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The Wager shear zone is an ∼450-km long zone of high strain hosted within Proterozoic and Archean rocks of the Rae domain in northwestern Hudson Bay, Nunavut. New field mapping and microstructural analyzes, combined with titanite and apatite geochronology, define the style, kinematics, and timing of Proterozoic ductile deformation. The results indicate that the Wager shear zone accommodated high-temperature deformation between ca. 1.75 and 1.74 Ga with post-kinematic cooling through apatite U–Pb closure (425–530 °C) at ca. 1705 Ma. These new data show that the Wager shear zone was last active later than inferred in previous work, at a time when the western Churchill Province was undergoing lithospheric delamination and exhumation following the terminal collisional events in the western portion of the Trans-Hudson Orogen.

Cenozoic tectono-magmatism in Iran is widely considered to be related to subduction of the Neo-Tethys Ocean. We employed whole-rock and mineral geochemistry and isotopic data of intrusive rocks from Tafresh, central Urumieh-Dokhtar magmatic arc, to evaluate the role of the mantle in magmatism, to assess the timing of emplacement, and to interpret the tectonic setting. Rock compositions range from gabbro or gabbro-diorite (plagioclase + pyroxene ± olivine), to diorite (plagioclase + amphibole ± pyroxene), to granodiorite (quartz + plagioclase + K-feldspar + amphibole + biotite), exhibiting high-alumina calc-alkaline affinity. Major oxide and trace element variations vary systematically from less to more evolved rocks suggesting a major role for fractional crystallization processes. Zircon LA-ICP-MS U–Pb ages of major rock types are in the range of 24–19 Ma, whereas those of gabbroic dikes are ~17.5 Ma. ԐNd values range between -1.8 and 3.7, and (87Sr/86Sr)i is narrowly restricted to 0.705–0.706, suggesting a common mantle source. The enrichment in light rare earth element (REE) enrichment and flat heavy REE patterns couple depletion of Nb–Ta–Ti indicate that subducting oceanic crust had interacted with the overlying mantle wedge. High-alumina, mid-Mg# Tafresh plutonic rocks formed from hydrous melts from which Ca-pyroxene and magnetite crystallized earlier than plagioclase, whereas late-crystallizing zircon nucleated while magma traversed through lithospheric mantle and Cadomian crust. Modelling of isotope and incompatible-element patterns suggests the contribution of no more than ~5% molten sediment or other crustal components in Tafresh magma, at a developmental stage before most plagioclase and amphibole had crystallized. The Miocene Tafresh plutons originated during the final stages of subduction, before the collision between the Arabian and Eurasian plates.

Reconstructions of the tectonometamorphic architectures of orogenic systems rely on petrogenetic indicator minerals, such as garnet, to trace the transport of rocks through space and time. We show the results of new garnet Lu–Hf geochronology and inclusion U-(Th-)Pb geochronology from exhumed midcrustal metamorphic rocks exposed in the Kanchenjunga region of the eastern Nepalese Himalaya. Garnet in three of the five specimens analyzed yielded pre-Himalayan Lu–Hf dates of ca. 292–230 Ma, which contrasts with Himalayan-aged inclusions therein. Garnet in these specimens either grew entirely during the Early Permian opening of the Neo-Tethys Ocean or grew partly during Cambrian–Ordovician orogenesis and partly during the Cenozoic. The remaining two specimens yield Lu–Hf ages of ca. 50 Ma, which are some of the oldest recorded dates for Himalayan metamorphism. The apparent discordance between the geochronological techniques highlights a potential issue with interpretations that rely on single-geochron-method inclusion records. These results further show that some pressure-temperature determinations used in Himalayan models may not be Himalayan in age.

The metamorphic core of the Himalayan orogen, once viewed as a structurally contiguous unit, is now known to be divided into multiple packages by a series of structural discontinuities. While these structures have been identified based on distinct metamorphic and kinematic histories of the bounding packages, the number of structures identified across the exhumed former midcrustal core varies throughout the Himalaya. The scarcity of field evidence for these structures not only impedes their identification, but also hinders our understanding of their role in the development of the Himalaya. This study characterizes the metamorphic and geochronological history of garnet, sillimanite, and kyanite bearing gneisses in Makalu-Arun region, Nepal. Phase equilibria modelling coupled with monazite U/Th-Pb petrochronology delineates four rock packages with distinct metamorphic histories separated by at least three thrust-sense structures. The earliest thrust activity and subsequent in-sequence thrusting lasted from ca. 23-14 Ma, with initiation of late out-of-sequence thrust after ca.14 Ma. These results are consistent with foreland thrust migration and juxtaposition consistent with models that incorporate underplating, metamorphism and exhumation of midcrustal rocks during orogenesis. Moreover, the demonstrated complex evolution of the metamorphic core in Makalu region is inconsistent with models that incorporate singular, orogenic-wide structures in the Himalaya. Supplementary material: https://doi.org/10.6084/m9.figshare.c.6420244

The Khumbu region of Nepal is host to numerous large leucogranite bodies that comprise some of the most iconic peaks in the Himalaya. These leucogranites occur throughout the high-metamorphic grade...

