Greenschist-facies Conditions Research Articles

New insights from 40 Ar/ 39 Ar laserprobe dating of white mica fabrics from the Pelion Massif, Pelagonian Zone, Internal Hellenides, Greece: implications for the timing of metamorphic episodes and tectonic events in the Aegean region

Abstract 40 Ar/ 39 Ar laserprobe dating of mylonites fabrics from the Pelion Massif in the Pelagonian Zone of mainland Greece has characterized its Mid-Late Alpine deformation history. Following high pressure (HP) metamorphism, ductile deformation occurred under greenschist-facies conditions from c. 54 Ma, and continued to affect the Pelion Massif until c. 15 Ma. The prolonged episode of ductile deformation in the Pelion Massif has resulted in the formation of an Oligocene-Early Miocene ductile domal structure. The new geochronological data obtained for the Pelion contribute to a detailed record of the Alpine kinematic history in the Pelagonian Zone and allow a discussion of P-T-t data from Aegean HP rocks to characterize the regional thermotectonic history. Comparison with the P-T-t data from the Cycladic region reinforces the point that Mid-Eocene phengite ages, commonly taken as the age of peak HP metamorphism in the Cyclades, do not always reflect the metamorphic culmination, but rather the retrograde paths of the HP rocks. It is shown that, on a regional scale, termination of HP metamorphism is a diachronous process in the Aegean region, being c. 54 Ma in the north (Pelagonian Zone) and shifting to younger ages, chiefly c. 40 Ma in the Cyclades, and c. 20 Ma on Crete, as the present-day subduction zone is approached. In contrast to the diachronous exhumation of Aegean HP assemblages, the well documented Miocene phase of ductile regional extension appears to be synchronous across the whole Aegean region and affected basement rocks until c. 15 Ma.

Magmatic crystallization, isobaric cooling, and decompression of the garnet-bearing assemblages of the Jijal sequence (Kohistan terrane, western Himalayas)

The Jijal ultramafic-mafic sequence is the lowermost exposed part of the Kohistan terrane. Petrography and geothermobarometry show that garnet-bearing assemblages of the Jijal sequence consist of a mosaic of magmatic and metamorphic equilibria. Garnet-bearing magmatic assemblages started their crystallization at a depth >50 km, either at the base of a thickened arc-type crust or within rising magmatic batches in the upper mantle. Quasi-isobaric cooling of magmatic assemblages occurred in the high- P , high- T granulite field without reaching the eclogite field. Subsequently, these assemblages were partly retrograded under amphibolite- to greenschist-facies conditions. This succession of equilibria attests to a switch from a high- P quasi-isobaric cooling regime to a major decompression followed by final cooling at the upper-crustal level. This P-T evolution is best explained by a yet unrecognized tectonic event that consists of partial exhumation of the base of the Kohistan terrane during its accretion to the Asian plate. Final cooling and unloading in turn, could be related to extension following the obduction of the Kohistan terrane onto the Indian plate.

Petrology and P– T history of the Wutai amphibolites: implications for tectonic evolution of the Wutai Complex, China

The Wutai Complex represents the best preserved granite-greenstone terrane in the North China Craton. The complex comprises a sequence of metamorphosed ultramafic to felsic volcanic rocks, variably deformed granitoid rocks, along with lesser amounts of siliciclastic and carbonate rocks and banded iron formations. Petrological evidence from the Wutai amphibolites indicates four metamorphic evolutionary stages. The M 1 assemblage is composed of plagioclase+quartz+actinolite+chlorite+epidote+biotite+rutile, preserved as mineral inclusions in garnet porphyroblasts. The metamorphic conditions for this assemblage cannot be quantitatively estimated. The M 2 stage is represented by garnet porphyroblasts in a matrix of quartz, plagioclase, amphibole, biotite, rutile and ilmenite. P– T conditions for this assemblage have been estimated using the program T weequ at 10–12 kbar and 600–650°C. The M 3 assemblage is shown by amphibole+plagioclase±ilmenite symplectic coronas around embayed garnets and yields P– T conditions of 6.0–7.0 kbar and 600–650°C. M 4 is represented by chlorite and epidote rimming garnet, chlorite rimming amphibole and epidote replacing plagioclase under greenschist-facies conditions of 400–500°C and relatively lower pressures. Taken together, the qualitative P– T estimates from M 1 and M 4 and the quantitative P– T estimates from M 2 and M 3 define a clockwise P– T path for the Wutai amphibolites. The estimated P– T path from the four stages suggests that the Wutai Complex underwent initial burial and crustal thickening (M 1+M 2), subsequent isothermal exhumation (M 3), and finally cooling and retrogression (M 4). This tectonothermal path, along with those of the Fuping and Hengshan complexes, which bound the southeast and northwest margins, respectively, of the Wutai Complex, is considered to record the early Paleoproterozoic collision between the eastern and western segments of the North China craton.

