U-Pb zircon and monazite geochronology

The Ossa-Morena zone in SW Iberia represents a section of the northern margin of West Gondwana that formed part of a Cordilleran-type orogenic system during the Neoproterozoic (Cadomian orogeny). The crustal section in this zone preserves the record of rifting that led to the opening of the Rheic Ocean in the early Paleozoic and the collision of Gondwana and Laurussia in the late Paleozoic (Variscan orogeny). We present U-Pb zircon data from three alkaline to peralkaline syenites that intruded Neoproterozoic and Cambrian strata and give crystallization ages ranging between ca. 490 Ma and 470 Ma. Lu/Hf isotopic data from these zircons give positive initial εHf values (0 ≤ εHf(t) ≤ +11.5) that approach the model values for the depleted mantle at the time of crystallization. This suggests that a significant proportion of the magma was derived from the mantle, with limited mixing/assimilation with crustal-derived melts. Alkaline/peralkaline magmatic suites of similar age and chemical composition intruded other sections of the northern margin of West Gondwana and along the boundaries of the continental blocks that today make up Iberia. These blocks are further characterized by the presence of high-pressure metamorphic belts that formed during accretion and subsequent collision of peri-Gondwanan domains against Laurussia during the Devonian and Carboniferous (Variscan orogeny). Our U-Pb and Lu-Hf data set indicates that during the Cambrian−Ordovician transition, lithosphere extension reached a stage of narrow intracontinental rifting, where deeply sourced magmas, probably coming from the lower crust and/or the upper mantle, intruded continental upper crust across various sections of previously stretched crust. We propose that necking of the Gondwana lithosphere into several continental microblocks with fertile mantle beneath them compartmentalized extension (multiblock model), which favored the onset of early Paleozoic peralkaline and alkaline magmas. The boundaries of microblocks represent zones of inherited crustal weakness that were later reactivated during the late Paleozoic as major accretionary faults related to the amalgamation of Pangea during the Variscan orogeny. Our dynamic model provides an explanation for the unusual spatial relationship between peralkaline and alkaline igneous provinces (usually shallow in the crust) and the occurrence of high-pressure rocks. Our observations suggest that Cordilleran-type orogens subjected to extension after long-lived subduction can develop wide continental platforms that feature multiple continental blocks. In addition, the formation of sequenced high-pressure belts in collisional orogens can be explained as the ultimate consequence of multiple necking events within continental lithosphere during previous collapse of a Cordilleran-type orogen.

Quantitative constraints on the rates of tectonic processes underpin our understanding of the mechanisms that form mountains. In the Sikkim Himalaya, late structural doming has revealed time-transgressive evidence of metamorphism and thrusting that permit calculation of the minimum rate of
movement on a major ductile fault zone, the Main Central Thrust (MCT), by a novel methodology. U-Th-Pb monazite ages, compositions, and metamorphic pressure-temperature determinations from rocks directly beneath the MCT reveal that samples from ~50 km along the transport direction of the thrust experienced similar prograde, peak, and retrograde metamorphic conditions at different times. In the southern, frontal edge of the thrust zone, the rocks were buried to conditions of ~550°C and 0.8 GPa between ~21 and 18 Ma along the prograde path. Peak metamorphic conditions of ~650°C and 0.8–1.0 GPa were subsequently reached as this footwall material was underplated to the hanging wall at ~17–14 Ma. This same process occurred at analogous metamorphic conditions between ~18–16 Ma and 14.5–13 Ma in the midsection of the thrust zone and between ~13 Ma and 12 Ma in the northern, rear edge of the thrust zone. Northward younging muscovite 40Ar/39Ar ages are consistently ~4 Ma younger than the youngest monazite ages for equivalent samples. By combining the geochronological data with the >50 km minimum distance separating samples along the transport axis, a minimum average thrusting rate of 10 ± 3 mm yr
1 can be calculated. This provides a minimum constraint on the amount of Miocene India-Asia convergence that was accommodated along the MCT.

