Metamorphic Muscovite Research Articles

Unmixing multiple metamorphic muscovite age populations with powder X-ray diffraction and 40Ar/39Ar analysis

A combination of modal estimates from powder X-ray diffraction (XRD) experiments and argon isotopic data shows that muscovite ⁴⁰Ar/³⁹Ar total gas age correlates with muscovite composition near the retrograde Bald Mountain shear zone (BMSZ) in Claremont, New Hampshire, and that the shear zone was active at ∼245 Ma. Petrologic study demonstrates that chemical disequilibrium is preserved in muscovite grains in these samples. The recognition of this preservation is critical to the interpretation of the ⁴⁰Ar/³⁹Ar step-heating experiments, which never produce age plateaus and yield spectra with steps that range in age by ∼20 Ma. Petrographic, compositional, and crystallographic data all indicate that the age spectra reflect dissolution of metastable Na-rich muscovite and precipitation of stable Na-poor muscovite associated with deformation in the BMSZ.Comparison of whole rock and muscovite concentrate XRD patterns from individual samples demonstrates that the mineral separation process can fractionate these muscovite populations. Therefore, four muscovite concentrates of varying magnetic susceptibility were prepared from a single hand sample, analyzed by XRD, and dated. These four splits define a mixing line that resolves end-member ages of 244.5 ± 4.2 Ma and 302.5 ± 12.5 Ma (1σ). Although the ages are imprecise, the petrologically supported conclusion that these schists preserve two discrete ages is distinct from an interpretation that the spectra reflect cooling through closure at ∼270 Ma, as might be concluded in the absence of petrologic characterization. The XRD results also demonstrate that, even well above anchizone conditions, petrologic information relevant to ⁴⁰Ar/³⁹Ar dating is observable in subtle variations in the crystallography of muscovite grains.

Petrology, geochemistry, Ar[sbnd]Ar isotopes of an arc related calk-alkaline pluton from Mamb (Pan-African Yaounde group, Cameroon): A testimony to the subduction of a hot oceanic crust

In Central Africa, the Pan-African Yaoundé group is made up of migmatites and gneisses exhumed and thrusted on micaschists. In a window in the micaschists, the Mamb metagabbro massif occurs as km-size and ca 20 m-high hill crosscut by a 15 m-thick dyke of meta-hornblendite. Minor facies are biotitites, garnetites, and granitic veins. Metagabbro comprises pargasite, biotite, labrador-andesine, pyrope- andradite- grossular-rich almandine, epidote, FeTi oxide, quartz, scapolite, rutile, zircon, and late metamorphic muscovite, whereas meta-hornblendite is made up of abundant edenite-pargasite, and minor amounts of plagioclase, biotite and epidote. Textural features such as epidote needles in plagioclase and rutile exsolutions in amphibole suggests that metamorphism (P-T conditions: 610 °C and 9.1 Kb) post-dated magma emplacement and crystallisation. Based on 40Ar/39Ar thermochronology of amphibole and micas, the proceedings of metamorphic processes extended from 601 to 586 Ma. Chemically, metagabbros and meta-hornblendites display hybrid adakitic and calk-alkaline characteristics. Mantle-normalised element patterns of metagabbros are marked by negative Nb, Ta, Zr, Hf, Ti, Y and positive Sr and Pb anomalies parallel to metadiorites of Yaoundé. Meta-hornblendite patterns show depletion in Th, K, and Pb similarly to alkaline rocks but high values of Y/Nb (> 3) and Zr/Nb (11–52) ratios point to a subalkalic affinity. Characteristics of these metagabbros and meta-hornblendites point to heterogeneous origins by partial melting of a dehydrated subducted hot oceanic crust and the fusion of a metasomatised mantle wedge. Metadiorites from Yaoundé are arc related rocks. Thus it appears that the island arc setting described at Boumnyebel extends eastwards up to Yaoundé, and probably up to Lomié in the south-eastern Cameroon. The northwards extension of this setting is yet to be established taking into account the presence of Archaean-Paleoproterozoic formations north of the continental scale Sanaga shear zone. At the regional scale, the best model to explain the presence of adakites and classic arc magmas at Mamb, Bape, Bangangté and Fomopea is a south-north subduction of the Pan-African Yaoundé hot oceanic crust under the Archaean Adamawa-Yade block.

