Abstract: 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 similar to 590 degrees 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 (similar to 550 degrees 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.
Abstract: The Variscan crystalline basement of the Calabria-Peloritani terrane (CPT) in southern Italy was partly reworked by ductile and brittle shear zones throughout the Alpine tectonic evolution (from thickening to exhumation). Although evidence of extensional tectonics in the CPT has already been found and roughly constrained to the Oligocene onward, no attempt has ever been made to directly date brittle fault movements. Structural (meso- and micro-scale), kinematic and petrographic analyses and Ar-40-Ar-39 laser experiments reveal that the pseudotachylyte-bearing shear zones of the Palmi area in southern Calabria formed in response to extensional shearing similar to 33.5 Ma ago and overprinted compressional tectonic structures. Results provide the first direct evidence of Middle Oligocene co-seismic faulting in the area and confirm the role of extensional tectonics in promoting the Oligocene exhumation of the Calabria basement.
Abstract: Two volcaniclastic successions intercalated in Pliocene basinal clays from the Southern Apennines have been analysed to determine their provenance and their relationship with the geodynamic evolution of the Western Mediterranean. The studied deposits are exclusively made up of ashy pyroclasts, dominated by fresh acidic to intermediate glass, mostly in the form of shards, pumice fragments and groundmass fragments with vitrophyric texture. Crystals include PI, Opx, Cpx, Hbl and rare Bt. Sedimentological features suggest that the volcanic material accumulated near the basin margin by primary fallout processes and was later remobilized by density currents. Ar-40-Ar-39 geochronology allowed dating of one succession at 2.24 +/- 0.06 Ma, corresponding to the Late Pliocene. Composition of the volcaniclastic material is typical of a transitional high-K calc-alkaline series. The age and chemical composition constrain the provenance of the volcaniclastic rocks from the Southern Tyrrhenian domain. Here, volcanic centres were active during Pliocene time, approximately at the northern end of a volcanic arc formed before the opening of the southernmost part of the sea. This paper shows that a detailed study of volcaniclastic products from the southern Apennines and Calabria can be very useful in collecting new pieces of information on the eruption history of the southern Tyrrhenian domain, since they record additional data not available from the study of exposed volcanic edifices.
Abstract: Syn-tectonic late-Hercynian granitoids, emplaced at mid-crustal level through multiple dyke injections, are well-exposed in a similar to 1 km-long section, in the southern Sila Massif (Calabria, Italy). The fieldwork study permitted us to reconstruct the following order of emplacement: (i) granodiorites with euhedral Kfs megacrystals (Grd1, in the main text) and heterogranular granodiorites with coarse rounded PI and Kfs in a fine-grained biotite-rich matrix (Grd2); (ii) granodiorites with a nearly homogeneous grain size (Grd3); (iii) tonalite; and (iv) granite. Both in the wall-rock, represented by migmatitic paragneisses, and in the deformed granitoids the foliation attitude is substantially homogeneous, striking about N140 with a dip of 50-60 degrees to the NE. The magmatic lineation is defined by the preferred alignment of euhedral feldspars. parallel to the mineral lineation observed in the host rocks. Micro- and meso-structural analyses indicate that foliation evolved from melt-present to solid-state conditions. Feldspar tiling, S/C structures. (sigma-type structures, c-axis quartz orientations commonly suggest a top-to-the-west sense of shear during magma injection and deformation under decreasing temperatures. Age of magma emplacement is constrained by U/Pb datings at 304-300 Ma, coeval with the regional metamorphic peak. Foliated granitoids and wall-rock are intruded by poorly foliated HbI-gabbro and, finally, by undeformed leucogranite, pegmatite and felsic porphyritic dykes. New U/Pb datings of the last intrusion event indicate an age of emplacement of 281 8 Ma, providing a minimum estimate for the end of the shear strain at mid-crustal level. (C) 2008 Elsevier Ltd. All rights reserved.
Abstract: The late Hercynian tectonic evolution of the Calabria crust is characterized by peak metamorphic conditions up to 800 degrees 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.
