Green Amphibole Research Articles

Skarn formation and Cu[sbnd]Ag mineralization in the McKenzie Gulch area, northern New Brunswick, Canada: Implication for the applications of mineral chemistry in exploration for porphyry copper and skarn deposits

The McKenzie Gulch (MG) copper–silver skarn occurrences are associated with the Middle Devonian intermediate to felsic dyke swarms. Mineralization occurs as veins and stockwork of veinlets, disseminations, patchy, and locally as replacement of calc-silicate skarns in argillaceous limestone. This skarn is interpreted to have developed in three stages: the earliest contact metamorphic stage resulting in hornfels developed from calcareous mudstone and very fine-grained clastic sedimentary rocks (Stage I); this was followed by metasomatic replacement resulting in prograde anhydrous skarn containing grossular-andradite (grandite), diopsidic pyroxene, and wollastonite (Stage II); increasing fluid/rock interaction results in retrograde skarn dominated by epidote, calcite, green amphibole, chlorite, sulfide minerals, titanite, andradite and hedenbergite commonly along veins and veinlets (Stage III). Subordinate sphalerite and pyrite occur in late veins cross-cutting both porphyry dykes, skarns and hornfels zone.Prograde garnets in the study area belong to the grossular-andradite solid solution that ranges from Adr16Gr82 to almost pure andradite - Adr99Gr1, whereas other garnet end-members constitute less than 5% collectively. Pyroxenes form a diopside-hedenbergite solid solution with composition between Di31Hd60 and Di93Hd7, while other pyroxenes constitute less than 10% collectively. These garnets and pyroxenes exhibit chemical zonation patterns generally characterized by a rim of Fe enrichment relative to cores of grains, which may suggest possible change in salinity (or other factor that promotes Fe transport). This means grossular and diopsidic cores are rimmed by more andradite and hedenbergitic compositions for garnet and pyroxene, respectively. In general, there is a tendency for increasing Fe component in both garnet and pyroxene from the prograde to retrograde stages.Epidote formed during the retrograde stage exhibits oscillatory zoning similar to retrograde andradite and indicates Fe enrichment increasing from core to rim. Based on the major element composition, textural and optical characteristics of garnets, we conclude that grandites (Al-rich) formed under low water/rock ratios, in equilibrium with metasomatic fluids whose composition was locally buffered by the host rocks, whereas andradite (Fe-rich) resulted from relatively high water/rock ratios that were in equilibrium with a magmatic-derived fluid. The results of this study indicate that there is a relationship between the composition of pyroxenes and garnets of the skarn alteration facies and the dominant metal of the mineralized skarns. These facies plot in the compositional field of Cu-dominated skarns. As a result, this relationship may be applied in exploration for skarn and porphyry copper mineralization.

Crystallisation and fast cooling of the (meta)gabbro from the Chenaillet ophiolite (Western Alps): In-situ U[sbnd]Pb dating of zircon, titanite, monazite and xenotime in textural context

The Chenaillet ophiolite, South of Montgenèvre (France), represents a preserved portion of the alpine ocean. The gabbros form lenticular bodies 50 to 200 m thick and a few hundreds of meters wide. They are intrusive in the serpentinites and often overlain by pillow-lavas. Plagiogranite/albitite veins are rare and volumetrically negligible. The petrology of the gabbro s.s. show great details of the progressive cooling of the massif, clinopyroxene are often rimmed by brown to green amphibole +/− ilmenite/titanite. Thermometry on these amphibole assemblages indicates a retrograde temperature evolution from late magmatic to subsolidus, between 950 and 900 °C, 800–750 °C, 600–500 °C. In a shear zone, a string of titanite associated with monazite and xenotime +/− ilmenite is located in a millimetric greenschist layer whose temperature is estimated at 600–550 °C. Accessories chemistry in this sample allowed in situ dating by LA-ICPMS on thin section in textural context. In the same sample, we performed UPb zircon dating, constraining the magmatic age, as well as UPb monazite, xenotime and titanite dating for the metamorphic evolution. In a Tera-Wasserburg diagram, monazite, xenotime and titanite yield 161.3 +/− 4.0 Ma, 161.5 +/− 2.4 Ma and 158.4 +/− 2.3 Ma lower intercept ages, respectively, while the zircon from the same sample indicates an age of 161.0 +/− 0.8 Ma. Finally, zircon dated in an albitite vein also yields a 161.8 ± 1.7 Ma age. These similar ages within the analytical uncertainties show that magmatism and metamorphism were concomitant. Thermal modelling predicts that a 150 m thick gabbroic massif crystallizes in a few hundred years and cools down in several thousand years. This is consistent with the ages reported in our study showing that the retrograde metamorphism in this massif records fast cooling. The development of this retrograde metamorphism required multiple fluid infiltrations during cooling. In the studied samples, 4 types of titanites could be distinguished based on their chemistries. 3 types variously depleted in REE are interpreted as related to water-driven fluids while another type has up to 10% REE, Y and Nb and suggest a possible felsic source. The two potential sources of water promoting this fast cooling are sea water-derived fluids and magmatic fluids exsolved from felsic veins.

