Dating Of White Mica Research Articles

Tectonic Evolution of the Nevado‐Filábride Complex (Sierra de Los Filábres, Southeastern Spain): Insights From New Structural and Geochronological Data

AbstractThe high‐pressure metamorphic Nevado‐Filábride Complex (NFC) in the Betics mountain range of southeastern Spain exhibits continental and ocean‐derived tectonic units, which are key for understanding the geodynamic evolution of the Western Mediterranean. We address the current debate in the definition of tectonic units, the emplacement of (ultra)mafic rocks, and the timing of burial metamorphism by conducting a structural study combined with single grain fusion 40Ar/39Ar dating of white micas in structurally critical outcrops of the eastern Sierra de Los Filábres. One older 40Ar/39Ar age population (38–27 Ma) is found at distance from the main shear zones in the relics of an early foliation, while a younger 40Ar/39Ar population (22–12 Ma) is dominant in the vicinity of these shear zones, where the early foliation is obliterated. Both age groups are interpreted as the record of deformation or fluid‐induced recrystallization during distinct fabric‐forming events, while alternative scenarios are discussed. A key observation is the presence of an ophiolitic mélange, which—together with new and published geochronological data—allows for a new tectonic hypothesis. This considers Paleogene subduction beneath a Jurassic oceanic lithosphere, followed by the continued subduction of NFC and overlying ophiolites below the Alpujárride Complex. Exhumation during westward slab roll‐back led to the formation of an extensional detachment system that obliquely cut nappe contacts. Although the timing constraints for high pressure‐low temperature (HP‐LT) metamorphism in the NFC remain inconclusive, the new tectonic hypothesis provides a solution that can account for both Paleogene and Miocene ages of HP‐LT metamorphism.

Exhumation of the high‐pressure Richarddalen Complex in NW Svalbard: Insights from 40Ar/39Ar geochronology

Abstract40Ar/39Ar dating of white mica from foliated metamorphic rocks of the Caledonian orogen of NW Svalbard was performed using the single‐grain fusion technique. Two samples, an augen orthogneiss and a mica schist, were collected from the high‐pressure Richarddalen Complex. Three additional mica schist samples were collected from the tectonically underlying Montblanc Unit. 40Ar/39Ar analysis of white mica separates yielded weighted average 40Ar/39Ar dates of 446.1 ± 1.1, 438.4 ± 1.1, 439.4 ± 1.2, 439.8 ± 1.5 and 439.3 ± 1.0 Ma, with the oldest date provided by the Richarddalen mica schist. The single‐grain fusion 40Ar/39Ar dating was accompanied by geochemical mapping of white mica using a microprobe. We interpret the oldest date as the age of cooling after a Late Ordovician high‐pressure event and the four younger dates to represent the timing of subsequent deformation associated with assembly of a Caledonian nappe stack.

Metamorphic stages in mountain belts during a Wilson cycle: A case study in the central Sanandaj–Sirjan zone (Zagros Mountains, Iran)

