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Dehydration reactions in the subducting slab liberate fluids causing major changes in rock density, volume and permeability. Although it is well known that the fluids can migrate and interact with the surrounding rocks, fluid pathways remain challenging to track and the consequences of fluid-rock interaction processes are often overlooked. In this study, we investigate pervasive fluid-rock interaction in a sequence of schists and mafic felses exposed in the Theodul Glacier Unit (TGU), Western Alps. This unit is embedded within metaophiolites of the Zermatt-Saas Zone and reached eclogite-facies conditions during Alpine convergence. Chemical mapping and in situ oxygen isotope analyses of garnet from the schists reveal a sharp chemical zoning between a xenomorphic core and a euhedral rim, associated to a drop of ~ 8â° in [delta].sup.18O. Thermodynamic and [delta].sup.18O models show that the large amount of low [delta].sup.18O H.sub.2O required to change the reactive bulk [delta].sup.18O composition cannot be produced by dehydration of the mafic fels from the TGU only, and requires a large contribution of the surrounding serpentinites. The calculated time-integrated fluid flux across the TGU rocks is 1.1 x 10.sup.5 cm.sup.3/cm.sup.2, which is above the open-system behaviour threshold and argues for pervasive fluid flow at kilometre-scale under high-pressure conditions. The transient rock volume variations caused by lawsonite breakdown is identified as a possible trigger for the pervasive fluid influx. The calculated schist permeability at eclogite-facies conditions (~ 2 x 10.sup.-20 m.sup.2) is comparable to the permeability determined experimentally for blueschist and serpentinites., Author(s): Thomas Bovay [sup.1], Daniela Rubatto [sup.1] [sup.2], Pierre Lanari [sup.1] Author Affiliations: (1) grid.5734.5, 0000 0001 0726 5157, Institute of Geological Sciences, University of Bern, , 3012, Bern, Switzerland [...]

Garnet is one of the most robust and ubiquitous minerals that record element zoning during crustal metamorphism. In addition to major elements, zoning in trace elements can provide a wealth of information to document the changing conditions of garnet growth and modification. However, mapping trace elements at low concentrations, over large areas and with high resolution has remained a major challenge. We present a comprehensive investigation of the TE distribution in garnet from three Alpine samples that underwent a complex evolution at different metamorphic conditions. The TE distribution in garnet grains is mapped in 2D in thin section with a novel approach using laser ablation inductively coupled plasma time of flight mass spectrometry (LA-ICP-TOFMS) to achieve a lateral resolution of 5 [micro]m and limits of detection for the heavy rare earth elements (REE) down to 0.2 [micro]g/g. Comparison with major element zoning measured by electron probe microanalysis and trace elements measured by conventional LA-ICPMS spot analysis testifies to the accuracy of the measurements. Garnet in an amphibolite-facies metapelite from Campolungo, Central Alps, that recorded metamorphism to 600 °C preserves Y + REE trace element zoning that closely matches that of Ca. In this sample, there is no notable diffusive modification for trace elements. Y + REE zoning is dominated by Rayleigh fractionation in the core and by the sporadic breakdown of accessory phases producing annuli in the rim of the garnet. A granulite-facies garnet from Malenco, Eastern Central Alps, formed during subsolidus heating, followed by peritectic melting reactions up to temperatures of 800-850 °C. Major and trace element zoning are decoupled indicating diffusional resetting of major elements, whereas trace elements still largely document the growth history. Enrichment of trace elements in the garnet mantle may be related to the consumption of biotite (V, Cr) and the dissolution of zircon (Zr) and monazite (Y + REE) in the melt. Diffusion of Y + HREE at the core-mantle boundary occurred over a length scale of ~ 200 [micro]m. Garnet in an eclogite from the Sesia Zone, Western Alps (P ~ 2 GPa, T ~ 600 °C), displays pronounced fluid-related veinlets, visible in FeO, MgO and MnO, which cross-cut the primary growth zoning. Surprisingly, complex Y + REE and Cr zoning is not affected by the veinlets, indicating that they did not form by a crack-seal mechanism but are rather related to a selective replacement process. The trace element maps provide a detailed insight into the growth and modification of garnet and thus allow assessment of equilibrium versus disequilibrium processes, and assist in determination of P-T conditions, garnet dating, diffusion modelling as well as documenting fluid-induced modifications., Author(s): Daniela Rubatto [sup.1], Marcel Burger [sup.2], Pierre Lanari [sup.1], Bodo Hattendorf [sup.2], Gunnar Schwarz [sup.2], Christoph Neff [sup.2], Peter Keresztes Schmidt [sup.2], Jörg Hermann [sup.1], Alice Vho [sup.1], Detlef [...]

