Metamorphic Envelope Research Articles

Titanite from the NYF-type pegmatites of Szklarska Poręba Huta quarry, Karkonosze granite massif, SW Poland

Titanite, an accessory mineral of pegmatite related to aplogranite, was identified in the Szklarska Poręba Huta quarry within the Karkonosze granite massif in Lower Silesia, Poland. It formed during pegmatitic to hydrothermal stages. Besides the isovalent substitution Sn → Ti, the chemical composition of the mineral is characterized by three coupled substitutions: (1) (Al, Fe, Sc)3+ + (OH, F)- → YTi + ZO, (2) XREE3+ + Y(Al, Fe, Sc)3+ →X)Ca2+ + YTi4+, and (3) (Al, Fe, Sc)3+ + (Nb, Ta)5+ → 2YTi. These substitutions are strongly dependent on the composition of the magma in terms of its Al2O3/TiO2 activity ratio, with the first one also influenced by the H2O/HF fugacity ratio. Fluorine, which induced the most common substitution (1), had its source in high-temperature F-bearing fluids released from rocks of the metamorphic envelope adjacent to the intruding granite. These fluids mobilized and transported various rock components (Sc, REE, Nb, Ta, etc.) among others in the form of fluoride complexes, enriching the aplogranite magma with some metallic elements. The substitution of Sn for Ti developed with decreasing temperature to the extent that in thin ore-mineralized quartz veins cutting aplogranite, titanite reaches Sn-bearing compositions up to the prevalence of Sn corresponding to malayaite.

Paleozoic Geodynamics and Architecture of the Southern Part of the Mongolian Altai Zone

AbstractThe Mongolian Altai Zone of the Central Asian Orogenic Belt has been traditionally interpreted as a mosaic of Paleozoic magmatic arcs, back‐arcs, and Precambrian continental terranes. In order to define its architecture and its tectonic evolution, three domains previously interpreted as terranes were investigated. The findings show that the Northern and Central domains are formed by a metamorphic sequence characterized by Barrovian S1 fabric transposed by recumbent folds and dominant sub‐horizontal amphibolite facies S2 schistosity. The latter is associated with the intrusions of late Devonian syntectonic granite sheets and anatexis in the south. The Southern domain is formed by early Permian migmatites and anatectic granites separated from the metamorphic envelope by amphibolite to green‐schist facies D3 shear zone cross‐cutting S2 fabrics. All domains have been reworked by E‐W upright folds associated with axial‐planar greenschist facies cleavage, reflecting the final mid‐Permian to Triassic D4 shortening. Lithological, geochemical, and U‐Pb zircon analyses of metasediments of all domains indicate that they are formed by Ordovician mature quartzite derived from Precambrian basement intruded by Cambrian‐Ordovician continental arc and Silurian immature graywacke which originated through erosion of an oceanic arc. Altogether, the whole sequence represents a fore‐arc basin in front of a migrating arc affected by thickening and late Devonian extension. The Southern domain is interpreted as an early Permian core complex amplified by mid‐Permian to Triassic compression. The apparent “terrane” architecture of the Mongol Altai Zone originated due to Devonian and Permian heterogeneous reworking of a giant Ordovician to Silurian fore‐arc basin.

Variscan stress regime rotation: Insights from the analysis of kink folds in the northern margin of the Bohemian Massif, South Poland

We aimed to determine variations in stress regimes during the youngest Variscan deformations in the northern part of the Bohemian Massif. For this purpose, we calculated the orientation of the principal stress and strain axes for kink folds observed in the metamorphic envelope of the Karkonosze Granite, using two methods: 1) the traditional method, incorporating structural diagrams (for conjugate kink folds only), and 2) butterfly diagram analysis. The use of both methods enabled us to determine the stress regime, based not only on conjugate but also on monoclinal kink bands. The obtained results prove that butterfly diagram analysis, when applied to monoclinal kink folds, yields reliable results, especially when calibrated using the internal friction angle (Ф) calculated for the conjugate structures.We identified two generations of kink folds: 1) an older one, developed under sublatitudinal shortening and most probably related to the Early Carboniferous terminal stages of the northwest-directed thrusting of the metamorphic units, and 2) a younger one; produced by north-south Variscan Carboniferous compression, and the emplacement and subsequent doming of the Karkonosze Granite. This is the first study on brittle-ductile structures observed commonly in the metamorphic units of the Bohemian Massif, showing their relation to the granitoid intrusion and complementing the tectonic models that usually omit kink folds.

