Biotite 40Ar Research Articles

High-Nb alkaline mafic dykes in the central Alps witness to the Triassic evolution of the Adria passive margin

The Edolo diabase dykes intruded the Variscan basement in the central Southalpine domain. Biotite 40Ar / 39Ar dating shows that the emplacement occurred during the final stage of Permian-Triassic lithospheric thinning. In this phase on the Adria passive margin, middle Triassic magmatism with disputed arc affinity is followed by a late Triassic alkaline event associated with continental rifting. The Edolo diabase dykes are Ti-rich and alkaline, with high Nb and Ta contents and ratios in igneous amphibole, biotite, and ilmenite. These features are typical of the late Triassic magmatism. In contrast, the early-crystallized clinopyroxene shows low Nb and Ta contents and ratios and an apparent arc affinity may be thus envisaged in the clinopyroxene parental melt. However, given the coexistence of both features in the same dykes, their composition cannot be interpreted in terms of transition between tectonic environments. Instead, it is likely attributed to the heterogeneity of the Adriatic lithosphere inherited from the Variscan to Permian evolution, which underwent asthenospheric upwelling due to Permian-Triassic extensional tectonics.

U–Pb SHRIMP and 40Ar/ 39Ar ages constrain the deformation history of the Karakoram fault zone (KFZ), SW Tibet

The ~ 800-km-long dextral Karakoram fault zone (KFZ) bounds SW Tibet and, despite a number of recent studies, its spatial and temporal evolution and consequent role in continental deformation remains under debate. In order to better constrain fault evolution we present U–Pb Sensitive High Resolution Ion MicroProbe (SHRIMP) and 40Ar/ 39Ar analyses of granitic samples from the southern portion of the Karakoram Fault. Zircons from the ductile shear zone in the Namru region of the Ayilari granite batholith record a crystallization age compatible with Gangdese magmatism belt (also known as the Trans-Himalayan magmatic Belt, THB) followed by a medium temperature cooling event between about 12.8–11.5 Ma. And zircons from the ductile shear zone in the Zhaxigang region of the Ayilari granite batholith record a crystallization age of 18.7 Ma. The Ayilari granite batholith near Namru appears to have experienced neither the 32 Ma nor 25–22 Ma magmatic events reported by previous studies. Biotite 40Ar/ 39Ar ages provide evidence that the 12.8–11.5 Ma cooling event coincides with the initiation of the KFZ cutting through the Ayilari granite, both at Namru and Zhaxigang. Consequently, we conclude that the previously reported 32 Ma, 25–22 Ma and 22–14 Ma events bear no relationship to the activity along the Karakoram fault, but resulted from either local metamorphic events caused by regional tectonics (such as activity on the south Kailas thrust fault), or from magmatism around Shiquanhe (also known as Gar). This study supports the conclusion that the KFZ, at least in the Zhaxigang–Namru region, propagated along strike to the Gar–Menshi area as a thin-skinned structure at around 13 Ma.

Late Miocene–Pliocene deceleration of dextral slip between Pamir and Tarim: Implications for Pamir orogenesis

The timing of the late Cenozoic collision between the Pamir salient and the Tien Shan as well as changes in the relative motion between the Pamir and Tarim are poorly constrained. The northern margin of the Pamir salient indented northward by ~ 300 km during the late Cenozoic, accommodated by south-dipping intracontinental subduction along the Main Pamir Thrust (MPT) coupled to strike-slip faults on the eastern flank of the orogen and both strike-slip and thrust faults on the western margin. The Kashgar–Yecheng transfer system (KYTS) is the main dextral slip shear zone separating Tarim from the Eastern Pamir, with an estimated cumulative offset of ~ 280 km at an average late Cenozoic dextral slip rate of 11–15 mm/a (Cowgill, 2010). In order to better constrain the slip history of the KYTS, we collected thermochronologic samples along the eastward-flowing, deeply incised, antecedent Tashkorgan–Yarkand River, which crosses the fault system on the eastern flank of the orogen. We present 29 new biotite 40Ar/ 39Ar ages, apatite and zircon (U–Th–Sm)/He ages, and apatite fission track (AFT) analysis, combined with published muscovite and biotite 40Ar/ 39Ar and AFT data, to create a unique thermochronologic dataset in this poorly studied and remote region. We constrain the timing of four major N-trending faults; the latter three are strands of the KYTS. The westernmost, the Kuke fault, experienced significant dip-slip, west-side-up displacement between > 12 and 6 Ma. To the east, within the KYTS, our new thermochronologic data and geomorphic observations suggest that the Kumtag and Kusilaf dextral slip faults have been inactive since at least 3–5 Ma. Long-term incision rates across the Aertashi dextral slip fault, the easternmost strand of the KYTS, are compatible with slow horizontal slip rates of 1.7–5.3 mm/a over the past 3 to 5 Ma. In summary, these data show that the slip rate of the KYTS decreased substantially during the late Miocene or Pliocene. Furthermore, Miocene–present regional kinematic reconstructions suggest that this deceleration reflects the substantial increase of northward motion of Tarim rather than a significant decrease of the northward velocity of the Pamir.

