Sm–Nd and U–Pb dating of high-pressure granulites from the Złote and Rychleby Mts (Bohemian Massif, Poland and Czech Republic)

In the Orlica‐?nie?nik complex at the NE margin of the Bohemian Massif, high‐pressure granulites occur as isolated lenses within partially migmatized orthogneisses. Sm–Nd (different grain‐size fractions of garnet, clinopyroxene and/or whole rock) and U–Pb [isotope dilution‐thermal ionization mass spectrometry (ID‐TIMS) single grain and sensitive high‐resolution ion microprobe (SHRIMP)] ages for granulites, collected in the surroundings of ?ervený D?l (Czech Republic) and at Stary Giera?tów (Poland), constrain the temporal evolution of these rocks during the Variscan orogeny. Most of the new ages cluster at c. 350–340 Ma and are consistent with results previously reported for similar occurrences throughout the Bohemian Massif. This interval is generally interpreted to constrain the time of high‐pressure metamorphism. A more complex evolution is recorded for a mafic granulite from Stary Giera?tów and concerns the unknown duration of metamorphism (single, short‐lived metamorphic cycle or different episodes that are significantly separated in time?). The central grain parts of zircon from this sample yielded a large spread in apparent ²⁰⁶Pb/²³⁸U SHRIMP ages (c. 462–322 Ma) with a distinct cluster at c. 365 Ma. This spread is interpreted to be indicative for variable Pb‐loss that affected magmatic protolith zircon during high‐grade metamorphism. The initiating mechanism and the time of Pb‐loss has yet to be resolved. A connection to high‐pressure metamorphism at c. 350–340 Ma is a reasonable explanation, but this relationship is far from straightforward. An alternative interpretation suggests that resetting is related to a high‐temperature event (not necessarily in the granulite facies and/or at high pressures) around 370–360 Ma, that has previously gone unnoticed. This study indicates that caution is warranted in interpreting U–Pb zircon data of HT rocks, because isotopic rejuvenation may lead to erroneous conclusions.     

Amphibolite facies retrograde metamorphism of the Zhujiachong eclogite,SE Dabieshan: 40Ar/39Ar age constraints from argon extraction using UV‐laser microprobe,in vacuo crushing and stepwise heating

The Zhujiachong eclogite in the south‐eastern Dabieshan ultra‐high‐P terrane has been overprinted during retrograde metamorphism, with the development of garnet‐amphibolite mineral assemblages in most rocks in the outcrop. This study is focused on providing age constraints for the retrograde amphibolite facies and greenschist facies mineralogy by ⁴⁰Ar/³⁹Ar dating. By applying a novel approach of combining three different techniques for extracting argon: laser stepwise heating of single grains and small separates, a spot fusion technique by UV‐laser ablation microprobe on polished sections and an in vacuo crushing technique for liberating radiogenic argon from fluid inclusions, it is demonstrated that an internally consistent thermal history can be derived. The ⁴⁰Ar/³⁹Ar ages indicate that phengite formed before 265 Ma, probably during the ultra‐high‐P event. Ages associated with amphibolite facies retrograde metamorphism range from 242 to 217 Ma by the analyses of amphibole. Ages of c. 230 Ma were found for the symplectite matrix that formed during retrogression from eclogite pyroxene. Late stage hydrothermal activity leading to the formation of coarse‐grained paragonite and fluid inclusions in vein amphibole was dated at c. 200 Ma. These age results agree well with the mineral crystallization sequence observed from thin‐sections of the retrograded eclogite: phengite → paragonite and amphibole in matrix → amphibole in the corona.     