Scheelite-powellite and paraniite-(Y) from the Fe-Mn deposit at Fianel, Eastern Swiss Alps

Small syngenetic exhalative Fe-Mn deposits embedded in Triassic marbles of the Suretta, Starlera, and Schams nappes (Eastern Swiss Alps) were subjected to a Tertiary regional metamorphism under blueschist- to greenschist-facies conditions. In one of the deposits (at Fianel, Val Ferrera), this polyphase metamorphism led to the formation of quartz+dolomite veinlets containing beryl, scheelite-powellite, paraniite-(Y), monazite- (Ce), fluorapatite, bergslagite, fluor-roméite, and antimonian betafite; these veinlets crosscut dolomite breccia lenses that are embedded in hematite-quartz-carbonate ores. Scheelitepowellite displays a continuous range of compositions between 28 and 70 mol% CaMoO 4 ; its As 2 O 5 contents range from 0.73 to 3.96 wt%, and are positively correlated with the Y 2 O 3 contents that vary between 0.33 and 2.47 wt%. The scheelite-powellite grains display a two-stage chemical zoning: stage A generally produced a core and a rim that, relative to the core, is richer in W, As, and Y. During the second stage (stage B), W-rich scheelitepowellite replaces stage-A grains along fractures and rims. Crystals of paraniite-(Y), ideally (CaWO 4 ) 2 ·YAsO 4 , occur as small inclusions (=1 μm) in stage-B scheelite-powellite. The Fianel deposit is only the second locality where paraniite-(Y) has been reported. The paraniite-(Y) from Fianel displays, like the type material, no polysomatic stacking fault in the scheelite-YAsO 4 layering. At Fianel, paraniite-(Y) is characterized by elevated Mo contents, and seems to have crystallized under influence of W- and LREE-rich fluids during stage B, i.e., during the metasomatic replacement of Y- and As-rich scheelite-powellite produced in stage A.

P– T– t paths and post-metamorphic exhumation of the Day Nui Con Voi shear zone in Vietnam

The Day Nui Con Voi belt in Vietnam is the southeasternmost part of the Red River shear zone in Asia. It is a narrow high-grade metamorphic core complex consisting of garnet–sillimanite–biotite gneisses, mylonite bands, amphibolite layers and migmatites. Geothermobarometric study of the complex revealed that the peak metamorphism took place under amphibolite-facies conditions of 690 −60 +30°C and 0.65±0.15 GPa and the subsequent mylonitization occurred under greenschist-facies conditions of ∼480°C and under 0.3 GPa. Fifteen synkinematic hornblende and biotite separates from gneisses, amphibolites and mylonites were dated with the K/Ar method. Hornblende separates from the Day Nui Con Voi give K–Ar ages of 26.4–28.5 Ma, and the biotite separates do give 24.5–24.7 Ma. Combination of thermobarometric and geochronological data yields the cooling history of 500°C at 28 Ma and 300°C at 24 Ma with a cooling rate of 70–110°C Ma −1, and 23 km post-metamorphic exhumation of the core complex. The first 16 km exhumation from the peak of metamorphism (at probably 31 Ma) to 28 Ma was triggered by the left-lateral strike-slip displacement of the Red River shear zone.

