Journal of Metamorphic Geology最新文献

Two types of aluminous paragneiss from the Loosdorf complex (Bohemian Massif, NE Austria) contain coarse-grained granulite assemblages and retrograde reaction textures that are investigated to constrain the post-peak history of the Gföhl unit in the southern Bohemian Massif. Both types have a peak assemblage garnet–biotite–sillimanite–plagioclase–K-feldspar–quartz–granitic melt ± kyanite ± ilmenite ± rutile, recording peak metamorphic conditions of $�\sim$0.9–1.1 GPa and $�\sim$780–820°C estimated by isochemical phase equilibrium modelling. The first sample type (Ysper paragneiss) developed (i) cordierite coronae around garnet and (ii) cordierite–spinel and cordierite–quartz reaction textures at former garnet–sillimanite interfaces. Calculated chemical potential relationships indicate that the textures formed in the course of a post-peak near-isothermal decompression path reaching $�\sim$0.4 GPa. Texture formation follows a two-step process. Initially, cordierite coronae grow between garnet and sillimanite. As these coronae thicken, they facilitate the development of local compositional domains, leading to the formation of cordierite–spinel and cordierite–quartz symplectites. The second sample type (Pielach paragneiss) exhibits only discontinuous cordierite coronae around garnet porphyroblasts but lacks symplectites. The formation of cordierite there also indicates near-isothermal decompression to 0.4–0.5 GPa and 750–800°C. This relatively hot decompression path is explained by the contemporaneous exhumation of a large HP–UHT granulite body now underlying the Loosdorf complex. The timing of regional metamorphism in the granulites and the southern Bohemian Massif in general is well constrained and has its peak at $�\sim$340 Ma. Monazite from Loosdorf paragneiss samples yield a slightly younger age of $�\sim$335 Ma. Although the ages overlap within error, they are interpreted to reflect near-isothermal decompression and exhumation resulting in the formation of the observed reaction textures.

Gneiss domes are an integral element of many orogenic belts and commonly provide tectonic windows into deep crustal levels. Gneiss domes in the New England segment of the Appalachian orogen have been classically associated with diapirism and fold interference, but alternative models involving ductile flow have been proposed. We evaluate these models in the Gneiss Dome belt of western New England with U-Th-Pb monazite, xenotime, zircon, and titanite petrochronology and major and trace element thermobarometry. These data constrain distinct pressure–temperature–time (P-T-t) paths for each unit in the gneiss dome belt tectono-stratigraphy. The structurally lowest units, Laurentia-derived migmatitic gneisses of the Waterbury dome, document two stages of metamorphism (455–435 and 400–370 Ma) with peak Acadian metamorphic conditions of ~1.0–1.2 GPa at 750–780°C at 391 ± 7 to 386 ± 4 Ma. The next structurally higher unit, the Gondwana-derived Taine Mountain Formation, records Taconic (peak conditions: 0.6 GPa, 600°C at 441 ± 4 Ma) and Acadian (peak: 0.8–1.0 GPa, 650°C at 377 ± 4 Ma) metamorphism. The overlying Collinsville Formation yielded a 473 ± 5 Ma crystallization age and evidence for metamorphic conditions of 650°C at 436 ± 4 Ma and 1.2–1.0 GPa, 750–775°C at 397 ± 4 to 385 ± 6 Ma. The structurally higher Sweetheart Mountain Member of the Collinsville Formation yielded only Acadian zircon, monazite, and xenotime dates and evidence for high-pressure granulite facies metamorphism (1.8 GPa, 815°C) at circa 380–375 Ma. Cover rocks of the dome-mantling The Straits Schist records peak conditions of ~1 GPa, 700°C at 386 ± 6 to 380 ± 4 Ma. Garnet breakdown to monazite and/or xenotime occurred in all units at circa 375–360 and 345–330 Ma. Peak Acadian metamorphic pressures increase systematically from the structurally lowest to highest units (from 1.0 to 1.8 GPa). This inverted metamorphic sequence is incompatible with the diapiric and fold interference models, which predict the highest pressures at the structurally lowest levels. Based upon P-T-t and structural data, we prefer a model involving, first, circa 380 Ma thrust stacking followed by syn-collisional orogen parallel extension, ductile flow, and rise of the domes between 380 and 365 Ma. Garnet breakdown at circa 345–330 Ma is interpreted to reflect further exhumation during collapse of the Acadian orogenic plateau. These results highlight the power of integrating petrologic constraints with paired geochemical and geochronologic data from multiple chronometers to test structural and tectonic models and show that syn-convergent orogen parallel ductile flow dramatically modified earlier accretion-related structures in New England. Further, the Gneiss Dome belt documents gneiss dome development in a syn-collisional, thick crust setting, providing an ancient example of middle to lower crustal processes that may be occurring today in the modern Himalaya and Pamir Range.