Cumulate Mush Hybridization by Melt Invasion: Evidence from Compositionally Diverse Amphiboles in Ultramafic–Mafic Arc Cumulates within the Eastern Gangdese Batholith, Southern Tibet

Abstract Amphibole plays an important role in the petrogenesis and evolution of arc magmas, but its role is not completely understood yet. Here, a field, petrological, geochronological and geochemical study is carried out on ultramafic–mafic arc cumulates with textural and chemical heterogeneities and on associated host diorites from the eastern Gangdese Batholith, southern Tibet to explore the problem. The cumulates occur as a large body in diorite host-rocks. The core of the body consists of coarse-grained Cpx hornblendite with a porphyritic texture. Towards the contact with the host diorite, the coarse-grained Cpx hornblendite grades to relatively homogeneous fine-grained melagabbro. Zircon U–Pb dating indicates that they all crystallized at 200 ± 1 Ma. Textural features and whole-rock and mineral chemical data reveal that both the Cpx hornblendite and the melagabbro are mixtures of two different mineral assemblages that are not in equilibrium: (1) brown amphibole and its clinopyroxene inclusions; (2) matrix clinopyroxene + green amphibole + plagioclase + quartz + accessory phases. Clinopyroxene and brown amphibole from the first assemblage are enriched in middle rare earth elements (MREE) relative to light REE (LREE) and heavy REE (HREE), and are weakly depleted in Ti, whereas clinopyroxene and green amphibole from the second assemblage are characterized by LREE enrichment over MREE–HREE and more marked Sr and Ti depletion. The higher Mg#, MgO and Cr of the late-formed green amphibole than the early formed brown amphibole suggest that the two assemblages are not on the same liquid line of descent. Given the close relations of the three rock types in the exposed crustal section, the cumulates are interpreted to have formed in an open system, in which an ultramafic cumulate body consisting of the first assemblage reacted with the host dioritic melt to form new clinopyroxene and amphibole of the second assemblage. The melt calculated to be in equilibrium with the first mineral assemblage resembles an average continental arc basalt, which is less evolved than the host dioritic melt, responsible for the second mineral assemblage. On the basis of whole-rock Sr–Nd–Hf isotopic similarity of the cumulates and a host diorite sample, we argue that the host diorites were formed through crystal fractionation from the parent melt of the first assemblage. Results of least-squares mass-balance calculations suggest that the quantities of the host dioritic melts, involved in the generation of these modified cumulates, vary from ~25 to ~44 %. The presence of magmatic epidote in the host diorites and Al-in-Hb geobarometry indicate that the reaction that occurred when the dioritic melts percolated through the cumulate body was at ~6 kbar. Both the brown and green amphiboles are enriched in MREE relative to HREE, and can impart to residual melts a strong geochemical signature of amphibole fractionation (low Dy/Yb). Thus, we conclude that fractional crystallization and melt–rock reaction are two mechanisms by which amphibole controls arc magma petrogenesis and evolution.

Amphibole geochemistry of the Baima layered intrusion, SW China: Implications for the evolution of interstitial liquid and the origin of Fe-Ti oxide ores

The Baima layered intrusion hosting a giant Fe-Ti oxide ore deposit in the Panxi region (SW China) is mainly composed of troctolite and olivine gabbro in the lower zone (LZ) and isotropic gabbro, olivine gabbro and apatite olivine gabbro in the upper zone (UZ). Amphibole, as a late-stage crystalized phase, occur throughout the Baima intrusion in several forms and can be used to trace the evolution of interstitial melt. The brown amphibole commonly occurs as rims on Fe-Ti oxides, interstitial granular grains, and inclusions in olivine and clinopyroxene. They are abundant in the Fe-Ti oxide ore layers. The different occurrences have similar major and trace element compositions (1.60–4.71 wt% TiO2, 68–160 ppm REE; 2.35–5.25 wt% TiO2, 55–159 ppm REE; and 5.24–5.33 wt% TiO2, 158–177 ppm REE, respectively). They crystallized in the magmatic stage (948–1033 °C, 954–1041 °C, and 983–1015 °C, respectively). The olive amphibole commonly occurs as granular grains at the base of the UZ. These crystals have 0.04–1.17 wt% TiO2 and 24–80 ppm REE, which are lower than those of the brown amphibole, and they crystallized at 634–755 °C. The green amphibole commonly occurs as granular grains, with <1.59 wt% TiO2, 5–144 ppm REE, and a formation temperature of 649–814 °C.The higher REE contents of the brown amphibole compared to the olive amphibole suggest that immiscibility occurred in the interstitial melt at the late magmatic stage of the Baima intrusion. The absence of negative Ti anomalies in the brown amphibole and their textural relationship indicate that the Fe-Ti oxides crystallized at almost the same time as the brown amphibole. Both the brown amphibole and the Fe-Ti oxides crystallized from an interstitial, immiscible Fe-rich liquid and then formed the Fe-Ti oxide ores. The evolved Si-rich melt migrated upward and mixed with the upper melts, which ended the olivine crystallization at the base of the UZ. The olive amphibole crystallized from the mixed melts. The green amphibole exhibits various REE patterns, suggesting secondary alteration.

