Sarah C. Sherlock

40Ar-39Ar and Rb-Sr geochronology of high-pressure metamorphism and exhumation history of the Tavsanli Zone, NW Turkey

Abstract Geochronological investigations in high- and ultra-high-pressure metamorphic rocks are problematic since firstly the low temperatures lead to fine grain size and disequilibrium assemblages, and secondly the problem of “excess argon” affects 40Ar-39Ar systematics, the most commonly used isotopic system. The Tavsanli Zone is a belt of high-pressure low-temperature (HP-LT) rocks spanning NW Turkey and is one such region where previous geochronological studies have produced a range of estimates for the age of HP-LT metamorphism, raising the question of whether they are geologically significant. This study presents new data from the Tavsanli Zone; 40Ar-39Ar ages are in the range 60 Ma to 175 Ma, whilst Rb-Sr ages are restricted to 79.7 Ma to 82.8 Ma, confirming the presence of excess argon. Detailed ultra-violet laser ablation microprobe (UVLAMP) studies have revealed younger 40Ar-39Ar ages in the cores of coarser white micas, which in conjunction with 40Ar-39Ar ages from the finest grained lithologies and the Rb-Sr white mica crystallisation ages, constrain the post-HP-LT metamorphism exhumation rates of these rocks. Petrological and regional constraints suggest that syn-subduction exhumation and cooling took place initially by synchronous subduction and exhumation by underplating. This is followed by a phase of syn-continent-continent collision at a rate of approximately 1.5 mma−1 and exhumation to the surface via thrusting. The 40Ar-39Ar hornblende data from a granodiorite intruding the HP-LT rocks constrain the later parts of exhumation path. This study highlights the importance of a multi-system geochronological approach when attempting to determine the history of HP-LT rocks.

Cretaceous and Triassic subduction-accretion, high-pressure–low-temperature metamorphism, and continental growth in the Central Pontides, Turkey

Biostratigraphic, isotopic, and petrologic data from the Central Pontides document major southward growth of the Eurasian continental crust by subduction-accretion during the Cretaceous and Triassic Periods. A major part of the accreted material is represented by a crustal slice, 75 km long and up to 11 km thick, consisting of metabasite, metaophiolite, and mica schist that represent underplated Tethyan oceanic crustal and mantle rocks. They were metamorphosed at 490 degrees C and 17 kbar in mid-Cretaceous time (ca. 105 Ma). The syn-subduction exhumation occurred in a thrust sheet bounded by a greenschist facies shear zone with a normal sense of movement at the top and a thrust fault at the base. A flexural Foreland basin developed in front of the south-vergent high-pressure-low-temperature (HP-LT) metamorphic thrust sheet; the biostratigraphy of the foreland basin constrains the exhumation of the HP-LT rocks to the lbronian-Coniacian, similar to 20 m.y. after the HP-LT metamorphism, and similar to 25 m.y. before the terminal Paleocene continental collision. The Cretaceous subduction-accretion complex is tectonically overlain in the north by oceanic crustal rocks accreted to the southern margin of Eurasia during the latest Triassic-earliest Jurassic. The Triassic subduction-accretion complex is made up of metavolcanic rocks of ensimatic arc origin and has undergone a high pressure, greenschist facies metamorphism with growth of sodic amphibole. Most of the Central Pontides consists of accreted Phanerozoic oceanic crustal material, and hence is comparable to regions such as the Klamath Mountains in the northwestern United States or to the Altaids in Central Asia.

Excess argon evolution in HP–LT rocks: a UVLAMP study of phengite and K-free minerals, NW Turkey

Excess argon is a common problem in high-pressure metamorphic rocks. Here, we present 40Ar/39Ar data from metabasite, metachert and metapelite lithologies and a range of minerals. The results allow us to discuss the origin of the excess argon and processes leading to its incorporation during high-pressure metamorphism. Samples were derived from blueschist and low-temperature eclogite facies rocks from the Tavsanli Zone of NW Turkey. Samples were analysed using the 40Ar/39Ar Ultra-Violet Laser Ablation Microprobe (UVLAMP) and yield ages ranging from 72±3 to 154±7 Ma. Detailed intra-grain profiling of phengites in situ, and analyses of sodic-amphibole, lawsonite, quartz and garnet, present a detailed picture of the spatial distribution of excess argon within different minerals in the same hand samples, revealing that they acted as near closed systems where excess argon was preferentially partitioned into phengite. Whole rock XRF data indicates a scattered relationship between bulk K2O content of the rock and the concentration of excess argon in phengite. We conclude that excess argon was derived in situ in a relatively closed, fluid-absent system, rather than introduced by fluids.