Contrasting views exist in regard of the evolution of metamorphic rocks in the southeastern Pohorje Mountains (Mts), located in the southeastern Eastern Alps. Major debated points are whether micaschists have experienced ultrahigh-pressure metamorphism in the Late Cretaceous (Eo-Alpine) and whether they were continuously exhumed or experienced a multiple subduction–exhumation process from that time on. Therefore, we studied micaschist sample 18Slo39 with two generations of garnet and phengitic muscovite from this area. Our detailed study of this rock included petrographic observations, chemical analyses of minerals with the electron microprobe, pseudosection modelling, conventional geothermometry, and monazite in-situ U-Th-Pb dating using laser-ablation inductively coupled plasma (ICP) mass spectrometry. The following results were obtained: The studied micaschist was subject to a peak pressure of 1.31 ± 0.14 GPa at 603 ± 26°C in Eo-Alpine times: 90.62 ± 2.78 (2σ) Ma (Stage I). Contact metamorphism at pressure–temperature conditions of 0.66 ± 0.10 GPa and 577 ± 23°C was induced by the intrusion of the Pohorje pluton (Stage III). We determined an early Miocene age of 18.33 ± 0.43 (2σ) Ma for this intrusion. Based on this study and the previously reported data for a micaschist (16Slo12) taken in the vicinity of sample 18Slo39, a geodynamic model is proposed for the region of the Pohorje Mts considering Eo-Alpine subduction of oceanic crust and European continental crust, of which the micaschist was part of. Another high-pressure event in the Eocene (Stage II) was the result of intracontinental subduction because of transpression by the Periadriatic fault system that separates the Eastern Alps from the Southern Alps. This type of subduction gave rise to magma generation and ascent to form the Pohorje pluton, which caused contact metamorphism in its vicinity.

In situ laser ablation and single-grain fusion ⁴⁰Ar/³⁹Ar geochronological techniques were directly compared using white mica from nine metasedimentary rocks from the Vaimok Lens of the Seve Nappe Complex (SNC) in the Scandinavian Caledonides. Seven of the rocks are from the eclogite-bearing Grapesvare nappe within the lens that is defined by D2 structures (S2 and F2), which were formed during exhumation following late Cambrian/Early Ordovician ultra-high pressure metamorphism. Two other rocks were obtained from ‘Scandian’ shear zones that delimit the nappes within the lens. The shear zones were active during terminal collision of Baltica and Laurentia in the Silurian to Devonian. The rocks exhibit variable deformation intensities and degrees of strain localization, expressed in particular by white mica. The in situ laser ablation and single-grain fusion ⁴⁰Ar/³⁹Ar dates both span from the late Cambrian to Middle Devonian. Results of both techniques generally show decreasing dates with increasing bulk deformation intensity and successive structural generations (i.e., D2 then Scandian structures). Furthermore, several discrepancies are evident when comparing the results of the two techniques for the same rocks, indicating the ⁴⁰Ar/³⁹Ar dates are not solely governed by bulk deformation intensities and structural generations. Instead, the discrepancies demonstrate the additional influence of white mica strain localization, which is illuminated by the different analytical volumes of the techniques. Thus, the ⁴⁰Ar/³⁹Ar datasets are altogether deciphered as a function of bulk deformation intensity and degree of strain localization that affected the overall white mica volume. The former controls the gross ⁴⁰Ar loss from the overall volume and the latter dictates the variability of ⁴⁰Ar loss within the volume. Exploiting the interplay of these two phenomena for the Vaimok Lens rocks with in situ laser ablation allows for the broad span of ⁴⁰Ar/³⁹Ar dates to be contextualized into a sequence of tectonic events: (1) cooling at 474 ± 3 Ma, (2) pre-collision deformation at 447 ± 2 Ma and (3) activation of crustal-scale shear zones in the SNC related to continental collision at 431 ± 3 Ma and 411 ± 3 Ma.

