Main Foliation Research Articles

Depositional Setting and Structural Evolution of the Archean Perseverance Volcanogenic Massive Sulfide Deposit, Matagami Mining District, Quebec, Canada

The Perseverance volcanogenic massive sulfide deposit (5.1 Mt at @ 15.8% Zn, 1.24% Cu, 29.4 g/t Ag, 0.4 g/t Au), Matagami district, Abitibi greenstone belt, consists of four pipelike orebodies discordant to local bedding in a shallow-dipping portion of the South Flank volcanic succession. Most of the ore is hosted by rhyolitic lavas of the Watson unit (2725.9 ± 0.8 Ma) and is overlain by a thinly laminated tuffaceous unit known as the Key Tuffite, as well as by rhyodacitic lavas of the Dumagami unit (2725.4 ± 0.7 Ma). The presence of sulfide zones predominantly hosted in the footwall rhyolite, relicts of Key Tuffite within the ore, and intricate sulfide replacement of the laminated tuff are all features consistent with the formation of most of the deposit via subseafloor replacement. The permeability of steep synvolcanic structures likely controlled the migration of metal-bearing fluids and simultaneously allowed the downward infiltration of seawater, causing mixing, cooling, and sulfide deposition in the subseafloor environment. Stratigraphic relationships suggest that massive sulfide formation was active at Perseverance while tuffaceous sedimentation was ongoing on the seafloor. Progressive growth of the massive sulfides along subvertical structures was accompanied by the development of pipelike sericite-chlorite (± talc) alteration halos. Both the hanging wall and the footwall units are characterized by significant MgO mass gains, K 2 O-Na 2 O mass losses, and high alteration index values (e.g., chlorite-carbonate-pyrite index, alteration index). However, these geochemical changes are less intense and widespread in the hanging wall than in the footwall. The record of a decreasing alteration above the orebodies was likely caused by the emplacement of the rhyodacite while massive sulfide deposition and hydrothermal activity were still ongoing. Primary relationships between mineralization and the host lithofacies were overprinted by intense deformation during the main event of compression induced by regional north-south shortening. Deformation is controlled by contrasting rheologies within the sulfide assemblage and between the orebodies and their host rocks. Structural modifications include the transposition of sulfide ores subparallel to the main foliation, the formation of piercement veins at the interface with the host rocks, and the generation of a series of secondary textures within the orebodies, including a vertical mineralogical banding. The distribution of Cu in the deposit is interpreted to be affected by the mechanical remobilization of chalcopyrite during deformation, whereas the zonation of Zn is likely primary. Hydrothermal alteration halos adjacent to the orebodies have accommodated significant strain, as indicated by the presence of tight folds in the Key Tuffite unit and a marked schistosity. Strain localization in the vicinity of the deposit highlights the relative ductility of the ore assemblage with respect to the host volcanic succession and resulted in a spatial association between hydrothermal alteration and deformation, despite the lack of a genetic relationship.

Petrographic, mineral and pressure–temperature constraints on phyllites from the Variscan basement at Punta Bianca, Northern Apennines, Italy

We present a petrological study of phyllites from the Variscan basement at Punta Bianca (Northern Apennines). According to the literature the studied rocks are believed to be the result of Alpine and Variscan metamorphism of pre-Carboniferous pelites, which consist of Fe-chlorite, potassic white mica, quartz, minor paragonite, and accessory hematite, rutile, monazite, zircon, xenotime, and florencite. Microstructural observations indicate a main S2 foliation that had overprinted an earlier S1 foliation. The mineral assemblage can be related to both S2 and S1.Large flakes of relatively Mg-rich potassic white-mica along the two foliations are probably detrital grains that preserved their composition. The calculation of isochemical phase diagrams, their contouring by various mineral chemical parameters and the deformation microstructures of quartz indicate intermediate pressure-low temperature conditions (5-7 kbar and 300-400 °C). The anhedral habit and the very low Pb contents (< 50 ppm) in monazite point to dissolution of this mineral probably during Tertiary times.

40Ar–39Ar laser dating of ductile shear zones from central Corsica (France): Evidence of Alpine (middle to late Eocene) syn-burial shearing in Variscan granitoids

