Deuteric Fluids Research Articles

Origin of bluish pectolite aka larimar from the Dominican Republic: Constraints from mineralogy and geochemistry

The commonly known larimar is usually a gem-quality, bluish pectolite (NaCa2Si3O8OH), intergrown with other associated minerals such as natrolite, prehnite, calcite, chlorite, found only in the Dominican Republic. Less frequently it shows consecutive and/or sequential green, blue, and white coloration within single amygdales/veins developed in altered (chloritized, serpentinized) Cretaceous basalts of the Dumisseau Formation. The origin of larimar is linked to the circulation of low-temperature (<170 °C) alkali-rich deuteric fluids that were later mixed with meteoric waters. Variations of CL colors, ranging from weak brown in green pectolite to strong yellow (Mn2+-activated) in blue and white species, were ascribed to the transition from oxic to suboxic crystallization environment within the single larimar bodies. These changes could be induced by the burial of magmatic rocks and resulted in the emergence of early zeolite (natrolite), and subsequent formation of native copper in a sulphur-deficient hydrothermal environment. Pectolite-forming elements (e.g. Ca and Na), followed by such transition metals as Cu, Cr, Co, and Ni, were released via the alterations of magmatic minerals (diopside, aegirine-augite, and chromite-magnetite) and/or leached from basaltic groundmass. Decoupling between δ18O values of blue (up to 10.67‰), green (up to 9.66‰), and white pectolite (up to 15‰) was maintained by a progressive temperature drop during ongoing pectolitization and/or presence of the boiling-related environment. Meanwhile, anomalously low δD values (from −390.51‰ to −294.02‰) indicate that the crystal structure of pectolite tends to incorporate light hydrogen isotope regardless of the original δD values in mineralizing fluids.

Origin, dynamics, and chemical evolution of garnet-bearing leucogranitic magma, Eastern Desert of Egypt: Controls on the rare-metal enrichment in the A-type magmatism

Egypt hosts numerous rare-metal granites, i.e., highly evolved granites enriched in rare metals (Ta, Nb, Be, Sn, Zr, Th, and REE). However, the processes involved in the rare-metal enrichment are not fully understood. We present new data on the textural characteristics and chemical composition of rare-metal mineralization associated with microgranite dikes in the Ras Abdah area of the Egyptian Eastern Desert. These dikes are garnet-bearing leucogranites (GLG) composed of perthitic alkali-feldspars and quartz. When compared to other Egyptian A-type granites, microgranite dikes are alkaline rocks with particularly higher HREE contents. Zircon, huttonite, fergusonite (Y), and Fe-Ti-Zn oxides (magnetite, Zn-bearing ilmenite and pyrophanite) are largely associated with the altered domains, which are also enriched in Nb, Zr, Y, Ta, Th, and REE. However, similarities between the chondrite-normalized REE patterns of the altered and unaltered domains of the GLG dikes favor the hypothesis of a unique magmatic signature. Moreover, the chemical and textural features of rare-metal minerals indicate that the alteration of primary minerals was caused by deuteric fluids or aqueous residual melt exsolved from the parental granitic magma (autometasomatism). Garnet compositions are rich in the spessartine component (up to 84 %), which is typical of garnet in highly evolved pegmatitic rocks. Furthermore, garnet exhibits no major element zoning but shows chemical fluctuations in trace element concentrations, suggesting correspondingly abrupt changes in melt composition due to sequential magma pulsing. This magma emplacement may cause crystal nucleation and oscillatory crystallization followed by magmatic segregation. Overall, parental magma type, dike injection, and magmatic-hydrothermal processes all play a role in the unusual enrichments of rare metals.

Feldspar traces mineralization processes in the Qongjiagang giant lithium ore district, Himalaya, Tibet

Recently, a large spodumene pegmatite deposit was discovered in the Higher Himalayan orogenic zone near Qongjiagang. The granites exposed on about 1500 vertical meters of the outcrop (with elevation varying from 4077 m to 5533 m) show an evolution from two-mica granite to tourmaline muscovite granite and tourmaline albite granite. Various types of pegmatites are dispersed throughout the granite, creating an opportunity to examine the evolutionary relationship between these rock types and their potential as ore deposits rich in lithium. This study collected feldspar samples from various granites and pegmatites along a vertical cross-section, and analyzed their structures using various techniques including Raman spectroscopy, and transmission electron microscopy. Elemental and isotopic compositions were determined using electron microprobe and laser ablation inductively coupled plasma mass spectrometry, providing valuable mineralogical information on the formation of widespread lithium deposits in the Qongjiagang area. The observed variations in feldspar composition suggest that changes in granite lithology and ore-bearing potential of pegmatites with increasing altitude result from upward fractional crystallization of magma formed by partial melting of Himalayan crystalline basement. This process is likely driven by decreasing temperature, resulting in a significant decrease in concentrations of alkaline earth metals (e.g., Ca, Sr, Ba), and gradual enrichment of trace elements including Rb, Cs, and Li. As a result, pegmatites attain a melt environment similar to that of the adjacent granite at similar elevations. Structural features of feldspar suggest that the decrease in Li concentration in K-feldspar and the formation of lithium-rich zonation of pegmatites are a result of simultaneous growth of spodumene and blocky orthoclase during the magmatic stage, as well as deuteric fluid extraction by syntectonic processes during the transition to the stable stage of microcline feldspar.