Polymetamorphic units are important constituents of continent–continent collisional orogens, and rift metamorphic assemblages are often overprinted by subsequent metamorphism during subduction and collision. This study reports the metamorphic conditions and evolution of the Dorud–Azna metamorphic units in the central part of the Sanandaj–Sirjan zone (SSZ), Iran. Here, new geothermobarometry results are integrated with 40Ar/39Ar mineral and Th–U–Pb monazite and thorite ages to provide new insight of polyphase metamorphism in the two different basement units of the SSZ, the lower Galeh-Doz orthogneiss and higher Amphibolite-Metagabbro units. In the Amphibolite-Metagabbro unit, staurolite micaschist underwent a prograde P–T evolution from 640 ± 20 °C/6.2 ± 0.8 kbar in garnet cores (M1) to 680 ± 20 °C/7.2 ± 1.0 kbar in garnet rims (M2). Three Th–U–Pb monazite ages of 306 ± 5 Ma, 322 ± 28 Ma and 336 ± 39 Ma from the garnet-micaschists testify the Carboniferous age of M1 metamorphism. In the same unit, the metagabbro records P–T conditions of 4.0 ± 0.8 kbar and 580 ± 50 °C in the (magmatic) amphibole core (Late Carboniferous intrusion) to 7.5 ± 0.7 kbar and 700 ± 20 °C in the amphibole rim indicating a prograde P–T path during subsequent burial (M1). New 40Ar/39Ar dating of white mica from the staurolite micaschist yielded a staircase pattern ranging from 36 ± 12 Ma to 170 ± 2 Ma. This implies polymetamorphism with a minimum Late Jurassic cooling age through the Ar retention temperature of ca. 425 ± 25 °C after M2 metamorphism and a Paleogene low-grade metamorphic overprint (M3), while 40Ar/39Ar white mica dating of garnet micaschist yielded a plateau age of 137.84 ± 0.65 Ma. We therefore interpret the amphibolite-grade metamorphism M2 to have predated 170 Ma and is likely between 180 and 200 Ma. Furthermore, it is overprinted at about 36 Ma under retrogressive low-grade M3 metamorphism (at temperatures of ~350–240 °C) during final shortening and exhumation. In the underlying Galeh-Doz unit, the Panafrican granitic orthogneiss intruded at P–T conditions of 3.2 ± 4 kbar and 700 ± 20 °C, then it was metamorphosed and deformed at 600 ± 50 °C and 2.0 ± 0.8 kbar (metamorphic stage M1) prior to Late Carboniferous intrusion of mafic dikes. 40Ar/39Ar dating of amphibole from the Galeh-Doz orthogneiss gave plateau-like steps between 260 and 270 Ma, representing the age of cooling through ca. 500 °C after the M1 metamorphic event. Interestingly, the results of this study demonstrate polyphase metamorphic histories in both the Galeh-Doz orthogneiss and Amphibolite-Metagabbro units at different P–T conditions and final thick-skinned Paleogene emplacement of these units over the underlying low-grade metamorphic June Complex. Our findings suggest that both units are affected by high-T/low-P Late Carboniferous orogenic metamorphism along with the bimodal magmatism, as result of rifting. We propose that the Early Jurassic amphibolite-grade M2 metamorphism of the SSZ is correlated with the initial subduction of the Neotethyan Ocean. Eventually, the investigated units reflect various stages of a Wilson cycle, from rifting to initiation of the subduction in final plate collision.

Reappraisal of the oldest high-pressure type schist in Japan: New zircon U-Pb age of the Kitomyo Schist of the Kurosegawa Belt

The Kitomyo Schist from Kurosegawa Belt, Shikoku, has been long considered as the oldest records of subduction metamorphism in Japan, based on an early 1970s K–Ar dating of white mica. The schist consists of mafic and pelitic layers and occurs as a tectonic block within serpentinite. Reappraisal of the schist confirmed the schist is characterized by an epidote-amphibolite peak metamorphic facies. The mafic portion is characterized by zoned amphibole + epidote + chlorite + titanite ± phengite ± rutile. The presences of relict rutile surrounded by titanite and the barroisitic cores of zoned amphibole suggest a high-pressure intermediate type metamorphism at the metamorphic peak (P = ~0.8–1.5 GPa and T = ~500–570 °C). The presence of Mn-rich garnet and the lack of biotite, oligoclase and paragonite also support high-pressure intermediate type metamorphism that eliminate the possibility of a typical blueschist-facies metamorphism. New SHRIMP and LA-ICPMS zircon U–Pb geochronology on a pelitic sample show detrital grains of Mesoproterozoic and Early Paleozoic ages, suggesting a maximum deposition age for the trench-fill sediment of ~440 Ma. Also the U–Pb data confirmed ~360 Ma overgrown rims that might have formed during the subduction zone epidote-amphibolite facies metamorphism. Reappraisal revealed that the Kitomyo Schist is not the oldest high-pressure type schist in Japan and rather comparable to the Late Paleozoic Renge Metamorphic Rocks and their equivalents in the Kurosegawa Belt. The Devono–Carboniferous high-pressure metamorphic rocks in Japan might have been paired with their coeval batholiths along the ‘Greater South China’ margin that was extensively eroded during later tectonic processes.

Deciphering the Jurassic\u2013Cretaceous evolution of the Hamadan metamorphic complex during Neotethys subduction, western Iran

