A Note on "Stress-Minerals"

The Kwoiek Area of British Columbia contains a pendant or screen of metamorphosed sedimentary and volcanic rocks almost entirely surrounded by a portion of the Coast Range Batholith, and intruded by several dozen stocks. The major metamorphic effects were produced by the quartz diorite batholithic rocks, with minor and later effects by the quartz diorite stocks. The sequence of important metamorphic reactions in the metasedimentary and metavolcanic rocks, ranging in grade from chlorite to sillimanite, is: 1. chlorite + carbonate + muscovite → epidote + biotite 2. chlorite + carbonate → actinolite + epidote 3. chlorite + muscovite → garnet + biotite 4. chlorite + epidote → garnet + hornblende 5. chlorite + muscovite → garnet + staurolite + biotite 6. chlorite + muscovite → aluminum silicate + biotite 7. muscovite + staurolite → garnet + aluminum silicate + biotite 8. staurolite → garnet + aluminum silicate Continuous reactions, occurring between reactions 5 and 7, are: A. chlorite + (high Ti) biotite + Al2O3 (from plagioclase?)→ garnet + staurolite + (low Ti) biotite + O2 B. muscovite (phengitic) → garnet + staurolite +muscovite (less phengitic) + O2 (?) Detailed electron microprobe work on garnet, staurolite, biotite, and chlorite shows that: (1) The garnet porphyroblasts are zoned according to a depletion model, called the Rayleigh depletion model, which assumes equilibrium between the edge of a growing garnet and the minerals which are unzoned, notably biotite, chlorite, and muscovite, but which assumes disequilibrium within the garnet. (2) The staurolite porphyroblasts are also zoned, and from their zoning patterns reactions A, B, and 5 are documented. Progressive reduction of iron with increasing grade of metamorphism is also inferred from the staurolite zoning patterns. (3) During a late period of falling temperature garnet continued to grow and the biotite and chlorite reequilibrated. The biotite, chlorite, and garnet edge compositions can vary from point to point in a given thin section, indicating that the volume of equilibrium at the final stage of metamorphism was only a few cubic microns. (4) The horizon within the garnet that grew at maximum temperature can be identified. The Mg/Fe ratio of this horizon, if the garnet composition is a limiting composition in the Al2O3 - K2O - FeO - MgO tetrahedron, increases systematically with increasing metamorphic grade. Biotite and chlorite compositions also show a general increase in Mg/Fe ratio with increasing metamorphic grade, but staurolite appears to show the reverse effect. (5) The Mg/Fe ratio at the maximum temperature horizon of the garnet porphyroblasts is a function of its Mn content as evidenced from the study of five garnet-bearing rocks, collected from one outcrop area, with the same assemblage but with differing proportions of minerals. An important implication of zoned minerals is that the effective composition of a system in a phase lies on the join between the homogeneous minerals (if there are two) and not within three-or- four-phase fields when a zoned mineral, such as garnet or staurolite, is present in the assemblage. Study of the three aluminum silicates found in the Kwoiek Area showed that a constant pressure change in polymorphs from andalusite to kyanite to sillimanite took place with increasing temperature. This transition series is best explained by the metastable formation of andalusite. Photographic materials on pages 15, 121, 160, 162, and 164 are essential and will not reproduce clearly on Xerox copies. Photographic copies should be ordered.

