Northward subduction of the leading edge of the Indian continental margin to depths greater than 100 km during the early Eocene resulted in high‐pressure (HP) quartz‐eclogite to ultrahigh‐pressure ...(UHP) coesite–eclogite metamorphism at Tso Morari, Ladakh Himalaya, India. Integrated pressure–temperature–time determinations within petrographically well‐constrained settings for zircon‐ and/or monazite‐bearing assemblages in mafic eclogite boudins and host aluminous gneisses at Tso Morari uniquely document segments of both the prograde burial and retrograde exhumation path for HP/UHP units in this portion of the western Himalaya. Poikiloblastic cores and inclusion‐poor rims of compositionally zoned garnet in mafic eclogite were utilized with entrapped inclusions and matrix minerals for thermobarometric calculations and isochemical phase diagram construction, the latter thermodynamic modelling performed with and without the consideration of cation fractionation into garnet during prograde metamorphism. Analysis of the garnet cores document (M1) conditions of 21.5 ± 1.5 kbar and 535 ± 15 °C during early garnet growth and re‐equilibration. Sensitive high resolution ion microprobe (SHRIMP) U–Pb analysis of zircon inclusions in garnet cores yields a maximum age determination of 58.0 ± 2.2 Ma for M1. Peak HP/UHP (M2) conditions are constrained at 25.5–27.5 kbar and 630–645 °C using the assemblage garnet rim–omphacite–rutile–phengite–lawsonite–talc–quartz (coesite), with mineral compositional data and regional considerations consistent with the upper P–T bracket. A SHRIMP U–Pb age determination of 50.8 ± 1.4 Ma for HP/UHP metamorphism is given by M2 zircons analysed in the eclogitic matrix and that are encased in the garnet rim. Two garnet‐bearing assemblages from the Puga gneiss (host to the mafic eclogites) were utilized to constrain the subsequent decompression path. A non‐fractionated isochemical phase diagram for the assemblage phengite–garnet–biotite–plagioclase–quartz–melt documents a restricted (M3) P–T stability field centred on 12.5 ± 0.5 kbar and 690 ± 25 °C. A second non‐fractionated isochemical phase diagram calculated for the lower pressure assemblage garnet–cordierite–sillimanite–biotite–plagioclase–quartz–melt (M4) documents a narrow P–T stability field ranging between 7–8.4 kbar and 705–755 °C, which is consistent with independent multiequilibria P–T determinations. Th–Pb SHRIMP dating of monazite cores surrounded by allanite rims is interpreted to constrain the timing of the M4 equilibration to 45.3 ± 1.1 Ma. Coherently linking metamorphic conditions with petrographically constrained ages at Tso Morari provides an integrated context within which previously published petrological or geochronological results can be evaluated. The new composite path is similar to those published for the Kaghan UHP locality in northern Pakistan, although the calculated 12‐mm a−1 rate of post‐pressure peak decompression at Tso Morari would appear less extreme.
The distribution of rheologically strong cratons, and their weakening by metamorphic hydration reactions, is of fundamental importance for understanding first‐order strength contrasts within the ...crust and the resulting controls on the tectonic evolution of the continents. In this study, the Douglas Harbor structural window within the Paleoproterozoic Trans‐Hudson orogen of Canada is used to study the hydration of the footwall Archean Superior craton basement by water released from the overlying Paleoproterozoic Cape Smith thrust‐fold belt. Phase equilibria modeling is applied to quantify the Archean and Paleoproterozoic metamorphic conditions, and to determine the effect of hydration on basement mineralogy. The amount of structurally bound water within the basement is calculated and shown to decrease as a function of distance below the basal décollement of the thrust‐fold belt. Applying a reactive fluid transport model to these results, the rate coefficient for fluid‐rock reaction is constrained to be 10−19 mol−1m3s−1, and the diffusivity of water through the grain boundary network to be 10−9 m2s−1 at the ambient metamorphic conditions of 570°C and 7.5 kbar. This newly documented rate of water diffusion is three orders of magnitude slower than thermal diffusion, implying that hydration by diffusion may be the rate‐limiting factor in the weakening of cratons, and therefore plays an important role in their geological persistence. This conclusion is consistent with field observations that Paleoproterozoic strain in the Douglas Harbor structural window is restricted to hydrated portions of the Archean Superior craton basement.