The Hamadan high-grade metapelites in the northwestern part of the Sanandaj–Sirjan zone, Iran, show a polymetamorphic evolution with relics of a garnet-bearing metamorphic mineral assemblage (M1), a contact metamorphic overprint (M2) related to the emplacement of the Middle to Late Jurassic Alvand composite pluton and a Buchan-type regional metamorphic event (M3) marked by 40Ar/39Ar ages in the 80–70 Ma range that is associated with penetrative ductile deformation producing a foliation and a thermal overprint onto the M2 assemblages. The M1 event is exclusively preserved as small garnet grains and mineral inclusions contained therein, incorporated into M2-stage cordierite porphyroblasts. Distinct metamorphic zones are developed over a region of ~ 600 km2, which are partly correlated with distance to the composite pluton: zones (1) cordierite + K-feldspar hornfels, and (2) andalusite ± cordierite hornfels that surround the Alvand composite pluton at a distance of up to 5 km. These two zones are clearly related to M2 metamorphism associated with pluton emplacement. Zones (3) staurolite schist, (4) andalusite schist, and (5) sillimanite schist are found outside of the contact aureole and are considered to be the result of regional M3 metamorphism in the eastern part distant to the Alvand composite pluton. Conventional thermobarometry shows that temperatures in the area vary between ~ 560 and 660 °C for zones 1 and 2 and ~ 490 and 690 °C for zones 3–5. Phase equilibria modelling in the MnNCKFMASHT system indicates two distinct isobaric prograde paths at low pressures, at ~ 2.7 kbar for zones 1 and 2 and slightly higher pressures of around 3.5–5.5 kbar for zones 3–5. U–Th–Pb monazite geochronology revealed overlapping ages of 168 ± 11 Ma and 149 ± 19 Ma in the hornfels (1 and 2) and schistose (3–5) zones, respectively. These ages are similar to the intrusion age of the Alvand composite pluton (153.3 ± 2.7 to 166.5 ± 1.8 Ma) and are interpreted to reflect heating due to the emplacement of the composite pluton (M2 contact metamorphic event). However, 40Ar/39Ar dating of white mica and amphibole yielded plateau ages ranging from 80 to 69 Ma over the entire transect. The formation of schistosity in zones 3–5 postdates the intrusion and is thus related to M3 metamorphism. The white mica fabric indicates formation of the foliation during M3 garnet growth, which is followed by local retrogression of garnet to chlorite during exhumation. Consequently, the 40Ar/39Ar white mica and amphibole ages likely indicate reheating during M3 to more than ca. 500 ± 25 °C (argon retention temperature in amphibole). These data establish the occurrence of a Cretaceous, Buchan-style regional metamorphic event that had not been firmly identified before. Subsequent Late Cretaceous exhumation of the Hamadan complex with its high-grade metapelites is due to extension along the Tafrijan–Mangavi–Kandelan fault, which represents a major ductile low-angle normal fault. Metamorphic temperatures coupled with mineral ages from this and published work suggest a fast stage of cooling with a rate of ~ 6 °C/Ma during exhumation after M3 metamorphism.

Cretaceous tectonic evolution of the Sava-Klepa Massif, Republic of North Macedonia – Results from calcite twin based automated paleostress analysis

The Sava-Klepa Massif represents an approximately 5 × 2 km sized fault-bounded block of dominantly basaltic rocks located within the Sava-Zone, an important suture zone between the Eurasian (Europe) and Gondwana (Adria) continental plates in the Balkans. Its nature and tectonic evolution is controversial: It is either interpreted as a remnant of the youngest Tethyan oceanic realm left behind after the main closure in the Late Jurassic or as the delimiter of a diffuse tectonic boundary between Adria and Europe, which had already collided in the Late Jurassic and was dominantly controlled by transtensional tectonics during Cretaceous times.In order to strengthen one or the other model, we concentrate on the Sava-Klepa Massif enclosing Paleozoic basement rocks, which mainly consist of a layered sequence of metamorphic marbles and mica schists. The metamorphic basement shows deformation structures clearly related to the Klepa event. Our results are based on paleostress and paleostrain analysis of twinned calcite from deformed marbles, and field observations, microstructural analysis, and 40Ar/39Ar dating of white mica. Paleostress and paleostrain analysis was enhanced by using an automated fabric analyser microscope, and developing the herein presented freeware standalone Windows® executable PACT software (Paleostress Analysis with Calcite Twins).Our results support the hypothesis that the Sava-Klepa Massif was formed during transtension as a pull-apart basin in the Late Cretaceous. This could have happened either after the collision of the European and Adriatic plates, or along a transtentional zone in the fore-arc region during subduction of the last Tethyan oceanic lithosphere.