The Grenvillian assembly of Rodinia: Timing of accretion on the western margin of the Kalahari (Kaapvaal) Craton

Abstract During the Grenvillian assembly of Rodinia, the Namaqua-Natal Metamorphic Province (NNMP) was formed as a result of the convergence of the Laurentia and Kalahari cratons. A detailed model for this accretion along the south-eastern margin of the Kalahari Craton has been established, but the tectonic history of the NNMP along the western margin of the Kalahari Craton has remained highly controversial. U-Pb SHRIMP zircon age dating of gneiss in the Kakamas Domain of the NNMP, as well as U-Pb SHRIMP age dating of detrital zircons and 40Ar/39Ar dating of metamorphic muscovite from sediments overlying the gneiss, confirms the presence of at least two separate events during the Namaqua-Natal Orogeny at ~1 166 Ma and 1 116 Ma. These events occurred after the Areachap Terrane was accreted onto the western margin of the Proto-Kalahari Craton during the Kheis Orogeny. 40Ar/39Ar ages derived from metamorphic muscovite formed in the metasediments of the Kheis terrane does not provide evidence for the timing of the Kheis Orogeny but suggests that it most likely only occurred after ~1 300 Ma and not at 1 800 Ma as commonly accepted. A U-Pb concordia age of ~1 166 Ma was derived from granitic gneiss in the Kakamas Domain of the Bushmanland Subprovince, possibly reflecting subduction and the initiation of continent-continent collision between the Proto-Kalahari Craton and the Bushmanland Subprovince. This granitic gneiss is nonconformably overlain by the metasediments of the Korannaland Group that contains metamorphic muscovite with 40Ar/39Ar ages of ~1 116 Ma. This age suggest that complete closure of the ocean between the Proto-Kalahari Craton and Bushmanland Subprovince probably occurred about 50 Ma after the intrusion of the ~1 166 Ma granitic gneisses.

Geology, mineralogy and geochemistry of the Shangxu orogenic gold deposit, central Tibet, China: Implications for mineral exploration

Shangxu is an orogenic gold deposit within the Bangong-Nujiang suture zone, central Tibet, China. It is hosted by turbidite sedimentary rocks of the Jurassic Mugagangri Group. The host rocks were metamorphosed to subgreenschist facies prior to being hydrothermally altered adjacent to mineralization. Hydrothermal minerals in wall rocks consist of muscovite, carbonate, sulfides and chlorite. Whole rock geochemistry indicates hydrothermal alteration is characterized by the introduction of K2O, CO2 and S, and leaching of Na2O. Increasing 3 K/Al and decreasing Na/Al alteration indexes approaching the ore correlate to muscovitization. CO2 concentration reflects the degree of carbonation, which in conjunction with molar (Mn + Fe)/(Fe + Ca + Mg + Mn) and CO2/CaO can be used to distinguish siderite from ankerite in carbonate alteration. Mass transfer analysis shows gains in As, Au, CO2, W, Ca, Cd, Ni, Cr, Sr, Rb, K and losses in Na, Pb, Cu during hydrothermal alteration. In the muscovite alteration zone, muscovite within shear zones (Ms1) shows an increase in Fe, Mg and Na towards the ore, whereas metamorphic muscovite along cleavage planes and in pressure shadows (Ms2), and hydrothermal muscovite (Ms3) change in composition from phengitic (Mg, Fe) distal to mineralization, to paragonitic (Na) adjacent to gold. In carbonate alteration, the composition in carbonate phases changes from early magnesian siderite to ankerite, and then to calcite and dolomite during the evolution of the hydrothermal system. Siderite spots in host rocks and ankerite in veins, wall rock or its replacement of siderite are all enriched in Fe, Mn and depleted in Mg from proximal to distal alteration zones. Pyrite is a dominant mineral in sulfide alteration of the Shangxu deposit. The earliest framboidal pyrite (Py1) formed during diagenesis, and overprinted by Py2, which is recrystallized pyrite associated with burial metamorphism. Py3 rims and enlarges previous pyrite aggregates, or recrystallizes into cubic grains with cracks and quartz pressure shadows during deformation. Py4 is ore-related euhedral pyrite, disseminated near gold, and formed in hydrothermal stage. Vein pyrite (Py5) coexists with marcasite, and fills cracks in quartz veins. Hydrothermal alteration minerals form overlapping alteration haloes surrounding the main auriferous lodes. Distal muscovitization, carbonatization and proximal sulfidation constitute a progressive alteration front from gold. The alteration envelope is tabular in shape, following the strike and dip of the mineralization, and tends to be the widest around the thickest parts of the ore, such that the occurrence and thickness of the alteration envelope can be used as an approximate guide to the size of gold mineralization. Shangxu shares many diagnostic features with turbidite-hosted orogenic gold deposits, such as Bendigo, Reefton and Meguma districts, in deposit geology, alteration mineralogy and lithogeochemistry.

Ore genesis and fluid evolution of the Kaladawan South Zn–Pb–Cu ore field, eastern Altyn Mountains (NW China): Evidence from fluid inclusions, H–O isotopes and geochronology

