Low-temperature Metamorphism Research Articles

Tectono-metamorphic evolution of the siliciclastic units in the Middle Tuscan Range (inner Northern Apennines): Mg–carpholite bearing quartz veins related to syn-metamorphic syn-orogenic foliation

This paper accounts for a new finding of High Pressure–Low Temperature (HP-LT) metamorphism documented in continental-derived rocks exposed in the central part of the Middle Tuscan Range, representing the easternmost location, in the Tyrrhenian Region, where blueschist facies metamorphism has been documented. It describes the development mechanism of Mg–carpholite bearing quartz veins in the framework of the deformational events described for the inner Northern Apennines. The HP-LT peak metamorphism can be estimated in P≥1.1GPa and T=370°–420°C as supported by the mineralogical assemblage consisting of quartz–muscovite–chloritoid–pyrophylite–hematite±chlorite±paragonite±Ti-oxides, documented in the aluminous metasediments (pelites and quartzose sandstones) of the Triassic Verrucano succession. This paragenesis defines the S1 schistosity, an axial planar tectonic foliation associated to map- and mesoscale isoclinals folds (F1); S1 schistosity is a composite foliation resulted from a non-coaxial progressive deformation associated to the DE1 collisional event (Late Oligocene–Early Miocene). Mg–carpholite bearing quartz veins developed as tension gashes in en-échelon arrays within brittle/ductile shear zones. The development of quartz veins was guaranteed by the high volumes of Si-rich fluids produced by the dynamic recrystallization process on the siliciclastic rocks.

Tectonic Implications of Early Paleozoic Metamorphism in the Anakie Inlier, Central Queensland, Australia

Well-defined metamorphic zones are developed in pelitic and psammitic rocks of the Late Neoproterozoic to Cambrian Anakie Metamorphic Group of the Anakie Inlier, central Queensland. They are defined by the incoming of biotite, garnet, and andalusite, with or without staurolite. Mineral assemblages indicate that low pressure–high temperature metamorphism is associated with D1, medium pressure–high temperature metamorphism with D2, and retrograde, low pressure–low temperature metamorphism with D3. A mean b cell parameter of 9.035 obtained from K-white micas in the lowest-grade rocks suggests upper intermediate pressure conditions during D2. The timing of the growth of the index minerals indicates that isotherms retreated to the southwest during D2. Phase diagram calculations for both Al-saturated and Al-undersaturated metapelites containing D1 associations indicate pressure-temperature (P-T) conditions of 0.4 GPa and 560°C. These changed to 0.64–0.79 GPa and 580°–640°C as a result of crustal thickening. The samples thus record a history of heating (D1), followed by near-isothermal compression (D2). This P-T pathway shows that contraction rather than extension, as suggested by some authors, occurred during D2. The contractional event is suggested to have taken place during the Delamerian Orogeny as a result of convergence and collisional processes along the former passive margin of Gondwana.

Magnetization carriers and remagnetization of bedded chert

Bedded chert is the only sediment type representative of the Paleozoic to early Mesozoic pelagic marine environment. Because of their association with ophiolites and island arc rocks, presence of datable microfossils and paleohorizontal reference provided by well-developed bedding surfaces, bedded chert sequences are often targeted for paleomagnetic tectonic studies. However, processes of magnetization acquisition in biosiliceous sediments, and consequently, the significance of their magnetic record, are not well understood. Our rock magnetic study of a Triassic–Jurassic radiolarian chert sequence, the Mino Terrane, Central Japan, shows that the ferrimagnetic assemblage of the gray chert units is of detrital origin, while the red chert's assemblage is dominated by authigenic phases – pigmentary hematite and biogenic magnetite – which contribute to the natural remanent magnetization. The presence of magnetofossils places red oxic chert in the category of prospective environmental archives. Magnetite-producing magnetotactic bacteria were apparently able to tolerate elevated concentrations of dissolved silica as well as a steep redox gradient in sedimentary pore-waters during the deposition of red chert layers. A strong uniaxial anisotropy due to chain-alignment of the biogenic magnetite grains likely contributes to the acquisition of anomalously stable partial thermoviscous magnetization by chert even at low metamorphic temperatures.

