Zircon U-Pb dating of lower crustal rocks from the G.ry Sowie Massif (Central Sudetes, SW Poland): new insights on the sedimentary origin and the tectono-thermal evolution

Devonian HP‒UHP lithotectonic associations represent pivotal element of Paleozoic evolution of European Variscan belt across the continent from Portugal to Poland. The Góry Sowie Massif (GSM), located in the Central Sudetes, represents one of the best preserved outcrops of lower crustal rocks that experienced protracted Devonian tectono-metamorphic history at the easternmost extremity of the belt. The area is surrounded by Devonian ophiolite remnants (c. 400 Ma; Kryza & Pin, 2010) and by Devonian and Silurian to Carboniferous sedimentary basins in the northern and southern part, respectively. The GSM is mainly composed of paragneisses and subordinate orthogneisses, metabasites and granulite. The dominantly sedimentary association and the overall geotectonic setting contrast with other km-scale granulite complexes in the Bohemian Massif that are dominated by felsic granulites and Late Cambrian orthogneisses that experienced 340 Ma HP metamorphism. Weak Carboniferous overprint makes the GSM a key locality to better understand Devonian stages of formation of HP granulites and provenance of the whole pre-Devonian lithological association. New U‒Pb analyses were carried out on zircons from 4 migmatitic paragneisses, 3 felsic biotite-poor granulites and two biotite-rich granulites in the northern part of the GSM, in order to constrain source provenance and tectono-thermal history of the area

Contrasting P-T-t-d paths of the polycyclic Palaeozoic tectono-metamorphic event in the Southern Chinese Altai: an example from Kalasu area

To understand the polycyclic Palaeozoic tectono-metamorphic evolution of the Southern Chinese Altai, petrological and structural studies together with thermodynamic modelling and dating were carried out in the Cambro-Ordovician metapelitic sequence of the Kalasu area. The sequence is divided into upper, middle and lower crustal orogenic levels according to their metamorphic grade and structural patterns. Metamorphism increases from low to high-grade towards the deeper crustal levels with garnet-biotite schists in the upper crustal level, sillimanite-garnet and staurolite-garnet-sillimanite schists and gneisses in the middle crustal level, and cordierite-sillimanite-K-feldspar migmatites in the lower crustal level. Structural succession involves a subhorizontal S1 foliation folded by NE-SW open to tight and upright F2 folds (with no metamorphism associated), reworking by an orthogonal D3 deformation, characterized by NW-SE open to close F3 folds with moderately plunging axes, steeply dipping S3 axial planes and S3 cleavage. Early Devonian calc-alkaline granitoids intruded the sequence parallel to S1 foliation, whereas Permian undeformed gabbroic bodies were emplaced in the lower crust and granites in the upper crust coevally with D3. The P-T-t-d paths indicate that the crystalline rocks underwent a clockwise evolution marked by Early Devonian burial associated with heating, followed by Permian decompression, in agreement with studies from other parts of the Chinese Altai. The burial is recorded in the middle and lower levels by the presence of g-st-ky-ru relics within the S1 fabric. This stage is related to crustal thickening, whereas heating is related to intrusions of Devonian granite sheets during an extensional setting. A subsequent decompression (around 3-5 kbar) is recorded in all crustal levels, associated with intrusions of gabbro and granite along the southern border of the Chinese Altai and coeval with the last Permian deformation. This last stage is related to the collision between the Junggar arc system and the Chinese Altai orogenic belt

Geochemistry, zircon U‒Pb and Hf isotopic compositions of lower crustal rocks from the Góry Sowie Massif (Central Sudetes, SW Poland): New insights on the sedimentary origin and tectono-thermal evolution

Devonian HP-UHP lithotectonic associations represent a pivotal element of Paleozoic evolution of the European Variscan belt across the continent from Portugal to Poland. The Góry Sowie Massif (GSM), located in the Central Sudetes, represents one of the best preserved outcrops of lower crustal rocks that experienced a protracted Devonian tectono-metamorphic history at the easternmost extremity of the belt. The area is surrounded by Devonian ophiolite remnants and Devonian to Carboniferous sedimentary basins in the northern and southern part, respectively. The GSM is mainly composed of paragneisses and subordinate orthogneisses, metabasites and granulite. The dominantly sedimentary association and the overall geotectonic setting contrast with the other km-scale granulite complexes in the Bohemian Massif that are dominated by felsic granulites and late Cambrian orthogneisses that experienced 340 Ma HP metamorphism. Weak Carboniferous overprint makes the GSM a key locality to better understand the Devonian stages of formation of HP granulites and provenance of the whole pre-Devonian lithological association. New U-Pb and Lu/Hf analyses were carried out on zircons from 4 migmatitic paragneisses, 3 felsic biotite-poor granulites and 2 biotite-rich granulites in the northern part of the GSM, and combined with geochemical analyses in order to constrain a source provenance and tectono-thermal history of the area. The paragneisses dominated by stromatic migmatite and felsic granulites occur as hundred meter-scale bodies associated with metric lenses of amphibolites, mafic and ultramafic rocks in the northern part of the massif

