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The Western Carpathians crystalline basement recorded common indications of the Cadomian/Avalonian basement precursors. There the pre-Mesozoic basement of the Tatric Unit was studied with respect to its lithological, structural, metamorphic and age characteristics. The obtained data proves that this basement is made of two different rock complexes—the older Lower Étage (Cambrian to Silurian) showing high-grade metamorphic evolution, and the younger Upper Étage (Upper Silurian to Devonian) presenting low-grade metamorphism and younger stage of development. First representative U–Th–Pb detrital zircon data from metamorphosed siliciclastics from the Tatric Unit of the Western Carpathians indicate a general dominance of Neoproterozoic—mainly Ediacaran source rocks. The latter are interpreted to have been located at the Cadomian arc supplied mostly from the Saharan Metacraton. Combined with less frequent Archean and Paleoproterozoic zircon populations a common Gondwanan provenance derived from reworked West African Craton is proposed for part of the samples. However, some samples contain Mesoproterozoic detrital zircons what is typical for the Avalonian microcontinent. The studied metamorphic rocks are interpreted to originate from the Rheic Ocean and also partly from the Rheno-Hercynian basin. Indeed, significant differences in the metamorphic evolution of both metamorphic rock assemblages suggest their present juxtaposition and or their thrusting during the Late Devonian to Carboniferous (Visean) Variscan subduction/collision processes. [ABSTRACT FROM AUTHOR]

Understanding mass transfer associated with fluids circulation and deformation in the Alpine orogeny is often complex due to common multistage crystallization. For example, in two emblematic and historic Pb-Ag deposits of the French Alps, Macôt-la Plagne (MP) and Peisey-Nancroix (PN), a sedimentary or orogenic origin is still debated. To discriminate between the metallogenic models of the two deposits, an integrative methodology combining field, microstructural, mineralogical, thermobarometrical, and geochronological data was here applied for establishing detailed Pressure–Temperature–Time–Deformation (P-T-t-d) mineralization conditions. Both deposits are located in Permo-Triassic quartzite of the External Briançonnais domain along the Internal Briançonnais Front (Internal Western Alps). The ore mainly occurs as veins and disseminated textures containing galena, pyrite, and variable content of tetrahedrite–tennantite and chalcopyrite. Quartz porphyroclasts and sulfide microstructures indicate a dynamic recrystallization of the quartzite during the main fluid mineralization episode. Chlorites and K-white micas (phengite) chemical analysis and thermodynamic modeling from compositional maps indicate an onset of the mineralization at 280 °C, with a main precipitation stage at 315 ± 35 °C and 6.25 ± 0.75 kbar. In situ U-Pb dating on monazite, cogenetic with sulfides, gives ages around 35 Ma for both deposits. The integrative dataset converges for a cogenetic MP-PN Alpine Pb-Ag mineralization during deformation in relation to the thrusting of the "Nappe des Gypses" and the Internal Briançonnais at the metamorphic peak. [ABSTRACT FROM AUTHOR]

In situ laser ablation inductively coupled mass spectrometry (LA ICP-MS) is used to determine the U-Th-Pb age of the phosphates ferromerrillite and apatite in the Los Angeles shergottitic meteorite. The initial 207Pb/206Pb was refined by analyzing K-rich diaplectic glass. LA ICP-MS mapping was used to document zones of elevated U and Th content and to establish textural controls on isotope ages. By critically assessing dispersion in the U-Th-Pb dataset due to Pb-diffusion in phosphates during high-temperature shock metamorphism, and as a result of subsequent terrestrial contamination, we obtain a best-estimate U-Pb age of 169 ± 5 Ma anchored at an initial 207Pb/206Pb of 0.98390 ± 0.00018. This is statistically indistinguishable from a joint-isochron age of 179 ± 6 with initial 208Pb/206Pb of 2.5151 ± 0.0028. These results complement previously determined Rb-Sr and Sm-Nd isotope ages and provide independent evidence for LA having crystallized as a medium-grained basic rock from a thick lava flow or high-level intrusion in the late Amazonian at ∼170 Ma. In the context of martian mantle evolution, the initial common-Pb values suggest that Los Angeles originated from a source (µ 2 ∼ 3.2) that is similar to enriched members of the shergottite meteorite clan. The U-Th-Pb systematics of both ferromerrillite and apatite were locally affected by diffusive Pb-loss in thin U-enriched marginal domains and more profoundly in shock-induced melt pockets where temperatures briefly exceeded 2000 °C. The results reveal: (1) how precise U-Pb ages can be attained from phosphates; (2) the importance of microtextural contextualization of isotope data; (3) that the timescales of cooling from shock conditions were sufficient to promote local diffusive re-equilibration of Pb over 10 s of microns; and (4) that LA ICP-MS mapping can be used to locate domains with the highest U/Pb and Th/Pb, which increases precision on lower intercept ages and isochron regression lines. [ABSTRACT FROM AUTHOR]

In recent decades, constraining the timing of shear activity has been one of the main topics of research about the tectono-metamorphic evolution of orogenic belts. We present a review of a combined structural and geochronological approach to two major ductile regional shear zones, in two collisional orogens: the first one affecting the Variscan basement in northern Sardinia (Italy) and the External Crystalline Massifs of the Alps (East Variscan Shear Zone; EVSZ), and the second one deforming the medium- to high-grade rocks of the metamorphic core of the Himalaya (High Himalayan Discontinuity). High-resolution, texturally and chemically controlled monazite geochronology applied in separated shear zones of the Variscan belt allowed recognizing a similar timing of activity ranging between c. 340–330 and 300 Ma. This approach led to a better understanding of the evolution of the EVSZ, supporting a model where several branches were active according to a growth by linkage model. Following a similar approach, in situ U-Th-Pb analysis of monazite constrained the timing of top-to-the-S/SW shearing of a regional-scale High Himalayan Discontinuity in the Himalayan belt to between c. 28 Ma and 17 Ma. Earlier exhumation of the hanging wall was triggered by shear zone activity, whereas at the same time, the footwall was still experiencing burial with increasing P-T conditions. The timing of shearing of this shear zone fits with an in-sequence shearing tectonic model for the exhumation of the Himalayan mid-crust. [ABSTRACT FROM AUTHOR]

Understanding mass transfer associated with fluids circulation and deformation in the Alpine orogeny is often complex due to common multistage crystallization. For example, in two emblematic and historic Pb-Ag deposits of the French Alps, Macôt-la Plagne (MP) and Peisey-Nancroix (PN), a sedimentary or orogenic origin is still debated. To discriminate between the metallogenic models of the two deposits, an integrative methodology combining field, microstructural, mineralogical, thermobarometrical, and geochronological data was here applied for establishing detailed Pressure–Temperature–Time–Deformation (P-T-t-d) mineralization conditions. Both deposits are located in Permo-Triassic quartzite of the External Briançonnais domain along the Internal Briançonnais Front (Internal Western Alps). The ore mainly occurs as veins and disseminated textures containing galena, pyrite, and variable content of tetrahedrite–tennantite and chalcopyrite. Quartz porphyroclasts and sulfide microstructures indicate a dynamic recrystallization of the quartzite during the main fluid mineralization episode. Chlorites and K-white micas (phengite) chemical analysis and thermodynamic modeling from compositional maps indicate an onset of the mineralization at 280 °C, with a main precipitation stage at 315 ± 35 °C and 6.25 ± 0.75 kbar. In situ U-Pb dating on monazite, cogenetic with sulfides, gives ages around 35 Ma for both deposits. The integrative dataset converges for a cogenetic MP-PN Alpine Pb-Ag mineralization during deformation in relation to the thrusting of the “Nappe des Gypses” and the Internal Briançonnais at the metamorphic peak.

In recent decades, constraining the timing of shear activity has been one of the main topics of research about the tectono-metamorphic evolution of orogenic belts. We present a review of a combined structural and geochronological approach to two major ductile regional shear zones, in two collisional orogens: the first one affecting the Variscan basement in northern Sardinia (Italy) and the External Crystalline Massifs of the Alps (East Variscan Shear Zone; EVSZ), and the second one deforming the medium- to high-grade rocks of the metamorphic core of the Himalaya (High Himalayan Discontinuity). High-resolution, texturally and chemically controlled monazite geochronology applied in separated shear zones of the Variscan belt allowed recognizing a similar timing of activity ranging between c. 340–330 and 300 Ma. This approach led to a better understanding of the evolution of the EVSZ, supporting a model where several branches were active according to a growth by linkage model. Following a similar approach, in situ U-Th-Pb analysis of monazite constrained the timing of top-to-the-S/SW shearing of a regional-scale High Himalayan Discontinuity in the Himalayan belt to between c. 28 Ma and 17 Ma. Earlier exhumation of the hanging wall was triggered by shear zone activity, whereas at the same time, the footwall was still experiencing burial with increasing P-T conditions. The timing of shearing of this shear zone fits with an in-sequence shearing tectonic model for the exhumation of the Himalayan mid-crust.