Abstract: 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. (c) 2006 Lavoisier SAS. All rights reserved.
Abstract: Hercynian basement rocks and Mesozoic ophiolites of the Calabria-Peloritani terrane drifted in the present position during the opening of western Mediterranean basins (namely Liguro-Provencal and Tyrrhenian basins) since the Oligocene. Basement rocks were partly involved by Alpine (late Cretaceous-Eocene) deformation and metamorphism before the onset of the drifting process. Even though the kinematics of the Alpine deformation in Calabria has been already defined, restoration of structural and kinematic data to the original position and orientation before the opening of the western Mediterranean has never been performed. In this work we present new structural and petrological data on a major tectonic contact of Alpine age exposed in central Calabria (Serre Massif). Structural and kinematic data are then restored at the original orientation in the early Oligocene time, to allow a correct tectonic interpretation. In the Serre Massif the Hercynian basement is sliced into three nappes emplaced during the Alpine orogeny. The upper nappe is formed by a nearly continuous section of the Hercynian crust, consisting of medium- to high-grade metamorphic rocks in the lower portion. The intermediate nappe mainly consists of orthogneisses, whereas the lower nappe is chiefly composed of phyllites. The contacts between the Alpine nappes are outlined by well developed mylonitic and cataclastic rocks. The Curinga-Girifalco Line is a well exposed shear zone that overprints mainly metapelitic rocks of the upper nappe and granitoid orthogneisses of the intermediate nappe. Mylonites of the intermediate nappe typically show overgrowths on garnet and hornblende with grossular-rich and tschermakitic composition, respectively. The Alpine mineral assemblage indicates that deformation took place in epidoteamphibolite facies at pressures ranging from 0.75 to 0.9 GPa. In the investigated area mylonites strike roughly WNW-ESE, with shallow dips towards SSW. Kinematic indicators in mylonites are mostly consistent with a top-to-the-SE shear sense in the present geographic coordinates. The mylonitic belt is affected by later extensional faults outlined by South-dipping cataclasite horizons. Published geochronological data indicate that mylonites and cataclasites developed in Eocene and early Miocene times, respectively. Considering rotational parameters coming from paleomagnetic studies and large-scale palinspastic reconstructions, the shear sense of the Curinga-Girifalco Line has been restored to the early Oligocene position and orientation. Through restoration a top-to-the-S shear sense is obtained. This result is in striking agreement with the convergence direction between Africa and W-Europe/Iberia during Eocene, computed from the North Atlantic magnetic anomalies. Our geodynamic reconstruction, combined with structural and petrological evidence, allows to relate the Curinga-Girifalco mylonites to a thrust related to the southeastern front of the double-verging Alpine chain. The adopted method could be used also for other exotic terranes, such as the Kabylie or the Corsica-Sardinia, to better constrain geometry and evolution of the southern Alpine belt. (c) 2006 Elsevier Ltd. All rights reserved.
Abstract: A pseudotachylite vein network crosscutting late Hercynian foliated tonalites can be observed along the Copanello cliffs (Calabria, Southern Italy). Pseudotachylites formed during the Oligocene-Miocene at intermediate crustal levels (ca. 10 km). They show variable thickness ranging from few mm up to 10 cm, as observed in injection veins branching from the fault plane. Microscopic observations indicate that pseudotachylite matrix mainly consists of plagioclase (An(46)-An(58)) and biotite microlites. Rounded clasts of quartz, plagioclase or of plagioclase-quartz lithic fragments are disseminated in the matrix. Intergranular, flow and spherulitic textures are commonly observed. Microstructural features are consistent with rapid crystallisation from melt. EDS analyses of rare and tiny glass veins indicated a trachyandesite or An(50) plagioclase melt composition. The conditions for pseudotachylite formation were reproduced by an analytical model taking into account the heat released by friction along a horizontal fault plane during a seismic event. The model is based on a three-stage rupture history that includes nucleation, propagation and stopping. In addition, by means of a numerical approach, the model reproduces cooling that follows the stopping stage. According to previous studies, the thermal perturbation induced by fault displacement is very intense. In fact, temperatures exceeding the tonalite and even An(50) plagioclase liquidus (1470 degrees C) are reproduced by small amount of slip (<= 6 cm) in suprahydrostatic regime. On the other hand, the thermal perturbation is strongly localised and of short duration. Peak temperatures abruptly decrease at a short distance from the fault plane (typically in few millimetres). In these conditions a thin film of melt can be produced. Therefore, the presence of cm-scale pseudotachylite veins can be only explained assuming an efficient and fast melt migration towards dilatant sites, such as pull-apart structures and injections veins. Results of the model may be useful to predict the thermal disturbance produced by earthquakes of low intensity. (c) 2005 Elsevier B.V. All rights reserved.
