Modelling white mica pressure‐temperature‐time (P‐T‐t) paths using thermobarometric and 40Ar/39Ar thermochronologic data

White micas are major rock forming minerals in igneous and metamorphic rocks, and their chemical and isotopic variations can be used to determine pressure, temperature and time (P‐T‐t) histories. We apply ⁴⁰Ar/³⁹Ar multi‐diffusion domain modelling to white micas from blueschists blocks in serpentinite matrix mélange from the exhumed Baja California subduction complex. Thermal history models yielded T‐t paths suggesting that ⁴⁰Ar* resides within multiple diffusion domains with varying ⁴⁰Ar* retentivity. Modelled white mica thermal histories and thermobarometric data were used to forward model continuous P‐T‐t paths. P‐T‐t paths are consistent with previous studies and are interpreted to constrain blueschist block exhumation paths within the Baja accretionary wedge. Our P‐T‐t models use temperature controlled ⁴⁰Ar/³⁹Ar step heat data in which argon loss by volume diffusion can be demonstrated, and for which the white mica petrogenesis is known.     

40Ar‐39Ar Geochronology and Tectonic Implications of the Blueschist from Northwestern Qiangtang,Northern Tibet,Western China

Blueschist exposed in the northwestern Qiangtang terrane, northern Tibet, western China (~84°30′ E, 34°24′ N), provides new constraints on the tectonic evolution of Qiangtang as well as northern Tibet. The blueschist represented by lawsonite- and glaucophane-bearing assemblages equilibrated at 375–400 °C and ~11 kbar. 40Ar-39Ar analysis on mineral separate from one blueschist sample yielded a well-defined plateau age of ~242 Ma. Geochemical studies show the blueschist is metamorphosed within-plate basalts. The high pressure-low temperature blueschist indicates a Triassic event of lithosphere subduction, and clearly represents an extension of the central Qiangtang metamorphic belt, and defines an in?situ suture between eastern and western Qiangtang.     

The relation between grain size and Ar/Ar date for Alpine white mica from the Siviez-Mischabel Nappe, Switzerland

Variations in Alpine white mica ⁴⁰Ar/³⁹Ar dates from the cover units of the Siviez-Mischabel Nappe relate to regional variations in the thermal history of the nappe. We focus on three regions within the nappe: the central Siviez-Mischabel (CSM), the southern Siviez-Mischabel (SSM), and the eastern Siviez-Mischabel (ESM). Our approach weaves together observations of quartz and mica textures in thin section, the variation of ⁴⁰Ar/³⁹Ar date with grain size, considerations of the effective diffusion dimension (EDD) of argon in white mica, and a comparison of dates with diffusion model results. In the CSM, pressure solution of quartz and dislocation glide in mica accommodated Alpine deformation. Dates record mica growth during nappe emplacement from 40 to 36 Ma and do not vary with grain size. In the SSM and ESM, both mica and quartz show textures associated with dynamic recrystallization, and dates decrease with grain size. In the SSM, dates also agree with the timing of nappe emplacement, but in the ESM, dates significantly post-date the timing of nappe emplacement. A comparison of dates with diffusion model results supports inferences from rock fabrics that the SSM experienced higher peak temperatures than the CSM, even though dates from both units approximate the timing of mica growth. Dates obtained from the ESM, however, do not compare well with simple models, and the thermal evolution of this region of the nappe, in the neighborhood of the Simplon Fault Zone, is not well understood.     

东准噶尔扎河坝超高压变质成因石英菱镁岩的^40Ar／^39Ar同位素年代学信息及地质意义

在扎河坝．阿尔曼泰蛇绿岩岩块内分布着高压-超高压成因的二辉橄榄岩、石榴辉石岩、石英菱镁岩及榴闪岩。石英菱镁岩内多硅白云母的Si（pfu）值均大于3．35,最高可达3．77,是典型高压-超高压成因的矿物。石英菱镁岩及其围岩蛇纹石化二辉橄榄岩的K2O含量极低,且多硅白云母的Na／（Na＋K）比值小于0．04,这些地球化学特征显示,扎河坝多硅白云母不应含有过剩Ar,是一个理想的^40Ar/^39Ar定年对象。精确的^40Ar/^39Ar年代学研究结果表明,扎河坝石英菱镁岩中多硅白云母的^40Ar/^39Ar同位素年龄为281．6±2．5Ma。而矿物化学特征表明,扎河坝石英菱镁岩中的多硅白云母曾经历了退变质作用的改造,因此,它代表的应该是超高压变质石英菱镁岩的折返年龄。多硅白云母的^40Ar/^39Ar年代学研究结果表明,扎河坝-阿尔曼泰蛇绿混杂岩内超高压石英菱镁岩的折返事件应发生在早二叠世。     

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.     

