Marian Janák

Subduction along and within the Baltoscandian margin during closing of the Iapetus Ocean and Baltica-Laurentia collision

The recent discovery of ultrahigh-pressure (UHP) mineral parageneses in the far-transported (greater than 400 km) Seve Nappe Complex of the Swedish Caledonides sheds new light on the subduction system that dominated the contracting Baltoscandian margin of continental Baltica during the Ordovician and culminated in collision with Laurentia in the Silurian to Early Devonian. High-grade metamorphism of this Neoproterozoic to Cambrian rifted, extended, dike-intruded outer-margin assemblage started in the Early Ordovician and may have continued, perhaps episodically, until collision of the continents at the end of this period. The recent discovery of UHP kyanite eclogite in northern Jamtland (west-central Sweden) yields evidence of metamorphism at depths of 100 km. Although UHP rocks are only locally preserved from retrogression during the long-distance transport onto the Baltoscandian platform, these high-pressure parageneses indicate that deep subduction played an important role in the tectonothermal history of the complex. Based on existing isotopic age data, this UHP metamorphism occurred in the Late Ordovician, shortly before, or during, the initial collision between the continents (Scandian orogeny). In some central parts of the complex, migmatization and hot extrusion occurred in the Early Silurian, giving way to thrust emplacement across the Baltoscandian foreland basin and platform that continued into the Early Devonian. Identification of HP/UHP metamorphism at different levels within the Scandian allochthons, definition of their pressure-temperature-time paths, and recognition of their vast transport distances are essential for an understanding of the deeper structural levels of the orogen in the hinterland (e.g., the Western Gneiss Region), where the attenuated units were reworked together during the Early Devonian.

Two types of metamorphic monazite with contrasting La/Nd, Th, and Y signatures in an ultrahigh-pressure metapelite from the Pohorje Mountains, Slovenia; indications for pressure-dependent REE; exchange between apatite and monazite?

Abstract Two monazite generations (M1; M2) were distinguished in a kyanite-garnet gneiss from the UHP terrain of the Pohorje Mountains, Slovenia. P-T estimates reveal a peak event at 760 °C/2.6 GPa and isothermal decompression down to 700 °C/0.6 GPa. M1 type provides a Th-U-Pb mean date of 100 ± 6 Ma, ThO2 contents between 3-7 wt%, Y2O3 values <0.3 wt%, and La/Nd ratios (1.2-1.4) that are clearly higher than for the whole-rock La/Nd (1.1). The absence of Y zoning in M1 and the lack of monazite inclusions in garnet indicate that M1 formed after the main stage of garnet growth (>1.2 MPa), probably close to the P-T peak. M2 type is slightly younger than M1 (74 ± 16 Ma), and has a lower La/Nd (0.3-0.9), lower ThO2 (0.1-5 wt%), and higher Y2O3 (up to 3.2 wt%). Most M2 monazites occur as tiny needles within apatite (subtype M2-a) or along apatite margins (M2-b). Parasitic growth of M2-a and -b from apatite is supported by its low ThO2 (<1 wt%) and La/Nd (<0.5). Isolated matrix grains (M2-c) and overgrowths around M1 (M2-d) have slightly higher La/Nd (0.5-0.9) and higher ThO2 (5 wt%) and were supplied from an apatite and M1 source. Elevated yttrium suggests that M2 formed during decompression, when garnet was consumed and Y was released. These observations imply that at UHP conditions MREE-rich apatite coexisted with low-MREE M1 monazite and reacted during decompression to Ca-F-apatite plus MREE-rich M2 monazite. This provides strong arguments that REE-partitioning between apatite and monazite is pressure-dependent.

