Thermal history vs. fabric anisotropy in granitoids emplaced at different crustal levels: an example from Calabria, southern Italy

The Sila and Serre granitoids of Calabria were emplaced in the late Carboniferous at depths ranging from 6 to 23 km in a postcollisional extensional regime. Their fabric, which developed during and after final crystallization up to the solid state, strongly increases in intensity with emplacement depth. This relationship is attributed to the thermal history of the Calabrian basement. Cooling histories of granitoids, constrained by geological data and Rb–Sr cooling ages on micas, demonstrate that residence times of rocks at temperatures greater than those of the brittle–ductile transition vary greatly as a function of initial emplacement depth. This explains why shallow-level granitoids, that remained for about 10 Myr at temperatures above those of the brittle domain, retain their original magmatic fabric. By contrast, the strong fabrics of the deep-seated granitoids are explained by solid-state strain overprint that lasted more than 100 Myr at temperatures above those of the brittle domain.     

Geochronological constraints on the timing of granitoid magmatism, metamorphism and post-metamorphic cooling in the Hercynian crustal cross-section of Calabria

Exposed cross‐sections of the continental crust are a unique geological situation for crustal evolution studies, providing the possibility of deciphering the time relationships between magmatic and metamorphic events at all levels of the crust. In the cross‐section of southern and northern Calabria, U–Pb, Rb–Sr and K–Ar mineral ages of granulite facies metapelitic migmatites, peraluminous granites and amphibolite facies upper crustal gneisses provide constraints on the late‐Hercynian peak metamorphism and granitoid magmatism as well as on the post‐metamorphic cooling. Monazite from upper crustal amphibolite facies paragneisses from southern Calabria yields similar U–Pb ages (295–293±4 Ma) to those of granulite facies metamorphism in the lower crust and of intrusions of calcalkaline and metaluminous granitoids in the middle crust (300±10 Ma). Monazite and xenotime from peraluminous granites in the middle to upper crust of the same crustal section provide slightly older intrusion ages of 303–302±0.6 Ma. Zircon from a mafic to intermediate sill in the lower crust yields a lower concordia intercept age of 290±2 Ma, which may be interpreted as the minimum age for metamorphism or intrusion. U–Pb monazite ages from granulite facies migmatites and peraluminous granites of the lower and middle crust from northern Calabria (Sila) also point to a near‐synchronism of peak metamorphism and intrusion at 304–300±0.4 Ma. At the end of the granulite facies metamorphism, the lower crustal rocks were uplifted into mid‐crustal levels (10–15 km) followed by nearly isobaric slow cooling (c. 3 °C Ma^?1) as indicated by muscovite and biotite K–Ar and Rb–Sr data between 210±4 and 123±1 Ma. The thermal history is therefore similar to that of the lower crust of southern Calabria. In combination with previous petrological studies addressing metamorphic textures and P–T conditions of rocks from all crustal levels, the new geochronological results are used to suggest that the thermal evolution and heat distribution in the Calabrian crust were mainly controlled by advective heat input through magmatic intrusions into all crustal levels during the late‐Hercynian orogeny.     

Tectonothermal history of the Hercynian continental crust of the Serre (southern Calabria, Italy) monitored by Rb–Sr biotite resetting

In the Serre mountains of Calabria, Italy, an exposed section of the continental crust, as left by the Hercynian orogeny, consists of intermediate-lower to upper crustal units. Huge masses of granitoids separate the lower from the upper crustal units. Many mica ages have been obtained from metamorphic and plutonic rocks, which have been interpreted as reflecting continuous cooling or discrete Mesozoic events. A reappraisal of previously determined isotopic data integrated with new Rb–Sr biotite ages is presented and assessed at regional scale to better constrain the post-Hercynian geological evolution of the continental crust of the Serre. The ages cover a wide span of time and form clusters which fit a model involving magmatic, hydrothermal and tectonic events preceding and accompanying the opening and closure of the Tethyan ocean.     