Exhumation of the Schistes Lustrés complex: in situ laser probe 40Ar/39Ar constraints and implications for the Western Alps

The Schistes Lustrés (SL) suture zone occupies a key position in the Alpine chain between the high‐pressure (HP) Brianconnais domain and the ultrahigh‐pressure (UHP) Dora Maira massif, and reached subduction depths ranging from c. 40–65 km (Cottian Alps). In order to constrain the timing of HP metamorphism and subsequent exhumation, several phengite generations were differentiated, on the basis of habit, texture, paragenesis and chemistry, as belonging to the first or second exhumation episode, respectively, D2 or D3, or to earlier stages of the tectono‐metamorphic evolution. Ten carefully selected samples showing D2, D3 (D2 + D3), or earlier (mostly peak temperature) phengite population(s) were subjected to laser probe ⁴⁰Ar/³⁹Ar analysis. The data support the results of the petrostructural study with two distinct age groups (crystallization ages) for D2 and D3 phengite, at 51–45 and 38–35 Ma, respectively. The data also reveal a coherent age cluster, at 62–55 Ma, for peak temperature phengite associated with chloritoid which were preserved in low strain domains. The age of the D3 event in the SL complex appears very similar to ages recently obtained for greenschist facies deformation on the border of most internal crystalline massifs. Exhumation rates of the order of 1–2 mm yr^?1 are obtained for the SL complex, which are compatible with velocities documented for accretionary wedge settings. Similarly, cooling velocities are only moderate (c.5 °C Myr^?1), which is at variance with recent estimates in the nearby UHP massifs.     

The geological significance of 40Ar/39Ar and Rb–Sr white mica ages from Syros and Sifnos,Greece: a record of continuous (re)crystallization during exhumation?

The Attic‐Cycladic crystalline belt in the central Aegean region records a complex structural and metamorphic evolution that documents Cenozoic subduction zone processes and exhumation. A prerequisite to develop an improved tectono‐metamorphic understanding of this area is dating of distinct P–T–D stages. To evaluate the geological significance of phengite ages of variably overprinted rocks, ⁴⁰Ar/³⁹Ar and Rb–Sr analyses were undertaken on transitional blueschist–greenschist and greenschist facies samples from the islands of Syros and Sifnos. White mica geochronology indicates a large age variability (⁴⁰Ar/³⁹Ar: 41–27 Ma; Rb–Sr: 34–20 Ma). Petrologically similar samples have either experienced greenschist facies overprinting at different times or variations in ages record variable degrees of greenschist facies retrogression and incomplete resetting of isotopic systematics. The ⁴⁰Ar/³⁹Ar and Rb–Sr data for metamorphic rocks from both islands record only minor, localized evidence for Miocene ages (c. 21 Ma) that are well documented elsewhere in the Cyclades and interpreted to result from retrogression of high‐pressure mineral assemblages during lower pressure metamorphism. Field and textural evidence suggests that heterogeneous overprinting may be due to a lack of permeability and/or limited availability of fluids in some bulk compositions and that retrogression was more or less parallel to lithological layering and/or foliation as a result of, possibly deformation‐enhanced, channelized fluid ingress. Published and new ⁴⁰Ar/³⁹Ar and Rb–Sr data for both islands indicate apparent age variations that can be broadly linked to mineral assemblages documenting transitional blueschist‐to‐greenschist‐ and/or greenschist facies metamorphism. The data do not record the timing of peak HP metamorphism, but may accurately record continuous (partial) resetting of isotopic systematics and/or (re)crystallization of white mica during exhumation and greenschist facies retrogression. The form of ⁴⁰Ar/³⁹Ar phengite age spectra are complex with the lowest temperature steps yielding Middle to Late Miocene ages. The youngest Rb–Sr ages suggest maximum ages of 20.6 ± 0.8 Ma (Syros) and 22.5 ± 0.6 Ma (Sifnos) for the timing of greenschist facies overprinting. The results of this study further accentuate the challenges of interpreting isotopic data for white mica from polymetamorphic terranes, particularly when mixing of populations and/or incomplete resetting of isotopic systematics occurs during exhumation. These data capture the full range of isotopic age variations in retrogressed HP rocks documented in previous isotopic studies, and can be interpreted in terms of the geodynamic evolution of the Aegean.     

