Muscovite Plateau Ages Research Articles

Early-Late Devonian Post-Collision Extension Due to Slab Breakoff Regime along the Northern Margin of North China Craton: Implications from Muscovite 40Ar-39Ar Dating

Ordovician-Silurian subduction, Early Devonian arc-contient collision and followed post-collision extension are recorded in the north of the North China Craton. Most previous research has focused on the first two processes. Discussion on the post-collisional extension and its tetonic regime is still limited. In this study, 40Ar-39Ar muscovite ages obtained from the central part of the northern North China Craton were analyzed to shed light on the timing of post-collision extension. Garnet muscovite schist and muscovite quartz schist in the Jianshan Formation yielded 40Ar-39Ar muscovite plateau ages of 412 ± 3 Ma and 391 ± 3 Ma, respectively. Two other 40Ar-39Ar muscovite plateau ages (389 ± 2 Ma and 397 ± 2Ma) were obtained for two Mesoproterozoic monzogranites intruding into the Jianshan Formation. Based on previous research, the northern North China Craton underwent a collision event with Bainaimiao arc at c. 415 Ma, followed by post-collision extension in early Late Paleozoic. Therefore, combined with the newly acquired muscovite 40Ar-39Ar dating results, the Jianshan Formation might go through regional metamorphism at c. 412 Ma during the collision process. Subsequently, the Jianshan Formation and monzogranites intruding into it went through rapid exhumation along with metamorphism at c. 397–389 Ma in a post-collision extensional setting. The muscovite 40Ar-39Ar ages provide new markers for the exhumation history and the post-collisional extension setting during Early-Late Devonian in the study area. Furthermore, slab breakoff as the cause for this extensional setting is argued by the emplacement of the Early-Late Devonian alkaline rocks.

A high-pressure folded klippe at Tehuitzingo on the western margin of an extrusion zone, Acatlán Complex, southern México

High-pressure (HP) rocks at Tehuitzingo, on the western margin of the HP belt within the Paleozoic Acatlán Complex (southern México), occur in a klippe that was thrust over low-grade clastic rocks. The youngest detrital zircon cluster in the low-grade rocks yielded U-Pb ages of 481±16Ma, which provide an older limit for deposition. The HP rocks are composed of metabasites, serpentinite, granite (482±3Ma) and mica schist (youngest concordant detrital zircon: 433±3Ma). The schist and granite are inferred to be high-grade equivalents of lower Paleozoic, low-grade rocks exposed elsewhere in the Acatlán Complex, from which they are inferred to have been removed by subduction erosion. Mineral analyses indicate that the subducted rocks underwent HP metamorphism and polyphase deformation at depths of ~50km (~16kbar and 750°C: eclogite facies). Subsequent retrogression passed through epidote-amphibolite to greenschist facies, which was synchronous with W-vergent thrusting over the low-grade clastic rocks. Deposition of the low-grade rocks and thrusting are bracketed between either 481–329Ma (Ordovician-Mississippian), and was followed by F3 synformal folding. Cooling through ca. 385°C is indicated by 329±1 and 316–317±2Ma, 40Ar/39Ar muscovite plateau ages in HP rocks, which are 5–17 my younger than those of the adjacent Piaxtla eclogites suggesting younger exhumation. The petrology, P-T conditions and ages of the Piaxtla Suite is consistent with an extrusion channel within the Acatlán Complex along the active western margin of Pangea during the Carboniferous. Detrital zircon populations in the low-grade psammite (ca. 481, 520–650, 720, 750, 815, 890, 1050 and 2750Ma) and the HP schist (ca. 457–480, 534, 908, 954–1150, 1265, 1845 and 2035Ma) indicate derivation from the Ordovician Acatlán granitoids, Neoproterozoic Brasiliano orogens, 900–750Ma Goiás arc (Amazonia), 1–1.3Ma Oaxaquia, and more ancient sources in Oaxaquia/Amazonia.