Structural style and metamorphic conditions of the Jinshajiang metamorphic belt: Nature of the Paleo-Jinshajiang orogenic belt in the eastern Tibetan Plateau

The Jinshajiang metamorphic belt is a Barrovian sequence distributed within the Jinshajiang Suture Zone (JSZ), which was formed in the thickened Paleo-Jinshajiang orogenic belt after closure of the Jinshajiang Ocean. We have identified three metamorphic phases, M1, M2 and M3, corresponding to deformation stages D1, D2 and D3, in the Barrovian sequences. The metamorphic belt exhibits a metamorphic field gradient from chlorite to biotite, garnet, staurolite–kyanite and sillimanite grades. Inclusions in garnet and staurolite (chlorite, mica, quartz, feldspar, ilmenite and graphite) indicate that M1 reflects greenschist facies metamorphism. Pervasive M2 metamorphism formed a dominant S1 schistosity within the Barrovian sequence. Peak metamorphic conditions for metapelites of the garnet–staurolite and staurolite–kyanite grade were ∼580°C and ∼0.65GPa according to petrogenetic grids. Peak metamorphism was in conditions of ∼635°C and 0.50GPa for the metapelites and ∼650°C and ∼0.61GPa for amphibolites in sillimanite grade. Greenschist facies retrograde metamorphism, M3, followed D2 deformation, a top-down-to-southeast shear in the JSZ. The D3 deformation is characterized by well-developed brittle faults with fault gouge and breccia. Zircon grains from an amphibolite sample have cores with igneous oscillatory zoning and metamorphic rims. However, the metamorphic rims are too narrow to analyze. Laser ablation-inductively coupled plasma mass-spectrometry (LA-ICPMS) analyses of the igneous cores of zircons yielded a crystallization age of 242Ma. 40Ar/39Ar dating of white mica from a garnet-schist gave a plateau age of 224Ma. The peak metamorphism is thus limited to be between 242 and 224Ma. We thus suggest a tectonic shift from collision to extension of for the Permian to Triassic Paleo-Jinshajiang orogenic belt.

Determining ages of multiple muscovite-bearing foliations in phyllonites using the 40Ar/39Ar step heating method: Applications to the Alleghanian orogeny in central New England

New ^40^Ar/^39^Ar data from chlorite-grade phyllonites in central New England demonstrate the effectiveness of the ^40^Ar/^39^Ar step heating method to date bulk separates containing multiple age populations of white mica. Detailed optical and scanning electron microscope (SEM) petrography of phyllonites collected along the Westminster West fault zone separating the Connecticut Valley Gaspé trough from the Bronson Hill anticlinorium shows that white mica and chlorite defining younger cleavages are recrystallized. The phyllonites are the product of retrograde shearing of higher-grade phyllites west of the fault zone. Samples dominated by two age populations outside of the fault zone yield minimum cooling ages and maximum growth ages. However, at low metamorphic grade, ^40^Ar/^39^Ar dating of white micas from the fault zone yields crystallization ages rather than cooling ages, constraining the timing of the latest movement of the fault zone. ^40^Ar/^39^Ar analyses using the step heating method indicate that west of the fault zone, muscovite cooling ages cluster at ∼365 Ma, suggesting cooling from Devonian (Acadian) metamorphism. To the east, ^40^Ar/^39^Ar analysis yields a muscovite cooling age of ∼318 Ma, suggesting cooling from a younger metamorphic event. In contrast, age steps from samples on the margins of the fault zone climb from minima of ∼306 Ma to \>340 Ma, reflecting a mixture of phyllonitic micas ≤306 Ma with relict Acadian muscovite cooling ages. A sample from the core of the fault zone yields a white mica age ∼300 Ma. These results illustrate the usefulness of the ^40^Ar/^39^Ar step heating method on bulk mineral separates regardless of complications arising from multiple age populations. Moreover, the data suggest that the boundary between the Connecticut Valley Gaspé trough (CVGT) and the Bronson Hill anticlinorium was likely a site of significant strain localization in late Pennsylvanian to early Permian times. These results imply that Late Paleozoic (Alleghanian) deformation, so well documented in coastal and southern New England, was transmitted across the composite Avalon-Ganderia crustal block to the boundary with Laurentian crust (underlying the CVGT) as far north as southeastern Vermont.

Post-Taconic tilting and Acadian structural overprint of the classic Barrovian metamorphic gradient in Dutchess County, New York

