Constraining cooling histories: rutile and titanite chronology and diffusion modelling in NW Bhutan

U–Pb analyses of rutile and titanite commonly yield ages that constrain the timing of cooling rather than the timing of their crystallization. Rutile which grew at or close to peak temperature conditions in a mafic granulite, intermediate granulite and mafic amphibolite within juxtaposed litho/tectonostratigraphic units in the Greater Himalayan Sequence (GHS) of NW Bhutan yield LA–MC–ICP–MS U–Pb lower intercept cooling ages of 10.1 ± 0.4, 10.8 ± 0.1 and 10.0 ± 0.3 Ma, respectively. Numerical finite‐difference diffusion models constrained by previously published temperature–time and Pb diffusion data suggest that these ages are best explained by rapid cooling from peak temperature conditions of ～800 °C at 14 Ma in the granulite‐bearing unit and ～650 °C at 12 Ma in the amphibolite‐bearing unit. The good fit between the model and analysed ages confirms the relatively high retention of Pb in rutile suggested by the experimental data. Titanite that grew during an exhumation‐related amphibolite facies overprint on an eclogite facies mineral assemblage from the neighbouring Jomolhari Massif yields a U–Pb lower intercept cooling age of 14.6 ± 1.2 Ma. Diffusion modelling suggests that this age is too old to be consistent with the temperature–time paths inferred for the rutile‐bearing samples. Instead, the titanite age suggests cooling from ～650 °C at an earlier time of 17–15 Ma, implying that the high‐grade rocks in the Jomolhari Massif experienced a different cooling history from the rest of the GHS in NW Bhutan. Together these data show that high‐grade rocks from three apparently different structural levels of the GHS in NW Bhutan experienced rapid cooling at >40 °C Ma^?1 at varying times. The highest grade granulite facies rocks were exhumed from deeper structural levels that are not exposed, not preserved, or not yet recognized west of eastern Nepal. A progressive along‐strike change in tectonic regime, metamorphic history and/or exhumation mechanism across the orogen is implied by these thermochronologic data.     

Metamorphic history of a syn‐convergent orogen‐parallel detachment: The South Tibetan detachment system,Bhutan Himalaya

The South Tibetan detachment system (STDS) in the Himalayan orogen is an example of normal‐sense displacement on an orogen‐parallel shear zone during lithospheric contraction. Here, in situ monazite U(–Th)–Pb geochronology is combined with metamorphic pressure and temperature estimates to constrain pressure–temperature–time (P–T–t) paths for both the hangingwall and footwall rocks of a Miocene ductile component of the STDS (outer STDS) now exposed in the eastern Himalaya. The outer STDS is located south of a younger, ductile/brittle component of the STDS (inner STDS), and is characterized by structurally upward decreasing metamorphic grade corresponding to a transition from sillimanite‐bearing Greater Himalayan sequence rocks in the footwall with garnet that preserves diffusive chemical zoning to staurolite‐bearing Chekha Group rocks in the hangingwall, with garnet that records prograde chemical zoning. Monazite ages indicate that prograde garnet growth in the footwall occurred prior to partial melting at 22.6 ± 0.4 Ma, and that peak temperatures were reached following c. 20.5 Ma. In contrast, peak temperatures were reached in the Chekha Group hangingwall by c. 22 Ma. Normal‐sense (top‐to‐the‐north) shearing in both the hangingwall and footwall followed peak metamorphism from c. 23 Ma until at least c. 16 Ma. Retrograde P–T–t paths are compatible with modelled P–T–t paths for an outer STDS analogue that is isolated from the inner STDS by intervening extrusion of a dome of mid‐crustal material.     

Mesozoic to Cenozoic tectono‐metamorphic history of the South Pamir–Hindu Kush (Chitral,NW Pakistan): Insights from phase equilibria modelling,and garnet–monazite petrochronology