Ordovician and Late Paleozoic–Early Mesozoic tectonothermal history of the La Noria area, northern Acatlán Complex, southern Mexico: Record of convergence in the Rheic and paleo-Pacific Oceans

The La Noria area lies in the northern part of the polydeformed Acatlán Complex, (southern Mexican Mixteca terrane), and the rocks record the following sequence of events: (i) Early–Middle Ordovician deposition of the volcaniclastic El Epazote and Las Calaveras units; (ii) late Middle Ordovician intrusion of the 467 ± 16 Ma megacrystic, peraluminous, rift-related granitoids; (iii) late Devonian, D 1, greenschist facies deformation; (iv) intrusion of the Los Malpasos leucogranite and associated minor intrusions; (v) Middle Mississippian, D 2, dextral N–S deformation also under greenschist facies metamorphic conditions; and (vi) undated D 3 kink band development. U–Pb LA-ICPMS detrital zircon ages: (a) the El Epazote unit yielded: a mean 206Pb/ 238U age from the youngest five concordant 206Pb/ 238Pb age of 488 ± 10 Ma, with other age peaks at ca. 506, ca. 1077, and ca. 1779 Ma and a few concordant Neoproterozoic ages: and (b) the Las Calaveras unit yielded a mean 206Pb/ 238U age from the sixteen youngest detrital zircons of 466 ± 10 Ma with other population age peaks ca. 1111, and ca. 1753 Ma. These data imply that granitoid intrusion was roughly synchronous with deposition of some of the host rocks. Whereas sources for most of the detrital zircons may be found in either the Acatlán and Oaxacan complexes, Laurentia or Gondwana, a Neoproterozoic source is most likely in Amazonia. The rocks record three low-grade deformational episodes: (i) D 1 produced a weak compositional banding and/or schistosity (S 1) under greenschist facies conditions; (ii) D 2, also occurred under greenschist facies conditions, and developed tight to isoclinal folds (F 2) in S 1 and an axial planar spaced-cleavage (S 2) that is co-planar with S 1; and (iii) D 3 produced reverse and conjugate kink bands (F 3) that deform the S 1/S 2 composite foliation. The leucogranite and related dikes that intrude the complex record only the latter two deformational events. Ca. 330 Ma 40Ar/ 39Ar muscovite plateau ages probably closely post-date the D 2 event. D 1 may be correlated with early Carboniferous deformation elsewhere in the Acatlán Complex. On the other hand, the initial 40Ar/ 39Ar steps at ca. 300, 220 and 172 Ma probably indicate thermal disturbances below 300 °C during the Permo-Carboniferous, Triassic and Jurassic, respectively. Whereas the Ordovician history of the plutons and volcano-sedimentary units coincides with the lifespan of both the Iapetus and Rheic oceans, the Late Paleozoic–Early Mesozoic deformation better reflects closure of the Rheic Ocean and convergence tectonics on the paleo-Pacific margin following the amalgamation of Pangea.

Superposed Neoproterozoic and Silurian magmatic arcs in central Cape Breton Island, Canada: geochemical and geochronological constraints

Isotopic and geochemical data indicate that intrusions in the eastern Creignish Hills of central Cape Breton Island, Canada represent the roots of arcs active at ∼ 540–585 Ma and ∼ 440 Ma. Times of intrusion are closely dated by (1) a nearly concordant U–Pb zircon age of 553±2 Ma in diorites of the Creignish Hills pluton; (2) a lower intercept U–Pb zircon age of 540±3 Ma that is within analytical error of 40Ar/39 Ar hornblende plateau isotope-correlation ages of 545 and 550±7 Ma in the River Denys diorite; and (3) an upper intercept U–Pb zircon age of 586±2 Ma in the Melford granitic stock. On the other hand, ∼ 441–455 Ma 40Ar/39 Ar muscovite plateau ages in the host rock adjacent to the Skye Mountain granite provide the best estimate of the time of intrusion, and are consistent with the presence of granitic dykes cutting the Skye Mountain gabbro–diorite previously dated at 438±2 Ma. All the intrusions are calc-alkaline; the Skye Mountain granite is peraluminous. Trace element abundances and Nb and Ti depletions of the intrusive rocks are characteristic of subduction-related rocks. The ∼ 540–585 Ma intrusions form part of an extensive belt running across central Cape Breton Island, and represent the youngest Neoproterozoic arc magmas in this part of Avalonia. Nearby, they are overlain by Middle Cambrian units containing rift-related volcanic rocks, which bracket the transition from convergence to extension between ∼ 540 and 505/520 Ma. This transition varies along the Avalon arc: 590 Ma in southern New England, 560–538 Ma in southern New Brunswick, and 570 Ma in eastern Newfoundland. The bi-directional diachronism in this transition is attributed to northwestward subduction of two mid-ocean ridges bordering an oceanic plate, and the migration of two ridge–trench–transform triple points. Following complete subduction of the ridges, remnant mantle upwelling along the subducted ridges produced uplift, gravitational collapse and the high-temperature/low-pressure metamorphism in the arc in both southern New Brunswick and central Cape Breton Island. The ∼ 440 Ma arc magmatism in the Creignish Hills extends through the Cape Breton Highlands and into southern Newfoundland, and has recently been attributed to northwesterly subduction along the northern margin of the Rheic Ocean.