^Ar／^39Ar Dating of Deformation Events and Reconstruction of Exhumation of Ultrahigh—Pressure Metamorphic Rocks in Donghai,East China

Abstract Recent investigations reveal that the ultrahigh‐pressure metamorphic (UHPM) rocks in the Donghai region of East China underwent ductile and transitional ductile‐brittle structural events during their exhumation. The earlier ductile deformation took place under the condition of amphibolite facies and the later transitional ductile‐brittle deformation under the condition of greenschist facies. The hanging walls moved southeastward during both of these two events. The ⁴⁰Ar/³⁹Ar dating of muscovites from muscovite‐plagioclase schists in the Haizhou phosphorous mine, which are structurally overlain by UHPM rocks, yields a plateau age of 218.0±2.9 Ma and isochron age of 219.8Ma, indicating that the earlier event of the ampibolite‐facies deformation probably took place about 220 Ma ago. The ⁴⁰Ar/³⁹Ar dating of oriented amphiboles parallel to the movement direction of the hanging wall on a decollement plane yields a plateau age of 213.1 ± 0.3 Ma and isochron age of 213.4±4.1 Ma, probably representing the age of the later event. The dating of pegmatitic biotites and K‐feldspars near the decollement plane from the eastern Fangshan area yield plateau ages of 203.4±0.3 Ma, 203.6±0.4 Ma and 204.8±2.2 Ma, and isochron ages of 204.0±2.0 Ma, 200.6±3.1 Ma and 204.0±5.0 Ma, respectively, implying that the rocks in the studied area had not been cooled down to closing temperature of the dated biotites and K‐feldspars until the beginning of the Jurassic (about 204 Ma). The integration of these data with previous chronological ages on the ultrahigh‐pressure metamorphism lead to a new inference on the exhumation of the UHPM rocks. The UHPM rocks in the area were exhumed at the rate of 3–4 km/Ma from the mantle (about 80–100 km below the earth's surface at about 240 Ma) to the lower crust (at the depth of about 20‐30km at 220 Ma), and at the rate of 1–2 km/Ma to the middle crust (at the depth of about 15 km at 213 Ma), and then at the rate of less than 1 km/Ma to the upper crust about 10 km deep at about 204 Ma.     

40Ar/39Ar dating,fluid inclusions and S–Pb isotope systematics of the Shabaosi gold deposit,Heilongjiang Province,China

The Shabaosi deposit is the only large lode gold deposit in the northern Great Xing'an Range. The gold ore bodies are hosted by sandstone and siltstone of the Middle Jurassic Ershi'erzhan Formation, and are controlled by three N–S‐trending altered fracture zones. The gold ore bodies are composed of auriferous quartz veinlets and altered rocks. Fluid inclusion studies indicate that the ore‐forming fluids belong to a H₂O–NaCl–CO₂–CH₄ system, with salinities between 0.83 and 8.28 wt% NaCl eq., and homogenization temperatures ranging from 180 to 320 °C. The δ³⁴S values of sulphides show a large variation from −16.9‰ to 8.5‰. The Pb isotope compositions of sulphides are characterized by a narrow range of ratios: 18.289 to 18.517 for ²⁰⁶Pb/²⁰⁴Pb, 15.548 to 15.625 for ²⁰⁷Pb/²⁰⁴Pb, and 38.149 to 38.509 for ²⁰⁸Pb/²⁰⁴Pb. The μ values range from 9.36 to 9.51. These results suggest that the ore‐forming fluids/materials were mainly of magmatic hydrothermal origin, derived from magmas produced by partial melting of the lower crust. The ⁴⁰Ar/³⁹Ar age of auriferous quartz veinlets from the Shabaosi gold deposit is about 130 Ma. The Shabaosi gold deposit has counterparts in similar orogenic gold deposits, and was formed during the post‐collisional setting of the Mongolia–Okhotsk Orogen. Copyright © 2014 John Wiley & Sons, Ltd.     