The island of Corsica (France) plays a central role in any reconstruction of Western Mediterranean geodynamics and paleogeography but several key aspects of its geological evolution are still uncertain. The most debated topics include the interpretation of the Corsican orogen as the result of an east- or west-directed subduction, and the actual involvement of the Variscan basement of Corsica in the Alpine orogenic cycle. This study integrates 40Ar–39Ar laserprobe, mesostructural, microtextural, and microchemical analyses and places relevant constraints on the style, P–T conditions, and timing of Alpine-age, pervasive ductile shear zones which affected the Variscan basement complex of central Corsica, a few kilometers to the west of the present-day front of the Alpine nappes. Shear zones strike ~NNE–SSW, dip at a high angle, and are characterized by a dominant sinistral strike-slip component. Two of the three investigated shear zones contain two texturally and chemically resolvable generations of white mica, recording a prograde (burial) evolution: (1) deformed celadonite-poor relicts are finely overgrown by (2) a celadonite-rich white mica aligned along the main foliation. White mica from a third sample of another shear zone, characterized by a significantly lower porphyroclast/matrix ratio, exhibits a nearly uniform high-celadonite content, compositionally matching the texturally younger phengite from the nearby shear zones. Mineral-textural analysis, electron microprobe data, and pseudosection modeling constrain P–T conditions attained during shearing at ~300°C and minimum pressures of ~0.6GPa. In-situ 40Ar–39Ar analyses of coexisting low- and high-celadonite white micas from both shear zones yielded a relatively wide range of ages, ~45–36Ma. Laser step-heating experiments gave sigmoidal-shaped age profiles, with step ages in line with in-situ spot dates. By contrast, the apparently chemically homogenous high-celadonite white mica yielded concordant in-situ ages at ~34Ma, but a hump-shaped age spectrum, with maximum ages of ~35Ma and intermediate- to high-temperature steps as young as ~33–32Ma. Results indicate that the studied samples consist of an earlier celadonite-poor white mica with a minimum age of ~46Ma, overgrown by a synshear high-celadonite white mica, developed at greater depth between ~37 and 35Ma; faint late increments in shearing occurred at ≤33–32Ma, when white mica incipiently re-equilibrated during exhumation. Results suggest that ductile shearing with a dominant strike-slip component pervasively deformed the Corsican basement complex during the emplacement and progressive thickening of the Alpine orogenic wedge and broaden the extent of the domain affected by the Alpine tectonometamorphic events. Integration of petrological modeling and geochronological data shows that the Variscan basement of central Corsica, close to the Alpine nappes, was buried during the late Eocene by ≥18km of Alpine orogenic wedge and foreland deposits. Our results, combined with previously published apatite fission-track data, imply an overburden removal ≥15km from the late Eocene (Priabonian) to the early Miocene (Aquitanian), pointing to a minimum average exhumation rate of 1.3–1.5mm/a.

Origin and evolution of the granitic intrusions in the Brusque Group of the Dom Feliciano Belt, south Brazil: Petrostructural analysis and whole-rock/isotope geochemistry

In the southern Brazilian state of Santa Catarina the Dom Feliciano Belt, formed by the tectonic juxtaposition of different crustal blocks during the Brasiliano-Pan African Orogenic cycle, can be divided into three domains. In the central domain, three granitic suites intrude the metavolcanosedimentary sequence of the Brusque Group: São João Batista (SJBS), Valsungana (VS) and Nova Trento (NTS), from the oldest to the youngest. This extensive magmatism, here referred to as granitic intrusions in the Brusqe Group (GIBG), is coeval with the thermal peak in the host metamorphic successions, but postdates its main foliation. A progressive deformation starting from the magmatic stage throughout the cooling history points to the influence of the late stages of deformation recorded in the Brusque Group.The SJBS consists of gray to white leucocratic, equigranular granites, with aluminous minerals such as muscovite, garnet and tourmaline. The porphyritic VS is the largest of the suites and is characterized by its cm-sized K-feldspar megacrysts in a coarse-grained biotite-rich matrix. The granites from the NTS are equigranular, light gray to pink in color and have biotite as the main mafic mineral, but magmatic muscovite, tourmaline and hornblende can occur as well.Geochemically, the GIBG are mildly peraluminous and show a calc-alkaline affinity. Most intrusions have a high REE fractionation, but some SJBS granites show a characteristic pattern with no fractionation and strong negative Eu anomalies (“seagull pattern”). Elevated Sr(i) values, between 0.707 and 0.735, and negative εNd values as low as −24 points to the melting of old evolved crust. The Nd (TDM) ages are scattered between 1.54 and 2.76 Ga, with a predominance of values around 2.0 Ga.The GIBG have a strong crustal signature that most closely connects, within the regional units, to that of the metasedimentary rocks of the Brusque Group and its crystalline basement, the Camboriú Complex. All three suites seem to have been produced during a same regional melting event, but at different crustal levels and reflecting heterogeneities within the same source rocks. Most evidences imply that sedimentary source rocks were especially important to the SJBS, which probably originated in a shallower environment, whilst the VS and NTS represent the melting of deeper crystalline crust, probably sharing some magmatic interaction.

