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Previously unrecognized mafic and felsic plutonic rocks, formerly interpreted as an anatectic part of the amphibolite-eclogite Marianske Lazně Complex (MLC, Bohemian Massif) were examined together with a possible upper-crustal equivalent represented by the Cista pluton intruding continental crust farther east. These plutonic rocks were studied by whole-rock geochemistry (major and trace elements, Sr-Nd isotopes) and zircon U-Pb geochronology. The MLC magmatic rocks range from pyroxene-amphibole gabbro to trondhjemite and oligoclasite. The Cista pluton consists of porphyritic granitoids. The calc-alkaline nature of these rocks, relative enrichment of fluid-mobile elements (including large-ion lithophile elements), strong fractionation of light rare earth elements over heavy rare earth elements and depletion of high field strength elements are evidence for active margin magmatism. U-Pb zircon dating indicates that the magmatism was Mid-Devonian in both the MLC (ca. 385 Ma) and Cista pluton (ca. 390 up to ca. 365 Ma). Both magmatic units show inherited cores probably linked to recycling of crustal material. Sr-Nd isotopic compositions indicate that the gabbro formed from a depleted MORB mantle source composition with a small contribution of a subduction and crustal component. A three-stage geodynamic model is proposed: 1) ca. 390–380 Ma recycling of a high-pressure accretionary wedge and migration of arc melts into more distal parts of the upper continental plate, 2) ca. 380–370 Ma compression and thickening of the accretionary wedge-arc edifice, and 3) ca. 370–365 Ma development of a giant east-dipping detachment that reworked the whole edifice in the west and also involved emplacement of the supracrustal Cista pluton along antithetic west-dipping shear zone in the east. The studied magmatic rocks record a Mid-Devonian magmatic arc related to the formation of a subduction wedge stacked beneath the upper plate that was followed by formation of a second and independent Carboniferous continental arc further east.

The onset of the Dead Sea transform has recently been reevaluated by U-Pb age-strain analyses of fault-related calcite taken from several fault strands along its main 500-km-long sector. The results suggest that the relative motion between Africa and Arabia north of the Red Sea was transferred northward to the Dead Sea transform as early as 20 Ma and along a ~10-km-wide deformation zone that formed the central rift with contemporaneous bounding sinistral motion. The Gishron fault is the western bounding fault with normal and sinistral fault offsets that placed Proterozoic crystalline rocks and a cover of Cambrian sandstones in fault contact with Cretaceous-Eocene carbonates. Fault-related calcite veins are common in the Gishron fault zone, and we report the results of a detailed study of one sample with nine calcite fillings. Low fluid inclusion entrapment temperatures

Titanite U–Pb geochronology is a promising tool to date high-temperature tectonic processes, but the extent to and mechanisms by which recrystallization resets titanite U–Pb dates are poorly understood. This study combines titanite U–Pb dates, trace elements, zoning, and microstructures to directly date deformation and fluid-driven recrystallization along the Coast shear zone (BC, Canada). Twenty titanite grains from a deformed calc-silicate gneiss yield U–Pb dates that range from ~ 75 to 50 Ma. Dates between ~ 75 and 60 Ma represent metamorphic crystallization or inherited detrital cores, whereas ~ 60 and 50 Ma dates reflect localized, grain-scale processes that variably recrystallized the titanite. All the analyzed titanite grains show evidence of fluid-mediated dissolution–reprecipitation, particularly at grain rims, but lack evidence of thermally mediated volume diffusion at a metamorphic temperature of > 700 °C. The younger U–Pb dates are predominantly found in bent portions of grains or fluid-recrystallized rims. These features likely formed during ductile slip and associated fluid flow along the Coast shear zone, although it is unclear whether the dates represent 10 Myr of continuous recrystallization or incomplete resetting of the titanite U–Pb system during a punctuated metamorphic event. Correlations between dates and trace-element concentrations vary, indicating that the effects of dissolution–reprecipitation decoupled U–Pb dates from trace-element concentrations in some grains. These results demonstrate that U–Pb dates from bent titanite lattices and titanite subgrains may directly date crystal-plastic deformation, suggesting that deformation microstructures enhance fluid-mediated recrystallization, and emphasize the complexity of fluid and deformation processes within and among individual grains.

In the Katha Range of central Myanmar, lithologic tracers and pressure-temperature-deformation time data identify Cambro-Ordovician, Indian-affinity Tethyan Himalaya Series, located ∼700 km from their easternmost outcrop in S-Tibet, and ∼450 km from Himalayan rocks in the Eastern Himalayan Syntaxis. Metamorphism began at ∼65 Ma, peaked at ∼45 Ma (∼510°C, 0.93 GPa), and exhumation/cooling (∼25°C/ Myr) occurred until ∼30 Ma in a subduction-early collision tectonic setting. When the Burma microplate-part of the intra-Tethyan Incertus arc-accreted to SE-Asia, its eastern boundary, the southern continuation of the Indus-Yarlung suture (IYS), was reactivated as the Sagaing fault (SF), which propagated northward into Indian rocks. In the Katha rocks, this strike-slip stage is marked by ∼4°C/Myr exhumation/cooling. Restoring the SF system defines a continental collision-oceanic subduction transition junction, where the IYS bifurcates into the SF at the eastern edge of the Burma microplate and the Jurassic ophiolite-Jadeite belts that include the Incertus-arc suture.