Geochemistry and petrogenesis of intrusive and extrusive ophiolitic plagiogranites, Central Anatolian Crystalline Complex, Turkey

Plagiogranites associated with the Sarikaraman ophiolite of the Central Anatolian Crystalline Complex, Turkey, closely resemble other plagiogranites from supra-subduction zone-type ophiolites of Neotethys. The ophiolite is remarkable in displaying a higher proportion of the plagiogranite suite (ca. 10% by volume) than is usually associated with such bodies. The Sarikaraman plagiogranites are represented by intrusive sheets and netvein trondhjemites largely developed at the top of the upper gabbros and as multiphase dykes within the sheeted dyke complex. The plagiogranite dykes are considered to feed extrusive silicified rhyolites associated with the basaltic lavas in the volcanic section of the ophiolite. Field relations suggest that the trondhjemites were probably generated from the roof section of a dynamic and evolving gabbroic magma chamber. Both the deep-seated trondhjemites and the volcanic rhyolites constitute the Sarikaraman plagiogranite suite. Geochemically there is complete overlap between the intrusive trondhjemites and extrusive rhyolites, which are characterised by (MORB-normalized) low HFS element contents with small negative NbTa anomalies and variably enhanced LIL element abundances. Unlike other plagiogranites, however, the Sarikaraman suite is not characterized by consistently low K 2O contents; a feature that reflects the variable mobilization of the LIL elements under lower greenschist facies conditions. The REE are uniformly enriched relative to the basic components of the complex, but have similar normalized patterns exhibiting mild light REE depletion. In terms of their origin, the initial or most primitive plagiogranite melts could have been generated by either fractional crystallization (70–85% of clinopyroxene-feldspar ± amphibole) or partial melting (5–15% batch melting) of a gabbroic ‘source material’, although only the first process can produce most of the range of the plagiogranite compositions. As a group the plagiogranites exhibit some degree of internal variation which can be generated by further fractionation largely dominated by feldspar with minor apatite and amphibole.

Extensional tectonics in the Caledonides of southern Norway, an overview

The extensional collapse of the Scandinavian Caledonides resulted in rapid tectonic denudation of the orogen, exhumation of high- and ultra-high-pressure metamorphic rocks and provided a structural template for the formation of Devonian supra-detachment sedimentary basins. The geometry and intensity of the extensional deformation show considerable variation vertically in the crustal section as well as horizontally from east to west across the orogen. The most prominent structural feature related to the extension in central-south Norway is the change in the direction of tectonic transport, from the easterly directed nappe translation during the Silurian Scandian Orogeny, to top-westerly directed sense of shear during the extension. The Fennoscandian basement was little affected by extension in the eastern Caledonides. In the west, however, top-to-the-west shear zones are commonly observed in basement windows. Deformation affecting the Cambrian to Late Silurian rocks in the Caledonian foreland developed a typical of foreland fold and thrust belt geometry. Deformation in the foreland was apparently contemporaneous with the extension-related decompression of the high-pressure rocks in the hinterland. Thrusting in the foreland may thus have been driven by gravitational collapse, and as such have important similarities to the foreland-hinterland relationships of the Himalayan-Tibetan region. The basal contacts of the Jotun and other major nappes constitute prominent shear zones in which fabrics related to thrusting have been mostly destroyed by extensional shearing. The high structural levels of the Western Gneiss Region, adjacent to the western margin of the Jotun Nappe, were only moderately affected by the extensional deformation. Consequently, the Proterozoic orthogneiss complexes are generally well preserved in this area. The westernmost and structurally lowermost parts of the Western Gneiss Region have, however, been subjected to extreme overburden during the Caledonian continental collision. Initial, near-isothermal decompression of the high-pressure rocks occurred by non-rotational vertical shortening-horizontal stretching at eclogite- to amphilbolite-facies conditions; at a later stage, decompression and cooling from amphibolite to greenschist facies occurred by rotational deformation associated with the large-scale extensional detachments. The initial extensional deformation in the hanging wall of the detachments in western Norway commenced at greenschist-facies conditions, and became progressively more brittle and localised as the complexes were exhumed in the Late Silurian to Middle Devonian. Major syn-depositional normal faults in the hanging wall of the extensional detachments eventually controlled sedimentation in the Devonian supra-detachment basins.