The Karakoram–Hindu Kush–Pamir and adjacent Tibetan plateau belt comprise a series of Gondwana‐derived crustal fragments that successively accreted to the Eurasian margin in the Mesozoic as the result of the progressive Tethys ocean closure. These domains provide unique insights into the thermal and structural history of the Mesozoic to Cenozoic Eurasian plate margin, which are critical to inform the initial boundary conditions (e.g. crustal thickness, structure and thermo‐mechanical properties) for the subsequent development of the large and hot Tibetan–Himalaya orogen, and the associated crustal deformation processes. Using a combination of microstructural analyses, thermobarometry modelling and U–Th–Pb monazite and Lu–Hf garnet geochronology, the study reappraises the metamorphic history of exposed mid‐crustal metapelites in the Chitral region of the South Pamir–Hindu Kush (NW Pakistan). This study also demonstrates that trace elements in monazite (especially Y and Dy), combined with thermodynamical modelling and Lu–Hf garnet dating, provides a powerful integrated toolbox for constraining long‐lived and polyphased tectono‐metamorphic histories in all their spatial and temporal complexity. Rocks from the Chitral region were progressively deformed and metamorphosed at sub‐ and supra‐solidus conditions through at least four distinct episodes from the Mesozoic to the Cenozoic. Rocks were first metamorphosed at ~400–500°C and ~0.3 GPa in the Late Triassic–Early Jurassic (210–185 Ma), likely in response to the accretion of the Karakoram during the Cimmerian orogeny. Pressure and temperature subsequently increased by ~0.3 GPa and 100°C in the Early‐ to Mid Cretaceous (140–80 Ma), coinciding with the intrusion of calcalkaline granitic plutons across the Karakoram and Pamir regions. This event is interpreted as the record of crustal thickening and the development of a proto‐plateau within the Eurasian margin due to a long‐lived episode of slab flattening in an Andean‐type margin. Peak metamorphism was reached in the Late Eocene–Early Oligocene (40–30 Ma) at conditions of 580–600°C and ~0.6 GPa and 700–750°C and 0.7–0.8 GPa for the investigated staurolite schists and sillimanite migmatites respectively. This crustal heating up to moderate anatexis likely resulted in the underthrusting of the Indian plate after a NeoTethyan slab‐break off or to the Tethyan Himalaya–Lhasa microcontinent collision and subsequent oceanic slab flattening. Near‐isothermal decompression/exhumation followed in the Late Oligocene (28–23 Ma) as marked by a pressure decrease in excess of ~0.1 GPa. This event was coeval with the intrusion of the 24 Ma Garam Chasma leucogranite. This rapid exhumation is interpreted to be related to the reactivation of the South Pamir–Karakoram suture zone during the ongoing collision with India. The findings of this study confirm that significant crustal shortening and thickening of the south Eurasian margin occurred during the Mesozoic in an accretionary‐type tectonic setting through successive episodes of terrane accretions and probably slab flattening, transiently increasing the coupling at the plate interface. Moreover, they indicate that the south Eurasian margin was already hot and thickened prior to Cenozoic collision with India, which has important implications for orogen‐scale strain‐accommodation mechanisms.     

Exhumation of the Main Central Thrust from Lower Crustal Depths, Eastern Bhutan Himalaya

Geothermometry and mineral assemblages show an increase of temperature structurally upwards across the Main Central Thrust (MCT); however, peak metamorphic pressures are similar across the boundary, and correspond to depths of 35–45 km. Garnet‐bearing samples from the uppermost Lesser Himalayan sequence (LHS) yield metamorphic conditions of 650–675 °C and 9–13 kbar. Staurolite‐kyanite schists, about 30 m above the MCT, yield P‐T conditions near 650 °C, 8–10 kbar. Kyanite‐bearing migmatites from the Greater Himalayan sequence (GHS) yield pressures of 10–14 kbar at 750–800 °C. Top‐to‐the‐south shearing is synchronous with, and postdates peak metamorphic mineral growth. Metamorphic monazite from a deformed and metamorphosed Proterozoic gneiss within the upper LHS yield U/Pb ages of 20–18 Ma. Staurolite‐kyanite schists within the GHS, a few metres above the MCT, yield monazite ages of c. 22 ± 1 Ma. We interpret these ages to reflect that prograde metamorphism and deformation within the Main Central Thrust Zone (MCTZ) was underway by c. 23 Ma. U/Pb crystallization ages of monazite and xenotime in a deformed kyanite‐bearing leucogranite and kyanite‐garnet migmatites about 2 km above the MCT suggest crystallization of partial melts at 18–16 Ma. Higher in the hanging wall, south‐verging shear bands filled with leucogranite and pegmatite yield U/Pb crystallization ages for monazite and xenotime of 14–15 Ma, and a 1–2 km thick leucogranite sill is 13.4 ± 0.2 Ma. Thus, metamorphism, plutonism and deformation within the GHS continued until at least 13 Ma. P‐T conditions at this time are estimated to be 500–600 °C and near 5 kbar. From these data we infer that the exhumation of the MCT zone from 35 to 45 km to around 18 km, occurred from 18 to 16 to c. 13 Ma, yielding an average exhumation rate of 3–9 mm year^?1. This process of exhumation may reflect the ductile extrusion (by channel flow) of the MCTZ from between the overlying Tibetan Plateau and the underthrusting Indian plate, coupled with rapid erosion.     