Insights on high-grade deformation in quartzo-feldspathic gneisses during the early Variscan exhumation of the Cabo Ortegal nappe, NW Iberia

Abstract. High-grade, highly deformed gneisses crop out continuously along the Masanteo peninsula and constitute the upper part of the lower crustal section in the Cabo Ortegal nappe (NW Spain). The rock sequence formed by migmatitic quartzo-feldspathic (qz-fsp) gneisses and mafic rocks records the early Ordovician (ca. 480–488 Ma) injection of felsic dioritic/granodioritic dykes at the base of the qz-fsp gneisses, and Devonian eclogitization (ca. 390.4 ± 1.2 Ma), prior to its exhumation. A SE-vergent ductile thrust constitutes the base of quartzo-feldspathic gneissic unit, incorporating mafic eclogite blocks within migmatitic gneisses. A NW-vergent detachment displaced metasedimentary qz-fsp gneisses over the migmatites. A difference in metamorphic pressure of ca. 0.5 GPa is estimated between both gneissic units. The tectono-metamorphic relationships of the basal ductile thrust and the normal detachment bounding the top of the migmatites indicate that both discrete mechanical contacts were active before the recumbent folding affecting the sequence of gneisses during their final emplacement. The progressive tectonic exhumation from eclogite to greenschist facies conditions occurred over ca. 10 Ma and involved bulk thinning of the high-grade rock sequence in the high pressure and high temperature (HP–HT) Cabo Ortegal nappe. The necessary strain was accommodated by the development of a widespread main foliation, dominated by flattening, that subsequently localized to a network of anastomosing shear bands that evolved to planar shear zones. Qz-fsp gneisses and neighbouring mafic granulites were exhumed at &gt; 3 mm yr−1, and the exhumation path involved a cooling of ∼ 20 °C/100 MPa, These figures are comparable to currently active subduction zones, although exhumation P–T trajectory and ascent rates are at the hotter and slower end in comparison with currently active similar settings, suggesting an extremely ductile deformation environment during the exhumation of qz-fsp gneisses within a coherent Cabo Ortegal nappe.

The Pampe gold deposit (Ghana): Constraints on sulfide evolution during gold mineralization

The Ashanti belt, a world-renowned gold producing region in southwest Ghana, has received renewed attention in recent years. Most studies, however, focused on the major deposits situated along the Ashanti shear zone and in the adjacent Tarkwa Basin to the east, neglecting smaller yet important occurrences, such as the Pampe deposit which occurs few kilometers to the west of this shear zone, on the Akropong Trend. Nevertheless, investigating such simpler smaller-scale mineralizing systems is attractive, in that this can help shedding light on the processes that control gold deposition at the regional scale.At Pampe, gold endowment has been estimated at approximately half a million ounces with an average gold grade of 2.8g/t. The mineralization is related to two sets of quartz veins; a first set (V1), which has a NE trend and is sub-parallel to the main foliation (S1), and a second set (V2), which crosscuts this foliation. The V2 veins have a NNW–SSE trend with local conjugate geometries indicating that they formed during NNW–SSE shortening, which, regionally, is linked to major orogenic gold deposition. Gold mineralization is systematically associated with sulfides, which occur disseminated in the vein walls and in the surrounding host rocks. The ore sulfide paragenesis consists of 1) a first generation of pyrite, which is associated with V1 veins; 2) a second generation of sulfides, consisting of an intergrowth of arsenopyrite and pyrite that crystallized contemporaneously with the formation of the second vein set; 3) a late phase of pyrite growth which occurs as overgrowths on phase-2 sulfides and formed during the waning stages of V2 emplacement. Invisible (sub-microscopic) gold was detected in all sulfide generations by LA-ICP-MS. The analytical profiles for the Au signal are mimicked by those for Pb, Cu, As, Ag, Te, and Bi. Invisible gold is thus interpreted to have precipitated within sulfides in the form of nanoparticles (colloidal gold alloys). Conversely, visible native gold grains were recognized exclusively in association with arsenopyrite from late V2 veins, either as inclusions or, more commonly, at the boundary with other sulfides, as well as in micro-fractures that crosscut the sulfides. Gold precipitation was likely induced by sulfidation of the wall rock during fluid–rock interaction. The Pampe deposit exemplifies the mineralization processes that took place at larger scale in neighboring world-class deposits such as Obuasi, Bogoso and Prestea.