The Dongjiu–Milin shear zone located on the northwest flank of the eastern Himalayan syntaxis, southeast Tibet, separates Indian and Asian plate rocks. It is characterized by a thick sequence of highly strained and ductilely deformed mylonite (sensu stricto and sensu lato) bound between a pair of sub-parallel transtensive brittle normal faults. An integrated geochronological, petrographic, and thermobarometric study of three samples of pelitic and semi-pelitic mylonite (sensu lato) from the shear zone provides new insights into its thermal and structural evolution, and hence the tectonic processes operating in the region since the India–Asia collision. U–Th–Pb in-situ dating of monazite, garnet–ilmenite thermometry, and textural relationships show that mylonitization and peak-thermal staurolite-grade metamorphism occurred at 23.4 ± 0.7 Ma at a temperature of at least ~ 610 ± 30 °C. Cooling of these units through 420–500 °C occurred at c. 11–8 Ma, as constrained by the retrograde breakdown of allanite to form monazite. These data show that the cooling history of mylonite (sensu lato) in the shear zone and the tectonic evolution of the syntaxis region can be divided into two distinct stages. Slow cooling at a rate of 6–12 °C/Myr during c. 23–8 Ma is attributed to exhumation during ductile deformation and mylonitization caused by the ongoing India–Asia collision. This was followed by a period of significantly faster cooling at a minimum rate of 57 °C/Myr since c. 8 Ma, most likely associated with brittle normal faulting that facilitated the final stages of excavation to the surface. These new thermochronological data provide evidence of deep-seated exhumation-related cooling processes occurring in a convergent margin orogenic setting.

The tectonothermal history of the four major phases of the Sierra San Pedro Mártir pluton and surrounding metamorphic rocks of the Mesozoic Peninsular Ranges batholith of Baja California is presented on the basis of U/Pb, 40 Ar/ 39 Ar step-heating, and fission-track dating, in combination with Al-in-hornblende geobarometry. A previous model proposed up to 90° of east-side-up tilting of the pluton, exposing >20 km of crustal section to account for its crescent shape, asymmetrical zon-ing, internal structure, the eastward younging of K-Ar dates across the intrusion and eastward increase in the metamorphic grade of the country rocks, from greenschist to amphibolite facies. The U/Pb data suggest that the different phases of the pluton were emplaced sequentially from west to east between 97.0 +4 / –1 Ma and 93.8 +1 / –1 Ma. All except one of the 105 40 Ar/ 39 Ar age spectra have well-defined plateaus and are interpreted as cooling ages. Samples from the pluton give hornblende and biotite 40 Ar/ 39 Ar plateau dates and apatite fission-track dates that young from west to east; thus, hornblende dates decrease from 95 to 91 Ma, biotite dates decrease from 94 to 88 Ma, and apatite dates decrease from 72 to 57 Ma. Muscovite, biotite, and plagio-clase from the same rock sample collected at the easternmost phase of the pluton yield concordant 40 Ar/ 39 Ar dates of 88 Ma. The exposed part of the pluton underwent rapid cooling (≈40 °C/Ma) down to ≈250 °C in the first 10 m.y. after intrusion. Modeling of track-length distribution in apatite is consistent with monotonic slow cooling from ca. 80 Ma to the present. The data do not support a history that includes major tilting of the pluton. Eastward younging of 40 Ar/ 39 Ar and fission-track dates may be explained by ≈15° of east-side-up tilting of the pluton at or after 88 Ma about a north-south horizontal axis. Furthermore, the fission-track data suggest that part or all of this tilting may have taken place at or after 57 Ma, and therefore may be a consequence of regional-scale crustal extension associated with the opening of the Gulf of California in Neogene time. Such tilting is in agreement with the Al-in-hornblende geobarometry for the hornblende-biotite intrusive phase that yields pressures of 5.2 ± 0.6 kbar. An ≈15° northeast-side-up tilt of the crustal block containing this pluton would explain the apparent paleomagnetic inclination discrepancies with cratonic North America and militates against large-scale northerly transport of Baja California.