Syn‐metamorphic folding in the Tauern Window,Austria dated by Th–Pb ages from individual allanite porphyroblasts

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.     

Middle Carboniferous crustal melting in the Variscan Belt: New insights from U–Th–Pbtot. monazite and U–Pb zircon ages of the Montagne Noire Axial Zone (southern French Massif Central)

In France, the Devonian–Carboniferous Variscan orogeny developed at the expense of continental crust belonging to the northern margin of Gondwana. A Visean–Serpukhovian crustal melting has been recently documented in several massifs. However, in the Montagne Noire of the Variscan French Massif Central, which is the largest area involved in this partial melting episode, the age of migmatization was not clearly settled. Eleven U–Th–Pb_tot. ages on monazite and three U–Pb ages on associated zircon are reported from migmatites (La Salvetat, Ourtigas), anatectic granitoids (Laouzas, Montalet) and post-migmatitic granites (Anglès, Vialais, Soulié) from the Montagne Noire Axial Zone are presented here for the first time. Migmatization and emplacement of anatectic granitoids took place around 333–326 Ma (Visean) and late granitoids emplaced around 325–318 Ma (Serpukhovian). Inherited zircons and monazite date the orthogneiss source rock of the Late Visean melts between 560 Ma and 480 Ma. In migmatites and anatectic granites, inherited crystals dominate the zircon populations. The migmatitization is the middle crust expression of a pervasive Visean crustal melting event also represented by the “Tufs anthracifères” volcanism in the northern Massif Central. This crustal melting is widespread in the French Variscan belt, though it is restricted to the upper plate of the collision belt. A mantle input appears as a likely mechanism to release the heat necessary to trigger the melting of the Variscan middle crust at a continental scale.     

Behaviour of geochronometers and timing of metamorphic reactions during deformation at lower crustal conditions: phase equilibrium modelling and U–Pb dating of zircon,monazite, rutile and titanite from the Kalak Nappe Complex,northern Norway

This study investigates the behaviour of the geochronometers zircon, monazite, rutile and titanite in polyphase lower crustal rocks of the Kalak Nappe Complex, northern Norway. A pressure–temperature–time–deformation path is constructed by combining microstructural observations with P–T conditions derived from phase equilibrium modelling and U–Pb dating. The following tectonometamorphic evolution is deduced: A subvertical S1 fabric formed at ~730–775 °C and ~6.3–9.8 kbar, above the wet solidus in the sillimanite and kyanite stability fields. The event is dated at 702 ± 5 Ma by high‐U zircon in a leucosome. Monazite grains that grew in the S1 fabric show surprisingly little variation in chemical composition compared to a large spread in (concordant) U–Pb dates from c. 800 to 600 Ma. This age spread could either represent protracted growth of monazite during high‐grade metamorphism, or represent partially reset ages due to high‐T diffusion. Both cases imply that elevated temperatures of >600 °C persisted for over c. 200 Ma, indicating relatively static conditions at lower crustal levels for most of the Neoproterozoic. The S1 fabric was overprinted by a subhorizontal S2 fabric, which formed at ~600–660 °C and ~10–12 kbar. Rutile that originally grew during the S1‐forming event lost its Zr‐in‐rutile and U–Pb signatures during the S2‐forming event. It records Zr‐in‐rutile temperatures of 550–660 °C and Caledonian ages of 440–420 Ma. Titanite grew at the expense of rutile at slightly lower temperatures of ~550 °C during ongoing S2 deformation; U–Pb ages of c. 440–430 Ma date its crystallization, giving a minimum estimate for the age of Caledonian metamorphism and the duration of Caledonian shearing. This study shows that (i) monazite can have a large spread in U–Pb dates despite a homogeneous composition; (ii) rutile may lose its Zr‐in‐rutile and U–Pb signature during an amphibolite facies overprint; and (iii) titanite may record crystallization ages during retrograde shearing. Therefore, in order to correctly interpret U–Pb ages from different geochronometers in a polyphase deformation and reaction history, they are ideally combined with microstructural observations and phase equilibrium modelling to derive a complete P–T–t–d path.     