Geochemistry,zircon U–Pb analysis,and fluid inclusion 40Ar/39Ar geochronology of the Yingchengzi gold deposit,southern Heilongjiang Province,NE China

The Yingchengzi gold deposit, located 10 km west of Shalan at the eastern margin of the Zhangguangcai Range, is the only high commercially valuable gold deposit in southern Heilongjiang Province, NE China. This study investigates the chronology and geodynamic mechanisms of igneous activity and metallogenesis within the Yingchengzi gold deposit. New zircon U–Pb data, fluid inclusion ⁴⁰Ar/³⁹Ar dating, whole‐rock geochemistry and Sr–Nd isotopic analysis is presented for the Yingchengzi deposit to constrain its petrogenesis and mineralization. Zircon U–Pb dating of the granite and diabase–porphyrite rocks of the igneous complex yields mean ages of 471.7 ± 5.5 and 434 ± 15 Ma respectively. All samples are high‐K calc‐alkaline or shoshonite rocks, are enriched in light rare earth elements and large ion lithophile elements, and are depleted in high field strength elements, consistent with the geochemical characteristics of arc‐type magmas. The Sr–Nd isotope characteristics indicate that the granite formed by partial melting of the lower crust, including interaction with slab‐derived fluids from an underplated basaltic magma. The primary magma of the diabase–porphyrite was likely derived from the metasomatized mantle wedge by subducted slab‐derived fluids. Both types of intrusive rocks were closely related to subduction of the ocean plate located between the Songnen–Zhangguangcai Range and Jiamusi massifs. However, fluid inclusion ⁴⁰Ar/³⁹Ar dating indicates that the Yingchengzi gold deposit formed at ~249 Ma, implying that the mineralization is unrelated to both the granite (~472 Ma) and diabase–porphyrite (~434 Ma) intrusions. Considering the tectonic evolution of the study area and adjacent regions, we propose that the Yingchengzi gold deposit was formed in a late Palaeozoic–Early Triassic continental collision regime following the closure of the Paleo‐Asian Ocean. In addition, the Yingchengzi deposit could be classified as a typical orogenic‐type gold deposit occuring in convergent plate margins in collisional orogens, and unlikely an intrusion‐related gold deposit as reported by previous studies. Copyright © 2015 John Wiley & Sons, Ltd.     

Neotethys closure history of Anatolia: insights from 40Ar–39Ar geochronology and P–T estimation in high‐pressure metasedimentary rocks

The multiple high‐pressure (HP), low‐temperature (LT) metamorphic units of Western and Central Anatolia offer a great opportunity to investigate the subduction‐ and continental accretion‐related evolution of the eastern limb of the long‐lived Aegean subduction system. Recent reports of the HP–LT index mineral Fe‐Mg‐carpholite in three metasedimentary units of the Gondwana‐derived Anatolide–Tauride continental block (namely the Afyon Zone, the Ören Unit and the southern Menderes Massif) suggest a more complicated scenario than the single‐continental accretion model generally put forward in previous studies. This study presents the first isotopic dates (white mica ⁴⁰Ar–³⁹Ar geochronology), and where possible are combined with P–T estimates (chlorite thermometry, phengite barometry, multi‐equilibrium thermobarometry), on carpholite‐bearing rocks from these three HP–LT metasedimentary units. It is shown that, in the Afyon Zone, carpholite‐bearing assemblages were retrogressed through greenschist‐facies conditions at c. 67–62 Ma. Early retrograde stages in the Ören Unit are dated to 63–59 Ma. In the Kurudere–Nebiler Unit (HP Mesozoic cover of the southern Menderes Massif), HP retrograde stages are dated to c. 45 Ma, and post‐collisional cooling to c. 26 Ma. These new results support that the Ören Unit represents the westernmost continuation of the Afyon Zone, whereas the Kurudere–Nebiler Unit correlates with the Cycladic Blueschist Unit of the Aegean Domain. In Western Anatolia, three successive HP–LT metamorphic belts thus formed: the northernmost Tav?anl? Zone (c. 88–82 Ma), the Ören–Afyon Zone (between 70 and 65 Ma), and the Kurudere–Nebiler Unit (c. 52–45 Ma). The southward younging trend of the HP–LT metamorphism from the upper and internal to the deeper and more external structural units, as in the Aegean Domain, points to the persistence of subduction in Western Anatolia between 93–90 and c. 35 Ma. After the accretion of the Menderes–Tauride terrane, in Eocene times, subduction stopped, leading to continental collision and associated Barrovian‐type metamorphism. Because, by contrast, the Aegean subduction did remain active due to slab roll‐back and trench migration, the eastern limb (below Southwestern Anatolia) of the Hellenic slab was dramatically curved and consequently teared. It therefore is suggested that the possibility for subduction to continue after the accretion of buoyant (e.g. continental) terranes probably depends much on palaeogeography.     