Fine-scale isotopic heterogeneities and fluids in the deep crust: a 40Ar/ 39Ar laser ablation and TEM study of muscovites from a granulite-eclogite transition zone

Spectacular exposures on Holsnøy Island, western Norway, reveal that eclogites formed in situ from adjacent anhydrous granulites as a result of the localized infiltration of fluids. Stepwise and laser 40Ar/ 39Ar experiments on muscovite from the eclogites have been used in conjunction with electron microprobe and transmission electron microscopy (TEM) analysis to examine the behavior of argon under high pressure metamorphism and subsequent cooling. Muscovites with 0.10–0.17Na/(Na + K) yield 40Ar/ 39Ar plateau and integrated ages in the range 450–465 Ma, 20–35 Myr older than 40Ar/ 39Ar muscovite plateau ages from adjacent eclogite samples. Laser 40Ar/ 39Ar analyses on ∼ 75 μm spots across two single muscovite grains from these samples are approximately uniform at 444.2 ± 1.9 (21 spots) and 443.3 ± 1.7 Ma (26 spots). However, incremental heating of high sodic (0.18–0.32 (Na/(Na + K)) muscovite failed to yield a plateau. Laser 40Ar/ 39Ar analyses of 100 areas from two single grains from this sample reveal complex two-dimensional patterns with variable apparent ages in the range of 445–625 Ma. Electron microprobe measurements obtained from one of the grains reveal that the paragonite content is also highly variable both parallel and perpendicular to (001). All the TEM bright field images obtained from this mica yield information consistent with a single homogeneous phyllosilicate phase, yet several of the selected area electron diffraction (SAED) analyses document locally intergrown muscovite and sodic muscovite, possibly in an exsolution relationship on a unit cell scale. Analytical electron microscopy (AEM) analyses obtained over a 5–100 nm scale on the same sample reveal great range and heterogeneity inNa/K, including pure muscovite, although no separate paragonite domains were found. In contrast, muscovite grains with consistent laser spot fusion and plateau ages show little heterogeneity inNa/K at the microprobe level. These data provide evidence of variable fluid-mediated incorporation of excess 40Ar under high pressure eclogite facies conditions at the time of the development of the sodic muscovite compositions. The data indicate that fine-scale heterogeneities in argon isotopic compositions were generated during eclogite facies equilibration and were preserved as the rocks were cycled back to the surface. Despite the influx of free fluids in the rocks at high pressure, the heterogeneities are best reconciled with local buffering and limited mobility of the fluids, at least during the final stages of equilibration.

Thick‐skinned versus thin‐skinned thrusting: Rheology controlled thrust propagation in the Variscan collisional belt (The southeastern Bohemian Massif, Czech Republic ‐ Austria)

