Muscovite Ages Research Articles

New 40Ar/39Ar laser single-grain ages of muscovites from mylonitic schists in the Rodna Mountains, Eastern Carpathians, Romania: correlations with microstructure

The Rodna Mountains afford the most internal structural window into the crystalline units of the Eastern Carpathians in Romania. The Rodna Mountains consist of Variscan metamorphic nappes that were restacked in the Alpine phase of Carpathian development forming the Subbucovinian and Infrabucovinian nappes. In order to evaluate age of deformation, ten samples were taken from the zone of greenschist facies mylonitic schist that marks the Alpine tectonic boundary between the Subbucovinian and Infrabucovinian nappes and 40Ar/39Ar laser single-grain ages determined for schistosity-forming muscovite. Microstructural assessment of quartz and muscovite distinguished two deformation events. Single-grain ages from the microstructurally most strongly reworked samples (four samples) give a tight clustering of ages at ca. 95 Ma. The least reworked schists have a broader clustering of ages spanning ca. 200–280 Ma with a late Permian peak and some samples showing outlier ages in the range 200–100 Ma. The relative development of the outliers, which correlates with evidence for increased microstructural reworking, is interpreted to mark progressive isotopic resetting. The ca. 95 Ma ages for the most reworked schists are estimates for the age of the Alpine nappe stacking. The ca. 200–280 Ma ages are similar to those of magmatism, metamorphism, and sedimentation thought to mark post-Variscan-pre-Alpine rifting and ocean basin formation in parts of the Alps and may be the thermal imprint of a related event in the Eastern Carpathians.

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.

Exhumation of the Diancang Shan metamorphic complex along the Ailao Shan-Red River belt, southwestern Yunnan, China: Evidence from 40Ar/ 39Ar thermochronology

Exhumation of the high-grade metamorphic rocks from deep to shallow crustal levels in the Diancang Shan (DCS) massif is evident by the temporal transition from shearing and mylonitization at the amphibolite facies in the lower to middle crust, through retrograde ductile–brittle faulting at greenschist facies in the middle crust and brittle faulting near the surface. New 40Ar/ 39Ar ages of amphibole, muscovite, biotite and K-feldspar, mostly from mylonitic rocks, help constrain the thermal evolution of the massif during its exhumation. The thermochronologic studies have shown at least three stages of exhumation and cooling from late Oligocene to Pliocene, e.g., 28–13 Ma, 13–4 Ma and 4–0 Ma, respectively. The initiation of the unroofing history of the DCS massif resulted from ductile left-lateral shearing along the Ailao Shan-Red River shear zone (ASRR) since the late Oligocene. Diachronous onset and subsequent parallel cooling and exhumation characterize the early thermal history of the massif. The first stage of exhumation might have stopped due to the cessation of the left-lateral shearing at ca. 21 Ma, although cooling continued until 13 Ma ago to relatively low-temperature conditions. The second and third stages of cooling started from 13 Ma and lasted until the recent active faulting. The diversity of cooling rates during the second stage from 13 to 4 Ma suggests differences in exhumation rates at various localities. Ductile to brittle shearing along the eastern flank retrograde normal shear zone is the best explanation for the exhumation during this stage. The inhomogeneous cooling and exhumation of the lower plate of the normal fault zone possibly resulted from the tilting of the lower plate or differential exhumation of slabs in the lower plate. During the third stage, cooling and exhumation of the DCS massif since 4 Ma ago is recognized when higher temperature cooling paths are extended and plotted at a lower temperature of about 16 °C, the present day average surface temperature. Therefore, the diachronous cooling and exhumation continuously spanned to the surface. The tectonic exhumation of the DCS massif constitutes a part of the tectono-thermal evolution and diachronous exhumation of the metamorphic complexes in the fan-shaped area bounded broadly in SE Asia by the Red River fault zone and Sagaing fault zone since 36 Ma. Extrusion-induced strike-slip shearing and rotation-associated normal faulting, played important roles in the diachronous cooling and exhumation of the DCS massif and other metamorphic complexes in SE Asia.