Variscan post-collisional cooling and uplift of the Tatra Mountains crystalline block constrained by integrated zircon, apatite and titanite LA-(MC)-ICP-MS U-Pb dating and rare earth element analyses

LA-ICP-MS U-Pb dating of apatite, titanite and zircon from the metamorphic cover of the Western Tatra granite was undertaken to constrain the timing of metamorphic events related to the final stages of Variscan orogenesis and subsequent post-orogenic exhumation. Zircon was found only in one sample from the northern metamorphic envelope. U-Pb ages from the outermost rims of zircons define a concordia age of 346 ± 6 Ma, while the inner rims yield a concordia age of 385 ± 8 Ma. Apatite from three samples from the northern metamorphic envelope yield U-Pb ages of 351.8 ± 4.4 Ma, 346.7 ± 5.9 Ma and 342.6 ± 7.1 Ma. Titanite from an amphibolite from the southern metamorphic envelope yields a U-Pb age of 345.3 ± 4.5 Ma. The age of c. 345 Ma is interpreted to represent the climax of metamorphism and the onset of simultaneous exhumation of the entire Tatra Mountains massif, and is recorded mainly in the northern part of the metamorphic cover.In the southern metamorphic envelope, distinct populations of apatite can be recognized within individual samples based on their rare earth element (REE) and actinide contents. One population of apatite (Ap1) yields a relatively imprecise U-Pb age of 340 ± 31 Ma. This population comprises apatite grains with very similar trace element compositions to apatite in the northern amphibolite samples, which suggests they crystallized under similar metamorphic conditions to their northern counterparts. A second apatite population (Ap2) yields an age of c. 328 ± 22 Ma, which is interpreted as neocrystalline apatite that formed during a late-Variscan (hydrothermal?) process involving (P, F, Ca, REE)-rich fluid migration. The youngest generation of apatite (Ap3) yields a U-Pb age of 260 ± 8 Ma and may have resulted from thermal resetting associated with the regional emplacement of Permian A-type granites. The proposed tectonic model assumes that rapid uplift (and cooling) of the Tatra block initiated at ca. 345 Ma, contemporaneous with anatexis. Subsequent fluid migration, possibly facilitated by extension related to the opening of Paleo-Tethys, affected only the southern part of the Tatra block.

Magmatic and post-magmatic phenomena in the Karkonosze granite and its metamorphic envelope (West Sudetes, SW Poland)

AbstractMineralogical studies of the Karkonosze granite (ca. 322-312 Ma) and its surroundings in West Sudetes (SW Poland) have provided data on Nb-Ta-REE minerals from pegmatites in the NE part of the pluton and several new finds of Ag minerals and 15 oxygenic Bi phases, hitherto not reported from the massif. The Karkonosze pegmatites are enriched in HREE as fergusonite-(Y) or xenotime-(Y) appear in almost every studied pegmatite, together with a subordinate assemblage of the aeschynite, euxenite or columbite group. The abundance of LREE minerals such as allanite-( Ce) and the monazite group, correlates inversely with the Nb-Ta-Ti minerals, whilst an early generation of monazite-(Ce) revealed an exceptionally high amount of Nd (up to 22 wt.% of Nd2O3). The physical and chemical conditions during the magmatic and post-magmatic processes were reconstructed and the effects of contact metamorphism in amphibolites from hornfelsed zones examined. Changes in solution composition and concentration at the early magmatic stage (825-920°C), pegmatitic stage overlapping with hydrothermal (560°C which ended at 160-90°C) and clearly hydrothermal stage (400 to 110°C) were studied in detail by means of melt and fluid inclusions in quartz. Furthermore, post-magmatic fluids, including some enriched in Li and B, were identified in rock-forming quartz from the whole pluton. In turn, study of the amphibolites indicates that the pair cummingtonite + anorthite or the presence of Ca-rich plagioclase with actinolite seem to be reliable mineral proxies of the thermal impact of the granitoid body on amphibolites in its envelope. The inferred conditions of the contact processes (450-550°C, 2.5- 4.8 kbar) point to an elevated geothermal gradient (ca. 32-45°C/km) probably reflecting the heat flow induced by the Karkonosze intrusion. Moreover, despite the textural and mineral changes imposed by regional and contact metamorphism, the amphibolites have their pre-metamorphic (magmatic) geochemical features undisturbed.