Despite ~50 Myr of continuous continent-continent collision, contractional structures in the Himalayan-Tibetan orogen are today limited to the northern and southern margins of the system, while extension dominates much of the interior. On the Tibetan Plateau, Cenozoic E-W extension has been accommodated by strike-slip faults and extensional grabens, while N-S extension at the Tibet-to-Himalaya transition has been accommodated by the South Tibetan fault system (STFS). The genetic relationship between N-S and E-W extension is disputed, although age constraints indicate temporal overlap of at least 7 Myr. In NW Bhutan the two intersect where the STFS basal detachment is cut by the Yadong cross structure (YCS), an extensional half graben that provides a rare opportunity to constrain relative timings. We report U-Pb zircon dates from four STFS footwall leucogranites consistent with episodic magmatism during the middle-late Miocene and in situ U(-Th)-Pb monazite and xenotime dates from three metasedimentary rocks ranging from late Oligocene to middle Miocene. We suggest that amphibolite facies footwall metamorphism was ongoing at the time the basal STFS detachment initiated as a ductile structure in the middle-late Miocene. Late-stage granitic intrusions may reflect footwall melting during extensional exhumation along the STFS, but post-metamorphic and post-intrusion fabrics suggest that most displacement occurred after emplacement of the youngest granites. Some of the oldest YCS-related fabrics are found in a deformed 14 Ma leucogranite, implying middle Miocene ductile deformation. This observation, along with evidence for subsequent brittle YCS deformation, suggests that N-S and E-W extensional structures in the area had protracted and overlapping deformation histories.

The H.U. Sverdrupfjella is part of the high-grade Maud Belt in Dronning Maud Land (East Antarctica), which was located in a central position of the Gondwana supercontinent. Here we study high-pressure granulites from the eastern H.U. Sverdrupfjella and present a detailed reconstruction of the P–T–t history based on a combination of Zr-in-rutile and Ti-in-zircon thermometry, zircon U–Pb dating, monazite chemical dating, garnet diffusion modelling and petrological modelling. Peak metamorphic conditions of $930 C and 1Á45 GPa persisted for less than 6 Myr and were attained at 570 6 7 Ma, embedded in a well-documented clockwise loading, heating, and decom-pression path. The rocks had already been rapidly exhumed to a crustal depth of $30 km at 556 6 7 Ma. In addition to the very short-lived ultrahigh-temperature peak, zircon preserves evidence for protracted granulite-facies conditions with temperatures above 800 C from as early as c. 590 Ma, persisting for c. 40 Myr. Constraints on prograde metamorphism are recorded in zircon and in rutile inclusions in garnet. Zr-in-rutile thermometry using rutile included in different generations of garnet is used to reconstruct the prograde P–T path, documenting burial followed by heating to ultrahigh temperatures at peak pressures. Complementary Ti zonation in prograde cores of zircon grains documents and dates heating and peak temperatures, whereas younger zircon rims show lower Ti-in-zircon temperatures and date the retrograde stages of metamorphism. Our results provide the first evidence for Neoproterozoic high-pressure granulite-facies metamorphism and ultrahigh-temperature conditions for this region. The clockwise loading–heating path and the peak P–T conditions strongly indicate that the rocks preserved in Dronning Maud Land were part of the lower plate during a continent–continent collision event related to Gondwana assembly at c. 570 Ma. The metamorphic evolution determined in this study and the correlation with similar P–T evolutions documented in adjacent terranes favour the continuation of the c. 580–560 Ma Mozambique Belt into Dronning Maud Land. Furthermore, the striking contemporaneity of the metamorphism in the different parts of central Gondwana suggests that the Coats Land Block was part of greater India prior to this collision.

A total of 4344 magmatic U-Pb ages in the range 2300 to 800 Ma have been compiled from the Great Proterozoic Accretionary Orogen along the margin of the Columbia / Nuna supercontinent and from the subsequent Grenvillian collisional orogens forming the core of Rodinia. The age data are derived from Laurentia (North America and Greenland, n = 1212), Baltica (NE Europe, n = 1922), Amazonia (central South America, n = 625), Kalahari (southern Africa and Dronning Maud Land in East Antarctica, n = 386), and western Australia (n = 199). Laurentia, Baltica, and Amazonia (and possibly other cratons) most likely formed a ca. 10 000-km-long external active continental margin of Columbia from its assembly at ca. 1800 Ma until its dispersal at ca. 1260 Ma, after which all cratons studied were involved in the Rodinia-forming Grenvillian orogeny. However, the magmatic record is not smooth and even but highly irregular, with marked peaks and troughs, both for individual cratons and the combined data set. Magmatic peaks typically range in duration from a few tens of million years up to around hundred million years, with intervening troughs of comparable length. Some magmatic peaks are observed on multiple cratons, either by coincidence or because of paleogeographic proximity and common tectonic setting, while others are not. The best overall correlation, 0.617, is observed between Baltica and Amazonia, consistent with (but not definitive proof of) their being close neighbours in a SAMBA-like configuration at least in Columbia, and perhaps having shared the same peri-Columbian subduction system for a considerable time. Correlation factors between Laurentia and Baltica, or Laurentia and Amazonia, are below 0.14. Comparison between the Grenville Province in northeastern Laurentia and the Sveconorwegian Province in southwestern Fennoscandia (Baltica) shows some striking similarities, especially in the Mesoproterozoic, but also exhibits differences in the timing of events, especially during the final Grenville-Sveconorwegian collision, when the Sveconorwegian evolution seems to lag behind by some tens of million years. Between the other cratons, the evolution before and during the final Grenvillian collision is also largely diachronous. After 900 Ma, magmatic activity had ceased in all areas investigated, attesting to the position of most of them within the stable interior of Rodinia.

