Peak metamorphic temperatures from cation diffusion zoning in garnet

Smit, Matthijs; Scherer, Erik E.; Mezger, Klaus

doi:10.1111/jmg.12024

Cited by 16 publications

(13 citation statements)

References 77 publications

(202 reference statements)

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“…Diffusion thermometry (after Smit et al, 2013a) provides peak temperatures (T peak ) from estimates of the length of diffusive zoning for Ca, Fe 2+ , and Mg in garnet, and the average cooling rate at high temperature. To constrain the latter, we performed combined Zr-in-rutile thermometry and rutile U-Pb thermochronology on two samples (SH-2, SH-3) from the Shakhdara dome ( Fig.…”

Section: Analytical Resultsmentioning

confidence: 99%

Early evolution of the Pamir deep crust from Lu-Hf and U-Pb geochronology and garnet thermometry

Smit

Ratschbacher

Kooijman

et al. 2014

Geology

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Determining early orogenic processes within the Pamir-Tibet orogen represents a critical step toward constructing a comprehensive model on the tectonic evolution of the region. Here we investigate the timing and cause of prograde metamorphism of Cenozoic metamorphic rocks from the Pamir plateau through Lu-Hf geochronology, U-Pb rutile thermochronology, and garnet thermometry. Regional prograde metamorphism and heating to 750-830 °C, as constrained by thermometry, occurred between 37 and 27 Ma. Prograde growth of garnet first occurred in the South Pamir and spread to the Central Pamir during the following 10 m.y. The early metamorphism is attributed to high mantle heat flow following the ca. 45 Ma break-off of the Indian slab south of the Pamir. Our investigation confirms a long-lived thermal history of the Pamir deep crust before the Miocene, and provides a causal link between break-off, enhanced mantle heat flow, and prograde heating of the subduction hanging wall. INTRODUCTIONThe Pamir-Tibet orogen is Earth's largest and highest plateau and a prime natural laboratory for investigating how plate dynamics, regional tectonics, and surface uplift and erosion interact. Refining models for this orogen, and collisional orogens in general, requires knowledge about how the crust thickens, and how its thermal and mechanical structure changes during and after collision. Tectonic processes occurring at depth particularly impact regional dynamics. However, investigating these processes in the Pamir-Tibet orogen is hindered by the scarcity of exposed Cenozoic metamorphic rocks and the general difficulty in reconstructing their prograde his-

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Section: Analytical Resultsmentioning

confidence: 99%

Early evolution of the Pamir deep crust from Lu-Hf and U-Pb geochronology and garnet thermometry

Smit

Ratschbacher

Kooijman

et al. 2014

Geology

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“…Peak metamorphic temperatures from this study are consistent with previous estimates for similar assemblages and suggest that the method proposed by Smit et al . () can provide meaningful results for low‐ T rocks. The combined mineral assemblages, peak metamorphic temperatures and ages from garnet and lawsonite within exotic blocks of the Franciscan Complex are consistent with previous observations of generally decreasing ages with apparent metamorphic grade for these rocks (e.g.…”

Section: Discussionmentioning

confidence: 99%

“…Metamorphic temperatures for garnet‐bearing rocks of the Franciscan Complex were determined by cation diffusion zoning in garnet following Smit et al . (). Elemental profiles were measured from individual garnet grains mounted in epoxy and polished to expose the garnet cores.…”

Section: Sample Descriptionsmentioning

confidence: 97%

Dating subduction‐zone metamorphism with combined garnet and lawsonite Lu–Hf geochronology

Mulcahy

Vervoort

Renne

2014

Journal Metamorphic Geology

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Studies of ancient and active subduction zones are critically important to understanding processes of interplate coupling, crust-mantle recycling and arc magmatism. Dating subduction metamorphism along prograde and retrograde paths in order to constrain such processes, however, has proved to be extremely difficult. The complex thermal history of subduction systems poses significant challenges to low-T geochronometers and subduction-zone assemblages commonly lack suitable minerals for higher temperature geochronology. Garnet and lawsonite, however, are critical index minerals of high-and low-T subduction-zone metamorphism and are well suited to Lu-Hf geochronology. In addition, the closure temperature for Lu-Hf in garnet and lawsonite should be sufficiently high that an age will date mineral crystallization and therefore the timing of subduction zone metamorphism. The relative stability and timing of garnet and lawsonite formation will be controlled by the bulk composition, peak metamorphic conditions and shape of the P-T path experienced by a particular sample. To test the influence of metamorphic conditions and P-T path on Lu-Hf geochronology, garnet and lawsonite-bearing samples were dated from rocks where lawsonite formed after garnet along a retrograde path, stabilized prior to garnet along a prograde path and formed contemporaneously with garnet. In the Franciscan Complex, the ages of garnet-epidote amphibolite, garnet-epidote blueschist, garnetlawsonite blueschist and lawsonite blueschist range from c. 166-130 Ma and generally decrease with decreasing metamorphic grade, consistent with previous studies. Garnet-lawsonite blueschist/eclogite formed along an apparent prograde path at Ward Creek records an apparent age of c. 152 Ma. Lower temperature lawsonite blueschist at Ward Creek, however, failed to provide a geologically significant date and likely reflects isotopic disequilibrium at low temperatures. The apparent temperature-time history from Franciscan Complex Lu-Hf ages most likely reflects samples derived from various portions of the subduction zone or that were subducted and metamorphosed at different times in the thermal evolution of the subduction zone. In the Sivrihisar Massif, lawsonite eclogite and garnet-lawsonite blueschist record distinctly different ages of 91.1 AE 1.3 Ma and 83.3 AE 1.8 Ma. The different ages date the timing of high-P metamorphism within each protolith and suggest that garnet-lawsonite eclogite metamorphism pre-dated garnet-lawsonite blueschist metamorphism in these samples by c. 8 Ma. The age of lawsonite eclogite metamorphism extends the timing of high-P metamorphism and requires subduction initiation prior to 91 Ma. The results indicate that the Lu-Hf system provides a reliable tool for dating the wide range in P-T conditions of subductionzone metamorphism. Lawsonite dating, in particular, provides a reliable method by which to date low-T retrograde and prograde metamorphism in the absence of garnet. Lawsonite may not be ideal for geochronology if sufficient garnet coexi...