ISOTOPE COMPOSITION OF CARBON AND OXYGEN IN CALCITES OF ALKALINE ULTRAMAPHIC DYKES WITHIN THE URIK-IYA GRABEN

The isotope composition of carbon and oxygen was studied in calcite of dykes and veins of ultramafic lamprophyres, kimberlite, alkaline mica picrites from the Yarma above-intrusion zone, and pyroxene-free picrites intruding the rocks of the Bolshetagninsky carbonatite massif within the Urik-Iya graben hosted by the East Sayan Mountains. The data on δ13C (from −6.6 to −3.9 ‰ relative to VPDB) disclose the ideas on the mantle origin of the carbonate substance of dykes. High values of δ18O (from +13.9 to +11.8 ‰ relative to VSMOW) suggest the impact of deuteric fluids, i.e. magmatic fluids separated from melts, at later stage of formation of the calcite-bearing alkaline ultramafic rocks.

Genesis of the Kiruna-type Nixintage iron deposit, Chinese Western Tianshan, NW China: Constrains of ore geology, geochemistry and geochronology

The Nixintage iron deposit is located in the central segment of the Awulale metallogenic belt, Chinese Western Tianshan. Iron ores are mainly hosted by andesitic tuff and andesite of the Carboniferous Dahalajunshan Formation. The paragenetic sequence includes a magmatic magnetite-apatite stage (I); a magmatic–hydrothermal stage (II), and three substages can be divided, including magnetite-albite substage (II-1), magnetite-chlorite-pyrite substage (II-2), magnetite-quartz-calcite substage (II-3); and a late hydrothermal quartz-calcite-sulfides stage (III). Anhedral, fine-grained magnetite associated with apatite in brecciated and matrix ores formed in stage I with relatively variable compositions, most probably indicate the fast crystallization under a disequilibrium condition. The high-grade magnetite orebodies mainly formed in the stage II, are featured by widespread hydrothermal mineral assemblages associated with the ores. Magnetite formed in this stage has medium-to-high concentrations of V, low-to-medium concentrations of Ti, Cr, relative depletion of Mg, Ni, K, and these data mainly plot in the Kiruna and porphyry fields of discriminant diagrams that comparable to magnetite from typical Kiruna-type iron deposits. Both stages I and II magnetites show similar wide δ18O ranges and with peak values at 6.0–10.0 ‰, heavier than typical Kiruna-type deposits, while could be attributed to the relatively low crystallization temperature inferred by microthermometric data of a previous study. The δ34S values of pyrite formed in different stages range between 0.1 and 2.4 ‰, comparable to the sulfur isotopic composition of the mantle-derived magma. These features indicate that stage I iron mineralization was generated by the extrusion of iron phosphorous oxide melts separated from the parental silicate magma. Subsequent deuteric fluids exsolved from basaltic to andesitic melts during the late stage of volcanism, generated magmatic–hydrothermal iron mineralization in stage II, and the continuous evolution of the deuteric fluids formed stage III hydrothermal veins. In summary, integrated evidence suggests the Nixintage is a Kiruna-type iron deposit.Zircon U-Pb ages of ore-hosting andesite and andesitic tuff are 317.6–314.4 Ma, and timing for iron mineralization was inferred at the same time, i.e., ca. 315 Ma. Previously published petro-geochemical data combined with our geochronological data of these rocks, suggest the ore-related volcanism in Nixintage probably originated from the partial melting of the mantle wedge modified by subducted fluids under a Late Carboniferous continental arc setting. Consequently, a genetic model for the studied deposit could be created. Furthermore, the southeast of the Nixintage nearer to the vent might have potential for prospecting large Kiruna-type iron deposits.

Age and genesis of the Jinshan gold deposit in the Chinese North Tianshan: A link to large-scale strike–slip shearing events

Large-scale transcrustal strike–slip shearing, orogenic gold deposits, and magmatic Ni–Cu sulfide deposits are prominent features in the Chinese North Tianshan in the northeastern segment of the Tianshan Orogenic Belt. Therelationshipbetween the shearing events and the different styles of mineralization, however, remains ambiguous. Here, structural data, zircon U–Pb–Hf isotopes, bulk-rock geochemistry, muscovite 40Ar/39Ar age dating, and S–Pb isotopes of pyrite from the Jinshan gold deposit are utilized in conjunction with the Pb isotope composition of mineralized mafic–ultramafic rocks of the Huangshandong Ni–Cu sulfide deposit to characterize thelinkbetweenstrike–slipshearing events, andorogenicAuandmagmaticNi–Cu sulfidemineralization. The new zircon U–Pb age results indicate that the host diorite of the gold orebodies formed at ca. 335 Ma. These diorites are enriched in light rare earth elements and large ion lithophile elements (e.g., Ba, U, K, and Pb), but depleted in high field strength elements (e.g., Nb, Ta, and Ti), and have low MgO (4.45–5.17 wt%) contents, Mg# (0.32–0.36), and (La/Yb)CN ratios (4.81–6.90). Based on the lack of ancient crustal basement rocks in the Qoltag Arc, the depleted Nd–Hf isotope composition (εHf(t) = 10.82–15.38; εNd(t) = 6.17–6.67), the juvenile Hf depleted-mantle model ages (350–530 Ma), and the enrichment in Na2O (4.45–5.17 wt%) relative to K2O (1.05–2.20 wt%) of the diorite, it is inferred that the primary magma from which the diorite crystallized was derived from partial melting of metasomatized mantle wedge in an oceanic arc environment. Two muscovite samples separated from an auriferous quartz lode yield 40Ar/39Ar plateau ages of 248.6 ± 0.8 Ma and 249.6 ± 0.9 Ma, representing the age of the gold mineralization event. The similarity in S–Pb isotope composition of pyrite in the Jinshan gold deposit and pyroxenite in the Huangshandong Ni–Cu sulfide deposit suggests that the auriferous fluids required for the formation of the Jinshan gold deposit could represent a mixture of deuteric fluids with limited upper crustal metamorphic fluids from the Kanggur–Huangshan Shear Zone.