The South Nepisiguit River Plutonic Suite consists of various phases of felsic to mafic plutonic rocks of early Devonian age in northern New Brunswick. The felsic portion of this intrusive suite includes a large pluton of homogeneous, peraluminous biotite granite (the Mount Elizabeth Granite), which is flanked on its western side by the alkaline Mount LaTour Granite. New in situ and mineral separate U-Pb Laser Ablation-Inductively Coupled Plasma Mass Spectrometry analyses of monazite and zircon grains from both sides of the suite define a crystallization age of 417.3 ± 0.96 Ma for the Mount Elizabeth Granite and 417.7 ± 4.4 Ma for the Mount LaTour Granite. The new data confirm previous geochronological work and indicate a close temporal relationship between these felsic phases of this plutonic suite.

The zircon intergrown with xenotime in pegmatite from Piława Górna (the Góry Sowie Block) was studied with respect to complex texture, and geochronological record in zircon. The zircon (ca. 2000 μm in size) demonstrates ca. 1100 μm wide, patchy zoned core and porous rim. The porosity also developed in the intergrown xenotime and mimics continuity along the zircon rim. The zircon core reflects homogeneous patches or patches with numerous up to several microns-sized inclusions of U, Th-rich phases and up to ca. 50 μm-sized inclusions of uraninite. The micro-Raman analyzes revealed increasing broadening of the ν 3 (SiO 4) band at ca. 1000 cm-1 that suggests increasing metamictization from little disordered outermost rim towards amorphous core. The EPMA results indicate high variations of UO 2 (0.28-0.95 wt.% in rim; 2.84-9.62 wt.% in core) and ThO 2 (<0.22 wt.% in rim; 0.33-0.84 wt.% in core) in the zircon. All these results indicate that the zircon domains were affected by a diffusion-reaction process in the core and coupled dissolution-reprecipitation process in the rim. The U-Pb LA-ICP-MS dating of zircon yielded discordant data with upper intercept age of 2086±35 Ma and lower intercept age of 373±18 Ma (n=65; MSWD=5.2). Concerning the youngest ages characterized by <10% discordance, three ages (0.1-2.9% discordant) in the rim yielded concordia age of 382±14 Ma (MSWD=1.4) interpreted as reflecting the timing of complete re-equilibration of the isotopic system due to fluid-mediated coupled dissolution-reprecipitation within the rim. The high radioactivity related to high U-Th concentration resulted in further Pb-loss in the core. The geochronological results strongly contrast with field observations. Although anatectic pegmatite dikes generally discordantly cut the tectonized migmatites and amphibolites related to ca. 380 Ma HT/MT-MP metamorphic event, the morphology, textural features and high-Th,U composition of the zircon suggest its origin related to crystallization in pegmatite at 2086±35 Ma, whereas 373±18 Ma age is related to re-equilibration of the whole system (i.e. Pb-loss and annealing).

Chemical U– Th – Pb dating of monazite from 12 schists throughout western and central Tasmania define a peak metamorphic age of ca 510 Ma. This age is very close to the age of arc – continent collision and ophiolite emplacement, implying very rapid uplift and cooling. To the south, along the western margin of the South Tasman Rise, metamorphism occurred later at 495 Ma, which correlates with a late stage of the Ross Orogeny, Antarctica. The Tyennan Orogeny in Tasmania has a three-stage history similar in age to the Delamerian Orogeny in South Australia. However, the Tyennan Orogeny only produced metamorphic rocks during the early stage associated with ophiolite obduction at 515 – 505 Ma. The intense compressional event recognised in the Delamerian and Ross Orogeny at 500 – 495 Ma is correlated with a mild basin inversion in Tasmania, and no metamorphism on mainland Tasmania has been recognised associated with this event. The westernmargin of the South Tasman Rise is a fragment of the Ross Orogen and does not correlate directly with Tasmania.