Oligo-Miocene thinning of the Beni Bousera peridotites and their Variscan crustal host rocks, Internal Rif, Morocco

Deciphering Variscan versus Alpine history in the internal Rif system is a key to constraining the tectonic evolution of the Alboran domain and hence the geodynamics of the western Mediterranean system during the Cenozoic. This study focuses on the evolution of the metamorphic envelope of the Beni Bousera massif and its relation to the underlying peridotites. Combining structural geology, metamorphic petrology, and LA-ICP-MS U-Th-Pb dating of monazite, this study contributes to the understanding of the tectonic history of the western internal Rif. The regional foliation (S2) is characterized by LP/HT mineral assemblages and obliterates a former foliation (S1) developed along a barrovian (MP/MT) metamorphic gradient. The dating of some metamorphic monazite grains from a micaschist and a migmatitic gneiss demonstrates that the crustal envelope of the peridotite recorded two distinct tectonometamorphic episodes. Data from monazite inclusions in S1 garnet suggest that the first event, D1, is older than 250-170 Ma and likely related to the Variscan collision, in agreement with the barrovian type of the metamorphic gradient. The second event, D2, is Alpine in age (at circa 21 Ma) and corresponds to a strong lithosphere thinning allowing subsequent subcontinental mantle exhumation. Such a tectonic context provides an explanation for the LP/HT metamorphic gradient that is recorded in the regional foliation of the western Betic-Rif system. This extension is probably related to a subduction slab roll-back in the western end of the Mediterranean realm during the Oligo-Miocene times. No evidences for a Tertiary HP/LT metamorphism have been identified in the studied area

P–T–t–D record of crustal‐scale horizontal flow and magma‐assisted doming in the SW Mongolian Altai

ABSTRACTThe Chandman massif, a typical structure of the Mongolian Altai, consists of a migmatite–magmatite core rimmed by a lower grade metamorphic envelope of andalusite and cordierite‐bearing schists. The oldest structure in the migmatite–magmatite core is a subhorizontal migmatitic foliation S1 parallel to rare granitoid sills. This fabric is folded by upright folds F2 and transposed into a vertical migmatitic foliation S2 that is syn‐tectonic, with up to several tens of metres thick granitoid sills. Sillimanite–ilmenite–magnetite S1 inclusion trails in garnet constrain the depth of equilibration during the S1 fabric to 6–7 kbar at 710–780 °C. Reorientation of sillimanite into the S2 fabric indicates that the S1–S2 fabric transition occurred in the sillimanite stability field. The presence of cordierite, and garnet rim chemistry point to decompression to 3–4 kbar and 680–750 °C during development of the S2 steep fabric, and post‐tectonic andalusite indicates further decompression to 2–3 kbar and 600–650 °C. Widespread crystallization of post‐tectonic muscovite is explained by the release of H2O from crystallizing partial melt. In the metamorphic envelope the subhorizontal metamorphic schistosity S1 is heterogeneously affected by upright F2 folds and axial planar subvertical cleavage S2. In the north, the inclusion trails in garnet are parallel to the S1 foliation, and the garnet zoning indicates nearly isobaric heating from 2.5 to 3 kbar and 500–530 °C. Cordierite contains crenulated S1 inclusion trails and has pressure shadows related to the formation of the S2 fabric. The switch from the S1 to the S2 foliation occurred near 2.5–3 kbar and 530–570 °C; replacement of cordierite by fine‐grained muscovite and chlorite indicates further retrogression and cooling. In the south, andalusite containing crenulated inclusion trails of ilmenite and magnetite indicates heating during the D2 deformation at 3–4 kbar and 540–620 °C. Monazite from a migmatite analysed by LASS yielded elevated HREE concentrations. The grain with the best‐developed oscillatory zoning is 356 ± 1.0 [±7] Ma (207Pb‐corrected 238U/206Pb), considered to date the crystallization from melt in the cordierite stability ~680 °C and 3.5 kbar, whereas the patchy BSE‐dark domains give a date of 347 ± 4.2 [±7] Ma interpreted as recrystallization at subsolidus conditions. The earliest sub‐horizontal fabric is associated with the onset of magmatism and peak of P–T conditions in the deep crust, indicating important heat input associated with lower crustal horizontal flow. The paroxysmal metamorphic conditions are connected with collapse of the metamorphic structure, an extrusion of the hot lower crustal rocks associated with vertical magma transfer and a juxtaposition of the hot magmatite–migmatite core with supracrustal rocks. This study provides information about tectono‐thermal history and time‐scales of horizontal flow and vertical mass and heat transfer in the Altai orogen. It is shown that, similar to collisional orogens, doming of partially molten rocks assisted by syn‐orogenic magmatism can be responsible for the exhumation of orogenic lower crust in accretionary orogenic systems.