The first results of U–Th–Pb and Lu–Hf isotope studies of detrital zircons from sedimentary rocks of the Daur Series (Argun continental massif of the Central Asian Fold Belt) are presented. It is established that the youngest detrital zircons from sandstones of the Urulungui and Dyrbylkei formations of the Daur Series have the ages of 899 and 771 Ma, respectively. These data determine the maximum depositional age of the Daur Series and agree with the current ideas about the Late Riphean age of this series. The majority of Late Riphean detrital zircons from sedimentary rocks of the Daur Series are characterized by Early Precambrian Lu–Hf model ages. In addition, the Archean and Early Proterozoic detrital zircons are present in the studied rocks. This suggests either the presence of ancient (Early Precambrian) recycled material in the sedimentary rocks of the Daur Series or the fact that the studied part of the Argun massif is an "exotic block," whose origin requires additional research. [ABSTRACT FROM AUTHOR]

The results of geological, geochemical, Sm–Nd isotopic geochemical studies of metamorphic rocks of the Djagdagle Formation and U–Pb geochronological studies of its zircons are presented. In correlation schemes of geological complexes of the Bureya Terrane, this formation is the oldest (conditionally, Late Archean) amid other formations of the terrane; thus, it is considered its basement. The biotite and two-mica garnet-bearing gneisses of the formation formed owing to transformation of primary sedimentary rocks. The maxima on the relative probability curve of zircon age correspond to 487, 541, 690, 778, and 896 Ma. Thus, the Djagdagle Formation is Paleozoic rather than Early Precambrian, as was considered before. The lower age boundary of the accumulation of the protolith is identified by the age of the youngest zircon population of ~487 Ma. Similar ages of some cores and rims of zircons indicate that the rocks of the protolith underwent structural metamorphic transformations under PT conditions of amphibolite facies. The results of Sm–Nd and Lu–Hf isotopic studies indicate that the geological complexes which originated by means of reworking of the Mesoproterozoic crust could have been the source of primary sedimentary rocks. Taking into account the lack of Mesoproterozoic (and older) rocks in the structure of the Bureya Terrane, it can be suggested that the sources included some amount of ancient recycled material. Similar geological evolution of the Bureya and Jiamusi terranes is suggested on the basis of similar age ranges of metasedimentary rocks of the Djagdagle Formation and Turan Group of the Bureya Terrane and the Kimkan Formation of the Jiamusi Terrane. This is also evident from their synchronous Neoproterozoic magmatism. [ABSTRACT FROM AUTHOR]

The Mogok metamorphic belt (MMB) extends for over 1,000 km along central Burma from the Andaman Sea to the East Himalayan syntaxis and represents exhumed lower and middle crustal metamorphic rocks of the Sibumasu plate. In the Mogok valley region, the MMB consists of regional high‐grade marbles containing calcite + phlogopite + spinel + apatite ± diopside ± olivine and hosts world class ruby and sapphire gemstones. The coarse‐grained marbles have been intruded by orthopyroxene‐ and clinopyroxene‐bearing charnockite‐syenite sheet‐like intrusions that have skarns around the margins. Syenites range from hornblende‐ to quartz‐bearing and frequently show layering that could be a primary igneous texture or a later metamorphic overprint. Calc‐silicate skarns contain both rubies and blue sapphires with large biotites. Rubies occur in marbles with scapolite, phlogopite, graphite, occasional diopside, and blue apatite. Both marbles and syenites have been intruded by the Miocene Kabaing garnet‐muscovite‐biotite peraluminous leucogranite. New mapping and structural observations combined with U‐Th‐Pb zircon, monazite, and titanite geochronology from syenites, charnockites, leucogranites, meta‐rhyolite‐tuffs, and skarns have revealed a complex multiphase igneous and metamorphic history for the MMB. U‐Pb zircon ages of the charnockite‐syenites fall into three categories, Jurassic (170–168 Ma), latest Cretaceous to early Paleocene (~68‐63 Ma), and late Eocene–Oligocene (44–21 Ma). New ages from five samples suggest that metamorphism in the presence of garnet and melt occurred between ~45 and 24 Ma. U‐Pb titanite ages from the ruby marbles and meta‐skarns at Le Oo mine in the Mogok valley are 21 Ma, similar to titanite ages from an adjacent syenite (22 Ma). U‐Th‐Pb dating shows that all the metamorphic ages are Late Cretaceous–early Miocene and related to the India‐Sibumasu collision. Key Points: Rubies and sapphires in granulite‐facies marbles from the Mogok metamorphic belt, Myanmar, are spatially associated with charnockite‐syenite sill‐like intrusions and surrounding skarnsU‐Th‐Pb LA‐ICPMS dating of zircon, monazite, and titanite shows that there were two groups of charnockite‐syenite dates, one Jurassic in age (170–168 Ma) and one latest Cretaceous to early Miocene (~68–21 Ma)Regional granulite‐facies metamorphism along the Mogok metamorphic belt is Late Cretaceous to Oligocene or early Miocene in age (~68‐21 Ma), peaking with garnet‐present melting between 45 and 21 Ma [ABSTRACT FROM AUTHOR]

Abstract—The paper reports first geochemical data on the Cambrian sedimentary rocks of the Mel'gin trough of the Bureya continental massif, as well as geochronological data on detrital zircons from these rocks. It is established, that sandstones of the Chergilen and Allin formations of the Mel'gin trough are dominated by detrital zircons with Late Riphean (peaks on relative probability plots at 0.78, 0.82, 0.94, and 1.04 Ga) and Early Riphean (peaks on relative probability plots at 1.38, 1.45, 1.64 Ga) ages. Single grains have Middle Riphean, Early Proterozoic and Late Archean ages. We can suppose that the sources of Late Riphean detrital zircons in sandstones of the Chergilen and Allin formations are rocks of gabbro‒granitoids (940‒933 Ma) and granite‒leucogranite (804‒789 Ma) associations identified in the Bureya continental massif. It is impossible to determine which rocks were sources for Middle Riphean and older detrital zircons in the Cambrian sedimentary rocks of the Bureya continental massif, because the pre-Late Riphean complexes have not been identified yet in its structure. The most probable geodynamic setting for the accumulation of the Cambrian deposits of the Mel'gin trough is an active continental margin, which is consistent with manifestation of the Early Cambrian granitoids magmatism. [ABSTRACT FROM AUTHOR]

The Adamello intrusive suite is a composite batholith in Northern Italy, with an estimated 2000 km3 volume, assembled incrementally over a time span of 10 to 12 million years. The history of crystallization has been studied in detail through laser ablation ICP-MS and SIMS U–Pb geochronology of zircon, which records prolonged crystallization of each of the different intrusive units at mid-crustal levels between 43·47 and 33·16 Ma. The magmas were episodically extracted from this storage area and ascended to the final intrusion level at ∼6 km paleo-depth. Each batch of melt cooled very rapidly down to the ambient temperature of 250°C, evidenced by distinct cooling paths recorded by amphibole, biotite and K-feldspar 40Ar/39Ar dates. The magma source area was moving from SW to NE with time, causing increasing thermal maturity in the mid-crustal reservoir. The resulting temporal trend of higher degrees of crustal assimiliation in the course of the evolution of the magmatic system can be traced through Hf and O isotopes in zircon. Rough estimates of magma emplacement rates ('magma flux') yield very low values in the range of 10-4 km3/yr, typical of mid-to-upper crustal plutons and increase with time. Although we cannot discern a decrease of magma flux from our own data, we anticipate that a dramatic decrease of magma flux between 33 and 31 Ma along the northern contact lead to cessation of magma emplacement. [ABSTRACT FROM AUTHOR]

The Oued Togba and Sebkha Gezmayet units of the Adrar Souttouf Massif in the southern Moroccan Sahara are thought to represent tectonic fragments that may have an affinity to the Avalonian and Meguma terranes of eastern North America. Here we study siliciclastic rocks of the Sebkha Gezmayet unit with respect to their detrital zircon spectra. Beside the commonly used U–Th–Pb ages, several aspects of zircon morphology (length, width, roundness, surficial indicators of sedimentary transport, morphotype) are described. The detrital zircon age spectrum of the Sebkha Gezmayet unit resembles that of the already dated underlying igneous rocks. Occurrences of Early Devonian zircon ages are at odds with the magmatic history of the West African Craton but are common in the Avalonian and Meguma terranes, which were affected by the Appalachian orogenies. The scarcity of Mesoproterozoic detrital zircon grains corroborates the previously suggested Meguma terrane affinity of this part of the Adrar Souttouf Massif. Combining zircon morphology and isotopic data, we propose first assumptions on the sedimentary environments of the Sebkha Gezmayet unit during different periods of the Palaeozoic. [ABSTRACT FROM AUTHOR]