The role of footwall deformation and denudation in controlling cooling age patterns of detachment systems: An application to the Kongur Shan extensional system in the Eastern Pamir, China

Determining the relationship between deformation and exhumation history in the footwall rocks of major low-angle normal faults (i.e., detachment faults) provides a way to quantify different models of continental extension. Although pressure–temperature–time ( P–T–t) datasets are commonly used to infer magnitude and rates of slip across detachment faults, correct use of these datasets requires a sound understanding of coupling effects between footwall deformation and advection of isotherms on the distribution of cooling ages. In this study we present thermo-kinematic modeling results which examine the relationship between different footwall kinematic models, erosion, and the spatial distribution of muscovite and biotite 40Ar/ 39Ar cooling ages. Our results show that large magnitudes of footwall erosion during normal faulting significantly affect cooling age patterns as footwall rocks at different distances from the fault surface pass through different thermal gradients. We apply our modeling results to simulating muscovite and biotite cooling-age patterns from the footwall of the Cenozoic Kongur Shan normal fault system in the Pamir. Previous studies had interpreted a documented ~5-fold increase in cooling rate at ~ 2 Ma to indicate an increase in exhumation rate during the Quaternary. However, our results show that the observed cooling age patterns and increase in cooling rate can be produced by a constant exhumation rate over the last 7 Ma due to the effect of changes in the rate of isotherm advection during exhumation.

Volcanic biotite-sanidine 40Ar/39Ar age discordances reflect Ar partitioning and pre-eruption closure in biotite

Research Article| October 01, 2010 Volcanic biotite-sanidine 40Ar/39Ar age discordances reflect Ar partitioning and pre-eruption closure in biotite John M. Hora; John M. Hora 1Department of Geoscience, University of Wisconsin, 1215 W. Dayton Street, Madison, Wisconsin 53706, USA2Geowissenschaftliches Zentrum der Universität Göttingen, Abteilung Geochemie, Goldschmidtstraße 1, 37077 Göttingen, Germany Search for other works by this author on: GSW Google Scholar Brad S. Singer; Brad S. Singer 1Department of Geoscience, University of Wisconsin, 1215 W. Dayton Street, Madison, Wisconsin 53706, USA Search for other works by this author on: GSW Google Scholar Brian R. Jicha; Brian R. Jicha 1Department of Geoscience, University of Wisconsin, 1215 W. Dayton Street, Madison, Wisconsin 53706, USA Search for other works by this author on: GSW Google Scholar Brian L. Beard; Brian L. Beard 1Department of Geoscience, University of Wisconsin, 1215 W. Dayton Street, Madison, Wisconsin 53706, USA Search for other works by this author on: GSW Google Scholar Clark M. Johnson; Clark M. Johnson 1Department of Geoscience, University of Wisconsin, 1215 W. Dayton Street, Madison, Wisconsin 53706, USA Search for other works by this author on: GSW Google Scholar Shan de Silva; Shan de Silva 3Department of Geosciences, Oregon State University, Corvallis, Oregon 97331-5506, USA Search for other works by this author on: GSW Google Scholar Morgan Salisbury Morgan Salisbury 3Department of Geosciences, Oregon State University, Corvallis, Oregon 97331-5506, USA Search for other works by this author on: GSW Google Scholar Author and Article Information John M. Hora 1Department of Geoscience, University of Wisconsin, 1215 W. Dayton Street, Madison, Wisconsin 53706, USA2Geowissenschaftliches Zentrum der Universität Göttingen, Abteilung Geochemie, Goldschmidtstraße 1, 37077 Göttingen, Germany Brad S. Singer 1Department of Geoscience, University of Wisconsin, 1215 W. Dayton Street, Madison, Wisconsin 53706, USA Brian R. Jicha 1Department of Geoscience, University of Wisconsin, 1215 W. Dayton Street, Madison, Wisconsin 53706, USA Brian L. Beard 1Department of Geoscience, University of Wisconsin, 1215 W. Dayton Street, Madison, Wisconsin 53706, USA Clark M. Johnson 1Department of Geoscience, University of Wisconsin, 1215 W. Dayton Street, Madison, Wisconsin 53706, USA Shan de Silva 3Department of Geosciences, Oregon State University, Corvallis, Oregon 97331-5506, USA Morgan Salisbury 3Department of Geosciences, Oregon State University, Corvallis, Oregon 97331-5506, USA Publisher: Geological Society of America Received: 27 Jan 2010 Revision Received: 18 May 2010 Accepted: 21 May 2010 First Online: 09 Mar 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 © 2010 Geological Society of America Geology (2010) 38 (10): 923–926. https://doi.org/10.1130/G31064.1 Article history Received: 27 Jan 2010 Revision Received: 18 May 2010 Accepted: 21 May 2010 First Online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation John M. Hora, Brad S. Singer, Brian R. Jicha, Brian L. Beard, Clark M. Johnson, Shan de Silva, Morgan Salisbury; Volcanic biotite-sanidine 40Ar/39Ar age discordances reflect Ar partitioning and pre-eruption closure in biotite. Geology 2010;; 38 (10): 923–926. doi: https://doi.org/10.1130/G31064.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The 40Ar/39Ar radioisotope system is widely used to date eruption and cooling of volcanic tephra–marker horizons that commonly provide the only means of correlating and assigning numerical ages to stratigraphy in which they are contained. This chronometer bridges the gap between 14C and longer-lived isotopic systems that are too imprecise for dating young samples. However, 40Ar/39Ar ages obtained from coevally erupted biotite and sanidine do not always match. Here, we use an independent chronometer, 238U-230Th disequilibrium, to demonstrate that 40Ar/39Ar age disparity is not caused by differences in pre-eruption crystallization times. Our findings indicate that the presence of extraneous 40Ar in biotite, and its absence in sanidine, may result from violations of two assumptions implicit in 40Ar/39Ar geochronology on volcanic samples: (1) Prior to eruption, minerals are devoid of 40Ar due to rapid loss to an “infinite reservoir” such as the atmosphere, and (2) closure to volume diffusion is geologically instantaneous and coincident with eruption. We propose a mechanism whereby the presence of extraneous Ar in certain minerals is explained by the relative sequence of four events in a magmatic system: (1) crystallization, (2) mineral closure with respect to Ar diffusion, (3) isotopic equilibration of magmatic and atmospheric Ar, and (4) quenching of the system by eruption. These data have potentially far-reaching implications for studies that depend on geochronological data, necessitating re-evaluation of interpretations based solely on biotite with no independent age control, particularly in young samples where the effects are most pronounced. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Episodic exhumation of the Greater Himalayan Sequence since the Miocene constrained by fission track thermochronology in Nyalam, central Himalaya