High-pressure rocks from the island of Ios in the Greek Cyclades were examined to resolve the P–T conditions reached during subduction of the two distinct lithotectonic units that are separated by the South Cycladic Shear Zone (SCSZ)—the footwall complex composed of Hercynian basement gneisses, schists and amphibolites, and the hangingwall complex composed of blueschists and eclogites. A combination of elastic tensor quartz inclusion in garnet (QuiG) barometry and Zr-in-rutile (ZiR) trace element thermometry was used to constrain minimum garnet growth conditions. Garnet from the hangingwall (blueschist) unit record formation pressures that range from 1.5 to 1.9 GPa and garnet from the footwall basement complex record garnet formation pressures of 1.65–2.05 GPa. ZiR thermometry on rutile inclusions within garnet establishes the minimum temperature for garnet formation to be ~480–500°C. That is, there is no evidence in the QuiG and ZiR results that the rocks of the blueschist hangingwall and basement experienced different metamorphic histories during subduction. This is the first reported observation of blueschist facies metamorphism in the Hercynian basement complex. A model is proposed in which initial subduction occurred along a relatively shallow P–T trajectory of ~11°C/km and then transitioned to a steeper, nearly isothermal trajectory at a depth of ~45 km reaching similar peak metamorphic conditions of ~500–525°C at 2.0 GPa for all samples. Such a change in the subduction path could be accomplished by either an increase in the rate of subduction or an increase in the angle of the subduction zone. The present juxtaposition of samples with contrasting mineral assemblages and garnet growth histories is interpreted to have arisen from differences in bulk compositions and variations in the preservation of high-pressure prograde mineral assemblages during exhumation. The existence of similar P–T conditions and prograde paths in the two units does not require that the rocks were all metamorphosed at the same time and that the SCSZ experienced little movement. Rather, it is suggested that the two units experienced prograde and peak metamorphism at different times and were subsequently juxtaposed along the SCSZ.

High-grade metamorphic rocks are widely exposed along the SE–NW- to E–W-trending shear zones in the Truong Son Belt, Central Vietnam, but few petrological studies have been conducted in this area. Herein, we report the occurrence of mylonitized granulites that crop out along the Dai Loc shear zone in the southernmost Truong Son Belt. Detailed petrographic analysis, geochemistry and P–T–t estimates of the evolution of two granulite samples are presented to elucidate the formation processes of these high-grade metamorphic rocks. The results indicate that the rocks underwent two distinct metamorphic cycles. The first cycle (M1) is characterized by coarse-grained granulite mineral assemblages, defining a tight clockwise P–T path with near-isobaric heating to a near ultrahigh-temperature peak at low pressure, followed by cooling. The prograde mineral assemblage (M1a) is indicated by inclusions of cordierite + sillimanite + biotite + quartz + spinel ± plagioclase in coarse-grained garnet, orthopyroxene and cordierite. The mineral assemblage of garnet + orthopyroxene + cordierite + plagioclase + K-feldspar + ilmenite + melt ± biotite (M1b) defines the peak P–T conditions of 5.3–6.3 kbar and 850–920°C. Post-peak cooling (M1c) is marked by the formation of quartz + biotite symplectites around garnet and orthopyroxene. The second cycle involved medium-pressure amphibolite facies metamorphism (M2), characterized by domainal development of fine-grained kyanite-bearing mineral associations. Petrographic observations indicate that these fine-grained associations were formed during mylonitization. Zircon U–Pb dating reveals that the timing of granulite facies metamorphism appears to be coeval with the intrusion of a post-collisional granitoid at 430–410 Ma. Granulite facies metamorphism and crustal melting were probably driven by asthenospheric mantle upwelling triggered by slab breakoff during the Early Palaeozoic. Considering previous structural and geochronological studies, the second metamorphic event likely occurred during the Triassic Indosinian orogeny.