This study investigates the behaviour of the Zrin-rutile and Ti-in-zircon thermometers in granulite facies metapelites from the Ivrea-Verbano Zone lower crustal section. U-Pb ages of zircon constrain the timing of regional amphibolite-granulite facies metamorphism to 316 ± 3 Ma and record zircon recrystallisation and resetting of U-Pb ages at 276 [+ or -] 4 Ma and 258 ± 3 Ma. Zr-inrutile thermometry records peak contact metamorphic temperatures related to intrusion of mafic magmatic rocks and gives peak temperatures between 900-930°C and 1,000-1,020°C that are consistent with the geological settings of the samples. Ti-in-zircon temperatures of 700-800°C and 810-870°C record growth or re-equilibration of zircon after cooling from peak temperatures. Ti-in-quartz thermometry for one sample records both peak and retrograde temperatures. Some rutiles in all samples record resetting of Zr-in-rutile temperatures at ~ 750-800°C. Electron microprobe profiles across individual rutiles demonstrate that Zr expulsion occurred by recrystallisation rather than by diffusive exchange. Ex-solution of small needles of baddelyite or zircon from rutile is an important method of Zr redistribution, but results in no net Zr loss from the grain. The demonstration that Zr-inrutile thermometry can robustly record peak temperatures that are not recorded by any other thermometer emphasises the relevance of this technique to investigating the evolution of high-grade metamorphic terranes, such as those that characterise the lower crust. Keywords Zr in rutile * Ti in zircon * Thermometry * Diffusion * Granulite facies * Ivrea-Verbano Zone * U-Pb geochronology, Introduction The temperature dependence of Zr incorporation into rutile grown in equilibrium with quartz and zircon provides a powerful single-mineral thermometer (Zack et al. 2004a; Watson et al. 2006; Ferry [...]

The petrology and timing of crustal melting has been investigated in the migmatites of the Higher Himalayan Crystalline (HHC) exposed in Sikkim, India. The metapelites underwent pervasive partial melting through hydrous as well as dehydration melting reactions involving muscovite and biotite to produce a main assemblage of quartz, K-feldspar, plagioclase, biotite, garnet ± sillimanite. Peak metamorphic conditions were 8-9 kbar and ~800 °C. Monazite and zircon crystals in several migmatites collected along a N-S transect show multiple growth domains. The domains were analyzed by microbeam techniques for age (SHRIMP) and trace element composition (LA-ICP-MS) to relate ages to conditions of formation. Monazite preserves the best record of metamorphism with domains that have different zoning pattern, composition and age. Zircon was generally less reactive than monazite, with metamorphic growth zones preserved in only a few samples. The growth of accessory minerals in the presence of melt was episodic in the interval between 31 and 17 Ma, but widespread and diachronous across samples. Systematic variations in the chemical composition of the dated mineral zones (HREE content and negative Eu anomaly) are related to the variation in garnet and K-feldspar abundances, respectively, and thus to metamorphic reactions and P-T stages. In turn, this allows prograde versus decompressional and retrograde melt production to be timed. A hierarchy of timescales characterizes melting which occurred over a period of ~15 Ma (31-17 Ma): a given block within this region traversed the field of melting in 5-7 Ma, whereas individual melting reactions lasted for time durations below, or approaching, the resolution of microbeam dating techniques (~0.6 Ma). An older ~36 Ma high-grade event is recorded in an allocthonous relict related to mafic lenses. We identify two sections of the HHC in Sikkim that traversed similar P-T conditions at different times, separated by a tectonic discontinuity. The higher structural levels reached melting and peak conditions later (~26-23 Ma) than the lower structural levels (~31-27 Ma). Diachronicity across the HHC cannot be reconciled with channel flow models in their simplest form, as it requires two similar high-grade sections to move independently during collision. Keywords U-Pb geochronology * Migmatites * Trace elements * Himalaya * Melting reactions, Introduction The dynamics of crustal melting (timing, duration and productivity) in collisional settings has major implications for the evolution of the overriding orogen. It has, for example, the capacity to [...]

