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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.

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

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

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.

Combined zircon geochronology and Hf isotopes of plutonic rocks from eastern Marie Byrd Land and Thurston Island, Antarctica, provide a detailed record of Phanerzoic arc magmatism along the paleo-Pacific margin of Gondwana. Magmatism along the Antarctic margin initiated in a dominantly contractional arc setting with an isotopically enriched lithospheric mantle source during the Ross Orogeny (c. 540–485 Ma). After termination of the Ross Orogeny through the Cretaceous, detrital zircon and zircon from igneous rocks record relative increases in zircon eHfi inferred to represent episodes of lithospheric-scale extension and relative decreases during inferred contractional episodes along the Antarctic margin. Comparison of this secular isotopic evolutionary trend with similar data from along the paleo-Pacific margin of Gondwana demonstrates a shared history among Marie Byrd Land, Australia, and Zealandia that contrasts with the shared record of Thurston Island, Antarctic Peninsula, and South America. These two contrasting histories highlight an early Permian along arc geochemical and inferred geodynamic switch from an isotopically enriched contractional arc system in South America, Antarctic Peninsula, and Thurston Island to an isotopically depleted extensional arc system in Marie Byrd Land, Zealandia, and Australia. Despite differences in timing, all segments of the paleo-Pacific margin underwent a similar secular isotopic evolution with dramatic shifts from enriched to juvenile isotopic compositions during extensional collapse.

A combined U-Pb geochronology, whole-rock geochemistry, and Sr-Nd isotopic study give new insights into the nature of the protolith, tectonic setting, and petrogenetic history of Cadomian orthogneisses from the Tutak metamorphic complex (TMC), Sanandaj-Sirjan Zone, Iran. U–Pb zircon dates of 535 ± 4 Ma suggest an Early Cambrian crystallization age for the orthogneiss protoliths. Geochemical data indicate that the orthogneisses have I-type, high-K calc-alkaline, and peraluminous signatures. Depletion in Nb, Ta, Ti, Sr, and enrichment in Nd, Th, K, U relative to the primitive mantle, suggest derivation in a subduction zone environment in an active continental margin. Initial 87Sr/86Sr and 143Nd/144Nd ratios vary from 0.7045 to 0.7092 and from 0.5116 to 0.5117, respectively. Moreover, eNd(t) values range from − 2.95 to − 5.29, and two-stage model ages vary from 1.4 to 1.6 Ga. Taken together, these data suggest derivation of TMC orthogneisses from a metasomatic lithospheric mantle with a contribution of a crustal component generated via partial melting of a metabasite (i.e., amphibolite).

Uranium-lead (U-Pb) zircon geochronology, whole-rock geochemistry, and petrographic observations indicate that specimens from a suite of variably deformed granite and orthogneiss from the Okhaldungha region of east-central Nepal share a common origin. Microtextural characterization and quartz crystallographic fabric preferred orientation analyses of these same specimens outline a strain gradient that marks the location of a shear zone boundary. The location of this boundary, at the base of the orthogneiss, coincides with one of the interpreted locations of the Main Central thrust, though it cannot be uniquely identified as such. This study provides the first steps toward empirical constraints on the location and geometry of thrust structures in the region, helping to clarify the complex local kinematic framework. These methods not only help in assessing orogenic models of the Himalaya, but may also be applied to investigating other orogenic systems where the potential location of shear structures is contested.