Abstract: By means of petrogrological, meso- and microstructural analyses, the fabric of a syn-tectonic late Hercynian K-feldspar megacryst-bearing granodiorite is described in this paper. The granodiorite was emplaced at 293 Ma within migmatitic paragneisses which had reached the regional peak metamorphic conditions at 304-300 Ma. The granodiorite and the migmatitic paragneisses are both affected by the same ductile shear zone. In the core of the shear zone, mylonites show a clear grain-size reduction and microstructures related to deformation at high to medium temperature conditions. Migmatitic paragneisses, foliated granodiorites and mylonites mostly show concordant lineation and foliation orientations. In addition, the preferred orientation of euhedral feldspars in granodiorites indicates that the fabric anisotropy started to develop in the magmatic state. These features strongly suggest that shear deformation was active during crystallisation of granitoids and continued under subsolidus conditions. In wall rocks and mylonites, kinematic indicators such as sigma- and delta-type porphyroclasts, S/C fabrics, shear bands and quartz (c) axis orientations suggest a top-to-the-W sense of shear. This is similar to the magma flow direction indicated by the tiling of euhedral feldspar megacrysts in granodiorites. Shear deformation developed, preferentially, by partitioning of strain in the granodioritic crystal mush. Geobarometry indicates that deformation took place at middle crustal levels (P=400-500 MPa). Whole rock-white mica Rb/Sr geochronological analysis of an undeformed pegmatite, crosscutting the mylonitic foliation, provided an age of 265 Ma. Timing of deformation is therefore bracketed between 293 Ma and 265 Ma.
Abstract: A Late-Hercynian intrusion of two-mica leucogranite from the Sila (Calabria, Italy) has been examined for petrographic, geochemical, and radiogenic isotope characteristics. The main objective was to analyse leucogranite genesis and its relation to partial melting of the Sila high-grade migmatitic paragneiss, which is affected by low-pressure metamorphism. The studied leucogranite is strongly peraluminous and is characterized by the presence of andalusite and sillimanite. It is very similar in major element composition to experimental melts produced by muscovite dehydration melting reaction from meta-sedimentary rocks. The leucogranite has high Rb and low Sr contents, whereas Zr and LREE concentrations are moderate to low. Variations in Sr, Rb, Zr, and LREE are inversely correlated to corresponding variations in migmatitic paragneiss. Maximum Zr and LREE contents in two leucogranite samples coherently indicate a saturation temperature of ca. 750 degrees C for both zircon and monazite. The estimate is in good agreement with peak temperatures of 740-770 degrees C obtained for high-grade rocks underlying the granite. These observations suggest a melt-restite connection between leucogranite and migmatitic paragneiss. Sr isotopic data are in agreement with this hypothesis whereas differences in the Nd isotopic composition between leucogranite and migmatitic paragneiss are attributed to disequilibrium retention of monazite in the residue. Mass-balance calculations, performed combining average major oxides analyses of leucogranite and migmatitic paragneiss, allow to reconstruct a composition similar to standard shale, except for the higher Na2O/CaO ratio.