Melt–fluid infiltration along detachment shear zones in oceanic core complexes: Insights from amphiboles in gabbro mylonites from the Godzilla Megamullion, Parece Vela Basin, the Philippine Sea

Multiple generations of amphibole may form in the lower crust due to magmatism and metamorphism during the development of oceanic core complexes. We investigated the occurrence and chemical compositions of amphibole in gabbro mylonites from the medial area of the Godzilla Megamullion along the Parece Vela Rift in the Philippine Sea. The samples contain brown and green amphiboles with a variety of different textures that may have different origins. The brown amphibole occurs mainly as blebs in clinopyroxene porphyroclasts (Bleb amphibole), the rims around clinopyroxene porphyroclasts (Coronitic amphibole), and as porphyroclasts and fine-grained amphibole within the matrix (Matrix amphibole). The trace element and Cl contents of the bleb and green amphiboles indicate magmatic and metamoprhic origins, respectively. The bleb amphibole is interpreted to have crystallized from a hydrous silicate melt derived from an oxide gabbro-forming melt prior to retrograde metamorphism. In contrast, the compositions of the coronitic amphibole and matrix amphibole vary between those of typical magmatic and metamorphic amphiboles, suggesting that the amphibole-forming reactions were continuously retrogressive. Retrograde metamorphism is generally interpreted to have involved seawater-derived fluids, but the trace element contents of the coronitic and matrix amphiboles do not differ significantly from those of the original minerals (i.e., clinopyroxene and plagioclase). One sample of gabbro mylonite (KH07–02-D18–1) contains amphiboles with high concentrations of light rare earth elements, indicating a large influx of externally derived LREE-enriched fluids. These fluids are interpreted to have formed from an interaction between hydrous silicate melt with LREE-enriched composition and seawater-derived fluid. Our results suggest that multiple phases of melt–fluid infiltration occurred during the development of the detachment fault at the Godzilla Megamullion.

Iron-copper mineralization associated with metagabbro in Mirawa Village, Kurdistan region, northeastern Iraq

Mirawa iron-copper mineralization in Mirawa village, Kurdistan region, occurs as a zone of 15 m thick within the metagabbro of the Mawat Ophiolite. It is hosted by brecciatedamphibolite facies gabbroic rocks with a northwest–southeast trending shear zone. Amphibole, magnetite, plagioclase, quartz, epidote, chlorite, clinopyroxene and chalcopyriterepresent the mineral constitutes of metagabbro. Metamorphic conditions have been estimated from the green metamorphic amphibole replacing primary magmatic clinopyroxene (M1, 500 &lt; T &lt; 560 °C; 4.3 &lt; P &lt; 5.2 kbar) and retrograde actinolite (white amphibole) replacing the green amphibole due to continuous decreasing pressure and temperature (M2, 415 &lt; T &lt; 480 °C; 1.9 &lt; P &lt; 2.3 kbar). Occurrence of quartz with ironcopper mineralization hydrothermal fluids played a significant role during metamorphism. Chlorite and epidote were formed at the late stage of metamorphism. Iron-copper mineralization occurred in two paragenetic stages. Coarse grains of magnetite, chalcopyrite and pyrite formed during the first stage of mineralization representing syngenetic mineralization. Magnetite is the main Fe-ore mineral and it is characterized by large euhedral to subhedral grains with ilmenite exsolution lamellae. The second stage is epigenetic where mineralization is characterized by the formation of secondary minerals, replacing the primary minerals. Hematite replaces primary magnetite along the rim. Chalcopyrite, pyrite and magnetite were replaced by goethite whereas covellite overgrows chalcopyrite forming a corona texture. Fine-grained idomorphic aggregates of magnetite are dominant in the second stage and seem to be formed from iron liberated from pyroxene and other iron-bearing minerals. Microstructural orientation of the coarse-grained magnetite indicates a NW-SE direction for the maximum stretching of strain ellipse.

New geochemical results indicate a non-alpine provenance for the Alpine Spectrum (epidote, garnet, hornblende) in quaternary Upper Rhine sediment

The heavy mineral assemblage of late Pleistocene to Modern Rhine river sediment in the Upper Rhine Graben is dominated by garnet, green amphibole and epidote. This so-called Alpine Spectrum has been taken to indicate an exclusive derivation from the Alps and has hitherto been investigated with optical methods only. We present the first single-grain geochemical data set of garnet and amphibole from the upper Rhine river and some of its main tributaries. We use the new data to test the alleged Alpine provenance in the Rhine.Our results show that the provenance of garnet, mostly pyrope-rich almandine is grain-size dependent. Particularly the pyrope content indicates that the 0.063–0.25 mm size fraction has an Alpine provenance whereas 0.25–0.5 mm grains derived from the Black Forest and the Vosges, which flank the Upper Rhine Graben. They mainly comprise of Paleozoic metamorphic and igneous rocks supplying a suite of heavy minerals similar to the Alpine Spectrum to the Rhine river. Amphibole, mostly Mg-hornblende, has an increasing TiO2 content from the south to the north of the graben, which also indicates an input from the graben shoulders. The new data indicate that the so-called Alpine Spectrum of heavy minerals in Rhine river sediment of the upper Rhine derived to significant degrees from non-Alpine sources. Furthermore, our results indicate a relatively uniform provenance for the Rhine River system during the past 1 Ma.