Genesis of the Jinbaoshan PGE-(Cu)-(Ni) deposit: Distribution of chalcophile elements and platinum-group minerals

ABSTRACT The Jinbaoshan platinum group element-(Cu)-(Ni) deposit in southwest China is a sulfide-poor magmatic platinum-group element deposit that experienced multiple phases of post-magmatic modification. The sulfide assemblages of most magmatic Ni-Cu-platinum-group element deposits in China and elsewhere in the world are dominated by pentlandite-pyrrhotite-chalcopyrite with lesser magnetite and minor platinum-group minerals. However, Jinbaoshan is characterized by (1) hypogene violarite-pyrite 1-millerite-chalcopyrite and (2) supergene violarite-(polydymite)-pyrite 2-chalcopyrite assemblages. The platinum-group minerals are small (0.5–10 μm diameter) and include moncheite Pt(Te,Bi)2, mertieite-I Pd11(Sb,As)4, the atokite Pd3Sn – rustenburgite Pt3Sn solid solution, irarsite IrAsS, and sperrylite PtAs2 hosted mainly by violarite, silicates (primarily serpentine), and millerite. The platinum-group minerals occur in two sulfide assemblages: (1) mertieite-I-dominant (with irarsite, palladium, and Pd-alloy) in the hypogene assemblage and (2) moncheite-dominant (with irarsite, sperrylite, and atokite) in the supergene assemblage. Palladium and intermediate platinum-group elements (Os, Ir, Ru) are concentrated mainly in violarite, polydymite, and pyrite 2. Platinum is seldom hosted by base metal sulfides and occurs mainly as discrete platinum-group minerals, such as moncheite, sperrylite, and merenskyite. Violarite and polydymite in the Jinbaoshan deposit contain more Pb-Ag than pentlandite and pyrrhotite in the Great Dyke and Lac des Iles deposit. The formation of the sulfide assemblages in Jinbaoshan can be interpreted to have occurred in three stages: (1) a magmatic Fe-Ni-Cu sulfide melt crystallized Fe-Ni monosulfide and Cu-rich intermediate solid solutions, which inverted to a primary pyrrhotite-pentlandite-chalcopyrite-magnetite assemblage; (2) an early-secondary hypogene voilarite-millterite-pyrite 1-chalcopyrite assemblage formed by interaction with a lower-temperature magmatic-hydrothermal deuteric fluid; and (3) a late-secondary supergene violarite-polydymite-pyrite 2-chalcopyrite assemblage formed during weathering. Late-magmatic-hydrothermal fluids enriched the mineralization in Pb-Ag-Cd-Zn, which are incompatible in monosulfide solid solution, added Co-Pt into violarite, and expelled Pd to the margins of hypogene violarite and millerite, which caused Pd depletion in the hypogene violarite and the formation of mertieite-I. Supergene violarite inherited Pd and intermediate platinum-group elements from primary pentlandite. Thus, the unusual sulfide assemblages in the Jinbaoshan platinum-group element-(Cu)-(Ni) deposit results from multiple overprinted post-magmatic processes, but they did not significantly change the chalcophile element contents of the mineralization, which is interpreted to have formed at high magma:sulfide ratios (R factors) through interaction of crustally derived sulfide and a hybrid picritic-ferropicritic magma derived from subduction-metasomatized pyroxenitic mantle during impingement of the Emeishan plume on the Paleo-Tethyan oceanic subduction system.

Combined zircon U-Pb dating and chemical Th–U–total Pb chronology of monazite and thorite, Abu Diab A-type granite, Central Eastern Desert of Egypt: Constraints on the timing and magmatic-hydrothermal evolution of rare metal granitic magmatism in the Arabian Nubian Shield