Investigating mid‐Ediacaran glaciation and final Gondwana amalgamation using coupled sedimentology and 40Ar/ 39Ar detrital muscovite provenance from the Paraguay Belt,Brazil

Understanding the evolution of the northern Paraguay Belt, Brazil, is critical in two current controversies: (i) the number, timing and significance of Ediacaran glaciations; and (ii) the timing of amalgamation of South American Gondwana. The Neoproterozoic Alto Paraguay Group forms much of the northern Paraguay Belt. The Serra Azul Formation, within this Group, contains unequivocal evidence for a glacial influence on sedimentation, including multi‐directional striations on sandstone clasts and striated, polished and bullet‐shaped mudstone clasts. However, the age of the Serra Azul Formation is not well‐constrained. The northern Paraguay Belt also formed after the traditionally accepted time for amalgamation of South American Gondwana. If the orogen represents closure of an ocean, then this traditional view is incorrect. A significant number of single grain ⁴⁰Ar/³⁹Ar detrital muscovite cooling ages (ca 120) from the Alto Paraguay Group are presented. The three youngest grains from the Serra Azul Formation yield a weighted mean age of 640 ± 15 Myr, providing a robust maximum depositional age for this formation. This age, when considered with other data, suggests that the Serra Azul Formation developed in a mid‐Ediacaran glaciation consistent with that expressed in the Gaskiers Formation of Newfoundland, Canada. Cryogenian ⁴⁰Ar/³⁹Ar detrital muscovite ages from the Alto Paraguay Group are hard to reconcile with the known geology of Amazonia and are interpreted as being sourced from the evolving orogen to the east – from an arc terrane, possibly the Goiás–Paranapanema Massif. Detrital muscovites in the upper part of the Alto Paraguay Group are as young as 544 ± 7 Myr, consistent with mounting evidence that indicates a Cambrian age for orogenesis within the Paraguay Belt during the final amalgamation of Gondwana. This article suggests that the data best support a model where ocean closure in the region continued until Ediacaran/Cambrian times, with final ocean closure represented by orogenesis in the Paraguay–Araguaia orogen.     

Tracing the exhumation of the Eclogite Zone (Tauern Window,Eastern Alps) by <Superscript>40</Superscript>Ar/<Superscript>39</Superscript>Ar dating of white mica in eclogites