Plain Language Summary
The ancient cores of continents are highly durable and have persisted for billions of years, in part due to the “dry” mineral assemblages they contain. The distribution of ancient continental cores is important in controlling how mountain ranges form and evolve, and where earthquakes occur. Therefore, it is important to understand how continental cores can be altered by geological processes. In this study, we analyze how continental cores can be weakened through the addition of water during mountain‐building and the formation of “wet” minerals. We do this by analyzing a 2 billion‐year old mountain belt in Arctic Canada, where the transition from “wet” to “dry” rocks is now exposed at the surface due to uplift and erosion from ∼20 km depth. By analyzing samples from across the region, and calculating their water contents, we are able to model the distribution of water and place constraints on the rate of fluid movement, and of fluid‐rock reaction. Our results show that the rates are very slow, which contributes to the geological persistence of the dry continental cores, and influences the behavior of the mountain belts that are formed when continents collide.
Key Points
The Trans‐Hudson orogen in Arctic Canada is used to investigate the hydration and weakening of cratonic basement during mountain‐building
The fluid‐rock reaction rate coefficient is 10−19 mol−1 m3 s−1 and the diffusivity of water in the grain boundary network is 10−9 m2 s−1
The rate of water diffusion is slower than thermal diffusion, and rate‐limiting for the weakening of cratons in orogenic settings
Phase equilibria modelling of post‐peak metamorphic mineral assemblages in (ultra)high‐P mafic eclogite from the Tso Morari massif, Ladakh Himalaya, northwest India, has provided new insights into ...the potential behaviour and source of metamorphic fluid during exhumation, and constrained the P–T conditions of hydration. A series of P–M(H2O) pseudosections constructed in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O (NCKFMASHTO) system show that a number of petrographically distinct hydration episodes occurred during exhumation from peak P–T conditions (~640 °C, 27–28 kbar), resulting in the formation of abundant compositionally zoned amphibole and minor clinozoisite poikiloblasts at the expense of a peak assemblage dominated by garnet and omphacite. Initial hydration is interpreted to have occurred as a result of the destabilization of talc following isothermal decompression to ~23 kbar, which led to the formation of barroisite–winchite amphibole core domains. An episode of fluid infiltration from an external source at ~19 kbar, with or without syn‐decompressional cooling to ~560 °C, resulted in further barroisitic–winchitic amphibole growth, followed by the formation of clinozoisite poikiloblasts. Continued buoyancy‐driven exhumation to the base of the lower crust is constrained to have taken place with no additional fluid input. A final hydration event is characterized by the formation of magnesiohornblende rims on the barroisite–winchite cores, with the former interpreted to have formed during later prograde overprinting in the middle crust associated with the final stages of exhumation. Notably, the vast majority of externally sourced H2O, comprising just over half of the current bulk rock fluid content, was added during this later hydration event. In a middle crustal setting, this is interpreted as the result of devolatilization reactions occurring in migmatitic host orthogneiss and/or metasedimentary units, or following the crystallization of partial melt.