Abstract: Thick granitoid sheets represent a considerable percentage of Palaeozoic crustal sections exposed in Calabria. High thermal gradients are recorded in upper and lower crustal regional metamorphic rocks lying at the roof and base of the granitoids. Ages of peak metamorphism and emplacement of granitoids are mostly overlapping, suggesting a connection between magma intrusion and low-pressure metamorphism. To analyse this relationship, thermal perturbation following granitoid emplacement has been modelled. The simulation indicates that, in the upper crust, the thermal perturbation is short-lived. In contrast, in the lower crust temperatures greater than 700degreesC are maintained for 12 Ma, explaining granulite formation, anatexis and the following nearly isobaric cooling. An even longer perturbation can be achieved introducing the effect of mantle lithosphere thinning into the model.
Abstract: 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 brittleductile 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.
Abstract: The thermal and exhumation history of late Hercynian granitoids from Calabria (Sila and Serre massifs) has been studied using thermobarometry and radiometric age determinations. The uplift and erosion which followed contractional tectonics of Tertiary age exposed in Calabria a nearly complete section of the Hercynian crust. Field data, constrained by igneous thermobarometrical data, have enabled us to draw simplified crustal profiles. In both the Sila and Serre massifs, granitoids make up the intermediate portions of the crustal sections and are stacked as tabular intrusions for up to 13 km cumulative thickness. Shallow granitoids are characterized by a weak fabric, mostly developed in the magmatic stage, whereas deep-seated granitoids display a strong fabric developed in the magmatic state and, with decreasing temperatures, in the subsolidus state. The intrusive bodies were emplaced at 300-290 Ma, at a time when the Calabrian crust was undergoing extensional tectonics and crustal thinning. The subsequent post-Hercynian evolution is recorded by Rb-Sr dates of micas and fission track ages of zircon and apatite obtained from granitoids emplaced at different depths. A decrease in Rb-Sr and fission track ages is observed as depth of emplacement increases. Data on the post-Hercynian geological evolution of Calabria were used to model in three stages the cooling and exhumation history of deep-seated and shallow granitoids. The first stage, in Permian to Triassic times, was characterized by slow erosion. It was followed by a second stage of extensional tectonics in Jurassic times. The third stage was exhumation during the Apenninic Orogeny. The model has generated two P-T-t arrays, one for deep-seated and the other for shallow granitoids of the Serre massif. The T-t paths. suggest that the dates of micas, zircon and apatite are cooling ages. They also show that deep-seated granitoids remained at temperatures above the brittle-plastic transition for a long time, whereas shallow granitoids cooled rapidly. Distinct P-T-t paths explain why deep-seated and shallow granitoids display different fabric and microstructural features. Copyright (C) 2000 John Wiley & Sons, Ltd.
Abstract: The Atesina Volcanic District, the Monte Luco volcanics, and the Cima dâAsta, Bressanone-Chiusa, Ivigna, Monte Croce and Monte Sabion intrusions, in the central-eastern Southern Alps, form a wide calc-alkaline association of Permian age (ca. 280-260 Ma). The magmatism originated during a period of post-orogenic extensional/transtensional faulting which controlled the magma ascent and emplacement. The magmatic products are represented by a continuum spectrum of rock types ranging from basaltic andesites to rhyolites, and from gabbros to monzogranites, with preponderance of the acidic terms. They constitute a metaluminous to weakly peraluminous series showing mineralogical, petrographic and chemical characteristics distinctive of the high-K calc-alkaline suites. In the MORB-normalized trace element diagrams, the most primitive volcanic and plutonic rocks (basaltic andesites and gabbros with Mg No. = 66 to 70; Ni = 25 to 83 ppm; Cr = 248 to 679 ppm) show LILE and LREE enriched patterns with troughs at Nb-Ta and Ti, a distinctive feature of subduction-related magmas. Field, petrographic, geochemical and isotopic evidence (initial Sr-87/Sr-86 ratios from 0.7057 to 