The Greater Himalayan Sequence (GHS), which makes up the core of the Himalayan orogen, has an uppermost tectonic contact defined by the South Tibetan Detachment System (STDS) and a lower tectonic contact defined by the Main Central Thrust (MCT). The GHS occurs as one of the most important tectostratigraphic units for deciphering processes related to tectonic and climatic exhumation across the orogen. Zircon and apatite fission track (ZFT, AFT) dating were carried out along a transect in Nyalam, central Himalaya in southern Tibet to constrain cooling driven by orogenic process since the middle Miocene. The hanging wall of the STDS yields an essentially unreset Jurassic ZFT age in the Jurassic strata. However, below the STDS within the GHS there is a clear and distinct thermal signal of cooling related to exhumation. In the footwall and within the GHS, the rocks have ZFT ages of middle Miocene to Pliocene, and AFT ages of late Miocene to Quaternary that get younger downward and away from the STDS. In combination with thermal structure modeling, a two-part episodic model, which is widely compatible with existing thermochronological data, is proposed for cooling and exhumation of the GHS since the middle Miocene: [1] middle Miocene; and [2] Pliocene to Quaternary (Recent). The middle Miocene cooling is suggested to have resulted from a rapid tectonic unroofing by down-to-the-north slip on the STDS. The tectonic exhumation was also recorded by several other thermochronological systems (e.g. biotite 40Ar/ 39Ar) with concordant middle Miocene cooling ages in different structural positions across the GHS. Post middle Miocene ZFT and AFT cooling ages in the lower part of the GHS suggest accelerated cooling by climate-enhanced erosional exhumation, which was initiated in the late Miocene to Pliocene and was dramatic in the Quaternary to Recent. Thermochronological data and modeling further imply that the present Himalayan topographic front may have been shaped essentially by surface erosion since the late Miocene, when the Himalayan divide might have been some 20–30 km to the south of its present position. However, these data do not preclude the possibility that the intense erosional exhumation may have triggered rock uplift to approach and/or maintain a steady topography in the GHS.

U-Pb and 40Ar/ 39Ar geochronological constraints on the exhumation history of the North Qinling terrane, China

The amphibolite facies grade North Qinling metamorphic unit forms the centre of the Qinling orogenic belt. Results of LA-ICP-MS U-Pb zircon, 40Ar/ 39Ar amphibole and biotite dating reveal its Palaeozoic tectonic history. U-Pb zircon dating of migmatitic orthogneiss and granite dykes constrains the age of two possible stages of migmatization at 517 ± 14 Ma and 445 ± 4.6 Ma. A subsequent granite intrusion occurred at 417 ± 1.6 Ma. The 40Ar/ 39Ar plateau ages of amphibole ranging from 397 ± 33 Ma to 432 ± 3.4 Ma constrain the cooling of the Qinling complex below ca. 540 °C and biotite 40Ar/ 39Ar ages at about 330–368 Ma below ca. 300 °C. The ages are used to construct a cooling history with slow/non-exhumation during 517– 445 Ma, a time-integrated cooling at a rate < 2.5 °C/Ma during the period of 445–410 Ma, an acceleration of cooling at a rate of 8 °C/Ma from 397 Ma to 368 Ma, and subsequently slow/non-cooling from 368 to 330 Ma. The data show a significant delay in exhumation after peak metamorphic conditions and a long period of tectonic quiescence after the suturing of the North China and South China blocks along the Shangdan suture. These relationships exclude classical exhumation models of formation and exhumation of metamorphic cores in orogens, which all imply rapid cooling after peak conditions of metamorphism.