The Great Unconformity is an iconic geologic feature that coincides with an enigmatic period of Earth's history that spans the assembly and breakup of the supercontinent Rodinia and the Snowball Earth glaciations. We use zircon (U-Th)/He thermochronology (ZHe) to explore the erosion history below the Great Unconformity at its classic Grand Canyon locality in Arizona, United States. ZHe dates are as old as 809 ± 25 Ma with data patterns that differ across both long (∼100 km) and short (tens of kilometers) spatial wavelengths. The spatially variable thermal histories implied by these data are best explained by Proterozoic syn-depositional normal faulting that induced differences in exhumation and burial across the region. The data, geologic relationships, and thermal history models suggest Neoproterozoic rock exhumation and the presence of a basement paleo high at the present-day Lower Granite Gorge synchronous with Grand Canyon Supergroup deposition at the present-day Upper Granite Gorge. The paleo high created a topographic barrier that may have limited deposition to restricted marine or nonmarine conditions. This paleotopographic evolution reflects protracted, multiphase tectonic activity during Rodinia assembly and breakup that induced multiple events that formed unconformities over hundreds of millions of years, all with claim to the title of a “Great Unconformity.”

Assigning correct protolith to high metamorphic-grade core zone rocks of large hot orogens is a particularly important challenge to overcome when attempting to constrain the early stages of orogenic evolution and paleogeography of lithotectonic units from these orogens. The Gurla Mandhata core complex in NW Nepal exposes the Himalayan metamorphic core (HMC), a sequence of high metamorphic-grade gneiss, migmatite, and granite, in the hinterland of the Himalayan orogen. Sm-Nd isotopic analyses indicate that the HMC comprises Greater Himalayan sequence (GHS) and Lesser Himalayan sequence (LHS) rocks. Conventional interpretation of such provenance data would require the Main Central thrust (MCT) to be also outcropping within the core complex. However, new in situ U-Th/Pb monazite petrochronology coupled with petrographic, structural, and microstructural observations reveal that the core complex is composed solely of rocks in the hanging wall of the MCT. Rocks from the core complex record Eocene and late Oligocene to early Miocene monazite (re-)crystallization periods (monazite age peaks of 40 Ma, 25–19 Ma, and 19–16 Ma) overprinting pre-Himalayan Ordovician Bhimphedian metamorphism and magmatism (ca. 470 Ma). The combination of Sm-Nd isotopic analysis and U-Th/Pb monazite petrochronology demonstrates that both GHS and LHS protolith rocks were captured in the hanging wall of the MCT and experienced Cenozoic Himalayan metamorphism during south-directed extrusion. Monazite ages do not record metamorphism coeval with late Miocene extensional core complex exhumation, suggesting that peak metamorphism and generation of anatectic melt in the core complex had ceased prior to the onset of orogen-parallel hinterland extension at ca. 15–13 Ma. The geometry of the Gurla Mandhata core complex requires significant hinterland crustal thickening prior to 16 Ma, which is attributed to ductile HMC thickening and footwall accretion of LHS protolith associated with a Main Himalayan thrust ramp below the core complex. We demonstrate that isotopic signatures such as Sm-Nd should be used to characterize rock units and structures across the Himalaya only in conjunction with supporting petrochronological and structural data.

Considerable debate persists regarding the petrogenesis of high-alumina basalts (HAB) which are purported to occur exclusively in subduction zones. Major and trace element, mineral chemistry, whole-rock Sr-Nd-Hf-isotopes and zircon U-Pb age data are reported for the Cenozoic Eshtehard HABs, in order to constrain the nature of mantle beneath the central Urumieh-Dokhtar magmatic arc, and further investigate the mechanism of HABs generation. Eshtehard HABs, chemically akin to those from continental arcs, include basaltic, basaltic andesite, andesitic and dacitic rocks. U-Pb geochronology of zircon yield ages of 47.2 ± 0.6 Ma, 43.9 ± 0.3, and 40.9 ± 0.5 to 39.4 ± 0.9 for basaltic, andesitic and dacitic samples, respectively. Basaltic andesitic dikes intruded into dacitic hosts yield age of ca. 20–18 Ma. eHf(t) values for Eocene zircons display a range variable from −6.4 to +6.5. Miocene zircons have higher eHf(t), ranging between −1.8 and +10.7. The studied rocks are characterized by enrichment in incompatible trace elements and have relatively homogeneous Sr-Nd isotopes. Integrated studies indicate that Eshtehard HABs were derived from the hydrated, dominantly depleted shallow asthenospheric mantle wedge (and possibly also in the lower lithosphere) overlying subducted oceanic lithosphere. Traversing lithospheric mantle and Cadomian crust, and assimilating crustal material while fractionating plagioclase, high-Ca pyroxene, magnetite, and amphibole, the primary melt formed Eshtehard high-alumina, low-Mg# basalts. We hypothesize that ponding of hydrous magma at the base of the crust allowed for further crustal assimilation and fractionation of Ca-pyroxene and magnetite without plagioclase and zircon nucleation; then, ascending magmas through the crust led to crystallization of plagioclase and eventually zircon. We suggest comparatively high water content (rather than high crystallization pressure), up to 4% sediment melt and less than 10% of continental crust materials were involved, as subordinate components, in the petrogenesis of Eshtehard HABs.

The Thakkhola graben is a large-scale N-S striking, E-W extensional structure located in west-central Nepal that was actively extending ca. 17 Ma. New multi-system geochronological data from the im...

Coupled zircon U-Pb age, trace-element, and Lu-Hf isotope analyses from central Sierra Nevada (eastern California, USA) metavolcanic strata reveal the expression of three Mesozoic arc flare-ups in the volcanic record over ∼150 m.y. of magmatic activity. Zircon εHf(i) values vary up to 20 epsilon units within individual samples and coeval sample populations but show no clear links with other geochemical indices, requiring both mixing and fractionation for arc magma genesis. Zircon compositions show repeated temporal trends across flare-ups: Hf isotopes spanning evolved to juvenile values converge to more juvenile compositions, middle rare earth element (MREE) depletion and heavy REE slopes increase during flare-up main phases, and highly variable U/Yb values converge to low values as flare-ups conclude. Despite pervasive contamination, juvenile source magmas dominate magmas erupted throughout the entirety of high-magma-addition episodes. Arc flare-ups thus represent ∼30 m.y. of increased mantle magma input that represents significant continental crust formation in Cordilleran arcs.