NE Baidrag block, Mongolia, records anticlockwise metamorphic paths at c. 890−790 Ma indicating peri-Rodinian back-arc compression followed with c. 560-520 Ma burial

The Barrovian type metamorphism affecting the peri-Siberian tract of the Central Asian Orogenic Belt is mostly dated indirectly on zircon from (syn-tectonic) magmatic rocks as Late Proterozoic - Early Cambrian. However, in-situ monazite geochronology in micaschists and migmatite gneisses at the northern part of the Precambrian Baidrag block, central Mongolia, revealed that the Baikalian Late Proterozoic - Early Cambrian cycle overprints an earlier Tonian phase of metamorphism. The apparent Barrovian-type zoning ranging from garnet, staurolite, kyanite to kyanite/sillimanite migmatitic gneisses is thus false and points to hidden metamorphic discontinuities and mixed metamorphic histories from different times. Therefore, to decipher and interpret the record of different tectono-metamorphic events it is necessary to unreveal complete P-T-t paths from individual samples. Two localities with Tonian-age monazite show anticlockwise P-T paths: 1) Grt−Sil−Ky gneiss records burial to the sillimanite stability (~720°C, 6.0 kbar) followed by burial to the kyanite stability (~750°C, 9 kbar) and, 2) The Grt−St schist records burial to the staurolite stability field (~620°C, 6 kbar), further followed by almost isothermal burial (~590°C, 8.5 kbar). Based on monazite textural positon, internal zoning, and REE patterns, the time of prograde burial under a thermal gradient of 27-32°C/km is estimated at c. 890−853 Ma and further burial under a geothermal gradient of 18-22°C/km is dated at c. 835−815 Ma. On the other hand three localities with Late Proterozoic to Cambrian monazite ages show clockwise metamorphic paths at variable P-T gradients: 3) P-T conditions of the Grt schist reaches ~5 kbar and 500 °C and 4) the Grt−St−Ky schist reaches conditions of 9 kbar and 670 °C, indicating burial under a geothermal gradient of 20-26 °C/km. 5) Grt-Sil gneiss shows peak of 6-7 kbar and 700-750 °C, indicating melting conditions at 30-32 °C/km gradient. Monazite included in porphyroblasts and in the matrix indicate that these P-T conditions reached under variable geothermal gradient were semi-contemporaneous and occurred between 570 and 520 Ma. By correlation with published zircon ages of 600-530 Ma from granitoid magmatic rocks we suggest that the areas with higher geothermal gradient may be explained by closer vicinity of magmatic intrusions. These P−T and geochronology data from a continuous Barrovian metamorphic section suggest that anticlockwise P−T evolution from c. 930 to 750 Ma can be interpreted as a result of thickening of peri-Rodinian supra-subduction extensional and hot edifice. This metamorphic event was followed by a clockwise P−T evolution from c. 570 to 520 Ma possibly related to the collision of the Baidrag continental active margin with peri-Siberian continental mass further north

Syn-deformational melt percolation through a high-pressure orthogneiss and the exhumation of a subducted continental wedge (Orlica-Śnieżnik Dome, NE Bohemian Massif)