The Danba Structural Culmination is a tectonic window into the late Triassic to early Jurassic Songpan‐Garzê Fold Belt of eastern Tibet, which exposes an oblique section through a complete ...Barrovian‐type metamorphic sequence. Systematic analysis of a suite of metapelites from this locality has enabled a general study of Barrovian metamorphism, and provided new insights into the early thermotectonic history of the Tibetan plateau. The suite was used to create a detailed petrographic framework, from which four samples ranging from staurolite to sillimanite grade were selected for thermobarometry and geochronology. Pseudosection analysis was applied to calculate P–T path segments and determine peak conditions between staurolite grade at ∼5.2 kbar and 580 °C and sillimanite grade at ∼6.0 kbar and 670 °C. In situ U–Pb monazite geochronology reveals that staurolite‐grade conditions were reached at 191.5 ± 2.4 Ma, kyanite‐grade conditions were attained at 184.2 ± 1.5 Ma, and sillimanite‐grade conditions continued until 179.4 ± 1.6 Ma. Integration of the results has provided constraints on the evolution of metamorphism in the region, including a partial reconstruction of the regional metamorphic field gradient. Several key features of Barrovian metamorphism are documented, including nested P–T paths and a polychronic field gradient. In addition, several atypical features are noted, such as P–T path segments having similar slopes to the metamorphic field gradient, and Tmax and Pmax being reached simultaneously in some samples. These features are attributed to the effects of slow tectonic burial, which allows for thermal relaxation during compression. While nested, clockwise P–T–t loops provide a useful framework for Barrovian metamorphism, this study shows that the effects of slow burial can telescope this model in P–T space. Finally, the study demonstrates that eastern Tibet experienced a significant phase of crustal thickening during the Mesozoic, reinforcing the notion that the plateau may have a long history of uplift and growth.
•Trans-Hudson Orogen P-T-d history is constrained by in situ U–Pb monazite ages.•U–Pb zircon depth profiling resolves ultrathin metamorphic zircon rims.•Monazite and zircon ages chronicle a ∼100m.y. ...tectonothermal evolution.•The data enable evaluation of tectonic correlations in the North Atlantic region.
Understanding the growth and recrystallization of zircon and monazite during regional metamorphism is important for identifying the nature and timescales of tectonic processes in continental crust. We present extensive in situ U–Pb monazite geochronology and U–Pb zircon depth profiling analyses from the mid-crust exposed on southeastern Baffin Island, Canada, in the internal zone of the Paleoproterozoic Trans-Hudson Orogen. In situ LA-ICP-MS U–Pb monazite dating on ten metasedimentary rocks (totalling >300 analyses) from different paleodepths reveals a dominant ca. 1860–1820Ma monazite population that records regional mid-amphibolite- to granulite-facies metamorphism with coeval fabric development. Subordinate monazite with ages between ca. 1800–1750Ma are interpreted to have formed through dissolution-reprecipitation. The SIMS U–Pb depth-profiling technique allowed dating of ca. 1861–1833Ma zircon rims and less abundant ca. 1800–1764Ma zircon rims that would be unresolvable using conventional spot dating. Trace element depth profiling via LA-ICP-MS shows that zircon rims typically have lower Th/U values, higher Hf contents and different REE compositions than zircon interiors. Zircon rim compositions are highly variable, with no apparent link to U–Pb age, and are interpreted as the combined result of metamorphic zircon growth and dissolution-reprecipitation. Our data demonstrate that the timing of peak metamorphism is recorded by both monazite and zircon, and that they are both sensitive to U–Pb age resetting at lower temperatures during late-orogenic tectonism. Regional mid-amphibolite- to granulite-facies metamorphism and east-directed crustal shortening are bracketed between ca. 1860–1820Ma. These events are interpreted to result from the accretion of cratonic blocks in the internal zone of the Trans-Hudson Orogen. The integrated geochronological approach on regionally-distributed samples enabled us to document ca. 1800–1750Ma monazite domains and zircon rims, which we interpret to result from the terminal collision with the lower-plate Superior craton followed by late- to post-orogenic tectonothermal activity that continued for at least 70 Myr after thermal peak. This prolonged history is characteristic of the northeastern Trans-Hudson Orogen, and implies that orogenic collapse may not be a hallmark of all hot, thickened orogens.