[1] An integrated structural, petrological, and geochronological/thermochronological study was undertaken to constrain the tectonic controls on the exhumation of the Voltri Massif high-pressure (HP) complex, located in the Ligurian portion of the Western Alps (Italy). Petrotextural analyses were performed to identify the pressure-temperature interval of (I) peak metamorphism (the [D.sub.1]-[M.sub.1] stage) to eclogitic/blueschist facies conditions and (2) the main retrogressive event (the [D.sub.2]-[M.sub.2] stage) to greenschist facies conditions. U-Pb SHRIMP dating on a zircon rim (33.8 [+ or -] 0.8 Ma) and titanite grains (29 [+ or -] 5 Ma), coupled with [sup.40]Ar-[sup.39]Ar analyses on phengite (~64 Ma to ~34 Ma) placed temporal constraints on the exhumation path from the [D.sub.1]-[M.sub.1] to the [D.sub.2]-[M.sub.2] stages at the Eocene-Oligocene boundary. Apatite fission track thermochronology, with older ages of 23.9 [+ or -]4.9 Ma, confirms the existence of a regional, Late Oligocene to Miocene cooling/exhumation event for the Voltri Massif (the [D.sub.3] - [M.sub.3] stage). The compilation of this pressure-temperature-deformation-time path supports a change in the exhumation history: (1) an initial stage, from the [D.sub.1] -[M.sub.1] to the [D.sub.2] -[M.sub.2], which was nearly isothermal with highly variable exhumation rates, and (2) a second stage, from [D.sub.2] -[M.sub.2] to upper crustal levels, which was accomplished by coolinlg and moderate exhumation rates of-1-2 mm yr-. This two-stage path can be reconciled with the dominant tectonic mechanisms responsible for exhumation of HP rocks in the Voltri Massif area. At the regional scale, this path is consistent with major geodynamic reorganization in the Mediterranean region at the Eocene-Oligocene time boundary, which involved a switch from synorogenic events during transpressive kinematics at the Alpine-Apennine plate boundaries, to postorogenic processes related to crustal thinning and opening of back-arc basins. Citation: Vignaroli, G., F. Rossetti, D. Rubatto, T. Theye, F. Lisker, and D. Phillips (2010), Pressure-temperature-deformation-time (P-T-d-t) exhumation history of the Voltri Massif HP complex, Ligurian Alps, Italy, Tectonics, 29, TC6009, doi: 10.1029/2009TC002621.

A lithostratigraphic association consisting of serpentinized mantle rocks, continent-derived allochthons, mid-oceanic ridge gabbros of Jurassic age and post-rift sediments, typical of an ocean-continent transition, is found in the eclogitic Piemonte units, in the Western Alps. In situ U-Pb geochronology was performed on zircons from an orthogneiss sampled at the bottom of a sliver of continental basement, in contact with serpentinites. Primary magmatic zircons of Permian age were overgrown by a second generation of zircon at ca. 166-150 Ma, likely related to melt infiltration associated with the intrusion of the underlying gabbroic body. This indicates that continental basement slices and oceanic basement rocks were already juxtaposed in the Jurassic and they were probably part of hyper-extended crust related to the opening of the Tethys. Therefore, the complex lithological association described here, which is also characteristic of several (ultra) high-pressure melange zones worldwide, was acquired prior to the orogenic event, during which it was only partly reworked. Ocean-continent transitions are in positions favorable to reach (ultra)high-pressure conditions, following negatively buoyant oceanic lithosphere into subduction, and then being accreted to the orogen, in response to the arrival of more buoyant continental lithosphere, resisting subduction. The ocean-continent transition is now found in the immediate footwall of a 500-m-thick shear zone, which accommodated multiple episodes of deformation during Eocene--Oligocene time, suggesting an important link between Alpine deformation and rift-related structures. doi: 10.1130/G30768.1

Byline: Daniela Rubatto (1), Jorg Hermann (1), Alfons Berger (2,3), Martin Engi (2) Abstract: The timing and dynamics of fluid-induced melting in the typical Barrovian sequence of the Central Alps has been investigated using zircon chronology and trace element composition. Multiple zircon domains in leucosomes and country rocks yield U--Pb ages spanning from ~32 to 22 Ma. The zircon formed during Alpine melting can be distinguished from the inherited and detrital cores on the basis of their age, Th/U ( Author Affiliation: (1) Research School of Earth Sciences, The Australian National University, Canberra, ACT, 0200, Australia (2) Institute of Geological Sciences, University of Bern, Baltzerstrasse 1-3, 3012, Bern, Switzerland (3) Institute for Geography and Geology, Aster Voldgade 10, 1350, Copenhagen, Denmark Article History: Registration Date: 14/04/2009 Received Date: 22/12/2008 Accepted Date: 14/04/2009 Online Date: 02/05/2009 Article note: Communicated by J. Hoefs. Electronic supplementary material The online version of this article (doi: 10.1007/s00410-009-0406-5) contains supplementary material, which is available to authorized users.