O Gnaisse alcalino de Serra do Matola - São Jõao Del Rei

The Variscan basement of Northern Apennines (Northern Italy) is a polymetamorphic portion of continental crust. This thesis investigated the metamorphic history of this basement occurring in the Cerreto Pass, in the Pontremoli well, and in the Pisani Mountains. The study comprised fieldwork, petrography and microstructural analysis, determination of the bulk rock and mineral composition, thermodynamic modelling, conventional geothermobarometry, monazite chemical dating and Ar/Ar dating of muscovite. The reconstructed metamorphic evolution of the selected samples allowed to define a long-lasting metamorphic history straddling the Variscan and Alpine orogenesis. Some general petrological issues generally found in low- to medium-grade metapelites were also tackled: (i) With middle-grade micaschist it is possible to reconstruct a complete P-T-D path by combining microstructural analysis and thermodynamic modelling. Prekinematic white mica may preserve Mg-rich cores related to the pre-peak stage. Mn-poor garnet rim records the peak metamorphism. Na-rich mylonitic white mica, the XFe of chlorite and the late paragenesis may constrain the retrograde stage. (ii) Metapelites may contain coronitic microstructures of apatite + Th-silicate, allanite and epidote around unstable monazite grains. Chemistry and microstructure of Th-rich monazite relics surrounded by this coronitic microstructure may suggest that monazite mineral was inherited and underwent partial dissolution and fluid-aided replacement by REE-accessory minerals at 500-600°C and 5-7 kbar. (iii) Fish-shaped white mica is not always a (prekinematic) mica-fish. Observed at high-magnification BSE images it may consist of several white mica formed during a mylonitic stage. Hence, the asymmetric foliation boudin is a suitable microstructure to obtain geochronological information about the shearing stage. (iv) Thermodynamic modelling of a hematite-rich metasedimentary rock fails to reproduce the observed mineral compositions when the bulk Fe2O3 is neglected or determined through titration. The mismatch between observed and computed mineral compositions and assemblage is resolved by tuning the effective ferric iron content by P-XFe2O3 diagrams.

Metamorfismo ercinico di bassa-pressione: evoluzione tettonico-metamorfica del complesso di mandatoriccio(massiccio della Sila - Calabria)