We present the first chemical age determination of monazite from a geochemically-specialized rare metal granite of the Central Eastern Desert (CED), Egypt. The Abu Diab composite A-type granite of the CED divides into three co-magmatic phases: two mica granite (TG), garnet-bearing muscovite granite (GMG), and muscovite granite (MG). The GMG granitic phase is a typical example of a rare metal, volatile-rich aqueous silicate magma. Its zircon has extremely high U-concentration (up to 45,000 ppm) and intense metamictization, resulting in significant radiogenic Pb-loss, as well as common Pb contamination, along fractures and other imperfections. Laser ablation (LA-ICP-MS) zircon U-Pb results yield imprecise to meaningless dates. On the other hand, the GMG exhibits two texturally and chemically distinct varieties of monazite, pristine (Mnz1) and altered (Mnz2), that yield similar electron microprobe (EMP) chemical ages of 587 ± 8 Ma (2σ) for Mnz1 and 587 ± 9 Ma (2σ) for Mnz2. These ages, combined with the inferred alteration temperature (T > 400 °C), indicate monazite alteration/replacement occurred during or soon after crystallization of the pluton, in response to expelled magmatic-hydrothermal fluids inferred to be F-rich, late-staged deuteric fluids. The alteration and elemental substitution of zircon and monazite reflect coupled dissolution-recrystallization processes during late-staged deuteric alteration when zircon and thorite were relatively vulnerable to post-magmatic hydrothermal effects because of their extremely metamict state. The ca. 587 Ma age is typical of A-type granite intrusion across the CED and the post-collisional stage of the Arabian Nubian Shield (ANS; 610 – 560 Ma), and is within analytical error of associated magmatic thorite (592 ± 8 Ma). These two ages agree with the timing of rare metal-rich intrusions (0.6−0.4 Ga) related to the Pan-African orogeny. Future research using the monazite chronometer may better constrain the timing of rare metal granitic magmatism of the ANS.

Silica‐Rich Vein Formation in an Evolving Stress Field, Atlantis Bank Oceanic Core Complex

AbstractDrilling 809‐m Hole U1473A in the gabbro batholith at the Atlantis Bank Oceanic Core Complex (OCC) found two felsic vein generations: late magmatic fractionates, rich in deuteric water, hosted by oxide gabbros, and anatectic veins associated with dike intrusion and introduction of seawater‐derived volatiles. Microtextures show a change from compressional to tensional stress during vein formation. Temperatures and oxidation state were obtained from amphibole‐plagioclase and oxide pairs in the adjacent gabbros. Type I veins generally have reverse shear‐sense, with restricted ΔFMQ, high Mt/Ilm ratios, and low‐amphibole Cl/F indicating deuteric fluids. They formed during percolation and fractionation of Fe‐Ti‐rich melts into the primary olivine gabbro. Type II veins are usually hosted by olivine gabbro, occur at dike contacts and the margins of normal‐sense shear zones. They are undeformed or weakly deformed, with highly variable ΔFMQ, low Mt/Ilm ratios, and high‐amphibole Cl/F, indicating seawater‐derived fluids. The detachment fault on which the gabbro massif was emplaced rooted near the base of the dike‐gabbro transition beneath the rift valley. The ingress of seawater volatiles began at &gt;800°C and penetrated at least ~590 m into the lower crust during extensional faulting in the rift valley and adjacent rift mountains. The sequence of the felsic vein formation likely reflects asymmetric diapiric flow, with a reversal of the stress regime, and a transition from juvenile to seawater‐derived volatiles. This, in turn, is consistent with fault capture leading to the large asymmetries in spreading rates during OCC formations and heat flow beneath the rift mountains.

Iron isotope fractionation in hydrous basaltic magmas in deep crustal hot zones

Mafic magmatism within deep crustal hot zones plays an important role in the crustal formation and evolution. High-pressure garnet crystallization drives magma differentiation and Fe isotope fractionation at the base of the lower crust. The ∼840 Ma intrusion and the associated ∼830 Ma dikes from the Tongde region in South China consist mainly of gabbro and diorite, which record complicated magmatic processes of the deep crustal zones and provide new insights into the Fe isotope fractionation. Rocks from the Tongde intrusion have high Sr/Y ratios (61–163) and variable δ57Fe values (+0.07 to +0.21‰ relative to IRMM-014) that initially increase then decrease with progressive magmatic differentiation. All samples do not display obvious imprint of deuteric fluid exsolution, and residual garnet in the mantle source did not fractionate the Fe isotope compositions of the primary magmas significantly. The variable Fe isotope signatures in this study can be reproduced by sequential fractional crystallization of olivine/pyroxene and garnet in the deep crust followed by amphibole saturation in the upper section. The host rocks of the intrusion underwent diffusive exchange with the dikes. Removal of both isotopically light- and heavy-Fe minerals well explains the small Fe isotope fractionation in mafic and intermediate rocks. The high-Sr/Y suites of the intrusion and dikes have an average δ57Fe of +0.11 ± 0.02‰ (2SD), similar to the value of arc crust at convergent continental margins. Therefore, garnet fractionation is an important process in arc roots, which drives the melts evolving to calc-alkaline series. Fractional crystallization of hydrous basaltic magmas under high pressures in deep crustal hot zones makes a significant contribution to the formation and evolution of continental crust.

Zircon in emplacement borders of post-collisional plutons compared to country rocks: A study on morphology, internal texture, U–Th–Pb geochronology and Hf isotopes (Araçuaí orogen, SE Brazil)