Byline: Bernardo Cesare (1,2), Daniela Rubatto (3), Maria Teresa Gomez-Pugnaire (4,5) Keywords: Anatexis; Geochronology; Monazite; SHRIMP; Southern Spain; Volcanism; Zircon Abstract: The high-K calc-alkaline volcanic rocks along the Neogene Volcanic Province of SE Spain represent crustal anatectic melts mixed with mantle components during the opening of the Alboran Sea. Partially melted metapelitic enclaves, along with the geochemical signature, provide evidence of their crustal source. U--Pb SHRIMP geochronology on monazite and zircon from enclaves and their hosting lavas in the localities of El Hoyazo, Mazarron and Mar Menor reveals variable delays between the melting at depth and the eruption of the volcanics. These data indicate that: (1) the most important event of anatexis in the Neogene spanned at least the 3 m.y. interval between 12 and 9 Ma (2) there is no trend in age of crustal melting and (3) the delay between magma generation and extrusion varies from more than 3 m.y. at El Hoyazo to ~0.5 m.y. and possibly 2.5 m.y. at Mar Menor, with no significant delay measurable at Mazarron. The variable time delay between anatexis and lava extrusion indicates that radiometric ages of volcanics may provide misleading information on the timing of magma genesis occurring at depth. This highlights the pitfall of basing detailed geodynamic models on volcanic extrusion ages alone. Author Affiliation: (1) Dipartimento di Geoscienze, Universita di Padova, Via Giotto 1, 35137, Padua, Italy (2) C.N.R. Istituto di Geoscienze e Georisorse, Corso Garibaldi, 37, 35137, Padua, Italy (3) Research School of Earth Sciences, The Australian National University, Canberra, ACT, 0200, Australia (4) Departamento de Mineralogia y Petrologia, Facultad de Ciencias, Universidad de Granada, Fuentenueva s/n, 18002, Granada, Spain (5) Instituto Andaluz de Ciencias de la Tierra (CSIC), Facultad de Ciencias, Universidad de Granada, Fuentenueva s/n, 18002, Granada, Spain Article History: Registration Date: 25/07/2008 Received Date: 24/04/2008 Accepted Date: 25/07/2008 Online Date: 21/08/2008 Article note: Communicated by J. Hoefs. Electronic supplementary material The online version of this article (doi: 10.1007/s00410-008-0333-x) contains supplementary material, which is available to authorized users.

The prograde sequence of rare earth minerals recorded in metapelites during regional metamorphism reveals a series of irreversible reactions among silicates and phosphates. In individual samples from the northern Lepontine (Central Alps), allanite is partly replaced by monazite at 560-580 [degrees]C. Relic allanite retains its characteristic growth zoning acquired at greenschist facies conditions (430-450 [degrees]C). Coexisting monazite and allanite were dated in situ to delimit in time successive stages of the Barrovian metamorphism. In situ sensitive high-resolution ion microprobe (SHRIMP) U-Th-Pb dating of allanite (31.5 [+ or -] 1.3 and 29.2 [+ or -] 1.0 Ma) and monazite (18.0 [+ or -] 0.3 and 19.1 [+ or -] 0.3 Ma) constrains the time elapsed between 430-450 [degrees]C and 560-580 [degrees]C, which implies an average heating rate of 8-15 [degrees]C/m.y. Combined with new fission track ages (zircon, 10-9 Ma; apatite, 7.5-6.5 Ma), metamorphic rates of the entire orogenic cycle, from prograde to final cooling, can be reconstructed.

An eclogite facies meta-plagiogranite from the Lanzo massif(western Alps, Italy) contains crystals of zircon intimately associated with allanite. Zircon displays different microtextures ranging from pristine, euhedral, and magmatic to fractured, porous varieties with mosaic zoning, and pervasive recrystallization into euhedral microcrystals. Fractures and voids in the recrystallized zircon microcrystals are mainly filled by high-pressure Na-rich pyroxene. Electron backscattered diffraction analysis revealed a similar crystallographic orientation for primary magmatic zircon crystals and microcrystals, with less than 2[degrees] misorientation among neighboring microdomains. The textural change is coupled with chemical and isotopic modifications: recrystallized zircon domains contain significantly less Th and light-to mid-REE, but are richer in Sr than magmatic zircon crystals. Magmatic zircon preserves the protolith U-Pb age of 163.5 [+ or -] 1.7 Ma, whereas zircon microcrystals have a mean age of 55 [+ or -] 1 Ma. The coexisting allanite also contains inclusions of Na-rich pyroxene and has chemical features (elevated Sr and Ni contents and lack of Eu anomaly) indicating formation at high pressure. Despite being associated texturally with zircon, allanite yields a younger Th-Pb age of 46.5 [+ or -] 3.0 Ma, suggesting that the Lanzo unit remained at relatively high pressure conditions for ~8 m.y. Zircon recrystallization proceeded with volume reduction and loss of material to an alkaline metamorphic fluid that acted as the agent for a coupled dissolution-reprecipitation process. Recrystallization occurred with minimum transport, in a low-strain environment, and was not significantly enhanced by metamictization. The source of the fluid for zircon recrystallization is most probably related to prograde devolatilization reactions in the surrounding serpentinite. Keywords: U-Th-Pb geochronology, allanite, EBSD, plagiogranite, Lanzo peridotite