The evolution of a late-Variscan high-T/low-P region: the southeastern margin of the Bohemian massif

A characteristic feature of the Moldanubian part of the central European Variscides is late-orogenic high-T/low-P metamorphism. Its past history and the possible reasons for this metamorphism are highlighted by the tectonometamorphic development at the south– eastern margin of the Bohemian massif. During the Variscan orogeny, at ca. 340 Ma, two different crustal segments were juxtaposed by thrusting (the Drosendorf unit on top of the Monotonous unit). This probably marks a collisional event that is widespread in the southeastern Moldanubian zone. The collision was followed by crustal uplift accompanied by strong heating in the lowermost structural unit (Monotonous unit). During the subsequent orogenic collapse, the Moldanubian nappe pile was thrust over parts of the Moravo-Silesian terrane. A late stage of crustal extension under greenschist-facies conditions is linked with pluton emplacement. In general, magmatic underplating as well as delamination of the lithospheric mantle explains the high-T/low-P metamorphism and the large-scale plutonism in the southeastern Moldanubian zone.

The transition from passive to active margin tectonics: a case study from the zone of Samedan (eastern Switzerland)

The Zone of Samedan is part of a fossil, early Mesozoic rift system originally situated in the distal, Lower Austro-Alpine domain of the Adriatic passive continental margin. An early Mesozoic configuration of asymmetrical rift basins bounded by relative structural highs compartmentalized Late Cretaceous active margin tectonics; Jurassic half-grabens were folded into arcuate synclines, whereas relative structural highs engendered thin, imbricated thrust sheets. West-directed thrusting and folding initiated at the surface and continued to depths favoring mylonitization under lower greenschist-facies conditions. At this time Liguria-Piemontese ophiolites were accreted to Lower Austro-Alpine units directly underlying the Zone of Samedan. Late Cretaceous orogenic collapse of the Adriatic active margin involved the reactivation of west-directed thrusts as low-angle, top-to-the-east, normal faults. These faults accommodated extensional uplift of Liguria-Piemontese ophiolites and Lower Austro-Alpine units beneath and within the Zone of Samedan. During Paleogene collision, some Late Cretaceous faults in the Zone of Samedan were reactivated under lower anchizonal conditions as north-directed thrusts. The latter stages of this early Tertiary thickening were transitional to brittle, high-angle normal faulting associated with top-to-the-east extension and spreading above the warm, uplifting Lepontine dome.

Tectono-Metamorphic Impact of a Subduction-Transform Transition and Implications for Interpretation of Orogenic Belts

Subduction-transform tectonic transitions were common in the geologic past, yet their impact on the evolution of orogenic belts is seldom considered. Evaluation of the tectonic transition in the Coast Ranges of California is used as an example to predict some characteristics of exhumed regions that experienced similar histories worldwide. Elevated thermal gradients accompanied the transition from subduction to transform tectonics in coastal California. Along the axis of the Coast Ranges, peak pressure-temperature (P/T) conditions of 700 to 1000° C at a pressure of ∼7 kbar, corresponding to granulite-facies metamorphism, and cooling to 500° C, or amphibolite facies, within 15 million years, are indicated by thermal gradients estimated from the depth to the base of crustal seismicity. Greenschist-facies conditions may occur at depths of 10 km or less. These P/T estimates are consistent with the petrology of crustal xenoliths and thermal models. Preservation of earlier subduction-related metamorphism is possible at depth in the Coast Ranges. Such rocks may record a greenschist or higher-grade overprint over blueschist assemblages, and late growth of metamorphic minerals may reflect dextral shear along the plate margin, with development of orogen-parallel stretching lineations. Thermal overprints of early-formed high-P (HP), low-T (LT) assemblages, in association with orogen-parallel stretching lineations, occur in many orogenic belts of the world, and have been attributed to subduction followed by collision. Alternatively, a subduction-transform transition may have caused the overprints and lineations in some of these orogenic belts. Possible examples are the Sanbagawa belt of Japan and the Haast schists of New Zealand. P/T conditions of inferred granulite-grade metamorphism in the Coast Ranges, and predicted cooling of these rocks through lower thermal gradients, resemble the P/T evolution of many granulite belts, suggesting that some granulite belts may have formed as a result of a subduction-transform transition. Arclike belts of plutons also can form as a consequence of subduction-transform transition.