Kinematics and metamorphism of a cretaceous core complex: The veporic unit of the Western Carpathians

Abstract The Veporic basement and its Permian-Mesozoic cover experienced medium-pressure, collision-related metamorphism during the Cretaceous. Geothermobarometric calculations of Alpine mineral assemblages indicate peak conditions of 8–12 kbar and 550–600°C in the deepest-exposed basement, and up to 8 kbar and 450–500°C in the Permian metasediments. After having reached the metamorphic peak conditions (at around 110 Ma, 40 Ar/ 39 Ar on amphiboles), the thermally softened Veporic unit was exhumed probably due to the underplating of a buoyant Tatric-Fatric crust. Exhumation was triggered by extensional denudation of former upper-crustal thrust units, overlying the Veporic unit. Unroofing was accomplished due to orogenparallel, top-to-east extension along low-angle, ductile normal shear zones. The area collapsed and rapidly cooled at 90-80 Ma ( 40 Ar/ 39 Ar on micas). As revealed by the structural record, the doming and tectonic exhumation of the Veporic core occurred in an overall contractional regime and was followed by additional Late Cretaceous—Early Tertiary shortening events.

Microdiamond discovered in the Seve Nappe (Scandinavian Caledonides) and its exhumation by the “vacuum-cleaner” mechanism

When a continent collides with an island arc or other continent, continental crust of the subducted continent may be buried to depths exceeding 100 km, and exposed to pressures that can cause formation of coesite and diamond. This process leads to substantial density increase in SiO2-rich rocks and, in turn, to a reduction of the buoyancy of the subducted material, which should inhibit exhumation. Nevertheless, coesite- and diamond-bearing continental crustal rocks are known from several occurrences worldwide. We report on the discovery of microdiamond in kyanite-garnet gneiss from allochthonous metasediments of the Seve Nappe Complex in the Scandinavian Caledonides. Our discovery calls for general reconsideration of existing exhumation models of deeply subducted continental crust. We propose that the diamond-bearing rocks were subducted in an arc-continent collision setting, and their exhumation was facilitated by local pressure reduction resulting from extraction of the forearc lithospheric block.

Eclogite-hosting metapelites from the Pohorje Mountains (Eastern Alps): P-T evolution, zircon geochronology and tectonic implications

Phase-equilibrium modelling, geothermobarometry, ion-microprobe dating and mineral chemistry of zircon have been used to constrain the P-T-t evolution of metapelitic kyanite-bearing gneisses from the ultrahigh-pressure (UHP) metamorphic terrane of the Pohorje Mountains in the Eastern Alps. These eclogite-hosting rocks are part of the continental basement of the Austroalpine nappes. Based on calculated phase diagrams in the system Na2O-CaO-K2O-FeO-MgO-MnO-Al2O3-SiO2-H2O (NCKFMMnASH) and conventional geothermobarometry, the garnet-phengite-kyanite-quartz assemblages of gneisses record metamorphic conditions of 2.2-2.7 GPa at 700-800 � C. These are considered as minima because of the potential for a diffusion-related modification and re- equilibration of the garnet and phengite during early stages of decompression. It is therefore most likely that the gneisses experienced the same peak UHP metamorphism at � 3 GPa as associated kyanite eclogites. Decompression and cooling to � 0.5 GPa and 550 � C led to the consumption of garnet and phengite, and the development of matrix consisting of biotite, plagioclase, K-feldspar � sillimanite and staurolite. Textures and phase diagrams suggest a low extent of partial melting during decompression. Cathodoluminescence images as well as zircon chemistry reveal cores encompassed by two types of metamorphic zircon rims. Ion probe U-Pb dating of three zircon cores yielded Permian (286 � 10, 258 � 7 Ma) and Triassic (238 � 7 Ma) concordia ages. The zircon rims are Cretaceous with a mean concordia age of 92.0 � 0.5 Ma and some cores gave a similar age. The Cretaceous zircons all exhibit very low Th/U ratio (,0.02) typical of metamorphic origin. In these zircons, nearly flat HREE patterns, (Lu/Gd)N ¼ 1-4, and only small negative Eu anomalies indicate formation in the presence of garnet and absence of plagioclase, which is corroborated by occurrence of Mg- and Ca-rich garnet inclusions. Therefore, these zircons are interpreted to record the Cretaceous HP/UHP metamorphism. The 92.0 � 0.5 Ma age obtained in this study agrees with that (93-91 Ma) determined earlier in the Pohorje eclogites from U/Pb zircon, Sm-Nd and Lu-Hf garnet-whole-rock dating. This implies that the eclogites and their country rocks were subducted and exhumed together as a coherent piece of continental crust. There is no evidence for a melange-like assemblage of rocks, which followed different P-T-t paths, or several subduction and exhumation cycles as proposed for some other UHP metamorphic terranes.