High‐P tectono‐metamorphic evolution of mylonites from the Variscan basement of the Northern Apennines, Italy

AbstractStrain localization within shear zones may partially erase the rock fabric and the metamorphic assemblage(s) that had developed before the mylonitic event. In poly‐deformed basements, the loss of information on pre‐kinematic phases of mylonites hinders large‐scale correlations based on tectono‐metamorphic data. In this study, devoted to a relict unit of Variscan basement reworked within the nappe stack of the Northern Apennines (Italy), we investigate the possibility to reconstruct a complete pressure (P)–temperature (T)–deformation (D) path of mylonitic micaschist and amphibolite by integrating microstructural analysis, mineral chemistry and thermodynamic modelling. The micaschist is characterized by a mylonitic fabric with fine‐grained K‐white mica and chlorite enveloping mica‐fishes, quartz, and garnet pseudomorphs. Potassic white mica shows Mg‐rich cores and Mg‐poor rims. The amphibolite contains green amphibole+plagioclase+garnet+quartz+ilmenite defining S1 with a superposed mylonitic fabric localized in decimetre‐ to centimetre‐scale shear zones. Garnet is surrounded by an amphibole+plagioclase corona. Phase diagram calculations provide P–T constraints that are linked to the reconstructed metamorphic‐deformational stages. For the first time an early high‐P stage at &gt;11 kbar and 510°C was constrained, followed by a temperature peak at 550–590°C and 9–10 kbar and a retrograde stage (&lt;475°C, &lt;7 kbar), during which ductile shear zones developed. The inferred clockwise P–T–D path was most likely related to crustal thickening by continent‐continent collision during the Variscan orogeny. A comparison of this P–T–D path with those of other Variscan basement occurrences in the Northern Apennines revealed significant differences. Conversely, a correlation between the tectono‐metamorphic evolution of the Variscan basement at Cerreto pass, NE Sardinia and Ligurian Alps was established.

Green amphibole distribution as a stratigraphic tool in loess sequences from Belgium: A review

The use of heavy minerals as a stratigraphic tool in the study of loess sequences from NW Europe originated some 70 years ago. One major problem in using the available data sets is the heterogeneous stratigraphic context of the samples, given the complex historic evolution of the stratigraphic framework of loess sequences. This paper aims at presenting a review of the use of heavy minerals for stratigraphic studies in loess sequences from Belgium, focusing on the important role of green amphiboles. We provide the first synthesis of the available data for Belgium in a common and accurate lithostratigraphic framework, i.e. the loess lithostratigraphic units recently reviewed by the National Commission for Stratigraphy. A total of 121 samples studied by 4 different researchers and collected from 13 different loess sections are considered. We also document the detailed mineralogical composition of these green amphiboles based on new microprobe analyses. Our results show that the green amphibole content of regional loess deposits presents a remarkable consistency in their stratigraphic distribution. Five groups are defined here, covering the entire Pleistocene loess sequence, from MIS 11 to Weichselian. While GA distribution used alone is not a discriminatory criterion, it becomes most of the time discriminatory when combined with the palaeoenvironmental signature of the sequence deduced from pedostratigraphy.

Fluid rock interactions as recorded by Cl-rich amphiboles from continental and oceanic crust of italian orogenic belts.

A number of samples of Cl-rich amphiboles coming from oceanic and sub-continental gabbro bodies has been studied in order to compare their microstructural and compositional peculiarities and to investigate the fluid-rock interactions in different geodynamic contexts. The development of a first group of amphiboles outcropping in the Northern Apennines was the result of an hydration event that has been ascribed to oceanic metamorphism. The second group was found in a slice of continental crust subducted during Alpine collision, in a subcontinental metagabbro from the Sesia-Lanzo Zone of Western Italian Alps. Their development has been ascribed to a hydrothermal event that took place after the exhumation of the metagabbro during pre-Alpine lithospheric extension. The Cl-amphiboles are either found in veins, as granoblastic aggregates in different microstructures or as rims of zoned amphiboles, where brown-amphibole cores (sometimes Ti-rich), and successive green amphibole, are rimmed by the Cl-rich amphibole. All amphiboles show edembergite to pargasite compositions up to glaucophane and crossites when reequilibrated under HP conditions, with a direct correlation between Fe and Na (A) vs. Cl content, and inverse correlation of Mg and Na (M4) vs. Cl. A comparison with other Cl-amphiboles that have been observed both in oceanic and continental settings, allow inferring the role played by Cl-rich fluids infiltration both in oceanic and continental crust, during lithospheric extension. The large variations in Si, Al IV , Al VI , Fe, Mg, K and Cl may be related to the combination of different factors, such as Cl-content and related cristal-chemical constraints, whole rock composition, PT conditions of reequilibration, the microdomains where the amphibole grows and the variable a HCl/fluid /a H2O/fluid ratio of the fluid in equilibrium with the amphiboles at various stages of the metamorphic evolution. Amphiboles that locally contain extremely high Cl contents (up to 4% wt) could have been in equilibrium with a locally enriched Cl-fluid. As suggested by the fact that the Cl content of amphibole into the veins is generally lower than in amphibole rims far from the veins, these equilibrium conditions probably were reached at places where the system was locally closed. In addition, hydration reaction consumed the H 2 O component of the fluid, leading to a re-equilibration of the crystallising amphibole with the remaining Cl-enriched fluid. Equilibration temperatures up to 350°C can be attributed to the Apennines amphiboles, and up to 550°C to the ones from the Sesia-Lanzo Zone.