Accessory phase geochronology records several phases of metamorphism that affected Moine metasedimentary rocks in the Northern Highlands Terrane of Scotland. Within the Sgurr Beag nappe, monazite and xenotime record initial metamorphism at c. 750 Ma, with later resetting at c. 600 Ma and c. 425 Ma. Monazite, xenotime, titanite and zircon also cryptically record c. 750 Ma and c. 600 Ma metamorphism in the Naver and Ben Hope nappes prior to the dominant 435-415 Ma Scandian orogeny. Textural, geochemical, and isotopic data indicate that c. 600 Ma and c. 425 Ma isotopic resetting in the Sgurr Beag nappe was caused by fluid-mediated dissolution and reprecipitation of monazite and xenotime. The compositions of fluid-altered monazite fall into two distinct trends, with high and low Y+HREE apparently reflecting the distinct timing and temperature conditions of dissolution and reprecipitation. Monazite-xenotime thermometry is applied to constrain the temperatures of fluid alteration. High Y+HREE, low Th/U monazite is related to high temperature (∼650°C) alteration at c. 600 Ma and low Y+HREE, high Th/U monazite is related to lower temperature (∼400°C) reactivity at c. 425 Ma. These data show that the composition of reprecipitated monazite is sensitive to metamorphic conditions during fluid alteration and is therefore useful in understanding the evolution of polyphase metamorphic terranes. These data suggest that a widespread metamorphic or metasomatic event affected northern Scotland at c. 600 Ma, likely related to higher heat flux and magmatic activity during the break-up of Rodinia. Additionally, the now adjacent Sgurr Beag and Moine nappes appear to have contrasting metamorphic records at least until Caledonian orogenesis, which suggests that final regional scale motion on the Sgurr Beag thrust must be of Caledonian age.

Extensive exposure of intrusive igneous rocks along the Ross orogen of Antarctica—an ancient accretionary orogen on the margin of East Gondwana—provides an exceptional opportunity to study continental arc magmatism. There is significant petrologic and geochemical variability in igneous rocks within a ~ 500-km-long segment of the arc in southern Victoria Land. The conspicuous occurrence of carbonatite and alkaline silicate rocks (nepheline syenite, A-type granite, and alkaline mafic rocks) adjacent to large complexes of subalkaline granitoids is not adequately explained by traditional models for continental arc magmatism. Extensive geochemical analysis (> 100 samples) and zircon U–Pb geochronology (n = 70) confirms that alkaline and carbonatitic magmatism was partially contemporaneous with the emplacement of large subduction-related igneous complexes in adjacent areas. Major pulses of subalkaline magmatism were compositionally distinct and occurred at different times along the arc. Large bodies of subalkaline orthogneiss and granite (sensu lato) were emplaced over similar time intervals (ca. 25 Myr) to the north (ca. 515–492 Ma) and south (ca. 550–525 Ma) of the alkaline magmatic province, although the initiation of these major pulses of magmatism was offset by ca. 35 Myr. Alkaline and carbonatitic magmatism spanned at least ca. 550–509 Ma, overlapping with voluminous subalkaline magmatism in adjacent areas. The most primitive rocks from each area have similarly enriched trace element compositions, indicating some common characteristics of the magma sources along the arc. The samples from the older subalkaline complex have invariably low Sr/Y ratios (

New U-(Th)/Pb geochronology and geochemical analyses of plutonic bodies in the Hindu Kush range, NW Pakistan, provide insight on the crustal growth and tectonic evolution of the southern Eurasian margin. These new data outline a protracted magmatic history that spans the Cambrian to the Neogene (ca. 538 to 23 Ma) and record a variety of petrogenetic associations variably influenced by within plate, volcanic arc, and collision tectonic environments. The Kafiristan pluton (538 ± 4 to 487 ± 3 Ma) yields geochemical signatures consistent with extensional plutonism and rifting of the Hindu Kush terrane from Gondwana. The Tirich Mir (127 ± 1 to 123 ± 1 Ma) and Buni-Zom (110 ± 1 to 104 ± 1 Ma) plutons have geochemical signatures that can be attributed to a subduction related continental volcanic arc system that developed along the southern margin of Eurasia in the Mesozoic. The Garam Chasma pluton, the youngest body in the study area (27.3 ± 0.5 to 22.8 ± 0.4 Ma), yields a geochemical signature consistent with widespread anatexis during crustal thickening related to the development of the Himalaya. The present geochemical and geochronological analysis from the Hindu Kush have produced important new constraints on the timing of tectonic events and variable tectonic settings along the south Eurasian margin before and after the continued India–Asia collision.