Integrated garnet and zircon–titanite geochronology constrains the evolution of ultra‐high–pressure terranes: An example from the Sulu orogen

Dating ultra‐high–pressure (UHP) metamorphic rocks provides important timing constraints on deep subduction zone processes. Eclogites, deeply subducted rocks now exposed at the surface, undergo a wide range of metamorphic conditions (i.e. deep subduction and exhumation) and their mineralogy can preserve a detailed record of chronologic information of these dynamic processes. Here, we present an approach that integrates multiple radiogenic isotope systems in the same sample to provide a more complete timeline for the subduction–collision–exhumation processes, based on eclogites from the Dabie–Sulu orogenic belt in eastern China, one of the largest UHP terranes on Earth. In this study, we integrate garnet Lu–Hf and Sm–Nd ages with zircon and titanite U–Pb ages for three eclogite samples from the Sulu UHP terrane. We combine this age information with Zr‐in‐rutile temperature estimates, and relate these multiple chronometers to different P–T conditions. Two types of rutile, one present as inclusions in garnet and the other in the matrix, record the temperatures of UHP conditions and a hotter stage, subsequent to the peak pressure (‘hot exhumation') respectively. Garnet Lu–Hf ages (c. 238–235 Ma) record the initial prograde growth of garnet, while coupled Sm–Nd ages (c. 219–213 Ma) reflect cooling following hot exhumation. The maximum duration of UHP conditions is constrained by the age difference of these two systems in garnet (c. 235–220 Ma). Complementary zircon and titanite U–Pb ages of c. 235–230 Ma and c. 216–206 Ma provide further constraints on the timing of prograde metamorphism and the ‘cold exhumation' respectively. We demonstrate that timing of various metamorphic stages can thus be determined by employing complementary chronometers from the same samples. These age results, combined with published data from adjacent areas, show lateral diachroneity in the Dabie–Sulu orogeny. Three sub‐blocks are thus defined by progressively younger garnet ages: western Dabie (243–238 Ma), eastern Dabie–northern Sulu (238–235 Ma) and southern Sulu terranes (225–220 Ma), which possibly correlate to different crustal slices in the recently proposed subduction channel model. These observed lateral chronologic variations in a large UHP terrane can possibly be extended to other suture zones.     

Two successive phases of ultrahigh temperature metamorphism in Rogaland,S. Norway: Evidence from Y‐in‐monazite thermometry

In Rogaland, South Norway, a polycyclic granulite facies metamorphic domain surrounds the late‐Sveconorwegian anorthosite–mangerite–charnockite (AMC) plutonic complex. Integrated petrology, phase equilibria modelling, monazite microchemistry, Y‐in‐monazite thermometry, and monazite U–Th–Pb geochronology in eight samples, distributed across the apparent metamorphic field gradient, imply a sequence of two successive phases of ultrahigh temperature (UHT) metamorphism in the time window between 1,050 and 910 Ma. A first long‐lived metamorphic cycle (M1) between 1,045 ± 8 and 992 ± 11 Ma is recorded by monazite in all samples. This cycle is interpreted to represent prograde clockwise P–T path involving melt production in fertile protoliths and culminating in UHT conditions of ~6 kbar and 920°C. Y‐in‐monazite thermometry, in a residual garnet‐absent sapphirine–orthopyroxene granulite, provides critical evidence for average temperature of 931 and 917°C between 1,029 ± 9 and 1,006 ± 8 Ma. Metamorphism peaked after c. 20 Ma of crustal melting and melt extraction, probably supported by a protracted asthenospheric heat source following lithospheric mantle delamination. Between 990 and 940 Ma, slow conductive cooling to 750–800°C is characterized by monazite reactivity as opposed to silicate metastability. A second incursion (M2) to UHT conditions of ~3.5–5 kbar and 900–950°C, is recorded by Y‐rich monazite at 930 ± 6 Ma in an orthopyroxene–cordierite–hercynite gneiss and by an osumilite gneiss. This M2 metamorphism, typified by osumilite paragenesis, is related to the intrusion of the AMC plutonic complex at 931 ± 2 Ma. Thermal preconditioning of the crust during the first UHT metamorphism may explain the width of the aureole of contact metamorphism c. 75 Ma later, and also the rarity of osumilite‐bearing assemblages in general.     