Origin of zircon in jadeitite from the Nishisonogi metamorphic rocks, Kyushu, Japan

On the basis of internal structures, laser ablation U–Pb ages and trace element compositions, the origin of zircon in jadeitite in the Nishisonogi metamorphic rocks was examined. The zircon comprises euhedral zoned cores overgrown by euhedral rims. The cores contain inclusions of muscovite, quartz, albite and possibly K-feldspar, yield 238U–206Pb ages of 126 ± 6 Ma (±2 SD, n = 45, MSWD = 1.0), and have Th/U ratios of 0.48–1.64. The rims contain inclusions of jadeite, yield 238U–206Pb ages of 84 ± 6 Ma (±2 SD, n = 14, MSWD = 1.1), and have Th/U ratios of <0.06. The cores are richer in Y, Th, Ti and rare earth elements (REEs), but the rims are richer in Hf and U. Chondrite-normalized REE patterns of the cores indicate higher SmN/LaN ratios, lower YbN/GdN ratios and larger positive Ce anomalies compared with those of the rims. Thus, the cores and rims have different 238U–206Pb ages and trace element compositions, suggesting two stages of zircon growth. Although the 238U–206Pb ages of the rims are consistent with the reported 40Ar/39Ar spot-fusion ages of matrix muscovite in the jadeitite, the 238U–206Pb ages of the cores are older. The mineral inclusions and high Th/U ratios in the cores are best explained by crystallization from felsic magma. Therefore, the cores are considered relicts from igneous precursor rocks. The rims surrounding the inherited cores possibly precipitated from aqueous fluids during jadeitite formation. The elevated U concentrations in the rims suggest that infiltration of external fluids was responsible for the precipitation. This study provides an example of jadeitite formation by metasomatic replacement of a protolith.

Geochronologic evidence for a cold arc–continent collision: The Taiwan orogeny

In order to constrain the tectonic consequences of the collision of the Luzon arc with the SE Chinese continental margin during the Neogene Taiwan orogeny, we assess the thermal history of the poly-metamorphic para-autochtthonous rocks of the continental margin in Taiwan. To accomplish this we report a new SIMS U–Pb age of prograde metamorphic zircons and interpret this age in the context of existing geochronology and thermochronology. Zircons extracted from the Fenniaolin amphibolite near Nanao, Taiwan, in the contact aureole of a granodiorite, are interpreted to have crystallized during the first lower amphibolite facies prograde metamorphism based on [1] their equant shape rather than the prismatic shape typical of magmatic zircon, [2] their microcrystalline or “micro-saccharoidal” aggregate structure and patchy and sector zoning rather than oscillatory zoning visible in cathodoluminescent imaging, [3] their low concentrations of U (most<40ppm), [4] their chlorite inclusions, and [5] their inclusion within prograde metamorphic amphiboles. The apparent absence of magmatic zircon is also consistent with the low Zr concentrations of the N-MORB metabasaltic protolith. SIMS analyses of 18 zircon grains yield a concordant 206Pb/238U age of 86±1Ma, analytically indistinguishable from the age of the adjacent pluton that caused the contact metamorphism. We thus conclude that the contact metamorphism of this intrusion caused the first lower amphibolite facies metamorphism of these rocks in the mid-Cretaceous.Existing 40Ar/39Ar and Rb–Sr cooling ages of both amphibole and muscovite at ~80Ma (e.g. Lan, 1990; Lo and Onstott, 1995) show that the granodiorite and its contact aureole cooled rapidly from ~700° to <400°C in~5m.y., indicating that the ambient temperature of the country rocks at the time of intrusion was less than the closure temperature of Ar diffusion in muscovite. Existing thermobarometry on rocks in the contact aureole yield pressures of ~5kb, equivalent to a depth of 15–20km. Cooling ages of biotite and K-feldspar in the literature reflect slow cooling at <2°C/m.y. for more than 50m.y. to at least 20Ma, suggesting atectonic cooling.The crustal history reconstructed from these data shows that the regional ‘gneissic’ terrane of the northern Tananao complex may have been caused by coalescing contact aureoles of multiple mid-Cretaceous intrusions (the ‘Nanao intrusive event’), and not by the Neogene Taiwan orogeny. On the contrary, the Tananao rocks probably achieved middle greenschist conditions and 15–20km depth in a forearc setting in the Early Cretaceous. This intrusive event caused ‘regional contact metamorphism’ at about 85Ma, after which the passive margin setting preserved these rocks at mid-crustal depths until the beginning of the Taiwan orogeny (~5Ma). These depths (10–12km) and temperatures (~200–250°C) were high enough to account for the late Miocene lower greenschist facies cleavages and the Pliocene–Pleistocene fission track cooling ages without additional loading and heating from the Taiwan orogeny.