The role of mantle hydration in continental crust recycling in the wedge region

Abstract In orogenic belts high pressure–low temperature (HP–LT) metamorphism can widely affect units derived from both the oceanic and the continental lithosphere. In order to verify whether high P / T (pressure/temperature) ratios recorded in the continental lithosphere can result from tectonic erosion, ablative subduction and recycling in the mantle wedge, we implemented a 2D numerical model to simulate oceanic subduction beneath a continent. Particular attention is paid to the role played by mantle hydration within the continental crust recycled in the wedge region. A comparison between hydrated and non-hydrated models highlights that hydration is fundamental in allowing the recycling of crustal material at shallow depths (≤150 km for a convergence rate of 1 cm year −1 ), making the uprising and exhumation of buried crustal material during active subduction possible. The recycled crustal material can originate from any crustal level. The T max and P max distributions within the final marker configuration show that crustal recycling induces the coupling of volumes that reached different depths during their paths in the corner flow. To verify the reliability of this model we compare predictions with natural geological data from the Austroalpine Sesia–Lanzo Zone (SLZ), the largest eclogite-facies crustal fragment of early Alpine age and whose Alpine tectonic evolution has been interpreted as compatible with a cycle of burial at depth and exhumation during active subduction of the oceanic lithosphere. The relationships between natural P – T estimates and predicted P – T values show that the simulated geodynamic scenario generates a thermal regime coherent with that affecting the subducted continental crust of the SLZ, which may have been stable for a long time during Alpine subduction, allowing the SLZ rocks to accomplish their burial and exhumation path under an active subduction regime.

Lawsonite Lu-Hf geochronology: A new geochronometer for subduction zone processes

We present a new method for Lu-Hf geochronology using lawsonite, an index mineral of high-pressure, low-temperature metamorphism. The method uses common cation exchange chromatography to purify Lu and Hf from the sample and analysis by multicollector inductively coupled plasma–mass spectrometry. Lawsonite 176 Lu/ 177 Hf ratios are elevated and are similar to other minerals commonly used in Lu-Hf geochronology such as garnet or apatite. We obtain an age of 145.5 ± 2.4 Ma for lawsonite-blueschist facies metamorphism at Ring Mountain within the Franciscan Complex, California, the lawsonite type locality. The age agrees well with published geochronology and thermochronology for the Franciscan Complex. The new method provides a powerful tool to investigate processes and time scales of blueschist to eclogite facies metamorphism, crust-mantle recycling via subduction, and the geodynamics of convergent margins.

Zircon Behaviour during Low-temperature Metamorphism

Zircon in greenschist-facies metasedimentary rocks from the Scottish Highlands displays a range of complex textures that reflect low-temperature alteration of original detrital grains. In situ back-scattered electron, cathodoluminescence, electron backscatter diffraction and chemical analyses show that altered zircon is porous, weakly luminescent, enriched in non-formula elements such as Al and Fe, and is associated with fractures within the host zircon. The low-temperature zircon appears to be nano-crystalline and to replace U-rich zircon via modification of whole grains or selective alteration of parts of grains, and is linked to the development of zircon outgrowths. The altered zircon is also associated with epitaxial xenotime outgrowths and inclusions. Low-temperature zircon is abundant in slates and other mica-rich samples and its formation is linked to a dissolution–reprecipitation mechanism. Zircon within quartz-rich host rocks typically shows evidence of deformation and the resulting fractures enhance its dissolution, creating rounded embayed morphologies. In contrast, zircon from phyllosilicate-rich rocks contains more new low-temperature growth. Zircon alters during both prograde and retrograde metamorphic events and its development is controlled by both the progressive accumulation of radiation damage in the host grain and the access of metamorphic fluids to the metamict zircon.