Zircon is a powerful tool to study the internal evolution of igneous bodies and their interactions with country rocks. At pluton borders, zircon may record the emplacement history from the crystallization onset to deuteric processes, as well as inheritance from country rocks. We present a detailed morphology and internal structure study coupled with isotopic analyses (U–Th–Pb and Lu–Hf) on a great number of zircon grains extracted from samples collected at the borders of three distinct post-collisional intrusions of the Araçuaí orogen: granites from the Aracê – Pedra Azul and Vitória plutons, and a tonalite from the Mestre Álvaro pluton. For comparison, we also present mineral and bulk-rock chemistry data from these samples of post-collisional intrusions, as well as zircon U–Pb-Hf data from their country rocks (the Nova Venécia migmatitic paragneisses and Ataléia granites) and a wide dataset compilation. Zircon saturation geothermometry suggests igneous temperatures above 800 °C for pluton borders. Zircon geochronology resulted in crystallization ages for borders of the plutons at 523 ± 2 Ma (Aracê – Pedra Azul), 505 ± 1 Ma (Vitória), and 527 ± 2 Ma (Mestre Álvaro). Lu–Hf data (Aracê – Pedra Azul pluton: εHf(t) −18.6 to −23.8, TDM ages from 2.25 to 2.47 Ga; Vitória pluton: εHf(t) −7.4 to −10.3, TDM ages from 1.58 to 1.71 Ga; Mestre Álvaro pluton: εHf(t) −0.7 to −8.8, TDM ages from 1.27 to 1.66 Ga; Nova Venécia migmatitic paragneiss: εHf(t) +4.1 to −39.2, TDM ages from 1.20 to 3.47 Ga; and Ataléia granite: εHf(t) −3.2 to −8.1, TDM ages from 1.42 to 1.64 Ga) indicate involvement of country rocks in the petrogenesis of post-collisional intrusions. Together, new and compiled data suggest: i) magma hybridization at high temperature, involving country rocks; ii) rapid growth of zircon crystals probably at rapid cooling rates; and iii) in situ dissolution–recrystallization and overgrowth processes in zircon crystals in response to interactions with residual (late-stage) melts and/or deuteric fluids.

Tracking chemical alteration in magmatic zircon using rare earth element abundances

Trace elements in magmatic zircon provide a wealth of petrogenetic information about host magmas. However, they are susceptible to alteration through post-magmatic interaction with hydrothermal fluids. Trace element analyses can also be biased by the inadvertent inclusion of exotic materials, such as mineral or glass inclusions, in analyzed volumes of zircon. In order to screen out samples with altered chemical signatures, zircons with high, flat light rare earth element (LREE) patterns are typically considered to be altered. However, visual selection of such patterns is qualitative and does not address ambiguous cases. The light rare earth element index (LREE-I = Dy/Nd + Dy/Sm) provides an approach for quantitative screening for aqueous alteration and contamination of zircon by exotic materials and was used to assess secondary processes in the greenschist facies Jack Hills detrital zircon suite. However, in addition to aqueous alteration, the LREE-I is also sensitive to melt compositional evolution, and its applicability to alteration in settings other than the Jack Hills quartzite is thus far undetermined. We investigate igneous zircon populations from a variety of geologic settings that show evidence for alteration by contact metamorphism during magma intrusion and deuteric fluid interactions during pluton crystallization. In suites with a high proportion of texturally altered zircons, low LREE-I values are common and this parameter correlates well with other contamination indicators (based on the observed secondary phases deposited in the zircons during fluid flow). Filtering zircon trace element compositions based on the LREE-I appears to remove the majority of chemically altered zircon, in many cases revealing previously obscured magmatic signals.

Crustal reworking at convergent margins traced by Fe isotopes in I-type intrusions from the Gangdese arc, Tibetan Plateau

Voluminous magmatism at active continental margins and collision zones is marked by I-type intrusions. Paleocene-Eocene intrusions, exposed in the Lhasa terrane of the Tibetan Plateau, are part of the Gangdese arc that operated during Neo-Tethyan ocean subduction and subsequent India-Asia collision. Arc-like geochemical trace element signatures and radiogenic isotope systematics are indicative of juvenile I-type magmatism with variable silica contents consequent to igneous differentiation. Here, we present the first Fe isotope data for this fossil arc terrane, which have highly variable δ57Fe values (relative to IRMM-014) of −0.05 to +0.57‰. The data show no obvious correlations with major and trace element systematics, except for highly evolved rocks with >70 wt% SiO2. Based on trace element systematics, deuteric fluid exsolution is excluded as a cause for the isotope variations in less evolved rocks. Furthermore, there is no apparent relation between Fe isotope values and the established tectonic evolution from syn- to post-collisional magmatism. Comparison with I-type granites of the Australian Lachlan Fold Belt and Cordilleran Snake River Plain reveals an, on average, lighter δ57Fe in the Gangdese suite in primitive lavas, yet distinctively heavier than intra-oceanic arcs. The heavier δ57Fe values for primitive Lachlan Fold Belt and Snake River Plain rocks are interpreted here as crustal contribution in the form of S-type magmas derived from crustal anatexis, which are absent in the Gangdese arc. Gangdese Belt data yield an average δ57Fe of +0.13 ± 0.02‰ (2σ), which is proposed as the best estimate for juvenile crust at active continental margins.Rhyolite-MELTS modelling suggests that the Gangdese Belt data can be reproduced through fractional crystallization along a liquid line of descent at oxygen fugacity between FMQ = 0 to +2, typical for arc-related melts. Crucially, the majority of data is coherent with partial melts of existing mafic crust as the major contributor to the juvenile component in the intrusions, with minor mantle-derived melts, evidenced through heavier primitive Fe isotopes. This indicates that for the Gangdese Belt samples, the primitive, parental melts are derived from lower crustal successions through predominantly crustal reworking with only subordinate crustal growth. Hence, the Fe isotope data indicates a two-step evolution with melting of underplated mafic material sometime in the geologic past. Although juvenile in nature, these successions are part of the existing crust, supporting scenarios in which crustal reworking in convergent margin intrusions is an important process in evolved magma generation.