The basal units of the allochthonous complexes of NW Iberia are used to examine the Lower Paleozoic geodynamic evolution of the northern Gondwana margin. These units represent the most external continental margin and the sequence of major magmatic events that affected them has been dated. Isotopic dating and field data highlight the existence of two magmatic pulses, dated at 489 ± 4 Ma (granodiorites) and 474 ± 3 Ma (alkali-granites), and a slightly younger alkaline/peralkaline pulse, dated at ca. 470–475 Ma (alkaline and peralkaline granites). Their framing into the regional background has allowed us to explore the major lithosphere-scale processes developed at the Gondwana periphery at that time, as well as to conceive a consistent model for the opening of the Rheic Ocean that reconciles the timing of sea opening and back-arc extension with the timing of intracontinental rifting. The sequence of events is framed in a Cambrian and Ordovician peri-Gondwanan subduction setting where we also explore how subduction may be linked to coeval intraplate magmatism far inboard of the arc-trench. This contribution discusses how such a scenario can be traced in basement areas through a modern analog perspective.

A total of 4344 magmatic U-Pb ages in the range 2300 to 800 Ma have been compiled from the Great Proterozoic Accretionary Orogen along the margin of the Columbia / Nuna supercontinent and from the subsequent Grenvillian collisional orogens forming the core of Rodinia. The age data are derived from Laurentia (North America and Greenland, n = 1212), Baltica (NE Europe, n = 1922), Amazonia (central South America, n = 625), Kalahari (southern Africa and Dronning Maud Land in East Antarctica, n = 386), and western Australia (n = 199). Laurentia, Baltica, and Amazonia (and possibly other cratons) most likely formed a ca. 10 000-km-long external active continental margin of Columbia from its assembly at ca. 1800 Ma until its dispersal at ca. 1260 Ma, after which all cratons studied were involved in the Rodinia-forming Grenvillian orogeny. However, the magmatic record is not smooth and even but highly irregular, with marked peaks and troughs, both for individual cratons and the combined data set. Magmatic peaks typically range in duration from a few tens of million years up to around hundred million years, with intervening troughs of comparable length. Some magmatic peaks are observed on multiple cratons, either by coincidence or because of paleogeographic proximity and common tectonic setting, while others are not. The best overall correlation, 0.617, is observed between Baltica and Amazonia, consistent with (but not definitive proof of) their being close neighbours in a SAMBA-like configuration at least in Columbia, and perhaps having shared the same peri-Columbian subduction system for a considerable time. Correlation factors between Laurentia and Baltica, or Laurentia and Amazonia, are below 0.14. Comparison between the Grenville Province in northeastern Laurentia and the Sveconorwegian Province in southwestern Fennoscandia (Baltica) shows some striking similarities, especially in the Mesoproterozoic, but also exhibits differences in the timing of events, especially during the final Grenville-Sveconorwegian collision, when the Sveconorwegian evolution seems to lag behind by some tens of million years. Between the other cratons, the evolution before and during the final Grenvillian collision is also largely diachronous. After 900 Ma, magmatic activity had ceased in all areas investigated, attesting to the position of most of them within the stable interior of Rodinia.

The Bafq mining district is in the Early Cambrian Kashmar-Kerman volcano-plutonic arc in Central Iran and hosts important ‘Kiruna-type’ magnetite-apatite deposits. The hydrothermal magnetite-apatite mineralization occurs mostly as massive orebodies, metasomatic replacements, veins and stockworks. Apatite (low-Sr fluorapatite containing small amounts of hydroxyl) has undergone a partial hydrothermal overprint which involved leaching of Na, Cl and REE. The REE were remobilized into monazite (and minor allanite, parisite and xenotime) which nucleated as inclusions within apatite or as individual crystals. The monazites have very small ThO2 contents (usually &lt;1 wt.%), but they occasionally show an inner core of high-Th monazite, with low-Th overgrowth rims. The chemical Th-U-total Pb dating of the high-Th monazites by electron microprobe analysis yields an isochron age of 515±21 Ma (initial PbO intercept = 68 ppm), or 529±21 Ma (forced initial PbO = 0), which is contemporaneous with ...