Olivine‐bearing symplectites in fractured garnet from eclogite,Moldanubian Zone (Bohemian Massif) – a short‐lived,granulite facies event

Recent petrological studies on high‐pressure (HP)–ultrahigh‐pressure (UHP) metamorphic rocks in the Moldanubian Zone, mainly utilizing compositional zoning and solid phase inclusions in garnet from a variety of lithologies, have established a prograde history involving subduction and subsequent granulite facies metamorphism during the Variscan Orogeny. Two temporally separate metamorphic events are developed rather than a single P–T loop for the HP–UHP metamorphism and amphibolite–granulite facies overprint in the Moldanubian Zone. Here further evidence is presented that the granulite facies metamorphism occurred after the HP–UHP rocks had been exhumed to different levels of the middle or upper crust. A medium‐temperature eclogite that is part of a series of tectonic blocks and lenses within migmatites contains a well‐preserved eclogite facies assemblage with omphacite and prograde zoned garnet. Omphacite is partly replaced by a symplectite of diopside + plagioclase + amphibole. Garnet and omphacite equilibria and pseudosection calculations indicate that the HP metamorphism occurred at relatively low temperature conditions of ~600 °C at 2.0–2.2 GPa. The striking feature of the rocks is the presence of garnet porphyroblasts with veins filled by a granulite facies assemblage of olivine, spinel and Ca‐rich plagioclase. These minerals occur as a symplectite forming symmetric zones, a central zone rich in olivine that is separated from the host garnet by two marginal zones consisting of plagioclase with small amounts of spinel. Mineral textures in the veins show that they were first filled mostly by calcic amphibole, which was later transformed into granulite facies assemblages. The olivine‐spinel equilibria and pseudosection calculations indicate temperatures of ~850–900 °C at pressure below 0.7 GPa. The preservation of eclogite facies assemblages implies that the granulite facies overprint was a short‐lived process. The new results point to a geodynamic model where HP–UHP rocks are exhumed to amphibolite facies conditions with subsequent granulite facies heating by mantle‐derived magma in the middle and upper crust.     

佳木斯地块南缘牡丹江地区高压变质作用：黑龙江杂岩的岩石学和地质年代学

牡丹江地区出露的黑龙江杂岩是由变质基性岩、泥质片岩、大理岩和变硅质岩系列组成的类似于蛇绿岩层序的构造混杂岩,代表了佳木斯地块西南缘碰撞增生的产物。钠长石和绿帘石变斑晶中共生的矿物组合(钠云母+蓝闪石+多硅白云母+绿帘石)的地质温压计估算结果表明,黑龙江杂岩的峰期变质的温压条件为 T=320~480℃, p=800~1600MPa,为典型的绿帘-蓝片岩相高压变质作用。黑龙江杂岩带中泥质片岩所含的多硅白云母单矿物给出的40Ar/39Ar坪年龄为164.9Ma±0.5Ma。根据白云母中K-Ar同位素体系的封闭温度(350~430℃)接近于得到的黑龙江杂岩的峰期变质温度,可以认为40Ar/39Ar坪年龄指示了这期高压变质事件的年龄,也证实了黑龙江杂岩带中普遍存在一期中侏罗世末期的变质-热事件。     

The juxtaposition of eclogite and mid‐crustal rocks in the Orlica–Śnieżnik Dome,Bohemian Massif