An integrated field, petrological and geochronological study of the Basong Tso region of south‐eastern Tibet has constrained the timing and P–T conditions of north–south Lhasa terrane accretion and ...provides new insight into the tectonothermal evolution of the Tibetan plateau. Two distinct high‐grade metamorphic belts are recognized in the region: a southern belt (the Basong Tso complex) that consists of sheared schist and orthogneiss; and a northern belt (the Zhala complex) that comprises paragneiss and granite. Combined pseudosection modelling and U–Pb geochronology of monazite and zircon indicates that the Basong Tso complex records peak metamorphic conditions of 9 ± 0.5 kbar and 690 ± 25 °C at c. 204–201 Ma, whereas the Zhala complex experienced peak metamorphic conditions of 5.0 ± 1.0 kbar and 740 ± 40 °C at c. 198–192 Ma. Microstructural analysis suggests that the two belts share a common early prograde history, after which the Basong Tso complex attained peak conditions following rapid burial, and the Zhala complex approached peak conditions along an isobaric path. Overall it is inferred that the Basong Tso and Zhala complexes represent the lower and upper structural levels of an evolving orogen that underwent Barrovian‐type metamorphism following collision (M1), followed by Buchan‐style overprinting at higher structural levels due to heat advection by syn‐tectonic granites (M2). Mylonitization (sensu lato) of the Basong Tso complex and juxtaposition of the two units occurred after attainment of peak conditions. The dominance of Mesozoic regional metamorphism across most of the Tibetan plateau indicates that Cenozoic crustal thickening processes, where present, are only manifested at depth.
The Sumdo complex is a Permian–Triassic eclogitic metamorphic belt in south-east Tibet, which marks the location of a suture zone that separates the northern and southern Lhasa terranes. An ...integrated geochronological and petrological study of a mafic eclogite from the complex has constrained its tectonometamorphic history and provides a case study of zircon growth in eclogite as a product of prograde dissolution–precipitation. In situ U–Pb geochronology indicates that the eclogite contains a single population of zircon with a crystallisation age of 273.6±2.8Ma. The morphology and chemistry of the zircon grains are consistent with growth by dissolution–precipitation of protolith magmatic zircon. The presence of zircon grains as inclusions in the cores of peak phases indicates that zircon dissolution–precipitation occurred during prograde metamorphism, and calculated pressure and temperature conditions over which mineral inclusions in zircon are stable suggest that the zircon most likely precipitated at ~15.5–16.5kbar and 500–560°C. Subsequent peak metamorphism is calculated to have reached pressure–temperature conditions of 27±1kbar and 670±50°C. Previous studies, which have documented a range of peak metamorphic conditions from high- to ultrahigh-pressure at c. 266–230Ma, indicate that the Sumdo complex is a composite belt that experienced protracted eclogite exhumation. The results of this study are consistent with this interpretation, and extend the age range of high-pressure metamorphism in the complex to over 40Myr. Analysis of published pressure–temperature–time data indicates two systematic behaviours within this spread. First, peak metamorphic temperatures declined over time. Second, eclogite exhumation occurred in two discrete intervals: soon after formation, and during the demise of the subduction zone. The latter behaviour serves as a reminder that eclogite exhumation is the exception rather than the rule.
•We analyse a sample of mafic eclogite from the Sumdo complex of south-east Tibet.•The eclogite contains a single zircon population with an age of 273.6±2.8Ma.•Zircon growth occurred by prograde dissolution-precipitation at ~16kbar.•The eclogite experienced peak metamorphic conditions of 27±1kbar and 670±50°C.•Eclogite exhumation in complex occurred early and late in subduction zone lifecycle.
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±0.7Ma at a temperature of at least ~610±30°C. Cooling of these units through 420–500°C occurred at c. 11–8Ma, 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. 23–8Ma 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. 8Ma, 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.
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•Syn-shear staurolite-grade metamorphism (>610±30°C) at 23.4±0.7Ma•Cooling rates of 6–12°C/Myr during c. 23–8Ma and 57°C/Myr since c. 8Ma•Significant tectonic transition from ductile to brittle exhumation style at c. 8Ma
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