we do it?

The length distributions of single fibrils of Coalinga, UICC-B and wet dispersed chrysotile were measured by transmission electron microscopy (TEM). It was found that the distributions significantly diverged above approximately 10 μm (μm) in length, corresponding to differences in published results of animal experiments. This result is in contrast to published data in which counting of an insufficient number of fibers resulted in an erroneous conclusion that the length distribution of Coalinga chrysotile fibrils was indistinguishable from those of other sources of chrysotile.The size distributions of the respirable particle size fractions from acknowledged tremolite asbestos samples were found to be dominated by elongate particles longer than 5 μm that are within the dimensional range of non-asbestiform amphiboles. Prior studies have shown that these elongate particles obscure a correlation between a specific size range of particles and results of animal implantation studies that used tremolite of various morphologies. In the prior studies, a reference protocol was developed from four crushed non-asbestiform amphiboles to differentiate the size range of amphibole particles that correlates with the mesothelioma frequencies observed in the animal studies. In the work reported here, this correlation was tested with TEM analyses of amphiboles from Libby, MT, Sparta, NJ and Homestake mine, Lead, SD, which represent known environmental/occupational situations. Further TEM analyses of the tremolite samples used in the original animal implantation studies have also shown that the numbers of elongate tremolite particles with lengths ≤5 μm implanted into the animals are not correlated with the observed mesothelioma frequencies.

Magmatic evolution and porphyry–epithermal mineralization in the Taftan volcanic complex, southeastern Iran

The Taftan volcanic complex is located above the Makran subduction zone in Sistan and Baluchestan province, southeastern Iran. The earliest volcanic activity at Taftan started in the late Miocene (∼8 Ma) with eruption of andesitic to dacitic lava onto a Cretaceous to Eocene volcanic and sedimentary paleosurface ∼20 km to the northwest of the current volcanic edifice. Later Plio-Pleistocene volcanism consisted of voluminous andesitic and dacitic lavas and pyroclastic flows (∼3.1 to 0.4 Ma). Taftan, and the neighbouring Bazman volcano, are the southeasternmost and youngest manifestations of arc-related volcanism in Iran, which began with the Eocene–Miocene Urumieh-Dokhtar Magmatic Arc (UDMA) in northwest and central Iran, and extends into western Pakistan (Chagai Arc).Major porphyry Cu±Mo±Au deposits are associated with Miocene to Pliocene intrusive rocks in the Kerman section of the Eocene–Neogene Urumieh-Dokhtar Magmatic Arc in southeastern Iran (e.g., Sar Cheshmeh, Meiduk), and at Reko Diq and Saindak in the Late Cretaceous–Quaternary Chagai Arc in western Pakistan. In a gap of ∼300 km between these two belts, several porphyry and epithermal prospects are exposed in the Miocene–Pliocene sections of the Taftan volcanic complex, including the Kharestan (6.10 ± 0.80 Ma) and Bidester porphyry-epithermal Cu-Au deposits (∼4.3 Ma), and the Siah Jangal epithermal Au deposit (late Miocene). In addition, several argillic to advanced argillic and fumarolic alteration zones occur in Plio-Pleistocene volcanic rocks around the current volcanic edifice. These deposits have received limited exploration.Electron microprobe analyses of amphibole (magnesio-hastingsite) phenocrysts and magnetite–ilmenite mineral pairs from the Taftan and Bazman volcanic rocks indicate a change of crystallization temperature and magmatic oxidation state from ∼1000 °C and ΔFMQ ≈ +1 in andesitic rocks, to ∼900 °C and ΔFMQ ≈ +2 in dacitic rocks. Magmatic water content was >4 wt%, as indicated by the ubiquitous presence of amphibole phenocrysts.Major and trace element compositions of the Taftan and Bazman volcanic rocks show calc-alkaline to high-K calc-alkaline affinity, with relative depletions of Nb, Ta, and Ti and enrichments of large-ion c elements (LILE), Th, and U on normalized diagrams. These trace element patterns, including listric-shaped normalized rare earth element profiles and relatively high Sr/Y and La/Yb ratios, are similar to those of fertile Miocene igneous rocks from the Kerman Belt and Chagai Arc, and suggest that the Taftan suite in particular should be prospective for porphyry Cu ore formation.Regionally, there is no clear geochemical difference between the Neogene Kerman Belt rocks, which are thought to post-date the onset of collision between the Afro-Arabian and Eurasian plates (late Oligocene to earliest Miocene), and the subduction-related Bazman–Taftan and Chagai Belt magmas. The porphyry deposits formed in these distinct settings are also virtually indistinguishable. This suggests that most of the processes affecting the geochemistry and metallogeny of the magmas in both settings take place in the lithosphere, albeit that the ultimate source of the magmas is in the supra-subduction zone asthenospheric mantle wedge. In collisional environments, subduction-related material previously crystallized in the deep lithosphere is simply being remobilized.