A review of the In Ouzzal granulitic terrane (Tuareg shield, Algeria): its significance within the Pan‐African Trans‐Saharan belt

ABSTRACTThe In Ouzzal terrane (IOT) or In Ouzzal granulite unit (IOGU) is an elongated Palaeoproterozoic block within the Neoproterozoic Pan‐African belt of north‐west Africa. The granulites derive from Archaean protoliths that include a large volume of metasediments which were deposited on a 3.2‐Ga gneissic basement. Near‐peak granulite facies conditions between 2.17 and 2 Ga were estimated at P=10 kbar and T rising from 800 to 1000°C. Premetamorphic orthogneisses were intruded at 2.5 Ga, and followed by the emplacement of syn‐ to late‐kinematic charnockites, syenites and carbonatites at around 2 Ga. Cooling of the granulites occurred till 1800 Ma. Shortly after its exhumation coeval with crustal extension and related alkaline magmatism in adjacent areas, the IOT was buried beneath late Palaeoproterozoic and Neoproterozoic cover sequences, and then behaved as a rigid block. Both margins are lithospheric faults, as evidenced by the occurrence of shear‐zone‐related mafic and felsic plutons. Pan‐African tectonothermal events were negligible in the north, but granulites in the south were significantly reworked under lower greenschist facies conditions during the northern motion of the block with respect to both the western and the eastern Pan‐African terranes. The Cambrian molasse, associated with widespread alkaline volcanism and subvolcanic granites, is horizontal in the north. The IOT, which was part of a larger continental mass including its counterpart in northern Mali, is interpreted as an exotic terrane which may have docked during Pan‐African plate convergence and lateral collision. The unchanged pediplain since c. 1.7 Ga in the north suggests that the IOT is underlain by thick Palaeoproterozoic lithospheric mantle, whereas its southern part is probably allochthonous and overlies Pan‐African structural units.

Left-lateral transpressive deformation and its tectonic implications, Sveconorwegian orogen, Baltic Shield, southwestern Sweden

The Mylonite Zone (MZ) is a major, ductile deformation zone in the Sveconorwegian orogen (Baltic Shield) of southwestern Sweden and southeastern Norway which has a strike length of over 400 km and an across-strike width which often exceeds 5 km. It is an orogen-parallel deformation zone which formed under retrogressive metamorphic conditions relative to the higher-grade structures in the surrounding crustal units. The MZ marks a conspicuous metamorphic break in the area south of lake Vänern and a distinct lithological break in the area north of this lake. Regional metamorphic considerations suggest that its surface exposure represents an oblique section through the crust with deeper levels exposed along the southern parts of the zone and shallower levels exposed farther north. Structural studies in three areas north of lake Vänern (Värmlandsnäs, Kil and Torsby-Charlottenberg) suggest that the MZ displays coeval, left-lateral, strike-slip and reverse, dip-slip senses of shear characteristic of a transpressive tectonic regime active under upper greenschist-facies conditions. In the northernmost Torsby-Charlottenberg area, at relatively shallow crustal levels, the MZ splays out to form two left-lateral, contractional strike-slip duplexes which together define a positive flower structure. Regionally, the MZ defines the western flank of an east-verging thrust system which formed in the eastern part of the Sveconorwegian orogen. The structures within this thrust system preceded the MZ, contain a subordinate, left-lateral strike-slip component of movement and were rotated into the Sveconorwegian Frontal Deformation Zone (SFDZ) in the frontal part of the orogen. The MZ is inferred to have formed in an extrusive (dispersive) tectonic environment which developed in connection with a slightly oblique collision and crustal shortening in a WNW-ESE direction. The initial stages of this collision produced the east-verging thrust system and shortening was absorbed, at this stage, by crustal thickening. Late, possibly out-of-sequence thrusting with a right-lateral, strike-slip sense of shear along the SFDZ marked the waning stages of crustal shortening in the Sveconorwegian orogen.

The tectono-metamorphic evolution of a dismembered ophiolite (Tinos, Cyclades, Greece)