Chloritoid and kyanite coexist in metapelites from the high-pressure/low-temperature Massa Unit in the Alpi Apuane metamorphic complex (Northern Apennines, Italy). The composition of chloritoid is extremely variable throughout the Massa Unit. Fe-chloritoid occurs in association with hematite-free, graphite-bearing schists, whereas strongly zoned Fe-Mg chloritoid is found with hematite and kyanite. We investigated the effect of different bulk Fe₂O₃ contents in controlling chloritoid composition through phase equilibria modelling of four selected samples, representative of the different chloritoid-bearing parageneses found in the Massa Unit. The ferric iron content, measured through wet chemical titration, ranges from 0 (graphite-chloritoid schist) to 73% of the total iron (hematite-chloritoid schist). We show that Mg-rich chloritoid compositions and stability of kyanite at greenschist to blueschist facies conditions can be reproduced in the MnO–Na₂O–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O–TiO₂–O (MnNKFMASHTO) chemical system only considering the presence of significant amounts of ferric iron as part of the bulk composition. The stabilization of kyanite at lower grade is directly linked to the presence of Fe₂O₃, which renders the reactive bulk rock composition effectively enriched in Al₂O₃ with respect to Fe and Mg. We also document that high Fe₂O₃ contents exacerbate the effect of chloritoid fractionation, producing strongly zoned Fe-Mg-chloritoid grains. Finally, the P–T modelling of the Massa Units performed in this study allows, for the first time, the recognition of a two-stage evolution at peak conditions, with an earlier pressure peak (1.2–1.3 GPa at 350–400°C), and a later thermal peak (0.7–1.1 GPa at 440–480°C), compatible with subduction, underthrusting and exhumation of the Adria continental margin during growth of the Northern Apennine orogenic wedge.

The integration of garnet-based petrologic constraints with multimineral geochronologic data in eclogites and blueschists allows the timing and rate of subduction zone metamorphism to be constrained. We present a combined garnet Lu–Hf/Sm–Nd and zircon/rutile U–Pb geochronology study on three eclogites, a garnet-bearing blueschist, and a micaschist from the Changning–Menglian orogenic belt, a newly discovered ultrahigh-pressure metamorphic belt in southeast Tibet, in order to characterize tectono-metamorphic events and determine the duration of Paleo-Tethys oceanic subduction. Integration of phase equilibrium modelling and conventional thermobarometry for the eclogites defines a clockwise P–T path evolving from blueschist facies conditions at ~1.4 GPa and ~505–530°C to peak eclogite facies conditions at ~2.8 GPa and ~630–640°C, followed by isothermal decompression to amphibolite facies at ~1.0 GPa and ~630–650°C. The Lu–Hf ages of c. 239–236 Ma obtained for the eclogites and the blueschist are indistinguishable from the rutile U–Pb age of c. 239 Ma obtained for the eclogites and, combined with the observation of well-preserved Rayleigh-fractionation-style Mn and Lu zoning profiles in garnet, reflect the timing of early prograde garnet growth. The Sm–Nd ages of c. 242–236 Ma reflect a later period of garnet growth, evidenced by flat and/or M-shaped Sm zoning profiles. Each of the Sm–Nd ages overlaps, within uncertainty, with its corresponding Lu–Hf age (i.e., from the same garnet fraction). The consistency of the Lu–Hf and Sm–Nd ages indicates a short overall duration of garnet growth from blueschist to eclogite facies metamorphism, reflecting rapid subduction of the oceanic slab. The magmatic zircon U–Pb dates of c. 247 Ma constrain the protolith age of these metabasaltic rocks. The close protolith and the high-pressure metamorphic ages, together with the consistent garnet Lu–Hf and Sm–Nd ages and the overlapping youngest and oldest metamorphic ages of the oceanic-type and continental-type eclogites, respectively, suggest a fast tectonic transition from divergence to convergence highlighted by rapid oceanic subduction, continuous transition from oceanic to continental subduction, and a rapid cooling of the subduction interface.