Introduction The Avan Cu-Fe skarn is located at the southern margin of Qaradagh batholith, about 60 km north of Tabriz. The Skarn-type metasomatic alteration is the result of Qaradagh batholith intrusion into the Upper Cretaceous impure carbonates. The studied area belongs to the Central Iranian structural zone. In regional scale, the studied area is a part of the Zangezour mineralization zone in the Lesser Caucasus. Several studies (Karimzadeh Somarin and Moayed, 2002; Calagari and Hosseinzadeh, 2005; Mokhtari, 2008; Baghban Asgharinezhad, 2012; Mokhtari, 2012) including master’s theses and research programs have been done on some skarns in the Azarbaijan area considering their petrologic and mineralization aspects. However, before this study, the Avan skarn aureole has not been studied in detail. In this paper, various geological aspects of the Avan skarn including mineralogy, bi-metasomatic alteration, metasomatism and mineralization during the progressive and retrograde stages of the skarnification processes have been studied in detail. Research Method This research consists of field and laboratory studies. Field studies include preparation of the geological map, identifying the relationship between the intrusion and the skarn aureole, identifying the relationship between different parts of the skarn zone and also collecting samples for laboratory studies. Laboratory studies include petrography, mineralography and microprobe studies. Cameca SX100 Microprobe belonging to Geological Survey of the Czech Republic was used in order to determine the chemical composition of the calc-silicate minerals such as pyroxene and garnet in garnet skarn and pyroxene- garnet skarn sub-zones. Discussion and conclusion Qaradagh batholith is composed of discrete acid to mafic phases including gabbro, diorite, quartz diorite, quartz monzonite, quartz monzodiorite, tonalite, granodiorite, monzogranite and granite porphyry which is dominated by granodiorite-quartz monzonite. Granitoids of this batholith are metaluminus, high K calc-alkaline I-type granite (Mokhtari, 2008). The Avan Cu-Fe skarn is related to the intrusion of granodioritic-quartz monzonitic part of the Qaradagh batholith into the Upper Cretaceous flysch- type rocks consisting of biomicrite, clay limestone, marl, siltstone and mudstone. The Avan skarn consists of three zones of endoskarn, exoskarn and marble. The main Cu-Fe mineralized zone is related to the exoskarn zone, which has 600 meters of length and 50 meters of thickness, respectively. The Exoskarn zone consists of garnet skarn, pyroxene-garnet skarn and ore skarn sub-zones. Garnet, belonging to ugrandite series (Ad53-89) with more than 50 percentage in volume, is the most important anhydrous calc-silicate mineral in the garnet skarn and the pyroxene-garnet skarn sub-zones. Some of the garnet crystals are zoned and their chemical composition changes toward the rim to almost pure andradite (Ad99). Clinopyroxene which has diopsidic composition (Di75-96), is another anhydrous calc-silicate mineral in the exoskarn zone with an abundance that reaches up to 50 percent in volume in pyroxene-garnet skarn sub-zone. The ore skarn sub-zone is located toward the outer part of the exoskarn zone and close to the border of the marble zone. The abundance of ore minerals in this sub-zone reaches up to 50 percentage in volume and includes magnetite, hematite, pyrite, chalcopyrite, bornite, malachite and goethite among which pyrite is the most abundant. In this sub-zone, anhydrous calc-silicate minerals of garnet and clinopyroxene have undergone intensive alteration and are replaced with hydrous calc-silicate (epidote and tremolite- actinolite), oxide (magnetite and hematite) and sulfide (pyrite, chalcopyrite and bornite) minerals. Based on the textural and mineralogical studies, the skarnification processes in the studied area can be categorized into two main stages: 1) prograde and 2) retrograde. During the prograde stage, the heat flow of the granitoid has caused isochemical metamorphism and changing more pure limestones to marble and marlly limestones to skarnoid (metamorphism and bi-metasomatism). The high temperature magmatic fluids have caused prograde metamorphism during which anhydrous calc-silicate minerals including garnet and pyroxene have appeared. During the early retrograde stage, i.e. the mineralization sub-stage, lower temperature hydrothermal fluids have caused hydrolysis and carbonization because of which anhydrous calc-silicate minerals along with their fractures and microfractures are changed to hydrous calc-silicate (epidote and tremolite-actinolite), oxide (magnetite and hematite), sulfide (pyrite, chalcopyrite and bornite) and carbonate (calcite) minerals. During the late retrograde stage, relatively low temperature fluids have altered anhydrous and hydrous calc-silicate mineral assemblage formed during the previous stages into a very fine grained mineral assemblage including clay minerals, chlorite and iron hydroxides. Presence of replacement textures in ore minerals and anhydrous calc-silicate minerals accompanied with open filling textures in the anhydrous calc-silicate minerals, for example oxide and sulphide veinlets within the garnet crystals, indicate that the mentioned ore minerals have been simultaneously generated with hydrous calc-silicate minerals (epidote and tremolite-actinolite) during the early prograde stage. The presence of minor amounts of wollastonite among the mineral assemblage of the Avan skarn, intergrowth of garnet and pyroxene, absence of reaction rim between garnet and clinopyroxene and absence of replacement textures indicate that these minerals have been simultaneously generated within the temperature ranges of 430–600 ºC and ƒO2 > 10-26, respectively. Acknowledgements The authors are grateful to the Journal of Economic Geology reviewers and editors for their constructive suggestions to the manuscript. Reference Baghban Asgharinezhad, S., 2012. Investigation of genesis, mineralogy and geochemistry of Fe-Cu skarn in Astamal area, NE Kharvana, Eastern Azarbaijan. MSc. Thesis, University of Tabriz, Tabriz, Iran, 185 pp. (in Persian with English abstract) Calagari, A.A. and Hosseinzadeh, G., 2005. The mineralogy of copper-bearing skarn to the east of the Sungun-Chay River, East-Azarbaijan, Iran. Journal of Asian Earth Sciences, 28(4-6): 423-438. Karimzadeh Somarin, A. and Moayed, M., 2002. Granite and gabbro-diorite associated skarn deposits of NW Iran. Ore geology reviews, 20(3-4): 127-138. Mokhtari, M.A.A., 2008. Petrology, geochemistry and petrogenesis of Qaradagh batholith (east of Syahrood, Eastern Azarbaijan) and related skarn with considering mineralization. Ph.D. Thesis, Tarbiat Modares University, Tehran, Iran, 347 pp. (in Persian with English abstract) Mokhtari, M.A.A., 2012. The mineralogy and petrology of the Pahnavar Fe skarn, in the Eastern Azarbaijan, NW Iran. Central European Journal of Geosciences, 4(4): 578-591.