Late-Hercynian shearing during crystallization of granitoid magmas (Sila massif,southern Italy): regional implications

Shearing of regional extent, involving granitoids and underlying mid-crustal rocks of the Sila massif (Calabria, Italy), is analysed in this paper. The deformed granitoids are affected by a wide NNW-SSE oriented deformation zone, stretching for about 60 km, from the neighbourhood of Cecita Lake to Cropani village. Meso- and micro-structures in granitoids, close to the boundary with underlying migmatitic paragneiss, indicate that deformation developed from melt-present to solid-state conditions. Simultaneous tectonics and magmatism activated a plutonic accretionary process at mid-crustal levels. This took place at about 300 Ma and involved hybrid magmas with a dominat contribution from a mantle source. The deformation regime remained steady for a long time during magma crystallization and cooling in subsolidus conditions. The regional top-to-the-W sense of shear in the present geographic coordinates, recorded in the deformed granitoids, seems geometrically consistent with the coeval direction of maximum extension found in another sector of the southern Hercynian belt, suggesting the original position of the Sila basement in this context. Magmatic ativity ended with the intrusion of mafic and felsic magams affected by a very weak deformation, ongoing during the final strain increments of the late-Hercynian stage.     

Pressure–temperature evolution of the late Hercynian Calabria continental crust: compatibility with post‐collisional extensional tectonics

The late Hercynian tectonic evolution of the Calabria crust is characterized by peak metamorphic conditions up to 800 °C and 1000 MPa, and coeval mid‐crustal granitoid emplacement at 304–300 Ma. To check if a post‐collisional extensional framework, similar to that of other Hercynian massifs, can explain petrologic data, we model the pressure–temperature evolution of the crust during extension following granitoid emplacement. Model parameters are constrained by petrologic, geochemical and structural data. Computed P–T paths are characterized by nearly isothermal decompression followed by isobaric cooling, which show a good fit to petrologic P–T paths for duration of extension between 5 and 10 Ma. The model results, therefore, support an interpretation of the magmatic and metamorphic evolution of the Calabria crust in terms of the late Hercynian extension. In this framework, slab break‐off is a reasonable explanation for the common evolution of the southern European Hercynian massifs.     

P–T–t path of the Hercynian low-pressure rocks from the Mandatoriccio complex (Sila Massif, Calabria, Italy): new insights for crustal evolution

The tectono‐metamorphic evolution of the Hercynian intermediate–upper crust outcropping in eastern Sila (Calabria, Italy) has been reconstructed, integrating microstructural analysis, P–T pseudosections, mineral isopleths and geochronological data. The studied rocks belong to a nearly complete crustal section that comprises granulite facies metamorphic rocks at the base and granitoids in the intermediate levels. Clockwise P–T paths have been constrained for metapelites of the basal level of the intermediate–upper crust (Umbriatico area). These rocks show noticeable porphyroblastic textures documenting the progressive change from medium‐P metamorphic assemblages (garnet‐ and staurolite‐bearing assemblages) towards low‐P/high‐T metamorphic assemblages (fibrolite‐ and cordierite‐bearing assemblages). Peak‐metamorphic conditions of ～590 °C and 0.35 GPa are estimated by integrating microstructural observations with P–T pseudosections calculated for bulk‐rock and reaction‐domain compositions. The top level of the intermediate–upper crust (Campana area) recorded only the major heating phase at low‐P (～550 °C and 0.25 GPa), as documented by the static growth of biotite spots and of cordierite and andalusite porphyroblasts in metapelites. In situ U–Th–Pb dating of monazite from schists containing low‐P/high‐T metamorphic assemblages gave a weighted mean U–Pb concordia age of 299 ± 3 Ma, which has been interpreted as the timing of peak metamorphism. In the framework of the whole Hercynian crustal section the peak of low‐P/high‐T metamorphism in the intermediate‐to‐upper crust took place concurrently with granulite facies metamorphism in the lower crust and with emplacement of the granitoids in the intermediate levels. In addition, decompression is a distinctive trait of the P–T evolution both in the lower and upper crust. It is proposed that post–collisional extension, together with exhumation, is the most suitable tectonic setting in which magmatic and metamorphic processes can be active simultaneously in different levels of the continental crust.     

Variations in the Nd isotopic ratios and canonical ratios of concentrations of incompatible elements as an indication of mixing sources of alkali granitoids and basites in the Khaldzan-Buregtei massif and the Khaldzan-Buregtei rare-metal deposit in western Mongolia