The identification of markers of subduction zones in orogenic belts requires the estimation of paleo-geothermal gradients through pressure-temperature-time (P-T-t) estimates in mafic rocks that potentially derive from former oceanic units once. However, such markers are rare in supracrustal sequences specially in deeply eroded and weathered Precambrian orogens, and reconstructing their metamorphic history is challenging because they are commonly retrogressed and only preserve a few mineral relicts of high-pressure metamorphism. Metamorphosed mafic rocks from Pouso Alegre region of the Neoproterozoic Southern Brasília Orogen outcrop as rare lenses within continental gneisses. They have previously been classified as retrograde eclogites, based on the presence of garnet and the characteristic symplectitic texture replacing omphacite. These rocks were interpreted to mark the suture zone between the Paranapanema and São Francisco cratons. To test the possible record of eclogitic conditions in the Pouso Alegre mafic rocks, samples including the surrounding felsic rocks have been investigated using quantitative compositional mapping, forward thermodynamic modeling and in-situ dating of accessory minerals to refine their P-T-t history. In the metamorphosed mafic rocks, the peak pressure assemblage of garnet and omphacite (Jd 20 , reconstructed composition) formed at 690 ± 35 °C and 13.5 ± 3.0 kbar, whereas local retrogression into symplectite or corona occurred at 595 ± 25 °C and 4.8 ± 1.5 kbar. The two reactions were coupled and thus took place at the same time. A zircon U-Pb age of 603 ± 7 Ma was obtained for metamorphic rims and linked to the retrogression stage. Monazite and metamorphic zircon U-Th-Pb ages for the surrounding rocks are at ca. 630 Ma and linked to peak pressure conditions similar to the one recorded by the mafic rocks. The low maximal pressure of 14 kbar and the high geothermal gradient do not necessarily support subduction process-related metamorphism but, more likely, metamorphism related to continental collision. [ABSTRACT FROM AUTHOR]

Detrital zircon provenance studies are an established tool to develop palaeogeographic models, mostly based on zircon of siliciclastic rocks and isotope data. But zircon is more than just istopes and features well definable morphological characteristics. The latter may indicate single grain transport histories independent of the individual grade of concordance. This additional tool for palaeogeoraphic reconstructions was tested on zircon from siliciclastic and carbonate sedimentary rocks of Palaeozoic age from the Aoucert and Smara areas of the Souttoufides, while findings of zircon in limestone generally open new archives for sedimentary provenance analysis. The morphologies-length, width, roundness, grain surfaces-of 834 detrital zircons from sediments of allochthonous Cambrian, and (par-)autochthonous Ordovician, and Devonian units were studied, while 772 of them were analysed for their U-Th-Pb isotopes by LA-ICP-MS. Mesoproterozoic zircon contents of more than 10% in the Cambrian sediments exclude the West African Craton (WAC) as exclusive source area. Thus, at least one additional external source is suggested. This is likely the western Adrar Souttouf Massif with its significant Mesoproterozoic zircon inheritance, or comparable, yet unknown sources. Decreasing Mesoproterozoic zircon age populations in Ordovician sediments are thought to be linked to the rifting of the terranes in the course of the Rheic Ocean opening and a predominant supply of WAC detritus. The Devonian sediments likely contain reworked material from the Cambrian siliciclastics, which is shown by the zircon age distribution pattern and the zircon morphologies. Therefore, multiple shifts in the direction of sedimentary transport are indicated. [ABSTRACT FROM AUTHOR]

The Gavilgarh-Tan Shear Zone (GTSZ) is a crustal-scale shear/fault zone that dissects the unclassified basement gneisses separating two major supracrustal belts, viz. the Paleo- to Mesoproterozoic (≥1.5 Ga) Betul Belt and the Neoproterozoic (∼1.0 Ga) Sausar Belt, of the Central Indian Tectonic Zone (CITZ). The GTSZ extends for more than 300 km strike length, partly covered by the Deccan Trap flows. Granitoid rocks ranging from syenogranite to granodiorite in composition, sheared at temperatures corresponding to the amphibolite facies metamorphic condition, define the GTSZ in the Kanhan River Valley. Earlier geological studies have suggested that the GTSZ underwent a sinistral-sense partitioned transpression in response to an oblique collision between two continental fragments, possibly related to crustal thickening and high-pressure granulite metamorphism (the Ramakona-Katangi granulite: RKG) in the northern part of the Sausar Belt. LA-ICP-MS U-Pb dating of zircon and EPMA U-Th-total Pb dating of monazite grains from four different types of syn-tectonic granitoids of the GTSZ carried out in the present study show that granitoids intruded the basement gneisses between 1.2 Ga and 0.95 Ga, given the error limit of the calculated ages. The age of transpression and mylonitization is more definitely bracketed between 1.0 Ga and 0.95 Ga, which correlates well with the published ages of deformation and metamorphism in the Sausar Belt. This age data strongly supports the suggested collisional tectonic model involving the GTSZ and the RKG granulites of the Sausar Belt and underlines a Grenvillian-age tectonic history for the southern part of the Central Indian Tectonic Zone (CITZ), which possibly culminated in the crustal assembly of the Neoproterozoic supercontinent Rodinia. [ABSTRACT FROM AUTHOR]

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), sillimanite (M) and cordierite-spinel grade (M). 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-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-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. [ABSTRACT FROM AUTHOR]

The Jagtiyal section exposes a part of the Precambrian granulite – facies terrane in the northeastern Dharwar Craton, specifically charnockite, orthogneiss, mafic granulite, garnet-cordierite gneiss and silica-deficient Mg-Al granulite. The silica-deficient Mg-Al granulites exhibit well-preserved metamorphic reaction microstructures and were used to elucidate the metamorphic history of the study area. Textural evidence exists for the prograde break-down of biotite and corundum and formation of a peak metamorphic assemblage of spinel, sapphirine, orthopyroxene, cordierite and K-feldspar. KFMASHTO system based P-T pseudosections as well as the application of multi-equilibrium calculation for mineral core compositions, gave P-T conditions of 7.5–8 kbar and 800–840 °C. The thermal peak was followed by near isothermal decompression that led to a variety of coronal textures including, successive rims on corundum, consisting of spinel + sapphirine + cordierite ± orthopyroxene, and symplectitic intergrowth of sapphirine-cordierite, orthopyroxene-cordierite, spinel-cordierite, and biotite-cordierite. Modelled P-T conditions for this stage are 4.3–4.5 kbar and 750–800 °C. Few samples show textural evidence for further retrogression leading to the break-down of sapphirine and cordierite to form chlorite, corundum, spinel and andalusite. The proposed clockwise P-T path implies that granulites from the Jagtiyal section likely resulted from thickened continental crust undergoing decompression. New U-Pb zircon geochronologic data provide evidence for a Neoarchaean (2604 ± 25 Ma) age that likely corresponds to the timing of accretion of the terrane that was subjected to high temperature metamorphism at that time. Moreover, there is evidence of a Neoproterozoic thermal overprint in the area around 638 Ma. . [ABSTRACT FROM AUTHOR]

Carbonatites are carbonate-dominated igneous rocks derived by low-degree partial melting of metasomatized mantle, although the geodynamic processes responsible for their emplacement into the crust are disputed. Current models favor either reactivation of lithospheric structures in response to plate movements, or the impingement of mantle plumes. Geochronology provides a means of testing these models, but constraining the age of carbonatites and related metasomatic events is rarely straightforward. We use in situ U-Th-Pb analysis of monazite by SHRIMP to constrain the emplacement age and hydrothermal history of the rare earth element-bearing Gifford Creek Carbonatite Complex in Western Australia, which has been linked to plume magmatism at ca. 1075 Ma. Monazite in carbonatites and related metasomatic rocks (fenites) from the carbonatite complex dates the initial emplacement of the carbonatite at 1361 ± 10 Ma ( n = 22, MSWD = 0.91). The complex was subjected to multiple stages of magmatic/hydrothermal overprinting from ca. 1300 Ma to 900 Ma during later regional tectonothermal events. Carbonatite emplacement at ca. 1360 Ma appears to be an isolated igneous event in the region, and occurred about 300 million years before intrusion of the ca. 1075 Ma Warakurna large igneous province, thus precluding a genetic connection. The Gifford Creek Carbonatite Complex occurs within a major crustal suture, and probably formed in response to reactivation of this suture during plate reorganization. Our study demonstrates the veracity of monazite geochronology in determining the magmatic and hydrothermal histories of a carbonatite complex, critical for evaluating competing geodynamic models for carbonatites. The approach involving in situ SHRIMP U-Th-Pb dating of monazite from a wide spectrum of rocks in a carbonatite complex is best suited to establishing the intrusive age and hydrothermal history of carbonatites. [ABSTRACT FROM AUTHOR]