0.7114; epsilon(Nd) values from -2.7 to -7.4; partial derivative(18)O values between 7.6 and 9.5 parts per thousand) support a hybrid nature for both volcanic and plutonic rocks, originating through complex interactions between mantle-derived magmas and crustal materials. Only the scanty andalusite-cordierite and orthopyroxene-cordierite bearing peraluminous granites in the Cima dâAsta and Bressanone-Chiusa Sr-87/Sr-86 = 0.7143-0.7167; initial epsilon(Nd) values intrusive complexes can be interpreted as purely crustal melts (initial between -7.9 and -9.6, close to average composition of the granulitic metasedimentary crust from the Ivrea Zone in the western Southern Alps). Although the Permian magmatism shows geochemical characteristics similar to those of are-related suites, palaeogeographic restorations, and geological and tectonic evidence, seem not to support any spatial and/or temporal connection with subduction processes. The magmatism is post-collisional and post-orogenic, and originated in a regime of lithospheric extension and attenuation affecting the whole domain of the European Hercynian belt. A change in the convergence direction between Gondwana and Laurasia, combined with the effects of gravitational collapse of the Hercynian chain, could have been the driving mechanism for lithosphere extension and thinning, as well as for upwelling of hot asthenosphere that caused thermal perturbation and magma generation. In the above context, the calc-alkaline affinity and the orogenic-like signature of the Permian magmatism might result from extensive contamination of basaltic magmas, likely derived from enriched lithospheric mantle source(s), with felsic crustal melts. (C) 1998 Elsevier Science B.V. All rights reserved.
Abstract: The Monte Croce pluton (MCP), in the central-eastern Southern Alps, belongs to a large calc-alkaline magmatic association of Permian age also including the Bressanone, Ivigna, Cima dâAsta and Monte Sabion granitoid intrusions, as well as the Atesina volcanics. The MCP was intruded into the low-medium grade basement of the Southern Alps, at depths corresponding to a crystallization pressure of about 3.1 +/- 0.6 kbar. Rb/Sr biotite dating yields a value of 273 +/- 3 Ma for the intrusion age. The MCP consists of medium-grained to porphyritic biotite +/- hornblende granodiorite/monzogranite (GD) containing mafic microgranular enclaves (MME) of tonalitic to granodioritic composition. Field, petrographic and mineralogical evidence strongly supports a model of extensive interaction between partially crystallized mafic and felsic magmas, with incorporation, fragmentation and dispersal (i.e. mingling +/- mixing) of the mafic component into the granitic host magma. The occurrence of mafic enclaves having distinct textures and particular compositions for the Fe-rich biotite and amphibole, suggests that at least two pulses of hybridized intermediate magmas were involved in the petrogenesis of the Monte Croce granitoids. The GD and the hosted MME are metaluminous to weakly peraluminous granitoids with high-K calc-alkaline affinity, mostly following linear trends on Harker diagrams. On chondrite-normalized spider diagrams, both GD and MME show patterns with low HFSE/(LILE, LREE) ratios and distinct Ba, Nb, Sr, P, and Ti negative spikes, similar to those of orogenic suites. Initial Sr-87/Sr-86 ratios range from 0.7086 to 0.7114 in the GD, and from 0.7080 to 0.7099 in the MME. epsilon(Ndi) values range from -6.0 to -7.6 in the GD, and from -4.9 to -7.1 in the MME. Field and petrographic evidence, as well as geochemical trends and isotopic data, are all consistent with a complex petrogenetic model involving magma mingling/mixing coupled with chemical exchange, plus local crystal fractionation. The tectonic significance of the MCP granitoids is interpreted in the context of the Permian magmatism of the Southern Alps. Geochemical features, similar to those of are-related suites, point to an orogenic signature. Palaeogeographic restorations, however, do not show evidence of active subduction processes. Furthermore, geological and tectonic evidence, as well as the timing of intrusion, indicate that the magmatism is related to late/post-orogenic lithospheric extension. Therefore, the calc-alkaline affinity and the orogenic signature of these Permian granitoids might reflect the involvement of mantle source(s) modified by previous subduction processes.
