Calibration of the garnet–biotite–Al 2 SiO 5 –quartz geobarometer for metapelites

The Blåhø Nappe on the island of Fjørtoft, which represents an isolated portion of the Seve Nappe Complex in the Western Gneiss Region, Norway, has been suggested to have experienced two deep burial cycles during the Caledonian orogeny. However, evidence on this multiple burial process by the derivation of a pressure–temperature–time (P–T–t) path has never been given in the literature. In this study, the ‘diamondiferous’ kyanite–garnet gneiss from the Blåhø Nappe on Fjørtoft was revisited to determine if such a process was correct. Two types of garnet, porphyroblastic garnet‐1 and fine‐grained garnet‐2, were recognized in the gneiss. The core of garnet‐1 is poor in Ca and documents P–T conditions of 1.2–1.3 GPa at c. 880°C based on pseudosection modelling. The inner rims of garnet‐1 and the core of garnet‐2 are both richer in Ca, recording P–T conditions of 1.35–1.45 GPa and 770–820°C. Application of conventional geothermobarometry on the outer rim of garnet‐1 and the rim of garnet‐2 yielded retrograde P–T conditions of 0.75–0.90 GPa and 610–685°C. These estimates define an anticlockwise P–T path at pressures below 1.5 GPa. Accessory monazite was dated with the electron microscope. Relicts of detrital monazite in the gneiss point to Sveconorwegian and possibly also Cryogenian provenance for the detritus of the sedimentary protolith. Metamorphic monazite in the gneiss records a wide age range from 460 to 380 Ma, with a peak c. 435 Ma and a shoulder at 395 Ma. These data suggest that the original (Ediacaran?) Baltica margin sediment (gneiss protolith) was transported to the base of an overlying plate during the early Caledonian (pre‐Scandian) orogeny. A long residence time of the metasedimentary rock at this base caused its heating to 880°C and homogenization of the early garnet chemistry. The late Caledonian (Scandian) collision between Baltica and Laurentia led to further burial, during which the studied gneiss was close to the former surface of the downgoing continental plate and, thus, cooled. The reconstructed P–T–t path confirms the multiple burial history of the Blåhø Nappe but contradicts previous ideas of deep burial of the Fjørtoft gneiss to more than 100 km.

Section: P–t Estimatesmentioning

confidence: 99%

“…Retrograde P-T conditions calculated for gneiss FJ-1B using the mineral pair Grt-Bt. The calculations were conducted by using the Grt-Bt geothermometer ofHoldaway (2000) and the Grt-Bt-Ky-Q geobarometer ofWu (2017…”

mentioning

confidence: 99%

An anticlockwise P–T–t path at high‐pressure, high‐temperature conditions for a migmatitic gneiss from the island of Fjørtoft, Western Gneiss Region, Norway, indicates two burial events during the Caledonian orogeny

Liu

Massonne

2019

“…Petrography shows that the equilibrium assemblage of subhedral biotite, plagioclase, quartz and garnet (M2) exists in all four samples; therefore, the Grt–Bt geothermometer (GB: Holdaway, 2000) and the Grt–Bt–Pl–Qz (GBPQ: Wu et al, 2004) geobarometer were coupled to estimate the pressure and temperature at peak metamorphic stage (M2). The Grt–Bt–Al 2 SiO 5 –Qz (GBAQ: Wu, 2017) and Grt–Bt–Ms–Pl (GBMP: Wu, 2015) geobarometers were also used if the sample contains muscovite, kyanite, or sillimanite at this stage. The Ti‐in‐biotite (Wu & Chen, 2015b), Ti‐in‐muscovite (Wu & Chen, 2015a) and two‐feldspar (Benisek et al, 2010) geothermometers were additionally utilized to estimate the temperature at related metamorphic stages (see Table 1 for detail).…”

Section: Geothermobarometrymentioning

confidence: 99%

“…The pressures adopted in Ti‐Bt, Ti‐Ms and Two‐feldspar are estimated according to the results in GB+GBPQ, GB+GBAQ and GB+GBMP. References and corresponding random errors/uncertainty (shown as error bars): GB thermometer: Holdaway (2000); ±25°C, GBPQ barometer: Wu et al (2004); ±1.2 kbar, GBAQ barometer: Wu (2017); ±1.8 kbar, GBMP barometer: Wu (2015); ±1.2 kbar, Ti–Bt thermometer: Wu and Chen (2015b); ±50°C, Ti–Ms thermometer: Wu and Chen (2015a); ± 65°C, two‐feldspar geothermometer: Benisek et al (2010), uncertainty of 5%.…”

Section: Geothermobarometrymentioning

confidence: 99%

Divergent metamorphism within the Namche Barwa Complex, the Eastern Himalaya, Southeast Tibet, China: Insights from in situ U–Th–Pb dating of metamorphic monazite

Peng

Gerdes

Zeng

et al. 2021

Geothermobarometry shows that metapelite samples from Namche Barwa Complex (NBC) reached upper amphibolite to near‐granulite facies during the peak metamorphic stage, with similar conditions of ~700–750°C/8–10 kbar, and then experienced retrograde metamorphism at ~630–700°C/4–7 kbar. In situ monazite laser ablation‐inductively coupled plasma‐mass spectrometry (LA‐ICP‐MS) U–Th–Pb dating suggests divergent metamorphism in the NBC: metapelite on the hanging wall of Namu–La thrust preserved a continuous metamorphic record of >19–3 Ma, whereas metapelite on the footwall yielded age ranges of >18–14 and 8–3 Ma, with a gap in recorded ages between 14 and 8 Ma. Monazite grains in the garnet porphyroblasts, more depleted in the heavy rare earth elements (HREE), yielded the youngest age of ~14 Ma. This is interpreted as the timing of upper amphibolite facies peak metamorphism in the metapelite from the NBC, with the NBC being exhumed coherently thereafter. Furthermore, the discrepancy between reported peak metamorphic ages of high‐pressure granulite (~40–30 Ma, ~25–20 Ma) and mid‐pressure metapelite (~14 Ma, this study) indicate asynchronous subduction–exhumation processes in the NBC. We suggest that crustal flow has played an essential role in exhumation since ~40 Ma, and recent surficial erosion (<8 Ma) intensified the exhumation of the NBC, with young leucogranite (<10 Ma) resulting from decompression melting. From ~3 Ma to the present, the interplay of erosion and tectonic movement caused ubiquitous rapid uplift, resulting in the concomitant exhumation of various types of rocks and the formation of the spectacular high relief between Yarlung Tsangpo Gorge and Namche Barwa Peak.

“…Monazite/Garnet 10 6 10 7 10 8 10 5 10 4 10 3 10 2 10 1 10 0 10 - Pressures were estimated using the garnet-aluminosilicate-quartz-plagioclase (GASP) barometer of Holdaway (2001) and the revised garnet-biotite-muscovite-plagioclase barometer (GBMP) of Wu (2017). The absolute uncertainty of the first method is ±0.08 GPa, while that of the second one is ±0.12 GPa.…”

Section: Conventional Thermobarometrymentioning

confidence: 99%

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

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...