This study addresses the question of how and where arc magmas obtain their chemical and isotopic characteristics. The Wooley Creek batholith and Slinkard pluton are a tilted, mid- to upper-crustal part of a vertically extensive, late-Jurassic, arc-related magmatic system in the Klamath Mountains, northern California. The main stage of the system is divided into an older lower zone (c. 159 Ma) emplaced as multiple sheet-like bodies, a younger upper zone (c. 158–156 Ma), which is gradationally zoned upward from mafic tonalite to granite, and a complex central zone, which represents the transition between the lower and upper zones. Xenoliths are common and locally abundant in the lower and central zones and preserve a ghost stratigraphy of the three host terranes. Bulk-rock Nd isotope data along with ages and Hf and oxygen isotope data on zircons were used to assess the location and timing of differentiation and assimilation. Xenoliths display a wide range of εNd (whole-rock) and εHf (zircon), ranges that correlate with rocks in the host terranes. Among individual pluton samples, zircon Hf and oxygen isotope data display ranges too large to represent uniform magma compositions, and very few data are consistent with uncontaminated mantle-derived magma. In addition, zoning of Zr and Hf in augite and hornblende indicates that zircon crystallized at temperatures near or below 800 °C; these temperatures are lower than emplacement temperatures. Therefore, the diversity of zircon isotope compositions reflects in situ crystallization from heterogeneous magmas. On the basis of these and published data, the system is interpreted to reflect initial MASH-zone differentiation, which resulted in elevated δ18O and lowered εHf in the magmas prior to zircon crystallization. Further differentiation, and particularly assimilation–fractional crystallization, occurred at the level of emplacement on a piecemeal (local) basis as individual magma batches interacted with partial melts from host-rock xenoliths. This piecemeal assimilation was accompanied by zircon crystallization, resulting in the heterogeneous isotopic signatures. Magmatism ended with late-stage emplacement of isotopically evolved granitic magmas (c. 156 Ma) whose compositions primarily reflect reworking of the deep-crustal MASH environment.

The Butcher Ridge igneous complex, Antarctica, is an ∼6000 km3 hypabyssal silicic intrusion containing rhythmically layered glassy rocks. Baddeleyite U-Pb geochronologic analysis on a sample of the Butcher Ridge igneous complex yielded an age of ca. 182.4 Ma, which confirms that it was emplaced synchronously with the Ferrar large igneous province. Rocks of the Butcher Ridge igneous complex vary from basaltic andesite to rhyolite, and so the inferred volume of the Butcher Ridge igneous complex makes it the most voluminous silicic component of the Ferrar large igneous province. Major-element, trace-element, and isotopic data combined with binary mixing, assimilation-fractional crystallization (AFC), and energy-constrained AFC models are consistent with formation of Butcher Ridge igneous complex silicic rocks by contamination of mafic Ferrar parental magma(s) with local Paleozoic plutonic basement rocks. Field and petrographic observations and evidence for alkali ion exchange suggest that the kilometer-long, meter-thick enigmatic rhythmic layering formed as a result of secondary hydration and devitrification of volcanic glass along parallel fracture networks. The regularity and scale of fracturing/layering imply a thermally driven process that occurred during shallow emplacement and supercooling of the intrusion in the upper crust. We suggest that layering observed in the Butcher Ridge igneous complex is analogous to that reported from terrestrial and Martian cryptodomes, and therefore it is an ideal locality at which to study layering processes in igneous bodies.

Extensive Mesoproterozoic magmatism occurred in the Karbi Hills Massif, NE India, providing a key to understanding the early geodynamic evolution of Karbi Craton during the Columbia (Nuna) supercontinent cycle. We present here LA-ICP-MS zircon U–Pb ages, Lu–Hf isotopic and geochemical data of Panbari-Geleki-Dallamara and Kuthori granitoids in the Kaziranga area in northern Karbi Craton. These granitoids occur as stock like bodies intruded into the Late Archean-Paleoproterozoic Basement Gneissic Complex, supracrustals and previously emplaced mafic magmatics. Due to subsequent tectono-thermal activities, the older magmato-sedimentary sequences suffered variable degrees of metamorphism and deformations. The U/Pb zircon geochronology indicates that these granitoids were emplaced over a span of about 94 m.y., subsequent to the emplacement of mafic dykes, sills and stocks in a post collision rift basin, now represented by the ‘Shillong Basin’. The Karbi Hills granitoids are younger (1644 ± 33–1550 ± 25 Ma) than the Meghalaya felsic magmatics (1778 ± 37–1620 ± 9.2). Basin formation and sedimentation began in the Shillong Basin (Shillong Group) as early as 1900 Ma and continued till 1400 Ma, coinciding with the assembly and dismantling of Columbia supercontinent. Early Proterozoic psammopelitic sedimentation prior to and/or after the mafic-felsic intrusions is now represented by the ‘Shillong Group’. The mafic magmatics occur in the craton, present a continental flood basalt character. The chemical makes up state that the granitoids bear mostly metaluminous but weakly peraluminous, A-type, within plate characters. The granitoids are highly potassic and silica saturated (up to 74.3 wt% SiO2), REE deficient, K, Rb and Ba enriched but Nb, Ti and P depleted. The zircon ɛHf(t) for granitoids show a variation up to 7 eHf units (+1.5 to +9.9), however, 13% grains possess negative ɛHf(t) values. This is particularly ascribed to minor mixing of magmas sourced from depleted mantle and metasomatised continental crust. There is, however, no record of any vestige of ophiolite preserved in the craton that would support an active subduction mechanism responsible for this part of early Mesoproterozoic magmatic episode.