Mafic replenishment of multiple felsic reservoirs at the Mono domes and Mono Lake islands, California

The Mono Basin has been the site of frequent volcanic activity over the past 60,000 years, including the emplacement of the Mono domes and Mono Lake islands. The Mono Basin lavas are the youngest and most poorly understood products of the Long Valley Volcanic Field. We have undertaken a study of Mono Basin volcanism encompassing whole-rock major and trace element, Sr, Nd, Pb, and O isotopic, and electron microprobe glass, plagioclase, and amphibole analyses. Variations in major and trace elements suggest that fractional crystallization of feldspar (Sr, K2O), apatite (P2O5), titanomagnetite (V), zircon (Zr), and allanite (La, Ce) has influenced the evolution of the Mono Basin lavas. Field observations, petrography, and chemistry together demonstrate that injection of more mafic magma is a common process throughout the Mono Basin. Mafic enclaves of the Mono domes are stretched and rounded, with chilled margins between enclave and host rhyolite. Thin sections reveal millimeter-scale inclusions of rhyolite in the enclaves and vice versa along the host-enclave border. Paoha Island dacite has glass with 67–72 wt% SiO2 and contains microscopic clots of more mafic glasses, with SiO2 contents as low as 64 wt%. Isotopically, the June Lake and Black Point basalts and the Mono dome enclaves represent the least evolved material in the Long Valley Volcanic Field, with 87Sr/86Sri 0.5126. The silicic Mono Lake lavas and Mono dome rhyolites display a significant crustal component, with 87Sr/86Sri >0.7058 and 143Nd/144Nd 19 and δ18O >+6.5‰. The Mono Lake lavas generally are younger and less evolved than the Mono domes, with enrichment in trace elements including Ba and Sr accompanied by lower 143Nd/144Nd and higher 206Pb/204Pb. This implies that the Mono domes and the Mono Lake lavas are derived from different magma batches, if not from separate magma chambers. There is no systematic relationship between the degree of chemical evolution and the lava ages, indicating that several magma batches have been involved in the development of the Mono domes complex. Pronounced differences in trace element composition (Nb, Y) and isotopic values between the Negit Island and Paoha Island lavas indicate that they, too, are produced by the evolution of at least two different batches of intermediate-composition magma.

Neoproterozoic Rosetta Gabbro from northernmost Arabian–Nubian Shield, south Jordan: Geochemistry and petrogenesis

An Ediacaran mafic intrusion of south Jordan is a distinctive appinitic igneous rock with a possibly unique texture, characterized by spherical clots up to 40mm in diameter composed of amphibole cores from which plagioclase euhedra radiate; we call it the Rosetta Gabbro. It is exposed as a small (ca. 750m2) outcrop in the Neoproterozoic basement of south Jordan. A second outcrop of otherwise similar gabbro is located about 400m to the north of the Rosetta Gabbro, but it lacks the distinctive texture. The Rosetta Gabbro could represent a magma pipe. It intrudes the Aqaba Complex (~600Ma) granitoids and metasediments of the Janub Metamorphic Complex (633–617Ma). The gabbro is an Ol- to QZ tholeiite with the following chemical characteristics: SiO2=46.2–47.8wt.%; Al2O3=16.4–17.7wt.%, TiO2=1.70–2.82wt.%, Na2O=1.27–2.83wt.%. K2O=0.82–1.63wt.%; Mg# 58–63; Σ REE=70–117ppm; La/Yb ~6 to 8; and Eu/Eu*=1.05–1.2. The investigated gabbro has the geochemical features of a continental flood tholeiitic basalt emplaced in a within-plate tectonic setting. Two varieties of amphiboles are found: 1) large, 3–5mm, brown ferri-titanian-tschermakite (K0.09Na0.28)(Na0.20Ca1.80)(Mn0.04Fe3+1.1Mg2.34Fe2+0.90Ti0.29Al0.22)(Al1.85Si6.15)O22(OH)1.95 of the calcic amphibole group which is riddled with opaques; and 2) acicular yellowish-light green ferrian-magnesiohornblende (K0.04Na0.153)(Ca1.755Na0.245) (Fe3+0.66Mn0.01Fe2+1.01Mg3.03Ti0.06Al0.22)(Al1.03Si6.97)O22(OH)1.95. Scattered flakes of phlogopite also occur. Tabular radiating plagioclase (An64–79) are complexly twinned, with broad lamellae that show no zoning. Laser-ablation ICP-MS analyses of amphibole and plagioclase reveal considerable variation in trace element abundance, in spite of more subtle major element variations except for TiO2 in amphibole. The REE in the amphibole shows an order of magnitude variation with a concave-downward pattern and a positive Eu anomaly Eu/Eu*=0.6–2, though far less pronounced compared to the Eu/Eu*=5–45 of plagioclase. The 3D dandelion-like texture of the rosettas is broadly similar to “Chrysanthemum Stone”, which is a diagenetic growth in sedimentary rock, but we can find no description of similar textures in igneous rocks. The formation of the rosettas is thought to reflect loss of magmatic water resulting in supersaturation of plagioclase, which grew rapidly around amphibole and may have floated in the magma. This implies magmatic evolution in shallow (10 to 12km deep) crust where temperatures were nevertheless in the range of ca. 750 to 900°C.