High-pressure/low-temperature metamorphism of basalts in Lavrion (Greece): implications for the preservation of peak metamorphic assemblages in blueschists and greenschists

The Upper Tectonic Unit of the Lavrion area is part of the Attic-Cycladic blueschist belt and was affected by high-pressure, low-temperature metamorphism. Blueschists and greenschists occur in the same outcrop and are believed to have experienced the same pressure-temperature (P-T) history which has been quantified using geothermobarometry and pseudosections for specific bulk-rock compositions. Calculated P-T conditions indicate minimum pressure of ∼ 0.9 GPa and temperature of ∼ 370 °C for the peak of metamorphism. The prograde and retrograde paths followed a very similar low geothermal gradient (10-12 °C/km) with cooling during decompression. Pseudosections show that both blueschists and greenschists can exist stably at the metamorphic peak, the dominant amphibole being a function of bulk composition: the blueschists, on average, have lower Mg# than the greenschists, which results in a larger P-T stability field of blue amphibole. A pseudosection analysis of the dehydration behaviour indicates that blueschists and some greenschists can preserve their peak assemblages (no dehydration along the retrograde path), whereas greenschist assemblages, in general, are rather prone to undergo dehydration and hence re-equilibration to lower P-T conditions during exhumation.

Paleoproterozoic bimodal post-collisional magmatism in the southwestern Amazonian Craton, Mato Grosso, Brazil: Geochemistry and isotopic evidence

This paper discusses geological and geochemical aspects of a Paleoproterozoic volcano-plutonic association that crops out in southwestern Amazonian Craton, Mato Grosso, Brazil. The study area was divided into undeformed and deformed domains, based on structural and geochronology studies. The undeformed domain is composed mainly of felsic explosive and effusive flows. Inter-layered mafic flows of basalts and sedimentary rocks are also present. The deformed domain is mainly composed of titanite and hornblende-bearing monzogranite to syenogranite and biotite monzogranite, while granodiorite is less common. U–Pb single zircon analyses yielded ages of 1.8–1.75 Ga in granites and felsic volcanic rocks for both domains. Basalts from the undeformed domain are phaneritic, fine-grained, and are often hydrothermally altered. They show tholeiitic affinity and are LREE enriched. Their trace element composition resembles those of within-plate associations. The ε Nd ( t = 1.75 Ga) for all these rocks are positive, ranging from 0.12 to 1.49, which reflect a juvenile source. The felsic volcanism comprises subalkaline rocks with high K contents and is divided into two groups: crystal enriched ignimbrites and effusive rhyolites. REE patterns of effusive rocks show negative-Eu anomalies and are smooth in the ignimbrites. Trace element patterns similar to those of the effusive rocks and ignimbrites are found in magmatic rocks derived from sources affected by subduction-related metasomatism. Hornblende and biotite granites occur in the deformed felsic plutonic domain. These rocks show evidence of low-temperature metamorphism and deformation, and in some places, of hydrothermal alteration. The LREE/Nb (or Ta) ratios of these rocks are consistent with those observed in granites of post-collisional settings. The ε Nd ( t = 1.75 Ga) values are slightly negative on average, with few positive values (−1.41 to +2.96). These data are interpreted as indicative of a magmatism produced during a post-collisional event from mixed sources: a metasomatised mantle and a Paleoproterozoic continental crust. An intracontinental shearing with age of 1.7–1.66 Ga created the Teles Pires–Juruena lineament which partially controlled this magmatism.

Complex chemical zoning in eclogite facies garnet reaction rims: the role of grain boundary diffusion

In metapelites of the Saualpe complex (Eastern Alps) continuous 10 µm to 20 µm wide garnet reaction rims formed along biotite-plagioclase and biotite-perthite interfaces. The pre-existing mineral assemblages are remnants of low pressure high temperature metamorphism of Permian age. The garnet reaction rims grew during the Cretaceous eclogite facies overprint. Reaction rim growth involved transfer of Fe and Mg components from the garnet-biotite interface to the garnet-feldspar interface and transfer of the Ca component in the opposite direction. The garnets show complex, asymmetrical chemical zoning, which reflects the relative contributions of short circuit diffusion along grain boundaries within the polycrystalline garnet reaction rims and volume diffusion through the grain interiors on bulk mass transfer. It is demonstrated by numerical modelling that the spacing of the grain boundaries, i.e. the grain size of the garnet in the reaction rim is a first order control on its internal chemical zoning.