AbstractThe six exposures of the Upper tectonic Unit of the Cycladic Massif occurring on the island of Tinos are shown to comprise a metamorphosed dismembered ophiolite complex. The common stratigraphic section consisting of tens-of-metres- thick tectonic slices of mafic phyllites overlain by serpentinites and gabbros is considered to have been derived by a combination of thrusting during obduction and subsequent attenuation by low-angle normal faults. All rock types show evidence of a phase of regional greenschist-facies metamorphism, which in the case of the phyllites is accompanied by penetrative deformation. The greenschist-facies metamorphism in gabbros is preceded by high temperature sea-floor amphibolite-facies alteration, whereas in the serpentinites, the antigorite + forsterite greenschist-facies assemblage overprinted an earlier low temperature lizardite serpentinite. Trace element patterns of the mafic phyllites and a harzburgitic origin of meta-serpentinites suggest a supra subduction zone (SSZ) affinity for the ophiolitic suite. ρ18O values of phyllites, gabbros and serpentinites range from 6 to 15%o. Model calculations indicate that such values are consistent with low temperature (50–200°C) alteration of parent rocks by sea-water at varying water/rock ratios. This would agree with the early low temperature mineralogy of the serpentinites, but the early high temperature alteration of the gabbros would require the presence of18O-enriched sea-water.The following overall history is suggested for Tinos ophiolitic slices. (1) Oceanic crust was generated at a supra-subduction zone spreading centre with high temperature alteration of gabbros. (2) Tectonic disturbance (its early hot stages recorded in an amphibolitic shear zone at the base of serpentinites) brought the already cooled ultramafics into direct contact with sea-water and caused low-Tserpentinization. (3) Tectonism after cooling involved thrusting which caused repetition and inversion of the original order of the oceanic suite. (4) Regional metamorphism of all the ophiolite components at greenschist-facies conditions (−450°C) overprinted the early alteration mineralogy. It was probably induced by continued thrusting and piling up of nappes. The Tinos ophiolite, dated as late Cretaceous and genetically related to other low pressure rock-units of the same age in the Aegean, differs in age and degree of dismemberment and metamorphism from ophiolites in mainland Greece.

The sediment-hosted Ngwako Pan copper deposit, Botswana

The strat-bound Ngwako Pan copper deposit is located in the northeastern part of the 250-km-long Neoproterozoic Ghanzi Ridge in northwestern Botswana. The sulfide mineralization occurs in the lowermost 25 m of the Middle Ghanzi Formation, which is composed of reduced facies (gray and greenish-gray) siliciclastic and subordinate carbonate metasediments. The 1- to 3.5-km-thick red-bed metasediments of the Lower Ghanzi Formation underlie the mineralized horizon. The distribution of copper ore in the Middle Ghanzi Formation shows a close spatial correlation with the occurrence of basaltic rocks in the Kgwebe Formation underlying the Lower Ghanzi Formation. The mineralized horizon is hosted by lacustrine or lagoonal sediments that were deposited on a paleohigh in a rift-related basin. The sediments were metamorphosed at lower greenschist facies conditions and tightly folded. The homogenization temperatures of fluid inclusions in secondary quartz are in the range of 57 degrees to 254 degrees C, the median being 179 degrees C. The major sulfide minerals, in order of decreasing abundance, are chalcocite- djurleite, bornite, chalcopyrite, pyrite, sphalerite, and galena. They occur as disseminations related to the intergranular porosity and permeability of the sediment and those related to fracture or shear porosity and permeability. Various load features indicate that at least part of the disseminated mineralization or its precursor sulfide minerals was deposited before major compaction was completed. Sulfurization of the sediment and possibly introduction of copper occurred early in the diagenetic history of the sediment. The mineralizing fluids were probably derived from oxidized ground water passing through red-bed footwall sediments. These fluids were enriched in copper where the aquifer was copper rich, i.e., where high Cu basaltic rocks constitute part of the source region of the siliciclastic sediments.