The paper reports data on the Nd isotopic composition and the evaluated composition of the sources of magmatism that produced massifs of alkali and basic rocks of the Khaldzan-Buregtei group. The massifs were emplaced in the terminal Devonian at 392–395 Ma in the Ozernaya zone of western Mongolia. The host rocks of the massifs are ophiolites of the early Caledonian Ozernaya zone, which were dated at 545–522 Ma. The massifs were emplaced in the following succession (listed in order from older to younger): (1) nordmarkites and dolerites syngenetic with them; (2) alkali granites and syngenetic dolerites; (3) dike ekerites; (4) dike pantellerites; (5) rare-metal granitoids; (6) alkali and intermediate basites and quartz syenites; and (7) miarolitic rare-metal alkali granites. Our data on the Nd isotopic composition [?_Nd(T)] and conventionally used (canonical) ratios of incompatible elements (Nb/U, Zr/Nb, and La/Yb) in rocks from the alkaline massifs and their host ophiolites indicate that all of these rocks were derived mostly from mantle and mantle-crustal enriched sources like OIB, E-MORB, and IAB with a subordinate contribution of N-MORB (DM) and upper continental crustal material. The variations in the ?_Nd(T) values in rocks of these massifs suggest multiple mixing of the sources or magmas derived from them when the massifs composing the Khaldzan-Buregtei group were produced. The OIB and E-MORB sources were mixed when the rocks with mantle signatures were formed. The occurrence of nordmarkites, alkali granites, and other rocks whose isotopic and geochemical signatures are intermediate between the values for mantle and crustal sources testifies to the mixing of mantle and crustal magmas. The crustal source itself, which consisted of rocks of the ophiolite complex, was obviously isotopically and geochemically heterogeneous, as also were the magmas derived from it. The model proposed for the genesis of alkali rocks of the Khaldzan-Buregtei massifs implies that the magmas were derived at two major depth levels: (1) mantle, at which the plume source mixed with an E-MORB source, and (2) crustal, at which the ophiolites were melted, and this gave rise to the parental magmas of the nordmarkites and alkali granites. The basites were derived immediately from the mantle. The mantle syenites, pantellerites, and rare-metal granitoids were produced either by the deep crystallization differentiation of basite magma or by the partial melting of the parental basites and the subsequent crystallization differentiation of the generated magmas. Differentiation likely took place in an intermediate chamber at depth levels close to the crustal (ophiolite) level of magma generation. Only such conditions could ensure the intense mixing of mantle and crustal magmas. The principal factor initiating magma generation in the region was the mantle plume that controlled within-plate magmatism in the Altai-Sayan area and the basite magmas related to this plume, which gave rise to small dikes and magmatic bodies in the group of intrusive massifs.     

Vertical extrusion and middle crustal spreading of omphacite granulite: a model of syn-convergent exhumation (Bohemian Massif, Czech Republic)

The exhumation of eclogite facies granulites (Omp–Plg–Grt–Qtz–Rt) in the Rychleby Mts, eastern Czech Republic, was a localised process initiated by buckling of crustal layers in a thickened orogenic root. Folding and post‐buckle flattening was followed by the main stage of exhumation that is characterized by vertical ductile extrusion. This process is documented by structural data, and the vertical ascent of rocks from a depth of c. 70 to c. 35 km is documented by metamorphic petrology. SHRIMP ²⁰⁶Pb/²³⁸U and ²⁰⁷Pb/²⁰⁶Pb evaporation zircon ages of 342 ± 5 and 341.4 ± 0.7 Ma date peak metamorphic conditions. The next stage of exhumation was associated with sideways flat thrusting associated with lateral viscous spreading of granulites and surrounding rocks over indenting adjacent continental crust at a depth of c. 35–30 km. This stage was associated with syntectonic intrusion of a granodiorite sill at 345–339 Ma, emplaced at a crustal depth of c. 25 km. The time required for cooling of the sill as well as for heating of the country rocks brackets this event to a maximum of 250 000 years. Therefore, similar ages of crystallization for the granodiorite magma and the peak of eclogite facies metamorphism of the granulite suggest a very short period of exhumation, limited by the analytical errors of the dating methods. Our calculations suggest that the initial exhumation rate during vertical extrusion was 3–15 mm yr^?1, followed by an exhumation rate of 24–40 mm yr^?1 during further uplift along a magma‐lubricated shear zone. The extrusion stage of exhumation was associated with a high cooling rate, which decreased during the stage of lateral spreading.     