Field mapping and ⁴⁰Ar/³⁹Ar age spectrum dating of white mica across the classic metamorphic gradient in Dutchess County, New York indicate that post-peak metamorphic and structural events complicate the interpretation of this sequence as an intact Taconic metamorphic field gradient. Oriented samples were collected and fabrics were measured along two ∼7 km transects. These transects extend from west of the biotite zone (west of Clove Mountain) to the sillimanite zone (Swamp River Valley) in eastern Dutchess County. Field and petrographic analysis reveals three pervasive foliations (S1, S2 and S3). S1 is folded in the lower grade rocks of the sequence, but is increasingly overprinted at garnet and higher zones by a penetrative foliation, S2. S2 dips steeply to the SE and is axial planar to folds in S1. S2 is overgrown by porphyroblasts of biotite, biotite + chloritoid, biotite + garnet ± staurolite ± kyanite ± sillimanite, at respective metamorphic grades, that appear to approximate peak temperature conditions. S1, S2, and peak temperature conditions are constrained as late Taconic based on a depositional age of ∼460 Ma (Potter, 1972) for the Walloomsac Formation and a staurolite age of ∼454 Ma (Lanzirotti and Hanson, 1997) from kyanite-sillimanite grade rocks. S3 is a greenschist facies foliation composed of muscovite ± biotite ± chlorite that truncates or overprints S2 in garnet through sillimanite grade rocks in the eastern part of the study area. S3 dips gently to the S-SE and becomes more penetrative to the east, such that in the garnet zone, S2 is overprinted by a weakly developed spaced S3 cleavage, while in many samples in the staurolite and higher metamorphic zones S2 micas are significantly overprinted by S3 micas. S3 is observed wrapping around and truncating peak temperature porphyroblasts. Three samples of lepidoblastic white mica (with varying proportions of S1, S2, and S3) from the biotite through staurolite zone all produce ⁴⁰Ar/³⁹Ar age spectra suggesting closure ages of ∼385 Ma. These white mica ages are interpreted to represent the time of cooling through white mica closure (∼350 °C). The ⁴⁰Ar/³⁹Ar ages in conjunction with field mapping and petrographic analysis suggest that S3 developed in the Devonian Acadian orogeny. Furthermore, the ⁴⁰Ar/³⁹Ar data constrains the timing of tilting and uplift of the Ordovician Barrovian sequence from the Middle Silurian to Early Devonian. This study suggests mid-crustal wedging during the Salinic orogeny as a possible mechanism for tilting the rocks of this region and exposing the Taconic-aged Barrovian field gradient at the surface today. We conclude the low-grade Acadian metamorphic overprint of these rocks involved deformation that produced a new (third) cleavage that was coincident with regional cooling from peak Taconic metamorphic conditions and thus required strain but no reheating during the Acadian.

Thickening and exhumation of the Variscan roots in the Iberian Central System: Tectonothermal processes and 40Ar/39Ar ages

40Ar/39Ar dating of white mica in metapelites, thermobarometric calculations and new structural and geological mapping have been carried out in the Somosierra region (Iberian Central System, ICS) to investigate the Variscan thickening–exhumation processes and the evolution of the paleo-thermal structure in a key region of the Central Iberian Zone. The rocks underwent an intermediate P/T event (M1) during a D1 stage of crustal thickening initiated between 354 and 347Ma. This event was probably related to a first S to SE-directed emplacement of the NW Iberian allochthon, and during the initial development of a mid-crustal subhorizontal shear zone accommodating the ductile flow of thermally weakened crust parallel to the main orogenic trend. Syn-convergent extension of the upper levels occurred by 335–327Ma, when tectonic unroofing led to D2 domal uplift of the orogenic roots and to recrystallization of rocks under intermediate to low P/T conditions (M2). Individualized gneiss domes developed along the orogenic trend, whereas some post-M2 thrusts dated at c. 319Ma reflect lateral flow related to limited gravitational spreading of the domes. Younger upright to SW verging folding of the ICS dome during D3 progressive renewal of contractive deformation took place after 316Ma. Final D4 gravitational collapse occurred under low P/T conditions (M3) at c. 300Ma, accompanied by the intrusion of large volumes of anatectic granitoids.

K-Ar Dating of White Mica from the Lesser Himalaya, Tansen-Pokhara Section, Central Nepal: Implications for the Timing of Metamorphism

The Lesser Himalaya in central Nepal comprises low- to medium-grade metasedimentary rocks. Metamorphic studies show that they have experienced at least two metamorphic events (M1 and M2). However, exact timing of metamorphism is still controversial. In the present study K-Ar dating of white micas in shales, slates, phyllites, schists and gneisses from the Tansen-Pokhara section was carried out to understand the timing of metamorphism. The muscovite in gneiss from the MCT zone shows an age of about 1255 Ma representing the age of crystallization of parent granite. Detrital mica from the Bhainskati Formation gives an age of about 2441 Ma. Probably this is the age of crystallization of muscovite in its provenance. The recrystallized white micas from slate and phyllite show older ages (Early Paleozoic) in the southern part (279 to 458 Ma). Most probably this represents the timing of M1. Age become gradually younger towards the north due to the mixing of older (M1 related) and younger (M2 related) white micas. Youngest age (10 Ma) was measured from the sample just below the Upper MCT. This age may be related to the M2 which was due to the Late Miocene-Pleistocene reactivation of the Upper MCT.DOI: http://dx.doi.org/10.3126/njst.v12i0.6509 Nepal Journal of Science and Technology 12 (2011) 242-251

