HYBRID ACCRETIONARY/COLLISIONAL MECHANISM OF PALEOZOIC ASIAN CONTINENTAL GROWTH: NEW PLATE TECTONIC PERSPECTIVE

Continental crust is formed above subduction zones by well-known process of “juvenile crust growth”. This new crust is in modern Earth assembled into continents by two ways: (i) short-lived collisions of continental blocks with the Laurussian or later Eurasian continent along the “Alpine Himalayan collisional/interior orogens” in the heart of the Pangean continental plates realm; and (ii) long lived lateral accretion of ocean-floor fragments along “circum-Pacific accretionary/peripheral orogens” at the border of the PaleoPacific and modern Pacific oceanic plate.Continental crust is formed above subduction zones by well-known process of “juvenile crust growth”. This new crust is in modern Earth assembled into continents by two ways: (i) short-lived collisions of continental blocks with the Laurussian or later Eurasian continent along the “Alpine Himalayan collisional/interior orogens” in the heart of the Pangean continental plates realm; and (ii) long lived lateral accretion of ocean-floor fragments along “circum-Pacific accretionary/peripheral orogens” at the border of the PaleoPacific and modern Pacific oceanic plate

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

MELTING OF ACCRETIONARY WEDGE AND BUILDING MATURE CONTINENTAL CRUST: INSIGHTS FROM THE MAGMATIC EVOLUTION OF THE CHINESE ALTAI OROGEN, CENTRAL ASIA

Tectonic-magmatic reworking of accretionary wedges is a key process responsible for differentiation and stabilization of continental crustal in accretionary orogens. This generic problem can be exemplified by magmatic evolution of the Chinese Altai which represents a high-grade core of the world's largest accretionary system, namely the Central Asian Orogenic Belt (CAOB). In the Chinese Altai, voluminous SilurianDevonian granitoids intruding a greywacke-dominated Ordovician flysch sequence. These intrusions are classically interpreted to originate from predominant (70‒90 %) juvenile (depleted mantle-derived) magma. However, their close temporal and spatial relationship with the regional anatexis of flysch rocks, allows us to examine the possibility that they were mainly derived from flysch rocks.Tectonic-magmatic reworking of accretionary wedges is a key process responsible for differentiation and stabilization of continental crustal in accretionary orogens. This generic problem can be exemplified by magmatic evolution of the Chinese Altai which represents a high-grade core of the world's largest accretionary system, namely the Central Asian Orogenic Belt (CAOB). In the Chinese Altai, voluminous SilurianDevonian granitoids intruding a greywacke-dominated Ordovician flysch sequence. These intrusions are classically interpreted to originate from predominant (70‒90 %) juvenile (depleted mantle-derived) magma. However, their close temporal and spatial relationship with the regional anatexis of flysch rocks, allows us to examine the possibility that they were mainly derived from flysch rocks

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

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