A study of igneous rocks related to Zn–Pb mineralization in the Shinyemi and Gagok deposits of the Taebaeksan Basin,South Korea

The Shinyemi and Gagok deposits, located in the Taebaeksan Basin, South Korea, display Zn–Pb mineralization along a contact between Cretaceous granitoids and Cambrian–Ordovician carbonates of the Joseon Supergroup. The Shinyemi mine is one of the largest polymetallic skarn‐type magnetite deposits in South Korea and comprises Fe and Fe–Mo–Zn skarns, and Zn–Cu–Pb replacement deposits. Both deposits yield similar Cretaceous mineralization ages, and granitoids associated with the two deposits displaying similar mineral textures and compositions, are highly evolved, and were emplaced at a shallow depth. They are classified as calc‐alkaline, I‐type granites (magnetite series) and were formed in a volcanic arc. Compositional variation is less in the Shinyemi granites and aplites (e.g., SiO₂ = 74.4–76.6 wt% and 74.4–75.1 wt%, respectively) than in the Gagok granites and aplites (e.g., SiO₂ = 65.6–68.0 wt% and 74.9–76.5 wt%, respectively). Furthermore, SiO₂ vs K/Rb and SiO₂ vs Rb/Sr diagrams indicate that the Shinyemi granitoids are more evolved than the Gagok granitoids. Shinyemi granitoids had been already differentiated highly in deep depth and then intruded into shallow depth, so both granite and aplite show the highly evolved similar chemical compositions. Whereas, less differentiated Gagok granitoids were separated into two phases in the last stage at shallow depth, so granite and aplite show different compositions. The amounts of granites and aplite are similar in the Shinyemi deposit, whereas the aplite appears in an amount less than the granite in the Gagok deposit. For this reason, the Shinyemi granitoids caused not only Fe enrichment during formation of the dolomite‐hosted magnesian skarn but also was associated with Mo mineralization in the Shinyemi deposit. Zn mineralization of the Gagok deposit was mainly caused by granite rather than aplite. Our data suggest that the variation in mineralization displayed by the two deposits resulted from differences in the compositions of their associated igneous intrusions.     

U-Pb and Rb-Sr radiometric dates and their correlation with metamorphic events in the granulite-facies basement of the serre,Southern Calabria (Italy)

An approximately 7 km thick, continuous sequence of granulite-facies rocks from the lower crust, which contains a lower granulite-pyriclasite unit and an upper metapelite unit, occurs in the NW Serre of the Calabrian massif. The lower crustal section is overlain by a succession of plutonic rocks consisting of blastomylonitic quartz diorite, tonalite, and granite, and is underlain by phyllonitic schists and gneisses.Discordant apparent zircon ages, obtained from granulites and aluminous paragneisses, indicate a minimum age of about 1,900 m.y. for the oldest zircon populations. The lower intersection point of the discordia with the concordia at 296±2 m.y. is also marked by concordant monazites. Therefore, the age of 296±2 m.y. is interpreted as the minimum age of granulite-facies metamorphism.Concordant zircon ages were obtained from a metamorphic quartz monzogabbronorite sill (298±5 m.y.) and an unmetamorphosed tonalite (295±2 m.y.); they are interpreted as the intrusion ages.Discordant zircon ages from a blastomylonitic quartz diorite gneiss, situated between the lower crustal unit and the non-metamorphosed tonalite, reveal recent or geologically young lead loss by diffusion. The ²⁰⁷Pb/²⁰⁶Pb ages of the two analysed size-fractions point to an intrusion age similar to that of the overlying tonalite.Rb-Sr mineral ages are younger in the granulite-pyriclasite unit than in the overlying metapelite unit. Feldspars from the granulite-pyriclasite unit yield ages of about 145 m.y. and those from the metapelite unit 176±5 m.y. In the same way, the biotite cooling ages range between 108 and 114 m.y. in the granulitepyriclasite and between 132 and 135 m.y. in the metapelite unit and the tonalite. Some still younger biotite ages are explained by the influence of tectonic shearing on the Rb-Sr systems. A muscovite from a postmetamorphic aplite in the metapelite unit yields a cooling age of 203±4 m.y.The Rb-Sr isotopic analyses from migmatite bands do not lie on an isochron, perhaps due to limited isotopic exchange between the small scale layers during the long cooling period after the peak of metamorphism.In the phyllonitic gneisses and schists a Hercynian metamorphism is indicated by a muscovite age of 268±4 m.y., whereas the biotite age of 43±1 m.y. from the same sample can be correlated with an Alpine greenschist-facies metamorphism.On the basis of the radiometric dates and of the P-T path of the lower crustal section deduced petrologically, the following model is presented: the end of the Hercynian granulite-facies metamorphism was accompanied by an uplift of the lower crustal rocks into intermediate crustal levels and by synchronous plutonic intrusions into the lower crust and higher crustal levels, but essentially into the latter. Substantial further uplift did not occur until after cooling from the temperature of the granulite-facies metamorphism to the biotite closing temperature. This cooling lasted for about 185 m.y. in the lower part and for about 160 m.y. in the upper part of the lower crust section.A comparison between the geologic evolutions of the NW Serre of Calabria and the Ivrea Zone of the Alps demonstrates striking similarities. The activity of deep seated faults in both areas at least since late Hercynian time raises the possibility that a fault precursor of the boundary of the Adriatic microplate already existed at this time.     