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“…Temperatures of all rocks analysed in this study lie well below the closure temperature for 176Hf* diffusion in garnet (Smit, Scherer, & Mezger, 2013) and are low enough that Lu diffusion may not be expected to a significant spatial extent (Bloch, Ganguly, Hervig, & Cheng, 2015). Considering that concentric growth zoning is preserved, the Lu-Hf dates would thus most plausibly represent crystallization ages.…”

Section: Timing Of Metamorphismmentioning

confidence: 81%

Mesozoic to Cenozoic tectono‐metamorphic history of the South Pamir–Hindu Kush (Chitral, NW Pakistan): Insights from phase equilibria modelling, and garnet–monazite petrochronology

Soret

Larson

Cottle

et al. 2019

Journal Metamorphic Geology

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The Karakoram–Hindu Kush–Pamir and adjacent Tibetan plateau belt comprise a series of Gondwana‐derived crustal fragments that successively accreted to the Eurasian margin in the Mesozoic as the result of the progressive Tethys ocean closure. These domains provide unique insights into the thermal and structural history of the Mesozoic to Cenozoic Eurasian plate margin, which are critical to inform the initial boundary conditions (e.g. crustal thickness, structure and thermo‐mechanical properties) for the subsequent development of the large and hot Tibetan–Himalaya orogen, and the associated crustal deformation processes. Using a combination of microstructural analyses, thermobarometry modelling and U–Th–Pb monazite and Lu–Hf garnet geochronology, the study reappraises the metamorphic history of exposed mid‐crustal metapelites in the Chitral region of the South Pamir–Hindu Kush (NW Pakistan). This study also demonstrates that trace elements in monazite (especially Y and Dy), combined with thermodynamical modelling and Lu–Hf garnet dating, provides a powerful integrated toolbox for constraining long‐lived and polyphased tectono‐metamorphic histories in all their spatial and temporal complexity. Rocks from the Chitral region were progressively deformed and metamorphosed at sub‐ and supra‐solidus conditions through at least four distinct episodes from the Mesozoic to the Cenozoic. Rocks were first metamorphosed at ~400–500°C and ~0.3 GPa in the Late Triassic–Early Jurassic (210–185 Ma), likely in response to the accretion of the Karakoram during the Cimmerian orogeny. Pressure and temperature subsequently increased by ~0.3 GPa and 100°C in the Early‐ to Mid Cretaceous (140–80 Ma), coinciding with the intrusion of calcalkaline granitic plutons across the Karakoram and Pamir regions. This event is interpreted as the record of crustal thickening and the development of a proto‐plateau within the Eurasian margin due to a long‐lived episode of slab flattening in an Andean‐type margin. Peak metamorphism was reached in the Late Eocene–Early Oligocene (40–30 Ma) at conditions of 580–600°C and ~0.6 GPa and 700–750°C and 0.7–0.8 GPa for the investigated staurolite schists and sillimanite migmatites respectively. This crustal heating up to moderate anatexis likely resulted in the underthrusting of the Indian plate after a NeoTethyan slab‐break off or to the Tethyan Himalaya–Lhasa microcontinent collision and subsequent oceanic slab flattening. Near‐isothermal decompression/exhumation followed in the Late Oligocene (28–23 Ma) as marked by a pressure decrease in excess of ~0.1 GPa. This event was coeval with the intrusion of the 24 Ma Garam Chasma leucogranite. This rapid exhumation is interpreted to be related to the reactivation of the South Pamir–Karakoram suture zone during the ongoing collision with India. The findings of this study confirm that significant crustal shortening and thickening of the south Eurasian margin occurred during the Mesozoic in an accretionary‐type tectonic setting through successive episodes of...

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Peak metamorphic temperatures from cation diffusion zoning in garnet

Cited by 16 publications

References 77 publications

Early evolution of the Pamir deep crust from Lu-Hf and U-Pb geochronology and garnet thermometry

Early evolution of the Pamir deep crust from Lu-Hf and U-Pb geochronology and garnet thermometry

Dating subduction‐zone metamorphism with combined garnet and lawsonite Lu–Hf geochronology

Mesozoic to Cenozoic tectono‐metamorphic history of the South Pamir–Hindu Kush (Chitral, NW Pakistan): Insights from phase equilibria modelling, and garnet–monazite petrochronology

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