The Ross orogen of Antarctica is an extensive (>3000 km-long) belt of deformed and metamorphosed sedimentary rocks and granitoid batholiths, which formed during convergence and subduction of palaeo-Pacific lithosphere beneath East Gondwana in the Neoproterozoic–early Palaeozoic. Despite its prominent role in Gondwanan convergent tectonics, and a well-established magmatic record, relatively little is known about the metamorphic rocks in the Ross orogen. A combination of garnet Lu–Hf and monazite U–Pb (measured by laser-ablation split-stream ICP-MS) geochronology reveals a protracted metamorphic history of metapelites and garnet amphibolites from a major segment of the orogen. Additionally, direct dating of a common rock-forming mineral (garnet) and accessory mineral (monazite) allows us to test assumptions that are commonly used when linking accessory mineral geochronology to rock-forming mineral reactions. Petrography, mineral zoning, thermobarometry and pseudosection modelling reveal a Barrovian-style prograde path, reaching temperatures of ~610–680 °C. Despite near-complete diffusional resetting of garnet major element zoning, the garnet retains strong rare earth element zoning and preserves Lu–Hf dates that range from c. 616–572 Ma. Conversely, monazite in the rocks was extensively recrystallized, with concordant dates that span from c. 610–500 Ma, and retain only vestigial cores. Monazite cores yield dates that overlap with the garnet Lu–Hf dates and typically have low-Y and heavy rare earth element (HREE) concentrations, corroborating interpretations of low-Y and low-HREE monazite domains as records of synchronous garnet growth. However, ratios of REE concentrations in garnet and monazite do not consistently match previously reported partition coefficients for the REE between these two minerals. High-Y monazite inclusions within pristine, crack-free garnet yield U–Pb dates significantly younger than the Lu–Hf dates for the same samples, indicating recrystallization of monazite within garnet. The recrystallization of high-Y and high-HREE monazite domains over >50 Ma likely records either punctuated thermal pulses or prolonged residence at relatively high temperatures (up to ~610–680 °C) driving monazite recrystallization. One c. 616 Ma garnet Lu–Hf date and several c. 610–600 Ma monazite U–Pb dates are tentatively interpreted as records of the onset of tectonism metamorphism in the Ross orogeny, with a more robust constraint from the other Lu–Hf dates (c. 588–572 Ma) and numerous c. 590–570 Ma monazite U–Pb dates. The data are consistent with a tectonic model that involves shortening and thickening prior to widespread magmatism in the vicinity of the study area. The early tectonic history of the Ross orogen, recorded in metamorphic rocks, was broadly synchronous with Gondwana-wide collisional Pan-African orogenies.

Geologic mapping combined with microstructural and geochronologic analyses from the lower Himalayan metamorphic core in west-central Nepal record along-strike similarity in flow style despite variability in the timing of metamorphism and deformation. The Main Central thrust zone at the base of the Himalayan metamorphic core varies in thickness, tectonostratigraphy, and metamorphic gradient along the 250 km of strike length studied. In situ U-Th/Pb geochronology of monazite sampled from an along-strike transect at the top of the high-strain zone records Eocene−Oligocene prograde metamorphism followed by Miocene retrograde metamorphism. The timing of prograde and retrograde metamorphism and the muscovite 40 Ar/ 39 Ar dates gradually decrease along strike from northwest to southeast. This age trend is punctuated by an abrupt ∼3−8 m.y. decrease in the age of prograde and retrograde metamorphism and muscovite 40 Ar/ 39 Ar dates near the Marsyandi River in central Nepal. Quartz crystallographic preferred orientation fabrics from a parallel transect along the base of the high-strain zone document similar flow style at ∼440 °C in central Nepal. Muscovite 40 Ar/ 39 Ar ages, interpreted to approximate the age of deformation at this structural level, decrease from ca. 7 to 4 Ma along strike from northwest to southeast. Diachronous deformation and metamorphism along strike in west-central Nepal demonstrate the necessity of incorporating more than a single transect into tectonic models. Along-strike tectonometamorphic variability in west-central Nepal spatially corresponds to faults in the Indian basement bounding the subsurface Faizabad ridge, highlighting the possible influence of inherited basement faults on the geometry of the basal Himalayan detachment, the Main Himalayan thrust, as well as the tectonic evolution of the structurally overlying Himalayan metamorphic core. This study highlights the potential influence of inherited structures on the overlying orogenic wedge and the probability of along-strike diachroneity of deformation in the Himalaya.