Copper Age axe-hammers, other tools and their raw materials in the sub-Apenninic area east of Bologna (Italy)

During this study, about two hundred stone tools from the Bologna territory—particularly from the S. Lazzaro di Savena area—were studied. They comprise perforated axe-hammers, axes/adzes and other variously polished complete or fragmentary tools of Copper Age typology, mostly coming from surface survey. They are mostly manufactured from basic magmatic rocks (in order of abundance: diabases, dolerites, gabbros, basalts and basic porphyrites) typical of the Apennine ophiolites outcropping upstream to the local mountains/hills, as well as in several areas of the Northern Apennines. Other lithologies, such as ophiolitic ultramafics (serpentinites, steatite and one Ca-pyroxenite) and non-ophiolitic lithologies, are poorly represented. The petrographic features of the studied artefacts are compatible with those of the ophiolitic rocks outcropping in the nearby Apennines and include varying degrees of low-grade, both of continental (various green and bright amphiboles, chlorite, saussurrite etc.) and oceanic metamorphism (typically brown hornblende and rare mylonitic textures). Some basic porphyrites with large phenocrysts, uncommon in the Apennine ophiolites, are also present. The few non-ophiolitic lithologies (siliciclastic arenites, calcarenites, limestones and siliceous stones) were probably supplied from the nearby Apennine, with the exception of a spotted slate of unknown origin. Based on their morphotypological features, some ophiolitic and not-ophiolitic artefacts may suggest some exchange and importation from—or simply some prehistoric cultural links with—Tuscany, Marche and occasionally Southern Italy and South Tyrol/Alto Adige.

The origin and age of the metamorphic sole from the Rogozna Mts., Western Vardar Belt: New evidence for the one-ocean model for the Balkan ophiolites

This study brings new geochronological and petrochemical data from the metamorphic sole beneath the Rogozna Mts., Western Vardar ophiolite belt. The Rogozna metamorphic sole is located at the base of a serpentinite nappe and consists of amphibolites and talc-chlorite schists. The Rogozna amphibolites are medium- to fine-grained rocks with nematoblastic texture and pronounced foliation. They consist of green amphibole (~70vol.%) with variable silica contents (6.4 to 7.8Siapfu), as well as Mg# (molMg/[Mg+Fetot]; 0.53 to 0.77) and variably albitized plagioclase (~30vol.%; Ab24–Ab98). Amphibolites are overprinted by a retrograde assemblage containing actinolite, epidote, clinoclore, sericite, chlorite, and magnetite. The amphibolites formed due to metamorphism of two basaltic suites: subalkaline/tholeiitic and alkaline. Subalkaline/tholeiitic amphibolites possess low Zr, Nb, Y, Th, Hf, TiO2, and P2O5 values and a LREE-depleted pattern, typical for the N-MORB (normal mid ocean ridge basalt) to BAB (back-arc basalt) origin. Alkaline amphibolites show elevated concentrations of Zr, Nb, Y, Th, Hf, TiO2, and P2O5 with a LREE-enriched pattern typically displayed by OIB (ocean island basalt). Amphibolites were crystallized during intra-oceanic thrusting at temperatures between 685°C and 765°C and at a depth of 12–17km. 40Ar/39Ar cooling ages of amphibole, ranging from 165 to 170Ma, slightly postdate the sole formation. Talc-chlorite schists are related to retrograde greenschist-facies metamorphism. They consist of Fe-rich talc and Cr-rich chlorite (peninite-diabantite) pseudomorphs after amphibole and MORB-type Cr-Al spinel, surrounded by Al- and Mg-poor ferrit chromite. The occurrence of ferrit chromite is related to earlier, amphibolite facies metamorphism. Chlorite pseudomorphs after amphibole were formed at 300–410°C.This study suggests that there is no essential difference in the emplacement age of the Dinaric and West Vardar ophiolite belts supporting the interpretation involving a single Mesozoic ocean in the Balkan sector.

On the Recycling of Amphibole-rich Ultramafic Intrusive Rocks in the Arc Crust: Evidence from Shikanoshima Island (Kyushu, Japan)

New insights into the role of amphibole in arc magma petrogenesis are provided by the mineral chemistry and U^Pb geochronology of Cretaceous amphibole-rich mafic rocks and associated granitoids from Shikanoshima Island (Kyushu, Japan). In the northeastern part of Shikanoshima Island a relatively large body (about 600 m in length) of amphibole-rich mafic rocks is found within granodiorite host-rocks.The core of the mafic body consists of amphibole-rich gabbrodiorite with a porphyritic texture.Towards the host granodiorite the porphyritic texture is progressively lost and a band of relatively homogeneous mediumto fine-grained mafic rock marks the boundary with the granitoid rocks.The amphibole-rich porphyritic gabbrodiorite consists of large amphibole grains (up to 60 vol. %) characterized by brown cores, occasional inclusions of clinopyroxene, and green rims. These large amphibole grains are dispersed in a fine-grained matrix consisting of green amphibole, clinopyroxene and plagioclase. Literature whole-rock data on the mafic rocks from Shikanoshima Island suggest that they are the intrusive counterparts of high-Mg andesite (HMA). Major and trace element mineral compositions reveal a marked chemical contrast between the brown amphibole (and its inclusions) and the matrix minerals, suggesting that they are not on the same liquid line of descent. The brown amphibole and its clinopyroxene inclusions were inherited from amphibole-rich ultramafic intrusive crustal rocks (e.g. hornblendites) crystallized from a melt with a chemical composition close to that of continental arc basalts. U^Pb geochronological data suggest that the xenocrystic material is about 20 Myr older than the matrix minerals. The matrix mineral crystallized from a parental liquid similar to sanukite-type HMA and with a trace element signature characterized by strong enrichment in elements with high crustal affinity and depletion in heavy rare earth elements. Green amphibole is a common mineral in all the studied lithologies; this allowed us to monitor the compositional variations in the liquid from which it crystallized moving from the core of the mafic complex to the host granodiorite.The data reveal that the interstitial melt had interacted with a melt enriched in elements with a high crustal affinity that, given the close association in the field, is inferred to be the host granitoid. These results favour an origin for sanukite-type HMA not from primary mantle melts but from mantle melts that have been affected by crustal processes and have been contaminated by crustal material. The major and trace element composition of the brown amphibole from the Shikanoshima Island mafic rocks is compared with that of brown amphibole from other amphibolite-rich intrusive rocks in orogenic settings worldwide (Alpine chain and Ross Orogen). The observed similarities suggest that the amphibole-rich mafic rocks are the expression of a magmatic process with a common geochemical affinity that is independent of the age and local geodynamic setting and thus related to a specific petrogenetic process. Amphibole-rich mafic and ultramafic intrusive rocks could be a common feature of all collisional systems and thus represent a ‘hidden’amphibole reservoir in the arc crust.We show that amphibole plays a major role in the petrogenesis of sanukite-type HMA but is also expected to play a major role in the differentiation of many other arc magmas.