Deducing the source and composition of rare earth mineralising fluids in carbonatites: insights from isotopic (C, O, 87Sr/86Sr) data from Kangankunde, Malawi

Carbonatites host some of the largest and highest grade rare earth element (REE) deposits but the composition and source of their REE-mineralising fluids remains enigmatic. Using C, O and 87Sr/86Sr isotope data together with major and trace element compositions for the REE-rich Kangankunde carbonatite (Malawi), we show that the commonly observed, dark brown, Fe-rich carbonatite that hosts REE minerals in many carbonatites is decoupled from the REE mineral assemblage. REE-rich ferroan dolomite carbonatites, containing 8–15 wt% REE2O3, comprise assemblages of monazite-(Ce), strontianite and baryte forming hexagonal pseudomorphs after probable burbankite. The 87Sr/86Sr values (0.70302–0.70307) affirm a carbonatitic origin for these pseudomorph-forming fluids. Carbon and oxygen isotope ratios of strontianite, representing the REE mineral assemblage, indicate equilibrium between these assemblages and a carbonatite-derived, deuteric fluid between 250 and 400 °C (δ18O + 3 to + 5‰VSMOW and δ13C − 3.5 to − 3.2‰VPDB). In contrast, dolomite in the same samples has similar δ13C values but much higher δ18O, corresponding to increasing degrees of exchange with low-temperature fluids (< 125 °C), causing exsolution of Fe oxides resulting in the dark colour of these rocks. REE-rich quartz rocks, which occur outside of the intrusion, have similar δ18O and 87Sr/86Sr to those of the main complex, indicating both are carbonatite-derived and, locally, REE mineralisation can extend up to 1.5 km away from the intrusion. Early, REE-poor apatite-bearing dolomite carbonatite (beforsite: δ18O + 7.7 to + 10.3‰ and δ13C −5.2 to −6.0‰; 87Sr/86Sr 0.70296–0.70298) is not directly linked with the REE mineralisation.

Magmatic, hydrothermal and subsolidus evolution of the agpaitic nepheline syenites of the Sushina Hill Complex, India: implications for the metamorphism of peralkaline syenites

AbstractThe Proterozoic Sushina Hill Complex is the only agpaitic complex, reported from India and is characterized by a eudialyte-rinkite-bearing nepheline syenite. The complex is considered a ‘metamorphosed agpaitic complex'. This study describes the mineral assemblages formed during successive stages of evolution from magmatic to hydrothermal stages and low-temperature subsolidus re-equilibration assemblage. The primary-late magmatic assemblage is characterized by albite, orthoclase, unaltered nepheline, zoned diopside-hedenbergite, rinkite, late magmatic eudialyte and magnesio-arfvedsonite formed at ∼700°C with maximum aSiO2 of 0.60. In contrast, a deuteric assemblage (400-348°C) is represented by aegirine-jadeite-rich clinopyroxene, post-magmatic eudialyte, sodalite, analcime and the decomposition assemblages formed after eudialyte with decreasing aSiO2 (0.52-0.48). A further low-temperature subsolidus assemblage (≤250°C) represented by late-forming natrolite could be either related to regressive stages of metamorphism or a continuum of the subsolidus processes. Considering the P/T range of the greenschist - lower-amphibolite facies of metamorphism it is evident that the incorporation of a jadeite component within pyroxene is related to a subsolidus process between ∼400°C and 348°C in a silica deficient environment. We emphasize that the deuteric fluid itself acted as an agent of metamorphism and the decomposition assemblage formed after eudialyte is retained even after metamorphism due to the convergence of subsolidus and metamorphic domains. The formation of jadeite-rich aegirine is not considered to result from metamorphism. Overall it is near-impossible to discern any bona fide metamorphic textures or mineral assemblages in these syenites which appear to preserve a relict mineralogy regardless of their occurrence in country rocks which have experienced greenschist - amphibolite facies metamorphism. The Sushina complex is very similar in this respect to the Norra Kärr complex (Sweden).

Genesis and Multi-Episodic Alteration of Zircon-Bearing Chromitites from the Ayios Stefanos Mine, Othris Massif, Greece: Assessment of an Unconventional Hypothesis on the Origin of Zircon in Ophiolitic Chromitites