RELACJE METODYCZNE I WIEKOWE POMIĘDZY ANALIZĄ TRAKOWĄ A POTASOWO-ARGONOWĄ NA PRZYKŁADZIE MASYWU KARKONOSZY I JEGO OSŁONY

METHODOLOGY AND AGE RELATIONS OF THE FISSION TRACK AND K-Ar METHODS THE CASE OF THE KARKONOSZE MASSIF AND ITS ENVELOPE Abstract. Isotopic methods have been gaining their importance in Poland in last decades. The understanding on the interpretation potential of isotopic determinations of minerals has been growing up as well. In that context, age relations and methodology of the fission track and K-Ar methods have been presented based on the case of minerals forming rocks of the Karkonosze Massif and its northern metamorphic envelope. The presented results are the compilation of earlier fission track determinations from the Karkonosze-Izera Block (Jarmolowicz--Szulc, 1984), K-Ar data from the same area (Jarmolowicz-Szulc et al. , 2009) and newer methodological data from the Karkonosze granitoid (Danisik et al. , 2010). Advantages of the fission track method have been shown, i.e., interpretation possibilities of burial and exhumation in geological history of rocks. Importance of the K-Ar method has been presented, in that determination of a cooling age after the pluton uplift.

Microstructural finite strain analysis and 40Ar/39Ar evidence for the origin of the Mizil gneiss dome, eastern Arabian Shield, Saudi Arabia

The Mizil antiform is a gneiss-cored culmination situated near the northern end of the Ar Rayn island arc terrane, which is the easternmost exposed tectonic unit of the Arabian Shield. This domal structure has a mantle of metamorphosed volcanosedimentary rocks belonging to the Al-Amar Group, and an igneous interior made up of foliated granodiorite-tonalite with adakitic affinity. The gneissic core has a SHRIMP U–Pb zircon age of 689±10Ma making it the oldest rock unit in the Ar Rayn terrane. An adakite diapir, formed by the melting of the subducted crust of a young marginal basin, and rising through the volcanosedimentary succession of the Ar Rayn island arc is thought to have caused the observed doming. Relatively uniform strain throughout the dome combined with strong vertical shortening and the roughly radial pattern of stretching lineation is consistent with diapirism; the absence of strain localization rules out detachment faulting as a causative mechanism. Amphibolites from the metamorphic envelope have an 40Ar/39Ar age of 615±2Ma; the age gap between core and cover is thought to reflect the resetting of metamorphic ages during the final suturing event, a phenomenon that is often observed throughout the eastern shield. Aeromagnetic anomalies beneath the Phanerozoic sedimentary cover indicate the presence of a collage of accreted terranes east of the Ar Rayn terrane that were probably amalgamated onto the Arabian margin during the latest stages of the closure of the Mozambique ocean; culminant orogeny is believed to have taken place between 620 and 600Ma as these terrane collided with a major continental mass to the east referred to here as the eastern Arabian block (EAB). The Mizil gneiss dome is therefore considered to have formed in a convergent contractional setting rather than being the outcome of extensional post-orogenic collapse.

The Moslavačka Gora crystalline massif in Croatia: a Cretaceous heat dome within remnant Ordovician granitoid crust