Cenozoic geodynamic evolution of the Aegean

The Aegean region is a concentrate of the main geodynamic processes that shaped the Mediterranean region: oceanic and continental subduction, mountain building, high-pressure and low-temperature metamorphism, backarc extension, post-orogenic collapse, metamorphic core complexes, gneiss domes are the ingredients of a complex evolution that started at the end of the Cretaceous with the closure of the Tethyan ocean along the Vardar suture zone. Using available plate kinematic, geophysical, petrological and structural data, we present a synthetic tectonic map of the whole region encompassing the Balkans, Western Turkey, the Aegean Sea, the Hellenic Arc, the Mediterranean Ridge and continental Greece and we build a lithospheric-scale N-S cross-section from Crete to the Rhodope massif. We then describe the tectonic evolution of this cross-section with a series of reconstructions from ~70 Ma to the Present. We follow on the hypothesis that a single subduction has been active throughout most of the Mesozoic and the entire Cenozoic, and we show that the geological record is compatible with this hypothesis. The reconstructions show that continental subduction (Apulian and Pelagonian continental blocks) did not induce slab break-off in this case. Using this evolution, we discuss the mechanisms leading to the exhumation of metamorphic rocks and the subsequent formation of extensional metamorphic domes in the backarc region during slab retreat. The tectonic histories of the two regions showing large-scale extension, the Rhodope and the Cyclades are then compared. The respective contributions to slab retreat, post-orogenic extension and lower crust partial melting of changes in kinematic boundary conditions and in nature of subducting material, from continental to oceanic, are discussed.

Multi-stage zeolite facies mineralization in the Hvalfjördur area, Iceland

The Hvalfjordur area, 30 km north of Iceland’s capital Reykjavik, belongs to the sequence of Late Tertiary to early Quaternary flood basalts with minor intercalations of hyaloclastites and rhyolites. The basalts are affected by progressive low-temperature metamorphism, caused by the burial of the lava succession and higher heat flow from nearby central volcanoes. Low-grade zeolite facies metamorphism of basaltic lavas in the Hvalfjordur area results in two distinct mineral parageneses that can be correlated to events in the burial and hydrothermal history of the lava pile. Stage Ia represents syn-eruptive near-surface alteration in which celadonite and silica were precipitated along primary pores. During regional burial metamorphism (stage Ib), hydrolysis of olivine and glass led to the formation of mixed-layer chlorite/smectite clays. The chlorite content of stage Ib phyllosilicate vesicle rims increases with increasing burial depth and temperature. Stage II occurred after the burial and is marked by zeolite mineralization caused by higher heat flow from the Laxarvogur and Hvalfjordur central volcanoes. Altogether 11 different zeolites were found in the Hvalfjordur area: analcime, chabazite, epistilbite, heulandite, laumontite, levyne, mesolite, stilbite, stellerite, thomsonite and yugawaralite. In total, three separate depth and temperature-controlled “zeolite zones” occur in the Hvalfjordur area.

High-pressure–low-temperature metamorphism of metasedimentary rocks, southern Menderes Massif, western Turkey

Kilometer-scale lenses of quartz-rich metasedimentary rocks crop out in a discontinuous belt along the southern margin of the Menderes Massif, Turkey, and preserve evidence for high-pressure–low-temperature (H P–L T) metamorphism related to subduction of a continental margin during Alpine orogeny. Kyanite schist, quartzite, and quartz veins contain kyanite + phengite + Mg-chlorite, and the veins also contain magnesiocarpholite. A deformed carbonate metaconglomerate juxtaposed with the quartzite-dominated unit does not contain H P index minerals, and likely represents the tectonized boundary of the siliceous rocks with adjacent marble. The H P–L T rocks (10–12 kbar, 470–570 °C) record different pressure conditions than the adjacent, apparently lower pressure Menderes metasedimentary sequence. Despite this difference there is disagreement as to whether these H P–L T rocks are part of the Menderes sequence or are related to the tectonically overlying Cycladic blueschist unit. If the former, the entire southern Menderes Massif experienced H P–L T metamorphism but the evidence has been obliterated from most rocks; if the latter, rocks recording different metamorphic-kinematic conditions experienced different tectonic histories and were tectonically juxtaposed during thrusting. Based on observations and data in this study, the second model better accounts for the differences in P– T-deformation histories of the southern Menderes Massif rocks, and suggests that the H P–L T rocks are not part of the Menderes cover sequence.