Geology of the Ramona pegmatites, San Diego County, California

The Precambrian Sierra Ancha sill complex, 700 to 800 feet thick, was intruded into flat-lying sedimentary rocks of the Apache Group in central Arizona. The bulk of the complex consists of a central layer of feldspathic olivine-rich diabase and upper and lower layers of olivine diabase. Diabasic rocks present in relatively minor quantity in the upper part of the complex include diabase pegmatite, albite diabase, and quartz diabase. Potassic granophyres locally form lenses up to two hundred feet thick near and at the roof of the complex. The intrusion was studied in the field and in the laboratory to determine the origins of the rock types and the conditions under which they formed. Extensive electron microprobe studies of mineral compositions and zoning are presented together with nineteen new whole rock chemical analyses. The Sierra Ancha olivine diabase has a high-alumina olivine basalt composition. Olivine diabase and olivine-rich diabase display a differentiation pattern characterized by moderate iron enrichment. Diabase pegmatite is relatively enriched in alkalis. The principle primary minerals in feldspathic olivine-rich diabase and olivine diabase include: plagioclase (An_(72)-An_(16)); augite (Wo_(43)En_(44)Fs_(13) to Wo_(40)En_(38)Fs_(22)); olivine (Fo_(74)-Fo_(54)); orthopyroxene (En_(77)-En_(44)); magnetite_ss (Mgt_(66)Usp_(34)-Mgt_(89)Usp_(11)); and ilmenite_(ss) (Ilm_(86)Hem_(14)-Ilm_(96)Hem_4). All of the orthopyroxene is primary. Fe- Mg fractionations between mafic mineral pairs increase with iron enrichment and declining crystallization temperatures. Ilmenite which formed by reaction-exsolution from magnetite was found to be consistently different in composition from primary ilmenite. The late-crystallizing diabase pegmatites contain an assemblage including iron-rich chlorite together with calcic pyroxene; from textural evidence the two phses appear primary. The calcic pyroxene has a compositional range from Wo_(49)En_(28)Fs_(23) to Wo_(49) En_(14)Fs_(37); its compositions define an iron-enriched trend in the pyroxene quadrilateral more calcic (i.e., closer to the diopsidehedenburgite join) than other iron-enriched igneous pyroxene trends described in the literature. Most diabasic rocks in the sill display some deuteric alteration. The mineral assemblage seemingly stable in the most-altered rocks includes albite (An_(2-0)), prehnite, calcic pyroxene (saute), chlorite, sphene, and apatite. Albite diabase contains this assemblage and apparently formed by recrystallization of normal diabase under deuteric conditions. The alteration assemblages are similar to those found in spilites. They provide an important example of the development of a spilitic assemblage by autometamorphism. The massive granophyres at and near the top of the sill appear to be igneous. The larger lenses occur at local high points in the roof of the complex near discordant contacts. The granophyres consist primarily of alkali feldspar with subordinate calcic pyroxene, iron-rich hornblende, biotite, and quartz and minor plagioclase and other phases. They have no relict sedimentary textural features, and they contain miarolitic cavities and rotated and displaced sedimentary rock inclusions. Locally, they occur as masses truncating overlying strata and as dikes in the overlying sedimentary rocks. Some of the dikes have apparent chilled contacts against the sedimentary rocks, suggesting that they were emplaced largely as melts. The granophyres formed as a result of the interaction of diabase magma with stratified rocks of the overlying Dripping Spring Quartzite. The massive granophyres are generally similar in composition to the overlying sedimentary rocks; both rock types have very unusual and distinctive high potassium contents. Contact metamorphism by the diabase has produced layered metasedimentary rocks with granophyric textures and mineral assemblages comparable to those in some massive granophyres. Consistent compositional differences between granophyres and sedimentary rocks may have been caused by metasomatic processes or by mixing of diabase magma with the sedimentary rock material which constitutes most of the granophyres. The interaction of diabase and sedimentary rocks may have occurred because magma in the upper part of the intrusion absorbed water from the overlying sedimentary rocks and solidified after magma in the central part of the intrusive. If this happened, the sedimentary rocks over the sill might have been melted to form the granophyres. No chilled facies of diabase occurs at the sill roof where granophyres are present. Compositional trends in mineral series indicate that the diabase magma in the upper part of the sill solidified towards the roof in at least one locality. Normal processes of magmatic differentiation produced feldspathic olivine-rich diabase, olivine diabase, and diabase pegmatite in the Sierra Ancha complex. The processes which produced the granophyres include recrystallization and fusion of rocks overlying the intrusion. The Sierra Ancha granophyres offer a superb opportunity to study these processes and others which may have produced many of the granitic rocks in the crust of the earth.