Eclogite, felsic orthogneiss and garnet–staurolite metapelite occur in a 5 km long profile in the area of Mi?dzygórze in the Orlica–?nie?nik dome (Bohemian Massif). Petrographic observations and mineral equilibria modelling, in the context of detailed structural work, are used to document the close juxtaposition of high‐pressure and medium‐pressure rocks. The structural succession in all lithologies shows an early shallow‐dipping fabric, S1, that is folded by upright folds and overprinted by a heterogeneously developed subvertical foliation, S2. Late recumbent folds associated with a weak shallow‐dipping axial‐plane cleavage, S3, occur locally. The S1 fabric in the eclogite is defined by alternation of garnet‐rich (grs = 22–29 mol.%) and omphacite‐rich (jd = 33–36 mol.%) layers with oriented muscovite (Si = 3.26–3.31 p.f.u.) and accessory kyanite, zoisite, rutile and quartz, indicating conditions of ～19–22 kbar and ～700–750 °C. The assemblage in the retrograde S2 fabric is formed by amphibole, plagioclase, biotite and relict rutile surrounded by ilmenite and sphene that is compatible with decompression and cooling from ～9 kbar and ～730 °C to 5–6 kbar and 600–650 °C. The S3 fabric contains in addition domains with albite, chlorite, K‐feldspar and magnetite indicating cooling to greenschist facies conditions. The metapelites are composed of garnet, staurolite, muscovite, biotite, quartz, ilmenite and chlorite. Chemical zoning of garnet cores that contain straight ilmenite and staurolite inclusion trails oriented perpendicular to the external S2 fabric indicates prograde growth, from ～5 kbar and ～520 °C to ～7 kbar and ～610 °C, during the formation of the S1 fabric. Inclusion trails parallel with the S2 fabric at garnet and staurolite rims are interpreted to be a continuation of the prograde path to ～7.5 and ～630 °C in the S2 fabric. Matrix chlorite parallel to the S2 foliation indicates that the subvertical fabric was still active below 550 °C. The axial planar S2 fabrics developed during upright folding are associated with retrogression of the eclogite under amphibolite facies conditions, and with prograde evolution in the metapelites, associated with their juxtaposition. The shared part of the eclogite and metapelite P–T paths during the development of the subvertical fabric reflects their exhumation together.     

A comparative U–Th–Pb (zircon–monazite) and 40Ar–39Ar (muscovite–biotite) study of shear zones in northern Victoria Land (Antarctica): implications for geochronology and localized reworking of the Ross Orogen

Mylonitic granites from two shear zones in northern Victoria Land (Antarctica) were investigated in order to examine the behaviour of the U–Th–Pb system in zircon and monazite and of the ⁴⁰Ar–³⁹Ar system in micas during ductile deformation. Meso‐ and micro‐structural data indicate that shear zones gently dip to the NE and SW, have an opposite sense of shear (top‐to‐the‐SW and ‐NE, respectively) and developed under upper greenschist facies conditions. In situ U–Pb dating by laser‐ablation inductively coupled plasma‐mass spectrometry of zircon areas with well‐preserved igneous zoning patterns (c. 490 Ma) confirm that granites were emplaced during the Early Cambrian to Early Ordovician Ross–Delamerian Orogeny. Monazite from the Bier Point Shear Zone (BPSZ) mainly yielded U–Th–Pb ages of c. 440 Ma, in agreement with in‐situ Ar laserprobe ages of syn‐shear muscovite and with most Ar ages of coexisting biotite. The agreement of ages derived from different decay schemes and from minerals with different crystal‐chemical features suggests that isotope transport in the studied sample was mainly controlled by (re)crystallization processes and that the main episode of ductile deformation in the BPSZ occurred at c. 440 Ma. Cathodoluminscence imaging showed that zircon from the BPSZ contains decomposed areas with faint relics of oscillatory zoning. These areas yielded a U–Pb age pattern which mimics that of monazite but is slightly shifted towards older ages, supporting previous studies which suggest that ‘ghost’ structures may be affected by inheritance. In contrast, secondary structures in zircon from the Mt. Emison Shear Zone (MESZ) predominantly consist of overgrowths or totally recrystallized areas and gave U–Pb ages of c. 450 and 410 Ma. The c. 450‐Ma date matches within errors most monazite U–Th–Pb ages and in‐situ Ar ages on biotite aligned along the mylonitic foliation. This again suggests that isotope ages from the different minerals are (re)crystallization ages and constrains the time of shearing in the MESZ to the Late Ordovician. Regionally, results indicate that shear zones were active in the Late Ordovician–Early Silurian and that their development was partially synchronous at c. 440 Ma, suggesting that they belong to a shear‐zone system formed in response to ～NE–SW‐directed shortening. Taking into account the former juxtaposition of northern Victoria Land and SE Australia, we propose that shear zones represent reactivated zones formed in response to stress applied along the new plate margin as a consequence of contractional tectonics associated with the early stages (Benambran Orogeny) of the development of the Late Ordovician–Late Devonian Lachlan Fold Belt.     