Dating Subduction Events in East Anatolia, Turkey

Metamorphic studies in the cover sequences of the Bitlis complex allow the thermal evolution of the massif to be constrained using metamorphic index minerals. Regionally distributed metamorphic index minerals such as glaucophane, carpholite, relics of carpholite in chloritoid-bearing schists and pseudomorphs after aragonite in marbles record a LT-HP evolution. This demonstrates that the Bitlis complex was subducted and stacked to form a nappe complex during the closure of the Neo-Tethys. During late Cretaceous to Cenozoic evolution the Bitlis complex experienced peak metamorphism of 1.0-1.1 GPa at 350-400°C. During the retrograde evolution temperatures remained below 460°C. 39Ar/40Ar dating of white mica in different parageneses from the Bitlis complex reveals a 74-79 Ma (Campanian) date of peak metamorphism and rapid exhumation to an almost isothermal greenschist stage at 67-70 Ma (Maastrichtian). The HP Eocene flysch escaped the greenschist facies stage and were exhumed under very cold conditions. These single stage evolutions contrast with the multistage evolution reported further north from the Amassia-Stepanavan Suture in Armenia. Petrological investigations and isotopic dating show that the collision of Arabia with Eurasia resulted in an assemblage of different blocks derived from the northern as well as from the southern plate and a set of subduction zones producing HP rocks with diverse exhumation histories.

Dating low-temperature deformation by 40Ar/39Ar on white mica, insights from the Argentera-Mercantour Massif (SW Alps)

In order to date low-temperature deformation, intensely strained muscovite porphyroclasts and neocrystallized shear band phengite from greenschist-facies shear zones have been dated by 40Ar/39Ar method in the Argentera-Mercantour massif. Shear zones are featured by gradual mylonitization of a Variscan granite, gneiss and Permian pelite protolith (300–315Ma) during the Alpine orogenic event. Mineralogical and textural observations indicate that phengites and chlorites developed from biotite and plagioclase in fluid system during deformation following dissolution–transport–precipitation reactions of the type biotite + plagioclase + aqueous fluid = chlorite + albite + phengite + quartz + titanite + K-bearing fluid in the granite-gneiss mylonite. Contrariwise, phengite developed at the expense of clays following substitution reaction in pelite mylonite. Based on conventional thermobarometry on phengite and chlorite and Pressure–Temperature-aqueous fluid (P–T-MH2O) pseudosections calculated with shear zone bulk compositions, the conditions during shear deformation were estimated at 375±30°C and 4.8–7±1kbar in an H2O-satured system. In this low temperature environment, 40Ar/39Ar analysis of the Variscan muscovite for various grades of ductile strain intensity shows a limited 40Ar/39Ar isotopic resetting, all ages scattering between 296 and 315Ma. Under conditions of intense ductile deformation and large-scale fluid circulation, muscovite grains formed during the Variscan retain their much older ages. 40Ar/39Ar dating of very fine grained synkinematic phengite grains, neoformed during the Alpine history, give consistent plateau ages (34–20Ma) for each shear zone. In detail, 40Ar excess can be detected in the pelite mylonitic sample where phengites crystallized by substitution process while the other mylonitic samples where phengites grow from fluid-induced reactions do not evidence any 40Ar excess. These results demonstrate that the 40Ar/39Ar dating of neocrystallized synkinematic white mica allows the determination of precise ages of deformation and fluid activity. Together with precise thermobarometry undertaken on the basis of mineral chemistry and whole-rock composition, 40Ar/39Ar dating of white mica leads to the reconstitution of precise depth-deformation history of low-grade (<400°C) metamorphic units. At the Argentera-Mercantour massif scale, several stages of shear zone development at 15–21km depth are dated between 33 and 20Ma. In the SE part of the massif shear zone ages are well constrained to be either (1) 33.6±0.6Ma or in the range (2) 26.8±0.7Ma–26.3±0.7Ma. In the West of the massif, younger shear zone ages range between (3) 22.2±0.3Ma and (4) 20.5±0.3Ma.