The newly-discovered Kaladawan South Zn–Pb–Cu orefield in eastern Altyn Mountains is composed of five Zn–Pb(–Cu) deposits, with the largest one containing 414.4 thousand tonnes (kt) Zn, 265.2 kt Pb and 149.2 kt Cu. Stratiform/lenticular orebodies are hosted by the Ordovician volcanic–sedimentary sequences subjected to greenschist-facies metamorphism. Two major ore-forming stages with significant cross-cutting relationship are identified at Kaladawan South: Stage (I) of the earlier massive, banded, disseminated barite-rich sulfide ores, and Stage (II) of the later polymetallic sulfide–quartz veins with economically less importance. Fluid inclusions (FIs) locked in quartz of Stage I massive sulfide ores are homogenized completely at 226–354 °C with calculated seawater-like salinities of 2.7–4.2 wt% NaCl equiv. These characteristics of ore-forming fluids coupled with ore textures of typical VMS deposits, are interpreted to be originated from the modified seawater by sea-floor volcanic-hydrothermal activities. In compared, the Stage II FIs yielded lower homogenization temperatures ranging 251 °C–357 °C with higher salinities of 0.8–14.4 wt% NaCl equiv. Laser Raman analyses identified large amounts of CO2 ± (CH4–N2) in the hydrothermal fluids of Stage II. The δ18OH2O values (0.5–5.7‰) and δDH2O values of (−68.1‰ to −32.3‰) of Stage II quartz are plotted in the range of metamorphic fluids in the D–O isotopic diagram, which indicating that the CO2-rich and mesothermal fluids are metamorphic-derived. The phlogopite paragenetic in the Stage II polymetallic vein yielded a 40Ar/39Ar plateau age of 435.4 ± 3.9 Ma, similar to the age of the metamorphic muscovite (451.0 ± 4.1 Ma) from the ore-hosting schist. Based on the ore deposit geology, mineral assemblages and fluid geochemistry, the Kaladawan South Zn–Pb–Cu mineralization is attributed to be an Ordovician VMS-type deposit overprinted by the Silurian metamorphic fluids.

Geochronology of Ashidani Formation in the Hida Gaien Belt at Kuzuryu, Central Japan

Previously a mystery, era in which the Ashidani Formation was formed in the Hida Gaien Belt at Kuzuryu is determined. The detrital zircon SHRIMP U–Pb age of Ashidani Formation sandstone is found to be within the range of 280 and 220 Ma. The most recent generation noted in samples collected is 224.2 ± 5.4 Ma (between the Carnian and Norian stages of the late Triassic Period). The metamorphic muscovite K–Ar age of sandstone schist is 182.6 ± 3.9 Ma (between the Pliensbachian and Toarcian stages of the Early Jurassic Age). There are no previous reports on metamorphic age values of 180 Ma in the Hida Gaien Belt. According to these dating results, geological development of the Ashidani Formation was limited to between the latter part of the late Triassic period and the start of the early Jurassic period, and can be compared to the Chizu metamorphic belt in the Chugoku region. Geological development of the Ashidani Formation is limited to the start of the early Jurassic period. Ashidani Formation Rocks were deposited in the forearc region of the eastern margin of the China craton at the beginning of the early Jurassic period. They then underwent high-pressure metamorphism alongside protoliths of the Chizu metamorphic belt in the middle part of the early Jurassic period due to subduction of the oceanic plate.

Late Paleozoic – Mesozoic tectonic evolution of the Eastern Taimyr-Severnaya Zemlya Fold and Thrust Belt and adjoining Yenisey-Khatanga Depression

Combined structural, thermochronological and geochronological studies were carried out to unravel the complex late Paleozoic – Mesozoic tectonic evolution of the eastern Taimyr – Severnaya Zemlya Fold and Thrust Belt, along with the adjoining Yenisey-Khatanga Depression and Olenek Fold Zone. New detailed field mapping, a fault and fold geometry and kinematic study, and paleostress reconstruction were undertaken for key areas within the Central and Southern Taimyr domains and the Olenek Fold Zone. Thirty seven samples were used for apatite fission track (AFT) analysis along with two samples for U-Pb zircon granite dating, and three samples for Ar-Ar metamorphic muscovite dating.The late Paleozoic (Early Carboniferous – Permian) tectonic event is recognized only in a relatively narrow zone including the Northern Taimyr Domain and part of the Central Taimyr Domain. It started in the Early Carboniferous (Visean) and is synchronous with the earliest stages of collision in the Uralian Orogen. Our results reveal a widespread distribution of latest Triassic – earliest Jurassic and Cretaceous deformations. The latest Triassic – earliest Jurassic tectonic event significantly overprinted the Northern and Central Taimyr domains and original unaltered late Paleozoic AFT ages are identified only locally. New completely reset AFT cooling ages group at 195-180 Ma, in reasonable agreement with structural data. Older Neoproterozoic and late Paleozoic Ar-Ar ages of metamorphic muscovite are retained, demonstrating that the dated rocks were not heated above 300 °C. An Early Cretaceous tectonic event is inferred from both the structural study and ca. 115–125 Ma AFT cooling ages, and is most widespread in the Southern Taimyr Domain, Olenek Fold Zone and adjoining parts of the Yenisey-Khatanga Depression, modifying older geological structures. A younger Late Cretaceous tectonic event at ca. 100-60 Ma is also inferred from AFT cooling ages, but its distribution is poorly constrained.Our study reveals that the distribution of late Mesozoic tectonic events has been significantly underestimated in previous structural studies of the region. The latest Triassic – earliest Jurassic and Cretaceous tectonic events are easily correlated with those in the Pai-Khoi – Novaya Zemlya and northern Verkhoyansk fold and thrust belts.