Studies of the high-pressure (HP) As Sifah eclogites in the NE Saih Hatat window, Oman, have used different combinations of radiometric dating results (Ar/Ar, Sm-Nd vs. U-Pb, Rb-Sr) to interpret disparate tectonic models for the timing, geometry, and cause of continental subduction – including its relationship to the Samail Ophiolite. To determine the timing of continental subduction, we coupled petrochronological analyses of major (garnet) and accessory phases (zircon, rutile) from the highest-grade metamorphic rocks (As Sifah eclogites) in the Saih Hatat. Early Permian (283.8±0.7 Ma) tuffaceous zircon cores are consistent with earlier interpretations that the As Sifah rocks were sourced from a distal portion of the Arabian continental margin. Data from a range of bulk compositions and metamorphic assemblages consistently suggest a single metamorphic event, with garnet growth from ∼81–77 Ma – though with slight, consistent offsets in the timing of metamorphic (re)crystallization between different lithologies. These new data confirm previous interpretations for continental HP metamorphism in a single NE-dipping subduction zone beneath the obducted Samail Ophiolite; there is no robust evidence for a ∼110 Ma event or for SW-dipping subduction beneath the Arabian plate. Combined with other constraints, our data suggest that the As Sifah unit was subducted at rates similar to other small continental HP bodies, followed by two stages of cooling and exhumation – likely associated with the dragging to mantle depths by a mafic root, an initial phase of rapid exhumation from mantle depths, and a lengthy residence (≤40 My) in the lower to middle crust.

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.

LA-ICP-MS U-Pb dates and EPMA trace-element maps of titanite were collected from calc-silicate gneisses of the Ikalamavony (peak T: 750–800 °C) and Anosyen (peak T: 900–950 °C) domains in southern Madagascar to evaluate how titanite responds to high-temperature metamorphism, cooling, and retrogression. Fluid-mediated replacement of precursor titanite by titanite domains of different composition (interface-coupled dissolution-precipitation, ICDR) was the primary mechanism by which titanite was reset following high-grade metamorphism. Comparison of titanite U-Pb dates (530–490 Ma) with independent petrology and thermochronology indicates that the alteration occurred at temperatures as low as 300–500 °C. Apparent Zr temperatures (temperatures calculated assuming titanite–quartz–zircon equilibrium) in altered titanite are less than or equal to the metamorphic peak, but higher than the inferred alteration temperature, implying that Zr was removed, but that titanite–quartz–zircon equilibrium was not achieved during alteration. Although evidence for ICDR was observed in titanite samples separated by 10's to ~100 km, differences in U-Pb dates among samples, and even among titanite grains in the same thin section suggest that alteration at any given time was localized.

MPS is grateful to the Oxford-Burma Aung San Suu Kyi trust for funding research and field visits to Mogok. U-Th-Pb geochronology was funded by UCSB and NSF grants EAR-1348003 and EAR-1551054 to BH. The Mogok Metamorphic Belt (MMB) in Myanmar is a polymetamorphic, mainly Paleogene granulite-uppermost amphibolite facies terrane consisting mainly of marbles and calc-silicates hosting spinel, ruby and sapphire. Jurassic charnockite-syenite intrusions, as well as Eocene-Miocene leucogranite intrusions are also present. Pelitic rocks are uncommon, and where present, have sillimanite, both as primary inclusions in garnet and as secondary Bt + Sil coronas around garnet. Core samples from the Kyi-Tauk-Pauk gold mine at Thabeikkiyin, north of Mandalay, are mostly Grt + Bt + Sill gneisses and migmatites with uncommon interbanded Opx + Grt + Bt gneisses. Pseudosection modelling suggests prograde garnet growth occurred by biotite-dehydration melting that reached peak P–T conditions of 870–970 °C and ~ 0.9 GPa, and was followed by garnet breakdown forming coarse retrograde Bt + Sil coronas. U[sbnd]Pb monazite data show an early high-grade granulite event at 43–32 Ma, and a later upper amphibolite sillimanite-grade event peaking at 23–20 Ma, with a change in monazite chemistry after c. 22 Ma that is consistent with fluid/(melt) interaction and garnet breakdown. Elevated Th/U ratios from ~35 to 22 Ma, and at ~18 Ma are compatible with melt influx at that time, timing that is similar to the age of the regional Kabaing leucogranite in the Mogok valley area. Our data show that peak granulite facies metamorphism along the Mogok Metamorphic belt was mainly Middle Eocene-Early Oligocene, with upper amphibolite facies metamorphism extending to earliest Miocene. The MMB is cut abruptly by the Sagaing fault, a large-scale dextral fault that extends from the Andaman Sea north to the East Himalayan syntaxis. Our new U[sbnd]Pb monazite data constrain the oldest age of initiation of the eastern branch of the cross-cutting Sagaing dextral strike-slip fault at Postprint

The thermal structure of the Tibetan plateau ‐‐ the largest orogenic system on Earth ‐‐ remains largely unknown. Numerous avenues, provide fragmentary pressure/temperature information, both at the present (predominantly informed though geophysical observation), and on the evolution of the thermal structure over the recent past (combining petrological, geochemical, and geophysical observables). However, these individual constraints have proven hard to reconcile with each other. Here, we show that models for the simple underthrusting of India beneath southern Tibet are capable of matching all available constraints on its thermal structure, both at the present day and since the Miocene. Many parameters in such models remain poorly constrained, and we explore the various trade‐offs among the competing influences these parameters may have. However, three consistent features to such models emerge: (i) that present day geophysical observations require the presence of relatively cold underthrust Indian lithosphere beneath southern Tibet; (ii) that geochemical constraints require the removal of Indian mantle from beneath southern Tibet at some point during the early Miocene, although the mechanism of this removal, and whether it includes the removal of any crustal material is not constrained by our models; and (iii) that the combination of the southern extent of Miocene mantle‐derived magmatism and the present‐day geophysical structure and earthquake distribution of southern Tibet require that the time‐averaged rate of underthrusting of India relative to central Tibet since the middle Miocene has been faster than it is at present.