Tectonic and metamorphic evolution of the Temsamane units, External Rif (northern Morocco): implications for the evolution of the Rif and the Betic–Rif arc

Located at an intermediate position in the External Rif nappe pile, the Temsamane units (northern Morocco) are characterized by an abnormally intense metamorphism and a penetrative ductile deformation. We present new metamorphic data showing that, in spite of their external position in the Rif, part of the Temsamane units underwent medium-pressure low-temperature (MP–LT) metamorphism (at c . 7–9 kbar and 330–430 °C), possibly during the Oligocene. Structural data show that the exhumation of these units, during Middle to Late Miocene times, was characterized by an intense approximately east–west stretching and by top-to-the-west shear senses. We tentatively propose two possible origins for the MP–LT Temsamane units: (1) an internal origin related to the subduction and the HP–LT event recorded in the Internal Rif (Alboran Domain), or (2) an external origin, implying a second subduction system within the External Rif, parallel to and almost contemporaneous with that of the Alboran Domain. This tectonometamorphic evolution of the Temsamane units is set within the context of the External Rif evolution. At a larger scale, we show that the exhumation history of the Temsamane units, which strongly resembles that documented in the core of the internal Betics, is compatible with the westward slab retreat occurring during the Middle to Late Miocene in the Betic–Rif region.

Record of mid-Archaean subduction from metamorphism in the Barberton terrain, South Africa

Although plate tectonics is the central geological process of the modern Earth, its form and existence during the Archaean era (4.0-2.5 Gyr ago) are disputed. The existence of subduction during this time is particularly controversial because characteristic subduction-related mineral assemblages, typically documenting apparent geothermal gradients of 15 degrees C km(-1) or less, have not yet been recorded from in situ Archaean rocks (the lowest recorded apparent geothermal gradients are greater than 25 degrees C km(-1)). Despite this absence from the rock record, low Archaean geothermal gradients are suggested by eclogitic nodules in kimberlites and circumstantial evidence for subduction processes, including possible accretion-related structures, has been reported in Archaean terrains. The lack of spatially and temporally well-constrained high-pressure, low-temperature metamorphism continues, however, to cast doubt on the relevance of subduction-driven tectonics during the first 1.5 Gyr of the Earth's history. Here we report garnet-albite-bearing mineral assemblages that record pressures of 1.2-1.5 GPa at temperatures of 600-650 degrees C from supracrustal amphibolites from the mid-Archaean Barberton granitoid-greenstone terrain. These conditions point to apparent geothermal gradients of 12-15 degrees C-similar to those found in recent subduction zones-that coincided with the main phase of terrane accretion in the structurally overlying Barberton greenstone belt. These high-pressure, low-temperature conditions represent metamorphic evidence for cold and strong lithosphere, as well as subduction-driven tectonic processes, during the evolution of the early Earth.