Application of low-temperature thermochronology to hydrothermal ore deposits: Formation, preservation and exhumation of epithermal gold systems from the Eastern Rhodopes, Bulgaria

New low-temperature thermochronological data have been used to quantify the protracted, Eocene–Miocene cooling histories of upper and lower plate rocks of the Kesebir–Kardamos extensional dome, Eastern Rhodopes, Bulgaria. 40Ar/ 39Ar and apatite fission-track data reveal that the lower plate has experienced continuous cooling and exhumation, since the Late Eocene. Muscovite 40Ar/ 39Ar plateau ages of 36.90 ± 0.16 Ma and 37.28 ± 0.19 Ma (2 σ) from metamorphic rocks of the footwall reveal the approximate time span during which they cooled below ∼ 350 °C during exhumation caused by detachment faulting. The sedimentary rock-hosted gold mineralization, which represents a thermal event at ∼ 250–220 °C, developed during the early stage of basin formation between 34.71 ± 0.16 Ma and 35.36 ± 0.21 Ma (adularia 40Ar/ 39Ar plateau ages; 2 σ). The termination of hydrothermal mineral deposition at Ada Tepe occurred contemporaneously with the earliest phase of calc–alkaline type magmatism at Iran Tepe (33.97 ± 0.36 Ma to 34.62 ± 0.46 Ma, hornblende and biotite 40Ar/ 39Ar plateau ages, 2 σ). Thermal history modelling of apatite fission-track data shows that the lower plate rocks cooled through ∼ 120 °C at ∼ 18.3 ± 1.9 Ma (1 σ). A time–temperature model obtained from zircon and apatite fission-track data from the upper plate reveals that it was being buried during the late Eocene. At ∼ 33–30 Ma, a dramatic change of the time–temperature path was caused by the initiation of horst–graben structures, resulting in rapid exhumation of the upper plate. Our new thermochronological data reveal many aspects of the mechanisms of formation of sedimentary rock-hosted gold deposits. The heat accumulated during sedimentary burial of the upper plate is a plausible heat source to drive hydrothermal fluid circulation and ore formation. The development of large half-graben basins in the hanging walls of detachment faults, accompanied by a favourable climate, may have created a situation that was adequate to capture a primary meteoric water supply that subsequently underwent hydrothermal convection. Our new 40Ar/ 39Ar age data indicate that magmatic activity was contemporaneous with gold deposition and must be considered as a source of heat, sulphur and/or a metal for the ore-forming hydrothermal system. The preservation and exhumation of sedimentary rock-hosted ore deposits is largely controlled by the exhumation history of the upper plate, which has been quantitatively constrained by low-temperature thermochronological methods, and commences with rapid exhumation at ∼ 33–30 Ma.

New radiometric dating constrains the time for initiation of the Karakorum fault zone (KFZ), SW Tibet

The Karakorum fault zone (KFZ), composed of strike-slip faults, has played an important role in intra-continental deformation during the Cenozoic convergence between the Indian and Eurasian plates. However, the spatial and temporal evolution of the KFZ remains under debate. This paper reports new zircon U–Pb Sensitive High Resolution Ion MicroProbe (SHRIMP) and biotite 40Ar/ 39Ar ages for samples from the fault zone to clarify the timing of events. The ages of Zircon U–Pb fall mainly around 47–50 Ma, which corresponds to a period of important magmatic activity of the Gangdese granitic belt (also known as the Trans-Himalayan magmatic belt). A metamorphic event followed in the period around 32 Ma, and that was then followed by a medium temperature cooling event between about 12 and 7.7 Ma. Biotite 40Ar/ 39Ar ages provide evidence that the 12–7.7 Ma cooling event coincides with the initiation of the KFZ cutting through the Ayilari granite. The Ayilari granite pluton does not seem to have experienced the separate 25–23 Ma cooling event, as shown in previous studies 60–80 km west of the current study area [Lacassin, R., Valli, F., Arnaud, N., Leloup, P.H., Paquette, J.L., Li, H., Tapponnier, P., Chevalier, M.L., Guillot, S., Maheo, G., Xu, Z., 2004. Large-scale geometry, offset and kinematic evolution of the Karakorum fault, Tibet. Earth and Planetary Science Letters 219 (3–4), 255–269.; Valli, F., Arnaud, N., Leloup, H.P., Sobel, E.R., Maheo, G., Lacassin, R., Guillot, S., Li, H., Tapponnier, P., Xu, Z., 2007. Twenty million years of continuous deformation along the Karakorum fault, western Tibet: a thermochronological analysis. Tectonics 26, doi: 10.1029/2005TC001913.; Valli, F., Leloup, P., Paquette, J., Arnaud, N., Li, H., Tapponnier, P., Lacassin, R., Guillot, S., Liu, D., Deloule, E., Xu, Z., Mahe´o, G., 2008. New U–Th/Pb constraints on timing of shearing and long-term slip-rate on the Karakorum fault. Tectonics 27, doi:10.1029/2007TC002184.]. We conclude that the 25–23 Ma cooling event bears no relationship to the activity of the KFZ, but resulted from a local metamorphic event caused either by a regional tectonic event (such as activity of the south Kailas thrust fault) or by magmatism around Shiquanhe (also known as Gar). The study of the KFZ around Namru supports the opinion that the KFZ propagated along the fault strike to the Gar-Menshi area around 12 Ma.