High-pressure granitic orthogneiss of the south-eastern Orlica-Śnieżnik Dome (NE Bohemian Massif) shows relics of a shallow-dipping S1 foliation, reworked by upright F2 folds and a mostly pervasive N-S trending subvertical axial planar S2 foliation. Based on macroscopic observations, a gradual transition perpendicular to the subvertical S2 foliation from banded to schlieren and nebulitic orthogneiss was distinguished. All rock types comprise plagioclase, K-feldspar, quartz, white mica, biotite and garnet. The transition is characterized by increasing presence of interstitial phases along like-like grain boundaries and by progressive replacement of recrystallized K-feldspar grains by fine-grained myrmekite. These textural changes are characteristic for syn-deformational grain-scale melt percolation, which is in line with the observed enrichment of the rocks in incompatible elements such as REEs, Ba, Sr, and K, suggesting open-system behaviour with melt passing through the rocks. The P-T path deduced from the thermodynamic modelling indicates decompression from ~15−16 kbar and ~650-740 ºC to ~6 kbar and ~640 ºC. Melt was already present at the P-T peak conditions as indicated by the albitic composition of plagioclase in films, interstitial grains and in myrmekite. The variably re-equilibrated garnet suggests that melt content may have varied along the decompression path, involving successively both melt gain and loss. The 6-8 km wide zone of vertical foliation and migmatite textural gradients is interpreted as vertical crustal-scale channel where the grain-scale melt percolation was associated with horizontal shortening and vertical flow of partially molten crustal wedge en masse

Structural and geochronological evolution of an accretionary orogenic wedge during Altai orogeny: An example from the Kalasu area (Chinese Altai)

A detailed geological and structural mapping was carried out in the Kalasu area located in the southern Chinese Altai, a small segment of the giant Altai accretionary orogenic wedge, in order to understand its polycyclic evolution. The Kalasu area is predominated by felsic metavolcanic rocks in the northeastern domain and mainly by a metapelitic sequence with subordinate metavolcanic layers, intruded by gneissic granitoids cropping out over the whole area. Structural investigations reveal the presence of three deformation events with variable intensity and degree of preservation. D1 is characterized by a metamorphic S1 foliation moderately to steeply dipping to the NW or locally to the SE, preserved in the northeastern and southwestern domain. The D2 deformation is responsible mainly for the variability in dip of the originally sub-horizontal S1 fabric due to NNW-SSE F2 folding, which is not associated with penetrative metamorphic reworking. Such a structural template was subsequently reworked by orthogonal D3 deformation. This last deformation is characterized by NW-SE steeply plunging isoclinal to close F3 folds, steeply dipping S3 axial planes and S3 foliation, which is delineated by cordierite-K-feldspar leucosome in the central domain where the S1 foliation is entirely transposed. To establish the absolute age of magmatic and metamorphic events, several samples were selected for U-Pb isotope analysis on zircon using the LA-ICPMS technique

Migmatite formation in a crustal-scale shear zone during continental subduction: an example from a high-pressure granitic orthogneiss from the Orlica-Śnieżnik Dome (NE Bohemian Massif)

Petrological study and pseudosection modelling have been carried out in high-grade orthogneisses of the southern domain of the Orlica- Snieznik Dome (NE Bohemian Massif). The studied samples are from an outcrop dominated by two deformation fabrics, a sub-horizontal S1 foliation defined by bands of recrystallized K-feldspar, quartz and plagioclase folded by centimetre- to several metre-scale close to isoclinal folds associated with development of a new subvertical N-S trending foliation S2. Based on field features and textural observations, a gradual transition from banded mylonitic orthogneiss (Type I) to stromatitic (Type II), schlieren (type III) and nebulitic (type IV) textures typical of migmatities can be distinguished. The banded orthogneiss is composed of almost monomineral recrystallized K-feldspar layers (2 to 10 mm thick) alternating with layers of plagioclase and quartz (1 to 4mm thick), parallel to the S1 limb and the axial planar S2 foliation. The stromatitic migmatite shows 1 to 4 mm thick layers with macroscopically diffuse boundaries between plagioclase, quartz and K-feldspar rich domains. Boundaries between quartz and feldspar layers are poorly defined and interlobed with adjacent minerals. The schlieren migmatite is almost isotropic preserving small K-feldspar-rich domains within a matrix characterized by random distribution of phases, whereas in the nebulitic migmatite the microstructure is completely isotropic characterized by random distribution of phases. The transition from the Type I to IV is characterized by increasing nucleation of interstitial phases along like-like grain boundaries, by a decrease of grain size of all phases and by progressive disintegration of recrystallized K-feldspar grains by embayments of fine-grained myrmekite. The mineral assemblage of all types consists of biotite, white micas, garnet, quartz, K-feldspar and plagioclase, and accessory apatite, ilmenite, zircon and monazite. In the mineral equilibria modelling, the core of garnet (alm0.58, py0.02-0.03, grs0.34, sps0.05) and phengite (Si = 3.38-3.20 p.f.u) is consistent with a P-T peak at 10-13 kbar and 720-750 C in the dominant grt-bt-ph-rt-qtz-pl-kfs mineral assemblage. The garnet rim (alm0.68, py0.02-0.03, grs0.11, sps0.21), white mica rim (Si = 3.10 p.f.u) together with unzoned biotite (XFe = 0.76-0.78) match the modelled isopleths in the middle-P part of the grt-bt-ph-ilm-qtz-pl-kfs field to reach the solidus at 78 kbar and 630650 C. In addition, the absence of prograde garnet zoning in the Type I to III suggests that the garnet was completely re-equilibrated during the retrograde history, whereas in the Type IV the HP garnet chemistry was preserved. This is discussed in frame of melt presence in different migmatite types along their P-T path. Based on mineral equilibria modelling it is argued for fluid/melt-fluxed melting at HP conditions and on exhumation. The migmatite textural types are a result of grain-scale melt migration process and not of a localized melt transport in dykes as known from metasediments