Metamorphism of the Central Anatolian Crystalline Complex, Turkey: influence of orogen-normal collision vs. wrench-dominated tectonics on P–T–t paths

The Central Anatolian Crystalline Complex (CACC) is a microcontinent in the Alpine–Himalayan belt. It has previously been considered as a coherent structural entity, but, although the entire CACC is comprised of similar rocks (primarily metasedimentary rocks and granitoids), it consists of at least four tectonic blocks characterized by different P–T–t paths. These blocks are the K?r?ehir (north‐west), Akda? (north‐east), Ni?de (south) and Aksaray (west) massifs. The northern massifs experienced thrusting and folding during collision and were slowly exhumed by erosion; metamorphic rocks are characterized by clockwise P–T paths at moderate P–T and local low‐P–high‐T (LP–HT) overprinting in the highest grade rocks. Apatite fission track ages are Eocene to Oligocene (47–32 Ma). The Aksaray block represents the hot, shallow mid‐crust of a Late Cretaceous–early Tertiary arc. It is dominated by intrusions; rare metapelitic rocks record low‐P (< 4 kbar) regional metamorphism overprinted by LP–HT contact metamorphism. Apatite fission track ages are 50–45 Ma. The Ni?de massif is different from the other CACC blocks because it evolved as a core complex in a wrench‐dominated setting. It is characterized by clockwise P–T paths at moderate P–T followed by widespread LP–HT metamorphism. Apatite fission track ages are Miocene (12–9 Ma), significantly younger than those in the northern massifs. Ni?de rocks resided in the mid‐crust at a time when the rest of the CACC was at or near the Earth's surface. Variations in P–T–t and tectonic histories — especially timing of exhumation — between the northern and southern CACC reflect the difference between head‐on collision vs. mid‐crustal wrenching.     

Exhumation mechanisms of melt‐bearing ultrahigh pressure crustal rocks during collision of spontaneously moving plates

A series of 2D petrological–thermomechanical numerical experiments was conducted to: (i) characterize the variability of exhumation mechanisms of ultrahigh pressure metamorphic (UHPM) rocks during collision of spontaneously moving plates and (ii) study the possible geodynamic effects of melting at ultrahigh pressure conditions for the exhumation of high‐temperature–ultrahigh pressure metamorphic (HT–UHPM) rocks. To this end, the models include fluid‐ and melt‐induced weakening of rocks. Five distinct modes of exhumation of (U)HPM rocks associated with changes in several parameters in the models of plate collision and continent subduction are identified as follows: vertical crustal extrusion, large‐scale crustal stacking, shallow crustal delamination, trans‐lithospheric diapirism, and channel flow. The variation in exhumation mechanisms for (U)HPM rocks in numerical models of collision driven by spontaneously moving plates contrasts with the domination of the channel flow mode of exhumation in a majority of the published results from numerical models of collision that used a prescribed plate convergence velocity and/or did not include fluid‐ and melt‐induced weakening of rocks. This difference in the range of exhumation mechanisms suggests that the prescribed convergence velocity condition and the neglect of fluid‐ and melt‐related weakening effects in the earlier models may inhibit development of several important collisional processes found in our experiments, such as slab breakoff, vertical crustal extrusion, large‐scale stacking, shallow crustal delamination and relamination, and eduction of the continental plate. Consequently, the significance of channel flow for the exhumation of UHPM rocks may have been overstated based on the results of the earlier numerical experiments. In addition, the results from this study extend over a larger proportion of the high‐temperature range of P–T conditions documented from UHPM rocks, including those retrieved from HT–UHPM rocks, than the results of experiments from previous numerical models. In particular, the highest peak metamorphic temperatures (up to 1000 °C) are recorded in the case of the vertical crustal extrusion model in which subducted continental crust is subjected to a period of prolonged heating by asthenospheric mantle abutting the continental side of the vertically hanging slab. Nonetheless, some extreme temperature conditions which have been suggested for the Kokchetav and Bohemian massifs, perhaps up to 1100–1200 °C, are still to be achieved in experiments using numerical models.     