An experimental study of amphibole stability in low-pressure granitic magmas and a revised Al-in-hornblende geobarometer

We report new experimental data on the composition of magmatic amphiboles synthesised from a variety of granite (sensu lato) bulk compositions at near-solidus temperatures and pressures of 0.8–10 kbar. The total aluminium content (Al^(tot)) of the synthetic calcic amphiboles varies systematically with pressure (P), although the relationship is nonlinear at low pressures (<2.5 kbar). At higher pressures, the relationship resembles that of other experimental studies, which suggests of a general relationship between Al^(tot) and P that is relatively insensitive to bulk composition. We have developed a new Al-in-hornblende geobarometer that is applicable to granitic rocks with the low-variance mineral assemblage: amphibole + plagioclase (An_(15–80)) + biotite + quartz + alkali feldspar + ilmenite/titanite + magnetite + apatite. Amphibole analyses should be taken from the rims of grains, in contact with plagioclase and in apparent textural equilibrium with the rest of the mineral assemblage at temperatures close to the haplogranite solidus (725 ± 75 °C), as determined from amphibole–plagioclase thermometry. Mean amphibole rim compositions that meet these criteria can then be used to calculate P (in kbar) from Al^(tot) (in atoms per formula unit, apfu) according to the expression: P (kbar)=0.5+0.331(8)×Al^(tot) +0.995(4)×(Al^(tot))^2. This expression recovers equilibration pressures of our calibrant dataset, comprising both new and published experimental and natural data, to within ±16 % relative uncertainty. An uncertainty of 10 % relative for a typical Al^(tot) value of 1.5 apfu translates to an uncertainty in pressure estimate of 0.5 kbar, or 15 % relative. Thus the accuracy of the barometer expression is comparable to the precision with which near-solidus amphibole rim composition can be characterised.

Exploring the potential of Raman spectroscopy for crystallochemical analyses of complex hydrous silicates: II. Tourmalines

Detailed Raman spectroscopic and electron microprobe studies of 33 amphibole (AB 2 C 5 T 8 O 22 W 2 , C 5 = M1 2 M2 2 M3) crystals from 6 amphibole subgroups were performed to check whether the Raman scattering arising from OH bond stretching vibrations can be used for the identification and site-occupancy analysis of amphiboles. Several results can be gained from this study. (1) C-site Mg and Fe 2+ on the M1M1M3 triplet linked to the W-site OH − lead to two-mode behavior of the OH bond stretching mode: up to four Raman peaks separated by ~16 cm −1 from each other can be observed and their fractional intensities can be used to quantify the concentration of M1M1M3 chemical species as well as to estimate the overall C-site occupancy by Mg and Fe 2+ . (2) The coexistence of Mg and Fe 2+ on the B site also leads to two-mode behavior but the peak splitting is only ~3 cm −1 . The fractional intensities of these split components can be used to refine the B-site occupation factors of Mg and Fe 2+ . (3) The presence of B-site Li, Na, and Ca results in one-mode behavior of the OH Raman peaks, i.e., slight shift of the peak positions depending on the concentration of distinct chemical species. The total replacement of B-site Ca by Na should reduce the peak positions by 6.5 cm −1 , whereas that of Li by ~13 cm −1 . (4) A-site occupancy leads to strong broadening as well as to a strong shift of the OH peaks toward higher wavenumbers. The full-width at half maximum (FWHM) of OH-stretching peaks associated with filled A sites is ~22 vs. ~8 cm −1 for OH peaks associated with vacant A sites. For T Al-poor amphiboles the M1M1M3-OH peaks shift toward higher wavenumbers by ~37 cm −1 in the case of W OH–A-cation– W F species and by 60 cm −1 in the case of W OH–A-cation– W OH species with respect to the peak position for W OH–A-vacancy– W OH species. (5) High content of C Al, i.e., M2 Al >1 atom per formula unit (apfu) seems to lead to one-mode behavior and an additional shift toward lower wavenumbers by ~4 cm −1 . (6) OH peaks with FWHM ~30–40 cm −1 may indicate high content of T Al (1 or 2 apfu), which can be verified by analyzing the FWHM of the Raman peak near 670 cm −1 , generated by vibrations of bridging oxygen atoms in TO 4 rings. (7) The W site occupancy factor of the OH groups can be estimated using the intensity ratio η between the total Raman scattering generated by OH bond stretching and the Raman peak near 670 cm −1 generated by TO 4 -ring vibrations. A small value of η ( −1 is indicative of oxo-amphiboles. Guidelines for non-specialists how to use the Raman scattering of OH bond stretching modes for “rough” but preparation-free, non-destructive, and easy-to-handle crystallochemical quantitative analyses of amphiboles are suggested.