The Incapillo Caldera and Dome Complex (∼ 28° S, Central Andes): A stranded magma chamber over a dying arc

The Plio-Pleistocene Incapillo Caldera and Dome Complex (5570 m) is situated at the southernmost termination of the Andean Central Volcanic Zone (∼ 28° S), where the more steeply dipping segment of the subducting Nazca plate transitions into the Chilean–Pampean “flat-slab” to the south. The ∼ 250 m high walls of the elliptical caldera (6 × 5 km) are composed of a homogeneous pumice-rich rhyodacitic ignimbrite that radiates ∼ 15 km from the caldera rim. New biotite 40Ar/ 39Ar ages along with previous K–Ar ages reveal that the final eruption of the Incapillo Caldera occurred at ∼ 0.51 ± 0.04 Ma. Flanking the caldera are ∼ 40 steep-sided rhyodacitic domes that predate the main “caldera-forming” ignimbrite. The morphology of these domes reflects the non-explosive effusive eruption of a crystal-rich gas-poor rhyodacitic magma. A volumetrically-minor debris flow composed of abundant pumice, dacites, and lithic clasts is topographically confined to a glacial valley south of the caldera and unconformably overlies the ignimbrite. Mixed pumices at the distal end of the ignimbrite reflect the convective physical condition within the magma chamber immediately prior to the pyroclastic eruption. Including the domes and ignimbrite, the total estimated volume of erupted Incapillo magma is ∼ 37 km 3. Based on its age, small size, and effusive eruptive style, the Incapillo Caldera and Dome Complex provides an endmember model for an Andean caldera erupting within a waning magmatic arc over a shallowing subduction zone.

Timing of post-obduction granitoids from intrusion through cooling to exhumation in central Anatolia, Turkey

The Middle to Late Cretaceous central Anatolian granitoids intrude the supra-subduction zone-type central Anatolian ophiolite and medium- to high-grade metasediments of central Anatolian crystalline complex and are overlain by Late Palaeocene to Early/Middle Eocene sediments. Their single-zircon 207Pb– 206Pb evaporation ages define three clusters: (1) Cenomanian–Turonian (weighted-mean age: 94.9 ± 3.4 Ma), (2) Turonian–Santonian (85.5 ± 5.5 Ma) and (3) Campanian (74.9 ± 3.8 Ma). Their hornblende and biotite 40Ar– 39Ar and K–Ar cooling ages cluster around 80–65 Ma. The close hornblende and biotite ages reflect rapid exhumation of a mid-crustal section during the Campanian–Maastrichtian. Early to Middle Palaeocene (57–62 Ma) apatite fission-track age clusters date the tail end of this exhumation episode. It is proposed that the central Anatolian granitoid melts were generated in a post-collisional extensional setting following the docking of an oceanic island arc onto the Tauride–Anatolide platform. Campanian–Maastrichtian to Early/Middle Palaeocene rapid exhumation event is considered to result from continent–continent collisions between Eurasian plate and the Tauride–Anatolide platform along the İzmir–Ankara–Erzincan suture zone.

Refinement of the age of the Middle Miocene Fort Ternan Beds, Western Kenya, and its implications for Old World biochronology

It has become increasingly obvious over the past two decades that the fossiliferous strata at Fort Ternan, Kenya, are probably somewhat younger than 14 Ma, an age which has long been attached to the deposits. This realisation flows from geological and biochronological observations. In order to test the hypothesis, resampling of all the lava flows in the region of Fort Ternan was undertaken in 2003, especially those underlying the Fort Ternan Beds in the Kipchorion Gorge where the sequence is the most complete. Samples obtained from lava flows underlying and overlying the fossil beds were analysed for anorthoclase K/Ar and 40Ar/ 39Ar and biotite 40Ar/ 39Ar age determinations. The results reveal that the age of the fossiliferous sediments is ca 13.7 ± 0.3 Ma . Since Fort Ternan yielded the ‘core fauna’ that defines Faunal Set IV of the East African biochronological sequence this refinement of its age will impact on age estimates of neighbouring Faunal Sets, as well as on other faunas correlated to Fort Ternan, including those in Europe belonging to MN Zones MN 5, MN 6 and MN 7/8. To cite this article: M. Pickford et al., C. R. Geoscience 338 (2006).