On the origin of multi‐layer coronas between olivine and plagioclase at the gabbro–granulite transition, Valle Fértil–La Huerta Ranges, San Juan Province, Argentina

AbstractTroctolitic gabbros from Valle Fértil and La Huerta Ranges, San Juan Province, NW‐Argentina exhibit multi‐layer corona textures between cumulus olivine and plagioclase. The corona mineral sequence, which varies in the total thickness from 0.5 to 1 mm, comprises either an anhydrous corona type I with olivine|orthopyroxene|clinopyroxene+spinel symplectite|plagioclase or a hydrous corona type II with olivine|orthopyroxene|amphibole|amphibole+spinel symplectite|plagioclase. The anhydrous corona type I formed by metamorphic replacement of primary olivine and plagioclase, in the absence of any fluid/melt phase at &lt;840 °C. Diffusion controlled metamorphic solid‐state replacement is mainly governed by the chemical potential gradients at the interface of reactant olivine and plagioclase and orthopyroxene and plagioclase. Thus, the thermodynamic incompatibility of the reactant minerals at the gabbro–granulite transition and the phase equilibria of the coronitic assemblage during subsequent cooling were modelled using quantitative μMgO–μCaO phase diagrams. Mineral reaction textures of the anhydrous corona type I indicate an inward migration of orthopyroxene on the expense of olivine, while clinopyroxene+spinel symplectite grows outward to replace plagioclase. Mineral textures of the hydrous corona type II indicate the presence of an interstitial liquid trapped between cumulus olivine and plagioclase that reacts with olivine to produce a rim of peritectic orthopyroxene around olivine. Two amphibole types are distinguished: an inclusion free, brownish amphibole I is enriched in trace elements and REEs relative to green amphibole II. Amphibole I evolves from an intercumulus liquid between peritectic orthopyroxene and plagioclase. Discrete layers of green amphibole II occur as inclusion‐free rims and amphibole II+spinel symplectites. Mineral textures and geochemical patterns indicate a metamorphic origin for amphibole II, where orthopyroxene was replaced to form an inner inclusion‐free amphibole II layer, while clinopyroxene and plagioclase were replaced to form an outer amphibole+spinel symplectite layer, at &lt;770 °C. Calculation of the possible net reactions by considering NCKFMASH components indicates that the layer bulk composition cannot be modelled as a ‘closed’ system although in all cases the gain and loss of elements within the multi‐layer coronas (except H2O, Na2O) is very small and the main uncertainties may arise from slight chemical zoning of the respective minerals. Local oxidizing conditions led to the formation of orthopyroxene+magnetite symplectite enveloping and/or replacing olivine. The sequence of corona reaction textures indicates a counter clockwise P–T path at the gabbro–granulite transition at 5–6.5 kbar and temperatures below 900 °C.

Heavy mineral content and radioactivity counts of beach sands west of oreti river mouth to blue cliffs, southland, new zealand

Abstract Beach and dune deposits on the south coast of Southland were sampled and the minerals of specific gravity greater than 2.9 were extracted, magnetically separated and counted for radioactivity; the percentages of heavy minerals are estimated and the predominant minerals noted. Total quantities of beach sands west of Orepuki are estimated. The predominant heavy mineral throughout is a green amphibole. No commercially attractive quantities of ore minerals are indicated.

Petrology and geochemistry of the Xiugugabu ophiolitic massif, western Yarlung Zangbo suture zone, Tibet