Monazite is a key accessory mineral for metamorphic geochronology, but interpretation of its complex chemical and age zoning acquired during high-temperature metamorphism and anatexis remains a challenge. We investigate the petrology, pressure-temperature and timing of metamorphism in pelitic and psammitic granulites that contain monazite from the Greater Himalayan Crystalline Complex (GHC) in Dinggye, southern Tibet. These rocks underwent isothermal decompression from pressure of >10 kbar to ~5 kbar at temperatures of 750-830 °C, and recorded three metamorphic stages at kyanite (M.sub.1), sillimanite (M.sub.2) and cordierite-spinel grade (M.sub.3). Monazite and zircon crystals were dated by microbeam techniques either as grain separates or in thin sections. U-Th-Pb ages are linked to specific conditions of mineral growth on the basis of zoning patterns, trace element signatures, index mineral inclusions (melt inclusions, sillimanite and K-feldspar) in dated domains and textural relationships with co-existing minerals. The results show that inherited domains (500-400 Ma) are preserved in monazite even at granulite-facies conditions. Few monazites or zircon yield ages related to the M.sub.1-stage (~30-29 Ma), possibly corresponding to prograde melting by muscovite dehydration. During the early stage of isothermal decompression, inherited or prograde monazites in most samples were dissolved in the melt produced by biotite dehydration-melting. Most monazite grains crystallized from melt toward the end of decompression (M.sub.3-stage, 21-19 Ma) and are chemically related to garnet breakdown reactions. Another peak of monazite growth occurred at final melt crystallization (~15 Ma), and these monazite grains are unzoned and are homogeneous in composition. In a regional context, our pressure-temperature-time data constrains peak high-pressure metamorphism within the GHC to ~30-29 Ma in Dinggye Himalaya. Our results are in line with a melt-assisted exhumation of the GHC rocks., Author(s): Jia-Min Wang [sup.1] [sup.2] , Fu-Yuan Wu [sup.1] [sup.3] , Daniela Rubatto [sup.4] , Shi-Ran Liu [sup.5] , Jin-Jiang Zhang [sup.5] , Xiao-Chi Liu [sup.1] , Lei Yang [sup.1] [...]

Recrystallized zircon grains in a phengite-epidote-chlorite schist from north-eastern New Caledonia contain as inclusions a mineral assemblage consisting of celadonite, kaolinite, quartz, Fe-oxy-hydroxide, smectite, chlorite, xenotime-(Y), thortveitite, yttrialite, and allanite-(Ce). This assemblage formed during low-temperature (

U-Pb geochronology with ion microprobe (SHRIMP) analysis has been carried out on eclogite-facies rocks of the Beigua Unit, an ophiolitic slice of the Voltri Massif, Western Alps. The investigated samples are eclogites and high-pressure metasomatic rocks (metarodingites and centimetre-sized Ti-clinohumite-bearing dykes). Zircon contained in an eclogitic metagabbro and a metarodingite preserves magmatic zoning patterns and trace element compositions. The zircon ages of 160 [+ or -] 1 and 161 [+ or -] 3 Ma are interpreted to date the crystallization of the gabbroic protoliths. Ti-clinohumite dykes in the same unit contain haddeleyite crystals in textural equilibrium with Ti-clinohumite, diopside, chlorite and magnetite, which form the eclogite-facies assemblage in these rocks. Baddeleyite also contains inclusions of such minerals, indicating its formation at high pressure. The baddeleyite has cathodoluminescence intensity and chaotic patterns similar to metamorphic zircon. It contains a significant amount of Hf (1.3-1.7 wt%), traces of Ti, Y, Nb, Ta, REE, U and Th. Its chondrite-normalised trace element pattern has strong enrichment in middle REE, positive Ce-anomaly and small negative Eu-anomaly. This represents the first report of baddeleyite formed during regional metamorphism, and suggests that this mineral could (re)crystallize easier than zircon under low-temperature, high-pressure conditions. The age of the baddeleyite is interpreted as likely dating the eclogite-facies metamorphism in the Beigua Unit at 33.6 [+ or -] 1.0 Ma. This age is very close to the Early Oligocene age of the overlying Tertiary continental breccias and conglomerates, which contains clasts of high-pressure rocks. This sedimentary record, which is unique for Alpine high-pressure units, is direct evidence of fast exhumation of the Beigua eclogites. The young age for the HP metamorphism of the Beigua ophiolite makes a revision of either the palaeogeography prior to collision, or of the subduction setting in the entire region, necessary.

Zircon and monazite from three restitic enclaves and one host dacite have been dated by ion microprobe (SHRIMP), with the aim of characterising their Miocene history and defining the timing relationships between crustal melting and eruption in the high-K calc-alkaline volcanics of the Neogene Volcanic Province of SE Spain. The studied samples are from the volcanic edifices of El Joyazo (Cerro del Hoyazo) and Mazarron. Zircons in the enclaves are characterized by a thin euhedral rim overgrowing a detrital core. The core-rim boundary is marked by tiny glass inclusions of S-type granitic composition, which attest to the growth of zircon rims during a crustal melting event. At El Joyazo, where lavas erupted at 6.3 Ma (Zeck and Williams 2002), U-Pb ages of zircon overgrowths define an age of anatexis of 9.63[+ or -]0.26 Ma (95% c.l.), in agreement with monazite ages of 9.74[+ or -]0.21 Ma (95% c.l.). At Mazarron, the age of anatexis provided by monazite at 9.13 [+ or -]0.18 Ma (95% c.l.) overlaps with that of melt-precipitated zircon in the host dacite, dated at 9.06[+ or -]0.53 Ma (95% c.l.). These results indicate that after partial melting, the enclaves and the syngenetic S-type melts resided at depth for > 3 m.y. at El Joyazo. Compared with the results from Mazarron, the long residence time obtained at El Joyazo is probably due to the greater depth of melting (c. 25 km vs. c. 15 km). At such depth, corresponding to the Miocene palaeo-Moho, the more ductile regime of the crust is likely to have favoured magma ponding. The thermal anomaly beneath the Neogene Volcanic Province, which generated the S-type crustal melts, is today visible from geophysical data and can be traced back to the Lower Miocene. As a consequence, residence times longer than determined in this work may be expected.