Kinematics and 40Ar/ 39Ar geochronology of the Gaoligong and Chongshan shear systems, western Yunnan, China: Implications for early Oligocene tectonic extrusion of SE Asia

The Gaoligong and Chongshan shear systems (GLSS and CSSS) in western Yunnan, China, have similar tectonic significance to the Ailaoshan–Red River shear system (ASRRSS) during the Cenozoic tectonic development of the southeastern Tibetan syntaxis. To better understand their kinematics and the Cenozoic tectonic evolution of SE Asia, this paper presents new kinematic and 40Ar/ 39Ar geochronological data for these shear systems. All the structural and microstructural evidence indicate that the GLSS is a dextral strike-slip shear system while the CSSS is a sinistral strike-slip shear system, and both were developed under amphibolite- to greenschist-grade conditions. The 40Ar/ 39Ar dating of synkinematic minerals revealed that the strike-slip shearing on the GLSS and CSSS at least began at ∼ 32 Ma, possibly coeval with the onset of other major shear systems in SE Asia. The late-stage shearing on the GLSS and CSSS is dated at ∼ 27–29 Ma by the biotite 40Ar/ 39Ar ages, consistent with that of the Wang Chao shear zone (WCSZ), but ∼ 10 Ma earlier than that of the ASRRSS. The dextral Gaoligong shear zone within the GLSS may have separated the India plate from the Indochina Block during early Oligocene. Combined with other data in western Yunnan, we propose that the Baoshan/Southern Indochina Block escaped faster southeastward along the CSSS to the east and the GLSS to the west than the Northern Indochina Block along the ASRRSS, accompanying with the obliquely northward motion of the India plate during early Oligocene (28–36 Ma). During 28–17 Ma, the Northern Indochina Block was rotationally extruded along the ASRRSS relative to the South China Block as a result of continuously impinging of the India plate.

40Ar/ 39Ar dating of strike-slip motion on the Tan–Lu fault zone, East China

NE-trending, sinistral ductile shear belts in the southern part of the Tan–Lu fault zone, East China are exposed along the eastern edge of the northern Dabie belt and southern Zhangbaling belt. Three hornblende, three phengite and 12 biotite separates from greenschist facies mylonites in the shear belts were dated by the 40Ar/ 39Ar method in this study. A hornblende plateau age of 143.3 Ma is interpreted as representing the time of sinistral motion on the Zhangbaling part of the Tan–Lu fault zone. Seven biotite samples from the same part yield 40Ar/ 39Ar cooling ages from 137 to 125 Ma. A phengite plateau age of 138.8 Ma from the eastern edge of the Dabie belt is considered as being close to the time of deformation in the Tan–Lu strike-slip zone. Five biotite 40Ar/ 39Ar ages from 120 to 110 Ma and one phengite age of 121 Ma from the same area represent cooling ages after the sinistral faulting. It is concluded on the basis of these 40Ar/ 39Ar ages that the sinistral ductile shear belts in the southern part of the Tan–Lu fault zone were produced in earliest Early Cretaceous. This sinistral motion on the Tan–Lu fault zone is considered to be related to high-speed, highly oblique subduction of the Pacific plate at this time.

Mesozoic cooling across the Yidun Arc, central-eastern Tibetan Plateau: A reconnaissance 40Ar/ 39Ar study

40Ar/ 39Ar dating of hydrous minerals from igneous and metamorphic rocks was undertaken with the aim of characterizing the thermal history of the Yidun Arc, eastern Tibetan Plateau. Regional metamorphism of Palaeozoic rocks of the western Yidun Arc occurred during the Early Triassic collision with the Qiangtang Block along the Jinsha Jiang Suture. Hornblende, muscovite, and biotite 40Ar/ 39Ar ages from syn-tectonic igneous rocks within this collision zone suggest that the region had cooled to below 300 °C by the Late Triassic. Muscovite samples from mica–schists within this collision zone yield variably discordant age spectra with generally Triassic 40Ar/ 39Ar ages, consistent with cooling of the region to below muscovite closure temperatures by the close of the Triassic. Hornblende and biotite 40Ar/ 39Ar ages from granite and granodiorite plutons in the eastern Yidun Arc suggest post-emplacement cooling of these plutons occurred rapidly, consistent with these granites being high-level intrusions. In sum, these results suggest that by the beginning of the Jurassic the Yidun Arc as a whole had cooled to below 300 °C. Two Cretaceous granites in the central Yidun Arc also experienced rapid cooling, suggesting they too were emplaced at shallow crustal depths. Post-Triassic deformation of the region must have occurred at or below the brittle–ductile transition within the crustal levels now exposed.