The Yarlung Zangbo Suture Zone (YZSZ), southern Tibet, is a discontinuous belt that is more than 2000km long, composed of the remnants of Neo-Tethyan Mesozoic ocean. One of these relicts is the Xiugugabu ophiolitic massif which is a mantle thrust sheet of more than 260km2 overlying the Cretaceous tectonic mélange south of the YZSZ in SW Tibet. The massif is composed of harzburgites and clinopyroxene–harzburgites with porphyroclastic and porphyromylonitic textures. In the southern part of the massif, peridotites were intruded by amphibole-bearing microgabbro and microgabbronorite sills. A diabase unit which is overlaid by a sedimentary sequence crops out on the NE flank of the massif.Mineral chemistry in harzburgites and clinopyroxene–harzburgites indicates compositions similar to abyssal and forearc peridotites. Peridotites are slightly LREE depleted to enriched with [La/Yb]CN 0.06–2.8 and [La/Sm]CN 0.34–2.64. These ultramafic rocks are inferred to be the residues of 5–25% of partial melting of a depleted mantle that has been enriched by percolating metasomatic melts in a suprasubduction environment.Amphibole–microgabbro and amphibole–microgabbronorite sills are mostly composed of brown to green amphibole, calcic plagioclase, clinopyroxene, ilmenite and orthopyroxene in gabbronorite. Textures and compositions of the brown amphiboles indicate a near-solidus high temperature hydrothermal origin (>800°C). These intrusive rocks are tholeiitic and show N-MORB type REE patterns ([La/Yb]NC 0.35–0.90), a LILE (mainly Th) enrichment and noticeable Nb, Ta and Ti negative anomalies. They have a suprasubduction affinity and were formed in a back-arc basin setting. The diabase unit outcropping to the NE of the massif is not directly related to the ultramafic and mafic ophiolitic rocks. The diabase shows LREE enriched patterns ([La/Yb]NC 8–8.9) and slight Nb, Ta and Ti negative anomalies. The diabase has an intraplate affinity and could have been derived from a mantle source enriched by subduction-related fluids. The absence of continental crustal assimilation indicates that these rocks were probably emplaced in the Jurassic, in an oceanic environment after the Triassic disaggregation of the Indian plate.The data are consistent with the recent geodynamic model proposed for the central part of the suture for the closure of the Neo-Tethys and suggest that the geodynamic evolution of the western part of the basin was comparable to the central part.

Mineral chemistry of eclogites to investigate the evolutionary metamorphic history of UHP rocks from Tso-Morari region, Ladakh, India

Micro textures of metabasics from the Tso-Morari region, Ladakh were studied in order to understand the evolutionary metamorphic history of eclogites. The mineral chemistry, paragenesis of mineral inclusions in garnet, and zoning in omphacite, garnet and amphibole suggest three main metamorphic stages: (i) an eclogite stage with late blueschist facies metamorphism, (ii) a medium-pressure amphibolite facies stage, and (iii) a low-pressure amphibolite to greenschist facies stage. The high Si content in phengite, presence of rutile besides almandine-rich garnet and omphacite in eclogites indicate the attainment of high pressures. Also, the textural features and composition of amphiboles indicate that blueschist facies conditions represented by growth of glaucophane at high pressure and low temperature were followed by a lower-pressure stage of metamorphism represented by partial and in some cases complete reaction of glaucophane to calcic green amphibole such as magnesio-hornblende. The relationships define a clock-wise P-T path with the involvement of an isothermal uplift path for the eclogites and associated garnet-amphibolites of Tso-Morari region.

Mineralogical and geochemical characteristics of scheelite-bearing skarns, and genetic relations between skarn mineralization and petrogenesis of the associated granitoid pluton at Sargipali, Sundergarh District, Eastern India

Skarn rocks occur at the contact between calcite-bearing dolomitic marbles and granitoids (massive varieties with pegmatites) in close spatial association with the mica schist-hosted Proterozoic Pb–Cu–Ag sulfide deposits at Sargipali, Sundergarh District, Eastern India. The exoskarn (pyroxene–garnet) of variable width (1 to 30 m) occurs in marble proximal to the granitic intrusion, and endoskarn (pyroxene–epidote) is variably developed (< 1 to 10 m). Molybdenum-free scheelite with minor pyrrhotite (0.2%) is found only in late garnet–clinopyroxene exoskarn assemblages. In the Sargipali area early regional and contact metamorphism converted impure carbonate lithologies to calc-silicate hornfels. Subsequent contact metasomatism formed the skarn rocks, which are well zoned geochemically, mineralogically, and texturally in the sequence pyroxene–garnet–amphibole from the calcite-bearing dolomitic marble to granitoid contact. The presence of zoning relative to igneous contacts indicates that skarn-forming fluids originated from the crystallizing magma. The skarns are composed of clinopyroxene, garnet, calcic amphiboles (K-rich ferropargasite, ferrohornblende, hastingsite, tschermakite, magnesiohornblende, and actinolite), wollastonite, plagioclase, potash feldspar, epidote, titanite, and quartz. The skarns are notably enriched in Al, Mg, and Fe. Garnets are grossularite–almandine with 9 to 10 mol% spessartine, whereas pyroxenes are hedenbergitic to diopsidic in composition. The variable Mg:Mn:Fe proportions in the skarn clinopyroxene suggest the formation of clinopyroxene compositions from relatively homogeneous fluids, which experienced local variations in their Mg:Mn:Fe proportion instead of from successively different compositions. The earliest hornfels assemblage (Stage I) formed initially above 500 °C. This was overprinted by prograde anhydrous skarn (Stage II) at about 500 °C–600 °C and of 3–4 kbar pressure in a mildly reducing environment under X(CO 2) = ~ 0.18. With increasing fluid/rock interaction, epidote, green amphibole ± quartz-bearing retrograde skarn (Stage III) formed as temperature decreased to approximately 480 °C at X(CO 2) = 0.05. Late hydrothermal alteration (Stage IV) caused the formation of actinolite. There is a correlation between intrusion composition and the metal contents of associated skarns. Calc-silicate mineral compositions in the Sargipali skarns are similar to those in other W skarn systems. This granitic complex is comprised of reduced, highly evolved, and metallogenically specialized S-type leucogranites, comparable to those commonly associated with Mo-poor W skarns. A syn-collisional tectonic setting is proposed, based on field evidence, the relative timing of the intrusions with respect to metasedimentary and carbonate rocks, and empirical trace-element geochemical evidence. Based on field evidence and geochemistry, two main intrusive phases have been recognized in the Sargipali granitoid pluton: (1) an undeformed massive granite in the west, and (2) a foliated granite along the eastern margin. Porphyritic granites are also recognized locally, which are older than the other units. A genetic link exists between granite magmatism, and the formation of pegmatites in the region. The granite–pegmatite system is highly peraluminous (Al-saturation index ranges from 1.2 to 1.8). The peraluminous character increases from the foliated and porphyritic granite through massive granite to pegmatite. The foliated granite has higher FeO t, TiO 2, MgO, Ba, Sr, Zr, Th, ∑ REE (~ 200 ppm), and lower SiO 2 contents than massive granite (∑ REE = ~ 22 ppm). Both of these granite phases are highly evolved, while the massive variety is more evolved, and is mostly dominated by variable source rock composition. The minimum crystallization temperature of granite magma is at 628 °C–695 °C and greater than the 3 kbar pressure. The granites may have been generated by partial melting of metasedimentary rocks of the Gangpur Group that might have been enriched in W. The exsolving W-rich magmatic fluids interacted extensively with carbonate rocks and formed disseminated scheelite mineralization. New CHIME U–Th–Pb monazite dates for foliated granites of Sargipali are 960 ± 10 Ma, while massive granite shows two ages; the U-poor population and U-rich population yielded 955 ± 15 Ma and 997 ± 13 Ma, respectively. These ages indicate that the deformed foliated granite is slightly older than the undeformed massive granite, while there exists a still older intrusion (porphyritic granite) that is syn- to post-kinematic to the major phase of deformation (D 1) of the enveloping country rock at ~ 1 Ga. Overall, the Sargipali granitoid pluton post-dates the syngenetic Pb–Cu–Ag sulfide mineralization in the region at ~ 1.69 Ga.