The Variscan nappe assembly within the southeastern Bohemian Massif includes (1) crystalline nappes which were transported from initial granulite to amphibolite facies conditions to uppermost crustal levels, (2) nappe assembly of footwall units under very low to low‐grade metamorphic conditions, and (3) frontal, thin‐skinned imbrication of flysch sequences which were deposited within a deep and narrow foreland basin during loading of the lower plate. Thick‐skinned thrusting of hinterland tectonic units resulted in an inverted metamorphic zonation and contrasts with the thin‐skinned tectonic character of foreland units. 40Ar/39Ar hornblende and muscovite plateau ages from hanging wall units reflect complete rejuvenation of intracrystalline argon isotopic systems during Late Variscan thrusting (between ∼ 340 Ma and 325 Ma). The 40Ar/39Ar mineral ages together with previously published 40Ar/39Ar and Sm/Nd data record systematically decreasing Variscan mineral ages structurally downward into footwall units. This is explained by rapid exhumation and cooling synchronously with thrusting along crustal‐scale ramps during foreland‐directed nappe propagation. The location of decollement zones within specific structural levels was apparently controlled by crustal‐scale rheological con‐trasts. Late Variscan cooling ages contrast with Cadomian 40Ar/39Ar hornblende and muscovite ages recorded within the structural basement immediately below the major Variscan thrust plane where intracrystalline isotopic systems were only partially rejuvenated during Variscan thrusting. These record a sequence of Cadomian magmatic and metamorphic events in foreland units including (1) pre intrusive metamorphism and deformation under amphibolite facies metamorphic conditions (>610 Ma), (2) greisen formation during a Cadomian collisional event (∼ 600 Ma), and (3) intrusion of calcalkaline granitoid bodies with I‐type affinities (∼ 590 Ma).

Microstructure, metamorphism, thermochronology and P‐T‐t‐deformation history of the Port aux Basques gneisses, south‐west Newfoundland, Canada*

Abstract The Port aux Basques gneisses comprise three lithostratigraphic units separated by major fault zones: the Grand Bay Complex; the Port aux Basques Complex; and the Harbor le Cou Group. A similar regionally developed polyphase history of penetrative deformation characterizes each of these units. Thickening during D1 produced rare recumbent folds (F1) and an axial planar schistosity (S1), overprinted by D2 recumbent folds (F2), and transposed during development of a locally penetrative, differentiated crenulation cleavage (S2). In western sectors of the area, D2 was associated with NW‐directed reverse shearing. The NE‐trending structural grain reflects D3 transpression, partitioned into dextral transcurrent movement along major shear zones and development of upright‐to‐steeply inclined, periclinal folds (F3) and a variably penetrative schistosity (S3).Amphibolite facies metamorphism increases in grade from west to east across the area. Microstructures, including porphyroblast‐matrix foliation relations and internal textural unconformities in garnet, indicate episodic porphyroblast nucleation and growth, which reflect a prograde traverse sequentially across univariant reactions during syntectonic metamorphism. Garnet, kyanite and staurolite porphyroblasts are wrapped by the S2 foliation, but each may contain trails of inclusions that define S1; commonly these trails preserve early stages of S2 crenulation cleavage development. Progressive and sequential reaction out of kyanite, staurolite and muscovite in favour of sillimanite, garnet, biotite and K‐feldspar, and the development of an increasing volume of anatectic migmatite in south‐eastern sectors of the area record syn‐ to late‐D2 peak metamorphic conditions. Microstructural relationships and petrogenetic grid considerations indicate clockwise trajectories in P‐T space for units of the Port aux Basques gneisses. Peak metamorphic conditions are estimated to have been 620–650° C at ≤8kbar in the west and 700–750° C at ≤8 kbar in the east.Titanite from an upper amphibolite facies calc‐silicate gneiss yields U‐Pb ages of c. 420 Ma, interpreted to date cooling shortly after the thermal peak in these gneisses. Variable D3 strain was associated with some recrystallization of hornblende and micas. 40Ar/39Ar hornblende plateau isotope correlation ages range from c. 419 to c. 393 Ma, from east to west across the area, and are interpreted to record cooling through c. 500° C coeval with or soon after D3 deformation. The range in ages may record the effects of heterogeneous D3 deformation and differential uplift from south‐east to north‐west associated with displacement on major shear zones. 40Ar/39Ar muscovite plateau ages cluster at c. 390 Ma, and date cooling through c. 375° C during regional exhumation. Cooling rates are moderate to fast and may indicate a component of tectonic exhumation.The Port aux Basques gneisses are a product of Silurian collisional tectonics. The higher grade of metamorphism in comparison with adjacent areas of the Canadian Appalachians is interpreted to reflect greater thickening due to juxtaposition of the St Lawrence promontory (Laurentian margin) with the Cabot promontory (Avalonian margin) during closure of the Iapetus Ocean.