佳木斯地块南缘牡丹江地区高压变质作用:黑龙江杂岩的岩石学和地质年代学

牡丹江地区出露的黑龙江杂岩是由变质基性岩、泥质片岩、大理岩和变硅质岩系列组成的类似于蛇绿岩层序的构造混杂岩,代表了佳木斯地块西南缘碰撞增生的产物.钠长石和绿帘石变斑晶中共生的矿物组合(钠云母+蓝闪石+多硅白云母+绿帘石)的地质温压计估算结果表明,黑龙江杂岩的峰期变质的温压条件为T=320～4800℃,p=800--1600MPa,为典型的绿帘-蓝片岩相高压变质作用.黑龙江杂岩带中泥质片岩所含的多硅白云母单矿物给出的~(40)Ar/~(39)Ar坪年龄为164.9Ma±0.5Ma.根据白云母中K~Ar同位素体系的封闭温度(350～430℃)接近于得到的黑龙江杂岩的峰期变质温度,可以认为~(40)Ar/~(39)Ar坪年龄指示了这期高压变质事件的年龄,也证实了黑龙江杂岩带中普遍存在一期中侏罗世末期的变质-热事件.     

Phanerozoic high-pressure eclogite and intermediate-pressure granulite facies metamorphism in the Gyeonggi Massif, South Korea: Implications for the eastward extension of the Dabie–Sulu continental collision zone

Petrological analysis, zircon trace element analysis and SHRIMP zircon U–Pb dating of retrogressed eclogite and garnet granulite from Bibong, Hongseong area, SW Gyeonggi Massif, South Korea provide compelling evidence for Triassic (231.4 ± 3.3 Ma) high-pressure (HP) eclogite facies (M1) metamorphisms at a peak pressure–temperature (P–T) of ca. 16.5–20.0 kb and 775–850 °C. This was followed by isothermal decompression (ITD), with a sharp decrease in pressure from 20 to 10 kb and a slight temperature rise from eclogite facies (M1) to granulite facies (M2), followed by uplift and cooling. Granitic orthogneiss surrounding the Baekdong garnet granulite and the ophiolite-related ultramafic lenticular body near Bibong records evidence for a later Silurian (418 ± 8 Ma) intermediate high-pressure (IHP) granulite facies metamorphism and a prograde P–T path with peak P–T conditions of ca. 13.5 kb and 800 °C. K–Ar ages of biotite from garnet granulites, amphibolites, and granitic orthogneisses in and around the Bibong metabasite lenticular body are 208–219 Ma, recording cooling to about 310 °C after the Early Triassic metamorphic peak. Neoproterozoic zircon cores in the retrogressed eclogite and granitic orthogneiss provide evidence that the protoliths of these rocks were  800 and  900 Ma old, respectively, similar to the ages of tectonic episodes in the Central Orogenic Belt of China. This, and the evidence for Triassic HP/UHP metamorphism in both China and Korea, is consistent with a regional tectonic link within Northeast Asia from the time of Rodinia amalgamation to Triassic continent–continent collision between the North and South China Blocks, and with an eastward extension of the Dabie–Sulu suture zone into the Hongseong area of South Korea.     

Timing of high-pressure metamorphism and exhumation of the eclogite type-locality (Kupplerbrunn–Prickler Halt, Saualpe, south-eastern Austria): constraints from correlations of the Sm–Nd, Lu–Hf, U–Pb and Rb–Sr isotopic systems