Water-present melting in the middle crust: The case of the Ollo de Sapo gneiss in the Iberian Massif (Spain)

The Variscan migmatitic region of Sanabria, in the inner zone of the Iberian Massif (Spain), provides a unique place to study the geochemistry and rheology of anatectic melts formed by water-present conditions. In Sanabria, the Ollo de Sapo gneiss underwent partial melting at 0.5–0.66GPa and 690–770°C. Leucosomes, leucogranitic bodies within the migmatite and leucogranites are widespread throughout the migmatite, with the leucogranitic bodies being concordant to discordant to the foliation. Mass balance calculations indicate that the partial melting of the gneiss produced 62.5wt.% of melt. Experiments and pseudosections made at the peak metamorphic P–T conditions under fluid-absent conditions yield much lower volumes of melt and therefore a fluid-present scenario is proposed. Major and trace element composition and structures of leucosomes and leucogranites support this scenario. Trace element composition of leucosomes is variable, with low to high REE, Zr, Y and Th content and positive to negative Eu anomalies. The composition of the most felsic, REE-depleted, Eu-rich leucosomes is closely similar to that of experimental melts with added water at T<800°C, implying both that these leucosomes represent pure melts and that partial melting took place under water-present conditions. A comparison between the composition of the most felsic leucosomes and melts modelled in pseudosections suggests that 4wt.% free water was present during partial melting, an amount of water that agrees with that needed to produce the calculated melt volume. The variability in the trace element composition of leucosomes and leucogranites is explained by the entrainment of residual phases, mainly biotite with inclusions of apatite and zircon. The source of the water is unknown and two candidates are proposed: water released from crystallization of mafic magmas emplaced in the migmatite and water fluxed from subhorizontal shear zones, with the latter option very likely being the main source. Migration of melt through the migmatite is supported by the location of leucosome bodies in strain shadows and in veins parallel to the main foliation and by mass balance calculations, which show that migmatites at the present outcrop level lost between one fourth and one half of the total amount of melt produced. The remaining melt is present as leucogranitic bodies within the migmatite and as the Hermisende–Padornelo leucogranitic pluton, a residuum-rich granitoid outcropping at the southern boundary of the migmatites that shows a continuous gradation from the migmatite to diatexites and the diatexitic granite. Leucosomes represent 37–55wt.% of the migmatites, very close to the second critical rheological boundary, which is interpreted as an upper limit of melt retention: when the local melt amount exceeded this rheological boundary, the stromatic structure broke and melt coalesced to form the residuum-bearing leucogranites found within the migmatite. The geology of the migmatites at Sanabria supports a model in which melts formed under water-present conditions are cool, leucocratic and REE-poor, that their geochemistry is dominated by entrainment of variable amounts of residuum and that the magmas are mobile only in the vicinity of the source.

Constraining P–T conditions during thrusting of a higher pressure unit over a lower pressure one (Gran Paradiso, Western Alps)

AbstractIdentifying higher pressure units overlying lower pressure ones is a first order argument to determine the presence of large‐scale thrusting. For the first time, petrology is used to quantify the pressure difference between two stacked units in the Western Alps. In the Gran Paradiso Massif, the Money unit crops out as a tectonic window below the Gran Paradiso unit. The reconstruction of the Alpine evolution of these two units and the history of their tectonic contact has been achieved using a multidisciplinary approach that combines meso‐ and microstructural analysis and pseudosection calculations. In both units, four stages of deformation and metamorphism have been identified. Stage 1 reflects the phase of continental crust subduction and P–T conditions of ~18–20 kbar, 480–520 °C and of ~13–18 kbar, 500–530 °C have been estimated for the Gran Paradiso and the Money units respectively. This yields a maximum difference of ~20 km in the depth reached by these two units during the early Alpine history. Thrusting of the Gran Paradiso unit over the Money unit (stage 2) led to the development of the main foliation and occurred in the high‐P part of the albite stability field at P–T conditions of ~12.5–14.5 kbar and 530–560 °C, identical in both units. The thrust contact was folded during stage 3 together with the entire Money unit, and then both units were exhumed together (stage 4). During this polyphase evolution, detrital garnet has been partially dissolved, while the earliest Na‐bearing phases (glaucophane, paragonite) have been overprinted by the low‐P mineral associations. The uncertainties on derived pressures between the two units are unfortunately larger than hoped, and this is attributed to the muscovite solid‐solution model not incorporating a pyrophyllite component.

Pan-African deformation markers in the migmatitic complexes of Parakou–Nikki (Northeast Benin)