U–Th–Pb zircon geochronology is an essential tool for quantifying the emplacement, differentiation and thermal evolution of crustal magmatic systems. However, the power of U–Pb zircon dates can be enhanced through complementary characterization of mineral texture and geochemistry, as this permits more detailed interpretations of geochronological datasets than conventionally achieved. Our approach to better relating zircon dates and geological processes consists of a multi-method analytical workflow, including cathodoluminescence imaging (CL), in situ LA-ICPMS/EPMA zircon geochemistry, U–Pb zircon ID-TIMS geochronology, and solution ICPMS zircon Trace Element Analysis (U–Pb TIMS-TEA). These methods are here applied to zircon from the Bergell Intrusion, a composite Alpine pluton preserving a ~ 10 km mid-crustal transect. Hand samples of tonalite, granodiorite and hybridized granitoid each record 250–700 kyr of autocrystic zircon growth. Bergell zircons are ubiquitously zoned with ca. 10 4 –10 6 yr growth histories, as evidenced by ID-TIMS analysis of microsampled fragments from single crystals. U–Pb TIMS-TEA data exhibit compositional trends on multiple spatiotemporal scales, including the handsample -scale, representing in situ differentiation at the emplacement level (e.g., Th/U); lithology -scale, defining trajectories corresponding to the production of tonalitic versus granodioritic magmas (Lu/Hf); and pluton -scale, indicating increasingly-evolved melts over ~1.6 Myr of pluton assembly (Zr/Hf). These absolute TIMS-TEA temporal trends are corroborated by relative LA-ICPMS/EPMA core-to-rim geochemistry. We compare records of trace element evolution from TIMS-TEA, Bergell whole-rock geochemistry, and a global compilation of whole-rock geochemical data. These findings support zircon compositional evolution as a robust indicator of differentiation at local and crustal scales, and provide key empirical constraints on melt differentiation and cooling timescales in the middle crust.

Re-examination of specimens collected across the Qomolangma Formation exposed in the summit pyramid on Mount Everest confirms a complex, multi-deformational history. Two pre-Oligocene deformational events are recorded within the calcite-dominated rocks from the main summit (8848 m). The first event is characterized by an RXZ of ~1.3–1.4 in detrital quartz grains, which increases to ~1.6 after the second event. The finite strain recorded in calcite in the same rocks is higher with an RXZ of ~2.7–2.8. The deformation in the main summit appears to have been accommodated dominantly through grain boundary sliding in calcite in a slightly constrictive environment with a vorticity of >0.81. Deformation in the South Summit (8750 m) contrasts with that of the top and is instead interpreted to reflect Miocene movement along the Qomolangma detachment with no earlier deformation preserved. U–Pb geochronology on synkinematic rutile indicates that movement on the structure occurred at 16.3 ± 5.0 Ma. Rocks within the Qomolangma Formation in the summit pyramid of Mount Everest appear to record the basic history of the Himalaya with early deformation likely related to crustal thickening and later, more spatially restricted deformation reflecting the Miocene extrusion of the metamorphic core.

The Hermit's Gorge area adjacent to the Rongbuk Glacier on the north side of Mount Everest is a critical location to establish the timing and duration of movement along the ductile strand of the South Tibetan Detachment system (STDS), a low-angle, north-dipping normal fault that bounds the upper part of the Greater Himalayan Sequence. Monazite from four leucogranite samples in the Hermit's Gorge that bracket the timing of ductile fabric development has been dated using both U/Th–Pb ID-TIMS and LA-MC-ICPMS. Results suggest that the earliest, ~ 16.4 Ma, leucogranite sills have been folded and deformed along with the host sillimanite gneisses and calc-silicates whereas structurally higher sills and dykes that post-date fabric development are slightly younger, all within uncertainty of one another at 15.6 to 15.4 Ma. Field relations combined with age data constrain ductile fabric formation associated with movement along this strand of the STDS as being on-going at 16.4 Ma but had ceased prior to 15.6 Ma, while brittle faulting along the STDS is younger than 15.4 Ma. Combined with data from the Everest massif and surrounding region, ages of granite crystallization and ductile shearing propagated up-structural section and northward with time.