The inner zone of the Sardinia Variscan segment consists of two metamorphic complexes: I) A polymetamorphic Migmatite Complex, with migmatites showing polyphase anatectic processes, in the presence of kyanite or sillimanite. The Migmatite complex preserved decametric lenses of eclogite relicts (eclogites A) affected by high T, high- to intermediate P recrystallization under granulite facies conditions The decompressional garnet + Ca-clinopyroxene + amphibole ± orthopyroxene-bearing assemblages developed in granoblastic textures generally in no stress conditions. In most cases, only symplectite textures provide evidence for the eclogitic event. II) A medium grade, mostly metapelitic complex consisting of Grt, Ky, Stau-bearing micaschists and paragneisses includes quartzites and garnet-bearing amphibolite boudins with N-MORB chemical affinity. Relicts of eclogite assemblages were locally found in the metabasite (eclogites B). In eclogites A, the geothermobarometric parameters yield temperatures in the range 690°-760°C for minimum pressure A1.3 GPa. Pyroxene compositions accord with temperatures in excess of 700°C. In eclogites B, the thermometric calibrations provide temperatures in the range 610°-700°C for pressures 1.3-1.5 GPa, based on the jadeite content. The temperatures are consistent with the biotite+muscovite+garnet+kyanite+staurolite assemblage in the host paragneisses, and with lack of anatectic processes. The age of 457±2 Ma, obtained by U/Pb dating on one sample of Type A eclogite is interpreted as a minimum estimate for the magmatism of the eclogite protolith. A second zircon population defined an age of 403±4 Ma interpreted as dating the zircon crystallization during the high-grade event. The relationships between Types A and B eclogites, and their bearing on the regional framework (Sardinia, Ligurian Alps) are discussed.

The study of late and post-orogenic sedimentary basins is a powerful tool to understand uplift, exhumation and erosion of an orogen. In the Ligurian Alps, high-pressure ophiolitic rocks are directly overlain by the sediments of the Tertiary Piedmontese Basin (TPB); the conglomerates at the bottom of the TPB succession contain clasts of metaophiolites and metasediments which display deformation structures acquired at peak metamorphic conditions ranging from eclogite- to blueschist- facies. The deformation events prevalently recorded by the highpressure rocks of the Ligurian Alps are either related to subduction and peak metamorphism, or to the late-stage greenschist collision. The main structures which accomplish the early exhumation of such high-pressure terrains are only locally recorded and poorly explored. Further information on these early tectonic events can be gathered through the study of the clasts in the conglomerates. In this paper we present a textural and petrologic outline and some thermobarometric estimates of the main types of high-pressure clasts sampled and their comparison with the high pressure equivalents presently exposed in the Ligurian Alps. In particular, some clasts displaying a Na-amphibole + white mica + epidote + sphene blueschist foliation superimposed to an eclogitic garnet + Na-clinopyroxene + rutile tectonitic assemblage have been studied in detail. These kind of rocks record a pressure-temperature path implying cooling during exhumation, whereas the high-pressure bedrocks are characterized by either isothermal decompression, or initial heating and subsequent cooling. We conclude that the blueschist foliation of these clasts most likely developed along shear zones and/or contacts among different slices, formed during tectonic coupling of warm, uprising eclogite units, with cooler blueschist slices during the early stages of exhumation.