The Barrovian inverted metamorphism of the Svratka dome developed within two nappes derived from the Brunia continent that was thrust beneath the Moldanubian orogenic root. The metamorphism increases from biotite–chlorite zone in the basement to very closely spaced staurolite, kyanite and sillimanite zones at the top of the nappe pile. The sequence of mineral growth, chemical zoning of garnet, and pseudosection modelling indicate prograde paths from 4.5 kbar/510 °C to 5.5 kbar/540 °C in the garnet zone, from 6 kbar/530 °C to 7 kbar/600 °C in the staurolite zone, and from 3.5 kbar/510 °C to 8.5 kbar/650 °C in the kyanite zone. The age of monazite inclusions in garnet and staurolite is interpreted to reflect prograde metamorphism at 338 ± 7 Ma and 336 ± 7 Ma, respectively. An older matrix monazite crystal is interpreted as dating prograde crystallization at 345 ± 7 Ma, whereas a younger monazite group records recrystallization at/or down to 334 ± 7 Ma. While these petrological and geochronological data are consistent with data from an inverted metamorphic sequence of the southern Thaya dome, the spacing and distribution of metamorphic zones, nappe thicknesses, and late structures are different in the two domes. An antiformal stack of imbricated basement sheets and the extreme attenuation of metamorphic isograds at the top of the nappe pile in the Svratka dome are explained by a relatively cold overthrusting Moldanubian domain, formed mainly of middle orogenic crust. The homogeneous thickening of the hinterland-dipping basement duplexes and the regular spacing of metamorphic isograds in the Thaya dome are explained by a hot overriding Moldanubian domain, which in this region has a high proportion of exhumed lower orogenic crust and formed a hot mid-crustal channel.

The thermal structure of the Tibetan plateau ‐‐ the largest orogenic system on Earth ‐‐ remains largely unknown. Numerous avenues, provide fragmentary pressure/temperature information, both at the present (predominantly informed though geophysical observation), and on the evolution of the thermal structure over the recent past (combining petrological, geochemical, and geophysical observables). However, these individual constraints have proven hard to reconcile with each other. Here, we show that models for the simple underthrusting of India beneath southern Tibet are capable of matching all available constraints on its thermal structure, both at the present day and since the Miocene. Many parameters in such models remain poorly constrained, and we explore the various trade‐offs among the competing influences these parameters may have. However, three consistent features to such models emerge: (i) that present day geophysical observations require the presence of relatively cold underthrust Indian lithosphere beneath southern Tibet; (ii) that geochemical constraints require the removal of Indian mantle from beneath southern Tibet at some point during the early Miocene, although the mechanism of this removal, and whether it includes the removal of any crustal material is not constrained by our models; and (iii) that the combination of the southern extent of Miocene mantle‐derived magmatism and the present‐day geophysical structure and earthquake distribution of southern Tibet require that the time‐averaged rate of underthrusting of India relative to central Tibet since the middle Miocene has been faster than it is at present.

Geochronological and thermobarometric data from a lower crustal xenolith suite in the Pamir offer a unique record of the transport of lower crust to mantle depths after an episode of slab breakoff. We compare petrologically constrained pressure-temperature-time paths from the xenoliths to pressure-temperature-time (P-T-t) paths of tracked markers in 2-D numerical geodynamic models of density foundering with thermodynamically calculated densities. We investigate whether gravitational “drip” instabilities or the peeling back of a dense layer—delamination—can reproduce the P-T-t paths seen in the xenoliths, with the ancillary goal of capturing the positive feedback between mechanical thickening and densification of the lower crust. Key thermobarometric observations from the xenoliths we try to match in our numerical study are: (1) initial heating at near-constant pressure followed by (2) a sharp increase in pressure with continued heating. We find that thick crustal sections develop P-T-t paths in numerical models of delamination that match the observations from xenoliths: the lower crust initially heats due to return flow from upwelling asthenosphere, and then foundering mantle lithosphere and crust show a marked increase in pressure with additional heating. Initial gravitational drip instabilities founder with relatively little heating yet may thin the mantle lithosphere sufficiently to allow for subsequent delamination or asymmetric drips to nucleate in the region of hotter, thinner mantle lithosphere. Such subsequent asymmetric drips or delamination entrain crust that closely follows the P-T-t path from xenoliths. This suggests that the xenoliths were not derived from an initial drip instability, but instead from later instabilities or delamination enabled by thinning of the lithosphere. In all models where density foundering occurs, the positive feedback between contraction and densification of the lower crust leads to the loss of initially positively buoyant lower crust. The combination of geological and numerical methods constrains the geometry and triggers of lower crustal foundering during collision. Contraction alone does not match the record of foundering; the lithosphere must have also been asymmetrically thinned.