An intercalibration study of the Fish Canyon sanidine and biotite 40Ar/ 39Ar standards and some comments on the age of the Fish Canyon Tuff

The age of the Fish Canyon Tuff (FCT) has been the subject of intense study due to its potential significance as a geochronological test to establish age concordance between the K/Ar and U/Pb isotopic systems. Minerals from the FCT (notably sanidine and biotite) are in widespread use as 40Ar/ 39Ar fluence monitors (“standards”) and have been employed as both primary and intercalibrated standards. In attempting to constrain the true age of FCT, the distinction between primary vs. intercalibrated ages is important, as only the primary age determinations are valid. An investigation of both Fish Canyon sanidine (FCs) and Fish Canyon biotite (FCT-3) was undertaken to assess the chemical and isotopic homogeneity of the minerals on a grain-to-grain scale. Electron microprobe results show that FCs is chemically homogeneous, verifying previous studies, although there are some very small zones of minor K depletion; FCT-3 biotite commonly contains numerous and small mineral inclusions. 40Ar/ 39Ar experiments further show that FCs is remarkably homogeneous, confirming previous observations, whereas FCT-3 shows much greater isotopic variability due to contamination from the mineral inclusions. By setting the age of FCs equal to 27.98±0.15 Ma (2 σ), as intercalibrated to the U/Pb system by Villeneuve et al. (2000), the resultant intercalibration factor between FCs and FCT-3 is 1.005±0.017 yielding an age of 28.13±0.47 Ma (2 σ) for FCT-3, although the significant intragrain chemical and isotopic variability precludes its use as an effective 40Ar/ 39Ar fluence monitor. The true age of the FCT is problematic; a survey of all primary ages for FCT shows that mean K/Ar ages for plagioclase, hornblende, biotite, and sanidine are consistently younger than mean U/Pb ages from titanite and zircon. In conjunction with recently published petrological results which indicate that the FCT contains material from multiple crystallization events, this significant discrepancy between the K/Ar and U/Pb data sets demonstrates that the Fish Canyon Tuff cannot serve as a golden standard between the K/Ar and U/Pb systems.

The Llullaillaco volcano, northwest Argentina: construction by Pleistocene volcanism and destruction by sector collapse

Llullaillaco is one of a chain of Quaternary stratovolcanoes that defines the present Andean Central Volcanic Zone (CVZ), and marks the border between Chile and Argentina/Bolivia. The current edifice is constructed from a series of thick dacitic lava flows, forming the second tallest active volcano in the world (6739 m). K–Ar and new biotite laser 40Ar/ 39Ar step-heating dates indicate that the volcano was constructed during the Pleistocene (≤1.5 Ma), with a youngest date of 0.048±0.012 Ma being recorded for a fresh dacite flow that descends the southern flank. Additional 40Ar/ 39Ar measurements for andesitic and dacitic lava flows from the surrounding volcanic terrain yield dates of between 11.94±0.13 Ma and 5.48±0.07 Ma, corresponding to an extended period of Miocene volcanism which defines much of the landscape in this region. Major- and trace-element compositions of lavas from Llullaillaco are typical of Miocene–Pleistocene volcanic rocks from the western margin of the CVZ, and are related to relatively shallow-dipping subduction of the Nazca plate beneath northern Chile and Argentina. Oversteepening of the edifice by stacking of thick, viscous, dacitic lava flows resulted in collapse of its southeastern flank to form a large volcanic debris avalanche. Biotite 40Ar/ 39Ar dating of lava blocks from the avalanche deposit indicate that collapse occurred at or after ∼0.15 Ma, and may have been triggered by extrusion of a dacitic flow similar to the one dated at 0.048±0.012 Ma. The avalanche deposits are exceptionally well preserved due to the arid climate, and prominent levées, longitudinal ridges, and megablocks up to 20-m diameter are observed. The avalanche descended ∼2.8 km vertically, and bifurcated around an older volcano, Cerro Rosado, before debouching onto the salt flats of Salina de Llullaillaco. The north and south limbs of the avalanche traveled 25 and 23 km, respectively, and together cover an area of approximately 165 km 2. Estimates of deposit volume are hampered by a lack of thickness information except at the edges, but it is likely to be between 1 and 2 km 3. Equivalent coefficients of friction of 0.11 and 0.12, and excess travel distances of 20.5 and 18.5 km, are calculated for the north and south limbs, respectively. The avalanche ascended ∼400 m where it broke against the western flank of Cerro Rosado, and a minimum flow velocity of ∼90 m s −1 can be calculated at this point; lower velocities of ∼45 m s −1 are calculated where distal toes ascend ∼200 m slopes. It is suggested that the remaining precipitous edifice has a high probability for further avalanche collapse in the event of renewed volcanism.