Ediacaran times witnessed a hemisphere-scale orogenesis, forming the extensive Pan-African mountain ranges and resulting in the final assembly of the Gondwana supercontinent. The Elat metamorphic association (S Israel), located at the northernmost tip of the Arabian Nubian Shield (ANS), comprises amphibolite-facies pelitic and psammitic schists and orthogneisses and was presumably shaped by Ediacaran East African orogenesis. However, the timing, number and duration of metamorphic events in Elat and elsewhere in the ANS are inconclusive and a major emphasis was previously given to pre-Ediacaran island-arc related tectonics. Here U-Th-Pb dating of monazite, a precise chronometer of metamorphic mineral growth, is systematically applied to the Elat schist and unveils the tectono-metamorphic evolution of the Elat association. In-situ analysis of metamorphic monazites by LASS-ICPMS (Laser-Ablation Split-Stream Inductively Coupled Plasma Mass Spectrometry) yields two concordant age clusters at ∼710 and 620 Ma, recording two regional metamorphic events. The corresponding REE patterns of the dated monazite grains indicate that garnet porphyroblast growth took place only during the younger event (M2). Likewise, the regional south-dipping penetrative foliation (S2), common to the Elat schist and the surrounding orthogneisses, formed during the Ediacaran event (M2). The peak M2 staurolite-garnet-sillimanite assemblage was overprinted by retrograde andalusite and cordierite during decompression, followed by the intrusion of andesitic dykes that were immediately metamorphosed at greenschist-facies conditions. This metamorphic P-T-d-t path corresponds to the collision of the already accreted arc terranes of the ANS with the Saharan Metacraton, as also suggested by the large geochronological database of 610–580 Ma post collisional granitoid batholiths throughout NE Africa and Arabia. The Cryogenian M1 metamorphism therefore records arc-arc collision during major convergence forming the proto-ANS to the south. [ABSTRACT FROM AUTHOR]

Baddeleyite has been recognized as a key mineral to determine the crystallization age of silica-undersaturated igneous rocks. Here we report a new occurrence of baddeleyite identified from REE-Nb-Th-rich carbonatite in the world's largest REE deposit, Bayan Obo, in the North China Craton (China). U-Th-Pb dating of three baddeleyite samples yields crystallization ages of 310–270 Ma with the best estimated crystallization age of ca. 280 Ma. These ages are significantly younger than the ca. 1300 Ma Bayan Obo carbonatites, but broadly coeval to nearby Permian granitoids intruding into the carbonatites. Hence, the Bayan Obo baddeleyite did not crystallize from the carbonatitic magma that led to the formation of the Bayan Obo carbonatites and related REE-Nb-Th deposit. Instead, it crystallized from hydrothermal fluids and/or a reaction involving zircon and dolomite during contact metamorphism related to the Permian granitoid emplacement. This is in agreement with the results of electron microprobe analysis that show humite inclusions in baddeleyite, since humite is a typical magnesian skarn mineral and occurs in close proximity to the intrusive contacts between carbonatites and granitoids. Our results show that baddeleyite can be used for dating hydrothermal and contact metamorphic processes. [ABSTRACT FROM AUTHOR]

The late Variscan Serre Batholith in central Calabria represents the middle portion, c. 13-km-thick, of a tilted crustal section continuously exposed from lower-crustal granulites to upper-crustal phyllites. The batholith is zoned, consisting of several granitoid types that were emplaced at depths ranging from c. 23 to c. 6 km. Deep, strongly foliated quartz diorites and tonalites were emplaced into migmatitic metapelites, the intermediate level granitoids are weakly foliated to unfoliated porphyritic granodiorites and monzogranites, and the shallowest bodies are two-mica granodiorites and granites, grading upward to biotite ± amphibole granodiorites, emplaced into paragneisses and phyllites. Five samples, representative of the main granitoid types in terms of both composition and emplacement depth, have been dated by SHRIMP. Zircon from a lower-crustal quartz diorite gave an emplacement age of 297.3 ± 3.1 Ma; two middle-crustal strongly peraluminous K-feldspar megacrystic granites were emplaced at 296.1 ± 1.9 Ma and 294.9 ± 2.7 Ma; a middle- to upper-crustal two-mica monzogranite was emplaced at 294.2 ± 2.6 Ma and finally, an upper-crustal weakly peraluminous granodiorite from the batholith roof was emplaced at 292.2 ± 2.6 Ma. These results are consistent with the development of the batholith by incremental multipulse overaccretion, each granitoid body being in a dominantly rigid state before intrusion of the next, with little or no possibility of magma mixing. The difference in age between the oldest and youngest granitoids, from the batholith floor and roof, respectively, is 5.1 ± 4.0 Ma, providing an upper limit of about 9 Ma on the time taken for batholith construction. The presence in the c. 296–294 Ma granitoids of c. 305–302 Ma anatectic zircon with varied Th/U and oxygen isotope compositions indicates a time interval of 8–9 Ma between incipient partial melting in the lower crust and magma emplacement in the middle crust. The emplacement of the first granitoid bodies into the top of the lower crust was controlled tectonically by the activation of a deep-seated shear zone. The shallowest granodiorites were emplaced during the waning stages of the shear zone activity, producing late- to post-tectonic contact metamorphism in the upper-crustal phyllites and mylonitic paragneisses. [ABSTRACT FROM AUTHOR]

The distribution of Th and U in a crystal of pegmatitic xenotime-(Y) (henceforth named xenotime) from Ås II feldspar quarry (Evje, S Norway) has been studied with transmission electron microscopy (TEM) to determine submicron- to nanoscale processes that might have affected U-(Th-)Pb age record. Xenotime contains high Th and U unaltered domains, and Th and U depleted altered domains, which contain numerous inclusions of (Th, U)-silicate and uraninite. A focused ion beam foil (FIB-foil) prepared from the unaltered xenotime, examined with transmission electron microscopy (TEM), revealed a network of ca. 15–20 nm-thick and up to several hundred nanometers-long dislocations. The dislocation cores contain U, Th and Si enriched material, and are surrounded on both sides by radiation-damaged xenotime. Rarely present "empty" dislocations are indicative of nanoscale-sized porosity in xenotime. The presence of a network of line defects in xenotime indicates plastic deformation of the xenotime crystal, which resulted in an initial stage of the formation of partially open phase boundaries. The dislocations network has been infiltrated by fluid, which mediated coupled dissolution-reprecipitation reactions. Segregation of U and Th into the dislocation cores, and selective transport of highly fluid-mobile U outside of the xenotime is suggested to be a source of Pb-excess and reverse discordance in the 207Pb/235U vs. 206Pb/238U concordia diagrams. The new finding of migration and finally segregation of U and Th into dislocation cores at the atomic-scale demonstrates the importance of nanoscale investigations for better understanding of the obtained age data and provides crucial implications for applications of xenotime geochronology. • dislocation cores in xenotime contain U, Th and Si enriched material. • dislocations are surrounded on both sides by radiation-damaged xenotime. • transport of U in dislocations network is a suggested source of age discordance. [ABSTRACT FROM AUTHOR]

The Pan-African Lufilian orogenic belt hosts world-class Cu deposits. In the Congolese Copperbelt (DRC), Cu(–Co) deposits, are mostly hosted within evaporitic and siliciclastic Neoproterozoic metasedimentary rocks (Mines Subgroup) and are interpreted as syn- to late-diagenetic deposits. In this paper, we present new data on Cu(–U) deposit hosted in metamorphic rocks of the internal zone of the Lufilian belt known as the Western Zambian Copperbelt in which a primary Cu mineralization is overprinted by a second syn-metamorphic Cu mineralizing event. This mineralizing event is synchronous with the Pan-African metamorphism affecting both the pre-Katanga basement and the Katanga metasedimentary sequence. Cu(–U) occurrences in the Western Zambian Copperbelt are hosted by kyanite-micaschists metamorphosed in the upper amphibolite facies. Mineral inclusions of graphite, micas and sulfides in kyanite porphyroblasts of the Cu-bearing kyanite-micaschists in the Lumwana Cu deposit point to a sedimentary protolith with relics of an inherited Cu stock. Based on petrologic, microstructural and geochronological evidence, we propose that this initial Cu-stock was remobilized during the Pan-African orogeny. Graphite, micas and sulfides preserved in a first generation of kyanite poikiloblasts (Ky 1 ) define an inherited S 0/1 foliation developed during the prograde part of the P – T path (D 1 deformation-metamorphic stage) reaching HP–MT metamorphic conditions. Remobilization during the retrograde part of the P – T path is evidenced by chalcopyrite–pyrrhotite and chalcopyrite–bornite delineating a steep-dipping S 2 schistosity and by chalcopyrite and bornite delineating a shallow-dipping S 3 schistosity associated with top to the south kinematic criteria. This retrograde path is coeval with ductile deformation in the kyanite field as evidenced by a second generation of synkinematic kyanite porphyroblasts (Ky 2 ) transposed in the S 3 schistosity (Ky 2–3 ), and is marked by progressive cooling from ca. 620 °C down to 580 °C (rutile geothermometry). Syn-S 2–3 metamorphic monazite grains yield U–Th–Pb ages ranging from ca. 540 to 500 Ma. Final retrogression and remobilization of Cu is marked by recrystallization of the sulfides in top to the north C 3 shear bands associated with rutile crystals yielding temperatures from ca. 610 to 540 °C. This final remobilization is younger than ca. 500 Ma (youngest U–Th–Pb age on syn-S 3 recrystallized monazite). These data are consistent with successive Cu remobilization for more than 40 Ma during Pan-African reworking of sediment-hosted deposits either from the basement of the Katanga sedimentary sequence or from the Katanga sequence itself marked by burial (D 1 ), syn-orogenic exhumation (D 2 ), and post-orogenic exhumation during gravitational collapse (D 3 ). [ABSTRACT FROM AUTHOR]