Subduction-related Jurassic andesites in the northern Great Caucasus

During the Jurassic the major tectonic units of the Great Caucasus (Bechasyn, Front Range, Main Range and Southern Slope zone) were affected by intensive magmatic activity. Magmatism within the Bechasyn zone, the northernmost unit, which represents the southern part of the Variscan-consolidated Skythian platform is considered here. With the beginning of the Early Jurassic this zone was reactivated by subsidence, accompanied by the deposition of epicontinental shallow water sediments. The Lower Jurassic portion of this sedimentary pile was intruded by numerous sills which display a clear temporal and spatial evolution. The older basic rocks are lower in the profile than the younger, more acidic rocks. A set of 75 samples, representing all exposed sills and their feeder-dikes, was analyzed for major and 21 trace elements. All samples appear more or less affected by alteration under lower greenschist facies conditions. However, these alterations essentially took place on local scales and did not affect the overall chemistry. According to their main element composition the rocks constitute a calc-alkaline series ranging from basaltic—andesitic to rhyolitic. Most of the samples are andesites. Chemically, these andesites closely resemble modern orogenic andesites occurring at convergent plate margins. Altogether, the field evidence and the chemical and mineralogical data obtained show the investigated rocks to be comagmatic and derived from basalt—andesitic initial melts by magmatic fractionation processes. Tholeiitic melts have to be considered as parental magmas, which according to the trace element characteristics of the basalt-andesitic rocks, were generated from an enriched peridotitic mantle source. 87Sr/86Sr isotope ratios and σ18O values confirm the mantle origin of this rock series. The observed compositional evolution can be explained as a result of olivine and clinopyroxene fractionation of the tholeitic melts followed by amphibole and plagioclase separation. 40Ar/39Ar measurements on biotite and plagioclase phenocrysts separated from these rocks vary between 190 and 180 Ma and thereby place the magmatic activity in the late Early Jurassic, in good agreement with the stratigraphic observations. Genetically, the calc-alkaline rocks are related to a subduction zone of the Andean type. Their chemical and isotopic compositions and their age setting corroborate the plate tectonic models for the evolution of the Caucasus orogenic belt during the Jurassic.

Metamorphism of cale-silicate and associated rocks in the Pan-African Kaaimans Group, Saldania Belt, South Africa

Phase relations and mineral compositions indicate a polymetamorphic history of carbonate and siliciclastic sediments constituting the Kaaimans Group of the Pan-African Saldania Belt along the south coast of South Africa. Regional metamorphism during Pan-African orogeny did not exceed greenschist facies conditions but was superimposed locally onto low- to medium-grade contact metamorphism related to the intrusion of syn-orogenic granitoids. Petrogenetic grids, constructed for the System CaO-MgO Al2O3-SiO2-H2O-CO2 and based on available thermodynamic data, reveal that vesuvianite, grossular-rich garnet and wollastonite bearing minerai assemblages, found in cale-silicate rocks adjacent to the syn-orogenic Rooiklip granite gneiss sheet, were formed at temperatures possibly as low as 450°C and pressures between 2 and 3 kbar. These mineral assemblages were in equilibrium with an H2O-CO2 fluid that was characterized by a CO2/(CO2 + H2O) mole fraction of less than 0.02. The fluid composition in the cale-silicate rocks was buffered entirely externally through magmatic/hydrothermal fluids related to the granite intrusion. The fluid flux was enhanced by syn-intrusive bedding-parallel shearing in the sedimentary rocks. Vesuvianite, which is of highly variable but generally Fe-rich composition, contains up to 2.1 wt.% F and hence acted as sink for fluorine. A regional increase in Pan-African metamorphic grade from the tectonically lowest to highest formation suggests an overturned position of the whole Kaaimans Group. Later metamorphism during the Cape Orogeny caused only very low- to low-grade retrogression.

Talc-chlorite-amphibole felses of the KTB pilot hole, Oberpfalz, Bavaria: Protolith characteristics and phase relationships

The metagabbro-amphibolite sequences in the KTB pilot hole contain intercalations of talc-chlorite-amphibole felses (or “hosbachites”), which show transitional contacts to the adjacent metagabbros. The hosbachites are characterized by relics of a primary igneous texture and still contain igneous minerals like clinopyroxene, biotite and pseudomorphs after olivine, while brown Ca-amphibole was presumably formed in a late-magmatic stage. The geological, textural, mineralogical and geochemical evidence indicates that the hosbachites were derived from ultramafic cumulates, differentiated from a basaltic magma, either in the inner parts of dolerite sills or in small gabbro intrusions. A pervasive metamorphic overprint under medium-pressure, amphibolite-facies conditions which was accompanied by penetrative deformation led to assemblages with green Ca-amphibole ± anthophyllite ± cummingtonite ± tremolite/actinolite + clinochlore + talc + olivine + ilmenite ± Cr-bearing spinel + sulfides. Phase relationships are consistent with a prograde P-T path leading to the formation of anthophyllite from olivine + talc at peak metamorphic temperatures of 640–700°C, at assumed pressures of 8-10 kbar, similar to those derived from mineral assemblages in the adjacent metabasites and metasediments. High-pressure relics locally present in coronitic metagabbros and retrograded eclogites of the KTB pilot hole were not recognized in the hosbachites. A retrograde overprint under greenschist-facies conditions led to the total replacement of igneous or metamorphic olivine by aggregates of antigorite + magnetite, chloritization of biotite and the formation of late tremolite/ actinolite.