The Oligo–Miocene Ardestan quartz diorite to tonalite is part of widespread Cenozoic magmatism within the Urumieh–Dokhtar Magmatic Assemblage of Iran. The Ardestan pluton is composed mainly of varying proportions of plagioclase feldspar (normally zoned from bytownite to andesine), amphibole (magnesio-hornblende) and biotite. Biotite exhibits a range of Al values (~2–2.8 apfu) over very restricted Fe# ratios (0.42–0.56) which are characteristic of continental arc magmatic suites. High Ti2O contents of biotite (TZr.Ti°C (716 ± 50°C) and similar to the average TZr.sat°C (735 ± 26°C). These results are consistent with the low bulk-rock SiO2 contents, which provide minimum estimates of temperature and indicate zircon crystallised from a fractionated magma. Zircons from the Ardestan pluton have high (Sm/La)N (>10) ratios suggesting a magmatic origin. T–$f_{{\rm O}_{\rm 2}}$ calculations of oxygen fugacity between –13.6 to –16.9 indicate oxidising crystallisation conditions between the Ni–NiO (NNO) and Fe2O3–Fe3O4 (HM) buffers. Tight linear trends of log (XF/XOH), log (XCl/XOH) and log (XCl/XOH) vs. XMg represent a narrow range of $f_{{\rm H}_2O}$, fHF and fHCl, clearly indicating that physico-chemical conditions were essentially constant throughout the formation of magmatic biotite. The shape of crystal size distribution curves along with the medium Al and Mg contents in amphibole and biotite, respectively, are consistent with a history of magma mixing involving injections of basic magma into the evolving felsic chamber. Calculated residence time for Ardestan plagioclase crystals of ~630 years support field evidence that these plutons were emplaced at shallow depths.

Few methods exist for directly dating orogenic events in the upper crust. Here we report in-situ dates from (∼10 μm diameter) monazite grown in quartz-calcite fibrous veins, which precipitated at very low-grade metamorphic conditions (about 250 °C) in volcaniclastic and calcareous rocks of the Early Cretaceous Trancas Fm. in the Zimapan Basin, Central Mexico. Our integrated study includes regional and local structural and kinematic analysis of vein and folds, detailed textural and mineralogical characterization of vein-bearing minerals, and texturally-controlled Th/Pb geochronologic analysis of monazite by LA-MC-ICPMS. We find that monazite in the veins preferentially crystallized at the interface between vein-forming calcite and quartz fibers, as well as at the vein-host rock interface. Monazite ages range from 154 to 68 Ma, with the youngest sub-population yielding a weighted mean age of 76.8 ± 0.8 Ma, coincident with the age of basin-wide folding as earlier constrained by Ar/Ar illite dating. The oldest monazite ages coincide with the timing of deposition of the volcaniclastic host-rock. Given that veins are ubiquitous in shallow crustal rocks, this approach to dating of veins with enclosed monazite has great potential to improve the constraints on temporal resolution of fluid-rock interaction during of deformation.

Accessory phosphate minerals, such as monazite and xenotime, are commonly used as geochronometers to provide timing constraints on tectonometamorphic events recorded by the host rocks within which they occur. However, the formation and recrystallization of accessory minerals, and their interaction with major mineral phases, are still poorly understood. As a consequence, linking ages obtained from accessory minerals to the metamorphic pressure-temperature (P-T) paths obtained primarily from major mineral phases, such as garnet, remains challenging. While there have been studies that have advanced our understanding of the behaviour of various accessory minerals through thermodynamic modelling, limited examples are available to test their reliability in natural metamorphic rocks. This study incorporates phosphate minerals into one of the most commonly used thermodynamic data sets for phase equilibria modelling. This refined methodology is tested by modelling the detailed P-T-time (t) paths for rocks from two different regions of the Himalaya, one subsolidus and one that experienced suprasolidus conditions. The results obtained from our integrated models yield direct information on the behaviour of monazite and xenotime growth/breakdown along the calculated P-T paths. This allows us to tie different age populations obtained from the accessory minerals directly to the P-T paths derived from major mineral phases and facilitates a refined understanding of the P-T-t histories of those rock specimens.

The Northern Highlands Terrane of Scotland hosts several thrust nappes that were deformed and metamorphosed during the Silurian Scandian orogeny. Quantitative petrological analysis of metamorphic assemblages indicates that the hinterland-positioned Naver nappe experienced decompression heating from 8–9 kbar and 600°C to 6–7 kbar and 700°C. Monazite–xenotime thermometry and geochronology delineate a detailed temperature–time history for the Naver nappe. Monazite often exhibits compositional zoning, which is used to establish multiple temperature–time points in several samples. These data indicate that the Naver nappe experienced relatively fast heating (c. 50°C myr−1) and relatively slow cooling (15–20°C myr−1), with peak temperatures occurring at c. 425 Ma. This temperature–time evolution is compatible with the early Emsian (407–403 Ma) deposition of unmetamorphosed conglomerates that rest on high-grade metamorphic rocks in the Naver nappe, but requires an acceleration in the cooling rate to 40–50°C myr−1 at 420–410 Ma. Geochronological constraints from this study and previous work suggest that deformation and metamorphism in the hinterland of the Scandian orogen in northern mainland Scotland are younger than the c. 430 Ma deformation in the foreland-positioned Moine thrust zone. We postulate that heat from pervasive granitic intrusions in the Naver nappe weakened the crust, allowing deformation to retreat to the hinterland of the orogen. Supplementary material: A description of our analytical methods, all U–Pb-trace element data, additional figures explaining our petrological analysis and other relevant data are available at: https://doi.org/10.6084/m9.figshare.c.4458041