Sm–Nd, Lu–Hf, Rb–Sr and SIMS U–Pb data are presented for meta‐gabbroic eclogites from the eclogite type‐locality ( Haüy, 1822 ) Kupplerbrunn–Prickler Halt and other areas of the Saualpe (SE Austria) and Pohorje Mountains (Slovenia). Mg‐rich eclogites derived from early gabbroic cumulates are kyanite‐ and zoisite rich, whereas eclogites with lower Mg contents contain clinozoisite ± kyanite. Calculated P–T conditions at the final stages of high‐pressure metamorphism are 2.2 ± 0.2 GPa at 630–740 °C. Kyanite‐rich eclogites did not yield geologically meaningful Sm–Nd ages due to incomplete Nd isotope equilibration, whereas Sm–Nd multifraction garnet–omphacite regression for a low‐Mg eclogite from Kupplerbrunn yields an age of 91.1 ± 1.3 Ma. The Sm–Nd age of 94.1 ± 0.8 Ma obtained from the Fe‐rich core fraction of this garnet dates the initial stages of garnet growth. Zircon that also crystallized at eclogite facies conditions gives a weighted mean U–Pb SIMS age of 88.4 ± 8.1 Ma. Lu–Hf isotope analysis of a kyanite–eclogite from Kupplerbrunn yields 88.4 ± 4.7 Ma for the garnet–omphacite pair. Two low‐Mg eclogites from the Gertrusk locality of the Saualpe yield a multimineral Sm–Nd age of 90.6 ± 1.0 Ma. A low‐Mg eclogite from the Pohorje Mountains (70 km to the SE) gives a garnet–whole‐rock Lu–Hf age of 93.3 ± 2.8 Ma. These new age data and published Sm–Nd ages of metasedimentary host rocks constrain the final stages of the eo‐Alpine high‐pressure event in the Saualpe–Pohorje part of the south‐easternmost Austroalpine nappe system suggesting that garnet growth in the high‐pressure assemblages started at c. 95–94 Ma and ceased at c. 90–88 Ma, probably at the final pressure peak. Zircon and amphibole crystallization was still possible during incipient isothermal decompression. Rapid exhumation of the high‐pressure rocks was induced by collision of the northern Apulian plate with parts of the Austroalpine microplate, following Jurassic closure of the Permo‐Triassic Meliata back‐arc basin.     

Constraining the P–T path of a MORB-type eclogite using pseudosections, garnet zoning and garnet-clinopyroxene thermometry: an example from the Bohemian Massif

A mid‐ocean ridge basalt (MORB)‐type eclogite from the Moldanubian domain in the Bohemian Massif retains evidence of its prograde path in the form of inclusions of hornblende, plagioclase, clinopyroxene, titanite, ilmenite and rutile preserved in zoned garnet. Prograde zoning involves a flat grossular core followed by a grossular spike and decrease at the rim, whereas Fe/(Fe + Mg) is also flat in the core and then decreases at the rim. In a pseudosection for H₂O‐saturated conditions, garnet with such a zoning grows along an isothermal burial path at c. 750 °C from 10 kbar in the assemblage plagioclase‐hornblende‐diopsidic clinopyroxene‐quartz, then in hornblende‐diopsidic clinopyroxene‐quartz, and ends its growth at 17–18 kbar. From this point, there is no pseudosection‐based information on further increase in pressure or temperature. Then, with garnet‐clinopyroxene thermometry, the focus is on the dependence on, and the uncertainties stemming from the unknown Fe³⁺ content in clinopyroxene. Assuming no Fe³⁺ in the clinopyroxene gives a serious and unwarranted upward bias to calculated temperatures. A Fe³⁺‐contributed uncertainty of ±40 °C combined with a calibration and other uncertainties gives a peak temperature of 760 ± 90 °C at 18 kbar, consistent with no further heating following burial to eclogite facies conditions. Further pseudosection modelling suggests that decompression to c. 12 kbar occurred essentially isothermally from the metamorphic peak under H₂O‐undersaturated conditions (c. 1.3 mol.% H₂O) that allowed the preservation of the majority of garnet with symplectitic as well as relict clinopyroxene. The modelling also shows that a MORB‐type eclogite decompressed to c. 8 kbar ends as an amphibolite if it is H₂O saturated, but if it is H₂O‐undersaturated it contains assemblages with orthopyroxene. Increasing H₂O undersaturation causes an earlier transition to SiO₂ undersaturation on decompression, leading to the appearance of spinel‐bearing assemblages. Granulite facies‐looking overprints of eclogites may develop at amphibolite facies conditions.     