For a long time the Moslavačka Gora Massif in Croatia has been regarded as a major outcrop of Variscan crystalline basement of the South Tisia block. However, new geochronological data indicate that this massif consists of a Cretaceous S-type granite pluton intruding a Cretaceous low-pressure/high-temperature (LP/HT) metamorphic envelope. The age of the LP/HT metamorphism is estimated at ~90–100 Ma using the method of electron microprobe based monazite dating. The Central Granite was dated at 82 ± 1 Ma (LA-SF-ICP-MS zircon age). The metamorphic complex comprises mainly felsic anatexites and orthogneisses of granitic composition, some metapelites (paragneisses and mica schists) and amphibolites. Zircons from three different samples of metagranite were dated at 486 ± 6, 483 ± 6, and 491 ± 1 Ma, suggesting that most of the metamorphic complex represents an Early Ordovician granitic series. The Cretaceous regional metamorphism culminated in granulite facies conditions of ~750°C and 3–4 kbar. A retrograde metamorphic event at lower amphibolite facies conditions overprinted the metamorphic complex. This event is probably related to the intrusion of the Central Granite. The southeastern sector of the massif was additionally affected by post-granitic, predominantly NE oriented shearing at greenschist facies conditions. As yet there is no clear evidence for Variscan events in the Moslavačka Gora Massif. Mineral relics of a medium-pressure amphibolite facies metamorphism are preserved in amphibolites. They are older than the Cretaceous LP/HT regional metamorphism, but their age is presently unknown. Some indications for a Permian regional metamorphic event are provided by inherited zircons in the Central Granite that have been dated with a Permian age, and by Permian monazite relics in metapelites. The Cretaceous high heat flow regime recorded in the Moslavačka Gora Massif is unique in the subcrop of the Pannonian Basin and may be a local feature triggered by a mafic intrusion in the lower crust.

Geotourist attractions of the Western Sudetic Geostrada

W artykule przedstawione zostały wybrane wyniki prac wykonanych w zachodniej części Geostrady Sudeckiej. Przeanalizowany został przebieg Geostrady Zachodniosudeckiej na tle geomorfologii i budowy geologicznej. Trasa ta okrąża szerokim łukiem od północy główne pasma Gór Izerskich i Karkonoszy oraz Kotlinę Jeleniogórską, a dalej biegnie przez wschodnią i południową część Rudaw Janowickich. Przebiega ona w większości w obrębie ekshumowanej (odsłoniętej) osłony metamorficznej (skał otaczających) batolitu Karkonoszy (ciała magmowego, powstałego w wyniku zakrzepnięcia magmy wewnątrz skorupy ziemskiej), bogatej w liczne wystąpienia mineralizacji rudnej. Przedstawiony został opis obiektów geomorfologicznych i geologicznych, pozostałości dawnej eksploatacji górniczej oraz oceniona została ich atrakcyjność turystyczna. Obszerniej opisane zostały wybrane, szczególnie atrakcyjne obiekty: formy geomorfologiczne, odsłonięcia interesujących kompleksów skalnych, struktur tektonicznych i pozostałości dawnego górnictwa złóż rud cyny w rejonie Krobicy-Kotliny-Gierczyna-Przecznicyoraz złóż uranu w Kowarach.//The paper presents selected results of the “Western Sudetic Geostrada” – a project, currently in preparation, which merges geosites, biotic nature objects, historical monuments and tourist infrastructure into a coherent tourist trail. The trail avoids main roads and famous tourist centers, leading to the less known parts of the Western Sudety Mts. and their foreland. Precisely, the “Western Sudetic Geostrada” runs around the main range of the Isera Mountains, surrounds from the north the Karkonosze Range and the Jelenia Góra Trough, and heads through the eastern and southern part of the Rudawy Janowickie Range. From geological point of view the trail runs through the exposed part of the exhumed metamorphic envelope the Karkonosze granite intrusion rich in numerous deposits and occurrences of ore mineralization. The selected geomorphological and geological objects and regions were described in details along with the relics of past mining operations and their tourist attractiveness were evaluated. Some especially attractive objects i.e. geomorphological forms, outcrops of interesting lithologic complexes, tectonic structures and remnants of past ore mining (tin mining in Krobica-Kotlina-Gierczyn-Przecznica area and uranium mining in Kowary) were described in details.

Aquatic bryophytes as bioindicators of arsenic mineralization in Polish and Czech Sudety Mountains

Abstract Concentrations of arsenic and its geochemically associated elements, Ag, Co, Cu and Ni, were measured together with Al, Ba, Cd, Fe, Mn, Pb, Sr and Zn in aquatic bryophytes. These bryophytes originated from some of the arsenic mineralization zones of the Klodzko-Zloty Stok granodiorite massif and its metamorphic envelope (Sudetes Mts., Poland) and from identical zones of the east Sudetic Rychlebske Mts. and Jesenik Mts. (Czech Republic). Concentrations of As in all examined populations were higher than background values (1 μg/g dry weight) culminating in an average maximum of 6270 μg/g in Chiloscyphus pallescens from a stream draining an area with former arsenic and gold mining near Zloty Stok. This survey has shown that aquatic bryophytes accumulate the examined elements to a high degree especially if growing in areas with mineral deposits. This was demonstrated by strong positive correlations between Ba, Sr, Zn concentrations in water and Platyhypnidium rusciforme .