Vertical extrusion and horizontal channel flow of orogenic lower crust: key exhumation mechanisms in large hot orogens?

A large database of structural, geochronological and petrological data combined with a Bouguer anomaly map is used to develop a two‐stage exhumation model of deep‐seated rocks in the eastern sector of the Variscan belt. An early sub‐vertical fabric developed in the orogenic lower and middle crust during intracrustal folding followed by the vertical extrusion of the lower crustal rocks. These events were responsible for exhumation of the orogenic lower crust from depths equivalent to 18?20 kbar to depths equivalent to 8?10 kbar, and for coeval burial of upper crustal rocks to depths equivalent to 8–9 kbar. Following the folding and vertical extrusion event, sub‐horizontal fabrics developed at medium to low pressure in the orogenic lower and middle crust during vertical shortening. Fabrics that record the early vertical extrusion originated between 350 and 340 Ma, during building of an orogenic root in response to SE‐directed Saxothuringian continental subduction. Fabrics that record the later sub‐horizontal exhumation event relate to an eastern promontory of the Brunia continent indenting into the rheologically weaker rocks of the orogenic root. Indentation initiated thrusting or flow of the orogenic crust over the Brunia continent in a north‐directed sub‐horizontal channel. This sub‐horizontal flow operated between 330 and 325 Ma, and was responsible for a heterogeneous mixing of blocks and boudins of lower and middle crustal rocks and for their progressive thermal re‐equilibration. The erosion depth as well as the degree of reworking decreases from south to north, pointing to an outflow of lower crustal material to the surface, which was subsequently eroded and deposited in a foreland basin. Indentation by the Brunia continental promontory was highly noncoaxial with respect to the SE‐oriented Saxothuringian continental subduction in the Early Visean, suggesting a major switch of plate configuration during the Middle to Late Visean.     

The Bocchigliero Palaeozoic sequence in the context of the Calabrian–Peloritan Hercynian Range (Italy)

The Calabrian–Peloritan Hercynian Range includes three weakly metamorphosed Palaeozoic sequences cropping out in north-eastern Sila (Bocchigliero sequence), southern Sila, Serre and Aspromonte (Stilo sequence), and in the Peloritan Mountains (Peloritan sequence). The work reported here considers the Bocchigliero sequence and comprises part of a geological, petrological and geochemical research programme on the Palaeozoic evolution of the Calabrian–Peloritan Arc. The Bocchigliero sequence constitutes the lower tectonic unit of the Hercynian Caiabrian–Peloritan Range and is overthrusted by the metamorphic Mandatoriccio Unit. The Bocchigliero sequence is a terrigenous–carbonate–volcanic association, is affected byclow grade metamorphism, contains Cambro-Ordovician fossils and extends in age from the Cambrian to the Devonian. The terrigenous material is represented by meta-arenites and metapelites (Cambrian–Devonian); the volcanics include metatuffites (Cambrian and Ordovician), metabasalts (Cambro-Ordovician), metaandesites and metarhyolites (Ordovician and Siluro-Devonian); limestone beds are present in the Devonian. It is believed that the Palaeozoic Bocchigliero basin formed in the Cambrian on a continental crust in which the rocks constituting today's Mandatoriccio Unit were located at 3–8 km depth. The crustal thinning in the Cambro-Ordovician led to fracturing and upwelling of alkaline within-plate basaltic magmas, whereas in the Ordovician the thinning took place under conditions of higher plasticity. In this latter period an increase in temperature resulting from mantle upwelling produced crustal partial melts of andesite and rhyolite composition. In addition, this thermal uprise was responsible for regional metamorphism characterized by low pressures and by the absence of penetrative deformation. The effects of this metamorphism are well developed in the rocks of the Mandatoriccio Unit. In the Silurian and Devonian, progressive closing of the basin took place. The Palaeozoic sequence was then subjected to Variscan low pressure–low temperature metamorphism and Alpine deformation.     