Isotope geochemistry of the mafic dikes from the Vazante nonsulfide zinc deposit, Brazil

The Vazante Group, located in the northwestern part of Minas Gerais, hosts the most important zinc mine in Brazil, the Vazante Mine, which represents a major known example of a hypogene nonsulfide zinc deposit. The main zinc ore is represented by willemite and differs substantially from other deposits of the Vazante-Paracatu region, which are sulfide-dominated zinc-lead ore. The age of the Vazante Group and the hosted mineralization is disputable. Metamorphosed mafic dikes (metabasites) that cut the metasedimentary sequence and are affected by hydrothermal processes recently were found and may shed light on the geochronology of this important geological unit. Zircon crystals recovered from the metabasites are xenocrystic grains that yield U–Pb conventional ages ranging from 2.1 to 2.4 Ga, so the basement of the Vazante Group is Paleoproterozoic or has metasedimentary rocks whose source area was Paleoproterozoic. Pb isotopes determined for titanite separated from the metabasites have common, nonradiogenic Pb compositions, which prevents determination of their crystallization age. However, the Pb signatures observed for the titanite crystals are in agreement with those determined for galena from the carbonate-hosted Zn–Pb deposits hosted by the Vazante Group, including galena from minor sulfide ore bodies of the Vazante deposit. These similarities suggest that the metalliferous fluids that affected the metabasites may have been those responsible for galena formation, which could imply a similar lead source for both nonsulfide and sulfide zinc deposits in the Vazante–Paracatu district. This common source could be related to deep-seated, basin-derived, metalliferous fluids associated with a long-lived hydrothermal system related to diagenesis and deformation of the Vazante Group during the Neoproterozoic.     

Thermal evolution of the Lesser Himalaya, central Nepal: Insights from K-white micas compositional variation

The Lesser Himalayan low- to medium-grade metamorphic rocks in central Nepal are rich in K-white micas occurring as porphyroclasts and in matrix defining S₁ and S₂. Porphyroclasts are usually zoned with celadonite-poor cores and celadonite-rich rims. The cores are the relics of igneous or high grade metamorphic muscovites, and the rims were re-equilibrated or overgrown under lower T metamorphic conditions. The matrix K-white micas defining S₁, pre-dating the Main Central Thrust activity, are generally celadonite-rich. They show heterogeneous compositional zoning with celadonite-rich cores and celadonite-poor rims. They were recrystallized at lower T condition prior to the Main Central Thrust activity, most probably prior to the India–Asia collision (pre-Himalayan metamorphism). The matrix K-white micas along S₂, synchronous to the Main Central Thrust activity (Neohimalayan metamorphism), are relatively celadonite-poor and were recrystallized under relatively higher T condition. K-white micas defining S₁ also were partially re-equilibrated during the Neohimalayan metamorphism. The average compositions of recrystallized K-white micas defining both S₁ and S₂ become gradually poor in (Fe + Mg)- and Si-contents and rich in Al- and Ti-contents from south to north showing an increase of metamorphic grade from structurally lower to higher parts in the Lesser Himalaya. This shows that the metamorphism is inverted throughout the inner Lesser Himalaya. The tectono-metamorphic significance of the published K–Ar and ⁴⁰Ar / ³⁹Ar K-white micas ages from the Lesser Himalaya need re-evaluation in the context of observed intrasample compositional variation and zoning, and possible higher closure temperature (500 °C) for K–Ar system.     