Evidence for submarine weathering from metamorphosed weathering profiles on basaltic rocks, Tananao Metamorphic Complex, Taiwan

The metamorphosed paleo-weathering horizon(s) developed on basaltic rocks in the Tananao Metamorphic Complex forms distinct Al-Fe-rich rocks and occurs as two parallel belts, demonstrating its potential as a stratigraphic correlation unit in this structurally complicated metamorphic terrain.These Al-Fe-rich rocks have been metamorphosed under amphibolite-/greenschist-facies conditions, leading to the formation of garnet-chloritoid rocks/chloritoid-rich rocks. The chemical changes along three profiles, from metabasite to garnet-chloritoid rock, show prominent K, Rb, Cs and Ba enrichment, yet, also, a large depletion of other elements if it is assumed that Ti, Zr, Nb, Ta and Hf remained relatively immobile. The REE's, however, behaved non-coherently. These chemical characteristics are attributed to pre-metamorphic processes and are shown to be similar to those observed with submarine weathering of basaltic rocks. In contrast, the chloritoid-rich rocks reveal distinct K, Rb, Cs and Ba depletion. It is suggested that this resulted from subaerial weathering processes.K enrichment is not an uncommon feature in Paleozoic/Precambrian paleosols. The present case study demonstrates that submarine weathering should be considered as a probable cause.

The Ronda peridotites (Los Reales nappe): An example of the relationship between lithospheric thickening by oblique tectonics and late extensional deformation within the Betic Cordillera (Spain)

The Internal (or metamorphic) Zone of the Betic Cordillera (southern Spain) is a 400 km long and ENE-elongated tectonic domain characterized by a widespread record of alternating compressional and extensional processes. In the westernmost part of the Internal Zone several orogenic Iherzolite massifs—the Ronda peridotites—were also involved in such processes. The Ronda peridotites form the lower portion of the Los Reales nappe and are overlain by a metamorphic sequence which varies from granulite- to greenschist-facies conditions. The tectono-metamorphic history of the Los Reales nappe involves two main synmetamorphic steps. A first extensional event at lithospheric scale is preserved within the attenuated continental crust which overlies the Ronda peridotites. It was related to the upwelling of an asthenospheric diapir below the thinned lithosphere, and led to the development of low-pressure-high-temperature metamorphism and of ductile shear zones with N110°E-trending stretching lineations. A subsequent compressional event is represented by the hot thrusting of the Ronda peridotites from the west-southwest to the east-northeast. Finally, late extensional deformations in a dominant N-S direction developed in response to the lithospheric thickening achieved during the former compressional event. The kinematics of the synmetamorphic deformations can be integrated into a model of sinistrally oblique tectonics compatible with the overall strike-slip motion of the African and Iberian plates from Jurassic to Tertiary times, whereas the later deformations should reflect the convergence between Africa and Iberia from Tertiary time onwards.

Diapirism, bulk homogeneous shortening and transcurrent shearing in the Archaean Dharwar craton: the Holenarsipur area, southern India

Qualitative strain data are spatially integrated in order to define Archaean foliation trajectories, and variations in strain intensity and ellipsoid type, in the Holenarsipur area, Dharwar craton, southern India. The study provides constraints on the bulk geometry, the progressive deformation and the kinematic history of this region of Archaean continental crust. Two major structural events have shaped the study area: (i) a regional diapiric event contemporaneous with a magmatic event, and (ii) a later sinistral transcurrent ductile shear event which has locally reoriented both foliations and stretching lineations. The entire Holenarsipur supracrustal belt underwent a common tectono-metamorphic history that included two metamorphic events. Each event corresponds temporally to a distinct strain field. The first, a regional amphibolite facies event, was contemporaneous with the main tectonic event and probably with the intrusion of Halekote trondhjemites. The second occurred under greenschist-facies conditions and was restricted to a late transcurrent shear zone at the eastern boundary of the greenstone belt. The strain pattern prior to transcurrent shearing cannot be explained by a thrusting event unless all trace of this event had been erased by the diapirism. We interpret all structures older than the transcurrent shearing to be the direct result of vertical movements linked to body forces, augmented by minor horizontal crustal shortening.