Samples from the Lomonosov Ridge place new constraints on the geological evolution of the Arctic Ocean

Abstract A number of rock samples were collected from two dredge positions on the Lomonosov Ridge at water depths of 2–3.5 km. The dredge samples are dominated by sediments deformed and metamorphosed under greenschist-facies conditions 470 myr ago according to 40 Ar/ 39 Ar dating of metamorphic muscovite. This shows that the Lomonosov Ridge was involved in a major Mid-Ordovician orogenic event that correlates with early arc–terrane accretion observed in northern Ellesmere Island, Svalbard, and other parts of the Caledonian belt. Detrital zircon age spectra of these metasediments span the Mesoproterozoic–Palaeoproterozoic with a main peak at around 1.6 Ga, and a pattern similar to that known from Caledonian metasedimentary rocks in East Greenland and northern Norway, as well as from Cambrian sediments in Estonia and Palaeozoic sediments on Novaya Zemlya. A second population of dredge samples comprises undeformed, non-metamorphic sandstones and siltstones. Detrital zircons in these sediments span the Palaeoproterozoic with a few Archaean zircons. Both rock types are covered by an up to 8 Ma ferromanganese crust and are evaluated to represent outcrop, and apatite fission-track data from three of the rock samples indicate that exposure at the seabed corresponds to a regional event of uplift and erosion that affected the Arctic in the Late Miocene. The data from the Lomonosov Ridge suggest that the 470 Ma orogenic event extended from Scotland and northern Scandinavia into the Arctic, including Svalbard, the Pearya Terrane and the Chukchi Borderlands.

Fault-gouge dating in the Southern Alps, New Zealand

We report two 40Ar/39Ar illite ages from fault gouge directly above the current trace of the Alpine Fault in New Zealand at Gaunt Creek (1.36±0.27Ma) and Harold Creek (1.18±0.47Ma), and one 40Ar/39Ar illite age from fault gouge from the Two Thumbs Fault on the east side of the Southern Alps. Metamorphic muscovite clasts inherited into the Alpine Fault gouge yielded 40Ar/39Ar ages of 2.04±0.3Ma at Gaunt Creek and 11.46±0.47Ma at Harold Creek. We also report Rb-Sr muscovite-based multimineral ages of Alpine Schist mylonite adjacent to the dated fault gouge at Harold Creek (13.1±4.3Ma) and Gaunt Creek (8.9±3.2Ma). 40Ar/39Ar muscovite ages from the Gaunt Creek mylonite yielded plateau ages of 1.47±0.08Ma and 1.57±0.15Ma. Finally, we report zircon fission track (0.79±0.11 and 0.81±0.17Ma) and zircon (U-Th)/He ages (0.35±0.03 and 0.4±0.06Ma) from Harold Creek.We interpret the fault gouge ages to date growth of newly formed illite during gouge formation at temperatures of ~300–350°C towards the base of the seismogenic zone. Simple backcalculation using current uplift/exhumation and convergence rates, and dip angles of 45–60° at the Alpine Fault support that interpretation. We infer that the fault gouge ages record faulting and gouge formation as the rocks passed very rapidly through the brittle-ductile transition zone on their way to the surface. Rb-Sr and 40Ar/39Ar ages on muscovite from Alpine Schist mylonite date muscovite growth at ~11Ma together with a younger phase of cooling/shearing at ~1.5–2Ma. Our ages from the Alpine Schist indicate extremely rapid cooling exceeding 200°C/Ma. The fault gouge age from the Two Thumbs Fault is significantly too old to have formed as part of the late Neogene/Quaternary Southern Alps evolution.

40Ar/39Ar constraints on the age and thermal history of the Urucum Neoproterozoic banded iron-formation, Brazil

Urucum is one of the youngest banded iron-formations (BIFs) yet its exact age remains uncertain. 40Ar/39Ar geochronology on late-diagenetic to early metamorphic cryptomelane from the Urucum sequence reveals a minimum depositional age of 587±7Ma. Metamorphic braunite age spectra yield flat segments defining apparent ages of 547±3Ma to 513±4Ma, interpreted as recrystallization or cooling ages. Metamorphic muscovite grains from a meta-arkose interbedded with the BIF yield reproducible plateau ages of 513±3Ma. Cryptomelane ages are interpreted to record the minimum timing of the extensional tectonic events that generated the graben systems at the southeastern border of the Amazon craton, including the Chiquitos-Tucavaca aulacogen and the Jacadigo Basin, which hosts the Urucum banded iron-formation. Structurally controlled hydrothermal alteration resulted in crystallization of braunite and muscovite during the interval between 547 and 513Ma coeval with post-collisional decompression that resulted in granite emplacement (e.g., São Vicente Granite) in the Paraguay Belt. Subsequent uplift and erosion of the Precambrian sequence possibly succeeded metamorphism. A chemically distinct cryptomelane generation identified in surface samples yield ca. 60Ma results. These ages provide evidence for supergene recrystallization after exhumation in the Mesozoic/Cenozoic.40Ar/39Ar systematics implies that the oldest cryptomelane samples experienced Ar-loss probably via thermally induced diffusion. This suggests that the Urucum BIF was buried and heated to significant temperatures in the late Proterozoic, consistent with present mineralogy and isotopic composition that do not reflect the original characteristics of the depositional environment.