Neoproterozoic hydrothermally altered basaltic rocks from Rajasthan, northwest India: Implications for late Precambrian tectonic evolution of the Aravalli Craton

The isolated volcano-sedimentary sequences of the Punagarh and Sindreth Groups occur along the western flank of the Delhi Fold Belt in northwest India, and include mafic rocks (pillow basalts and dolerite dykes) that are dominantly olivine tholeiites with minor quartz-normative and alkali basalts. Sindreth samples appear to have higher primary TiO₂ and P₂O₅ abundances relative to those from Punagarh. Both suites of mafic rocks show variable, but profound hydrothermal alteration effects, with loss on ignition (LOI) values up to 10.3 wt.%, and extensive secondary minerals including albite, sericite, chlorite and calcite. Despite this, there is excellent preservation of magmatic textures, but there has been extensive albitization of plagioclase phenocrysts, a hallmark of hydrothermal alteration processes in oceanic crust. Supporting evidence for such hydrothermal alteration comes from correlations of LOI abundances with CaO/Na₂O, and evidence for U mobility is apparent on diagrams of Nb/Th vs. Nb/U. Felsic volcanic rocks (rhyolite, dacite) interlayered with the Sindreth basalts yield U–Pb zircon ages (TIMS method) between 761 ± 16 and 767 ± 3 Ma, which we interpret as representing the time of primary magmatic activity. We infer that the volcano-sedimentary rocks of the Punagarh also formed at this time, on the basis of similarities in lithology, stratigraphy, field relations and geochemistry. Intermediate granitoid rocks yield older U–Pb ages of 800 ± 2 and 873 ± 3 Ma, which we correlate with the post-Delhi Supergroup Erinpura Granites. Taken together, the features of the Punagarh and Sindreth Groups are consistent with their formation in a back-arc basin setting. Their coevality with other magmatic systems in NW India (Malani Igneous Suite), the Seychelles and Madagascar, for which a continental arc setting has also been proposed, supports the notion of an extensive convergent margin in western Rodinia at 750–770 Ma.     

Tectonometamorphic evolution of the tip of the Himalayan metamorphic core in the Jajarkot klippe,west Nepal

New phase equilibrium modelling, combined with U–Th/Pb petrochronology on monazite and xenotime, and ⁴⁰Ar/³⁹Ar geochronology on white mica, reveal the style of deformation and metamorphism near the southern tip of the extruded Himalayan metamorphic core (HMC). In the Jajarkot klippe, west Nepal foreland, greenschist to lower amphibolite facies metamorphism is entirely constrained to the Cenozoic Himalayan orogeny, in contrast with findings from other foreland klippen in the central Himalaya. HMC rocks exposed in the Jajarkot klippe yield short‐lived, hairpin pressure–temperature–time–deformation paths that peaked at 550–600°C and 750–1,200 MPa at 25 Ma. The Main Central thrust (MCT) and the South Tibetan detachment (STD) bound the base and the top of the HMC, respectively, and were active simultaneously for at least part of their deformation history. The STD was active at c. 27–26 Ma and possibly as late as c. 19 Ma, while the MCT may have been active as early as 27 Ma and was still active at c. 22 Ma. The tectonometamorphic conditions in the Jajarkot klippe are characteristic of crustal thickening and footwall accretion of new material at the tip of the extruding metamorphic orogenic core. Our new results reveal that collisional processes active in the middle to late Miocene at the base of the HMC now exposed in the hinterland were also active earlier, during the Oligocene, at the tip of the southward‐extruding middle crust.     

Ultrafast, > 50 Hz LA‐ICP‐MS Spot Analysis Applied to U–Pb Dating of Zircon and other U‐Bearing Minerals

LA‐ICP‐MS U–Pb detrital zircon studies typically analyse 50–200 grains per sample, with the consequent risk that minor but geologically important age components (e.g., the youngest detrital zircon population) are not detected, and higher abundance age components are misrepresented, rendering quantitative comparisons between samples impossible. This study undertook rapid U–Pb LA‐ICP‐MS analyses (8 s per 18–47 μm diameter spot including baseline and ablation) of zircon, apatite, rutile and titanite using an aerosol rapid introduction system (ARIS). As the ARIS resolves individual single pulses at fast sampling rates, spot analyses require a high repetition rate (> 50 Hz) so the signal does not return to baseline and mass sweep times (> 80 ms) that span several laser pulses (i.e., major undersampling of the signal). All rapid U–Pb spot analyses employed 250–300 pulses, repetition rates of 53–65 Hz (total ablation times of 4.1–5.7 s) and low fluence (1.75–2.5 J cm^?2), resulting in pit depths of ca. 15 μm. Zircon, apatite, rutile and titanite reference material data yield an accuracy and precision (2s) of < 1% for pre‐Cenozoic reference materials and < 2% for younger reference materials. We present a detrital zircon data set from a Neoproterozoic tillite where > 1000 grains were analysed in < 3 h with a precision and accuracy comparable to conventional LA‐ICP‐MS analytical protocols, demonstrating the rapid acquisition of huge detrital data sets.     