Al- and F-rich titanite (3.86 < [Al.sub.2][O.sub.3] < 9.33 wt%, 0.93 < F < 2.53 wt%) from a polymetamorphic marble of the Sesia Zone (Western Alps) has been investigated in order to determine the behaviour of titanite during high-pressure metamorphism. Mesostructural to micro-structural relationships, mineral assemblages and petrological data indicate a pre-Alpine (low-pressure, high-temperature) to early-Alpine (high-pressure, medium-temperature) pressure-temperature-time evolution. Backscattered electron images, X-ray qualitative elemental maps and electron microprobe analysis show that most Al- and F-rich titanite that occurs as isolated crystals is rather homogeneous in composition. In contrast, titanite associated with, or rimmed by, typical early-Alpine, high-pressure minerals is characterized by variable [Al.sub.1][(F, OH).sub.1][Ti.sub.-1][O.sub.-1] substitution within single crystals, particularly at grain boundaries with omphacite and/or phengite. In-situ ion microprobe U-Pb analysis of titanite domains that have various Al and F contents yielded apparent [sup.206]Pb/[sup.238]U ages scattering between 283 and 153 Ma. Chemical and petrological data are indispensable to interpret this complex age distribution, and the good correlation between [sup.206]Pb/[sup.238]U ratios and Al content indicates that the Al- and F-rich titanite was formed during pre-Alpine metamorphism ([greater than or equal to] 281 [+ or -] 11 Ma). Progressively younger ages are obtained in domains with decreasing Al and F content, suggesting that partial chemical re-equilibration was responsible for the incomplete isotopic resetting during Alpine metamorphism. Petrological and U-Pb data show that Al- and F-rich titanite should be used with caution to infer high-pressure conditions in polymetamorphic carbonate systems.

Diamondiferous rocks from the Kokchetav Massif, Kazakhstan, represent deeply subducted continental crust. In order to constrain the age of ultra high pressure (UHP) metamorphism and subsequent retrogression during exhumation, zircons from diamondiferous gneisses and metacarbonates have been investigated by a combined petrological and isotopic study. Four different zircon domains were distinguished on the basis of transmitted light microscopy, cathodoluminescence, trace element contents and mineral inclusions. Mineral inclusions and trace element characteristics of the zircon domains permit us to relate zircon growth to metamorphic conditions. Domain 1 consists of rounded cores and lacks evidence of UHP metamorphism. Domain 2 contains diamond, coesite, omphacite and titanian phengite inclusions providing evidence that it formed at UHP metamorphic conditions (P > 43 kbar; T ~ 950 [degrees] C). Domain 3 is characterized by low-pressure mineral inclusions such as garnet, biotite and plagioclase, which are common minerals in the granulite-facies overprint of the gneisses (P ~ 10 kbar; T ~ 800 [degrees] C). This multi-stage zircon growth during cooling and exhumation of the diamondiferous rocks can be best explained by zircon growth from Zr-saturated partial melts present in the gneisses. Domain 4 forms idiomorphic overgrowths and the rare earth element pattern indicates that it formed without coexisting garnet, most probably at amphibolite-facies conditions (P ~ 5 kbar; T ~ 600 [degrees] C). The metamorphic zircon domains were dated by SHRIMP ion microprobe and yielded ages of 527 [+ or -] 5, 528 [+ or -] 8 and 526 [+ or -] 5 Ma for domains 2, 3 and 4 respectively. These indistinguishable ages provide evidence for a fast exhumation beyond the resolution of SHRIMP dating. The mean age of all zircons formed between UHP metamorphic conditions and granulite-facies metamorphism is 528 [+ or -] 3 Ma, indicating that decompression took place in less than 6 Ma. Hence, the deeply subducted continental crust was exhumed from mantle depth to the base of the crust at rates higher than 1.8 cm/year. We propose a two-stage exhumation model to explain the obtained P-T-t path. Fast exhumation on top of the subducted slab from depth > 140 to ~ 35 km was driven by buoyancy and facilitated by the presence of partial melts. A period of near isobaric cooling was followed by a second decompression event probably related to extension in a late stage of continental collision.