Synmetamorphic carbon mobility and graphite enrichment in metaturbidites as a precursor to orogenic gold mineralisation, Otago Schist, New Zealand

The Macraes orogenic gold deposit is hosted by a graphitic micaceous schist containing auriferous porphyroblastic sulphides. The host rock resembles zones of unmineralised micaceous graphitic pyritic schists, derived from argillaceous protoliths, that occur locally in background pelitic Otago Schist metasediments. This study was aimed at determining the relationship between these similar rock types, and whether the relationship had implications for ore formation. Argillites in the protolith turbidites of the Otago Schist metamorphic belt contain minor amounts of detrital organic matter ( 50 million years of evolution of the metamorphic belt, from development of sheared argillite in the Jurassic, to postmetamorphic ductile extension in the Cretaceous. Introduced graphite is structurally controlled and occurs with metamorphic muscovite and chlorite as veins and slicken-sided shears, with some veins having >50% noncarbonate carbon. Graphitic foliation seams in low-grade micaceous schist and metamorphic quartz veins contain equant graphite porphyroblasts up to 2 mm across that are composed of crystallographically homogeneous graphite crystals. Graphite reflectance is anisotropic and ranges from ~1% to ~8% (green light). Texturally similar porphyroblastic pyrite has grown in micaceous schist (up to 10 wt.% S), metamorphic quartz veins and associated muscovite-rich shears. These pyritic schists are weakly enriched in arsenic (up to 60 ppm). The low-grade metamorphic mobility and concentration of graphite in micaceous schists is interpreted to be a precursor process that structurally and geochemically prepared parts of the Otago Schist belt for later (more restricted) gold mineralisation. Economic amounts of gold, and associated arsenic, were subsequently introduced to carbonaceous sulphidic schists in the Macraes gold deposit by a separate metamorphic fluid derived from high-grade metaturbidites. Fluid flow at all stages in these processes occurred at metamorphic rates (mm/year), and fluids were broadly in equilibrium with the rocks through which they were passing.

When can muscovite 40Ar/ 39Ar dating constrain the timing of metamorphic exhumation?

Metamorphic cooling rates determined using muscovite 40Ar/ 39Ar ages underpin many tectonic models, but are based on several simplifying approximations. We demonstrate that a number of these approximations, especially those relating to the assignment of a single “closure temperature”, might not hold for certain metamorphic scenarios, and particularly for metamorphism during rapid orogenic cycles. Using numerical diffusion models that include a recently reported significant pressure dependence of Ar diffusion in muscovite, we systematically interrogate the approximations associated with linking 40Ar/ 39Ar dates to pressure–temperature histories. The results of the simulations are presented in a simple graphic form allowing evaluation of the pressure–temperature (PT) regions in which Ar diffusion is efficient, and hence those in which the muscovite may yield true 40Ar/ 39Ar cooling ages. We suggest that a robust method for determining whether metamorphic muscovite 40Ar/ 39Ar ages relate to the timing of cooling involves: (1) the determination of the PT conditions and relative timing of muscovite crystallisation during the metamorphic cycle, (2) the collection of high precision and high spatial resolution 40Ar/ 39Ar data from muscovite grains and (3) the comparison between analytical data and numerical diffusion models which test different post-crystallisation pressure–temperature histories.

Zircon and muscovite ages, geochemistry, and Nd–Hf isotopes for the Aktyuz metamorphic terrane: Evidence for an Early Ordovician collisional belt in the northern Tianshan of Kyrgyzstan