La-ICP-MS U–Pb and 40Ar/ 39Ar geochronology of the sheared metamorphic rocks in the Wuyishan: Constraints on the timing of Early Paleozoic and Early Mesozoic tectono-thermal events in SE China

Mylonite and ultramylonite have been well documented in Pre-Devonian meta-volcanic and meta-sedimentary rocks in the Wuyishan, SE China. Field observations and thin section analyses show dextral strike-slip shearing. Deformation temperature is estimated in 400 °C–500 °C and the pressure between 7 kbar and 10 kbar. La-ICP-MS U–Pb zircon and 40Ar/ 39Ar step-heating biotite–muscovite ages are used to constrain the timing of the dextral shear zone and its tectonothermal history. La-ICP-MS U–Pb dating of zircons from three gneissic granitic samples provides magmatic crystallization ages between 441 and 431 Ma, and dating of the growth rims of zircons from one ultramylonite sample provides metamorphic age of 433.8 ± 6.8 Ma. These dates provide additional constraint on the timing of the Early Paleozoic tectono-thermal event in the Wuyishan region. 40Ar/ 39Ar step-heating dating of biotite and muscovite separated from two mylonite samples yields ages of 235.3 ± 2.8 Ma and 238.5 ± 2.8 Ma, respectively; and an undeformed granitic dyke intruded into the shear zone was dated, 229.8 ± 2.2 Ma, by La-ICP-MS U–Pb zircon method. These dates reveal that metamorphic rocks in the Wuyishan suffered from dextral shearing at the time span between 230 Ma and 239 Ma, during the Triassic Indosinian tectono-thermal event.

Obduction of Tethyan-type ophiolites—A case-study from the Thetford-Mines ophiolitic Complex, Quebec Appalachians, Canada

Oceanic plagiogranites and peridotite-hosted granites (PHG) of the Thetford-Mines ophiolite (TMO) yield mean U–Pb zircon ages of 479.2±1.6Ma and 469.5±2.8Ma, and are interpreted to be the products of fractionation of basaltic magma and melting of the Laurentian margin, respectively. The existing 40Ar/39Ar age for the ophiolitic metamorphic sole of the TMO is a hornblende isochron age of 477±5Ma for amphibolite facies rocks. Sites previously sampled for U–Pb and 39Ar–40Ar analyses were resampled for more accurate 39Ar–40Ar dating, in order to better constrain the thermo-tectonic evolution of the ophiolite and underlying Laurentian margin rocks on which they were overthrust, and quantify the time required for obduction of Tethyan-type ophiolites in the Canadian Appalachians. Amphiboles from TMO gabbro and plagiogranite yield 39Ar–40Ar plateau ages of 477–475Ma. In contrast, muscovites from PHG yield cooling ages of 466–465Ma, and show evidence for thermal resetting at ca. 460Ma for both a granitic mylonite and a crosscutting pegmatite facies. The amphibolitic metamorphic sole yields 39Ar–40Ar ages of ca. 471Ma in amphibole, and high-temperature ages of ca. 466Ma in muscovite, with evidence for recrystallization to ages as young as 457Ma. Muscovites from the sub-ophiolitic Laurentian metasedimentary basement, and micaschist clasts in supraophiolitic debris flows of the TMO, yield similar pseudo-plateau and high-temperature ages of ca. 463–464Ma. The U–Pb and 39Ar–40Ar data suggest that the obduction of peri-Laurentian oceanic crust lasted approximately 5 to 10 m.y. Since both facies formed approximatelym.y. after the ophiolitic crust, this supports models whereby the still-hot ophiolitic mantle provided the heat needed to generate the PHG and the metamorphic sole between ca. 475 and 470Ma. Muscovite ages of PHG record cooling below ca. 450–500°C by ca. 465Ma, but 39Ar–40Ar ages from both a mylonitized PHG facies and micaschist clasts suggest that the emplacement of the oceanic lithosphere on the Laurentia margin had nearly ended by ca. 460Ma, likely due to foreland-directed downward thrust propagation and related exhumation of the collisional wedge. The new age data imply that the southern Quebec ophiolites were thrust 100s of km over the Laurentian margin, which has major implications for the location of the subduction suture and the early stages of the Taconic orogeny in the Northern Appalachians.