Granitoids associated with the Neoproterozoic–early Paleozoic Ross orogeny are extensively exposed in the Dry Valleys region of southern Victoria Land, Antarctica, affording an exceptional opportunity to gain insight into the temporal and spatial scales of continental arc magmatism. Samples spanning 150 km along strike and 50 km across strike were selected for isotopic and geochemical analysis. Zircon U-Pb geochronology and the first Hf isotope data for Dry Valleys granitoids, coupled with whole-rock elemental data, reveal mixing between enriched lithospheric mantle and Precambrian crustal components and indicate that the principal phase of magmatism in the Dry Valleys area was restricted to a period of 23 m.y., from ca. 515 to 492 Ma. This relatively short period of magmatism contrasts with other segments of the Ross orogen, in which magmatism spanned greater than 100 m.y. Most calc-alkaline intrusions spanned 515–500 Ma, while postkinematic granitoids with alkali-calcic geochemical signatures spanned 505–492 Ma, indicating a transitional shift to an overall extensional tectonic regime. Zircon e Hf(i) values range between −0.3 and –7.2, with two-stage depleted-mantle model ages ranging from 1.5 to 1.9 Ga. Low e Hf(i) values in mafic samples are consistent with derivation from an enriched subcontinental lithospheric mantle source, while the large-volume granitic intrusions show evidence for increasing assimilation of old crust over time. A broadening of the e Hf(i) range to more negative values in the younger intrusions may reflect crustal thickening or underplating of fertile continental material into the source region of the arc.

Author(s): Browne, Nicoletta Christina | Advisor(s): Cottle, John M. | Abstract: A bimodal suite of lamprophyric and felsic porphyry dikes, forming part of the Neoproterozoic-Ordovician Ross orogeny, occurs throughout the Dry Valleys of Southern Victoria Land, Antarctica. Lamprophyres are mafic dikes enriched in incompatible elements and hydrous minerals and, in orogenic settings, are frequently associated with post-subduction extension, marking the end of subduction-related magmatism. Although the geologic units comprising the Ross orogen have been extensively mapped and studied, only sparse geochemical and geochronological data for these dikes exist; the Ross orogen provides an opportunity to gain insight into orogenic processes, particularly those underlying the end of subduction. A primary goal of this project was to use field relationships and U-Pb zircon geochronology to clarify cross-cutting relationships between lamprophyres, porphyry dikes, and other intrusive units in order to document the timing of the youngest magmatism and end of the Ross orogeny in Southern Victoria Land. Another was to use whole rock major and trace-element geochemistry to determine the petrogenetic relationship between lamprophyre and porphyry dikes and finally improve our understanding of the geochemistry of lamprophyre dikes. Geochronological data from 19 samples in tandem with field observations indicate that lamprophyres are cross-cut by, and older than, porphyry dikes, and that emplacement of these dikes occurred in quick succession following the cessation of voluminous calc-alkaline magmatism. Dates are consistent with similar dikes from Northern Victoria Land, suggesting similar emplacement ages for dikes along much of the Ross orogen margin. Whole-rock geochemical data from 213 samples indicate that the most primitive lamprophyres are enriched in Mg, Cr, and Ni as well as incompatible elements such as LILE and LREE, indicating that they drive from an enriched mantle source. The presence of negative HFSE anomalies and Pb enrichment suggests a subcontinental lithosphere source enriched by previous addition of upper continental crust-derived sediments, with the LILE enrichment and hydrous character being consistent with metasomatism of the source region. The results are similar to other “orogenic” lamprophyres that are sourced from metasomatized mantle contaminated by continental detritus. Porphyry dikes form co-linear arrays with lamprophyres in major and trace element trends, but are separated from the majority of lamprophyres by a gap of approximately 10 wt. % SiO2, indicating a bimodal suite of magmas with limited, local mixing. Lamprophyres and porphyry dikes have similar trace element distributions, and this, in tandem with cross-cutting relationships, is consistent with the porphyry dikes representing a melt from a lamprophyre-like source. The rarity of porphyry dikes described at a reconnaissance level south of the Dry Valleys suggests that this may have been a localized event, while the absence of porphyry dikes coexisting with lamprophyres in otherwise similar tectonic settings elsewhere in the world suggests that the occurrence of bimodal suites may relate to specific occurrences of underplating rather than crustal mixing.