Diachronic and different metamorphic evolution in the fossil Variscan lower crust of Calabria

Different P–T–t paths and Variscan tectonic evolution have been described for the lower crust of Calabria. New data have been collected through retrieval technique and construction of pseudosections to control the validity of the previous data and to check the appropriate model to describe the tectono-thermal evolution of the lower crust of the Serre (southern Calabria). The time-period from ~350 and ~270?Ma has been considered to depict the evolution from Variscan crustal thickening to exhumation as happens in the peri-Mediterranean blocks of south European Variscides and consistently with the available geochronological data. It results that: (1) P-peak at 0.9 and 1.03 GPa at the top and bottom, respectively, was reached earlier than T-peak, (2) crustal thickening developed likely earlier than 325?Ma within the stability field of kyanite, in agreement with previous studies, up to the P-peak along a geothermal gradient of about 21–22°C?km^?1, (3) the T-peak of 700 and 880°C at the top and bottom, respectively, was reached in the stability field of sillimanite after a nearly isobaric heating and (4) Variscan exhumation occurred under increasing T/depth ratio and stopped 270–280?Ma ago. The P–T paths for the upper and lower portions of the section, qualitatively comparable to the numerical simulation, reflect different styles of exhumation, cooling and, according to the available geochronological data, diachronic evolution.     

Low-pressure metamorphism of Palaeozoic pelites in the Aspromonte, southern Calabria: constraints for the thermal evolution in the Calabrian crustal cross-section during the Hercynian orogeny

During Hercynian low-pressure/high-temperature metamorphism of Palaeozoic metasediments of the southern Aspromonte (Calabria), a sequence of metamorphic zones at chlorite, biotite, garnet, staurolite–andalusite and sillimanite–muscovite grade was developed. These metasediments represent the upper part of an exposed tilted cross-section through the Hercynian continental crust. P–T information on their metamorphism supplements that already known for the granulite facies lower crust of the section and allows reconstruction of the thermal conditions in the Calabrian crust during the late Hercynian orogenic event. Three foliations formed during deformation of the metasediments. The peak metamorphic assemblages grew mainly syntectonically (S2) during regional metamorphism, but mineral growth outlasted the deformation. This is in accordance with the textural relationships found in the lower part of the same crustal section exposed in the northern Serre. Pressure conditions recorded for the base of the upper crustal metasediments are c. 2.5 kbar and estimated temperatures range from <350 °C in the chlorite zone, increasing to 500 °C in the lower garnet zone, and reaching 620 °C in the sillimanite–muscovite zone. Geothermal gradients for the peak of metamorphism indicate a much higher value for the upper crust (c. 60 °C km^?1) than for the granulite facies lower crust (30–35 °C km^?1). The small temperature difference between the base of the upper crust (620 °C at c. 2.5 kbar) and the top of the lower crust (690 °C at 5.5 kbar) can be explained by intrusions of granitoids into the middle crust, which, in this crustal section, took place synchronously with the regional metamorphism at c. 310– 295 Ma. It is concluded that the thermal structure of the Calabrian crust during the Hercynian orogeny – as it is reflected by peak metamorphic assemblages – was mainly controlled by advective heat input through magmatic intrusions into all levels of the crust.     

Alpine metamorphic evolution and cooling history of the Veporic basement in northern Hungary: new petrological and geochronological constraints

Petrological and geochronological investigations were carried out on metamorphic rocks of the Veporic unit (Inner Western Carpathians) in northern Hungary. K/Ar and Ar/Ar data on micas and amphibole show only Alpine ages (mostly in the range of 87-95 Ma) in this basement unit. Thermobarometric calculations yield lower amphibolite facies peak conditions (ca. 550냴 °C and 9ǃ kbar) for the Eoalpine metamorphic event. Complex evolution of gneissic rocks is reflected by the presence of discontinuously zoned garnets, the cores of which may represent relics of a pre-Alpine (presumably Variscan) thermal event. Zircon fission track (FT) data in the narrow range of 75-77.5 Ma indicate that this portion of the Veporic unit was emplaced to shallow crustal levels already during the Senonian time. The relative minor difference between zircon FT and K/Ar or Ar/Ar ages suggests very rapid cooling during the Late Cretaceous, most probably related to the extensional unroofing of the Veporic core complex. The obtained cooling ages do not support previous models of Tertiary uplift and exhumation of the Veporic unit along the Hurbanovo-Diósjeni Line.     