Abstract: Major, trace element composition and Sr isotopic data were collected for gabbroic rocks, plagiogranites and albitites in the ophiolite assemblage from Swat Valley (NW Frontier Province, Pakistan). Petrographic study revealed that these rocks were subjected to important structural and mineralogical modifications due to greenschist-epidote-amphibolite facies sub-sea-floor metamorphism and to brecciation. On the other hand, the examination of whole rock chemical composition and of chemical trends showed that these rocks were affected by some chemical modifications, concerning especially Na2O, K2O and Rb. The very low contents of HFS (high field strength) and RE elements found in gabbroic rocks and plagiogranites were considered to be a primary magmatic feature pointing in part to their cumulitic nature and in part to an origin from a refractory parental magma. The Sr isotopic data indicate that gabbroic rocks and plagiogranites were subjected to exchange with sea water. The particular chemical features shared by gabbroic rocks and plagiogranites suggested that fractional crystallization was a possible evolution process. In contrast, albitites are characterized by anomalously high contents in HFSE and LREE and by values of the Sr-87/Sr-86 ratio very close to sea water. These features suggest a more complex origin with respect to gabbroic rocks and plagiogranites.
Abstract: Hercynian gabbroic, dioritic and tonalitic rocks crop out in the neighbourhood of Rovale (Sila Grande, Calabria). They make up a crude rectangular outcrop with the western part consisting of gabbroic rocks and the eastern of dioritic and tonalitic rocks. They come into contact with medium to high grade metapelites on the western side and with heterogeneous granodiorites on the other sides. In the gabbroic body both opx +/- ol bearing cumulates and amphibole differentiates occur and are characterized by the widespread presence of brown pargasite. Sporadic magmatic to subsolidus corona textures between olivine and plagioclase or orthopyroxene and plagioclase can be observed and their preservation clearly suggests a post-tectonic emplacement for the gabbroic magma. Diorites and tonalites display hypidiomorphic textures free of olivine and orthopyroxene and bearing green Mg-hornblende. The granitoids, on the basis of chemical data, display orogenic features of the continent-continent collision type. The gabbroic rocks have high Al tholeiitic composition and fractionation of orthopyroxene and plagioclase played an important part in their evolution. The Rb/Sr isochron method did not give a precise emplacement age for the granitoids as a whole. Initial Sr-87/Sr-86 ratios (at 290 Ma) are higher in the gabbroic body (0.7091-0.7095) than in diorites and tonalites (0.7083-0.7092). Thus gabbroic rocks appear more displaced than diorites and tonalites towards crustal isotopic composition. The epsilonNd data seem to confirm this feature, thus suggesting that the gabbroic rocks and diorites derived from distinct mantle magma batches. Interestingly, small isotropic gabbroic masses occur within the diorites and show general features that allow them to be considered as possibly parental with respect to the host diorites. The evolution to the dioritic composition might have occurred through fractionation and minor mixing with a more acidic component such as the northern granodiorites. Geochemical, Sr and Nd isotopic data indicate a scenario of a composite plutonic body formed by distinct magma batches of mixed crust and mantle origin.