A laser-ablation multi-collector inductively coupled plasma mass spectrometry (LA-MC-ICPMS) study of seven thorite and uranothorite [(Th,U)SiO 4 ] samples ranging in age from ~13 to ~500 Ma provides new insights into the U-Th/Pb isotope systematics of these geologically significant, high-Th mineral species. Despite extreme actinide enrichment and complex intra-crystal zonation in actinides and rare earth elements, this study demonstrates that radiogenic-lead loss and/or metamictization is minimal and restricted to domains that have undergone significant hydration. Dating of four igneous uranothorites yields ages that are concordant in U/Pb and Th/Pb space, consistent with other high-temperature chronometers, and are inferred to accurately reflect the timing of crystallization of each rock. Similarly, Th/Pb ages of three thorite and/or huttonite-bearing samples yield geologically plausible dates consistent with other mineral chronometers. No evidence of isotopic inheritance was observed in any of the samples. Data presented here demonstrate for the first time the feasibility of extracting accurate and precise U/Pb and Th/Pb ages from Phanerozoic thorite, uranothorite and huttonite using LA-MC-ICPMS at the 5 μm spatial resolution. These phases have the potential to be robust chronometers in igneous and metamorphic rocks as well as to provide important provenance information complementary to more widely used minerals such as detrital zircon.

The exhumed Himalayan midcrustal core exposed in the Likhu Khola region of east-central Nepal includes upper-greenschist- to upper-amphibolite-grade metamorphic rocks that record pervasive, top-to-the-south sense deformation. Metamorphic temperature estimates are within error across the mapped area ranging from 772 ± 37 °C in the structurally lower, southern part of the study area to 853 ± 58 °C in the structurally higher, northern area. Estimated metamorphic pressures are relatively constant at lower structural levels, but they decrease from 11.8 ± 1.4 kbar to a minimum of 6.5 ± 1.3 kbar up structural section. The decrease in pressures coincides with an abrupt change in pressure estimates up structural section that is interpreted to mark a tectonometamorphic discontinuity that separates two domains with distinct structural, thermal, and metamorphic histories. In situ laser-ablation split-stream monazite U-Th/Pb and rare earth element (REE) petrochronology outlines dates ranging from ca. 27.8 Ma to 15.1 Ma in the hanging wall of the interpreted discontinuity; monazite REE data indicate the spread in ages is the result of a protracted metamorphic history and late-stage anatexis. Metamorphic and petrochronologic data from the Likhu Khola are consistent with a kinematic model in which material structurally above the discontinuity was metamorphosed in the deep hinterland of the orogen and was subsequently incorporated into the foreland of the orogen. The transition from hinterland- to foreland-style processes was marked by a shift to discrete deformation and the development of the discontinuity. Movement across the discontinuity is interpreted to have driven metamorphism and deformation of the rocks structurally below at or after 15 Ma. Discontinuities similar to that identified in this study are being identified and described across the orogen, indicating they are important, orogenwide features.

Granulitized eclogite-facies rocks exposed in the Ama Drime Massif, south Tibet, were dated by Lu-Hf garnet geochronology. Garnet from the three samples analyzed yielded Lu-Hf ages of 37.5 ± 0.8 Ma, 36.0 ± 1.9 Ma, and 33.9 ± 0.8 Ma. Eclogitic garnet growth is estimated at ca. 38 Ma, the oldest age for burial of the lower Indian crust beneath Tibet reported from the central-eastern Himalaya. Granulite-facies overprinting followed at ca. 15–13 Ma, as indicated by U-Pb zircon ages. Unlike ultrahigh-pressure eclogites of the northwest Himalaya, the Ama Drime eclogites are not characteristic of rapid burial and exhumation of a cold subducted slab. The rocks instead resulted from crustal thickening during the early stages of continental collision, and resided in the lower-middle crust for >20 m.y. before they were exhumed and reheated. These new data provide solid evidence for the Indian crust having already reached at least ∼60 km thickness by the late Eocene.