We thank the Oxford–Burma Aung San Suu Kyi trust for funding research and fieldwork visits to Myanmarfor MS,NG and LR. Geochronology was funded by UCSB and NSF grants EAR-1348003and EAR-1551054. The Mogok metamorphic belt (MMB) extends for over 1,000 km along central Burma from the Andaman Sea to the East Himalayan syntaxis and represents exhumed lower and middle crustal metamorphic rocks of the Sibumasu plate. In the Mogok valley region, the MMB consists of regional high‐grade marbles containing calcite + phlogopite + spinel + apatite ± diopside ± olivine and hosts world class ruby and sapphire gemstones. The coarse‐grained marbles have been intruded by orthopyroxene‐ and clinopyroxene‐bearing charnockite‐syenite sheet‐like intrusions that have skarns around the margins. Syenites range from hornblende‐ to quartz‐bearing and frequently show layering that could be a primary igneous texture or a later metamorphic overprint. Calc‐silicate skarns contain both rubies and blue sapphires with large biotites. Rubies occur in marbles with scapolite, phlogopite, graphite, occasional diopside, and blue apatite. Both marbles and syenites have been intruded by the Miocene Kabaing garnet‐muscovite‐biotite peraluminous leucogranite. New mapping and structural observations combined with U‐Th‐Pb zircon, monazite, and titanite geochronology from syenites, charnockites, leucogranites, meta‐rhyolite‐tuffs, and skarns have revealed a complex multiphase igneous and metamorphic history for the MMB. U‐Pb zircon ages of the charnockite‐syenites fall into three categories, Jurassic (170–168 Ma), latest Cretaceous to early Paleocene (~68‐63 Ma), and late Eocene–Oligocene (44–21 Ma). New ages from five samples suggest that metamorphism in the presence of garnet and melt occurred between ~45 and 24 Ma. U‐Pb titanite ages from the ruby marbles and meta‐skarns at Le Oo mine in the Mogok valley are 21 Ma, similar to titanite ages from an adjacent syenite (22 Ma). U‐Th‐Pb dating shows that all the metamorphic ages are Late Cretaceous–early Miocene and related to the India‐Sibumasu collision. Publisher PDF

Neogene, syn‐collisional extensional exhumation of Asian lower–middle crust produced the Shakhdara–Alichur gneiss‐dome complex in the South Pamir. The

The Eastern Ghats Province (EGP) of India, characterized by ∼1 Ga granulite metamorphism (M1), collided with the Archaean Indian craton in either Grenvillian (∼1 Ga) or Pan-African (∼0.6–0.5 Ga) time. At ∼850 Ma, the Koraput Alkaline Complex (KAC) intruded into M1 EGP granulites, and was metamorphosed during a second granulite event (M2) in the EGP. U-Pb zircon dates from gneisses in Koraput yield magmatic ages ∼1500 Ma, with overgrowths at 1100–900 Ma and 800–700 Ma. Cratonic zircons from the EGP contact zone have Archaean 2.8–2.6 Ga cores with overgrowths at ∼700 Ma. Titanites from a calc-silicate gneiss within the EGP define a U-Pb isochron around 700 Ma, whereas titanites from nepheline syenites within the KAC yielded a range of dates from ∼622 to 520 Ma. The c. 1500 Ma age in the EGP either implies Columbia break-up or proximity to a Columbia margin. The ∼700 Ma age is interpreted as the age of M2 granulite metamorphism that followed intracontinental thrusting of the EGP over ∼2.8 Ga granulites of the Jeypore Province. The ∼520 Ma titanite ages are correlated with final juxtaposition of the EGP-Jeypore Province amalgam with the Indian craton during Gondwana formation.

We report the discovery of osumilite in ultrahigh‐temperature (UHT) metapelites of the Anosyen domain, southern Madagascar. The gneisses equilibrated at ~930°C/0.6 GPa. Monazite and zircon U–Pb dates record 80 Ma of metamorphism. Monazite compositional trends reflect the transition from prograde to retrograde metamorphism at 550 Ma. Eu anomalies in monazite reflect changes in fO_2 relative to quartz–fayalite–magnetite related to the growth and breakdown of spinel. The ratio Gd/Yb in monazite records the growth and breakdown of garnet. High rates of radiogenic heat production were the primary control on metamorphic grade at the regional scale. The short duration of prograde metamorphism in the osumilite gneisses (

Xenoliths that erupted in the SE Pamir of Tajikistan from 1000 to 1050°C and 90 km depth illuminate what happens when crust founders into the mantle. 40Ar/39Ar dating of minerals from the xenoliths and volcanic host rocks of the shoshonitic Dunkeldik pipe and dike field indicates eruption at 11.2 ± 0.2 Ma. U-Pb and trace element laser-ablation split stream inductively coupled plasma mass spectrometry of zircon shows that the igneous and metasedimentary xenoliths were likely derived from the crustal section into which they were intruded: the Jurassic-Cretaceous Andean-style magmatic arc and its Proterozoic-Mesozoic host rocks along the southern margin of Asia. Recrystallization of these zircons was extensive, yielding a range of dates down to 11 Ma. The zircons show distinct changes in Eu anomaly, Lu/Gd ratio, and Ti concentrations compatible with garnet growth and minimal heating at 22–20 Ma and then 200-300°C of heating, ~25 km of burial, and alkali-carbonate melt injection at 14–11 Ma. These changes are interpreted to coincide with (i) heat input due to Indian slab breakoff at ~22–20 Ma and (ii) rapid thickening and foundering of the Pamir lithosphere at 14-11 Ma, prior to and synchronous with collision between deep Indian and Asian lithospheres beneath the Pamir.

Large domes of crystalline, mid–deep crustal rocks of Asian provenance make the Pamir a unique part of the India–Asia collision. Combined major-element and trace-element thermobarometry, pseudosections, garnet-zoning deconstruction, and geochronology are used to assess the burial and exhumation history of five of these domes. All domes were buried and heated sufficiently to initiate garnet growth at depths of 15–20 km at 37–27 Ma. The Central Pamir was then heated at ~10–20°C/Myr and buried at 1–2 km/Myr to 600–675°C at depths of 25–35 km by 22–19 Ma. The Shakhdara Dome in the South Pamir was heated at ~20°C/Myr and buried at 2–8 km/Myr to reach 750–800°C at depths of ≥50 km by ~20 Ma. All domes were exhumed at >3 km/Myr to 5–10 km depths and ~300°C by 17–15 Ma. The pressures, temperatures, burial rates, and heating rates are typical of continental collision. Decompression during exhumation outpaced cooling, compatible with tectonic unroofing along mapped large-scale, normal-sense shear zones, and with advection of near- or suprasolidus temperatures into the upper crust, triggering exhumation-related magmatism. The Shakhdara Dome was exhumed from greater depth than the Central Pamir domes perhaps due to its position farther in the hinterland of the Paleogene retrowedge and to higher heat input following Indian slab breakoff. The large-scale thickening and coincident ~20 Ma switch to extension throughout a huge area encompassing the Pamir and Karakorum strengthens the idea that the evolution of orogenic plateaux is governed by catastrophic plate-scale events.