This work attempted at clarifying the structure of Ikaria using primarily intensive geological mapping combined with structural analysis and a geothermometry approach of Raman spectrometry of carbonaceous material. Foliation over the whole island defines a structural dome cored by high-grade to partially molten rocks. Its exhumation was completed by two top-to-the-N ductile extensional shear zones, operating in the ductile and then the brittle fields, through a single extensional event coeval with progressive strain localization. The thermal structure of the dome with regard to position of ductile shear zones was retrieved using the Raman spectroscopy of carbonaceous material. Peak-metamorphic temperatures range from 390 °C in the upper parts of the structure down to 625 °C in the core of the dome in the vicinity of migmatites and S-type granite. Pioneer in situ U–Th–Pb analyses on monazite performed on the leucosome parts of these rock yielded a 15.7 ± 0.2 Ma age. Ikaria Island thus completes the series of Miocene migmatite-cored Metamorphic Core Complex in the central part of the Aegean domain where a genuine high-temperature zone can be defined as the central Aegean H T zone. There, the extreme stretching of the continental crust is associated with dominantly top-to-the-N kinematics. [ABSTRACT FROM AUTHOR]

High-precision 232Th-208Pb dates have been obtained from allanite porphyroblasts that show unambiguous microstructural relationships to fabrics in a major syn-metamorphic fold in the SE Tauern Window, Austria. Three porphyroblasts were analysed from a single garnet mica schist from the Peripheral Schieferhülle in the core of the Ankogel Synform, one of a series of folds which developed shortly before the thermal peak of Alpine epidote-amphibolite facies metamorphism: allanite grain 1 provided two analyses with a combined age of 27.7 ± 0.7 Ma; grain 2, which was slightly bent and fractured during crenulation, provided two analyses with a combined age of 27.7 ± 0.4 Ma; a single analysis from grain 3, which overgrew an already crenulated fabric, gave an age of 28.0 ± 1.4 Ma. The five 232Th-208Pb ages agree within error and define an isochron with an age of 27.71 ± 0.36 Ma (95% confidence level; MSWD = 0.46). The results imply that the crenulation event was in progress in a short interval (<1 Ma) c. 28 Ma, and that the Ankogel Synform was forming at this time. The thermal peak of regional metamorphism in the SE Tauern Window was probably attained shortly after 28 Ma, only c. 5 Ma after eclogite facies metamorphism in the central Tauern Window. Metasediment may contain allanite porphyroblasts with clear-cut microstructural relationships to fabric development and metamorphic crystallization; for such rocks, 232Th-208Pb dating on microsamples offers a powerful geochronological tool. [ABSTRACT FROM AUTHOR]

Apatite is an important common U- and Th-bearing accessory mineral in igneous, metamorphic and clastic sedimentary rocks. The advent of in situ U–Th–Pb apatite geochronology by the SIMS and LA-(MC)-ICP-MS methods has demonstrated the importance of having uniform and homogeneous reference materials. Recently, it has been shown that Sr and Nd isotopic data combined with U–Pb age and trace element concentration data can provide important constraints on apatite paragenesis because this phase usually exhibits high Sr and REE concentrations but has low Rb/Sr ratios which result in negligible corrections for the ingrowth of radiogenic Sr. However, as apatite can potentially have complex internal structures resulting from multiple thermal events, such as inherited cores and metamorphic overgrowths, requires that the Sr and Nd isotopic data should be measured with high spatial resolution. However isobaric interferences hamper the precise determination of Sr or Nd isotopic compositions in LA-MC-ICP-MS analysis. In this work we undertook in situ measurements of Sr and Nd isotopic compositions of eleven apatite reference materials (AP1, AP2, Durango, MAD, Otter Lake, NW-1, Slyudyanka, UWA-1, Mud Tank, McClure Mountain and SDG) commonly used in U–Th–Pb geochronology. Our obtained Sr and Sm–Nd isotopic compositions for these apatite samples are consistent with those values obtained by solution-based methods (isotope dilution and ion chromatography) using MC-ICP-MS or TIMS, which demonstrates the reliability and robustness of our analytical protocol. [ABSTRACT FROM AUTHOR]

We present textural, chemical and U-Th-Pb-age data on monazites from garnet--kyanite (Grt-Ky) metapelites, from the Chepelare Shear Zone (Bulgaria) and the Nestos Shear Zone (Greece), in the Central Rhodope. Samples from both locations have experienced two stages of high temperature metamorphism during Alpine times. The first event involved mid-Mesozoic granulite facies dehydration melting. The second event involved mid-Cenozoic lower-grade fluid-assisted partial melting. The latter is well expressed in adjacent felsic rocks but had limited impact on the Grt-Ky metapelites. Most samples display evidence for strong ductile shearing in the presence of fluids. Monazite is present in the highly foliated matrix and as inclusions in garnet and kyanite. Unlike the inclusions, matrix monazites display features of fluid-assisted dissolution--recrystallization. Y-poor domains with U-Th-Pb ages of between ca. 115 and 165 Ma represent the largest part of the grains. Y-rich domains with mid-Cenozoic ages occur as rims, or as small satellite grains surrounding the Mesozoic grains. The Cenozoic monazite domains crystallized at the expense of the Mesozoic ones and simultaneously incorporated Y provided by the fluid-assisted resorption of garnet. An age of ca. 36 Ma is obtained for the samples of both shear zones, interpreted as dating the main episode of monazite growth during the Cenozoic. Similar ages exist for the crystallization of leucosomes and pegmatites in the adjacent migmatitic gneisses, indicating that the fluids responsible for the precipitation of the Cenozoic monazites were probably released during the crystallization of nearby anatectic melts. Together with associated rutile and biotite, many newly grown monazites show a preferred orientation paralleling the matrix foliation. This supports the hypothesis of dynamic dissolution--precipitation as an efficient mean to promote renewed monazite crystallization during ductile deformation of the host rock at ca. 36 Ma. [ABSTRACT FROM AUTHOR]

Detailed quartz lattice preferred orientation (LPO) data define two structural discontinuities in the exhumed high-grade metamorphic core of the Himalaya exposed in the upper Tama Kosi region of east central Nepal. The structures are marked by abrupt breaks in a general trend of up structural section increasing quartz LPO-defined deformation temperatures. Deformation associated with the upper structural discontinuity, which occurs within sillimanite grade rocks, is postpeak metamorphism in both the hanging wall and the footwall. New geochronologic data constrain the timing of metamorphism in the hanging wall of the upper discontinuity to between 24 and 16 Ma, indistinguishable from previously published ages for the footwall. Movement across this structure represents Early Miocene strain localization and thickening in the Himalayan midcrust. Movement across the lower discontinuity, which occurs between staurolite and kyanite grade rocks, appears to be synmetamorphic with material in its footwall at approximately 10 Ma, but postpeak metamorphism for material in its hanging wall. This movement is interpreted to reflect the underplating and incorporation of material into the metamorphic core. The recognition of two thrust-sense discontinuities in the exhumed Himalayan core in the Tama Kosi region is consistent with other similar structures recognized along the Himalaya. The widespread nature of these structures reinforces that they are important to our understanding of the evolution of the kinematics of large, hot orogens. [ABSTRACT FROM AUTHOR]

Abstract: Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) has been employed for U–Th–Pb geochronology of allanite, using various analytical parameters. Opinions diverge on analytical protocols and data reduction, and particularly, on the possibility of using non-matrix-matched standards and samples to obtain accurate and precise ages. In this study, ablation experiments were carried out in spot mode using an ArF 193nm excimer laser coupled to a quadrupole ICP-MS, in order to characterise the ablation behaviour of allanite under various fluence (4, 7.8 and 12J/cm2) and ablation rate conditions (4, 8 and 12Hz). The U–Th–Pb data were compared to the Plešovice zircon (337.13±0.37Ma) in order to investigate the effects of the different matrices. The ratio-of-the-mean intensities method was used for data reduction. Analyses in spot mode at low frequency and fluence provide accurate and precise allanite 206Pb/238U and 208Pb/232Th ages, using a matrix-matched external standard. Matrix effects between allanites with different ages (30.1 to 417Ma), common 208Pb (0.5–33%) and 206Pb (5.8–90%) proportions, and FeOtotal (11.5–16.1%) and REE+Th (0.52–0.90apfu) compositions do not affect the age accuracy. The difference of mass bias correction coefficients between allanites and zircon shows evidence of matrix effects, which are often responsible for systematic errors on ages. Matrix-matched standardisation with an allanite standard is required for precise U–Th–Pb allanite ages. Even though laser-induced elemental fractionation can be minimised for non-matrix-matched standards and samples, other sources of matrix effects still affect age accuracy and precision. [Copyright &y& Elsevier]