Metamorphism and diachronous cooling in a contractional orogen: the Strandja Massif, NW Turkey

AbstractThe southern part of the Strandja Massif, northern Thrace, Turkey, comprises a basement of various gneisses, micaschists and rare amphibolite, and a cover of metaconglomerate and metasandstone, separated from each other by a pre-metamorphic unconformity. Metamorphic grade decreases from the epidote–amphibolite facies in the south to the albite–epidote–amphibolite/greenschist-facies transition in the north. Estimated P–T conditions are 485–530°C and 0.60–0.80 GPa in the epidote–amphibolite facies domain, and decrease towards the transitional domain between greenschist- and epidote–amphibolite facies. Rb–Sr muscovite ages range from 162.9 ± 1.6 Ma to 149.1 ± 2.1 Ma, and are significantly older (279–296 Ma) in the northernmost part of the study area. The Rb–Sr biotite ages decrease from 153.9 ± 1.5 Ma in the south to 134.4 ± 1.3 Ma in the north. These age values in conjunction with the attained temperatures suggest that the peak metamorphism occurred at around 160 Ma and cooling happened diachronously, and Rb–Sr muscovite ages were not reset during the metamorphism in the northernmost part. Structural features such as (i) consistent S-dipping foliation and SW to SE-plunging stretching lineation, (ii) top-to-the-N shear sense, and (iii) N-vergent ductile shear zones and brittle thrusts suggest a N-vergent compressional deformation coupled with exhumation. We tentatively ascribe this metamorphism and subsequent diachronous cooling to the northward propagation of a thrust slice. The compressional events in the Strandja Massif were most probably related to the coeval N-vergent subduction/collision system in the southerly lying Rhodope Massif.

Early–Middle Ordovician nappe tectonics of the junction between the Kokchetav HP-UHP metamorphic belt and the Stepnyak paleoisland arc ( northern Kazakhstan)

The North Kokchetav tectonic zone is located between the Kokchetav HP-UHP metamorphic belt and the Stepnyak zone of Ordovician island arc and oceanic complexes. The Kokchetav zone is a collage of nappes (thrust sheets) that consist of basement gneiss and sedimentary rocks of the Kokchetav microcontinent, granite gneiss, mica schists with eclogite blocks, the Shchuch’e ophiolite, Middle Proterozoic felsic volcanics, and Arenigian siliceous-terrigenous sediments with olistostromes. The latter are of gravity-sliding origin and their clastic material includes quartz-muscovite and quartz-garnet-muscovite schists, gneiss, dolomite, and amphibolite. The sheet boundaries are marked by mylonite and Early Ordovician mica schists ( 40Ar/ 39Ar ages of syntectonic muscovite are 489–469 Ma). The North Kokchetav collage of compositionally diverse thrust sheets can be interpreted as a collisional zone. According to geological evidence, tectonic activity in the zone lasted as late as the Middle Ordovician. Syncollisional thrusting in the North Kokchetav zone was coeval with the latest dynamic metamorphic event in the Kokchetav belt. All events of retrograde metamorphism and exhumation of HP and UHP rocks in the belt have Cambrian ages, i.e., the rocks had been exhumed prior to the Early–Middle Ordovician collisions and the related orogeny.