The onset and evolution of the Dead Sea transform are re-evaluated based on new in situ U-Pb dating and strain analyses of mechanically twinned calcites. Direct dating of 30 syn-faulting calcites from 10 different inactive fault strands of the transform indicates that the oceanic-to-continental plate boundary initiated between 20.8 and 18.5 Ma within an ∼10-km-wide distributed deformation zone in southern Israel. Ages from the northern Dead Sea transform (17.1–12.7 Ma) suggest northward propagation and the establishment of a well-developed >500-km-long plate-bounding fault in 3 m.y. The dominant horizontal shortening direction recorded in the dated twinned calcites marks the onset of left-lateral motion along the evolving plate boundary. The observed changes in the strain field within individual fault strands cannot be simply explained by local "weakening effects" along strands of the Dead Sea transform or by gradual changes in the Euler pole through time.

Geothermochronologic data outline the temperature-deformation-time evolution of the Muskol and Shatput gneiss domes and their hanging walls in the Central Pamir. Prograde metamorphism started before ~35 Ma and peaked at ~23–20 Ma, reflecting top-to- ~N thrust-sheet and fold-nappe emplacement that tripled the thickness of the upper ~7–10 km of the Asian crust. Multimethod thermochronology traces cooling through ~700–100°C between ~22 and 12 Ma due to exhumation along dome-bounding normal-sense shear zones. Synkinematic minerals date normal sense shear-zone deformation at ~22–17 Ma. Age-versus-elevation relationships and paleoisotherm spacing imply exhumation at ≥3 km/Myr. South of the domes, Mesozoic granitoids record slow cooling and/or constant temperature throughout the Paleogene and enhanced cooling (7–31°C/Myr) starting between ~23 and 12 Ma and continuing today. Integrating the Central Pamir data with those of the East (Chinese) Pamir Kongur Shan and Muztaghata domes, and with the South Pamir Shakhdara dome, implies (i) regionally distributed, Paleogene crustal thickening; (ii) Pamir-wide gravitational collapse of thickened crust starting at ~23–21 Ma during ongoing India-Asia convergence; and (iii) termination of doming and resumption of shortening following northward propagating underthrusting of the Indian cratonic lithosphere at ≥12 Ma. Westward lateral extrusion of Pamir Plateau crust into the Hindu Kush and the Tajik depression accompanied all stages. Deep-seated processes, e.g., slab breakoff, crustal foundering, and underthrusting of buoyant lithosphere, governed transitional phases in the Pamir, and likely the Tibet crust.

Asian deep crust exposed in the Pamir permits determination of the amount, sequence, and interaction of shortening, extension, and lateral extrusion over ~30 km of crustal section during the India-Asia collision. In the Central Pamir, gneiss domes and their hanging walls record Paleogene tripling of the 7–10 km thick Phanerozoic upper crustal strata; total crustal thickness may have amounted to 90 km. Two thrust sheets, comprising Cambro-Ordovician, respectively, Carboniferous to Paleogene strata, straddle the domes. Amphibolite-facies metamorphic rocks within the domes—equivalent to lower grade rocks outside the domes—form fold nappes with dome-scale wavelengths. E-W stretching occurred contemporaneously with top-to- ~ N imbrication and folding. At ~22–12 Ma, bivergent (top-to-N and top-to-S), normal-sense shear zones exhumed the crystalline rocks; most of the extension occurred along the northern dome margins. Shortening resumed at ~12 Ma with opposite-sense thrusting and folding focused along the dome margins. Throughout the building of the Central and South Pamir, dominant ~N-S shortening interacted with ~E-W extension along mostly dextral shear/fault zones. In the Neogene, shear is concentrated along a dextral wrench corridor south of the domes. We interpret the Paleogene shortening to record thickening and northward growth of the Pamir-Tibetan Plateau and short-lived Miocene crustal extension as gravitational adjustment, i.e., collapse, of the thickened Asian crust to Indian slab breakoff. Synconvergent Paleogene lateral extrusion thickened the Afghan Hindu Kush crust west of the India-Asia collision, and the Miocene-Recent dextral shear and ~E-W extension have accommodated collapse of the Pamir Plateau into the Tajik depression.

Length scales of compositional heterogeneity in titanite from 750 to 1000 °C metamorphic rocks from southern Madagascar were measured to provide empirical constraints on elemental diffusivities. The calculated Pb diffusivity is comparable to experimental estimates of Sr diffusivity; because of this, U–Pb dates from rocks that reached peak temperatures 900 °C; thus, Zr-in-titanite thermobarometry should not be reset by diffusion in all but the smallest grains in the hottest rocks. Al and Nb diffuse at similar rates to Zr. Ce and Fe diffuse slower than Pb, but faster than Zr. Differences in empirical and experimental estimates of elemental diffusivities might be related to the complexity of most natural titanite solid solutions compared to the near-end-member titanite used in experiments.