The analysis of structural markers collected from the petrostrutural units of Parakou-Nikki, in Northeast Benin, indicates that the Pan-African remobilization or rejuvenation (600 ± 50 Ma) of the Benino-Nigerian Metacraton consits of a wide migmatization accompanied by five tectogenetic phases (Dn–Dn+4). The Dn phase is penecontemporaneous with the collision between the Benino-Nigerian shield and the oriental margin of the West-African Craton. This phase is associated with the granulitization or migmatization episode, and materialized by the Sn foliation on which the Dn+1 deformation markers (major Pan-African structuring phase in the Dahomeyide orogenic belt) are superimposed. Thus, the Sn+1 plane and the Ln+1 lineations indicate a perfect transposition of the Sn plane and a syn-Dn paragenesis remobilization in the syn-Dn+1 amphibolite facies retromorphosis. The strong resumption of the main foliation (Sn+1) corresponds to the Dn+2 deformation phase. The latter, associated with a second retromorphosis, is materialized by Pn+2 folds, with submeridian to NE–SW axes, and by a Sn+2 foliation or strain-slip cleavage. It is also the phase of emplacement of the Kandi mega-shear zone associated with the development of Cn+2 dextral shear plane very generalized over all the petrostructural units of Parakou–Nikki. At a large scale, the Dn+3 phase is expressed as Pn+3 kilometric synform and antiform structures with NE–SW to ENE–WSW trending axes. This Dn+3 phase can be considered as a transpression responsible for the development of the Cn+3 sinistral shear plane and flat shear structures displaying a westward overlapping. It ends with a fracturing episode materialized by strike-slip fault systems defining a main stress σ1 trending ESE–WNW to SE–NW. The late Dn+4 phase only corresponds to conjugated strike-slip faults which resulted from a major SE–NW to SSE–NNW compression.

Petrografia e geoquímica das rochas metamáficas intercaladas em gnaisses do Arco Magmático de Goiás na região de Indiara (GO)

In Indiara region (GO), dozens of metamafic rocks lenses of various dimensions (metric to kilometric) occur, intercalated in gneisses of the Goias Magmatic Arc and oriented according to the direction of the main foliation (Sn). These lenses consist of amphibolite (amphibole schists and amphibolites), have chemical compositions of sub-alkaline basalts to andesitic basalts and sub-alkaline tholeiitic affinity. The distribution of major and trace elements, as well as rare earth elements (REE), suggests that these rocks are derived from a source with similar geochemical signature. The analyzed rocks are enriched in large ion litophile elements (Cs, Rb, Ba, K, Th and U) in relation to elements of high field strength (Nb, Ta, Zr, Hf and Y) and in relation to REE, Sr, P and Ti, showing weak to medium negative anomalies of Nb, Ta and P and suggesting that these rocks were generated in a magmatic arc environment.

Numerical modeling of failure mechanisms in phyllite mine slopes in Brazil

This paper presents three case studies comprising failure mechanisms in phyllite mine slopes at Quadrilátero Ferrífero, State of Minas Gerais, Brazil. Numerical modeling techniques were used in this study. Failure mechanisms involving discontinuities sub parallel to the main foliation are very common in these mines. Besides, failure through the rock material has also been observed due to the low strength of phyllites in this site. Results of this work permitted to establish unknown geotechnical parameters which have significant influence in failure processes, like the in situ stress field and the discontinuity stiffness.

Geochemistry and petrology of metamorphosed submarine basic ashes in the Edough Massif (Cap de Garde, Annaba, northeastern Algeria)

The study presents the first evidence of metamorphosed submarine ashes in the Edough Massif, in northeastern Algeria. It occurs below the greenschist-facies Tellian units that represent the thrusted Mesozoic to Eocene passive paleomargin of northern Africa deposited on thinned continental crust. The metamorphic complex consists of tectonically superposed units composed of gneisses (lower unit) and micaschists (upper unit). At the Cap de Garde, these units enclose an “intermediate unit” composed of micaschists and meter-thick layers of marbles, which are sometimes intercalated with amphibolites. The latter occur as discontinuous small lenses and layers. The amphibolites are parallel to the primary bedding of the marbles and the main foliation. Chemical markers and field observations indicate that they are metamorphic equivalents of basic igneous rocks. The lenticular character, low thickness and multiple intercalations with marine sediments and the unusual high lithium concentrations suggest subaqueous near-source basaltic ash-fall deposits in a marine environment.

From Mesoproterozoic magmatism to collisional Cretaceous anatexis: Tectonomagmatic history of the Pelagonian Zone, Greece

AbstractThe magmatic history of the Pelagonian Zone, in northern Greece, is constrained with secondary ion mass spectrometer (SIMS) U‐Pb dating on zircons of various granitoids whose structural positions were defined with respect to the regional main foliation. Ages pertain to four groups: (i) Mesoproterozoic (circa 1430 Ma) crystallization of granites inferred from inherited magmatic zircon cores that have been partially molten during the (ii) Neoproterozoic at circa 685 Ma (metamorphic zircon rims) and subsequently intruded by a Neoproterozoic leucogranite (circa 600 Ma). (iii) Late‐ or post‐Variscan calc‐alkaline granitoids (315–301 Ma) were in turn intruded by a subvolcanic dike at about 280 Ma. In the Early Permian the εNd(t) in magmas decreased from −7.3 to −1.3, hinting to mantle‐derived melts produced during extension. Rifting is further heralded by two acidic and one mafic dike containing Lower‐Middle Triassic zircons (246–242 Ma). (iv) Early Cretaceous anatectic melts at 117 ± 8 Ma formed during regional metamorphism. This age is the first report of in situ anatexis in the Pelagonian Zone. Cretaceous anatexis developed during the Mesozoic collision of Pelagonia with the Eurasian margin. Major‐ and trace‐element geochemistry of amphibolites further attests for the complex pre‐Alpine tectonic history with Neoproterozoic calc‐alkaline and back‐arc geochemical signature and Triassic alkali‐magmatism.