The Leo Pargil dome (LPD) in northwest India exposes an interconnected network of pre-, syn-, and post-kinematic leucogranite dikes and sills that pervasively intrude amphibolite-facies metapelites of the mid-crustal Greater Himalayan sequence. Leucogranite bodies range from thin (5-cm-wide) locally derived sills to thick (2-m-wide) crosscutting dikes extending at least 100 m. Three-dimensional exposures elucidate crosscutting relations between different phases of melt injection and crystallization. Combined laser ablation inductively coupled plasma mass spectrometry U–Th/Pb geochronology and trace element analysis on well-characterized monazite grains from nineteen representative leucogranites yields a large, internally consistent data set of approximately 700 U–Th/Pb and 400 trace element analyses. Grain-scale variations in age correlate with trace element distributions and indicate semi-continuous crystallization of monazite from 30 to 18 Ma. The youngest U–Th/Pb ages in a given sample are consistent with the outcrop-scale crosscutting relations, whereas older ages within individual samples record inheritance from partially crystallized melt and source metapelites. U–Th/Pb isotopic and trace element data are incorporated into a model of melting within the LPD that involves (1) steady-state equilibrium batch melting of compositionally homogeneous metapelitic sources; (2) pulses of increased melt mobility lasting 1–2 m.y. resulting in segregation of melt from its source and amalgamation into mixed magmas; and (3) rapid emplacement and final crystallization of leucogranite bodies. Melt systems in the LPD evolved from locally derived, in situ melt in migmatitic source rocks into a vast network of dikes and sills in the overlying non-migmatitic host rocks.

Laser-ablation split-stream (LASS) analysis—high-speed, high spatial-resolution, simultaneous isotopic and elemental analysis—enables petrochronology at a new level, through the interpretation of isotopic dates combined with elemental abundances and/or isotopic tracers. This contribution begins with an introduction to petrochronology, presents a new LASS technique using dual multi-collector–single-collector inductively-coupled plasma mass spectrometry, and offers examples of how this technique is used to decipher the evolution of rocks with complex geologic histories.

[1] The eastern Himalaya is characterized by a region of granulites and local granulitized eclogites that have been exhumed via isothermal decompression from lower crustal depths during the India-Asia collision. Spatially, most of these regions are proximal to the South Tibetan detachment system, an orogen-parallel normal-sense detachment system that operated during the Miocene, suggesting that it played a role in their exhumation. Here we use geo- and thermochronological methods to study the deformation and cooling history of footwall rocks of the South Tibetan detachment system in northern Sikkim, India. These data demonstrate that the South Tibetan detachment system was active in Sikkim between 23.6 and ~13 Ma, and that footwall rocks cooled rapidly from ~700 to ~120 °C between ~15-13 Ma. While active, the South Tibetan detachment system exhumed rocks from mid-crustal depths, but an additional heat source such as strain heating, advected melt and/or crustal thinning is required to explain the observed isothermal decompression. Cessation of movement on the South Tibetan detachment system produced rapid cooling of the footwall as isotherms relaxed. A regional comparison of temperature-time data for the eastern South Tibetan detachment system indicates a lack of synchronicity between the Sa'er-Sikkim-Yadong section and the NW Bhutan section. To accommodate this requires either strike-slip tear faulting or local out-of-sequence thrusting in the younger segment of the orogen.