The PALMEIROPOLIS Cu-Zn (Pb) volcanogenic massive sulfide deposit, Brazil, consists of three ore bodies enclosed by hydrothermal alteration zones. The ore bodies and the alteration zones were metamorphosed under amphibolite fades conditions. The Palmeirdpolis alteration zones are characterized by a great diversity of bulk rock composition that originated a wide variety of low variance mineral assemblages. These assemblages are composed of orthoamphiboles (anthophyllite and gedrite), hornblende, biotite, garnet, staurolite, sillimanite, gahnite and, rarer, cordierite. Based on analyses of mineral chemistry aad mineral assemblages, temperatures are estimated to have been 550 - 625°C and pressures 2 - 5.5 kbar. The temperature of metamorphism that prevailed at the Palmeirdpolis deposit is comparable to other amphibolite fades massive sulfide deposits, such as Geco and Linda, Canada; Falun, Sweden; and Bleikvassli, Norway. The mineralogy of the alteration zones is similar in all these deposits even though they were metamorphosed at different pressure conditions, reflected by the crystallization of one of Al2 SiO5 phase (andalusite, sillimanite or kyanite).

The objective of this research is to elucidate the isotopic, geochemical und structural characteristics of minerals in microstructures to understand the evolution of Rb/Sr isotope system within minerals in the course of microstrucutral setting during deformation. Isotopic analyses using the Rb/Sr method on microsamples enable to distinguish similar mineral samples and a textural controlled separartion under microscopic observation. This offers new insights into submillimeter scale under isotopic aspects. The determination of mineral ages was not confined to the geochronological discribtion of geological processes. Beyond this, mineral ages are terms for isotopic evolution in time. Since age values are easier to compare among to each other than time induced change of isotopic composition the discussion of results and interpretion become more obvious. For age determinations using the Rb/Sr micromethod samples from the attic-cycladic Blueschist belt on South Evia (Cyclades, Greece) were used. The similarities of the rocks in terms of composition and structure allow direct comparison, an important precondition for this approach. Mineral ages were calculated on white mica using Sr/Sr of calcite as initial isotopic composition. Age values widely scatter in all lithologies but vary in different amounts between 15 93 Ma. There is no corelation between microstructural Postion and mineral age. The chemical characterisation of mineral populations using microprobe analyses focussed on the relations between microstructre and chemical composition of shape fabrics to correlate with isotopic characteristcs. The inhomogeneous distribution of main elements within K-white mica refers to discontinuous mineral evolution. High Si content indicates high pressure conditions of 10 -17 kbars. The pre-hp metamorphic aquired Sr composition in Sr reservoirs is almost completely preserved and homogeneous over within cm-scale with values of 70 4 ppm. Sr composition depends on lithology. Extreme gradients exist on lithologic boundaries. The results of isotopical investigations yield new aspects to understand the evolution of subduction and exhumation in the attic-cycladic Blueschist belt in the Aegaeis. Age values of 15 25 Ma obtaines from phengites with high Si content. Summarizing, a so far unknown very young high-pressure event in the course of metamorphic evolution of the south Evian formations must be postulated.