Timing of metamorphism,melting and exhumation of the Leo Pargil dome,northwest India

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.     

U–Pb zircon geochronology of ferrodiorites and quartz diorites from the Turkel Anorthosite Complex: a Neoarchaean convergent margin in eastern India

The Turkel anorthosite Complex (TAC) in the Eastern Ghats Belt in India is composed of anorthosites and leuconorites at the centre and ferrodiorites and quartz diorites at the periphery. Here we report whole‐rock geochemistry, and zircon U–Pb data and REE geochemistry from a co‐spatial ferrodiorite and two quartz diorites from the TAC. The diorites have low abundance of High Field Strength Elements (HFSE) and REE, exhibit a flat chondrite‐normalized pattern with slight LREE enrichment and negligible or no Eu anomaly. Our results show weighted mean ²⁰⁷Pb/²⁰⁶Pb ages of 2433 ± 33 Ma for the ferrodiorite. Two quartz diorite samples from Turkel yield mean²⁰⁷Pb/²⁰⁶Pb ages of 2419 ± 32 Ma and 2505 ± 31 Ma. The zircons from all the analysed samples show high REE contents, prominent HREE enrichment and a conspicuous positive Eu anomaly, suggesting a common magmatic source. The prominent Neoarchaean to early Palaeoproterozoic magmatic ages from the anorthosite complex deviate from the late Neoproterozoic ages reported from other anorthosite suites in the Eastern Ghats Belt, and suggest an active convergent margin along SE India during Archaean–Proterozoic transition. Copyright © 2014 John Wiley & Sons, Ltd.     

U–Pb SHRIMP zircon geochronology and T–t–d history of the Kampa Dome, southern Tibet

Structural, petrographic and geochronologic studies of the Kampa Dome provide insights into the tectonothermal evolution of orogenic crust exposed in the North Himalayan gneiss domes of southern Tibet. U–Pb ion microprobe dating of zircons from granite gneiss exposed at the deepest levels within the dome yields concordia ²⁰⁶Pb/²³⁸U age populations of 506 ± 3 Ma and 527 ± 6 Ma, with no evidence of new zircon growth during Himalayan orogenesis. However, the granite contains penetrative deformation fabrics that are also preserved in the overlying Paleozoic strata, implying that the Kampa granite is a Cambrian pluton that was strongly deformed and metamorphosed during Himalayan orogenesis. Zircons from deformed leucogranite sills that cross-cut Paleozoic metasedimentary rocks yield concordant Cambrian ages from oscillatory zoned cores and discordant ages ranging from ca. 491–32 Ma in metamict grains. Since these leucogranites clearly post-date the metasedimentary rocks they intrude, the zircons are interpreted as xenocrysts that are probably derived from the Kampa granite. The Kampa Dome formed via a series of progressive orogenic events including regional ~ N–S contraction and related crustal thickening (D₁), predominately top-to-N ductile shearing and crustal extension (D₂), top-to-N brittle–ductile faulting and related folding on the north limb of the dome, localized top-to-S faulting on the southern limb of the dome, and crustal doming (D₃), and continued N–S contraction, E–W extension and doming (D₄). Structural and geochronologic variability amongst adjacent North Himalayan gneiss domes may reflect changes in the magnitude of crustal exhumation along the North Himalayan antiform, possibly relating to differences in the mid-crustal geometry of the exhuming fault systems.     

The Temaguessine Fe-cordierite orbicular granite (Central Hoggar, Algeria): U–Pb SHRIMP age, petrology, origin and geodynamical consequences for the late Pan-African magmatism of the Tuareg shield