A geochronological and petrological study of anatectic paragneiss and associated granite dykes from the Day Nui Con Voi metamorphic core complex,North Vietnam: constraints on the timing of metamorphism within the Red River shear zone

The Red River shear zone (RRSZ) is a major left‐lateral strike‐slip shear zone, containing a ductilely deformed metamorphic core bounded by brittle strike‐slip and normal faults, which stretches for >1000 km from Tibet through Yunnan and North Vietnam to the South China Sea. The RRSZ exposes four high‐grade metamorphic core complexes along its length. Various lithologies from the southernmost core complex, the Day Nui Con Voi (DNCV), North Vietnam, provide new constraints on the tectonic and metamorphic evolution of this region prior to and following the initial India–Asia collision. Analysis of a weakly deformed anatectic paragneiss using P–T pseudosections constructed in the MnO–Na₂O–CaO–K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O–TiO₂–O (MnNCKFMASHTO) system provides prograde, peak and retrograde metamorphic conditions, and in situ U–Th–Pb geochronology of metamorphic monazite yields texturally controlled age constraints. Tertiary metamorphism and deformation, overprinting earlier Triassic metamorphism associated with the Indosinian orogeny and possible Cretaceous metamorphism, are characterized by peak metamorphic conditions of ~805 °C and ~8.5 kbar between c. 38 and 34 Ma. Exhumation occurred along a steep retrograde P–T path with final melt crystallizing at the solidus at ≥~5.5 kbar at ~790 °C. Further exhumation at ~640–700 °C and ~4–5 kbar at c. 31 Ma occurred at subsolidus conditions. U–Pb geochronological analysis of monazite from a strongly deformed pre‐kinematic granite dyke from the flank of the DNCV provides further evidence for exhumation at this time. Magmatic grains suggest initial emplacement at 66.0 ± 1.0 Ma prior to the India–Asia collision, whereas grains with metamorphic characteristics indicate later growth at 30.6 ± 0.4 Ma. Monazite grains from a cross‐cutting post‐kinematic dyke within the core of the DNCV antiform provide a minimum age constraint of 25.2 ± 1.4 Ma for the termination of fabric development. A separate and significant episode of monazite growth at c. 83–69 Ma is suggested to be the result of fluid‐assisted recrystallization following the emplacement of magmatic units.     

The ultra‐high pressure coronitic and pseudomorphous reactions in a metagranodiorite from the Brossasco‐Isasca Unit,Dora‐Maira Massif,western Italian Alps: a petrographic study and equilibrium thermodynamic modelling

Metagranodiorite samples from the Brossasco‐Isasca Unit, Dora‐Maira Massif, western Alps, show pseudomorphous and coronitic textures where igneous minerals were partially replaced by ultra‐high pressure (UHP) metamorphic assemblages. The original magmatic paragenesis consisted of quartz, plagioclase, K‐feldspar, biotite and minor phases. During UHP metamorphism, the plagioclase (site P) was replaced by zoisite, jadeite, quartz, K‐feldspar and kyanite, and coronitic reactions developed between biotite and adjacent minerals. At the original igneous biotite–quartz contact (site A), a single corona of poorly zoned garnet is developed, whereas at the biotite–K‐feldspar (site B) and biotite–plagioclase (site C) contacts, composite coronas are formed. Integration of results from petrographic observations, calculations of mineral stoichiometry and thermodynamic calculations of mineral stability has allowed the determination of the metamorphic reactions involved and the estimation of the metamorphic conditions, which reached as high as 24 kbar and 650 °C. Accurate microanalysis by energy‐dispersive spectroscopy (EDS) and statistical analysis of the data allowed us to identify, for the first time in a natural Na‐pyroxene of metagranitoid rocks, the end‐member Ca‐Eskola.     