The large (~700km2) composite Karkonosze (Krkonoše) pluton in the West Sudetes, on the border between Poland and the Czech Republic, consists mainly of porphyritic and equigranular granitoids, but contains a range of lithologies from lamprophyre to leucogranite. The absolute age and duration of the plutonism have proved difficult to determine. Previous age measurements by Rb–Sr, Ar–Ar and U–Pb range from ~330 to 290Ma, with more recent results converging to ~320–300Ma. Dating of zircon and monazite from samples of a variety of major and minor lithologies by SIMS U–Th–Pb, several from the geochemical study of Słaby and Martin (2008), has narrowed the possible age range further. U–Pb ages measured on eight of ten zircon and monazite samples are in the range ~314–311Ma. Zircon ages measured on the two major types of porphyritic granitoid are 313±3 and 311±4Ma, and monazite ages are 312±2, 313±3 and 311±3Ma. Monazite from one hybrid granitoid has an age of 314±3Ma, and zircon from another an age of 314±4Ma. Zircon from a composite dyke has an age of 311±6Ma. The monazite U–Pb age of an equigranular granite, at 318±6Ma, is consistent with geological evidence that it is older than the porphyritic granitoids but, because of the relatively large uncertainties, is not conclusive. Zircon from one microgranular enclave is anomalously young, 302±4Ma. Evidence is mounting that the main porphyritic granitoids, hybrid granitoids and composite dykes were emplaced within a short time interval between 314±4 and 311±3Ma. Given the uncertainties, emplacement of these units could have been effectively simultaneous. The larger difference between the ages from the equigranular granite and microgranular enclave, however, indicates that the whole Karkonosze thermal episode possibly lasted as long as 15Ma. [ABSTRACT FROM AUTHOR]

Abstract: Rock specimens collected from lower, middle, and upper portions of the exhumed Himalayan mid-crust in the upper Tama Kosi region of east-central Nepal yield pressure–temperature–time–deformation (P–T–t–D) paths that demonstrate the presence of a structural, metamorphic, and geochronologic discontinuity. A P–T–t–D path from below the discontinuity defines a loading path with deformation at staurolite-grade metamorphic conditions occurring at ~10–8Ma. P–T–t–D paths from above the discontinuity, in contrast, record an earlier and protracted metamorphic history that includes decompressional heating. Monazite ages from specimens collected above the discontinuity constrain prograde garnet growth to be >21Ma with decompression-related garnet breakdown initiating at c.19Ma and likely continuing until c.15Ma. These contrasting P–T–t–D histories separated by a tectonometamorphic discontinuity are consistent with an evolutionary model for the Himalaya in which rocks above the discontinuity were metamorphosed in the deep hinterland of the orogen and deformed during lateral extrusion from beneath Tibet while rocks below the discontinuity were deformed and metamorphosed later within the shallower foreland of the orogen. These results demonstrate the spatial and temporal relationships and compatibility between lateral midcrustal extrusive flow and wedge taper processes within an evolving orogen. [Copyright &y& Elsevier]

Abstract: In this study we apply laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) to the accessory minerals apatite, baddeleyite, monazite, pyrochlore, titanite, zircon and zirconolite from five meta-carbonatite and one meta-alkaline rock samples in order to obtain U–Th–Pb ages. We thereby assess the effect of amphibolite facies metamorphism on their U–Th–Pb isotopic systems. The results indicate that for the investigated samples the relative retentivity of Pb follows the order baddeleyite>zircon≥apatite≥titanite>zirconolite>monazite≥pyrochlore. Accordingly, the emplacement ages of the magmatic protoliths are best constrained by data from baddeleyite, zircon, apatite and titanite. In contrast, age data obtained from zirconolite, monazite and pyrochlore are interpreted to constrain the timing of metamorphism and/or cooling. [Copyright &y& Elsevier]

Abstract: U–Th–Pb geochronology of titanite by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) is a promising technique to constrain the history of igneous and metamorphic rocks. However, the quality of the resulting ages depends strongly on protocol adopted for the analyses and data reduction. There is no general agreement on the laser ablation settings and methodology that should be applied for titanite LA-ICP-MS geochronology. In particular it is essential to define an analytical procedure that could minimize the elemental fractionation for titanite U–Th–Pb geochronology, and to evaluate if non matrix-matched standards and samples (e.g. zircon and titanite) are suitable to obtain precise and accurate ages. In this study, ablation experiments were carried out in spot mode using an ArF 193nm excimer laser coupled to a quadrupole ICP-MS, with varying fluence, spot size and repetition rate conditions. The ablation behavior of the Khan titanite reference material was described in details and compared to the Plešovice zircon standard. The ratio-of-the-mean intensity method was used for data reduction. Three sources of fractionation and systematic errors between zircon and titanite are considered together: mass bias coefficients, shape of the time-dependent fractionation, and differences of ablated volumes. Even if the laser-induced elemental fractionation and matrix effects can be minimized between the Plešovice zircon standard and the Khan titanite, a matrix-matched standardization with a titanite standard is required for precise U–Th–Pb titanite ages, as well as at low frequency and fluence conditions. [Copyright &y& Elsevier]

Abstract: We report ion microprobe U–Th–Pb geochronology of in situ zircon from the Himalayan high- and ultrahigh-pressure eclogites, Kaghan Valley of Pakistan. Combined with the textural features, mineral inclusions, cathodoluminescence image information and the U–Th–Pb isotope geochronology, two types of zircons were recognized in Group I and II eclogites. Zircons in Group I eclogites are of considerably large size (>100μm up to 500μm). A few grains are euhederal and prismatic, show oscillatory zoning with distinct core–rim luminescence pattern. Several other grains show irregular morphology, mitamictization, embayment and boundary truncations. They contain micro-inclusions such as muscovite, biotite, quartz and albite. Core or middle portions of zircons from Group I eclogites yielded concordant U–Th–Pb age of 267.6±2.4Ma (MSWD=8.5), have higher U and Th contents with a Th/U ratio>1, indicating typical magmatic core domains. Middle and rim or outer portions of these zircons contain inclusions of garnet, omphacite, phengite and these portions show no clear zonation. They yielded discordant values ranging between 210 and 71Ma, indicating several thermal or Pb-loss events during their growth and recrystalization prior to or during the Himalayan eclogite-facies metamorphism. Zircons in Group II eclogites are smaller in size, prismatic to oval, display patchy or sector zoning and contain abundant inclusions of garnet, omphacite, phengite, quartz, rutile and carbonates. They yielded concordant U–Th–Pb age of 44.9±1.2Ma (MSWD=4.9). The lower U and Th contents and a lower Th/U ratio (<0.05) in these zircons suggest their formation from the recrystallization of the older zircons during the Himalayan high and ultrahigh-pressure eclogite-facies metamorphism. [Copyright &y& Elsevier]

The Leo Pargil dome, northwest India, is a 30 km-wide, northeast-trending structure that is cored by gneiss and mantled by amphibolite facies metamorphic rocks that are intruded by a leucogranite injection complex. Oppositely dipping, normal-sense shear zones that accommodated orogen-parallel extension within a convergent orogen bound the dome. The broadly distributed Leo Pargil shear zone defines the southwest flank of the dome and separates the dome from the metasedimentary and sedimentary rocks in the hanging wall to the west and south. Thermobarometry and in-situ U-Th-Pb monazite geochronology were conducted on metamorphic rocks from within the dome and in the hanging wall. These data were combined with U-Th-Pb monazite geochronology of leucogranites from the injection complex to evaluate the relationship between metamorphism, crustal melting, and the onset of exhumation. Rocks within the dome and in the hanging wall contain garnet, kyanite, and staurolite porphyroblasts that record prograde Barrovian metamorphism during crustal thickening that reached ∼530-630 °C and ∼7-8 kbar, ending by c. 30 Ma. Cordierite and sillimanite overgrowths on Barrovian assemblages within the dome record dominantly top-down-to-the-west shearing during near-isothermal decompression of the footwall rocks to ∼4 kbar by 23 Ma during an exhumation rate of 1.3 mm year−1. Monazite growth accompanied Barrovian metamorphism and decompression. The leucogranite injection complex within the dome initiated at 23 Ma and continued to 18 Ma. These data show that orogen-parallel extension in this part of the Himalaya occurred earlier than previously documented (>16 Ma). Contemporaneous onset of near-isothermal decompression, top-down-to-the-west shearing, and injection of the decompression-driven leucogranite complex suggests that early crustal melting may have created a weakened crust that was proceeded by localization of strain and shear zone development. Exhumation along the shear zone accommodated decompression by 23 Ma in a kinematic setting that favoured orogen-parallel extension. [ABSTRACT FROM AUTHOR]