Several small chromium (Cr) ore bodies are hosted within a unit of tectonically thinned dunite in the retired Ayios Stefanos mine of the western Othris ophiolite complex in Greece. Chromium ores consist of tectonically imprinted bodies of semi-massive to massive, podiform and lenticular chromitites composed of chromian spinel [Cr-spinel] with high Cr# [Cr/(Cr + Al) = 0.51–0.66] and Mg# [Mg/(Mg + Fe2+) = 0.58–0.76], low Fe3+# [Fe3+/(Fe3+ + Fe2+) ≤ 0.43] and low TiO2 (≤0.21 wt %) content. This composition is characteristic of Cr-spinels in equilibrium with melts of intermediate affinity between island-arc tholeiites (IATs) and mid-ocean ridge basalts (MORBs). Several Cr-spinel crystals in these ores exhibit imperfect zones made up of spinel hosting oriented lamellae of Mg-silicates (mostly chlorite) locally overgrown by porous domains along grain boundaries and fractures. From the Cr-spinel core to the lamellae-rich rim Cr#, Mg# and Fe3+# generally increase (0.68–0.87, 0.78–0.88 and 0.55–0.80, respectively), whereas from the core or the spinel zones with oriented lamellae to the porous domains Mg# and Fe3+# generally decrease (0.45–0.74 and ≤0.51, correspondingly). The lamellae-rich rims formed at oxidizing conditions, whereas the porous rims resulted from a later reducing event. Several tiny (≤30 μm), subhedral to anhedral and elongated Zr-bearing silicate mineral grains were discovered mainly along open and healed fractures cutting Cr-spinel. Most of the Zr-bearing silicate minerals (30 out of 35 grains) were found in a chromitite boulder vastly intruded by a complex network of gabbroic dykes. The dominant Zr-bearing silicate phase is by far zircon displaying a homogeneous internal texture in cathodoluminescence (CL) images. Raman spectroscopy data indicate that zircons have experienced structural damage due to self-irradiation. Their trace-element contents suggest derivation from a plagioclase-bearing, low-SiO2 intermediate to mafic source. Combined micro-textural and minerochemical data repeat the possibility of zircon derivation from limited volumes of high-T fluids emanating from the gabbroic intrusions. Once zircon is precipitated in cracks, it may be altered to Ca-rich Zr-bearing silicate phases (i.e., armstrongite, calciocatapleiite). Almost all zircons in these samples show evidence of gains in solvent compounds (CaO, Al2O3 and FeO) possibly due to re-equilibration with late deuteric fluids.

Cretaceous ongonites (topaz-bearing albite-rich microleucogranites) from Ongon Khairkhan, Central Mongolia: Products of extreme magmatic fractionation and pervasive metasomatic fluid: rock interaction

Ongonites were defined at their type locality at Ongon Khairkhan, central Mongolia, as pristine magmatic topaz-bearing albite–quartz-keratophyres with up to 4wt. % F and containing phenocrysts of albite, K-feldspar, quartz and rare mica and topaz hosted in a groundmass composed of the same minerals. However, detailed petrographic and SEM-EDS studies indicate that these rocks underwent considerable subsolidus exchange with deuteric fluids, as evidenced by the presence of albitic plagioclase (Ab~100) and end-member orthoclase (Or~100), secondary Li–Fe-rich mica (zinnwaldite) enriched in rare metals (Sn, W, Ta, Nb), pitted feldspars containing fluid inclusions, and disseminated fluorite. The ~120Ma old dyke rocks, emplaced at a high structural level in the crust, are strongly peraluminous leucogranites characterized by high Al and alkalis that are also enriched in Rb, Cs, Ga and Ta, depleted in Mg, Ca, Zr, Ba, Sr and Eu, and have anomalous K/Rb, Rb/Sr, Zr/Hf and Nb/Ta ratios compared to the average continental crust. However, the suite has Nd isotopic ratios (ɛNd(120) ~−1) similar to those of contemporaneous A-type granites of the Mongolian–Transbaikalian igneous province of the Central Asian Orogenic Belt. The inferred primary δ18O (~+6 to +7‰) and Pb isotopic values are consistent with a granitic parent magma and interaction with orthomagmatic fluids. The ongonites and constituent minerals record (1) an extensive and protracted crystal fractionation history, in part due to the presence of volatiles (particularly F) which depressed the solidus temperature of the felsic rocks and extended its duration of crystallization and (2) subsolidus exchange with fluids which includes late flux of heated meteoric water as indicated by modified whole rock δ18O values (+0.5 to +2.7‰). The interaction of the ongonites with internally derived orthomagmatic fluids is considered to result in enrichment and/or redistribution of several incompatible elements, but not to have greatly modified their original major element chemistry which indicates that this suite represents the last stages of fractionation of a highly differentiated, F-rich granitic magma. The escape of these evolved melts from the apical part of the underlying pluton is now represented by the ongonite dykes. Late-stage magmatic, water-rich fluids enriched in incompatible elements including Nb, Ta, Sn and W were responsible for the late- to post-magmatic alteration and associated W mineralization.

The Axum–Adwa basalt–trachyte complex: a late magmatic activity at the periphery of the Afar plume

The Axum–Adwa igneous complex consists of a basalt–trachyte (syenite) suite emplaced at the northern periphery of the Ethiopian plateau, after the paroxysmal eruption of the Oligocene (ca 30 Ma) continental flood basalts (CFB), which is related to the Afar plume activity. 40Ar/39Ar and K–Ar ages, carried out for the first time on felsic and basaltic rocks, constrain the magmatic age of the greater part of the complex around Axum to 19–15 Ma, whereas trachytic lavas from volcanic centres NE of Adwa are dated ca 27 Ma. The felsic compositions straddle the critical SiO2-saturation boundary, ranging from normative quartz trachyte lavas east of Adwa to normative (and modal) nepheline syenite subvolcanic domes (the obelisks stones of ancient axumites) around Axum. Petrogenetic modelling based on rock chemical data and phase equilibria calculations by PELE (Boudreau 1999) shows that low-pressure fractional crystallization processes, starting from mildly alkaline- and alkaline basalts comparable to those present in the complex, could generate SiO2-saturated trachytes and SiO2-undersaturated syenites, respectively, which correspond to residual liquid fractions of 17 and 10 %. The observed differentiation processes are consistent with the development of rifting events and formation of shallow magma chambers plausibly located between displaced (tilted) crustal blocks that favoured trapping of basaltic parental magmas and their fractionation to felsic differentiates. In syenitic domes, late- to post-magmatic processes are sometimes evidenced by secondary mineral associations (e.g. Bete Giorgis dome) which overprint the magmatic parageneses, and mainly induce additional nepheline and sodic pyroxene neo-crystallization. These metasomatic reactions were promoted by the circulation of Na–Cl-rich deuteric fluids (600–400 °C), as indicated by mineral and bulk rock chemical budgets as well as by δ18O analyses on mineral separates. The occurrence of this magmatism post-dating the CFB event, characterized by comparatively lower volume of more alkaline products, conforms to the progressive vanishing of the Afar plume thermal effects and the parallel decrease of the partial melting degrees of the related mantle sources. This evolution is also concomitant with the variation of the tectono-magmatic regime from regional lithospheric extension (CFB eruption) to localized rifting processes that favoured magmatic differentiation.