Progradation of the Alpidic Central Western Carpathians orogenic wedge related to two subductions: constrained by 40Ar/39Ar ages of white micas

To reveal a model of the polystage evolution of an orogenic wedge subsequent to two subductions, 40Ar/39Ar dating of white micas (Wmca) across the shear zones in the Central Western Carpathians (CWC) has been carried out. This wedge is limited by the two suture zones, as a remnant after subducted oceanic crust of the Neotethyan Meliatic (in the S) and South-Penninic (in the N) oceans. It is composed of a system of basement and cover nappes, separated by major shear zones. Wmca aggregates from different microstructural and grain-size domains of lower- to medium-grade blastomylonites were dated by the 40Ar/39Ar method. The minimum age of NW-ward thrusting of the Gemeric and Meliatic nappes over the South-Veporic nappes in the southern part of this wedge was detected around 100 Ma. The NE-SW trending sinistral transpression, combined with a top-to-the SE extensional exhumation was dated at 90 – 85 Ma in the South-Veporic unit. Collision continued at ca. 95-90 Ma in the central part of the North-Veporic unit, while in the frontal part and in the Tatric and Infratatric units it was identified at 80 – 70 Ma. Lateral extrusion of the Tatric unit towards the NE occurred in Early Eocene, at ca. 50 Ma, indicating a final evolutionary stage of this wedge. The younging of ages from S to N revealed that footwall-propagating thrust-faults were responsible for the growth of the orogenic wedge.

Tracing the exhumation of the Eclogite Zone (Tauern Window, Eastern Alps) by 40Ar/39Ar 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 in terpreted 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 omphacite 2 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.

A Late Tournaisian synmetamorphic folding and thrusting event in the eastern Variscan foreland: 40Ar/39Ar evidence from the phyllites of the Wolsztyn–Leszno High, western Poland

A Variscan foreland in western Poland comprises two NW-trending basement highs, which are concealed under Carboniferous through Triassic strata of the Fore-Sudetic Monocline (FSM). Both highs consist of multiply deformed quartz-sericite ± albite ± chlorite phyllites of unknown protolith age. 40Ar/39Ar laser probe dating of white micas in up to 0.5-mm-thick mica layers, which form the S2 axial-plane foliation in phyllites of the Wolsztyn–Leszno High, yielded an age of 340.1±2.6 Ma for the lower greenschists facies metamorphism and probably also for the F2 folding. This deformation was associated and followed by thrusting, which brought about the basement highs. The latter delivered clasts to overlying late Visean–early Namurian flysch basin that was mainly sourced from the Saxothuringian Sudetes in which most of the deformation occurred between 345–335 Ma. The FSM basement may represent an independent terrane, referred to as the Wielkopolska terrane, belonging to the Armorican Terrane Assemblage.

Dating the evolution of C– S microstructures: a combined 40Ar/ 39Ar step-heating and UV laserprobe analysis of the Alpine Roffna shear zone

A technique combining 40Ar/ 39Ar step-heating analyses, in situ UV laserprobe dating and electron microprobe chemical characterization has been developed to unravel the relationships between microstructures, metamorphism and K/Ar systematics. This technique has been applied to white micas of a discrete shear zone in the Roffna metarhyolite in the Suretta nappe (SE Switzerland). Neoformed phengites from the Roffna metarhyolite crystallised in shear zones during Tertiary Alpine deformation under blueschist- and greenschist-facies conditions. In situ 40Ar/ 39Ar UV laser dating of white micas defining the schistosity and C-planes yield systematically younger isotopic dates in the C-planes, which are also correlated with decreasing celadonite content (i.e., Si/Al ratio). These results suggest that the timing of the high-pressure deformation event is preserved in the schistosity; high-Si micas appear to have lost no Ar. Later deformation was localised preferentially in C-planes giving younger 40Ar/ 39Ar dates probably related to a later period of phengite growth. The age sequence is broadly consistent with the textural constraints and geological field relations.