Abstract: The occurrence of ferric oxides in Archean rocks has played an important role in discussions on the amount of free oxygen in the atmosphere of the ancient Earth. Recognizing that post-Archean weathering may also produce oxide minerals, drill cores have been used to obtain samples beneath the depth of Phanerozoic weathering. The first core of the Archean Biosphere Drilling Project (ABDP-1) documented hematite as alteration products in 3.4Ga basalts from the Marble Bar area of the Pilbara Craton, NW Australia, and this has been used to infer the presence of an O2-bearing atmosphere in the Archean. It is possible, however, that despite recovery of samples from >100m depth, oxidation of the basalts occurred much younger than the depositional age. In this study, the age of oxidation of the Apex Basalt from the ABDP-1 drill core at Marble Bar is constrained by U–Th–Pb geochronology. Lead and U concentrations of the basalts from the ABDP-1 drill core vary greatly, between <1–58ppm and 0.08–1.04ppm, respectively, whereas Th contents are more restricted (0.24–0.71ppm). 206Pb/204Pb ratios are non-radiogenic and vary from 12.44 to 14.69. The linear array in terms of 206Pb/204Pb–207Pb/204Pb variations does not reflect an age but reflects two-component mixing between a non-radiogenic “ore lead” end member and a radiogenic “basalt lead” end member. The samples do not form isochrons on 238U/204Pb–206Pb/204Pb, 235U/204Pb–207Pb/204Pb, or 232Th/204Pb–208Pb/204Pb diagrams, indicating post-formation U and Pb addition. Comparison of measured U/Th ratios with “model” U/Th ratios calculated based on 208Pb/204Pb–206Pb/204Pb variations indicates that U enrichment most likely occurred in the last 200Ma. The degree of U enrichment in the samples is correlated with Fe(III)/FeTotal ratios, indicating that U addition and oxidation were related, most likely reflecting penetration of oxygenated surface waters in the Phanerozoic along bedding planes and shear zones. These results, therefore, indicate that oxidation of the Apex Basalt did not occur in the Archean, and therefore cannot be used to infer an oxygenated atmosphere at that time. [Copyright &y& Elsevier]

Abstract: We present a petrological and geochronological study of “silvery micaschists” that crop out in the northern side of the Gran Paradiso Massif, Western Alps, with the aim of constraining P–T conditions and age of the Alpine high-pressure metamorphism. The studied “silvery micaschists”, which are the products of metasomatic alteration of granitoids along ductile shear zones, consist of Mg-chlorite, talc, chloritoid, minor phengite, and accessory minerals. Microstructural relationships indicate the following prograde sequence in the growth of U–Th bearing accessory minerals: florencite → monazite → allanite. Thermobarometric calculations indicate that the Mg-chlorite+talc+chloritoid+allanite peak assemblage was stable at P =1.9–2.7 GPa and T =515–600 °C, while monazite formed earlier at pressures over 2.0 GPa. SHRIMP dating of allanite yielded 33.7±1.6 Ma, interpreted as the age of the high-pressure metamorphic peak. Prograde monazite yielded an age of 37.4±0.9 Ma, implying a minimum duration of ∼2–4 Ma for the Alpine subduction event. Combining our ages with previous constraints, it can be concluded that the initial exhumation of the Gran Paradiso Massif occurred at a fast rate higher than 2 cm/year, and slowed down to ∼1 cm/year in the final stages. In a regional context, the new geochronological data align the subduction of the Gran Paradiso Massif with the other Internal Crystalline Massifs of the Western Alps. This supports a subduction model marked by alternating compressional events, related to the accretion of continental terranes, and extensional events, related to the episodic retreat of subduction zone hinges. [Copyright &y& Elsevier]

Abstract: The monazite Th–Pb chronometer is widely used in a variety of geological contexts but may be affected by interaction with fluids. In the central part of the Rhodope Massif (Greece, Bulgaria), synfolial pegmatite veins were emplaced within two major synmetamorphic shear zones. In the Chepelare shear zone (CZS), which displays no evidence of post-metamorphic fluid flow, consistent monazite Th–Pb ages of ∼36 Ma are interpreted as dating vein emplacement. Higher in the metamorphic pile, zircon and monazite give U–Th–Pb ages of ∼42 Ma for vein emplacement. In the Nestos Shear Zone (NSZ) pegmatites show evidence of post-emplacement interaction with fluids during greenschist facies ductile deformation. Evidence for this includes the precipitation of calcite in microscale tension gashes within feldspar porphyroclasts, and the oxygen isotope disequilibrium between quartz and feldspar. Aqueous carbonic fluids deriving from carbonate lithologies in the footwall of the NSZ are identifiable in the pegmatites by the high δ 18O value of quartz (up to 21.8‰) and the presence of calcium in calcite veinlets. In these samples, some of the monazite grains show large intragrain scattering of Th–Pb age spanning up to ∼12 Ma, with intergrain scattering reaching ∼16 Ma (from ∼39 to ∼55 Ma). Within individual grains, age domains correlate with chemical heterogeneities, and some show a characteristic Ca-excess. These chemical and isotopic alterations are interpreted to be caused by interaction with fluids derived from carbonate lithologies. Complementary U–Pb data on zircons from the NSZ pegmatites yield mostly Mesozoic ages related to an older metamorphic cycle, and an age of ∼48 Ma for one grain with typical magmatic zoning. The Th–Pb ages of 49.3±1.6 to 54.9±1.7 Ma probably relate to the emplacement of the pegmatites. The spread of younger Th–Pb ages (from 38.6±1.1 to 46.2±1.6 Ma) probably reflects the period of fluid circulation during progressive cooling to greenschist facies conditions, or variable perturbations of the monazite isotope system during fluid-assisted greenschist facies deformation. This is consistent with 39Ar–40Ar mica ages of ∼32 to 34 Ma, interpreted as a tighter constraint for the timing of the greenschist facies metamorphism. This study illustrates the capacity of monazite to record distinct events in a single rock and highlights the need for identifying the potential involvement of fluids in order to interpret monazite ages. [Copyright &y& Elsevier]

The eastward transition from penetrative ductile deformation, metamorphism, and plutonism in the hinterland of the southern Canadian Cordillera to the 'thin-skinned' deformation of the foreland represents a significant change in tectonic style and process. The transition is also marked by a zone of structural divergence across which SW vergent hinterland structures pass into NE vergent foreland structures. The Selkirk fan within the Selkirk Mountains of the southern Canadian Cordillera is considered a type locality for this zone. This structure trends SE-NW for more than 120 km in greenschist to upper amphibolite facies rocks that have at least three generations of superposed deformation. The kinematic development of the Selkirk fan has been controversial, but its age generally has been inferred to be Middle to Late Jurassic. However, new U-Th-Pb dates obtained by ID-TIMS and SHRIMP indicate a more protracted history. The data demonstrate that a protofan developed in the Middle Jurassic (172-167 Ma) and that the west flank of the fan was exhumed to upper crustal levels at this time. Conversely, the eastern flank was progressively buried to >25 km depth and pervasively overprinted by Cretaceous deformation (104-84 Ma) and Cretaceous-Paleocene metamorphism (144-56 Ma). A new tectonic model is proposed to reconcile the composite nature and protracted development (>100 Ma) of the Selkirk fan within an orogenic system that evolved in response to periods of terrane accretion on the western margin of the North American craton. [ABSTRACT FROM AUTHOR]

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° 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 ± 1.7 Ma, whereas zircon microcrystals have a mean age of 55 ± 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 ± 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. [ABSTRACT FROM AUTHOR]

Integration of in situ and conventional ion microprobe U-Th-Pb zircon and monazite geochronology with 40Ar/39Ar hornblende, biotite, and muscovite thermochronology provides a temporal record of metamorphism, partial melting, fluids, and cooling of exhumed midcrustal to lower crustal rocks. The Valhalla complex, southeastern British Columbia, is one of a series of elongate gneiss domes within the Omineca belt of the Canadian Cordillera. We have performed U-Th-Pb and 40Ar/39Ar age measurements on migmatitic gneisses from a range of structural levels and settings within the Valhalla complex in an effort to better document the timing of migmatization and exhumation. Stromatic migmatites and leucosomes crystallized in boudin necks yield concordant U-Pb zircon ages that cluster near 60 Ma and Th-Pb monazite ages of circa 60-50 Ma. In situ analyses of monazite in samples that contain <10% leucosome yield a bimodal distribution at 70-65 and 62-57 Ma. The younger ages are similar to migmatite zircon ages. 40Ar/39Ar analyses from the same rocks document cooling at 51-49 Ma. These new data, combined with field, structural, and petrologic data from this and previous studies of the Omineca domes, indicate that a large region (thousands of km2) of orogenic crust may have been partially molten during the Early Tertiary (>60-50 Ma). However, the combined data also suggest that melt crystallization (circa 60 Ma) in the Valhalla complex occurred rapidly and before the other domes within the Omineca belt. Cooling at 49 Ma throughout the Omineca is associated with regional extension and exhumation of migmatites in the domes. The presence of abundant migmatites and the similarity in timing of melt crystallization and cooling in domes along the 400-km-long Omineca belt is consistent with localization of exhumation at the boundary between the flowing crust of the hinterland and the rigid, colder crust of the Rocky Mountain foreland. [ABSTRACT FROM AUTHOR]