Very low-grade metamorphism in the Portage Lake Volcanics on the Keweenaw Peninsula, Michigan, USA

The Western Vardar ophiolite crops out along the entire Balkan Peninsula. In northeast Albania near Bajram Curri, the Jurassic plate interface reveals a metamorphic sole below peridotites of the Mirdita Ophiolite. There, I mapped an area of around 35 km2 along the southeast-dipping fossil plate interface and performed various micro-scale analyses such as polarisation microscopy, Raman-spectrometry, Raster-electron microscopy and Micro-x-ray fluorescence on metamorphic sole as well as mantle lithologies. Three major units are distinguishable. The mantle harzburgites overthrust a tectonic melange which itself can be separated into a non-metamorphic broken formation and a metamorphic sole. The metamorphic sole itself was accreted at a lower structural level than the broken formation and later emplaced on top of it. The tectonic melange is tightly folded with fold axes plunging gently towards NNW or NNE. The metamorphic sole shows an inverse metamorphic field gradient up to greenschist-facies conditions near Bajram Curri and granulite-facies conditions near Shkelzen. Granulite-facies conditions are preserved as relictic blasts with albite, margarite & muscovite as pseudomorphs after feldspar; as well as garnet and monazite. S-c’-fabrics in calcschists show top to the west shear sense direction, suggesting westward emplacement of the overlying ophiolites. All lithologies in the metamorphic sole are characterised by a later lower-greenschist-facies overprint, evolving from dynamic to static recrystallisation. Stilpnomelane, quartz and white mica form a typical mineral assemblage giving temperatures between 280°-450°C and pressures between 4.3-5.5 kbar. Right above the southeast-dipping plate interface, foliated upper plate harzburgites with southeast dips crop out. Mylonitic layers preserve a mineral assemblage with major olivine and minor orthopyroxene, clinopyroxene, as well as chromium-rich spinel (Cr# 72). Clinopyroxene lamellae in Aluminium- depleted (Al2O3 0.85 wt%) orthopyroxene clasts give lower temperature limits for recrystallisation of 1042°-1063°C under low-pressure conditions. The mantle source was either already depleted or the primary harzburgite was melted in a second stage. In the later evolving ultramylonitic layers, the mineral phases exhibit average grain sizes of 5 µm. There, phases of two clinopyroxene with low-calcium and high-calcium content respectively occur. Serpentinisation decreases upsection and developed in three texturally distinct types that bear different serpentine minerals and opaque phases.

The Tanno pegmatitic field, placed southward of Chiavenna (Central Alps, Sondrio, Italy), develops a large number of subplanar dykes that crosscut the Chiavenna Unit, an ophiolitic complex mainly composed, in the study area, of amphibolite rocks. This study focuses on the contact between a pegmatitic dyke and the amphibolitic country rock. We distinguished four zones across the contact: I) inner amphibolite, II) contact amphibolite, III) contact pegmatite, IV) inner pegmatite. The inner amphibolite, not affected by melt-rock interaction, is composed of amphibole, phlogopite, ilmenite, titanite and rutile. Two amphibole generations occur, both of them showing a patchy compositional zoning. Amphibole I are Mg-hornblende, whereas Amphibole II have a pargasitic composition. The contact amphibolite shows an enrichment of mica belonging to the phlogopite-biotite series, titanite and the presence of fluorapatite and plagioclase (Ab(4)(5-)(60)), that is absent in the inner amphibolite. Close to the contact, amphiboles display no zoning and gain an Mg-horneblenditic composition. The contact pegmatite has quartz, albitic plagioclase. garnet (almandine-spessartine series). muscovite, K-feldspar and Iluorapatite. It shows a comb texture, with elongation of plagioclase crystals normal to the contact itself. Far from the contact, the inner pegmatite has an increasing grain-size and a less organized texture. In this zone several accessory phases occur, including gahnite, columbite-(Fe), monazite-(Ce), xenotime-(Y), uraninite and betafite. Whole rock analyses suggest that a chemical exchange, concerning both major elements and trace elements, occurred between the pegmatitic melt and the hosting amphibolite. A considerable increase of SiO2, Na2O and, to a lesser extent, of Al2O3 is observed from the amphibolite towards the pegmatite; K2O and CaO show a decrease at the same extent. The REE pattern in the pegmatite highlights an enrichment in HREE at the contact. Mineral chemistry confirms this trend with variations observable in plagioclase, gradually more albitic from the amphibolite to the pegmatite. Mineralogical characters and geochemical features allow to classify the Tanno pegmatite in the LCT (Lithium, Cesium, Tantalium) family. Based on the metamorphic peak conditions reported from the Lepontine Dome the ambient conditions during pegmatite intrusion were ca. 550 degrees C and 5 kbar. The reduced thermal difference between pegmatite and wall rock explains the diffuse contact observed by X-ray micro-computed tomography. The collected data suggest a chemical interaction between melt and wall rock, according to the following reaction taking place in the amphiboliteAmphibole I + Amphibole II + Ilmenite + Pegmatitic melt -> Amphibole III + Plagioclase + Phlogopite + Titanite + Fluorapatite