Late Eocene crustal thickening followed by Early-Late Oligocene extension along the India-Asia suture zone: Evidence for cyclicity in the Himalayan orogen

The timing of geologic events along the India-Asia suture in southern Tibet remains poorly understood because minimal denudation prevents widespread exposure of structurally deep rocks. In this study, we present geologic maps of two structurally deep domes, cored by mylonitic orthogneisses, across the India-Asia suture zone in southwestern Tibet. New U-Pb zircon ages and rock textures indicate that core orthogneisses are originally Gangdese arc rocks that experienced Late Eocene prograde metamorphism, probably during crustal thickening. Crosscutting leucogranite sills underwent northwest-southeast extension related to slip along a brittle ductile shear zone here designated the Ayi Shan detachment. The timing of shear along detachment is bracketed by zircon U-Pb ages of 26–32 Ma for these pre- to syn(?)-extensional leucogranites, and by a 40 Ar/ 39 Ar muscovite age of 18.10 ± 0.05 Ma for a rhyolitic dike. This rhyolite dike crosscuts a widespread siliciclastic unit that was deposited across the detachment, which we correlate to the Kailas Formation. The Great Counter thrust defines the surface trace of the India-Asia suture zone; it cuts the Kailas Formation, and is in turn cut by the Karakoram fault. A new 40 Ar/ 39 Ar muscovite age of 10.17 ± 0.04 Ma for the Karakoram fault footwall is consistent with published thermochronologic data that indicate Late Miocene transtension in southwestern Tibet.

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 &amp; Sons, Ltd.

Timing of ophiolite obduction in the Grampian orogen

This study addresses the timing and pressure-temperature (P-T) conditions of ophiolite obduction, one of the proposed causes of the ca. 470 Ma Grampian orogeny of Scotland and Ireland. This event gave rise to the main structural and metamorphic characteristics of the Grampian terrane-the type area for Barrovian metamorphism, the cause of which remains enigmatic despite a century of research. Zircons from the Highland Border ophiolite, Scotland, define a 499 +/- 8 Ma U-Pb concordia age, which is interpreted as dating magmatism. Its metamorphism is dated by a 490 +/- 4 Ma Ar-40-Ar-39 hornblende age, and a 488 +/- 1 Ma Ar-40-Ar-39 muscovite age from a metasedimentary xenolith within it, from which P-T estimates of 5.3 kbar and 580 degrees C relate to ophiolite obduction. Metamorphism of the Deerpark complex ophiolitic melange (Irish correlative of the Highland Border ophiolite) is constrained by a 514 +/- 3 Ma Ar-40-Ar-39 hornblende age, while mica schist slivers within it yield detrital zircon U-Pb ages consistent with Laurentian provenance and Rb-Sr and Ar-40-Ar-39 muscovite ages of ca. 482 Ma. P-T values of 3.3 kbar and 580 degrees C for the mica schist constrain the conditions of ophiolite obduction. Metamorphic mineral ages from the Grampian terrane (Dalradian Supergroup) are substantially younger (ca. 475-465 Ma) than those from the ophiolites. If conductive heating in overthickened crust was the cause of Barrovian metamorphism, then collisional thickening must have started soon after ophiolite obduction at ca. 490 Ma in order to generate the ca. 470 Ma metamorphic peak in the Grampian terrane. (Less)

Late Paleozoic subduction and exhumation of Cambro-Ordovician passive margin and arc rocks in the northern Acatlán Complex, southern Mexico: Geochronological constraints