Orthogneiss within the Paleoproterozoic strata of Lesser Himalayan sequence across the Himalaya has been variably linked to development in a continental arc setting, Indian basement, or a continental rift. New whole rock and trace element geochemical data and U/Pb zircon geochronology indicate that the granitoid protoliths to these rocks were derived from upper crustal sources in the Paleoproterozoic and have within-plate, A-type affinities. This is consistent with their generation in a rifted margin and is compatible with paleogeographic reconstructions that indicate an open boundary for present-day northern India in the Paleoproterozoic. Keywords: Himalaya, Geochemistry, Paleogeography, Rifting, Geochronology

Present-day along-strike heterogeneities within the Himalayan orogen are seen at many scales, from variations within the deep architecture of the lithospheric mantle, to differences in geomorphologic surface processes. Here, we present an internally consistent petrochronologic dataset from the Himalayan metamorphic core (HMC), in order to document and investigate the causes of along-strike variations in its Oligocene–Miocene tectonic history. Laser ablation split-stream analysis was used to date and characterise the geochemistry of titanite from 47 calc-silicate rocks across >2000 km along the Himalaya. This combined U-Pb-REE-Zr single mineral dataset circumvents uncertainties associated with interpretations based on data compilations from different studies, mineral systems and laboratories, and allows for direct along-strike comparisons in the timing of metamorphic processes. Titanite dates range from ∼30 Ma to 12 Ma, recording (re-)crystallization between 625 °C and 815 °C. Titanite T-t data overlap with previously published P-T-t paths from interleaved peltic rocks, demonstrating the usefulness of titanite petrochronology for recording the metamorphic history in lithologies not traditionally used for thermobarometry. Overall, the data indicate a broad eastward-younging trend along the orogen. Disparities in the duration and timing of metamorphism within the HMC are best explained by along-strike variations in the position of ramps on the basal detachment controlling a two-stage process of preferential ductile accretion at depth followed by the formation of later upper-crust brittle duplexes. These processes, coupled with variable erosion, resulted in the asymmetric exhumation of a younger, thicker crystalline core in the eastern Himalaya. Keywords: Himalaya, Petrochronology, Titanite, Metamorphic petrology

New metamorphic petrology and geochronology from the Loe Sar dome in the Swat region of northern Pakistan place refined constraints on the pressure, temperature and timing of metamorphism and deformation in that part of the Himalayan orogen. Thermodynamic modelling and monazite petrochronology indicate that metamorphism in the area followed a prograde evolution from ∼525 ± 25 °C and 6 ± 0.5 kbar to ∼610 ± 25 °C and 9 ± 0.5 kbar, between ca. 39 Ma and 28 Ma. Partitioning of heavy rare earth elements between garnet rims and 30–28 Ma monazite are interpreted to indicate coeval crystallization at peak conditions. Microtextural relationships indicate that garnet rim growth post-dated the development of the main foliation in the area. The regional foliation is folded about large-scale N–S trending fold axes and overprinting E–W trending folds to form km-scale domal culminations. The textural relationships observed indicate that final dome development must be younger than the 30–28 Ma monazite that grew with garnet rims post-regional foliation development, but pre-doming-related deformation. This new timing constraint helps resolve discrepancy between previous interpretations, which have alternately suggested that N–S trending regional folds must be either pre- or post-early Oligocene. Finally, when combined with existing hornblende and white mica cooling ages, these new data indicate that the study area was exhumed rapidly following peak metamorphism. Keywords: Pakistan, Himalaya, Monazite petrochronology, Metamorphism, Phase equilibria modelling, Deformation

The Ekomedion two-mica granite, southwestern Cameroon, has potential for uranium and molybdenum mineralization. Here, we present LA-ICP-MS U–Pb ages, Lu–Hf isotope characteristics, trace element concentrations and Ti-geothermometry of zircon from this granite hosting U–Mo mineralization in pegmatitic pods. The majority of zircon are CL-dark though some CL-bright cores were also identified. U–Pb zircon age data range from 121 ± 3 to 743 ± 11 Ma with only 5 of 34 ages being near concordant. The concordant mean age of 603 ± 12 Ma is similar to ages of granitic intrusions along the Central African Shear Zone in Cameroon. Apparent ages with mean of 261 ± 6 Ma reveal open system behavior with respect to Pb and/or U. Zircon eHfi values range from − 20.3 to − 0.3. This implies that U–Mo was remobilized during partial melting of the surrounding gneiss. Zircon Th/U > 0.1 as well as an increasing Hf with decreasing Th/U indicates that fractional crystallization was the main factor that controlled U–Mo mineralization in pegmatitic pods. Y and Y/Ho ratios cluster from 29 to 33 close to the chondritic ratio of 28 and indicate fractionation of Y and Ho with low F contents during the earliest stages of crystallization. Late stage accumulation of F-rich magmatic-hydrothermal fluids impacted U–Mo mineralization as a ligand. Zircon contains a prominent negative Eu anomaly pointing to a fractionating system rich in plagioclase. Calculated Ti-in-zircon temperatures span 672 °C to 1232 °C with the temperatures at the high end reflecting interference from mineral inclusions in the zircon grains while the lower temperature values are linked to crystallization.