The Paleozoic French Variscan Belt in Massif Central and Massif Armoricain is a collision belt that provides a good example of a suture zone where ophiolites are rare, and the frontal (i.e., the magmatic arc) part of the upper plate is not present. In the lower plate (or Gondwana), the continental rocks are subdivided into an Upper Gneiss Unit (UGU) and a Lower Gneiss Unit (LGU). The UGU experienced a high-pressure (and likely ultra-high-pressure) metamorphism followed by crustal melting during their exhumation. New chemical U-Th-Pb monazite ages and ion-probe U-Pb zircon ages on migmatites allow us to constrain the P-T-t paths followed by the UGU and LGU. By comparison with thermomechanical experiments, a possible geodynamic evolution scenario can be proposed for the Variscan convergence. The high-compression regime of continental subduction developed during the initial subduction of the northern margin of Gondwana under Armorica in Silurian times. This induced the formation of a new subduction zone in the back-arc basin, which is the youngest, hottest, and thus mechanically the weakest part of the overriding plate. As a result, the arc-back-arc basin domain has been almost totally subducted below Armorica. Only a limited part of the back-arc basin rocks remains exposed in the Devonian St-Georges-sur-Loire Unit. Subsequently, the continental subduction of Gondwana resumed with a steeper dip associated with low-compression regime that in turn allowed the high-pressure rocks to be exhumed and partly melted in Late Devonian times. Such a scheme depicts quite well the complexity of the Variscan Belt. [ABSTRACT FROM AUTHOR]

Abstract: Structural transects through the South Tibetan Detachment system (STDS) in the Dzakaa Chu valley, Tibet reveal a ∼1000-m thick, low-angle (<35°) zone of distributed ductile shear that displaces Paleozoic sediments over amphibolite facies gneisses, calc-mylonites and leucogranites of the Greater Himalayan Series (GHS). Within the shear zone, grain-size reduction with dynamic recrystallisation of quartz and growth of secondary phyllosilicates accommodated ductile deformation at elevated temperatures. Small-scale brittle normal faults and extensional shear veins overprint ductile features recording deformation at lower temperatures. Our structural data indicate that the Dzakaa Chu STDS records a progression from ductile- to brittle-deformation without development of a discrete detachment fault(s) that is common to many STDS sections. U(–Th)–Pb dating of post-kinematic leucogranites suggest that, in the lower part of the shear zone, mylonitic fabric development occurred prior to ∼20Ma. By integrating structural and geochronological evidences we propose that the Dzakaa Chu STDS represents a deeper structural position than elsewhere in the Himalaya and provides important insight into the early ductile exhumation of the GHS that was dominated by movement along a 1-km wide shear zone without discrete brittle detachments. These findings are an important step towards understanding the development of low-angle detachment fault systems active during continental collision. [Copyright &y& Elsevier]

Abstract: Strongly migmatitic sapphirine-bearing granulites occur as dm- to m-sized enclaves in garnetiferous leucogranites in the southeastern part of the Madurai Block (MB), the largest section of Neoproterozoic crust in the Southern Granulite Terrain of India (SGT). Early mineral assemblages (biotite–sillimanite–quartz, garnet–biotite–quartz) break down via successive dehydration melting reactions to high-T and high-P phase assemblages (e.g. garnet-melt, orthopyroxene–sillimanite-melt, orthopyroxene–cordierite-melt). Melt-consuming reactions along the retrograde path led to partial resorption of garnet to orthopyroxene–sillimanite, orthopyroxene–sapphirine and/or orthopyroxene–cordierite as well as orthopyroxene–sillimanite assemblages to cordierite–biotite–quartz symplectites. The occurrence of UHT metamorphism is indicated by the presence of small inclusions of sapphirine–quartz intergrowths in garnet. This reaction sequence is interpreted in a KFMASH partial petrogenetic grid taking both melt-producing and melt-consuming reactions into account which yields a clockwise heating–cooling–decompression P–T path culminating at 8–11 kbar and 1000–1100 °C. In-situ U–Th–Pb monazite dating has been performed to more precisely constrain the ages given by chemical electron-microprobe on monazite in various textural positions. Monazite inclusions in garnet often yield a bimodal distribution of early (950–850 Ma) and late Neoproterozoic (600–500 Ma) ages which is in good agreement with the results of U–Pb and EPMA zircon dating in other places of the southeastern Madurai Block. By contrast, monazite inclusions in other mafic minerals (orthopyroxene, cordierite, biotite) or as interstitial phase exclusively yield ages between 550 and 480 Ma. The early Neoproterozoic population is interpreted to reflect the timing of migmatisation and peak UHT metamorphism, whereas the late Neoproterozoic ages are inferred to date cooling and exhumation with granitic magmas to a higher crustal level of the Madurai Block. [Copyright &y& Elsevier]

Thermochemical properties have been either measured or estimated for synthetic monazite, LaPO4, and dissakisite, CaLaMgAl2(SiO4)3OH, the Mg-equivalent of allanite. A dissakisite formation enthalpy of −6,976.5 ± 10.0 kJ mol−1 was derived from high-temperature drop-solution measurements in lead borate at 975 K. A third-law entropy value of 104.9 ± 1.6 J mol−1 K−1 was retrieved from low-temperature heat capacity ( C p) measured on synthetic LaPO4 with an adiabatic calorimeter in the 30–300 K range. The C p values of lanthanum phases were measured in the 143–723 K range by differential scanning calorimetry. In this study, La(OH)3 appeared as suitable for drop solution in lead borate and represents an attractive alternative to La2O3. Pseudo-sections were calculated with the THERIAK-DOMINO software using the thermochemical data retrieved here for a simplified metapelitic composition (La = ∑REE + Y) and considering monazite and Fe-free epidotes along the dissakisite-clinozoïsite join, as the only REE-bearing minerals. Calculation shows a stability window for dissakisite-clinozoïsite epidotes ( T between 250 and 550°C and P between 1 and 16 kbar), included in a wide monazite field. The P– T extension of this stability window depends on the bulk-rock Ca-content. Assuming that synthetic LaPO4 and dissakisite-(La) are good analogues of natural monazite and allanite, these results are consistent with the REE-mineralogy sequence observed in metapelites, where (1) monazite is found to be stable below 250°C, (2) around 250–450°C, depending on the pressure, allanite forms at the expense of monazite and (3) towards amphibolite conditions, monazite reappears at the expense of allanite. [ABSTRACT FROM AUTHOR]

The Mogok metamorphic belt (MMB) extends for over 1500 km along the western margin of the Shan-Thai block, from the Andaman Sea north to the eastern Himalayan syntaxis. Previous geochronology has suggested that a long-lasting Jurassic-early Cretaceous subduction-related event resulted in emplacement of granodiorites and orthogneisses (171-120 Ma) and a poorly constrained Tertiary metamorphic event. On the basis of new U-Pb isotope dilution thermal ionization mass spectrometry and U-Th-Pb laser ablation-multicollector-inductively coupled plasma mass spectrometer geochronology presented here, we propose two Tertiary metamorphic events affected the MMB in Burma. The first was a Paleocene event that ended with intrusion of crosscutting postkinematic biotite granite dikes at ∼59 Ma. A second metamorphic event spanned late Eocene to Oligocene (at least from 37, possibly 47, to 29 Ma). This resulted in the growth of metamorphic monazite at sillimanite grade, growth of zircon rims at 47-43 Ma, sillimanite + muscovite replacing older andalusite, and synmetamorphic melting producing garnet and tourmaline bearing leucogranites at 45.5 ± 0.6 Ma and 24.5 ± 0.7 Ma. These data imply high-temperature sillimanite + muscovite metamorphism peaking at 680°C and 4.9 kbar between 45 and 33 Ma, to around 606-656°C and 4.4-4.8 kbar at 29.3 ± 0.5 Ma. The later metamorphic event is older than 24.5 ± 0.3 Ma, the age of leucogranites that crosscut all earlier fabrics. Our structural and geochronological data suggest that the MMB links north to the unexposed middle or lower crust rocks of the Lhasa terrane, south Tibet, and east to high-grade metamorphic core complexes in northwest Thailand. [ABSTRACT FROM AUTHOR]