Byline: Daniela Rubatto (1), Ian S. Williams (1), Ian S. Buick (2) Abstract: We report an extensive field-based study of zircon and monazite in the metamorphic sequence of the Reynolds Range (central Australia), where greenschist- to granulite-facies metamorphism is recorded over a continuous crustal section. Detailed cathodoluminescence and back-scattered electron imaging, supported by SHRIMP U--Pb dating, has revealed the different behaviours of zircon and monazite during metamorphism. Monazite first recorded regional metamorphic ages (1576+-5Ma), at amphibolite-facies grade, at [proportional to]600degC. Abundant monazite yielding similar ages (1557+-2 to 1585+-3Ma) is found at granulite-facies conditions in both partial melt segregations and restites. New zircon growth occurred between 1562+-4 and 1587+-4Ma, but, in contrast to monazite, is only recorded in granulite-facies rocks where melt was present (aY=700degC). New zircon appears to form at the expense of pre-existing detrital and inherited cores, which are partly resorbed. The amount of metamorphic growth in both accessory minerals increases with temperature and metamorphic grade. However, new zircon growth is influenced by rock composition and driven by partial melting, factors that appear to have little effect on the formation of metamorphic monazite. The growth of these accessory phases in response to metamorphism extends over the 30Ma period of melt crystallisation (1557--1587Ma) in a stable high geothermal regime. Rare earth element patterns of zircon overgrowths in leucosome and restite indicate that, during the protracted metamorphism, melt-restite equilibrium was reached. Even in the extreme conditions of long-lasting high temperature (750--800degC) metamorphism, Pb inheritance is widely preserved in the detrital zircon cores. A trace of inheritance is found in monazite, indicating that the closure temperature of the U--Pb system in relatively large monazite crystals can exceed 750--800degC. Author Affiliation: (1) Research School of Earth Sciences, Australian National University, ACT 0200, Canberra, Australia (2) School of Earth Sciences and VIEPS, La Trobe University, 3038, Bundoora, Victoria, Australia Article History: Registration Date: 01/01/2001 Received Date: 07/04/2000 Accepted Date: 12/08/2000 Article note: Electronic Supplementary Material The online version of this article (doi: 10.1007/PL00007673 contains supplementary material, which is available to authorized users.

The collision between the Adriatic and European continental plates during the Cretaceous and Tertiary periods caused the formation of the Alpine chain and subduction of continental and oceanic crust under the Adriatic margin. A new study uses a cathodoluminescence-based ion microprobe to determine the dating of formation and subduction of the Zermatt-Saas-Fee ophiolites (ZSFO) found in the Alpine chain. It is suggested that ZSFO formation is inhibited by the intrusion ages of the ophiolitic metagabbros.

The Ivrea-Verbano Zone (IVZ), northern Italy, exposes an attenuated section through the Permian lower crust that records high-temperature metamorphism under lower crustal conditions and a protracted history of extension and exhumation associated partly with the Jurassic opening of the Alpine Tethys ocean. This study presents SHRIMP U-Pb geochronology of rutile from seven granulite facies metapelites from the base of the IVZ, collected from locations spanning ~35 km along the strike of Paleozoic fabrics. Rutile crystallised during Permian high-temperature metamorphism and anatexis, yet all samples give Jurassic rutile U-Pb ages that record cooling through 650-550 °C. Rutile age distributions are dominated by a peak at ~160 Ma, with a subordinate peak at ~175 Ma. Both ~160 and ~175 Ma age populations show excellent agreement between samples, indicating that the two distinctive cooling stages they record were synchronous on a regional scale. The ~175 Ma population is interpreted to record cooling in the footwall of rift-related faults and shear zones, for which widespread activity in the Lower Jurassic has been documented along the western margin of the Adriatic plate. The ~160 Ma age population postdates the activity of all known rift-related structures within the Adriatic margin, but coincides with extensive gabbroic magmatism and exhumation of sub-continental mantle to the floor of the Alpine Tethys, west of the Ivrea Zone. We propose that this ~160 Ma early post-rift age population records regional cooling following episodic heating of the distal Adriatic margin, likely related to extreme lithospheric thinning and associated advection of the asthenosphere to shallow levels. The partial preservation of the ~175 Ma age cluster suggests that the post-rift (~160 Ma) heating pulse was of short duration. The regional consistency of the data presented here, which is in contrast to many other thermochronometers in the IVZ, demonstrates the value of the rutile U-Pb technique for probing the thermal evolution of high-grade metamorphic terrains. In the IVZ, a significant decoupling between Zr-in-rutile temperatures and U-Pb ages of rutile is observed, with the two systems recording events ~120 Ma apart. Electronic supplementary material The online version of this article (doi:10.1007/s00410-015-1135-6) contains supplementary material, which is available to authorized users., Author(s): Tanya A. Ewing[sup.1] [sup.2] , Daniela Rubatto[sup.1] , Marco Beltrando[sup.3] , Jorg Hermann[sup.1] Author Affiliations: (1) Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia (2) [...]