We report the major-element, trace-element, and 87 Sr/ 86 Sr compositions of six plagioclase crystals from two Samoan lavas with extreme EM2 isotopic compositions (ALIA-115-18 with whole-rock 87 Sr/ 86 Sr of 0.718592, and ALIA-115-21 with whole-rock 87 Sr/ 86 Sr of 0.720469). We employed laser-ablation split-stream mass spectrometry (LASS) to simultaneously measure 87 Sr/ 86 Sr ratios, major-element concentrations, and trace-element concentrations in the same plagioclase crystal volume. We find that two plagioclase crystals have extreme 87 Sr/ 86 Sr heterogeneity in excess of 5000 ppm (where ppm of 87 Sr/Sr variability86=10 6 ⋅[Sr/8687Sr max − 87 Sr/ 86 Sr min ]/ 87 Sr/ 86 Sr avg ). In two of the plagioclase crystals, we identify the highest 87 Sr/ 86 Sr ratios (0.7224) ever measured in any fresh, mantle-derived ocean island basalt (OIB) or OIB-hosted mineral phase. We find that in 87 Sr/ 86 Sr-versus-Sr concentration space, the six plagioclase crystals overlap in a “common component” region with higher 87 Sr/ 86 Sr than has been previously identified in whole-rock Samoan lavas or mineral separates. We use the occurrence of olivine mineral inclusions (Fo=74.5±0.8, 2 SD) in the high- 87 Sr/ 86 Sr zone of one plagioclase crystal to infer the bulk composition (Mg#=46.8±0.8, 2 SD) of the extreme EM2 magma from which the olivine and high- 87 Sr/ 86 Sr plagioclase crystallized. We argue that a relatively evolved EM2 endmember magma mixed with at least one lower- 87 Sr/ 86 Sr melt to generate the observed intra-crystal plagioclase isotopic heterogeneity. By inferring that subducted terrigenous sediment gives rise to EM2 signatures in Samoan lavas, we estimate that the quantity of sediment necessary to generate the most-elevated 87 Sr/ 86 Sr ratios observed in the Samoan plagioclase is ∼7% of the mantle source. We also estimate that sediment subduction into the mantle over geologic time has generated a sediment domain that constitutes 0.02% of the mass of the mantle, a much lower proportion than required in the EM2 mantle source. Even if subducted sediment is concentrated in large low-shear-velocity provinces (LLSVPs) at the base of the mantle (which constitute up to 7.7% of the mantle's mass), then only 0.25% of the LLSVPs are composed of sediment. This requires that the distribution of subducted sediment in the mantle is heterogeneous, and the high relative abundance of sediment in the Samoan EM2 mantle is an anomalous relic of ancient subduction that has survived convective attenuation.

Author(s): Swanson-Hysell, NL; Avery, MS; Zhang, Y; Hodgin, EB; Sherwood, RJ; Apen, FE; Boerboom, TJ; Keller, CB; Cottle, JM | Abstract: The ca. 1.83nGa Trans-Hudson orogeny resulted from collision of an upper plate consisting of the Hearne, Rae, and Slave provinces with a lower plate consisting of the Superior province. While the geologic record of ca. 1.83nGa peak metamorphism within the orogen suggests that these provinces were a single amalgamated craton from this time onward, a lack of paleomagnetic poles from the Superior province following Trans-Hudson orogenesis has made this coherency difficult to test. We develop a high-quality paleomagnetic pole for northeast-trending diabase dikes of the post-Penokean orogen East-Central Minnesota Batholith (pole longitude: 265.8°; pole latitude: 20.4°; A95: 4.5°; K: 45.6nN: 23) whose age we constrain to be 1,779.1n±n2.3nMa (95% CI) with new U-Pb dates. Demagnetization and low-temperature magnetometry experiments establish dike remanence be held by low-Ti titanomagnetite. Thermochronology data constrain the intrusions to have cooled below magnetite blocking temperatures upon initial emplacement with a mild subsequent thermal history within the stable craton. The similarity of this new Superior province pole with poles from the Slave and Rae provinces establishes the coherency of Laurentia following Trans-Hudson orogenesis. This consistency supports interpretations that older discrepant 2.22–1.87nGa pole positions between the provinces are the result of differential motion through mobile-lid plate tectonics. The new pole supports the northern Europe and North America connection between the Laurentia and Fennoscandia cratons. The pole can be used to jointly reconstruct these cratons ca. 1,780nMa strengthening the paleogeographic position of these major constituents of the hypothesized late Paleoproterozoic supercontinent Nuna.

Shallow high-silica granites provide a window into processes that may precede many styles of rhyolitic volcanic eruptions., Rhyolitic melt that fuels explosive eruptions often originates in the upper crust via extraction from crystal-rich sources, implying an evolutionary link between volcanism and residual plutonism. However, the time scales over which these systems evolve are mainly understood through erupted deposits, limiting confirmation of this connection. Exhumed plutons that preserve a record of high-silica melt segregation provide a critical subvolcanic perspective on rhyolite generation, permitting comparison between time scales of long-term assembly and transient melt extraction events. Here, U-Pb zircon petrochronology and 40Ar/39Ar thermochronology constrain silicic melt segregation and residual cumulate formation in a ~7 to 6 Ma, shallow (3 to 7 km depth) Andean pluton. Thermo-petrological simulations linked to a zircon saturation model map spatiotemporal melt flux distributions. Our findings suggest that ~50 km3 of rhyolitic melt was extracted in ~130 ka, transient pluton assembly that indicates the thermal viability of advanced magma differentiation in the upper crust.