The Aktyuz metamorphic terrane in the Kyrgyz northern Tianshan consists of granitoid gneisses and migmatites with subordinate paragneisses, greenschists, presumed meta-ophiolites, and garnet amphibolite dykes that contain HP eclogite relicts. The gneisses and migmatites were previously considered to be Archaean and Palaeoproterozoic in age on the basis of α-Pb and U–Pb multigrain zircon dating. Zircons from a post-tectonic granite were previously dated at 692±15Ma, constraining the time of main deformation and metamorphism in the Aktyuz terrane to the Precambrian.The chemical characteristics of most granitoid samples are consistent with melting of chemically evolved crustal material, which is supported by Nd and Hf isotopic data. Zircon U–Pb SHRIMP ages were obtained for the main varieties of metamorphic rocks, for a gabbro of a low-grade ophiolite complex and for several post-kinematic igneous rocks. In addition, metamorphic muscovite was dated by the 40Ar–39Ar method, and whole-rock Sm–Nd isotopic systematics were obtained on several granitoid rocks. Our magmatic zircon crystallization ages for granitoid gneisses in the Aktyuz and Kemin Complexes range from 778±6 to 844±9Ma which we interpret to reflect the time of magmatic emplacement of the gneiss protoliths. These rocks reflect an episode of Neoproterozoic granitoid magmatism, which is also documented in southern Kazakhstan, the Kyrgyz Middle Tianshan, the Chinese Central Tianshan and the Tarim Craton. Nd and Hf isotopic systematics show these rocks to be derived from Mesoproterozoic to Archaean sources. The calc-alkaline composition of these rocks seems compatible with a subduction setting, but is most likely inherited from the source, therefore the tectonic scenario for emplacement of the gneiss protolith remains unknown. Two ages of 562±7 and 541±3Ma and negative εNd(t)-values for granitoid gneisses document a later crustal melting episode. Muscovite 40Ar/39Ar ages of ca. 470Ma for Aktyuz gneisses constrain the main fabric-forming metamorphism to the Early Palaeozoic. A migmatitic paragneiss, which was previously interpreted as Palaeoproterozoic, contains detrital zircons with an age spectrum from 503 to 1263Ma; the youngest grain suggests a maximum Cambrian age of protolith deposition. An ophiolitic metagabbro of the Kemin Complex yielded an Early Cambrian age of 531±4Ma, which is close to the age of ophiolites in the adjacent Djalair–Naiman belt of Kazakhstan, suggesting a possible genetic link. Two samples of quartz diorite from the post-kinematic Dolpran pluton yielded Early Ordovician zircon ages of 471.9±3.5 and 472.0±3.1Ma. The presence of a 783±7Ma xenocrystic zircon points at Precambrian crust at depth, which may explain an earlier, discordant apparent age obtained by multigrain zircon dating of this pluton. Undeformed rhyolite and basalt in the East Kyrgyz Range, previously classified as Neoproterozoic and Cambrian, yielded Late Ordovician ages of 451.9±4.6 and 448.9±5.6Ma respectively.Our data imply that the Aktyuz terrane is not a single segment of continuous Precambrian continental crust but represents a complex amalgamation of Neoproterozoic continental and Early Palaeozoic ophiolitic slivers, which were stacked together in a subduction and collisional setting. Large fragments of continental crust were subducted to HP eclogite-facies conditions, which led to eclogite metamorphism in the mafic dykes. The main deformational event occurred during the latest Cambrian to earliest Ordovician between 503 and 472Ma.

UPb and 40Ar39Ar geochronology of Palaeozoic units in the northern Apennines: determining protolith age and alpine evolution using the Calamita Schist and Ortano Porphyroid

AbstractIn the northern Apennines, the Palaeozoic basement involved in the Late Oligocene–Middle Miocene nappe stack contains metamorphic units for which hypothetical ages have been assigned on the basis of lithological correlations with the Palaeozoic formations of the Variscan chain in Sardinia. This uncertainty concerning the age poses limitations to reconstructing the Palaeozoic stratigraphy, defining the Alpine and pre‐Alpine histories and correlations with other domains of the Variscan chain. We present the UPb age of detrital zircon and the 40Ar39Ar age of metamorphic muscovite for the Calamita Schist and Ortano Porphyroid, two metamorphic units of undetermined Palaeozoic age cropping out in the eastern Elba Island. The radioisotopic data allows us to: (i) define the Early Carboniferous and Middle Ordovician ages for the Calamita Schist and Ortano Porphyroid, respectively, as well as their derivation (flysch deposit and magmatic rocks); (ii) pose some constraints concerning their alpine tectonic and metamorphic histories. These new data generate a more precise reconstruction of the Palaeozoic sequence in the northern Apennines, and they document that the Palaeozoic basement involved in the alpine deformation underwent internal stacking with an inversion of the original sequence. Copyright © 2010 John Wiley & Sons, Ltd.

Sediment sources and dispersion as revealed by single-grain 40Ar/39Ar ages of detrital muscovite from Carboniferous and Cretaceous rocks in mainland Nova ScotiaGeological Survey of Canada contribution 20090289.

