Metamorfismo ercinico di bassa-pressione: evoluzione tettonico-metamorfica del complesso di mandatoriccio(massiccio della Sila - Calabria)

The Kwoiek Area of British Columbia contains a pendant or screen of metamorphosed sedimentary and volcanic rocks almost entirely surrounded by a portion of the Coast Range Batholith, and intruded by several dozen stocks. The major metamorphic effects were produced by the quartz diorite batholithic rocks, with minor and later effects by the quartz diorite stocks. The sequence of important metamorphic reactions in the metasedimentary and metavolcanic rocks, ranging in grade from chlorite to sillimanite, is: 1. chlorite + carbonate + muscovite → epidote + biotite 2. chlorite + carbonate → actinolite + epidote 3. chlorite + muscovite → garnet + biotite 4. chlorite + epidote → garnet + hornblende 5. chlorite + muscovite → garnet + staurolite + biotite 6. chlorite + muscovite → aluminum silicate + biotite 7. muscovite + staurolite → garnet + aluminum silicate + biotite 8. staurolite → garnet + aluminum silicate Continuous reactions, occurring between reactions 5 and 7, are: A. chlorite + (high Ti) biotite + Al2O3 (from plagioclase?)→ garnet + staurolite + (low Ti) biotite + O2 B. muscovite (phengitic) → garnet + staurolite +muscovite (less phengitic) + O2 (?) Detailed electron microprobe work on garnet, staurolite, biotite, and chlorite shows that: (1) The garnet porphyroblasts are zoned according to a depletion model, called the Rayleigh depletion model, which assumes equilibrium between the edge of a growing garnet and the minerals which are unzoned, notably biotite, chlorite, and muscovite, but which assumes disequilibrium within the garnet. (2) The staurolite porphyroblasts are also zoned, and from their zoning patterns reactions A, B, and 5 are documented. Progressive reduction of iron with increasing grade of metamorphism is also inferred from the staurolite zoning patterns. (3) During a late period of falling temperature garnet continued to grow and the biotite and chlorite reequilibrated. The biotite, chlorite, and garnet edge compositions can vary from point to point in a given thin section, indicating that the volume of equilibrium at the final stage of metamorphism was only a few cubic microns. (4) The horizon within the garnet that grew at maximum temperature can be identified. The Mg/Fe ratio of this horizon, if the garnet composition is a limiting composition in the Al2O3 - K2O - FeO - MgO tetrahedron, increases systematically with increasing metamorphic grade. Biotite and chlorite compositions also show a general increase in Mg/Fe ratio with increasing metamorphic grade, but staurolite appears to show the reverse effect. (5) The Mg/Fe ratio at the maximum temperature horizon of the garnet porphyroblasts is a function of its Mn content as evidenced from the study of five garnet-bearing rocks, collected from one outcrop area, with the same assemblage but with differing proportions of minerals. An important implication of zoned minerals is that the effective composition of a system in a phase lies on the join between the homogeneous minerals (if there are two) and not within three-or- four-phase fields when a zoned mineral, such as garnet or staurolite, is present in the assemblage. Study of the three aluminum silicates found in the Kwoiek Area showed that a constant pressure change in polymorphs from andalusite to kyanite to sillimanite took place with increasing temperature. This transition series is best explained by the metastable formation of andalusite. Photographic materials on pages 15, 121, 160, 162, and 164 are essential and will not reproduce clearly on Xerox copies. Photographic copies should be ordered.