Occurrence and significance of blueschist in the southern Lachlan Orogen

Serpentinite/talc‐matrix mélanges, bearing blocks of blueschist metavolcanics, occur within the Heathcote and Governor Fault Zones of the southern Lachlan Orogen. In the Heathcote Fault Zone, serpentinite‐matrix mélange consists of blocks or small pods of boninite, andesite, ultramafic rocks, chert and volcanogenic sandstone variably metamorphosed to prehnite‐pumpellyite, greenschist, or greenschist to blueschist facies. In the Governor Fault Zone, blueschist metavolcanics occur as blocks within serpentinite/talc matrix that is interleaved with prehnite‐pumpellyite to greenschist facies, intermediate pressure slate and phyllite. Ar/Ar dating of white mica from slaty mud‐matrix (broken formation) indicates that the main fabric development occurred at 446 ± 2 Ma. U–Pb (SHRIMP) dating of titanite from blueschists in the Governor Fault Zone indicates that metamorphism occurred at approximately 450 Ma, close to the time of mélange formation. Previously published, Ar/Ar dating of white mica from phyllite and biotite from metadiorite in the Heathcote Fault Zone suggest that blueschist metamorphism occurred at a similar time. These ages are supported by field relationships. Illite crystallinity and b0 data from white mica, and the preservation of blueschist blocks indicate that these fault zones maintained low temperatures both during and after intermediate‐ to high‐pressure metamorphism. Occurrences of blueschists in the Arthur Lineament of the Tyennan (Delamerian) Orogen in Tasmania, and in the New England Orogen, have different ages, and in conjunction with the occurrences described here, suggest that subduction‐accretion processes contributed significantly to the development of the Tasmanides from Cambrian through to Carboniferous times.

DIKE-HOSTED ORES OF THE BEAST DEPOSIT AND THE IMPORTANCE OF EOCENE MAGMATISM IN GOLD MINERALIZATION OF THE CARLIN TREND, NEVADA--A REPLY

We appreciate the comments of G.D. Langstaff and the opportunity to elaborate on the origin of Carlin-type gold deposits. Langstaff focuses on two of the critical issues regarding these deposits, their age and spatial association with igneous rocks. Although his comment mentions almost every known Carlin-type deposit, our reply focuses on deposits in major districts. Direct dating of hydrothermal minerals and indirect dating based on crosscutting relations with pre-, syn-, or postmineral rocks in Nevada have demonstrated that major deposits in all trends are Eocene (Emsbo et al., 1996; Leonardson and Rahn, 1996; Phinisey et al., 1996; Groff et al., 1997; Hall et al., 1997, 2000; Hofstra et al., 1999; Hofstra and Cline, 2000; Johnston, 2000; Ressel et al., 2000a, b; Tretbar et al., 2000). Although many Carlin-type deposits are not dated, we emphasize that no deposit has been demonstrated to be any other age. Preliminary 40Ar/39Ar dating of white micas from mineralized Eocene dikes at Meikle and Griffin in the Carlin trend gives Eocene ages (Ressel et al., 2000b). Any discussion of the origin, especially spatial association, of Carlin-type deposits with igneous rocks must emphasize the northern and central Carlin trend (Fig. 1). Deposits there constitute ~60 percent of the total estimated contained gold in Carlin-type deposits of Nevada and Utah (Hofstra and Cline, 2000). We interpreted the 700 km2 aeromagnetic anomaly (Fig. 1) there to reflect an underlying composite Eocene intrusion emplaced between about 40 …

K-Ar dating of white micas from the Ventersdorp Contact Reef of the Witwatersrand Basin, South Africa: timing of post-depositional alteration

White micas from metamorphosed and hydrothermally-altered basaltic lavas, conglomerates, quartzites and shales in and around the Ventersdorp Contact Reef of the Witwatersrand Basin, South Africa, were dated by the K-Ar method to constrain post-depositional thermal and mineralization processes. The minerals were separated into various grain sizes between < 0.4 and 10μm, and characterized in terms of composition, paragenetic sequence and texture, by XRD, SEM and TEM techniques. The K-Ar isochron of all white micas in the basaltic lavas suggests an age of 1994 ± 60 Ma, and that of the smaller mica particles (< 2μm) in the quartzites defines a younger age of 1917 ± 66 Ma. This range is considered to reflect the timing of long-lived hydrothermal alteration in the Ventersdorp Contact Reef. The older age is slightly younger than the intrusion of the Bushveld Complex (2060–2054 Ma) and the Vredefort catastrophism (2020 Ma), which are well-documented events that were superimposed onto the Witwatersrand Basin. The younger age may be associated with the Eburnian orogenesis along the western edge of the Kaapvaal Craton resulting in continental-scale fluid migration and hydrothermal activity that extended throughout the Griqualand Basin and even into the Witwatersrand Basin. The K-Ar ages obtained here for the white mica fractions of the Ventersdorp Contact Reef in the Witwatersrand Basin confirm that the period between 2.0 and 1.9 Ga was significant, as far as alteration, and possibly also gold mobilization, was concerned.