Mineralization events in the Xiaokele porphyry Cu (–Mo) deposit,NE China: Evidence from zircon U–Pb and K‐feldspar Ar–Ar geochronology and petrochemistry

The Great Xing'an Range (GXR), Northeast (NE) China, is a major polymetallic metallogenic belt in the eastern segment of the Central Asian Orogenic Belt. The newly discovered Xiaokele porphyry Cu (–Mo) deposit lies in the northern GXR. Field geological and geochronological studies have revealed two mineralization events in this deposit: early porphyry‐type Cu (–Mo) mineralization, and later vein‐type Cu mineralization. Previous geochronological studies yielded an age of ca. 147 Ma for the early Cu (–Mo) mineralization. Our ⁴⁰Ar/³⁹Ar dating yielded ⁴⁰Ar/³⁹Ar plateau ages of 124.8 ± 0.4 to 124.3 ± 0.4 Ma on K‐feldspar in altered Cu‐mineralized diorite porphyrite dikes that represent the overprinting vein‐type Cu mineralization, consistent with zircon U–Pb ages of the diorite porphyrite (126.4 ± 0.5 to 125.0 ± 0.5 Ma). The Cr and Ni contents and Mg^# of the Xiaokele diorite porphyrites are high. The diorite porphyrites at Xiaokele are enriched in light rare‐earth elements (REEs), and large‐ion lithophile elements (e.g., Rb, Ba, and K), are depleted in heavy REEs and high‐field‐strength elements (e.g., Nb, Ta, and Ti), and have weak negative ε_Hf(t) values (+0.29 to +5.27) with two‐stage model ages (T_DM2) of 1,164–845 Ma. Given the regional tectonic setting in Early Cretaceous, the ore‐bearing diorite porphyrites were likely formed in an extensional environment related to lithospheric delamination and asthenospheric upwelling induced by subduction of the Paleo‐Pacific Plate. These tectonic events caused large‐scale magmatic activity, ore mineralization, and lithospheric thinning in NE China.     

Cretaceous evolution of a metamorphic core complex,the Veporic unit,Western Carpathians (Slovakia): P–T conditions and in situ40Ar/39Ar UV laser probe dating of metapelites

Alpine metamorphism, related to the development of a metamorphic core complex during Cretaceous orogenic events, has been recognized in the Veporic unit, Western Carpathians (Slovakia). Three metamorphic zones have been distinguished in the metapelites: 1, chloritoid + chlorite + garnet; 2, garnet + staurolite + chlorite; 3, staurolite + biotite + kyanite. The isograds separating the metamorphic zones have been modelled by discontinuous reactions in the system K₂O–FeO–MgO–Al₂O₃–SiO₂–H₂O (KFMASH). The isograds are roughly parallel to the north‐east‐dipping foliation related to extensional updoming along low‐angle normal faults. Thermobarometric data document increasing P–T conditions from c. 500 °C and 7–8 kbar to c. 620 °C and 9–10 kbar, reflecting a coherent metamorphic field gradient from greenschist to middle amphibolite facies. ⁴⁰Ar/³⁹Ar data obtained by high spatial resolution in situ ultraviolet (UV) laser ablation of white micas from the rock slabs constrain the timing of cooling and exhumation in the Late Cretaceous. Mean dates are between 77 and 72 Ma; however, individual white mica grains record a range of apparent ⁴⁰Ar/³⁹Ar ages indicating that cooling below the blocking temperature for argon diffusion was not instantaneous. The reconstructed metamorphic P–T–t path is ‘clockwise’, reflecting post‐burial decompression and cooling during a single Alpine orogenic cycle. The presented data suggest that the Veporic unit evolved as a metamorphic core complex during the Cretaceous growth of the Western Carpathian orogenic wedge. Metamorphism was related to collisional crustal shortening and stacking, following closure of the Meliata Ocean. Exhumation was accomplished by synorogenic (orogen‐parallel) extension and unroofing in an overall compressive regime.