Abstract: The Capo Rasocolmo (CR) and Villa San Giovanni (VSG) granitoids, in the Southern Calabrian Arc (CLA), form post-tectonic unzoned intrusions in migmatitic paragneisses. They are strongly peraluminous (two-mica aluminosilicate-bearing), showing significant small-scale mineralogical and chemical heterogeneities with compositions ranging from leucotonalite to monzogranite. The CR and VSG granitoids are characterized by SiO2 contents varying from 67 to 75%, low mafics (TiO2 + FeO(t) + MgO < 3.9%) and high Al2O3 (A/CNK = 1.201. 40) with low Rb and high Ba contents and strongly fractionated REE patterns (La(n)/Yb(n) = 34 and Yb(n) = 3.7, on average). They show broad trends in major and trace element compositions which define linear regressions on variation diagrams. The granitoids have an ââSâ-type signature in terms of their enclaves, mineralogy and zircon typology, whereas their geochemical characteristics are similar to those of late- to post-collision granites. Initial Sr isotopic ratios (0.7094 - 0.7107) are lower than that of the regionally exposed basement rocks and similar to that of the coeval hybrid calc-alkaline granitoids; is-an-element-of NdI values range from -6.7 to -9.1. Petrographic and geochemical evidence indicate a crystal-liquid unmixing process as the dominant mechanism involved in granitoid evolution. A two-stage model was tested on the CR granitoids, involving fractional crystallization plus assimilation followed by partial solid-liquid segregation during the emplacement of the granite magma; the model accounts very well for the observed geochemical and isotopic variations. The ultimate origin of the CR and VSG granitoids investigated in the present study. A direct derivation from shale-like protoliths, suggested by field data, mineralogy, is consistent with REE patterns and experimental melting data, but is not supported by Ba-Rb decoupling and the Sr isotopic data. A comparison with the classical S-type granites of the Lachlan Fold Belt also excludes an S-type source for the CR and VSG granitoids. A direct derivation of the CR and VSG granitoids from a coeval calc-alkaline suite, through low pressure fractionation processes, is not consistent with the geochemical trends and REE patterns. An origin from a distinct calc-alkaline magma batch, involving both crustal and subcrustal components, is favoured as the most realistic petrogenetic model.
Abstract: Corona textures between olivine and plagioclase or orthopyroxene and plagioclase are present in Hercynian gabbroic rocks from the Calabrian Sila Massif They have been studied through optical and SEM investigations together with EDS and WDS analyses. Textural features indicate the existence of two extreme corona types formed during late magmatic stages or during subsolidus cooling. Magmatic coronas are characterized by an inner orthopyroxene layer and an outer orange-brown amphibole layer that might be in optical continuity with orthopyroxene and amphibole poikilites respectively. Subsolidus coronas consist of an inner layer of colourless amphibole and an outer layer of amphibole +/- spinel. They sometimes form a collar also around plagioclase enclosed in olivine. A large spectrum in the composition of corona amphiboles from Ti-bearing pargasite to Mg-hornblende was observed. The variation in Ti content of amphibole was interpreted as a consequence of the different conditions of crystallization from late magmatic to subsolidus with temperatures ranging from 880-degrees-C to 580-degrees-C. The significant gahnite component in spinel possibly indicates that subsolidus reactions occurred in an open system. The pressure of formation constrained by the mineral assemblage of metamorphic basement rocks and by the neighbouring diorites has been estimated at 4 kbar.
Abstract: In order to evaluate whether clay minerals or accessory phases control the REE pattern of the < 2-mum fraction of pelitic sediments, a set of samples from a borehole in the southern Apennines, Italy, was analysed for mineralogy and chemical composition (major elements, Rb, Sr, Y, Zr, Nb and REE). SEM analyses showed the presence of accessory phases such as Ti-oxides, apatite, zircon and pyrite. XRD procedures revealed that the < 2-mum fraction essentially consists of illite, smectite, dickite, kaolinite and chlorite. Illite and smectite probably derived by leaching from a micaceous âbiotite-likeâ mineral while dickite took origin from post-depositional, interstitial fluids. Only illite is correlated with REE, while a smectite-rich sample is characterized by the lowest content in REE. Strong correlations exist between REE and TiO2, Nb, Zr and a less significant one with Sr; no correlations exist between these elements and Y. R-mode factor analysis indicated that three factors account for 87% of the total variance. The first factor (Var = 43.2%) seems to express mainly the tendency of some resistate minerals to concentrate REE in the fine fraction. The second factor (Var = 30.0%) may be related to the effect of circulating fluids responsible for dickite genesis. The third factor may be linked to the weathering of micaceous phases. The REE do not have a significant weight in the second and third factors. Resistate minerals appear dominant in controlling, at least, the LREE and Nb. It is proposed that in the studied samples, illite likely conveyed REE via the included minerals.