Petrology and U–Pb Zircon Geochronology of Amphibole-rich Cumulates with Sanukitic Affinity from Husky Ridge (Northern Victoria Land, Antarctica): Insights into the Role of Amphibole in the Petrogenesis of Subduction-related Magmas

A microanalytical trace element and geochronological study was carried out on mafic amphibole-rich cumulates (quartz diorites) cropping out in northern Victoria Land (Antarctica). Associated tonalites and basement rocks were also investigated. Rock textures and major and trace element mineral compositions reveal the presence in quartz diorites of two mineral assemblages: (1) clinopyroxene-I + brown amphibole ± dark mica; (2) clinopyroxene-II + green amphibole + plagioclase + quartz. Both mineral assemblages contain mafic phases with elevated Mg-number, but their trace element signatures differ significantly. In situ U–Pb zircon geochronology was carried out to support petrogenetic and geological interpretations. Quartz diorites were emplaced in the mid-crust probably at 516 ± 3 Ma. Parental melts of quartz diorites were computed by applying solid/liquid partition coefficients. The melt in equilibrium with the first mineral assemblage (melt-I) is extremely depleted in heavy rare earth elements (HREE), Y, Ti, Zr and Hf (at about 0·2 times normal mid-ocean ridge basalt) and enriched in B, Th, U, the large ion lithophile elements and light REE (LREE). It shares many similarities with sanukitic melts (e.g. Setouchi andesites), which originated by equilibration of subduction-derived sediment melts with a refractory mantle. The melt in equilibrium with the second mineral assemblage (melt-II) is characterized by a steep LREE enrichment (LaN/YbN up to 39), a U-shaped HREE pattern and low Ti, which is depleted relative to HREE. The trace element signature of melt-II can be acquired through amphibole crystallization starting from a sanukitic melt similar to melt-I, probably in a deeper magma chamber. Our results allow us to constrain that melts from the subducted slab were produced on a regional scale, in accordance with literature data, below a large sector of the east Gondwana margin during the mid-Cambrian. Implications for the role of amphibole in petrogenesis of subduction-related magmas are also discussed.

Exsolution microstructures in amphiboles from metabasalts of the Betic ophiolitic association (SE Spain)

Exsolution microstructures in a complex amphibole assemblage from metabasalts of the Betic Ophiolitic Association (SE Spain) have been studied using transmission and analytical electron microscopy (TEM, AEM). Three main types of amphibole were identified, filling veins and vesicles: brown amphibole, with mean composition of pargasite, originated during an early ocean-floor metamorphism, and two types of green amphiboles with compositions of edenite and actinolite-magnesiohornblende, respectively formed during a later orogenic metamorphic event. The exsolution in the brown amphibole include minute rutile and amphibole rods. The brown amphibole also contains numerous halite inclusions. Amphibole exsolutions, with compositions of magnesiohornblende and actinolite appear to have formed paired rutile + amphibole intergrowths. All the exsolution products appear oriented with respect to the lattice of the host amphibole. Green amphiboles do not contain either submicroscopic rutile or halite inclusions, but they show oriented lamellae of magnesiokatophorite, exsolved in the edenite. These thin lamellae (300 to 500 A in thickness) are coherently intergrown with the host. The studied amphibole pairs evidence two types of miscibility gaps: the one gap between amphiboles of the calcic group (brown amphibole) and the other gap between amphiboles of the calcic and sodiccalcic groups (green amphibole).