Subduction zones are crucial features of Earth’s plate tectonics, yet subduction initiation remains enigmatic and controversial. Herein, we reappraise the timing of formation of the first fragments detached from the leading edge of the downgoing slab during subduction initiation (i.e., the Semail metamorphic sole; Oman–United Arab Emirates). Based on geochronology and phase equilibrium modeling, we demonstrate that subduction initiated prior to 105 Ma and at a slow pace (< cm/yr). Subduction stagnated at relatively warm conditions (15–20°C/km) for at least 10 Myr before evolving into a faster (≥ 2–5 cm/yr) and colder (~7°C/km) self-sustained regime. Subduction unlocking at 95-96 Ma, through the progressive change of the interplate thermo-mechanical structure, triggered the onset of slab retreat, large-scale corner flow and fast ocean spreading in the overriding plate. These results reconcile conflicting analogue and numerical subduction initiation models and reveal the thermal, mechanical and kinematic complexity of early subduction steps.

[The Mio‐Pliocene fluvial systems in the Calchaqui region changed their location and geometry, associated with pro‐ and retrograding gravel wedges in response to intrabasin structural growth of ranges. , Abstract Foreland basins are sensitive recorders of spatiotemporal variations in tectonic and climatic forcing associated with an approaching orogenic front. Thus, analysis of foreland deposits and their associated deformation patterns and provenance signals allows assessment of tectonic and sedimentary processes during orogeny, providing clues to past environmental conditions. The Calchaqui region in the southern part of the northwest Argentinian Eastern Cordillera (ca. 25–26°S lat) structurally evolved from a contiguous Paleogene foreland of the Andes into a broken foreland and finally an intermontane basin landscape. This history is recorded in the sedimentary sequences of the Mio‐Pliocene Angastaco and Palo Pintado Formations. We combine sedimentological methods, U‐Pb zircon and K‐Ar glass geochronology, clay mineralogy, and geochemical weathering indices with apatite fission track and (U‐Th‐Sm)/He thermochronology, structural data, and fault modelling to document the stratigraphic response to the fluvial and tectonic processes that followed the formation of the broken foreland. Our observations suggest that fluvial systems in the Calchaqui region repeatedly changed their location and geometry. These fluvial systems were associated with pro‐ and retrograding gravel wedges that most likely formed in response to the structural growth of the Aguas de Castilla and Altos de Vinaco ranges that bound the basin to the east. A compartmentalisation of the foreland with restricted fluvial networks must have occurred by ca. 9 Ma. Our results demonstrate that the reconstruction of stratigraphic architectures constitutes a powerful means to better understand intrabasin tectonics and surface uplift in foreland basins.]

This study presents a re-examination of historical specimens (DG136 and DG167) from the Monashee complex in the southeastern Canadian Cordillera that are critical to the current understanding of rare earth element (REE) distribution between garnet and monazite (and other accessory minerals) during metamorphism. Nine-hundred and fifty-one new monazite petrochronology spot analyses on 29 different grains across two specimens outline detailed (re)crystallization histories. Trace element data collected from the same ablated volume, interpreted in the context of new phase equilibria modelling that includes monazite, xenotime and apatite, link ages to specific portions of the pressure–temperature (P-T) paths followed by the specimens. These linkages are further informed by garnet Lu-Hf geochronology and xenotime petrochronology. The clockwise P-T paths indicate prograde metamorphism was ongoing by ca. 80 Ma in both specimens. The structurally deeper specimen, DG136, records peak P-T conditions of ~755–770 ℃ and 8.8–10.4 kbar, interpreted to coincide with (re-)crystallization of low Y monazite at ∼75–70 Ma. Near-rim garnet isopleths from DG167 cross in the observed peak assemblage field at ∼680 °C and 9.3 kbar. These conditions are interpreted to correspond with low Y monazite (re-)crystallisation at ∼65 Ma. Both specimens record decompression along their retrograde path coincident with high Y 70–55 Ma and 65–55 Ma monazite populations in DG136 and DG167, respectively. These findings broadly agree with those initially reported ∼20 years ago and confirm early interpretations using trace elements in monazite as generally reliable markers of metamorphic reactions. Modern phase equilibria modelling and in situ petrochronological analysis, however, provide additional insight into monazite behaviour during anatexis and the effects of potential trace element buffering by REE-bearing phases such as apatite.

U–Pb petrochronology of deep crustal xenoliths and outcrops across northeastern Tanzania track the thermal evolution of the Mozambique Belt and Tanzanian Craton following the Neoproterozoic East African Orogeny (EAO) and subsequent Neogene rifting. At the craton margin, the upper–middle crust record thermal quiescence since the Archean (2.8–2.5 Ga zircon, rutile, and apatite in granite and amphibolite xenoliths). The lower crust of the craton documents thermal pulses associated with Neoarchean ultra-high temperature metamorphism (ca. 2.64 Ga, > 900 °C zircon), the EAO (600–500 Ma rutile), and fluid influx during rifting ( 650 °C (above Pb closure of rutile and apatite) at the time of eruption. Zoned titanite records growth during cooling of the lower crust at 550 Ma, followed by fluid influx during slow cooling and exhumation (0.1–1 °C/Myr after 450 Ma). Permissible lower-crustal temperatures for the craton and orogen suggest variable mantle heat flow through the crust and reflect differences in mantle lithosphere thickness rather than advective heating from rifting.