Multimethod geochronology (U-Pb zircon; 40 Ar/ 39 Ar hornblende, biotite, feldspar; apatite fission track) on granitoids, gneisses, and Cenozoic intramontane basin clastics of the Gissar-Alai ranges, South Tian Shan collisional belt, west of the Talas-Fergana fault, elucidates a history of Neoproterozoic magmatism, late Paleozoic magmatism and metamorphism, and Mesozoic−Cenozoic thermal reactivation. Zircon-core and grain-interior U-Pb ages of ca. 2.7−2.4, 2.2−1.7, 1.1−0.85, and 0.85−0.74 Ga tie the early evolution of the Gissar-Alai ranges to that of the Tarim craton. At least part of the Gissar range crystalline basement—the Garm massif—shows U-Pb zircon crystallization ages of ca. 661‒552 Ma (median ca. 609 Ma), again suggesting a Tarim craton connection. Tarim collided with the Middle Tian Shan block at ca. 310‒305 Ma, completing the protracted formation of the South Tian Shan collisional belt. The massive Gissar range granitoids intruded later (ca. 305‒270 Ma), contemporaneous with peak Barrovian-type metamorphism in the Garm massif rocks. Major- and trace-element compositions suggest that the Gissar granitoid melts have continental arc affinity. Zircon e Hf and whole-rock e Nd values of −2.1 to −6.9 and −2.7 to −7.2, respectively. and Hf-isotope crustal model and Nd-isotope depleted mantle model ages of ca. 1.0‒1.2 and ca. 1.1‒2.2 Ga, respectively, suggest significant input of Precambrian crust in the Gissar granitoid and Garm orthogneiss melts, consistent with the U-Pb ages of inherited and detrital zircons. The distinct ca. 661‒552 Ma Garm gneiss crystallization ages and the ca. 1.0−2.2 Ga model ages (and the lack of 2.4−3.4 Ga model ages) tie the Garm gneisses and the reworked crust of the Gissar range to the northern rim—the Kuqa and Kolar sections—of the Tarim craton, suggesting a united Karakum-Tarim craton. Although about contemporaneous with widespread postcollisional magmatism in the entire Tian Shan, the large volume and short duration of the Gissar range magmatism, including crustal thickening and prograde metamorphism during Tarim craton‒Middle Tian Shan block collision, and formation and closure of an oceanic back-arc basin (the Gissar basin), indicate its origin in a distinct setting. Combined, this likely resulted in midcrustal melting and upper-crustal batholith emplacement. Mafic dikes and pipes intruded at ca. 256−238 Ma (median ca. 241 Ma); the source region of the parental melts was within the asthenospheric mantle. The simplest interpretation for these basanites is that they were part of the Tarim flood basalt province; this would extend this province westward from the Tarim craton into the southwestern Tian Shan and imply that the relatively short-lived flood basalt event (ca. 290‒270 Ma) was followed by much less voluminous but longer-lasting hotspot magmatism. The 40 Ar/ 39 Ar and detrital apatite fission-track dates outline post−Gissar-Alai range granitoid emplacement cooling, Cimmerian collision events at the southern margin of Asia, Late Cretaceous crustal extension and local magmatism, and early Cenozoic shortening and burial in the far field of the India-Asia collision.

The exhumed Fiordland sector of Zealandia offers a deep-crustal view into the life cycle of a Cordilleran-type orogen from final magmatic construction to extensional orogenic collapse. We integrate U-Pb thermochronologic data from metamorphic zircon and titanite with structural observations from >2000 km 2 of central Fiordland to document the tempo and thermal evolution of the lower crust during the tectonic transition from arc construction and crustal thickening to crustal thinning and extensional collapse. Data reveal that garnet granulite facies metamorphism and partial melting in the lower crust partially overlapped with crustal thickening and batholith construction during emplacement of the Western Fiordland Orthogneiss (WFO) from 118 to 115 Ma. Metamorphic zircons in metasedimentary rocks yield 206 Pb/ 238 U (sensitive high-resolution ion microprobe–reverse geometry) dates of 116.3–112.0 Ma. Titanite laser ablation split stream inductively coupled plasma–mass spectrometry chronology from the same rocks yielded complex results, with relict Paleozoic 206 Pb/ 238 U dates preserved at the margins of the WFO. Within extensional shear zones that developed in the thermal aureole of the WFO, titanite dates range from 116.2 to 107.6 Ma and have zirconium-in-titanite temperatures of ∼900–750 °C. A minor population of metamorphic zircon rims and titanites in the Doubtful Sound region yield younger dates of 105.6–102.3 Ma with corresponding temperatures of 740–730 °C. Many samples record Cretaceous overdispersed dates with 5–10 m.y. ranges. Core-rim traverses and grain maps show complex chemical and temporal variations that cannot easily be attributed to thermally activated volume diffusion or simple core-rim crystallization. We interpret these Cretaceous titanites not as cooling ages, but rather as recording protracted growth and/or crystallization or recrystallization in response to fluid flow, deformation, and/or metamorphic reactions during the transition from garnet granulite to upper amphibolite facies metamorphism. We propose a thermotectonic model that integrates our results with structural observations. Our data reveal a clear tectonic break at 108–106 Ma that marks a change in processes deep within the arc. Prior to this break, arc construction processes dominated and involved (1) emplacement of mafic to intermediate magmas of the Malaspina and Misty plutons from 118 to 115 Ma, (2) contractional deformation at the roof of the Misty pluton in the Caswell Sound fold-thrust belt from 117 to 113 Ma, and (3) eclogite to garnet granulite facies metamorphism and partial melting over >8 m.y. from 116 to 108 Ma. These processes were accompanied by complex patterns of lower crustal flow involving both horizontal and vertical displacements. After this interval, extensional orogenic collapse initiated along upper amphibolite facies shear zones in the Doubtful Sound shear zone at 108–106 Ma. Zircon and titanite growth and/or crystallization or recrystallization at this time clearly link upper amphibolite facies metamorphism to mylonitic fabrics in shear zones. Our observations are significant in that they reveal the persistence of a hot and weak lower crust for ≥15 m.y. following arc magmatism in central Fiordland. We propose that the existence of a thermally weakened lower crust within the Median Batholith was a key factor in controlling the transition from crustal thickening to crustal thinning and extensional orogenic collapse of the Zealandia Cordillera.