In the Cerreto area metamorphic rocks belonging to the basement of the Northern Apennine are exposed in association with Triassic evaporites and quartzites. The Cerreto basement rocks consist of lenses and plurimetric bodies of amphibolites included in a sequence of metapsammites (biotite and/or muscovite schists). Both metasediments and amphibolites underwent a retrograde evolution starting from an initial medium-pressure amphibolite stage (M1: P ~ 8 kbar, T ~ 650°C) down to greenschist condition. The amphibolites show a well preserved earlier foliation associated with development of green amphibole (aluminotschermakite) + plagioclase + quartz + ilmenite assemblage followed by partial retrograde recrystallization in epidote-amphibolite facies under static conditions and along shear zones (M2: ~ P 4-5 kbar, T ~ 530°C); pre-kinematic garnet porphyroblasts rimmed by amphibole + plagioclase coronas are widespread. Metasediments show a pervasive mylonitic deformation associated with amphibolite to greenschist facies assemblages and rare relics of an earlier fabric, testified by inclusions of quartz + biotite in plagioclase and garnet porphyroclasts. The latest stages of deformation are accompanied in both metasediments and amphibolites by retrograde development of greenschist-facies assemblages in millimeter-scale shear zones crosscutting the former structures. Major and trace element compositions of the amphibolites suggest that most protoliths are cogenetic and could be related through fractional crystallization of plagioclase + olivine and/or pyroxene from variably evolved tholeiitic liquids. Geochemical data also indicate that most of the amphibolites have protolith features resembling Enriched-type MORB. 40Ar/39Ar dating performed on two hornblende samples (312 and 328 Ma) indicate a Variscan age for the amphibolite-facies metamorphism implying that the Cerreto metamorphic rocks belong to a unit deformed and metamorphosed at deep structural levels during the main deformational event (i.e. collisional stage) of the Southern Europe Variscan belt. The present tectonic setting of the Cerreto rocks, i.e. rootless slices within cover unit, could be related to the Apenninic thrusting. The Apenninic tectonic history likely occurred at very high structural levels without development of pervasive syn-metamorphic structures.

The PALMEIROPOLIS Cu-Zn (Pb) volcanogenic massive sulfide deposit, Brazil, consists of three ore bodies enclosed by hydrothermal alteration zones. The ore bodies and the alteration zones were metamorphosed under amphibolite fades conditions. The Palmeirdpolis alteration zones are characterized by a great diversity of bulk rock composition that originated a wide variety of low variance mineral assemblages. These assemblages are composed of orthoamphiboles (anthophyllite and gedrite), hornblende, biotite, garnet, staurolite, sillimanite, gahnite and, rarer, cordierite. Based on analyses of mineral chemistry aad mineral assemblages, temperatures are estimated to have been 550 - 625°C and pressures 2 - 5.5 kbar. The temperature of metamorphism that prevailed at the Palmeirdpolis deposit is comparable to other amphibolite fades massive sulfide deposits, such as Geco and Linda, Canada; Falun, Sweden; and Bleikvassli, Norway. The mineralogy of the alteration zones is similar in all these deposits even though they were metamorphosed at different pressure conditions, reflected by the crystallization of one of Al2 SiO5 phase (andalusite, sillimanite or kyanite).

Coaxial flattening at deep levels of orogenic belts: evidence from blueschists and eclogites on Syros and Sifnos (Cyclades, Greece)