Fluid transfer in anorthosites and related rocks from Rogaland (Southwest Norway): Evidence from stable isotopes

C and H contents and C, H and O isotopic compositions have been measured in sixteen samples of the Åna-Sira massif-type anorthosite (Rogaland, Southwest Norway) and in 30 surrounding rocks, including the metamorphic envelope and magmatic intrusions related to the anorthosite emplacement. Oxygen isotopic compositions of the Åna-Sira anorthositic massif ( δ 18 O = 6.1 ± 0.3‰ ) and the Tellnes orebody ( δ 18 O = 5.2 ± 0.2‰ ) are consistent with a pure mantle origin and show no evidence for exchange with hydrothermal fluids. The associated rocks (norites to charnockites) have δ 18O values ( 5.2 ± 0.9‰) similar to uncontaminated mantle-derived magmas. The H 2O content of the rocks (magmatic or metamorphic) is lower than 0.54%, suggesting the preservation of original granulite facies conditions during cooling and uplift. Due to their cumulate origin, the anorthosites are very poor in carbon (bulk C < 50 ppm), the volatiles having been concentrated in the residual melt. Higher carbon concentrations (up to 1484 ppm) are found in the associated intrusions. Anorthosites and gneisses close to the contacts (less than 500 m) with later intrusions show an increase in their carbon content (up to 700 ppm C), and this carbon is mainly in the form of interstitial carbonate. The carbon distribution observed at the contact between later intrusions and anorthosites as well as gneisses indicates that some CO 2 has fluxed from the later magmas over a distance of a few hundred metres. This suggests only minimal lateral propagation of the fluids by diffusion or infiltration. Tentative carbon budgets suggest that most of the volatiles were transferred upwards into the overlying continental crust.

Stable isotopic and petrological constraints on scapolitization of the Whitestone meta‐anorthosite, Grenville Province, Ontario*

The Whitestone Anorthosite (WSA), located in the Central Gneiss Belt of the south‐western Grenville Province, Ontario, exhibits a nearly concentric metamorphic envelope characterized by an increase in modal scapolite, hornblende, epidote and garnet, developed around a core of granulite facies clinopyroxene ± orthopyroxene ± garnet meta‐anorthosite. Scapolite‐ and hornblende‐bearing assemblages develop mainly at the expense of plagioclase and pyroxene within the envelope.Stable isotopic and petrological data for scapolite‐bearing mineral assemblages within meta‐anorthosite constrain the source of carbon responsible for CO3‐scapolite formation and the extent of fluid/rock interaction between the anorthosite and adjacent lithologies. Stable isotopic data indicate increasing δ18O and δ13C from core to margin of the meta‐anorthosite and for samples from the southern extension of the WSA, where it is ductilely deformed within the Parry Sound Shear Zone (PSSZ). The average δ18OSMOW value (whole rock) for the WSA core is 6.9‰, increasing to 11.5‰ where the WSA is in tectonic contact with marble breccia. The average δ13CPBD value of scapolite in meta‐anorthosite from the centre of the WSA is ‐3.4‰, increasing to ‐0.5‰ at the eastern (marble) contact. Average values of δ13C for scapolite and whole‐rock δ18O for samples from the shear zone are ‐1.0 and 8.0‰, respectively. Marbles have average δ18O and δ13C values of 19.2 and ‐0.4‰, respectively.The sulphate content of texturally primary scapolite decreases from the core of the WSA (XSO4= 0.48) to the eastern contact (≤0.05). Texturally late scapolite after plagioclase and garnet tends to be CO3‐rich relative to texturally primary scapolite, and some scapolite grains show zoning in the anion site with CO3‐enriched rims. Scapolite composition may vary at any scale from a single grain to outcrop.The pattern of isotopic enrichment in 13C and 18O preserved in the eastern margin of the WSA is consistent with marble as the major source of fluid contributing to the formation of the metamorphic envelope. The decrease in XSO4 and increase in XCO3 in scapolite toward the margin of the WSA indicate that the volatile content was reset by, or developed from, a CO2‐bearing fluid. Assuming derivation of fluid from marble, minimum fluid/rock values at the margin of the WSA range from 0.03 for the least enriched, to 0.30 for the most isotopically enriched samples. Although marble is not found in immediate contact with samples of sheared meta‐anorthosite from the PSSZ, a marble source is also consistent with the C and O isotope composition and anion chemistry of scapolite within these samples.