The structural and topographic evolution of orogenic plateaus is an important research topic because of its impact on atmospheric circulation patterns, the amount and distribution of rainfall, and resulting changes in surface processes. The Puna region in the north-western Argentina (between 13°S and 27°S) is part of the Andean Plateau, which is the world's second largest orogenic plateau. In order to investigate the deformational events responsible for the initial growth of this part of the Andean plateau, we carried out structural and stratigraphic investigations within the present-day transition zone between the northern Puna and the adjacent Eastern Cordillera to the east. This transition zone is characterized by ubiquitous exposures of continental middle Eocene redbeds of the Casa Grande Formation. Our structural mapping, together with a sedimentological analysis of these units and their relationships with the adjacent mountain ranges, has revealed growth structures and unconformities that are indicative of syntectonic deposition. These findings support the notion that tectonic shortening in this part of the Central Andes was already active during the middle Paleogene, and that early Cenozoic deformation in the region that now constitutes the Puna occurred in a spatially irregular manner. The patterns of Paleogene deformation and uplift along the eastern margin of the present-day plateau correspond to an approximately north-south oriented swath of reactivated basement heterogeneities (i.e. zones of mechanical weakness) stemming from regional Paleozoic mountain building that may have led to local concentration of deformation belts. Fil: Montero López, Carolina. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Salta. Instituto de Bio y Geociencias del NOA. Universidad Nacional de Salta. Facultad de Ciencias Naturales. Museo de Ciencias Naturales. Instituto de Bio y Geociencias del NOA; Argentina Fil: Hongn, Fernando Daniel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Salta. Instituto de Bio y Geociencias del NOA. Universidad Nacional de Salta. Facultad de Ciencias Naturales. Museo de Ciencias Naturales. Instituto de Bio y Geociencias del NOA; Argentina Fil: López Steinmetz, Romina Lucrecia. Universidad Nacional de Jujuy. Instituto de Ecorregiones Andinas. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Salta. Instituto de Ecorregiones Andinas; Argentina Fil: Aramayo, Alejandro José. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Salta. Instituto de Bio y Geociencias del NOA. Universidad Nacional de Salta. Facultad de Ciencias Naturales. Museo de Ciencias Naturales. Instituto de Bio y Geociencias del NOA; Argentina Fil: Pingel, Heiko. Universitat Potsdam; Alemania Fil: Strecker, Manfred R.. Universitat Potsdam; Alemania Fil: Cottle, John M.. University of California; Estados Unidos Fil: Bianchi, Carlos Luis. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Salta. Instituto de Bio y Geociencias del NOA. Universidad Nacional de Salta. Facultad de Ciencias Naturales. Museo de Ciencias Naturales. Instituto de Bio y Geociencias del NOA; Argentina

Lavas erupted at hotspot volcanoes provide evidence of mantle heterogeneity. Samoan Island lavas with high 87Sr/86Sr (>0.706) typify a mantle source incorporating ancient subducted sediments. To further characterize this source, we target a single high 87Sr/86Sr lava from Savai’i Island, Samoa for detailed analyses of 87Sr/86Sr and 143Nd/144Nd isotopes and major and trace elements on individual magmatic clinopyroxenes. We show the clinopyroxenes exhibit a remarkable range of 87Sr/86Sr—including the highest observed in an oceanic hotspot lava—encompassing ~30% of the oceanic mantle’s total variability. These new isotopic data, data from other Samoan lavas, and magma mixing calculations are consistent with clinopyroxene 87Sr/86Sr variability resulting from magma mixing between a high silica, high 87Sr/86Sr (up to 0.7316) magma, and a low silica, low 87Sr/86Sr magma. Results provide insight into the composition of magmas derived from a sediment-infiltrated mantle source and document the fate of sediment recycled into Earth’s mantle., Subduction of oceanic crust and sediments contributes to heterogeneities in the mantle, which are sampled by mantle plumes. Here, the authors find that extreme isotopic heterogeneity in Samoan clinopyroxenes can help constrain the composition of mantle sources containing sediment recycled into the Earth’s mantle.

Metamorphic soles are m to ~500 m thick tectonic slices welded beneath most large- scale ophiolites (usually ~20 km thick). They typically show a steep inverted metamorphic structure where the pressure and temperature (T) conditions of crystallization increase upward, from the base of the sole (500 ± 100°C at 0.5 ± 0.2 GPa) to the contact with the overlying peridotite (800 ± 100°C at 1.0 ± 0.2 GPa). The inverted T gradient was historically interpreted as a result of heat transfer from the incipient mantle wedge toward the nascent slab synchronously with the overlying ophiolite formation (within only 1-2 Myrs). Their mineralogical assemblage and deformation pattern provide major constraints on the nature and the timing of the processes controlling the dynamics of the plate interface during early subduction.Soret et al. (2017, 2019) recently reappraised the tectonic–petrological model for the formation of metamorphic soles below ophiolites, showing that the present-day structure of the sole results from the successive stacking of several homogeneous oceanic crustal slivers (without internal T gradient). This stacking marks the evolution of rheological properties of slab material and peridotites of the upper plate as the plate interface progressively cools (Agard et al., 2016). These findings outline the thermal and mechanical complexity of early subduction dynamics, and highlight the need for refined numerical modelling studies.Lu-Hf geochronology on garnet from the Oman metamorphic sole has recently shown that the earliest accreted subunit, found directly against the upper plate mantle, was initially buried ≥ 8 Ma earlier than previously estimated (Guilmette et al., 2017). These results imply initiation ≥ 8 Ma before the formation of the ophiolite, which underscores the common belief that ophiolite-sole couples record spontaneous subduction initiation and rather indicates far-field forcing long before upper plate extension and mantle upwelling.We herein present new U-Pb titanite and monazite petrochronology across the different sub-units of the Oman metamorphic sole. Our results confirm the time lag of several million years between subduction initiation and the ophiolite formation, therefore supporting the recently proposed model of far-field forced subduction initiation. They also reveal a significant time lag between the underplating and exhumation of each sub-unit of the sole.