Retrograde breakdown products of cordierite are commonly termed pinite, but their modes, compositions and formation conditions are only poorly known. A systematic study on pinitised cordierite from high-temperature metamorphic pelites of the Schwarzwald and the Bayerische Wald using electron microprobe (EM), transmission electron microscope (TEM), scanning electron microscope (SEM), x-ray diffraction, petrographic microscope and Fourier Transformation Infrared Spectroscopy (FTIR) investigations was carried out. On the basis of composition, phase assemblage, textural position and grain size four pinite types (border, mat, fissure, isotropic type) were distinguished that formed by distinct allochemical processes under different pressure-temperature conditions at different times. They probably represent general features of cordierite breakdown. Border-type (b-type) pinite consists of muscovite and biotite, formed at 350 - 550 °C from a K+-bearing fluid, most likely derived from the breakdown of K-feldspar to muscovite and quartz. B-type pinitisation may be related to granite intrusion in the Carboniferous. Mat-type (m-type) pinite encompasses chlorite-muscovite pinite and complex m-type pinite, bearing clay minerals, and is in general considered to represent the alteration of cordierite by an alkli-bearing fluid. Petrogenetic grids define an upper T limit for chlorite-muscovite of around 500 - 550 °C. Complex clay m-type pinite of this study has the principal constituents chlorite, berthierine, I/S R1 and I/S R0, Na/K-illite and random chlorite/berthierine mixed-layers. The observed features are similar to those of non-equilibrium assemblages from the early and the late diagenetic zones as complex m-type pinite lacks stable equilibrium and should have been formed much below 200 °C. In this case, its genesis is a complex two-stage stage process, with chlorite and I/S R1 representing the primary product of m-type pinitisation. Secondary berthierine and I/S R0 are formed by the replacement of chlorite and I/S R1 driven by the infiltration of an additional very low-grade fluid. M-type pinitisation could be related to low-temperature meteoric alteration of granites. F-type pinites form alteration veins penetrating intact cordierite. They are filled with tiny (< 20 - 250 Å in thickness), randomly oriented and randomly related, highly imperfect crystals, floating in an amorphous matrix. The principal crystals or packets of layers are berthierine, illite and/or I/S R1, smectite and chlorite, mixed-layers including 7/14 Å, 10/14 Å, I/S and complex mixed-layering of 7 Å, 10 Å and 14Å layers. Randomness in orientation of the crystallites points to in situ crystallisation of packets of layers directly either from a solution present in alteration veins or from a pre-existing gel-like material as more stable secondary phases. The occurrence of corresponding optical and chemical zonation pattern let the author assume that corresponding processes of leaching and repolymerisation acting in discrete band-like zones are the principle mechanism of f-type pinitisation. Lack of perfect and homogeneous structures and enhanced thickness of individual packets of layers indicate f-type pinitisation to result from cordierite alteration at very low-grade conditions. I-type pinite is fairly homogenous in BSE images and isotropic under crossed polars. Compared to cordierite, i-type pinite is depleted in most elements but networkforming species (Si and Al) and potential interlayer cations with K and Ca being most abundant (Ca > K). A dramatic fall in Si counting rates occurred during electron beam exposure at standard conditions, suggesting that i-type pinites may represent amorphous gel-like highly hydrated material. This assumption is confirmed by the occurrence of fissures penetrating i-type pinite, which are strongly suspected to represent dehydration shrinkage fissures. Unfortunately, TEM could not give further evidence. Therefore, the phase inventory and structural state of i-type pinite remain unclear. I-type pinitisation is probably a leaching process at very low temperatures (weathering) with network-modifying species being preferentially dissolved. H2O and CO2 contents of partly pinitised cordierite were determined by in situ FTIR measurements in order to detect late-stage modifications of the cordierite channel compositions, possibly related to pinitisation. The volatile contents of all investigated samples were found to be consistent with equilibration with a H2O undersaturated melt, during high-grade metamorphism. Therefore, a systematic change of cordierite channel volatile contents as a key step within the process of pinitisation is not indicated. Nevertheless, the crystallographic control of f-type pinitisation implies that the channel structure determines the c axis to be the preferred direction of cordierite dissolution/leaching.

Applicability of the RSCM geothermometry approach in a complex tectono-metamorphic context: The Jebilet massif case study (Variscan Belt, Morocco)

Metamorphic heterogeneities within a single HP unit: Overprint effect or metamorphic mix?

Ordovician metasedimentary rocks are the oldest and most extensive sedimentary sequence in the Chinese Altai. They experienced two major episodes of deformation (D1 and D2) resulting in the formation of juxtaposed Barrovian‐type and migmatite domains. D1 is characterized by a penetrative sub‐horizontal fabric (S1), and D2 is marked by upright folds (F2) withNW–SE‐trending axial planes in shallow crustal levels and by sub‐vertical transposition foliations (S2) in the high‐grade cores of large‐scale F2 antiforms. In the Barrovian‐type domain, successive growth of biotite, garnet and staurolite is observed in the S1 fabric. Kyanite included in garnet and plagioclase in the migmatite domain is interpreted to have formed also in S1. In the biotite and garnet zones, the spaced S2 cleavage is marked by biotite and muscovite, and in the staurolite and kyanite zones, the penetrative S2 fabric is characterized by sillimanite, locally with late cordierite. Phase equilibria modelling indicates that the S1 fabric was associated with an increase in pressure and temperature under Barrovian‐type conditions in both domains. The S2 fabric was related to decompression, in which rocks in the biotite and garnet zones well preserve the peak assemblage, and the higher grade rocks in the staurolite and kyanite zones re‐equilibrated to different degrees under high‐temperature/low‐pressure (HT/LP) conditions. The D1 metamorphic history is attributed to the progressive burial related to Early–Middle Palaeozoic crustal thickening and the metamorphism associated with D2 is interpreted to result from exhumation by vertical extrusion. The extrusion of hot rocks was contemporaneous with the formation of gneiss domes accompanied by the intrusion of juvenile magmas at middle crustal levels during the Middle Palaeozoic. Consequently, there is a genetic link between the Barrovian‐type and migmatite domains related to continuous transition of the Barrovian‐type fabric into theHT/LPone during development of domal structures in the southern Altai orogenic belt. This study has a broad impact on the understanding of the thermo‐mechanical behaviour of accretionary orogenic systems worldwide. The lower crustal flow and doming of hot crust, so far reported only in continental collisional settings, seems to be also an integral mechanism responsible for both horizontal and vertical redistribution of accreted material prior to continental collision.