A modified single-shot laser ablation inductively coupled plasma mass spectrometry (SS-LA-ICPMS) technique is presented that enables rapid acquisition of in-situ U–Th/Pb dates on accessory minerals from a single laser pulse. Advances over existing techniques include formulation of a semi-automated data reduction method and robust error propagation. The effects of pulse-to-pulse laser stability and analyte transport configurations on the precision and accuracy of the analyses are systematically assessed. The validity of the SS-LA-ICPMS approach is demonstrated by repeat analyses of multiple zircon and monazite reference materials, which consistently reproduce to within uncertainty of accepted reference values. Application of the method to detrital zircon and monazite samples with a wide range of ages and variable parent and daughter nuclide concentrations results in U–Th/Pb age spectra that are statistically indistinguishable from conventional laser ablation protocols that employ continuous laser pulsing. The SS-LA-ICPMS technique is minimally destructive, offers increased sample and grain throughput and enables rapid dating of large numbers of grains—enabling, e.g., to identify grains that are geologically significant yet rare.

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.

On the north side of the Mulock Glacier and at Cape Teall in southern Victoria Land, Antarctica, deformed greenschist facies Skelton Group metasediments are intruded by the post‐kinematic A‐type Mulock Granite. Geochemical data indicate that this intrusive complex is similar to A‐type granitoids previously described both to the north (Glee In‐trusives and Penny Hill Granite), where they form part of the Koettlitz Glacier Alkaline Province, and to the south (Foggy‐dog Granite Suite). A U‐Pb zircon date of 546 ± 3 Ma on the Mulock Granite places a minimum constraint on the age, and timing of deformation of the Skelton Group rocks in this area, and indicates that the Mulock Granite was emplaced during the initial stages of the Ross Orogeny. This age also confirms that crystallisation of this granite body was synchronous with that of A‐type granitoids from the Royal Society Range to Skelton Glacier area immediately to the north. During the late Neoproterozoic, extension or transtension along this par...

U(-Th)-Pb dating of zircon, monazite, and xenotime from metamorphic and igneous rocks at two outcrops along a north-south transect in the Mount Everest region of southern Tibet provide new constraints on the timing and duration of thermal events associated with channel flow and the ductile extrusion of the Greater Himalayan Series (GHS). At the southernmost outcrop in the Kangshung Valley, Th-Pb ages from monazite indicate that prograde metamorphism associated with crustal thickening following the India-Asia collision occurred at least as early as 38.9±0.9 Ma. A subsequent sillimanite-grade metamorphic event at 28.0±1.2 Ma was followed by two phases of leucogranite emplacement at 20.8±0.8 and 16.7±0.4 Ma. At Thongmön, ∼40 km to the northeast of the Kangshung Valley, prograde metamorphism was occurring at ∼25.4 Ma and lasted until 16.1±0.1 Ma, reaching ∼740°C and 5 kbar at 22.4± 0.2 Ma. Immediately following metamorphism, two phases of leucogranite were emplaced at 15.2±0.2 and 12.6±0.2 Ma, with an intervening phase of ductile deformation. These data combined with ages from the Rongbuk glacier and Ama Drime range, north and east of Everest and the North Himalayan Mabja dome 100-140 km to the northeast, suggest that GHS metamorphism lasted ∼20 m.yr. and that migmatization and south-directed channel flow peaked around ∼23-20 Ma and ended by ∼16 Ma. The youngest leucogranites crosscut all ductile fabrics related to the Miocene channel flow.

[1] The Ama Drime Massif (ADM) is an elongate north-south trending antiformal feature that extends ∼70 km north across the crest of the South Tibetan Himalaya and offsets the position of the South Tibetan Detachment system. A detailed U(-Th)-Pb geochronologic study of granulitized mafic eclogites and associated rocks from the footwall of the ADM yields important insights into the middle to late Miocene tectonic evolution of the Himalayan orogen. The mafic igneous precursor to the granulitized eclogites is 986.6 ± 1.8 Ma and was intruded into the paleoproterozoic (1799 ± 9 Ma) Ama Drime orthogneiss, the latter being similar in age to rocks previously assigned to the Lesser Himalayan Series in the Himalayan foreland. The original eclogite-facies mineral assemblage in the mafic rocks has been strongly overprinted by granulite facies metamorphism at 750°C and 0.7–0.8 GPa. In the host Ama Drime orthogneiss, the granulite event is correlated with synkinematic sillimanite-grade metamorphism and muscovite dehydration melting. Monazite and xenotime ages indicate that the granulite metamorphism and associated anatexis occurred at

[1] 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.