Excess Ar in biotites from the Broderick Falls (Webuye) area, western Kenya: implications for the tectonothermal history of the Mozambique Belt and its Archaean foreland

RbSr whole rock and KAr mineral age determinations were made on rocks from the Broderick Falls (Webuye) area, western Kenya. Granitic rocks yielded a RbSr whole rock isochron age of 2555 ± 101 Ma with an initial 87 Sr 86 Sr ratio of 0.70121 ± 0.00038. This age represents the time of granitoid emplacement. KAr mineral ages range from 574 to 3420 Ma, which is very variable with respect to mineral type and locality. Mylonitic granodiorite very close to the Nandi Escarpment gave a KAr age of 916 Ma from biotite, suggesting the time of the activity of the Nandi Fault, which may be an earlier phase of the Pan-African Orogeny. Ages of biotites in a zone between 4 and 6 km northeast of the Nandi Fault are anomalously high compared to those of coexisting hornblende and the RbSr isochron age, confirming the existence of excess 40Ar in biotite. Excess 40Ar was probably introduced into biotite under the appropriate temperature conditions prevailing near the Nandi Fault. Taramite, a rare sodic-calcic amphibole, was found in a cordierite-biotite gneiss of the Kavirondian Supergroup and gave a typical Pan-African KAr age of 574 Ma. The last Pan-African metamorphism occurred in the terrane east of the Surongai Thrust.

Excess 40Ar in biotite and hornblende from the Noril'sk 1 intrusion, Siberia: implications for the age of the Siberian Traps

The Siberian Traps represent the largest known Phanerozoic manifestation of continental flood volcanism. The general agreement is now that the volcanism was of short duration (∼ 1 m.y.) and was synchronous with the Permian-Triassic boundary. Discrepancies between different geochronologic results prevent rigorous evaluation of the age and duration of magmatism. Several alkalic mafic intrusions associated with the Siberian Traps represent subvolcanic feeders for the Trap lavas. The Noril'sk 1 intrusion is intruded into the earliest flows and is chemically and isotopically related to stratigraphically overlying flows. Biotites from the Noril'sk 1 intrusion have previously yielded 40Ar 39Ar plateau dates (254-251 Ma) that are significantly older than seemingly reliable 40Ar 39Ar plateau dates (250-247 Ma) on plagioclase and whole-rock samples from Trap basalts that are geologically constrained to be older than the intrusion. Possible explanations of this discrepancy include: (1) plagioclase and whole-rock samples are anomalously young due to 40Ar loss via alteration or episodic outgassing, and/or (2) the biotites contain excess 40Ar. Detailed laser heating 40Ar 39Ar analysis of six individual grains of biotite in a total of 214 steps yield data that permit characterization of multiple components of Ar through isotope correlation patterns. An isochron age of 250.1 ± 1.5 Ma (2σ) and an excess Ar component with 40Ar 36Ar = 565 ± 39 , are obtained by regression of the 115 highest temperature steps. Step heating of a bulk hornblende sample also reveals excess Ar, with 40Ar 36Ar = 327 ± 10 and an isochron age of 249.3 ± 1.6 Ma. Neglecting uncertainties in the ages of neutron fluence monitors, the two dates just overlap at 2σ, but are distinctly younger than the previous mean plateau date (253.9 ± 1.6 Ma) for biotite. The different initial 40Ar 36Ar for the two phases is interpreted as reflecting the evolution of an initial lower mantle composition towards increasing crustal contamination.

Location of extraneous argon in granulitic-facies minerals: A paired microprobe-laser probe [formula omitted] analysis

We have investigated minerals from the In Ouzzal granulitic unit (Sahara, Algeria). As excess 40Ar frequently occurs in these rocks, we have applied the 40 Ar 39 Ar laser probe technique in an attempt to determine the location of the excess component in the minerals. Combining both electron microprobe and laser probe analyses, the excess 40Ar component was found to be located on the rim of the biotite and in the microcracks of the pyroxene and garnet which are filled by symplectic secondary minerals such as cordierite, secondary pyroxene, spinel and sapphirine. The cores of the biotites yield geologically meaningful ages as indicated by the lack of excess 40Ar in this part of the crystal. In plagioclase, the location of the excess component could not be pinpointed, indicating distribution throughout the mineral. The excess 40Ar is irregularly distributed throughout the perthitic feldspars. However, for the perthitic and plagioclase phases, conclusions are limited by the spatial resolution of the laser beam and size of the lasered areas. These results are compared with age spectra obtained by incremental heating applied to pure mineral separates. The interpretation in terms of energy site occupancy is enlightened by laser probe results. The incorporation of the excess 40Ar in biotite appears to have originated from the outgassing of 1.8-Ga-old perthitic phases during the Pan-African orogenic (600 Ma) which had resulted in an increase of Ar partial pressure in the granulitic rocks. Incorporation of excess 40Ar in garnet and pyroxene was contemporaneous with the formation of symplectites ∼2 Ga ago.