Mantle-rooted fluid pathways and world-class gold mineralization in the giant Jiaodong gold province: Insights from integrated deep seismic reflection and tectonics

The Variscan basement of Northern Apennines (Northern Italy) is a polymetamorphic portion of continental crust. This thesis investigated the metamorphic history of this basement occurring in the Cerreto Pass, in the Pontremoli well, and in the Pisani Mountains. The study comprised fieldwork, petrography and microstructural analysis, determination of the bulk rock and mineral composition, thermodynamic modelling, conventional geothermobarometry, monazite chemical dating and Ar/Ar dating of muscovite. The reconstructed metamorphic evolution of the selected samples allowed to define a long-lasting metamorphic history straddling the Variscan and Alpine orogenesis. Some general petrological issues generally found in low- to medium-grade metapelites were also tackled: (i) With middle-grade micaschist it is possible to reconstruct a complete P-T-D path by combining microstructural analysis and thermodynamic modelling. Prekinematic white mica may preserve Mg-rich cores related to the pre-peak stage. Mn-poor garnet rim records the peak metamorphism. Na-rich mylonitic white mica, the XFe of chlorite and the late paragenesis may constrain the retrograde stage. (ii) Metapelites may contain coronitic microstructures of apatite + Th-silicate, allanite and epidote around unstable monazite grains. Chemistry and microstructure of Th-rich monazite relics surrounded by this coronitic microstructure may suggest that monazite mineral was inherited and underwent partial dissolution and fluid-aided replacement by REE-accessory minerals at 500-600°C and 5-7 kbar. (iii) Fish-shaped white mica is not always a (prekinematic) mica-fish. Observed at high-magnification BSE images it may consist of several white mica formed during a mylonitic stage. Hence, the asymmetric foliation boudin is a suitable microstructure to obtain geochronological information about the shearing stage. (iv) Thermodynamic modelling of a hematite-rich metasedimentary rock fails to reproduce the observed mineral compositions when the bulk Fe2O3 is neglected or determined through titration. The mismatch between observed and computed mineral compositions and assemblage is resolved by tuning the effective ferric iron content by P-XFe2O3 diagrams.