Middle Carboniferous crustal melting in the Variscan Belt: New insights from U–Th–Pbtot. monazite and U–Pb zircon ages of the Montagne Noire Axial Zone (southern French Massif Central)

In France, the Devonian–Carboniferous Variscan orogeny developed at the expense of continental crust belonging to the northern margin of Gondwana. A Visean–Serpukhovian crustal melting has been recently documented in several massifs. However, in the Montagne Noire of the Variscan French Massif Central, which is the largest area involved in this partial melting episode, the age of migmatization was not clearly settled. Eleven U–Th–Pb_tot. ages on monazite and three U–Pb ages on associated zircon are reported from migmatites (La Salvetat, Ourtigas), anatectic granitoids (Laouzas, Montalet) and post-migmatitic granites (Anglès, Vialais, Soulié) from the Montagne Noire Axial Zone are presented here for the first time. Migmatization and emplacement of anatectic granitoids took place around 333–326 Ma (Visean) and late granitoids emplaced around 325–318 Ma (Serpukhovian). Inherited zircons and monazite date the orthogneiss source rock of the Late Visean melts between 560 Ma and 480 Ma. In migmatites and anatectic granites, inherited crystals dominate the zircon populations. The migmatitization is the middle crust expression of a pervasive Visean crustal melting event also represented by the “Tufs anthracifères” volcanism in the northern Massif Central. This crustal melting is widespread in the French Variscan belt, though it is restricted to the upper plate of the collision belt. A mantle input appears as a likely mechanism to release the heat necessary to trigger the melting of the Variscan middle crust at a continental scale.     

Prograde and retrograde metamorphic fabrics – a key for understanding burial and exhumation in orogens (Bohemian Massif)

In the Orlica–?nie?nik Dome (NE Bohemian massif), alternating belts of orthogneiss with high‐pressure rocks and belts of mid‐crustal metasedimentary–metavolcanic rocks commonly display a dominant subvertical fabric deformed into a subhorizontal foliation. The first macroscopic foliation is subvertical, strikes NE–SW and is heterogeneously folded by open to isoclinal folds with subhorizontal axial planes parallel to the heterogeneously developed flat‐lying foliation. The metamorphic evolution of the mid‐crustal metasedimentary rocks involved successive crystallization of chlorite–muscovite–ilmenite–plagioclase–garnet, followed by staurolite‐bearing and then kyanite‐bearing assemblages in the subvertical fabric. This was followed by garnet retrogression, with syntectonic crystallization of sillimanite and andalusite parallel to the shallow‐dipping foliation. Elsewhere, andalusite and cordierite statically overgrew the flat‐lying fabric. With reference to a P–T pseudosection for a representative sample, the prograde succession of mineral assemblages and the garnet zoning pattern with decreasing grossular, spessartine and X_Fe are compatible with a P–T path from 3.5–5 kbar/490–520 °C to peak conditions of 6–7 kbar/～630 °C suggesting burial from 12 to 25 km with increasing temperature. Using the same pseudosection, the retrograde succession of minerals shows decompression to sillimanite stability at ～4 kbar/～630 °C and to andalusite–cordierite stability at 2–3 kbar indicating exhumation from 25 km to around 9–12 km. Subsequent exhumation to ～6 km occurred without apparent formation of a deformation fabric. The structure and petrology together with the spatial distribution of the metasedimentary–metavolcanic rocks, and gneissic and high‐pressure belts are compatible with a model of burial of limited parts of the upper and middle crust in narrow cusp‐like synclines, synchronous with the exhumation of orogenic lower crust represented by the gneissic and high‐pressure rocks in lobe‐shaped and volumetrically more important anticlines. Converging P–T–D paths for the metasedimentary rocks and the adjacent high‐pressure rocks are due to vertical exchanges between cold and hot vertically moving masses. Finally, the retrograde shallow‐dipping fabric affects both the metasedimentary–metavolcanic rocks and the gneissic and high‐pressure rocks, and indicates that the ～15‐km exhumation was mostly accommodated by heterogeneous ductile thinning associated with unroofing of a buoyant crustal root.     

兴凯湖花岗杂岩体的锆石U——Pb 年龄及其地质意义

佳木斯地块东南缘的兴凯湖花岗杂岩体主要由花岗闪长岩、二长花岗岩和正长花岗岩组成,其锆石的LA-ICP-MS U-Pb 测年结果显示,杨田寨南山岩体形成于257 ± 2 Ma,双子山岩体形成于215 ～212 Ma; 花岗岩的时空展布表明,两期花岗质岩浆的就位可能分别与古亚洲洋板块的俯冲作用和后造山伸展进程密切相关。结合本区和黑龙江东部其他花岗质岩石的年龄,揭示了佳木斯地块和兴凯地块在早古生代、二叠纪及三叠纪共同经历了三期大规模岩浆活动,暗示两者具有相同的构造属性,共同组成了同一陆块。