The origin and age of high pressure (HP) rocks are crucial for paleogeographic reconstructions because they either define an oceanic suture or an extrusion zone within the upper plate. HP rocks in the San Miguel Las Minas area in the northern part of the Acatlán Complex, southern Mexico have been inferred to be of early Paleozoic age and to mark oceanic sutures. However, blueschists in the northern part of the complex have yielded Mississippian 40Ar/ 39Ar plateau ages of 344 ± 5 Ma for glaucophane and 338 ± 3 Ma and 337 ± 2 Ma for muscovite. These ages are slightly younger than recently published ages: a U–Pb zircon age of 353 ± 1 Ma from associated eclogite, and a 347 ± 3 Ma muscovite age from the tectonically overlying, greenschist facies Las Minas Unit. Taken together, these data indicate rapid cooling between 700° and 340 °C in ca. 17 Myr. On the other hand, associated Ordovician Anacahuite Amphibolite cooled through ca. 500 °C at 299 ± 6 Ma ( 40Ar/ 39Ar on hornblende) suggesting a second, Permian phase of exhumation. Protoliths of the high grade rocks include Cambrian–Ordovician, rift-passive margin, psammites, pelites, and tholeiitic dykes, an Ordovician mafic intrusion (Anacahuite Amphibolite dated at 470 ± 10 Ma: U–Pb zircon) and megacrystic granite (dated at 492 ± 12 Ma: U–Pb zircon), and arc-related mafic rocks of unknown age. These rocks are interpreted to have been part of the upper plate rocks that was removed by subduction erosion and taken to depths between 35 and 55 km where they underwent blueschist–eclogite facies metamorphism. This was followed by rapid extrusion along a channel bounded by an easterly dipping, Mississippian, listric normal shear zone, and a thrust modified by a Permian dextral fault. Rocks above and below the extrusion zone are mainly Cambro-Ordovician rift-passive margin units, but a small vestige of the arc is preserved as dikes cutting rocks lying unconformably beneath the fossiliferous latest Devonian–Lower Permian Patlanoaya Group. Since faunal data indicate that Pangea had begun amalgamation by the Mississippian, at which time the Acatlán Complex lay 1500–2000 km south of the Ouachita collisional orogen between Gondwana and Laurentia, it is inferred that subduction and extrusion of the high pressure rocks occurred on the active western margin of Pangea.

Monazite CHIME/EPMA dating of Erinpura granitoid deformation: Implications for Neoproterozoic tectono-thermal evolution of NW India

The in-situ “chemical” Th–U–Pb dating of monazite with the electron microprobe is used to unravel the Neoproterozoic tectono-thermal history of the “Erinpura Granite” terrane in the foreland of the Delhi Fold Belt (DFB) in the NW Indian craton. These granitoids are variably deformed and show effects of shearing activity. Monazites from the Erinpura granitoids recorded two main events; (1) protolith crystallization at 863 ± 23 Ma and (2) recrystallization and formation of new Th-poor monazite at 775 ± 26 Ma during shear overprint. Some components of the Erinpura granitoids, such as the Siyawa Granite and granites exposed near Sirohi town, show evidence of migmatization. This migmatization event is documented by anatexis and associated monazite crystallization at 779 ± 16 Ma. The age data indicate an overlap in timing between anatectic event and ductile shear deformation. The end of the tectono-thermal event in the Sirohi area is constrained by a 736 ± 6 Ma Ar–Ar muscovite age data from the ductile shear zone.

Out-of-sequence deformation and expansion of the Himalayan orogenic wedge: insight from the Changgo culmination, south central Tibet