This thesis investigates metamorphic controls on the distribution of metals and metal-transporting ligands in the mid to lower continental crust. The metal content of the mid crust is investigated by evaluating the mobility of elements during chlorite devolatilisation in the upper greenschist facies. This interval of metamorphic fluid production was found to coincide with the pyrite-pyrrhotite transition; a reaction that liberates sulfur into the metamorphic fluid and is thought to be important for scavenging gold to form orogenic Au deposits. The pyrite-pyrrhotite transition field was found to extend to higher temperatures (500 – 550 °C) in a natural system than previously described by thermodynamic modelling. Mass balance calculations suggest that S-bearing metamorphic fluids produced during simultaneous chlorite and pyrite breakdown partially extracted Au, As, Bi, Sb, Mn, and W from the rock. Conversely, the uptake of Cu, Pb, Zn, Ni, Co, Ga, Ge, Mo, Tl, V, and Cr by silicate and sulfide minerals acted to retain these metals in the deep crust.At greater crustal depths, biotite devolatilisation causes melting of lower-crustal rocks. Biotite is stabilised to higher temperatures by uptake of the metal-transporting ligand F, which is controlled by Mg#, and factors that affect Mg# in biotite, including bulk composition, coexisting mineralogy, and preferential removal of Fe over Mg during partial melting. Chlorine retention in biotite is inversely correlated to F, such that initial biotite dehydration produces a slightly Cl-enriched, F-poor melt, enriching residual biotite in F. At final dehydration of biotite, especially in MgO- and K2O-rich bulk compositions, F-rich biotite breaks down at lower-crustal conditions to produce a hot, dry, F-rich and Cl-poor granitic melt. The amount of F released into such a melt from only moderately F-enriched biotite is sufficient to form highly F-enriched melt, typical of A-type granites. These metamorphogenicmelts may be important for transport of REE and some HFSE to the upper crust due to the tendency of these elements to be enriched in F-rich melts.An investigation into mobilisation/retention of base metals during ultra-high temperature metamorphism found that element mobility is primarily controlled by the tendency of elements to partition into residual minerals. Sulfide and major silicate minerals in Rogaland metamorphic aureole rocks are sources of metals for uptake into melts produced by biotite dehydration. Copper, Pb, and Zn, are hosted in major silicate minerals at high to ultra-high temperatures, and therefore these are primarily controlled by metamorphic reactions amongst the major silicate minerals. Retention of base and precious metals can occur at high to ultra-high temperatures by shielding in quartz veins, preferential removal of silicate melt, and reaction with residual minerals. These findings suggest that metapelitic rocks in the lower crust may be viable source region for ore deposits.The work presented in this thesis identifies metamorphism as a key driver of metal redistribution in the mid and lower continental crust. Our understanding of the formation of metamorphogenic and crust-derived ore deposits is improved by understanding the relationships between fluid/melt loss, residual mineral growth, and consequent metal partitioning.