Two types of antigorite serpentinite controlling heterogeneous slow-slip behaviours of slab–mantle interface

It is known that plate boundaries in subduction zones show heterogeneous slip nature with strongly coupled seismogenic zones and various types of episodic tremor and slip (ETS) zones. In order to examine the petrological controls on the large-scale structure, we compared recent geophysical observations in the Shikoku area, southwest Japan with petrological models of the hanging wall mantle wedge. As a result, we found a close relationship between mineral assemblages in the mantle wedge and the characteristics of slow slip behaviour recorded in the Shikoku area: Short-term ETSs take place in the antigorite + olivine stability field and silent long-term slow slip events (SSEs) take place in the antigorite+brucite stability field. Based on observations of natural antigorite serpentinites, we propose a model that the dominant serpentinization reaction in the mantle wedge changes with increasing depth resulting in variable extents of pore fluid pressures along slip planes. The serpentinization reaction in the antigorite+brucite stability field (olivine + H2O → antigorite + brucite) proceeds at the expense of water. This is consistent with moderately elevated pore pressures inferred for long-term SSEs. The existence of weak brucite enhances the development of shear zones oblique to the main foliation. The resultant anastomosing network provides fluid pathways that may help reduce pore pressures on slip planes. In contrast, progress of the serpentinization reaction in the antigorite + olivine stability field (olivine + H2O + SiO2 → antigorite) results in a large amount of residual water that contributes to further increase pore fluid pressures on slip planes of short-term SSEs. Our results imply that understanding of serpentinization reactions and their contributions to fluid networks in mantle wedge is important in constructing quantitative 3-D models for strain evolutions along plate boundaries.

Structurally-controlled Melt Accumulation: Himalayan Migmatites and Related Deformation – Dhauli Ganga Valley, Garhwal Himalaya

Abstract In the uppermost parts of the Higher Himalayan Crystallines (HHC) of the Great Himalaya, widespread in situ partial melting of sillimanite+K-feldspar gneiss resulted in the formation of migmatite and resultant melt accumulation near the South Tibetan Detachment System (STDS) during various deformation events along the Dhauli Ganga valley in Garhwal. The oldest migmatite phase, designated as the Me1, parallels the main foliation Sm as the stromatite layers and concordant leucogranite bands. Younger melt phases Me2, Me3 and Me5 are recorded along small-scale ductile thrusts, extensional fabric and structureless patches, respectively. It is only the Me4 melting phase that is evidenced by large-scale melt migration along cross-cutting irregular veins. These were possible conduits for migration and accumulation of melt into larger leucogranite bodies like the Malari granite (19.0± 0.5 Ma).

Structural control and geological significance of the Zn–Pb ores formed in the Benasque Pass area (Central Pyrenees) during the post-late Ordovician extensional event of the Gondwana margin