A method for depth profiling using single-shot laser-ablation split stream (SS-LASS) ICP-MS is developed to simultaneously measure U-Pb age and trace-element concentrations in titanite. Simple semi-infinite, 1-D half-space diffusion models were applied to near-rim, trace-element zoned domains in titanite to distinguish between cooling and (re)crystallization ages and investigate the potential for preservation of thermally mediated diffusive loss profiles. These data illustrate the need to measure multiple trace elements with varying diffusivities to interpret a mineral's thermal history resulting from the non-unique nature of 1-D diffusion models where both temperature and time are unknown. A case study of titanites from two Pamir Plateau gneiss domes indicates they underwent ≥ 25 Myr of amphibolite and granulite facies metamorphism yet did not experience significant volume diffusion modification following (re)crystallization. The interpretation of prolonged (re)crystallization rather than diffusion allows for high-resolution, near-rim temperature-time histories to be extracted using U-Pb dates and Zr4 + apparent temperatures by SS-LASS.

To interpret seismic images, rock seismic velocities need to be calculated at elevated pressure and temperature for arbitrary compositions. This technical report describes an algorithm, software, and data to make such calculations from the physical properties of minerals. It updates a previous compilation and Excel® spreadsheet and includes new MATLAB® tools for the calculations. The database of 60 mineral end-members includes all parameters needed to estimate density and elastic moduli for many crustal and mantle rocks at conditions relevant to the upper few hundreds of kilometers of Earth. The behavior of α and β quartz is treated as a special case, owing to its unusual Poisson's ratio and thermal expansion that vary rapidly near the α-β transition. The MATLAB tools allow integration of these calculations into a variety of modeling and data analysis projects.

Internal heating can cause melting, metamorphism, and crustal weakening in convergent orogens. This study evaluates the role of radiogenic heat production (RHP) in a Neoproterozoic ultrahigh-temperature metamorphic (UHTM) terrane exposed in southern Madagascar. Monazite and zircon geochronology indicates that the Paleoproterozoic Androyen and Anosyen domains (i) collided with the oceanic Vohibory Arc at ~630 Ma, (ii) became incorporated into the Gondwanan collisional orogen by ~580 Ma, and (iii) were exhumed during crustal thinning at 525–510 Ma. Ti-in-quartz and Zr-in-rutile thermometry reveals that UHTM occurred over >20,000 km^2, mostly within the Anosyen domain. Assuming that U, Th, and K contents of samples from the field area are representative of the middle to lower crust during orogenesis, RHP was high enough—locally >5 μW/m^3—to cause regional UHTM in

Following the c. 50 Ma India–Kohistan arc–Asia collision, crustal thickening uplifted the Himalaya (Indian Plate), and the Karakoram, Pamir and Tibetan Plateau (Asian Plate). Whereas surface geology of Tibet shows limited Cenozoic metamorphism and deformation, and only localized crustal melting, the Karakoram–Pamir show regional sillimanite- and kyanite-grade metamorphism, and crustal melting resulting in major granitic intrusions (Baltoro granites). U/Th–Pb dating shows that metamorphism along the Hunza Karakoram peaked at c. 83–62 and 44 Ma with intrusion of the Hunza dykes at 52–50 Ma and 35 ± 1.0 Ma, and along the Baltoro Karakoram peaked at c. 28–22 Ma, but continued until 5.4–3.5 Ma (Dassu dome). Widespread crustal melting along the Baltoro Batholith spanned 26.4–13 Ma. A series of thrust sheets and gneiss domes (metamorphic core complexes) record crustal thickening and regional metamorphism in the central and south Pamir from 37 to 20 Ma. At 20 Ma, break-off of the Indian slab caused large-scale exhumation of amphibolite-facies crust from depths of 30–55 km, and caused crustal thickening to jump to the fold-and-thrust belt at the northern edge of the Pamir. Crustal thickening, high-grade metamorphism and melting are certainly continuing at depth today in the India–Asia collision zone.

Single-shot laser ablation split stream (SS-LASS) petrochronology offers spatial resolution of

Combining high-spatial resolution and high-precision geochronology and geochemistry of zircon provides constraints on the timing and duration of ultrahigh-pressure (UHP) metamorphism resulting from the collision of Baltica–Avalonia and Laurentia during the Scandian orogeny in the Western Gneiss Region of Norway. Zircons were extracted from a layered eclogite in the Saltaneset region (southern UHP domain) and from an eclogite in the Ulsteinvik region (central UHP domain). Zircons were first analyzed for U–Pb and trace element compositions by laser ablation split-stream (LASS) inductively coupled plasma mass spectrometry (ICP-MS), followed by analysis of those same zircons that yielded Scandian dates by integrated U–Pb isotope dilution–thermal ionization mass spectrometry and Trace Element Analysis (TIMS–TEA). LASS results from a garnet–quartz layer within the Saltaneset eclogite give Scandian dates of ca. 413–397 Ma, with subsequent ID–TIMS analyses ranging from 408.9 ± 0.4 Ma to 401.4 ± 0.2 Ma (2σ). An omphacite-rich layer from the same eclogite yields LASS dates of ca. 414–398 Ma and a single ID–TIMS date of 396.7 ± 1.4 Ma. In comparison, the Ulsteinvik eclogite LASS results give dates spanning ca. 413–397 Ma, whereas ID–TIMS analyses range from 409.6 ± 0.6 Ma to 401.3 ± 0.4 Ma. ID–TIMS zircon data from the eclogites reveals two age populations: 1) ca. 409–407 Ma and 2) ca. 402 Ma. Both in situ and solution trace element data show a distinct pattern for Scandian zircons, with strongly-depleted HREE and weakly-negative Eu anomalies (Eu/Eu*), whereas inherited zircon REE patterns are distinguished by steep HREE slopes and marked negative Eu/Eu*. When coupled with partition coefficients calculated for zircon and garnet, these REE patterns indicate that zircon (re)crystallized during eclogite-facies metamorphism at ca. 409–407 Ma and ca. 402 Ma at two widely separated UHP localities.