Anticlockwise metamorphic paths at ca. 890−790 Ma from the NE Baidrag block, Mongolia, indicate back-arc compression at the Rodinia periphery

The processes leading to the assembly of the Rodinia supercontinent through Grenvillian collisional orogeny are relatively well known. In contrast, accretionary orogenic processes occurring at the supercontinent periphery following Rodinia assembly are poorly understood. To fill this gap, we have identified metamorphic rocks in the Mongolia collage of the Central Asian Orogenic Belt, where numerous data testify for Meso- to Neo-proterozoic magmatic reworking. The tectono-metamorphic evolution of the peri-Siberian tract of the Central Asian Orogenic Belt is mainly characterized by the late Proterozoic-early Cambrian (Baikalian) cycle. However, we document here a Tonian age metamorphism at the northern part of the Precambrian Baidrag block, previously considered as a typical example of the Baikalian metamorphic belt. This study incorporates zircon and in-situ monazite geochronology linked to P-T modelling of Grt-Sil-Ky migmatite gneiss and Grt-St micaschist. Grt-Sil-Ky gneiss records initial burial to the sillimanite stability field at ~720 °C and 6.0 kbar followed by further burial to the kyanite stability field at ~750 °C and ~9 kbar and decompression to ~650 °C and ~8 kbar. The Grt-St schist records initial burial to the staurolite stability field at ~620 °C and 6 kbar, followed by further burial to ~590 °C and 8.5 kbar. The monazite data yield a continuum of 207Pb-corrected 238U/206Pb dates of ca. 926−768 Ma in the Grt-Sil-Ky gneiss, and ca. 937−754 Ma in the Grt-St schist. Based on monazite textural positon, internal zoning, and REE patterns, the time of prograde burial to 6.0 kbar under a thermal gradient of 27-32 °C/km is estimated at ca. 890−853 Ma. It is not clear whether such high-grade conditions prevailed until a phase of further burial under a geothermal gradient of 18-22 °C/km dated at ca. 835−815 Ma. The late monazite recrystallization at ca. 790 Ma is related to decompression. Additionally, monazite with dates of ca. 568−515 Ma occur as whole grains or as rims with sharp boundaries on Tonian monazite in Grt-St schist suggesting a minor Baikalian overprint. Metamorphic zircon rims with Th/U ratios of ~0.01-0.06 in Grt-Sil-Ky gneiss with 877±7 Ma age, together with lower intercepts of detrital zircon discordia lines in both Grt-Sil-Ky gneiss and Grt-St schist further support the Tonian age of high-grade metamorphism. The anticlockwise P-T evolution is interpreted as a result of thickening of a supra-subduction extensional and hot edifice - probably of back-arc or arc type. This kind of prograde metamorphism has so far only been described on the northern part of the Tarim block and was interpreted to be a result of initiation of peri- Rodinian subduction of the Mirovoi Ocean. The geodynamic consequences of a unique discovery of Tonian metamorphism are discussed in terms of tectonic switch related to initiation of peri-Rodinian oceanic subduction during supercontinent assembly, followed by strong mechanical coupling potentially related to onset of Rodinia dispersal

P-T-t-d evolution of orogenic middle crust of the Roc de Frausa Massif (Eastern Pyrenees): a result of horizontal crustal flow and Carboniferous doming?