The Kokchetav complex in Kazakhstan contains garnet-bearing gneisses that formed by partial melting of metasedimentary rocks at ultrahigh-pressure (UHP) conditions. Partial melting and melt extraction from these rocks is documented by a decrease in [K.sub.2]O and an increase in FeO + MgO in the restites. The most characteristic trace element feature of the Kokchetav UHP restites is a strong depletion in light rare earth elements (LREE), Th and U. This is attributed to complete dissolution of monazite/ allanite in the melt and variable degree of melt extraction. In contrast, Zr concentrations remain approximately constant in all gneisses. Using experimentally determined solubilities of LREE and Zr in high-pressure melts, these data constrain the temperature of melting to ~1,000 °C. Large ion lithophile elements (LILE) are only moderately depleted in the samples that have the lowest U, Th and LREE contents, indicating that phengite retains some LILE in the residue. Some restites display an increase in Nb/Ta with respect to the protolith. This further suggests the presence of phengite, which, in contrast to rutile, preferentially incorporates Nb over Ta. The trace element fractionation observed during UHP anatexis in the Kokchetav gneisses is significantly different from depletions reported in low-pressure restites, where generally no LREE and Th depletion occurs. Melting at UHP conditions resulted in an increase in the Sm/Nd ratio and a decoupling of the Sm-Nd and Lu-Hf systems in the restite. Further subduction of such restites and mixing with mantle rocks might thus lead to a distinct isotopic reservoir different from the bulk continental crust. Keywords Anatexis * UHP metamorphism * REE * Accessory minerals, Introduction At active plate margins, the crust and its sedimentary cover are subducted into the mantle and undergo heating, dehydration, mixing with mantle material and partial melting. This is one [...]

[1] We present new U-Pb sensitive, high-resolution ion microprobe (SHRIMP) dates for zircon from eclogite facies samples from northern New Caledonia, which have important consequences for the Late Cretaceous to Eocene tectonic evolution of the southwest Pacific region. Two high-pressure metapelites contain zoned zircon crystals. Cores of these crystals represent inherited magmatic zircon grains, which yield an age of 85 Ma in both samples. Igneous zircon domains from a metamorphosed mafic cumulate and a felsic metasedimentary rock were also dated at 55 Ma. The igneous zircon of uniform age in rocks of sedimentary origin indicates that sedimentation took place from a restricted source area soon after magmatism. Therefore the protoliths of the eclogite facies rocks are considered to have formed between 85 and 55 Ma in a back-arc basin. This time period matches the age of the unmetamorphosed Poya Terrane of western New Caledonia and provides further evidence for a direct link between the two terranes. In the metapelites, irregular zircon rims that cross cut the oscillatory zoning of the cores have low trace element and Th/U contents and contain inclusions of high-pressure minerals providing evidence that these zircon rims formed during eclogite facies metamorphism. Therefore the 44.1 [+ or -] 0.9 Ma and 44.5 [+ or -] 1.2 Ma ages obtained from these rims represent burial related to subduction of the back-arc basin. It is suggested that subduction initiated soon after deposition of the youngest sediments at 55 Ma and stopped at 44 Ma during attempted subduction of continental crust, now represented as the Norfolk Ridge. Therefore the subduction system that produced the high-pressure rocks could only have operated for a maximum of 11 m.y. and would have been relatively hot. The 44 Ma age constraint for peak metamorphism also significantly predates (by 10 m.y.) the timing of obduction of the New Caledonia Ultramafic Nappe, discounting any link between obduction and high-pressure metamorphism. Combining our results with previously documented geological data, we present a revised model for the geological development of New Caledonia, which involves multiple episodes of compression and extension during the Eocene. Citation: Spandler, C., D. Rubatto, and J. Hermann (2005), Late Cretaceous-Tertiary tectonics of the southwest Pacific: Insights from U-Pb sensitive, high-resolution ion microprobe (SHRIMP) dating of eclogite facies rocks from New Caledonia, Tectonics, 24, TC3003, doi:10.1029/2004TC001709.

We produced a pressure-temperature-time path in order to determine the exhumation rate of the deepest subducted Alpine rocks. In situ dating of peak-metamorphic titanite in an eclogite facies calc-silicate rock indicates that subduction to pressures of ~3.5 GPa was reached at 35.1 [+ or -] 0.9 Ma. Titanite formed during two decompression stages, at 1 [+ or -] 0.15 GPa and ~0.4-0.5 GPa, and yielded ages of 32.9 [+ or -] 0.9 Ma and 31.8 [+ or -] 0.5 Ma, respectively. Combining the age data and making assumptions about the conversion of pressure to depth yield mean exhumation rates of 3.4 cm/yr and 1.6 cm/yr. These rates imply that exhumation acted at plate tectonic speeds similar to subduction, and was significantly faster than erosion. We suggest that fast exhumation is driven by a combination of tectonic processes involving buoyancy and normal faulting. Keywords: titanite, exhumation, high-pressure metamorphism, U/Th/Pb, geochronology, Western Alps.