Eight-phase alkali feldspars: low-temperature cryptoperthite, peristerite and multiple replacement reactions in the Klokken intrusion

Eight feldspar phases have been distinguished within individual alkali feldspar primocrysts in laminated syenite members of the layered syenite series of the Klokken intrusion. The processes leading to the formation of the first four phases have been described previously. The feldspars crystallized as homogeneous sodian sanidine and exsolved by spinodal decomposition, between 750 and 600 °C, depending on bulk composition, to give fully coherent, strain-controlled braid cryptoperthites with sub-μm periodicities. Below ~500 °C, in the microcline field, these underwent a process of partial mutual replacement in a deuteric fluid, producing coarse (up to mm scale), turbid, incoherent patch perthites. We here describe exsolution and replacement processes that occurred after patch perthite formation. Both Or- and Ab-rich patches underwent a new phase of coherent exsolution by volume diffusion. Or-rich patches began to exsolve albite lamellae by coherent nucleation in the range 460–340 °C, depending on patch composition, leading to film perthite with ≤1 μm periodicities. Below ~300 °C, misfit dislocation loops formed, which were subsequently enlarged to nanotunnels. Ab-rich patches (bulk composition ~Ab91Or1An8), in one sample, exsolved giving peristerite, with one strong modulation with a periodicity of ~17 nm and a pervasive tweed microtexture. The Ab-rich patches formed with metastable disorder below the peristerite solvus and intersected the peristerite conditional spinodal at ~450 °C. This is the first time peristerite has been imaged using TEM within any perthite, and the first time peristerite has been found in a relatively rapidly cooled geological environment. The lamellar periodicities of film perthite and peristerite are consistent with experimentally determined diffusion coefficients and a calculated cooling history of the intrusion. All the preceding textures were in places affected by a phase of replacement correlating with regions of extreme optical turbidity. We term this material ultra porous late feldspar (UPLF). It is composed predominantly of regions of microporous very Or-rich feldspar (mean Ab2.5Or97.4An0.1) associated with very pure porous albite (Ab97.0Or1.6An1.4) implying replacement below 170–90 °C, depending on degree of order. In TEM, UPLF has complex, irregular diffraction contrast similar to that previously associated with low-temperature albitization and diagenetic overgrowths. Replacement by UPLF seems to have been piecemeal in character. Ghost-like textural pseudomorphs of both braid and film parents occur. Formation of patch perthite, film perthite and peristerite occurred 104–105 year after emplacement, but there are no microtextural constraints on the age of UPLF formation.

Petrogenesis of the Barcroft pluton, northern White-Inyo Mountains, east-central California

The White-Inyo Range lies within the regional transition from Paleozoic-Precambrian North American continental basement to outboard Mesozoic and younger accreted terranes and a superimposed Andean-type arc. In the central White Mountains, the metaluminous Barcroft granodiorite invaded a major NE-striking, SE-dipping high-angle reverse fault—the Barcroft break. Because it is a relatively isolated igneous body and is well exposed over an elevation range of 1,500–4,000 m, its thermal history and that of the surrounding superjacent section are clearer than those of nearly coeval, crowded plutons emplaced in the hotter Sierra Nevada belt. The Barcroft pluton was emplaced as a compositionally heterogeneous series of areally scattered melt pulses episodically injected over the approximate interval 167–161 Ma. The oldest dated rocks are relatively quartzofeldspathic, whereas the youngest is more ferromagnesian, suggesting progressive partial fusion of a relatively mafic protolith. Heavy rare earth-enriched zircons indicate that Barcroft melts were derived at mid-crustal depths from a previously emplaced metabasaltic protolith containing plagioclase but lacking garnet. Granodioritic magma genesis involved the possible mixing of mafic and felsic melts, as well as very minor assimilation of country rocks, but mainly by fractional fusion and crystallization. Bulk chemical, rare earth, and isotopic data suggest that analyzed Barcroft rocks are members of a single suite. Granodioritic rocks are slightly more magnetite-rich at higher elevations on the NE, nearer the roof of the pluton. Earlier thermobarometry chronicled cooling and re-equilibration of the Barcroft pluton from its margins inward, as well as from mid-crustal generation depths of ~25 km through ascent and stalling at ~10–12 km. Refractory phase assemblages crystallized along the pluton margins, whereas subsolidus minerals in the interior of the of body continued to exchange with upper crustal deuteric and surficial aqueous fluids during exhumation and cooling.