40Ar/39Ar dating of metamorphic and igneous rocks of the Liscomb Complex, Meguma Terrane, southern Nova Scotia, Canada

Recent geological mapping in the Meguma Terrane of southern Nova Scotia has revealed the presence of previously unrecognized high-grade gneisses and gabbroic rocks that form, along with previously recognized granitic rocks, the Liscomb Complex. Field relationships indicate emplacement of the suites, in the sequence gneisses, gabbros, and granites, after the ca. 400 Ma Acadian deformation of the Meguma Group metasedimentary rocks. 40Ar/39Ar dating of amphibole and mica phases from these suites indicates (1) an amphibole inverse isochron age of 375 ± 3 Ma for the gneisses, and amphibole plateau ages of 371 ± 4 and 372 ± 5 Ma for the gabbros; (2) muscovite plateau ages of 367 ± 3 Ma for the gneisses, and 369 ± 3 to 373 ± 4 Ma for the granites; and (3) variably discordant biotite age spectra with integrated ages sometimes greater than the amphibole and muscovite plateau ages (371 – 383 Ma (n = 5) gneisses; 373–385 Ma (n = 3) gabbros; 367 and 368 Ma in granites). The spectral discordance in the biotites is attributed to irradiation-induced recoil in the presence of sub-microscopic chlorite, whereas the high integrated ages reflect excess argon.Collectively, the age data indicate rapid cooling of the Liscomb Complex through the argon retention temperatures of amphibole (approx. 500 °C) and micas (approx. 350–300 °C), consistent with previous 40Ar/39Ar dating of amphibole and micas in this area. This cooling history is attributed to rapid uplift and exhumation of the area postemplacement of the units because of its close proximity (i.e., along its northern boundary) to the Cobequid – Chedabucto Fault System.

Tectonic evolution of Proterozoic rocks in the Cimarron Mountains, northern New Mexico, USA

Abstract Portions of three Proterozoic tectonostratigraphic sequences are exposed in the Cimarron Mountains of New Mexico. The Cimarron River tectonic unit has affinities to a convergent margin plutonic/volcanic complex. Igneous hornblende from a quartz diorite stock records an emplacement pressure of 2–2.6 kbar. Rocks within this unit were subsequently deformed during a greenschist facies regional metamorphism at 4–5 kbar and 330 ± 50° C.The Tolby Meadow tectonic unit consists of quartzite and schist. Mineral assemblages are indicative of regional metamorphism at pressures near 4 kbar and temperatures of 520 ± 20° C. A low‐angle ductile shear zone separates this succession from gneisses of the structurally underlying Eagle Nest tectonic unit. Gneissic granite yields hornblende pressures of 6–8 kbar. Pelitic gneiss records regional metamorphic conditions of 6–7 kbar and 705 ± 15° C, overprinted by retrogression at 4 kbar and 530 ± 10° C. Comparison of metamorphic and retrograde conditions indicates a P–T path dominated by decompression and cooling. The low‐angle ductile shear zone represents an extensional structure which was active during metamorphism. This extension juxtaposed the Tolby Meadow and Eagle Nest units at 4 kbar and 520° C. Both units were later overprinted by folding and low‐grade metamorphism, and then were emplaced against the Cimarron River tectonic unit by right‐slip movement along the steeply dipping Fowler Pass shear zone.An argon isotope‐correlation age obtained from igneous hornblende dates plutonism in the Cimarron River unit at 1678 Ma. Muscovite associated with the greenschist facies metamorphic overprint yields a 40 Ar/39 Ar plateau age of 1350 Ma. By contrast, rocks within the Tolby Meadow and Eagle Nest units yield significantly younger argon cooling ages. Hornblende isotope‐correlation ages of 1394–1398 Ma are interpreted to date cooling during middle Proterozoic extension. Muscovite plateau ages of 1267–1257 Ma appear to date cooling from the low‐grade metamorphic overprint. The latest ductile movement along the Fowler Pass shear zone post‐dated these cooling ages. Argon released from muscovites of the Eagle Nest/Tolby Meadow composite unit, at low experimental temperatures, yields apparent ages of c. 1100 Ma. Similar ages are not obtained north‐east of the Fowler Pass shear zone, suggesting movement more recently than 1100 Ma.