Structural, petrological and textural studies are combined with phase equilibria modelling of metapelites from different structural levels of the Roc de Frausa Massif in the Eastern Pyrenees. The pre‐Variscan lithological succession is divided into the Upper, Intermediate and Lower series by two orthogneiss sheets and intruded by Variscan igneous rocks. Structural analysis reveals two phases of Variscan deformation. D1 is marked by tight to isoclinal small‐scale folds and an associated flat‐lying foliation (S1) that affects the whole crustal section. D2 structures are characterized by tight upright folds facing to the NW with steep NE-SW axial planes. D2 heterogeneously reworks the D1 fabrics, leading to an almost complete transposition into a sub‐vertical foliation (S2) in the high‐grade metamorphic domain. All structures are affected by late open to tight, steeply inclined south‐verging NW-SE folds (F3) compatible with steep greenschist facies dextral shear zones of probable Alpine age. In the micaschists of the Upper series, andalusite and sillimanite grew during the formation of the S1 foliation indicating heating from 580 to 640 °C associated with an increase in pressure. Subsequent static growth of cordierite points to post‐D1 decompression. In the Intermediate series, a sillimanite-biotite-muscovite‐bearing assemblage that is parallel to the S1 fabric is statically overgrown by cordierite and K‐feldspar. This sequence points to ~1 kbar of post‐D1 decompression at 630-650 °C. The Intermediate series is intruded by a gabbro-diorite stock that has an aureole marked by widespread migmatization. In the aureole, the migmatitic S1 foliation is defined by the assemblage biotite-sillimanite-K‐feldspar-garnet. The microstructural relationships and garnet zoning are compatible with the D1 pressure peak at ~7.5 kbar and ~750 °C. Late‐ to post‐S2 cordierite growth implies that F2 folds and the associated S2 axial planar leucosomes developed during nearly isothermal decompression to 5 kbar at suprasolidus conditions. Almost complete consumption of garnet and late cordierite growth points to post‐D2 equilibration at <4 kbar and <750 °C. The early metamorphic history associated with the S1 fabric is interpreted as a result of horizontal middle crustal flow associated with progressive heating and possible burial. The upright F2 folding and S2 foliation are associated with a pressure decrease coeval with the intrusion of mafic magma at mid‐crustal levels. The D2 tectono‐metamorphic evolution may be explained by a crustal‐scale doming associated with emplacement of mafic magmas into the core of the dome

Are The Chinese Altai 'terranes' the result of juxtaposition of different crustal levels during Late Devonian and Permian orogenesis?

The general structure of the Chinese Altai has been traditionally regarded as being formed by five tectono- stratigraphic 'terranes' bounded by large-scale faults. However, numerous detrital zircon studies of the Paleozoic volcano-sedimentary sequences shown that the variably metamorphosed Cambro-Ordovician sequence, known as the Habahe Group, is present at least in four 'terranes'. It structurally represents deepest rocks unconformably covered by Devonian and Carboniferous sedimentary and volcanic rocks. Calc-alkaline, mostly Devonian, granitoids that intruded all the terranes revealed their syn-subduction related setting. Geochemistry and isotope features of the syn-subduction granitoids have shown that they originated mainly from the melting of youthful sediments de- rived from an eroded Ordovician arc further north. In contrast, Permian alkaline granitoids, mostly located in the southern part of the Chinese Altai, reflect a post-subduction intraplate setting. The metamorphic evolution of the metasedimentary sequences shows an early MP-MT Barrovian event, followed by two Buchan events: LP-HT mid-Devonian (ca. 400-380 Ma) and UHT-HT Permian (ca. 300-270 Ma) cycles. The Barrovian metamorphism is linked to the formation of a regional sub-horizontal possibly Early Devonian fabric and the burial of the Cambro- Ordovician sequence. The Middle Devonian Buchan type event is related to intrusions of the syn-subduction granitoids during an extensional setting and followed by Late Devonian-Early Carboniferous NE-SW trending upright folding and crustal scale doming during a general NW-SE shortening, responsible for the exhumation of the hot lower crust. The last Permian deformation formed NW-SE trending upright folds and vertical zones of deformation related to the extrusion of migmatites, anatectic granitoids and granulite rocks, and to the intrusions of gabbros and granites along the southern border of the Chinese Altai. Finally, the Permo-Triassic cooling and thrust systems affected the whole mountain range from ca. 265 to 230 Ma. In conclusion, the Chinese Altai represents different crustal levels of the lower, middle and upper orogenic crust of a single Cambro-Ordovician accretionary wedge, heterogeneously affected by the Devonian polyphase metamorphism and deformation followed by the Permian tectono-thermal reworking event related to the collision with the Junggar arc. It is the interference of Devonian and Permian upright folding events that formed vertical boundaries surrounding the variously exhumed and eroded crustal segments. Consequently, these crustal segments should not be regarded as individual suspect terranes