The architecture of the crust in the French Massif Central (FMC) is described as a nappe stack composed, from top to bottom, of the Upper Gneiss Unit (UGU), the Lower Gneiss Unit (LGU), and the Para-Autochthonous Unit (PAU), which are intruded by Carboniferous granitic plutons. In the Pontgibaud region, the Upper Gneiss Unit/Lower Gneiss Unit (UGU/LGU) attribution and the relationship between metamorphic and magmatic rocks are uncertain. This hinders both our understanding of the geodynamic processes that led to the construction of the Variscan orogenic belt, and a precise assessment of the deep geothermal potential of this high-heat flow area. Our field investigations document a metamorphic gradient with, from South to North, micaschists and paragneiss with a gently, dominantly southward–dipping foliation grading into migmatites that display a steeper foliation. A maximum late Ediacaran deposition age of sedimentary protoliths is determined from LA-ICPMS U–Pb dates of the youngest detrital zircon populations that range from 588.9 ± 7.1 to 571.8 ± 8.2 Ma. Equilibrium phase diagrams calculated for selected samples document a metamorphic gradient from 2.3– 5.3 kbar, and 570°–660°C, characterizing a mid- to low pressure-high temperature event dated from 357.6 ± 1.5 to 342.2 ± 4.6 Ma by LA-ICP-MS U–Pb analyses of metamorphic monazite. The contact between the metasedimentary rocks and migmatites is crosscut by peraluminous, cordierite-biotite-bearing Claveix and Gelles granitic plutons dated at 338.0 ± 7.3 Ma and 323.3 ± 9.7 Ma, respectively by U–Pb on zircon. The distribution of U–Pb dates and Lu–Hf isotopic compositions of detrital zircon in metamorphic rocks; the absence of HP mineral relics; the age of HT metamorphism of ca. 357–345 Ma; and the lack of a major mylonitic tectonic contact, which would typify the base of UGU, collectively support that the exposed crust of the Pontgibaud area corresponds to LGU metamorphic rocks. Moreover, the mineralogy, bulk chemistry and zircon Lu–Hf isotopic composition of the granites and migmatites are consistent with an origin by partial melting of LGU metasedimentary rocks. These data show that the Pontgibaud area exposes a section along a Carboniferous MP-LP/HT metamorphic gradient from micaschists to migmatites. Genesis and extraction of a large volume of granitic magma require higher temperatures (>800 °C) that were not reached by the exposed rocks. Accordingly, we propose that the local mid- and lower crustal levels are made of rocks with a similar protolith as those exposed at the surface, but affected by higher metamorphic grade and magma extraction.

The External Ligurian Units of the Northern Apennine are interpreted as derived from the continent-ocean transition domain at the northern thinned continental margin of the Adria microplate, i.e. the External Ligurian domain. The evolution of this paleogeographic realm from pre-orogenic times to the Eoalpine and Meso-alpine tectonics is presented here, through a review of stratigraphic, petrological and structural data. The tectono-metamorphic evolution started in the Late Carboniferous-Early Permian (about 290 Ma), with the emplacement at deep crustal levels of the gabbroic protholits of mafic granulites. These lower continental crust rocks subsequentely underwent Permo-Triassic tectonic exhumation and were finally exhumed at shallow crustal levels in middle Jurassic. The latter period was characterized by extensive brittle faulting at shallow crustal levels, giving rise to extensional allochtons formed by stretched slices of upper continental crust (mainly granitoids). At deep structural levels high temperature shearing of ophiolitic gabbros took place. Opening of the Ligurian Tethys is finally testified by the basalt emplacement and radiolarian chert sedimentation in the Late Jurassic. During Late Cretaceous, development of Alpine intraoceanic subduction led to the inversion of the External Ligurian domain: the Eo-alpine deformation is recorded by syn-tectonic sedimentation of the Complessi di Base Auct., by development of very low-grade metamorphism and deformation at about 80 Ma. Middle Eocene deformation related with collision and indentation of the Adria with the Alpine accretionary wedge can be subdivided in two main stages: the first one (Ligurian Phase 1) implies large-scale, westward displacement of the EL Units, whereas the second stage (Ligurian Phase 2) is characterized by eastverging structures probably driven by the thinning of the preexisting nappe pile associated with exumation of underplated HP/LT Alpine units.

Crustal-scale structures in the Proterozoic Mount Isa Inlier of north Australia: their seismic response and influence on mineralisation