Chemical Geology 409 (2015) 28–41 Contents lists available at ScienceDirect Chemical Geology journal homepage: www.elsevier.com/locate/chemgeo Monazite response to ultrahigh-pressure subduction from U–Pb dating by laser ablation split stream Bradley R. Hacker a, ⁎ , Andrew R.C. Kylander-Clark a , Robert Holder a , Torgeir B. Andersen b , Emily M. Peterman c , Emily O. Walsh d , Jonathan K. Munnikhuis e a Earth Science, University California, Santa Barbara, CA 93106-9630, USA University of Oslo, Physics of Geological Processes, PO Box 1048, Blindern, 0316 Oslo, Norway Earth and Oceanographic Science, Bowdoin College, 6800 College Station, Brunswick, ME 04011, USA d Geology, Cornell College, Mount Vernon, IA 52314, USA e Geological Sciences, University of North Carolina, Chapel Hill, Campus Box #3315 Chapel Hill, NC 27599-3315, USA b c a r t i c l e i n f o Article history: Received 5 February 2015 Received in revised form 6 May 2015 Accepted 11 May 2015 Available online 22 May 2015 Editor: K. Mezger Keywords: Monazite Laser ablation Split stream Petrochronology a b s t r a c t To assess the response of monazite during subduction of continental crust to mantle depths, U–Pb isotopic ratios and elemental abundances were measured simultaneously by laser-ablation split-stream inductively-coupled plasma mass spectrometry (LASS) in rocks from the ultrahigh-pressure Western Gneiss Region of the Scandinavian Caledonides. Nearly seventy different samples of quartzofeldspathic basement and overlying metasedimentary rocks were studied. Pre-subduction monazite (chiefly 1.6 Ga and 1.0 Ga) is preserved locally in the structurally lowest, basement rocks because earlier, Precambrian tectonism produced coarse-grained, high-grade rocks that were resistant to further recrystallization in spite of syn-subduction temperatures and pressures of 650–800 °C and 2–3.5 GPa. A few of the monazite in the metasedimentary rocks atop the basement preserve syn-subduction U–Pb dates, but the majority continued to recrystallize during post-subduction exhumation and record a general westward decrease in age related to westward-progressing exhumation. The absence of Precambrian monazite in the metasedimentary rock atop the basement suggests that sedimentation postdated the 1.0–0.9 Ga high-grade metamorphism and was late Proterozoic to early Paleozoic. © 2015 Elsevier B.V. All rights reserved. 1. Introduction The development of techniques to analyze monazite U/Th–Pb dates in situ by electron probe [Suzuki et al., 1991], SIMS [Harrison et al., 1995], and LA-ICP-MS [Kosler et al., 2001] has spurred renewed interest in the petrogenesis of monazite, including understanding neocrystal- lization [Kingsbury et al., 1993], recrystallization [Finger, 1998], compo- sition [Franz et al., 1996], and thermometry [Gratz and Heinrich, 1997; Pyle et al., 2001]. Advances in these complementary tracks of analytical methods and petrologic interpretation have enabled increasing use of monazite as a petrochronometer—i.e., the interpretation of isotopic dates in the context of elemental or complementary isotopic informa- tion collected from the same mineral. One might infer from recent models [Kelsey, 2008] [Yakymchuk and Brown, 2014] that monazite is routinely recrystallized during high- temperature orogenesis and that its U–Pb age records only the final, cooling stage. Here we apply LASS (laser-ablation split-stream inductively-coupled plasma mass spectrometry) [Kylander-Clark et al., 2013] to monazite from the ultrahigh-pressure (UHP) Western Gneiss ⁎ Corresponding author. Tel.: +1 805 893 7952. E-mail address: hacker@geol.ucsb.edu (B.R. Hacker). http://dx.doi.org/10.1016/j.chemgeo.2015.05.008 0009-2541/© 2015 Elsevier B.V. All rights reserved. Region (WGR). Our objective in using in situ, combined U–Pb and trace-element analysis of monazite is to understand the broad-scale metamorphic response of monazite and its host rocks during subduc- tion to, and exhumation from, UHP conditions. The premise behind collecting trace-element and isotopic data simultaneously is that the trace elements provide insight into the petrological context of the isoto- pic dates: depressed heavy rare earth element concentrations may indi- cate that an interpreted U–Pb date coincides with the stability of garnet [Rubatto, 2002], elevated Eu and Sr may indicate HP recrystallization and plagioclase instability [Finger and Krenn, 2007; Holder et al., 2015], and Th and U help understand fluid availability [Hoskin and Schaltegger, 2003] and its potential role in monazite recrystallization [Seydoux-Guillaume et al., 2002]. We show that some monazite sur- vived amphibolite–granulite-facies metamorphism, partial melting, and subduction with its original age intact. 2. Geologic setting The Western Gneiss Region (WGR) of Norway contains one of Earth's largest ultrahigh-pressure terranes (Fig. 1). Like most well-exposed and extensively investigated UHP terranes, the WGR is dominated by quartzofeldspathic gneiss; eclogite and (U)HP