The Benasque Pass area (Central Pyrenees) is located in the eastern part of the Variscan Lys-Caillaouas Massif, where Cambro-Ordovician rocks of the Hesperian massif crop out. In the area several Zn–Pb deposits are located in Variscan structures. Lead isotopes indicate a late Ordovician–early Devonian model age for the deposits. In the region, an unconformity between pre-late Ordovician and late Ordovician rocks has been defined by several authors and has been identified in the study area. The first structures observed in the Benasque Pass area consist of a pre-Variscan cleavage, which only affects the pre-late Ordovician rocks. In the study area, the main structures correspond to two Variscan generations: (i) E–W trending, north-verging folds, which are associated with the main foliation S1 (D1); (ii) E–W trending upright folds and associated small south-directed thrusts (D2). D2 structures are the ones hosting the zinc–lead ore. In the Benasque Pass area, D1 and D2 Variscan structures have developed under low grade metamorphic conditions. Some faults observed in the north and the south of the study area may have Alpine movement. In the Benasque Pass area, the strongly silicified and brecciated zinc–lead ore is mainly developed in the D1 short limb of the Mina Peak Syncline, within which S1 foliation is intensely folded and cross-thrust by D2 structures. Taking into account their model age, all these zinc–lead deposits seem to be stratiform, stratabound or filling open spaces in origin and related to the pre-Variscan extension described for the Central Pyrenees. However, the zinc–lead ore is hosted in strongly silicified and brecciated Variscan structures.The δ34S isotopic composition of sulfides indicates that the metamorphism and deformation did not completely obliterate the original isotopic composition, suggesting that sulfur in the Benasque Pass has mainly come from the reduction of marine sulfate extracted from the rocks of the metasedimentary sequence. In the Benasque Pass area, the lead isotope ratios are homogeneous and have a pre-Variscan model age, which is also the age of the host rocks. In the case of the Benasque Pass, the deposits occurred in Variscan structures, making it impossible to recognize their original pre-Variscan characteristics indicated by lead isotopes model age.The correlation between the Pb isotope data for deposits from the Benasque Pass and Aran Valley, also located in the Central Pyrenees, and those for other deposits from the Southern European Gondwana is evident and the isotopic data set closely fits the two-stage Pb evolution model for Sardinia terrane. The coherence of the Pb isotope trends in all the deposits suggests that Pb was extracted at various times from sources with similar time-averaged U/Pb, Th/Pb and Th/U values, through a process of leaching of the basement rocks by large volumes of hydrothermal fluids. In the case of the Pyrenees (Central and Eastern), the lead and other metals present in the deposit were extracted by hydrothermal fluids from the upper crust rocks during the post-late Ordovician extensional event of the Gondwana margin.

Granite emplacement by crustal boudinage: example of the Calmayo and El Hongo plutons (Córdoba, Argentina)

AbstractThis study deals with the structure and emplacement of the Calmayo and El Hongo trondhjemite plutons (Famatinian belt of Córdoba, Argentina). It provides structural data from the granites and the country rocks and a study of the magnetic fabric in the plutons. New U/Pb geochronological data yield intrusion ages of 512.1 ± 3.4 Ma and 500.6 ± 4.5 Ma for the Calmayo and El Hongo plutons respectively. The El Hongo massif and the southern part of the Calmayo trondhjemite preserve magmatic structures, whereas the northern domain of Calmayo shows the imprint of solid‐state deformation. The main foliation in the country rocks outlines a boudin‐like pattern at the map scale and the granites are located along boudin necks, suggesting that the emplacement of these trondhjemite plutons was linked to large‐scale boudinage of the country rocks.

Long-lived LREE mobility in the cratonic, rift and foredeep to foreland sedimentary cover at the western margin of the Karelia Province

The western margin of the Karelia Province of the Fennoscandian Shield consists of Archean crust covered by remnants of Paleoproterozoic (c. 2.5–1.9Ga) metasedimentary rocks. The study area in the eastern part of the margin shows 2.3–2.1Ga cratonic sedimentation followed by 2.1–2.05Ga rift sedimentation and c. 1.91–1.92Ga foredeep to foreland sedimentation related to continent–arc/continent collision. Extreme LREE mobility (loss up >95% of La) is recorded by many samples from these units. Monazite, bastnäsite and, in some cases, allanite have been the prealteration LREE host phases in the LREE depleted samples. Monazite was altered to Th-silicates with variable loss of the LREE, the remaining part of which was incorporated in Th-poor monazite.The depositional age of the sedimentary units was studied by dating (LA-MC-ICPMS) detrital zircon grains, and the age of LREE mobility by dating monazite and xenotime and by whole-rock Sm–Nd modeling. The maximum deposition age of the foredeep to foreland basin is 1.91–1.92Ga based on detrital zircon data and the minimum age of the main foliation in the study area is 1.87–1.86Ga based on the younger age limit of late-to post-tectonic xenotime and the ages of few probable metamorphic zircons. Coeval bastnäsite, with xenotime in a LREE enriched sample, suggests that this was also one important stage of LREE mobility. Monazite crystallizations have occurred 1.76–1.78Ga ago while the Rb–Sr whole rock data suggest that the closure of this isotope system took place 1732±21Ma ago. The Sm–Nd modeling gives ages at 0.4–1.0Ga for LREE depletion. The Sm–Nd age of 412±27Ma can be considered as the age of the latest major LREE depletion event.We propose that alkali-bearing oxidizing fluids started to form in the cratonic sequences due to diagenetic reactions maybe as early as 2.3–2.1Ga. Alkalinity of fluids was later increased by interaction with alkaline 2.05Ga source materials before and during the c. 1.91–1.92Ga foredeep to foreland basin stage. Basin inversion and metamorphic crystallization occurred at 1.91–1.87Ga. Episodes of fluid migration at 1.78–1.73Ga and afterwards were focused along fracture zones and in microfracture networks. The 0.4Ga stage is the most prominent of the <1.7Ga events and is best explained by assuming shield-scale foreland basin related to the Caledonian orogenic front, causing heat redistribution and also fluid circulation in the underlying crystalline basement. The 2.1Ga formed marginal fault has been a very important shield-scale tectonic element and a pathway for fluid circulation for almost 2billion years.