Pressure–temperature estimates on the Tjeliken eclogite: new insights into the (ultra)-high-pressure evolution of the Seve Nappe Complex in the Scandinavian Caledonides

Abstract The metamorphic evolution of the Tjeliken eclogite, occurring within the Seve Nappe Complex of northern Jämtland (Swedish Caledonides), is presented here. The prograde part of the pressure and temperature (P–T) path is inferred from the mineral inclusions (pargasitic amphibole) in garnet and intracrystalline garnet exsolutions in omphacite. Peak metamorphic conditions of 25–26 kbar at 650–700 °C are constrained from geothermobarometry for the peak-pressure assemblage garnet+omphacite+phengite+quartz+rutile, using the garnet–clinopyroxene Fe–Mg exchange thermometer in combination with the net-transfer reaction (6 diopside+3 muscovite=3 celadonite +2 grossular+pyrope) geobarometer, the average P–T method of Thermocalc and pseudosection modelling. Quartz inclusions with well-developed radial cracks were identified within omphacite, which suggest that the studied rock could have been buried down to the coesite stability field. Post-peak P–T evolution is inferred from diopside–plagioclase symplectites and amphibole coronas around garnet. Previous studies in northern Jämtland suggest a substantial gap between the P–T conditions of the Lower and Middle Seve nappes: 14–16 kbar and 550–680 °C and 20–30 kbar and 700–800 °C, respectively. The Tjeliken eclogite has been considered previously to be a part of Lower Seve by most authors, but the new P–T data suggest that it may be an isolated klippe of Middle Seve.

Pressure–temperature evolution of a kyanite–garnet pelitic gneiss from Åreskutan: evidence of ultra-high-pressure metamorphism of the Seve Nappe Complex, west-central Jämtland, Swedish Caledonides

Abstract New evidence is presented for ultra-high-pressure metamorphism of kyanite–garnet pelitic gneiss in the Åreskutan Nappe of the Seve Nappe Complex, in the central part of the Scandinavian Caledonides. Modelled phase equilibria for a peak pressure assemblage garnet+phengite+kyanite+quartz (coesite) in the NCKFMMnASH system record pressure and temperature conditions of c. 26–32 kbar at 700–720 °C, possibly up to ultra-high-pressure conditions. Subsequent decompression, simultaneous with an increase of temperature to c. 800–820 °C, led to partial melting largely owing to the dehydration and breakdown of phengite. Based on existing isotope age data, we conclude that the Middle Seve Nappe in central Jämtland experienced deep subduction in the late(st) Ordovician, prior to decompression and partial melting of the pelitic protoliths during Early Silurian extrusion, giving way in the Mid to Late Silurian to thrusting on to the Baltoscandian platform. Nappe emplacement probably continued into and through the Early Devonian.

Nitrogen-bearing fluids, brines and carbonate liquids in Variscan migmatites of the Tatra Mountains, Western Carpathians - heritage of high-pressure metamorphism

Gaseous inclusions in migmatites of the Tatra Mountains contain 5–100 mol.% nitrogen, contrasting with essentially the nitrogen-absent, mostly CO 2 -CH 4 composition of fluid inclusions in other migmatites. While CO 2 predominates in the Tatra metapelite migmatites, pure nitrogen inclusions are typical of the associated metabasite migmatites. CaCl 2 -rich brines (>30 wt.% NaCl eq.), halite, graphite and carbonate globules are the additional phases accompanying the nitrogen-rich fluids in the migmatite leucosomes. Monophase, crack-bound carbonate inclusions, consisting of calcite in metabasites and of Mg-bearing siderite in Fe-rich metapelites, are believed to represent a carbonate liquid, coexisting with and exsolving from the CO 2 -N 2 -brine fluid at temperatures between 550–700°C. Compositions and densities of the N 2 -CO 2 fluids and associated brines are similar to those in high-pressure granulites and eclogites. Therefore, these volatiles are interpreted to have been inherited from a high-pressure stage, pre-dating the migmatisation. Together with mineral textures and assemblages, the fluid-inclusion record is indicative of a substantial crustal thickening in the Western Carpathians during the Variscan orogeny. Essentially pure CO 2 -N 2 , graphite-saturated fluids in metapelites have been generated at dry conditions ( X H 2 O = 0.2-0.25), resembling those of typical granulite-facies metamorphism.