The Dongjiu–Milin shear zone located on the northwest flank of the eastern Himalayan syntaxis, southeast Tibet, separates Indian and Asian plate rocks. It is characterized by a thick sequence of highly strained and ductilely deformed mylonite (sensu stricto and sensu lato) bound between a pair of sub-parallel transtensive brittle normal faults. An integrated geochronological, petrographic, and thermobarometric study of three samples of pelitic and semi-pelitic mylonite (sensu lato) from the shear zone provides new insights into its thermal and structural evolution, and hence the tectonic processes operating in the region since the India–Asia collision. U–Th–Pb in-situ dating of monazite, garnet–ilmenite thermometry, and textural relationships show that mylonitization and peak-thermal staurolite-grade metamorphism occurred at 23.4 ± 0.7 Ma at a temperature of at least ~ 610 ± 30 °C. Cooling of these units through 420–500 °C occurred at c. 11–8 Ma, as constrained by the retrograde bre...

ABSTRACT The Dongjiu–Milin shear zone located on the northwest flank of the eastern Himalayan syntaxis, southeast Tibet, separates Indian and Asian plate rocks. It is characterized by a thick sequence of highly strained and ductilely deformed mylonite (sensu stricto and sensu lato) bound between a pair of sub-parallel transtensive brittle normal faults. An integrated geochronological, petrographic, and thermobarometric study of three samples of pelitic and semi-pelitic mylonite (sensu lato) from the shear zone provides new insights into its thermal and structural evolution, and hence the tectonic processes operating in the region since the India–Asia collision. U–Th–Pb in-situ dating of monazite, garnet–ilmenite thermometry, and textural relationships show that mylonitization and peak-thermal staurolite-grade metamorphism occurred at 23.4 ± 0.7 Ma at a temperature of at least ~ 610 ± 30 °C. Cooling of these units through 420–500 °C occurred at c. 11–8 Ma, as constrained by the retrograde breakdown of allanite to form monazite. These data show that the cooling history of mylonite (sensu lato) in the shear zone and the tectonic evolution of the syntaxis region can be divided into two distinct stages. Slow cooling at a rate of 6–12 °C/Myr during c. 23–8 Ma is attributed to exhumation during ductile deformation and mylonitization caused by the ongoing India–Asia collision. This was followed by a period of significantly faster cooling at a minimum rate of 57 °C/Myr since c. 8 Ma, most likely associated with brittle normal faulting that facilitated the final stages of excavation to the surface. These new thermochronological data provide evidence of deep-seated exhumation-related cooling processes occurring in a convergent margin orogenic setting.

ABSTRACT The Dongjiu–Milin shear zone located on the northwest flank of the eastern Himalayan syntaxis, southeast Tibet, separates Indian and Asian plate rocks. It is characterized by a thick sequence of highly strained and ductilely deformed mylonite (sensu stricto and sensu lato) bound between a pair of sub-parallel transtensive brittle normal faults. An integrated geochronological, petrographic, and thermobarometric study of three samples of pelitic and semi-pelitic mylonite (sensu lato) from the shear zone provides new insights into its thermal and structural evolution, and hence the tectonic processes operating in the region since the India–Asia collision. U–Th–Pb in-situ dating of monazite, garnet–ilmenite thermometry, and textural relationships show that mylonitization and peak-thermal staurolite-grade metamorphism occurred at 23.4 ± 0.7 Ma at a temperature of at least ~ 610 ± 30 °C. Cooling of these units through 420–500 °C occurred at c. 11–8 Ma, as constrained by the retrograde breakdown of allanite to form monazite. These data show that the cooling history of mylonite (sensu lato) in the shear zone and the tectonic evolution of the syntaxis region can be divided into two distinct stages. Slow cooling at a rate of 6–12 °C/Myr during c. 23–8 Ma is attributed to exhumation during ductile deformation and mylonitization caused by the ongoing India–Asia collision. This was followed by a period of significantly faster cooling at a minimum rate of 57 °C/Myr since c. 8 Ma, most likely associated with brittle normal faulting that facilitated the final stages of excavation to the surface. These new thermochronological data provide evidence of deep-seated exhumation-related cooling processes occurring in a convergent margin orogenic setting.