Large volumes of anatectic melt retained in granulite facies migmatites: An injection complex in northern Quebec

The Opinaca Subprovince in the Superior Province of northern Québec is a metasedimentary belt characterized by metagreywacke rocks that record a late Archean granulite facies metamorphism and contains abundant thin veins of leucogranite. Regional-scale study of this subprovince provides insight to the net budget of anatectic melt (loss versus accumulation) in the lower middle continental crust during anatexis.A petrological study indicates that the metagreywackes partially melted at ~820°C and ~7kbar and generated <10% melt with orthopyroxene±garnet as peritectic phases. The metagreywackes contain various amounts of leucogranite in thin veins, sills and dykes injected sub-parallel to the subvertical E–W striking main foliation. Study of more than 1070 outcrops indicates that the “typical outcrop” contains ~63% of leucogranite; far more than the maximum 10% of melt produced in-situ from the host rocks. Crystallization ages obtained by U–Pb SHRIMP analysis of zircons from both the migmatites and the leucogranite veins indicate that the granulite facies metamorphism and the injection of veins were contemporary and occurred during slow cooling (6°CMy−1) from the metamorphic peak at 2666Ma to the granite solidus at 2636Ma.The large discrepancy (about 50%) between the volume of anatectic melt produced locally (in situ) and the observed volume of leucogranite in the terrain indicates that anatectic melt accumulated in the Opinaca Subprovince; it is an injection complex. This finding that granulite facies terranes can be enriched in melt contrasts with the general view that granulite terranes are melt-depleted.The presence of an injection complex in the deep crust has several broad implications for the continental crust. 1) H2O released as the injected melt crystallized rehydrated the adjacent granulite facies rocks which is evident from orthopyroxene replaced by amphibole and biotite. Consequently, these deep rocks would be more fertile in a subsequent anatectic event than non-rehydrated residuum. 2) The retention of voluminous leucogranite in the injection complex has thermal consequences for the crust. First, the bulk composition of the crust containing the injection complex is necessarily more felsic, thus the budget of heat-producing elements (Th, U and K) is locally enhanced and radiogenic heat production is higher. Secondly, latent heat of crystallization is released at a deeper level where the leucogranite solidifies. The combined effect of both maintains the lower crust several tens of degrees warmer for longer period of time.

Lithofacies characteristics of diatreme deposits: Examples from a basaltic volcanic field of SW Sardinia (Italy)

A deeply eroded diatreme field, consisting in several, decametric-sized, vertical, mainly clastic volcanic bodies of basaltic composition is described for the first time in the Variscan basement of SW Sardinia. The recognition and description of four different lithofacies in these diatremes allowed discussion of the role of the different processes which control magma eruption and conduit infilling, and making general inferences about diatremes. The studied diatremes have a cross-sectional shape from elliptical to sub-triangular, and are slightly elongated nearly parallel to the main foliation of the intruded meta-sedimentary rocks. Foliation of host rocks is locally reoriented or folded close to the contact with the diatremes, suggesting that magma possibly rose to the surface through fissures oriented nearly parallel to host rock foliation. Textural features of the volcanic bodies show many analogies with kimberlitic diatremes, despite the difference in petrography and composition. Juvenile lapilli are mainly made by ghosts of mafic phenocrysts (olivine and clinopyroxene) set in a groundmass formed by plagioclase microlites immersed in a cryptocrystalline, chlorite-rich matrix. The four lithofacies were described mainly based on the shape and physical features of the clasts and textural anisotropy: a globular, juvenile-rich, lapilli tuff facies (GJLt); an angular, juvenile-rich, lapilli tuff facies (AJLt); a lithic-rich, lapilli tuff facies LiRLt), and a coherent, lava-like facies (COH). All the clastic lithofacies are generally well sorted and typically lack a fine-grained matrix. Juvenile fragments are lapilli sized and from equant to oblate in axial ratio, and from rounded-globular to very angular in shape. Conversely, lithic clasts are largely variable in shape and size, and are mainly represented by basement-derived clasts. The absence of bedding, the scarcity of the coherent facies and the dominance of clast supported, structureless, volcaniclastic facies suggest that the outcropping portion of these volcanic bodies represents the lower diatreme zone. The presence of diffuse welding and the globular shapes of some juvenile fragments, together with their vesicularity, suggest that magma fragmentation was mainly driven by magmatic gas exsolution occurring at a deeper level respect to classical, basaltic explosive activity. Textural features, facies association and facies architecture of the studied deposits are suggestive of an important affinity with kimberlitic and other ultramafic diatremes.