[1] The Changgo culmination, one of the North Himalayan domes in south central Tibet, consists of a multiphase granite core surrounded by a deformed metasedimentary carapace. The granitic core records general non-coaxial shear with a top-to-the-south sense shear component. The contact between the core and the carapace is a shear zone, characterized by general non-coaxial shear with a top-to-the-north shear sense, interpreted to be the northern continuation of the South Tibetan detachment system (STDS). The shear zone contains lenses of leucogranite dated at 35.4 Ma. This is interpreted to reflect Eocene crustal thickening, coeval with the earliest shortening event recorded in the carapace. The main phase of the Changgo granite crystallized at 23.5 Ma, while undeformed aplite dikes, the youngest phase observed in the granite, were intruded at 22.1 Ma. Aplite dikes crosscut the main deformation fabric within the Changgo granite; therefore, that deformation and associated south directed shearing must have ended between 23.5 Ma and 22.1 Ma. The dikes are strained within the STDS, indicating that final displacement along the STDS must post-date 22 Ma, yet be older than 18.4 Ma, the cooling age of muscovite in the shear zone. It is proposed that the exhumation of the Changgo culmination is related to tectonically driven erosion in response to crustal thickening and rebuilding of the orogenic critical taper wedge. Subsequently, deformation in the wedge migrated toward the foreland, expanding the orogenic wedge laterally and moving the locus of displacement from the Main Central thrust structurally downward to the Main Boundary thrust.

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.

Laser 40Ar/39Ar age constraints on Miocene sequences from the Bengal basin: Implications for middle Miocene denudation of the eastern Himalayas

Petrographic, mineral‐chemistry and subsurface studies reveal that orogenic sedimentation had already begun in the Bengal basin by the early Miocene. Laser 40Ar/39Ar age determinations were made for detrital muscovite grains (145 total, among 4 samples) from the lower‐to‐middle Miocene Bhuban Formation. The laser fusion ages range from circa 12 Ma to 516 Ma, and thus suggest derivation from a combination of sources: the Himalayas, Indo‐Burman ranges and possibly the Indian shield and Tibetan plateau. Modes of circa 16 Ma, 18 Ma, 26 and 40 Ma in the age distributions of these samples are most consistent with unroofing of the Higher Himalayas since the early Miocene. Detrital micas of such an early age (16 Ma) for the Bhuban Formation are interpreted to indicate that little time elapsed between the isotopic closure of 40Ar in the muscovite and its ultimate deposition in middle Miocene strata. The detrital ages of circa 16 and 22 Ma in this study, most prominent in the highest stratigraphic levels sampled in this study, are younger than those previously reported in the western Himalayan foreland basins. These younger detrital ages are consistent with rapid middle Miocene unroofing and erosion as has been proposed for crystalline rocks of the eastern Himalayas. The minimum 40Ar/39Ar ages for muscovite in a particular sample seem proportional to the stratigraphic level sampled, i.e., younger ages tend to occur for samples of higher stratigraphic level. These results support earlier studies indicating that detrital geochronology can be used as an effective tool in evaluating stratigraphic ages.

Hot metamorphic core complex in a cold foreland

The Montagne Noire forms the southernmost part of the French Massif Central. Carboniferous flysch sediments and very low-grade metamorphic imprint testify to a very external position in the orogen. Sedimentation of synorogenic clastic sediments continued up to the Visean/Namurian boundary (≤320 Ma). Subsequently, the Palaeozoic sedimentary pile underwent recumbent folding and grossly southward thrusting. An extensional window exposes a hot core of Carboniferous HT/LP gneisses, migmatites and granites (Zone Axiale), which was uplifted from under the nappe pile. After the emplacement of the nappes on the Zone Axiale (Variscan D1), all structural levels shared the same tectonic evolution: D2 (extension and exhumation), D3 (refolding) and post-D3 dextral transtension. HT/LP-metamorphism in the crystalline rocks probably started before and continued after the emplacement of the nappes. Peak metamorphic temperatures were attained during a post-nappe thermal increment (M2). M2 occurred during ENE-directed bilateral extension, which exhumed the Zone Axiale and its frame as a ductile horst structure, flanked to the ENE by a Stephanian intra-montane basin. Map patterns and mesoscopic structures reveal that extension in ENE occurred simultaneously with NNW-oriented shortening. Combination of these D2 effects defines a bulk prolate strain in a “pinched pull-apart” setting. Ductile D2 deformation during M2 dominates the structural record. In wide parts of the nappes on the southern flank of the Zone Axiale, D1 is only represented by the inverted position of bedding (overturned limbs of recumbent D1 folds) and by refolded D1 folds. U–Pb monazite and zircon ages and K–Ar muscovite ages are in accord with Ar–Ar data from the literature. HT/LP metamorphism and granitoid intrusion commenced already at ≥330 Ma and continued until 297 Ma, and probably in a separate pulse in post-Stephanian time. Metamorphic ages older than c. 300 Ma are not compatible with the classical model of thermal relaxation after stacking, since they either pre-date or too closely post-date the end of flysch sedimentation. We therefore propose that migmatization and granite melt generation were independent from crustal thickening and caused, instead, by the repeated intrusion of melts into a crustal-scale strike-slip shear zone. Advective heating continued in a pull-apart setting whose activity outlasted the emplacement of the Variscan nappe pile. The shear-zone model is confirmed by similar orogen-parallel extensional windows with HT/LP metamorphism and granitoid intrusion in neighbouring areas, whose location is independent from their position in the orogen. We propose that heat transfer from the mantle occurred in dextral strike-slip shear zones controlled by the westward propagating rift of the Palaeotethys ocean, which helped to destroy the Variscan orogen.