Thermal history of the enstatite chondrites from silica polymorphs

Here we report the results of our petrologic and mineralogical study of enstatite (E) chondrites in order to explore their thermal histories. We studied silica phases in 20 E chondrites by laser micro Raman spectroscopy to determine the silica polymorphs they contain. Silica phases are commonly present in E chondrites and their polymorphs reflect the physical conditions of formation. The samples studied here include EH3-5, EL3-6, E chondrite melt rocks, and an anomalous E chondrite. We identified quartz, tridymite, cristobalite, and silica glass in the samples studied. EH4 5 and EH melt rocks are divided into high and low temperature classes based on niningeritealabandite solid solutions. EH3, EL3, and some EH melt rocks of the high temperature class contain tridymite and cristobalite. We suggest that tridymite and cristobalite crystallized in chondrules and E chondrite melts, followed by rapid cooling, leading to the survival of these silica polymorphs. EH4 and EL4 chondrites also contain tridymite and cristobalite in their chondrules, indicating that these silica polymorphs survived low temperature metamorphism (as estimated from opaque mineral geothermometers) because of the sluggishness of the transition to a more stable polymorph. Tridymite and cristobalite in EL6 chondrites reflect the high temperature processes experienced by these meteorites. On the other hand, some EH5 chondrites and EH melt rocks of the low temperature class contain quartz, which may be a product of the transition from tridymite or cristobalite during a long period of low temperature metamorphism. Although the thermal history of E chondrites have been previously estimated from opaque minerals, such compositions mainly reflect low temperature processes. However, we can reconstruct the primordial thermal processes and subsequent cooling histories of E chondrites from their silica polymorphs. The E chondrites have complicated thermal histories, which produced the observed variations among them.

Recent advances in Alpine studies: tracking the Caledonian–Variscan belt in the internal western Alps

Recent advances in western Alps knowledge haveconcerned several key geodynamic issues. The deepstructure of the belt, and then the implication ofthe lower crust and lithospheric mantle in the colli-sional deformation were recognized through seismic–reflection and tomographic studies. The origin ofthe Piedmont–Ligurian Ocean and adjacent palaeo-margins was thoroughly elaborated through compar-ison with central Atlantic and the Galicia margin(it is worth to point here another striking similarity,i.e. with the oceanic margin of southwestern Aus-tralia [4]). A wealth of field and laboratory stud-ies have been devoted to the Alpine synkinematicmetamorphism, and particularly to the early, high-pressure–low-temperature (HP–LT) metamorphism,which increases from the Fe–Mg carpholite-bearing,low-grade blueschist-facies units of internal Briancon-nais, to the eclogite and ultra-high pressure (UHP)units of the internal massifs (IM) and overthrust-ing ophiolites [9,22 (and references therein)]. Thepeak pressure mineral assemblages from the ophio-lites and IM are now clearly dated from the Earlyto Late Eocene. Kinematic and thermal models havebeen coined taking into account the chemical compo-sition of the crust [14]. The HP–UHP metamorphism

Diagenesis and low-temperature metamorphism of Mt. Medvednica, Croatia: Mineral assemblages and phyllosilicate characteristics

New mineral paragenetic, illite Kübler index, chlorite “crystallinity”, apparent crystallite thickness, lattice strain, and K-white mica geobarometric data proved that the Eoalpine (Paleozoic-Mesozoic) metamorphic complex was affected by medium-pressure, high-temperature anchizonal regional metamorphism, whereas the Jurassic ophiolitic mélange and the Late Cretaceous-Paleocene sedimentary sequence of Mt. Medvednica were diagenetically altered. Mineral chemical investigations carried out on phyllosilicate flakes found in various microstructural positions revealed complete mineral chemical homogenization of chlorite and K-white mica of selected slate samples from the Eoalpine (Paleozoic-Mesozoic) metamorphic complex. One possible explanation of this feature is an Alpine (Cretaceous) regional metamorphic event with polyphase deformational history. Variscan low-temperature metamorphism overprinted by an Alpine (Cretaceous) event, with temperatures at least as high as those of the Variscan one, may be an alternative, although more complicated explanation. However, no isotope geochronological evidence supports this assumption. At present only one metamorphic event can be detected. Its physical conditions were ca. 350-400 °C on the basis of illite Kübler index and chlorite Al(IV) empirical thermometers and 3-4 kbar using K-white mica b cell dimension measurements.