Single-grain ages of detrital muscovite from 15 sand(stone) samples from the Lower Carboniferous Horton Group and the Lower Cretaceous Chaswood Formation of central Nova Scotia were used to infer the nature of the Early Carboniferous unroofing of the Meguma terrane and the reworking of Carboniferous rocks in the Early Cretaceous. In the western Windsor Basin, a sample from the oldest Horton Group rocks yielded ages principally between ca. 400 and 380 Ma, suggesting that most of the muscovite present came from the metamorphic rocks of the Meguma terrane but was variably reset by the intrusion of the South Mountain Batholith at ca. 380 Ma. Other samples in this part of the basin show partial post-depositional resetting. Younger Horton Group metamorphic rocks in the eastern Windsor Basin contain many grains with ages of ca. 370–360 Ma, suggesting derivation from the central core of the South Mountain Batholith or the Musquodoboit Pluton. Horton Group sandstones from the western part of the St. Marys Basin contain muscovite derived from the Liscomb Complex along with metamorphic muscovite variably reset by the intrusion of this complex. In general, our data suggest predominant northward dispersion of muscovite from the Meguma terrane to the Horton Group and a lack of axial transport along the Horton grabens through central Nova Scotia, a pattern compatible with tectonic models in which the Meguma terrane is ramped over the Avalon terrane. Muscovite ages obtained for the Chaswood Formation compare well with those from the Horton Group rocks in the western St. Marys Basin. These rocks may have been exposed to rapid erosion by reactivation of the Cobequid–Chedabucto fault zone in the Early Cretaceous and the resulting sediments were perhaps transported to depositional sites along northeast-trending faults. Unlike the detrital monazites in these rocks, there is no evidence that any of the detrital muscovites came from distal sources outside the Meguma terrane.

Zircon dissolution in a ductile shear zone, Monte Rosa granite gneiss, northern Italy

AbstractThe sizes, distributions and shapes of zircon grains within variably deformed granite gneiss from the western Alps have been studied. Zircon shows numerous indicators of a metamorphic response in both the host gneiss and a 5 cm wide continuous ductile shear zone, within which the zircon grain sizes range from <1 urn to >50 μm. However, the very fine grain sizes are virtually absent from grain boundaries. Within this zone, zircons consistently have more rounded and embayed margins, which are interpreted as evidence of dissolution in response to fluid influx during shearing. Zircons are preferentially located near metamorphic muscovite in both the host gneiss and the shear zone and tend to show the poorest crystal shape, indicating that fluids linked to the formation and presence of muscovite may enhance both the crystallization of zircon and its subsequent dissolution. Larger zircon crystals typically show a brittle response to deformation when adjacent to phyllosilicates, with fractures consistently perpendicular to the (001) mica cleavage. The variety of metamorphic behaviour observed for zircon indicates that it may be highly reactive in sub-solidus mid-crustal metamorphic environments.

Geochemical signatures of mesothermal Au-mineralized late-metamorphic deformation zones, Otago Schist, New Zealand

Hydrothermal processes along two regional-scale shear zones in the Otago Schist were dominated by structurally controlled fluid flow and mineralization in the host schist, with relatively minor quartz vein formation, and mineralized rocks are only subtly different from unmineralized rocks. Most Au in the shear zones is associated with sulphide minerals (pyrite and arsenopyrite) disseminated through the host schist or along microshears. Minor enrichment of Sb, Mo and Bi (ppm level) is detectable in the Hyde-Macraes Shear Zone (HMSZ). Hydrothermal muscovite is slightly more aluminous (1–2 wt%) than metamorphic muscovite in both shear zones. HMSZ muscovite averages >900 ppm N, in contrast to metamorphic muscovite that averages c. 200 ppm N. In both shear zones, rutile has replaced metamorphic titanite and epidote has altered to carbonate and phyllosilicates, but these reactions were nearly isochemical. Structurally controlled hydrothermal graphite in the HMSZ occurs in microshears (up to 3 wt%, above background <0.2 wt%). Alteration in the Rise & Shine Shear Zone (RSSZ) was accompanied by addition of abundant ankerite. The two shear zones have subtly different geochemical signatures and are not directly genetically related. However, As enrichment is a key exploration target for both shear zones.

Variscan amphibolite-facies rocks from the Kurtoğlu metamorphic complex (Gümüşhane area, Eastern Pontides, Turkey)

The Kurtoglu metamorphic complex, that forms part of the pre-Liassic basement of the Sakarya zone in northern Turkey, consists of at least two tectonic units. Blueschist-facies rocks of unknown metamorphic age in the southern part of the complex are tectonically overlain by Variscan low-pressure high-temperature metamorphic rocks. The latter comprise mica schists and fine-grained gneisses, cut by metaleucogranitic dikes, as well as migmatitic biotite gneisses and subordinate amphibolite intercalations. Structural data indicate that metamorphism and penetrative deformation occurred after dyke intrusion. Peak metamorphic conditions of the mica schists, fine-grained gneisses and metaleucogranites are estimated to ∼650°C and ∼0.4 GPa, based on phase relationships in the system NCKFMASH, Fe–Mg partitioning between garnet and biotite as well as garnet-aluminosilicate-quartz-plagioclase (GASP) and garnet-plagioclase-biotite-quartz (GBPQ) barometry. Peak temperatures of the migmatitic biotite gneisses and amphibolite intercalations are not well constrained but might have been significantly higher (690–740°C), as suggested from hornblende-plagioclase thermometry. 40Ar–39Ar incremental dating on muscovite and biotite fractions from the mica schists and fine-grained gneisses yielded plateau ages of ∼323 Ma. Significantly older model ages of ∼329 and ∼337 Ma were obtained on muscovite fractions from two metaleucogranite samples. These fractions contain both relict igneous and newly formed metamorphic muscovite.