Abstract: Granulite-facies rocks, of metasedimentary and metaigneous type are exposed in the Serre (Southern Calabria). They are representative of the intermediate - lower crust remaining after the Hercynian orogeny and exhumed during the Alpine orogeny. Huge masses of calc-alkaline and peraluminous granitoids intrude both the lower crust and the overlying shallower crustal levels. In this paper, we deal with the composition of the lower crust and its relationship with granitoids along the Serre section, on the basis of major, minor, trace elements, REE and Sr, Pb, Nd isotope systematics. The metapelites, which make up a large part of the lower crust section, have a restite character due to the extraction both of in situ crystallized leucosomes and of some of the late-Hercynian peraluminous granites which intrude the upper crust. The dominant calc-alkaline granitoids seem to require the contribution of hydrated basic lower crust and/or mantle-derived magmas for their genesis. As a result it appears that the melting processes which affected the lower crust about 300 Ma ago, at the same time as decompression, produced a crustal differentiation contributing to the generation of granitoids which migrated toward high crustal levels. The occurrence of some gabbroic rocks and microgranular mafic enclaves associated with the calc-alkaline plutonic rocks indicates that the mantle contributed both thermally and chemically to granitoid genesis. Nd model ages in granitoids (1.0 - 2.4 Ga) indicate significant addition of juvenile material in late Proterozoic - early Palaeozoic times.
Abstract: Major, trace element and isotopic data are reported for Hercynian granodiorites and tonalites from Capo Vaticano, Calabria, with the aim of clarifying the relationships between acidic and intermediate lithologies in orogenic intrusive suites. These granodiorites (SiO2 = 70.5%-73.5%) are peraluminous, relatively depleted in Rb, U, Th, and Pb, and display a large variation in many geochemical and isotopic parameters. REE show fractionated, cross-cutting chondrite-normalized patterns with small but significant negative Eu anomalies. Initial (290 Ma) Sr and Nd, and present-day Pb isotope ratios are: (Sr-87/Sr-86)290 = 0.7105-0.7110; (Nd-143/Nd-144)290 = 0.51 175-0.51221; Pb-206/Pb-204 = 18.35-18.50; Pb-207/Pb-204 = 15.64-15.76; Pb-208/Pb-204 = 38.51-39.03. Pb isotope ratios determined for separated feldspars show similar Pb-206/Pb-204 and Pb-208/Pb-204 and higher Pb-207/Pb-204 ratios compared to the present-day values for the whole rock. The tonalites (SiO2 = 57.6%-67.3%) are typically calc-alkaline in composition and most of them are weakly peraluminous. They display large heterogeneities in trace-element abundances, REE patterns and Nd and Pb isotope ratios. both between and within individual intrusive units. Sr. Nd and Pb isotopic compositions display a range of values close to that of the associated granodiorites (Sr-87/Sr-86)290 = 0.7099-0.7111; (Nd-143/Nd-144)290 = 0.51185-0.51224; Pb-206/Pb-204 = 18.31-18.56; Pb-207/Pb-204 = 15.65-15.72, Pb-208/Pb-204 = 38.41-39.59. The obtained data indicate that the tonalites and granodiorites share many compositional features which point to a genetic linkage. Likewise, many petrological, geochemical and field data exclude the possibility that all the granodiorites derive from intermediate magmas by any common evolutionary process, such as crystal-liquid fractionation or assimilation-fractional crystallization (AFC), and point to a different genesis for the two rock types. An origin of the granodiorites by the mixing of two separate acidic melts, one of crustal anatectic origin and one probably derived from an intermediate magma by crystal/liquid fractionation or AFC, is suggested by the data presented. The tonalites appear to have been generated by interaction of a mafic magma with crustal end-member(s) with isotopic and geochemical signatures similar to those of the granodiorites. The large involvement of crustal material in the genesis of the Capo Vaticano granitoids is responsible for the many common compositional characteristics observed in the intermediate and acidic rocks. However, a contribution from mantle components is also indicated by petrological and geochemical data, as well as by the occurrence of mafic enclaves commonly present in the tonalites. Nevertheless, the geochemical and isotopic features of such a mantle component appear to be greatly diluted by a large quantity of crustal material involved in the genesis of the studied granitoids.