ABSTRACT A combined geochronological and petrological study of pelitic migmatites from the northwestern flank of the eastern Himalayan syntaxis has constrained the timing and P–T conditions of two high-grade metamorphic events that affected the south Lhasa block (Asian margin) and provides new insight into the tectonothermal evolution of the India–Asia collision. U(–Th)–Pb dating of in situ monazite shows that upper amphibolite-facies sillimanite-grade metamorphism and consequent partial melting occurred between c. 71 and 50 Ma at P–T conditions above 6.3 ± 1.2 kbar and 750 ± 30 °C. Further partial melting at upper amphibolite-facies kyanite-grade conditions occurred between c. 44 and 33 Ma at minimum P–T conditions of 10.4 ± 1.0 kbar and 698 ± 20 °C. These data are interpreted to record a south Lhasa block mid-crustal sillimanite-grade melting event in the Late Cretaceous to Early Eocene related to regional heat advection caused by coeval and prolonged emplacement of Gangdese batholith units. This was followed by a higher pressure and lower temperature kyanite-grade melting event during the Middle Eocene to Early Oligocene associated with deformation and crustal thickening in the south Lhasa block, coeval with kyanite-grade metamorphism along the Himalaya, as a result of the on-going India–Asia collision. These partially-melted crustal lithologies offer potential sources (or otherwise analogs for sources) for the Miocene emplacement of adakitic intrusions previously documented in the eastern Himalayan syntaxis region.

Greenschist to amphibolite grade Haimanta metasediments of the NW Himalaya preserve much of the prograde metamorphic history of Eohimalayan crustal thickening, which has been erased by Oligo-/Miocene migmatization elsewhere in the Himalaya. Our zircon and monazite U/Th–Pb data unravel a multi-stage prograde metamorphic evolution. The earliest evidence of prograde Barrovian metamorphic monazite growth is ~ 41 Ma. Peak metamorphic conditions (~ 8-8.5 kbar, ~ 600-700 °C) were attained at 37–36 Ma and followed by a prolonged evolution at high temperatures with at least three distinct episodes of monazite growth, which may be related to formation of the northern Himalayan nappes (e.g., Shikar Beh nappe, Nyimaling nappe). Rapid exhumation of the crystalline started at ~ 26 Ma and resulted in cooling through the muscovite 40Ar/39Ar closure temperature by 21.8 Ma. Although a local continuation of the South Tibetan detachment is not unambiguously identified in central Himachal Pradesh extrus...

The age and timing of the Permian-Triassic mass extinction have been difficult to determine because zircon populations from the type sections are typically affected by pervasive lead loss and contamination by indistinguishable older xenocrysts. Zircons from nine ash beds within the Shangsi and Meishan sections (China), pretreated by annealing followed by partial attack with hydrofluoric acid, result in suites of consistent and concordant uranium/lead (U/Pb) ages, eliminating the effects of lead loss. The U/Pb age of the main pulse of the extinction is 252.6  0.2 million years, synchronous with the Siberian flood volcanism, and it occurred within the quoted uncertainty.

Mygatt-Moore Quarry in the Brushy Basin Member of the Upper Jurassic Morrison Formation is one of the more important large dinosaur quarries in the formation, yielding fossils of sauropod dinosaurs including Apatosaurus, Camarasaurus, and an indeterminate diplodocine (cf. Diplodocus or Barosaurus), the theropod dinosaurs Allosaurus and Ceratosaurus, and the ornithischians Othnielosaurus and Mymoorapelta. Fossil wood and carbonized plants are known from the deposit, and carbonized dinosaur skin has also been discovered.

We report here on a new U/Pb radiometric age for the Mygatt-Moore Quarry, from a smectitic mudstone collected at the quarry horizon. We isolated zircons using an ultrasonic separation technique and standard heavy liquid and magnetic methods, analyzed single zircons using a chemical abrasion method (CA-TIMS). The age determined for this dinosaur quarry, 152.18 ±0.29 Ma, places the quarry at the boundary between the Kimmeridgian and Tithonian stages of the Late Jurassic Period. This age can now be used to place the quarry into stratigraphic position with respect to other dated horizons in the Morrison Formation and other rock units worldwide. We will also use this age along with other ages to create a radiometrically based stratigraphic framework for the Morrison Formation.