The Temaguessine high-level subcircular pluton is intrusive into the LATEA metacraton (Central Hoggar) Eburnian (2 Ga) basement and in the Pan-African (615 Ma) granitic batholiths along a major NW–SE oriented major shear zone. It is dated here (SHRIMP U–Pb on zircon) at 582 ± 5 Ma. Composed of amphibole–biotite granite and biotite syenogranite, it comprises abundant enclaves: mafic magmatic enclaves, country-rock xenoliths and remarkable Fe-cordierite (#Fe = 0.87) orbicules. The orbicules have a core rich in cordierite (40%) and a leucocratic quartz–feldspar rim. They are interpreted as resulting from the incongruent melting of the meta-wacke xenoliths collapsed into the magma: the breakdown of the biotite + quartz assemblage produced the cordierite and a quartz–feldspar minimum melt that is expelled, forming the leucocratic rim. The orbicule generation occurred at T < 850° and P < 0.3 GPa. The Fe-rich character of the cordierite resulted from the Fe-rich protolith (wacke with 4% Fe₂O₃ for 72% SiO₂). Strongly negative ε_Nd (−9.6 to −11.2), Nd T_DM model ages between 1.64 and 1.92 Ga, inherited zircons between 1.76 and 2.04 Ga and low to moderately high I_Sr (0.704–0.710) indicate a Rb-depleted lower continental crust source for the Temaguessine pluton; regional considerations impose however also the participation of asthenospheric material. The Temaguessine pluton, together with other high-level subcircular pluton, is considered as marking the end of the Pan-African magma generation in the LATEA metacraton, resulting from the linear delamination along mega-shear zones, allowing asthenospheric uprise and melting of the lower continental crust. This implies that the younger Taourirt granitic province (535–520 Ma) should be considered as a Cambrian intraplate anorogenic event and not as a very late Pan-African event.     

990 and 1100 Ma Grenvillian tectonothermal events in the northern Oaxacan Complex, southern Mexico: roots of an orogen

Inliers of 1.0–1.3 Ga rocks occur throughout Mexico and form the basement of the Oaxaquia microcontinent. In the northern part of the largest inlier in southern Mexico, rocks of the Oaxacan Complex consist of the following structural sequence of units (from bottom to top), which protolith ages are: (1) Huitzo unit: a 1012±12 Ma anorthosite–mangerite–charnockite–granite (AMCG) suite; (2) El Catrı́n unit: ≥1350 Ma orthogneiss migmatized at 1106±6 Ma; and (3) El Marquez unit: ≥1140 Ma para- and orthogneisses. These rocks were affected by two major tectonothermal events that are dated using U–Pb isotopic analyses of zircon: (a) the 1106±6 Ma Olmecan event produced a migmatitic or metamorphic differentiation banding folded by isoclinal folds; and (b) the 1004–978±3 Ma Zapotecan event produced at least two sets of structures: (Z1) recumbent, isoclinal, Class 1C/3 folds with gently NW-plunging fold axes that are parallel to mineral and stretched quartz lineations under granulite facies metamorphism; and (Z2) tight, upright, subhorizontal WNW- to NNE-trending folds accompanied by development of brown hornblende at upper amphibolite facies metamorphic conditions. Cooling through 500 °C at 977±12 Ma is documented by ⁴⁰Ar/³⁹Ar analyses of hornblende. Fold mechanisms operating in the northern Oaxacan Complex under Zapotecan granulite facies metamorphism include flexural and tangential–longitudinal strain accompanied by intense flattening and stretching parallel to the fold axes. Subsequent Phanerozoic deformation includes thrusting and upright folding under lower-grade metamorphic conditions. The Zapotecan event is widespread throughout Oaxaquia, and took crustal rocks to a depth of 25–30 km by orogenic crustal thickening, and is here designated as Zapotecan Orogeny. Modern analogues for Zapotecan granulite facies metamorphism and deformation occur in middle to lower crustal portion of subduction and collisional orogens. Contemporaneous tectonothermal events took place throughout Oaxaquia, and in various parts of the Genvillian orogen in Laurentia and Amazonia.     

Age of the Wirgane granodiorite intrusions (Western High-Atlas, Morocco): New U–Pb constraints

U–Pb geochronological data, obtained on single zircon grains using the ion microprobe Cameca IMS 1270 (CRPG-CNRS, Nancy), indicate a Neoproterozoic age (625 ± 5 Ma) for the two intrusions of Wirgane granodiorite. This age is in variance with the previously published conclusions where the intrusions were supposed Variscan. Consequently, the Wirgane intrusions are interpreted as remnants of the Neoproterozoic basement of the Variscan belt. They are similar to the Anti-Atlas calc-alkaline intrusions of the collisional to post-collisional stages of the Pan-African orogeny. At that time, the Wirgane region was an area of continental growth to the north of the major South Atlasic Fault.