Magnetic fabric constraints on oroclinal bending of the Texas and Coffs Harbour blocks: New England Orogen, eastern Australia

Abstract We have carried out a magnetic fabric study on deformed, clay-rich rocks from the Carboniferous Texas beds and Coffs Harbour Association in order to test the hypothesis of oroclinal bending of the Texas and Coffs Harbour blocks of the southern New England Orogen, eastern Australia. The magnetic susceptibility is dominated by paramagnetic phyllosilicates, with site-dependent contributions from the ferromagnetic minerals pyrrhotite, magnetite and hematite. Pyrrhotite is ubiquitous in the Texas block and in the central part of the Coffs Harbour block, and may be important as an indicator of the grade of low-temperature metamorphism. The magnetic fabric results, in general, show good agreement with structural observations in the Texas and Coffs Harbour blocks. The magnetic foliation is related to the pervasive cleavage associated with accretionary deformation prior to oroclinal bending. The magnitudes of the anisotropy parameters and the plunge of the magnetic lineation indicate an increase in the intensity of deformation from east of the Coffs Harbour orocline towards the Texas orocline. An imprint from oroclinal deformation is suggested by a significant increase in the anisotropy parameter and by the development of steeply plunging magnetic lineations towards the hinges of the Texas and Coffs Harbour oroclines. The Terrica beds from the Early Permian (Allandale) Terrica inlier in the Texas orocline also show another, pre-tilt induced, tectonic imprint. Remanence data from volcanics in the Alum Rock inlier (293 Ma) and magnetic fabric data from this inlier tentatively constrain the onset of oroclinal bending as prior to extrusion of the Alum Rocks, and the completion of bending as post-Terrica beds deposition and pre-Illawarra Reversal (est. 265 Ma).

Caledonian low-temperature granulite-facies metamorphism in the West Kunlun orogenic belt—SHRIMP Chronological evidence from zircon

The West Kunlun orogenic belt is located at the conjunction of the paleo-Asian tectonic system and the Tethys tectonic system. Petrological and mineralogical studies of the Early Cambrian metamorphic surface crust in this region have shown that in case the metamorphism reached low-temperature granulate facies, the typical mineral assemblage is biotite-garnet-silimanite-K feldspar-plagioclase-quartz. The peak metamorphic temperatures are within the range of 720–740°C and the pressure is 0.6 GPa ±. Three types of metamorphic zircon have been detected in the metamorphic rocks: the complex inclusion-bearing type ; the early relic zircon inclusion-bearing type; and the inclusion-free type. SHRIMP age determination of these three types of metamorphic zircon have revealed that these zircons were formed principally during 400–460 Ma, indicating that pre-Cambrian metamorphic surface crust rocks underwent low-temperature granulite facies metamorphism during the Caledonian. In combination with the geological characteristics of this region, it is considered that when the oceanic basin was closed, there occurred intense intracontinental subduction (type A), bringing part of the Early Cambrian metamorphic basement in this region downwards to the lower crust. Meanwhile, there were accompanied with tectonic deformation at deep levels and medium- to high-grade metamorphism. This study provided important chronological and mineralogical evidence for the exploration of the evolutionary mechanism and process of the West Kunlun Early Paleozoic.

Effect of the chemical composition of the crust on the metamorphic evolution of orogenic wedges

AbstractPetrological data provide a good record of the thermal structure of deeply eroded orogens, and, in principle, might be used to relate the metamorphic structure of an orogen to its deformational history. In this paper, we present two‐dimensional thermal modelling of various subduction models taking into account varying wedge geometry as well as variation of density and topography with metamorphic reactions.The models clearly show that rock type accreted in the wedge has important effects on the thermal regime of orogenic wedges. The thermal regime is dominated by radiogenic heat production. Material having high radioactive heat production, like the granodioritic upper crust, produces high temperature metamorphism (amphibolitic conditions). Material with low radioactive heat production results in low temperature metamorphism of greenschist or blueschist types depending on the thickness of the wedge.Application of this model to seemingly unrelated areas of the Central Alps (Lepontine Dome, Grisons) and Eastern Alps (Tauern Window) explains the coexistence and succession of distinct Barrovian and blueschist facies metamorphic conditions as the result of a single, continuous tectonic process in which the main difference is the composition of the incoming material in the orogenic wedge. Accretion of the European upper continental crust in the Lepontine and Tauern Domes produces Barrovian type metamorphism while accretion of oceanic sediments results in blueschist facies metamorphism in the Valaisan domain.