New radiometric ages from the Subpenninic nappes (Eclogite Zone and Rote Wand – Modereck Nappe, Tauern Window) show that phengites formed under eclogite-facies metamorphic conditions retain their initial isotopic signature, even when associated lithologies were overprinted by greenschist- to amphibolite-facies metamorphism. Different stages of the eclogite-facies evolution can be dated provided ⁴⁰Ar/³⁹Ar dating is combined with micro-structural analyses. An age of 39 Ma from the Rote Wand – Modereck Nappe is interpreted to be close to the burial age of this unit. Eclogite deformation within the Eclogite Zone started at the pressure peak along distinct shear zones, and prevailed along the exhumation path. An age of ca. 38 Ma is only observed for eclogites not affected by subsequent deformation and is interpreted as maximum age due to the possible influence of homogenously distributed excess argon. During exhumation deformation was localised along distinct mylonitic shear zones. This stage is mainly characterised by the formation of dynamically recrystallized omphacite2 and phengite. Deformation resulted in the resetting of the Ar isotopic system within the recrystallized white mica. Flat argon release spectra showing ages of 32 Ma within mylonites record the timing of cooling along the exhumation path, and the emplacement onto the Venediger Nappe. Ar-release patterns and ³⁶Ar/⁴⁰Ar vs.³⁹Ar/⁴⁰Ar isotope correlation analyses indicate no significant ⁴⁰Ar-loss after initial closure, and only a negligible incorporation of excess argon. From the pressure peak onwards, eclogitic conditions prevailed for almost 8–10 Ma.     

Detrital K‐feldspar 40Ar/39Ar Ages: Source Constraints of the Lower Miocene Sandstones in the Pearl River Mouth Basin,South China Sea

The South China Sea began to outspread in the Oligocene. A great quantity of terraneous detritus was deposited in the northern continental shelf of the sea, mostly in Pearl River Mouth Basin, which constituted the main paleo-Pearl River Delta. The delta developed for a long geological time and formed a superimposed area. Almost all the oil and gas fields of detrital rock reservoir distribute in this delta. Thirty-three oil sandstone core samples in the Zhujiang Formation, lower Miocene (23–16 Ma), were collected from nine wells. The illite samples with detrital K feldspar (Kfs) separated from these sandstone cores in four sub-structural belts were analysed by the high-precision 40Ar/39Ar laser stepwise heating technique. All 33 illite 40Ar/39Ar data consistently yielded gradually rising age spectra at the low-temperature steps until reaching age plateaus at mid-high temperature steps. The youngest ages corresponding to the beginning steps were interpreted as the hydrocarbon accumulation ages and the plateau ages in mid-high temperature steps as the contributions of the detrital feldspar representing the ages of the granitic parent rocks in the provenances. The ages of the detrital feldspar from the Zhujiang Formation in the four sub-structural belts were different: (1) the late Cretaceous ages in the Lufeng 13 fault structural belt; (2) the late Cretaceous and early Cretaceous-Jurassic ages in the Huizhou 21 buried hill-fault belt; (3) the Jurassic and Triassic ages in the Xijiang 24 buried hill-fault belt; and (4) the early Cretaceous – late Jurassic ages in the Panyu 4 oil area. These detrital feldspar 40Ar/39Ar ages become younger and younger from west to east, corresponding to the age distribution of the granites in the adjacent Guangdong Province, Southern China.     

Peak‐temperature patterns of polyphase metamorphism resulting from accretion,subduction and collision (eastern Tauern Window,European Alps) – a study with Raman microspectroscopy on carbonaceous material (RSCM)

Raman microspectroscopy on carbonaceous material (RSCM) from the eastern Tauern Window indicates contrasting peak‐temperature patterns in three different fabric domains, each of which underwent a poly‐metamorphic orogenic evolution: Domain 1 in the northeastern Tauern Window preserves oceanic units (Glockner Nappe System, Matrei Zone) that attained peak temperatures (T_p) of 350–480 °C following Late Cretaceous to Palaeogene nappe stacking in an accretionary wedge. Domain 2 in the central Tauern Window experienced T_p of 500–535 °C that was attained either within an exhumed Palaeogene subduction channel or during Oligocene Barrovian‐type thermal overprinting within the Alpine collisional orogen. Domain 3 in the Eastern Tauern Subdome has a peak‐temperature pattern that resulted from Eo‐Oligocene nappe stacking of continental units derived from the distal European margin. This pattern acquired its presently concentric pattern in Miocene time due to post‐nappe doming and extensional shearing along the Katschberg Shear Zone System (KSZS). T_p values in the largest (Hochalm) dome range from 612 °C in its core to 440 °C at its rim. The maximum peak‐temperature gradient (≤70 °C km^?1) occurs along the eastern margin of this dome where mylonitic shearing of the Katschberg Normal Fault (KNF) significantly thinned the Subpenninic‐ and Penninic nappe pile, including the pre‐existing peak‐temperature gradient.     

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.