Eclogites of the Late Cambrian–Early Ordovician North Kokchetav tectonic zone ( northern Kazakhstan): structural position and petrology

We consider the structural position and petrology of eclogites in the North Kokchetav accretion-collision zone located north of the Kokchetav metamorphic belt formed by high- and ultrahigh-pressure rocks. In the Early Ordovician North Kokchetav tectonic zone, thin sheets of mylonite and diaphthoric gneisses with eclogites are tectonically conjugate with the volcanic and sedimentary rocks of the Stepnyak paleoisland-arc zone. Eclogites have been revealed at two sites of the North Kokchetav tectonic zone—Chaikino and Borovoe. The Chaikino eclogites formed at 800–850 °C and 18–20 kbar, and the Borovoe eclogites, at 750–800 °C and 17–18 kbar. Study of pyroxene-plagioclase symplectite replacing omphacite of the eclogites at both sites has recognized three stages of regressive magmatism: (1) formation of coarse-grained clinopyroxene-plagioclase symplectite at 760–790 °C and 11–12 kbar, (2) formation of fine-grained clinopyroxene-plagioclase symplectite at 700–730 °C and 7–8 kbar, and (3) amphibolization of pyroxene at 570–600 °C and 5–6 kbar. The Ar-Ar age of muscovite from the Borovoe mica schists hosting eclogites is 493 ± 5 Ma, which corresponds to the time of cooling of metamorphic rocks to <370 °C. Hence, the peak of high-pressure metamorphism and all recognized stages of retrograde changes are dated to the Cambrian. The geological data evidence that eclogite-schist-gneiss sheets were localized in the accretion-collision zone and became conjugate with sedimentary and volcanic rocks no later than in the Middle Ordovician.

Intracontinental subduction: a possible mechanism for the Early Palaeozoic Orogen of SE China

AbstractThe Early Palaeozoic Orogen of SE China consists of three litho‐tectonic elements, from top to bottom: a sedimentary Upper Unit, a metamorphic Lower Unit and a gneissic basement. The boundaries between these units are flat lying, south directed, ductile decollements. The lower one is coeval with an amphibolite facies metamorphism (M1). The belt is reworked by migmatite–granite domes, high‐temperature metamorphism (M2) and granitic plutons related to post‐orogenic crustal melting. We date here the syn‐M1 ductile shearing at 453 ± 7 Ma by U‐Th/Pb method on monazite. Previous ages and our new 40Ar/39Ar ages of biotites and muscovites show that the metamorphic rocks experienced syn‐M2 exhumation from 440 to 400 Ma. The Early Palaeozoic Orogen of SE China is an intracontinental belt in which decollements accommodated the north‐directed subduction of the Cathaysian continent. This orogen is an example of intracontinental subduction that was not preceded by oceanic subduction.