Mantle fragments in the Scandinavian caledonides

Mantle fragments of ultramafic composition are widespread in the Scandinavian Caledonides (SC). Lenses and boudins of Alpine-type peridotites in the Scandinavian Caledonides represent parts of dismembered ophiolite sequences and fragments of sub-continental upper mantle. Metaperidotites of nappes in internal positions are generally isofacial with the metamorphic envelope, usually Caledonian metasediments but in places also Precambrian metagranitoids forming the basement cores of the nappes. Caledonian metamorphism strongly modified the texture and mineralogy of the peridotites and resulted in a systematic metamorphic pattern which is consistent with the pattern observed in the envelope. Metaperidotites of the external massifs display at least a two-stage metamorphic history: an early Caledonian high-pressure high-temperature phase related to early crustal stacking and a late Caledonian regional metamorphic overprint which produced a regular Barrovian-type metamorphic pattern of in-situ metamorphism. Metaperidotites from nappes in intermediate positions (Iapetus Ocean ophiolites and ultramafic rocks from island arc environments) show strongly diverging histories. Metaperidotites from internal ophiolites (oceanic ophiolites, Köli) lack any evidence of subduction metamorphism, are serpentinized to various degrees, show abundant primary mantle relic mineralogies and the Caledonian metamorphic overprint is low. Metaperidotites from external (island arc) ophiolites and other associations (Seve) often show relic high-pressure metamorphism related to the Finnmarkian phase of the Caledonian orogeny. The Seve metaperidotites are occasionally associated with eclogites and show a weak overprint of late Caledonian regional metamorphism. Alpine-type peridotites are absent in the foreland of the Baltic Shield and in the innermost nappes (Lofoten). The metamorphic characteristics and evolution recorded by the metaperidotites in the Scandinavian Caledonides allow a general reconstruction of the dynamics of collision belt formation.

Pb isotope geochemistry of a massif-type anorthositic-charnockitic body: The Hidra Massif (Rogaland, S.W. Norway).

The Hidra Massif (Rogaland complex, S.W. Norway) is a massif-type anorthositic-charnockitic body. It consists of undeformed anorthosites and leuconorites, grading into fine-grained jotunites at the contact with the granulite facies gneisses of the metamorphic envelope. A stockwork of charnockitic dykes cross-cuts the massif. The Pb isotopic compositions of the anorthosites and leuconorites are comparable or slightly less radiogenic than those of the jotunites ( 206Pb 204Pb from 18.079 to 19.307,( 207Pb 204Pb from 15.568 to 15.657 and 208Pb 204Pb from 37.617 to 38.493). These values are compatible with an upper mantle origin for the parental magma of jotunitic composition and for the plagioclasic cumulates, but show the incorporation of lower crustal material (U-depleted and thus less radiogenic). The charnockitic dykes have significantly less radiogenic Pb isotopic compositions ( 206Pb 204Pb from 17.472 to 19.171, 207Pb 204Pb from 15.489 to 15.620and 208Pb 204Pb from 36.991 to 40.922) which can be explained by a larger proportion of lower crustal contamination material. The contaminant could be the granulite facies gneisses of the metamorphic envelope. This interpretation is compatible with the K-Rb relationships of these rocks and with the O and Sr isotopic geochemistry. The proportion of contaminating lead in the charnockitic dykes can be estimated at 55 ± 15% considering the border facies jotunite as the uncontaminated parental magma and the least radiogenic gneiss of the metamorphic envelope as the contaminant.

The Geology of the Islands of South Connemara

AbstractThe exposed part of the metamorphic envelope on the south side of the Galway granite is described. It is characterized by the presence of pillow lavas and a thick greywacke series containing intraforma-tional conglomerates. This South Connemara Series is tentatively assigned to the Ordovician; such a correlation would require the Galway granite to be a laconic intrusion.