The Lavrion area corresponds to the northwestern end of the Attic-Cycladic Complex and mainly consists of metamorphic rocks formed during the Eocene high-pressure/low-temperature (HP/LT) event and the Upper Oligocene-Lower Miocene medium-pressure metamorphic event. These metamorphic rocks are found in two superimposed tectonic units: the Kamariza Unit, which includes metavolcanic rocks and the overlying Lavrion blueschist Unit, which is largely represented by metaophiolites. Protoliths of metavolcanic rocks in the Kamariza Unit are calc-alkaline basalts displaying a marked enrichment in Th, U, and LREE and depletion in Ta, Nb, Hf and Ti, which point to a genesis from a depleted mantle source further enriched by subduction components. The Lavrion blueschist Unit mainly includes metavolcanic rocks with tholeiitic composition, as well as subordinate metagranites and metavolcanic rocks with calc-alkaline affinity. The tholeiitic metavolcanic rocks are mainly represented by enriched-type (E-) mid-ocean ridge basalts (MORB) and subordinately by normal-type (N-) MORB. E-MORB chemistry implies a genesis from a depleted asthenospheric source modified by an OIB component, whereas the N-MORB has chemical features typical for rocks generated in a mid-ocean ridge setting from a primitive asthenospheric source. Previous works suggested that the magmatic protoliths of similar HP/LT metamorphic rocks from elsewhere in the Cyclades reflect an arc-back-arc tectonic setting, which developed during the Cretaceous closure of the Pindos oceanic basin. However, recent geological studies have shown that the Lavrion metamorphic Units, unlike similar units from the Cycladic zone, represent Triassic Pelagonian sequences metamorphosed under HP/LT conditions typical of the Cycladic zone. The geochemical and petrological characteristics of the Lavrion metamorphic rocks support this conclusion. In particular, calc-alkaline protoliths display many similarities with the Triassic calc-alkaline rocks associated with the rift of Gondwana, whereas MORB-type protoliths are similar to the Triassic MORB found in the Subpelagonian ophiolitic melanges. The magmatic protoliths of the Lavrion HP/LT metamorphic rocks are thus compatible with a paleotectonic evolution which encompasses the Triassic continental rift, followed by the early oceanization stage of the Pindos ocean, and emplaced on the border between the Pelagonian continental margin and the Pindos basin. These rocks were probably included into melanges during the Jurassic closure of the Pindos basin, and finally they were involved in the Eocene and Upper Oligocene-Lower Miocene metamorphic events that affected the Cycladic zone.

The complex structural history of the Dabie-Sulu terrane is deduced from various scales of structural features studied in UHP metamorphic units combined with metamorphic and thermal data. Excluding pre-UHP events, the following sequence of distinct tectonometamorphic stages is suggested: (1) The first deformation, D1, produced weak foliation and lineation in massive eclogite in the P-T stability field of coesite/diamond under low differential stress. (2) The D2 event is mainly inferred from a dominant coesite eclogite-facies texture characterized by a stretching mineral lineation, mesoscale sheath-like folds, and a network of ductile shear zones. (3) D3 structures and fabrics developed shortly after the formation of granulite/amphibolite—facies symplectites. These structures are characterized by a regional, steeply dipping foliation and heterogeneous compositional layering, eclogite boudinages of various dimensions, intrafolial folds, and ductile shear zones forming an anastomosing pattern, leading to tectonic juxtaposition or nappe-like structures defining shear zone-bounded crustal slices. The D3 deformation event was associated with decompressional partial melting and intense retrogressive metamorphism. (4) Post-collisional ductile crustal thinning and extension affected the D3 foliation and compositional layering, producing a regional, gently dipping D4 main amphibolite-facies foliation and stretching lineation, dome- and arc-shaped structures, and low-angle and extensional detachments typified by different stretching directions. All of these structures formed prior to intrusion of Late Mesozoic plutons, faulting, basin development, and tectonic unroofing at shallow crustal levels (D5). The first two stages of ductile structures (D1 and D2) were related to subduction and collision between the Sino-Korean and Yangtze cratons at UHP (>27 kbar) and HP metamorphic conditions at 230-250 Ma. In contrast, D3 and D4 stages of Late Triassic to Jurassic (~230-170 Ma) ductile deformation accompanied exhumation of UHP and HP metamorphic rocks to crustal levels, initially driven by compression and later by extension. D4 structures dominate the map pattern of most UHP and HP metamorphic rocks in the orogen. The final D5 deformation largely controls the present-day geomorphology of the Dabie-Sulu region. A modified tectonic evolution model for the UHP and HP metamorphic belts is proposed. It involves continental subduction and collision between the Yangtze and Sino-Korean cratons, and subsequent polyphase exhumation of the UHP and HP metamorphic rocks.