Ultramafic cumulates of oceanic affinity in an intracontinental subduction zone: ultrahigh-pressure garnet peridotites from Pohorje (Eastern Alps, Slovenia)

Publisher Summary Continental subduction and exhumation have been recognized as processes common to continental collision, which has led to the widespread occurrence of ultrahigh-pressure (UHP) metamorphosed rocks in collision belts. Garnet peridotites are subordinate but common constituents in nearly all UHP terranes. They are classified as crustal or mantle derived depending on their emplacement within the crust prior to or during continental subduction. The origin of peridotites is diverse and includes ultramafic cumulates and residual mantle of subcontinental, oceanic, or sub-arc mantle affinity. Identification of this origin is not always straightforward due to complex, often multiphase metamorphic histories, but these rocks may provide important information about the geodynamic and premetamorphic history of their host terranes. The shallow level of intrusion of the garnet peridotite protolith indicates that the SBUC mantle rocks were exhumed at the time of magma emplacement. Remnants of igneous minerals allow identification of the composition of melts from which cumulates crystallize. The rocks are of oceanic affinity and form the more primitive equivalent to associated eclogitized metagabbros within the SBUC and surrounding continental crust. The oceanic affinity of mafic and ultramafic units in Pohorje indicates a depleted asthenospheric mantle source.

Rare-element granitic pegmatite of Miocene age emplaced in UHP rocks from Visole, Pohorje Mountains (Eastern Alps, Slovenia): accessory minerals, monazite and uraninite chemical dating

Abstract The granitic pegmatite dike intruded the Cretaceous UHP rocks at Visole, near Slovenska Bistrica, in the Pohorje Mountains (Slovenia). The rock consists mainly of K-feldspar, albite and quartz, subordinate muscovite and biotite, while the accessory minerals include spessartine-almandine, zircon, ferrocolumbite, fluorapatite, monazite- (Ce), uraninite, and magnetite. Compositions of garnet (Sps48-49Alm45-46Grs + And3-4 Prp1.5-2), metamict zircon with 3.5 to 7.8 wt. % HfO2 [atom. 100Hf/(Hf + Zr) = 3.3-7.7] and ferrocolumbite [atom. Mn/(Mn + Fe) = 0.27-0.43, Ta/(Ta + Nb) = 0.03-0.46] indicate a relatively low to medium degree of magmatic fractionation, characteristic of the muscovite - rare-element class or beryl-columbite subtype of the rare-element class pegmatites. Monazite-(Ce) reveals elevated Th and U contents (≤11 wt. % ThO2, ≤5 wt. % UO2). The monazite-garnet geothermometer shows a possible precipitation temperature of ~495 ± 30 °C at P~4 to 5 kbar. Chemical U-Th-Pb dating of the monazite yielded a Miocene age (17.2 ± 1.8 Ma), whereas uraninite gave a younger (~14 Ma) age. These ages are comtemporaneous with the main crystallization and emplacement of the Pohorje pluton and adjacent volcanic rocks (20 to 15 Ma), providing the first documented evidence of Neogene granitic pegmatites in the Eastern Alps. Consequently, the Visole pegmatite belongs to the youngest rare-element granitic pegmatite populations in Europe, together with the Paleogene pegmatite occurrences along the Periadriatic (Insubric) Fault System in the Alps and in the Rhodope Massif, as well as the Late Miocene to Pliocene pegmatites in the Tuscany magmatic province (mainly on the Island of Elba).