Deformation history of the northwestern Selwyn Basin, Yukon, Canada: Implications for orogen evolution and mid-Cretaceous magmatism

Neoproterozoic to Paleozoic slope-to-basin facies continental margin strata underlie area 700 x 200 km across central Yukon Territory, Canada, and collectively define the Selwyn Basin. In a Cordilleran framework, Selwyn Basin strata form a strongly deformed and thrust-faulted package located between the Mackenzie foreland fold-and-thrust belt, and accreted terranes and displaced elements of the ancient North American continental margin. Orogeny commenced in the Jurassic as exotic elements of the composite Yukon-Tanana terrane overrode the ancient continental margin. Collision-related deformation had ceased by ca. 100 Ma, and was followed by a Late Cretaceous (post–85 Ma) dextral transcurrent regime, which laterally displaced elements of the newly assembled continental margin along the orogen-parallel Tintina fault. In western Selwyn Basin, more than 100 km of structural overlap was accommodated on two main detachments, the Robert Service and underlying Tombstone thrust faults. Internal deformation within the thrust sheets is intense, characterized by shear-related folds and fabrics. Metamorphic grade reaches lower to middle greenschist facies at the deepest structural levels exposed, and is characterized by chlorite-muscovite schists. The onset of deformation is constrained by the Late Jurassic age of the youngest units deformed during orogeny. The end of ductile deformation is constrained by new 40Ar/39Ar ages for metamorphic muscovite that range from 104 to 100 Ma. Due to the low metamorphic grade, these ages are interpreted to closely follow the waning of deformation. At ca. 93 ± 3 Ma, isolated granitic intrusions of the Tombstone-Tungsten magmatic belt were emplaced across the western Selwyn Basin in a tensional, postcollisional regime. Restoration of displacement on the Tintina fault places the western Selwyn Basin adjacent to the Yukon-Tanana terrane uplands of east-central Alaska in the Early to mid-Cretaceous. Despite their adjacent positioning in cross-orogen section during orogenesis, the two elements feature some significant differences in Jurassic-Cretaceous deformation. Most notably, the Yukon-Tanana terrane uplands record a significant extensional event at 120–105 Ma, which resulted in NW-SE–oriented extension, exhumation of deep structural levels, and voluminous felsic plutonism. In contrast, western Selwyn Basin did not undergo equivalent uplift and extension, and features temporally and spatially restricted plutonism. Within an orogenic framework, the Yukon-Tanana terrane uplands can therefore be considered to represent an exhumed core characterized by high heat flow, whereas the western Selwyn Basin represents an immediate northeastern salient to the exhumed core. These differences have important implications for the geodynamic setting of mid-Cretaceous plutonism across these two major lithologic-tectonic entities of the northern Cordillera.

SIGNIFICANCE OF ALTERATION ASSEMBLAGES FOR THE ORIGIN AND EVOLUTION OF THE PROTEROZOIC NABARLEK UNCONFORMITY-RELATED URANIUM DEPOSIT, NORTHERN TERRITORY, AUSTRALIA—A DISCUSSION

Sir: In their recent paper on the Nabarlek uranium deposit, Polito et al. (2004) appear to have misunderstood Wilde and Wall’s (1987) description of outer and inner halo alteration. Wilde and Wall (1987) defined the outer alteration as pseudomorphous replacement of amphibole by chlorite, biotite by chlorite, and rutile (or anatase) and feldspar by finegrained white mica (sericite) and not as chlorite-hematite schist, chlorite-muscovite ± hematite schist and chloritesericite schist up to 1 km from the Nabarlek fault (Polito et al., p. 117). Contrary to the assertion by Polito et al. (2004, p. 117) that is ... relatively rare in the outer alteration zone, metamorphic rocks of the outer halo have been pervasively silicified beneath and along the margins of the Oenpelli Dolerite and quartz veining is common along pre-ore faults beyond the ore zone. The suggestion of Polito et al. (2004) that pervasive silicification was coincident with intrusion of the Oenpelli Dolerite lacks credible supporting evidence. An equally plausible interpretation is that the lithologic boundary between the competent and homogeneous Oenpelli Dolerite and its strongly foliated and inhomogeneous host rocks focused fluid flow during post-intrusion deformation. The inner halo as defined by Wilde and Wall (1987) extends for no more than 50 m from ore and is manifested as nearcomplete pervasive replacement of metamorphic quartz by fine-grained (clay-sized) chlorite and white mica involving extreme desilicification of the host rocks (Fig. 1a, b). In many samples from Nabarlek and other uranium deposits from the East Alligator Rivers uranium field there is evidence of preservation of metamorphic (and outer halo) phases, notably coarse metamorphic muscovite and biotite (often with interlayered chlorite) in pervasive fine-grained alteration products (Fig. 1a-d). Fig. 1. a. Photomicrograph, plane-polarized light, of altered schist from inner alteration zone at Nabarlek (DH96 60m). Ch …