Flat plateau and impossible isochrons: apparent 40Ar-39Ar geochronology in a high-pressure terrain

Until recently most ages determined by the 40Ar-39Ar technique have been represented by the age spectrum and isochron. Early in the history of the technique a series of criteria were outlined in order to discriminate between spectra which were considered to reflect disturbed isotopic systems and spectra which were considered to reflect the accurate age of the rock or mineral. With the advancement of 40Ar-39Ar instrumentation and the introduction of laser-ablation techniques, it is now possible to obtain an age from single grains and on a sub-grain scale. In this case there is no production of an age spectra or determination of a plateau age; however, isochrons are still readily produced and used in the discussion of Ar isotope ratios, and specifically excess argon. Here, new data illustrate the possible drawback first of the age spectrum and plateau age in the absence of an independent geochronological constraint in high-pressure terranes, and second, of the accompanying isochron, which we demonstrate can be both incorrect and geologically meaningless.

The response of vegetation on the andean flank in western amazonia to pleistocene climate change

Pleistocene climate fluctuations caused major shifts in the altitudinal distribution of forest plant species. A reconstruction of past environmental change from Ecuador reveals the response of lower montane forest on the Andean flank in western Amazonia to glacial-interglacial global climate change. Radiometric dating of volcanic ash indicates that deposition occurred ~324,000 to 193,000 years ago during parts of Marine Isotope Stages 9, 7, and 6. Fossil pollen and wood preserved within organic sediments suggest that the composition of the forest altered radically in response to glacial-interglacial climate change. The presence of Podocarpus macrofossils ~1000 meters below the lower limit of their modern distribution indicates a relative cooling of at least 5°C during glacials and persistence of wet conditions. Interglacial deposits contain thermophilic palms suggesting warm and wet climates. Hence, global temperature change can radically alter vegetation communities and biodiversity in this region.

Precise dating of low-temperature deformation: Strain-fringe analysis by 40Ar-39Ar laser microprobe

Pyritized graptolites from the Welsh Basin (United Kingdom) slate belt acted as rigid bodies during cleavage formation, and epizonal white micas formed within the resulting. strain shadows, orthogonal to the principal stress orientation. Although the quantities of mica are small, they are a pure synkinematic mineral and have been dated by Ar-40-Ar-39 infrared laser microprobe as a means to dating cleavage. Four samples of strain-fringe mica from different hand samples yielded ages ranging from 394.4 +/- 3.1 to 397.8 +/- 1.8 Ma (2sigma), with a mean age of 396.1 +/- 1.4 Ma (2sigma). By focusing on minerals that are unequivocally synkinematic, this technique provides a novel solution to the problems of isotopically dating slaty cleavage. Previous studies have predominantly relied on dating whole-rock slate samples or separated illite grains by Ar-40-Ar-39 techniques; problems encountered included (1) separating the effects of isotopic contamination by detrital phases, (2) Ar-39 loss during the irradiation of illite mineral separates, and (3) thermally induced Ar-40 loss in nature from fine-grained minerals. By circumventing these problems, this new method provides the first unequivocal and high-precision age data for Acadian deformation in the well-characterized Welsh Basin slate belt. With such precision, the method may afford geologists the opportunity to track tectonic fronts across orogens and assess the rates of accretion processes in areas that are peripheral to sites of continent-continent collision.

First field evidence of southward ductile flow of Asian crust beneath southern Tibet

There is lively debate on whether Asian plate material was involved in southward flow of mid-lower crust in a ductile channel beneath southern Tibet. One argument against such involvement is the apparent absence of material derived from Asian lithosphere within the High Himalayan Series (Indian plate) that could represent the putative channel. A north-south–trending mid-Miocene dike swarm that intrudes the Tethyan sedimentary cover of the Sakya gneiss dome (Indian plate) yields new Sr-Nd isotopic data (87Sr/86Sr = 0.7071–0.7079; ϵNd −4 to −6) indicating that these melts share the same source as Miocene dacitic dikes from north of the Indus-Tsangpo suture. Moreover, dikes on both sides of this suture represent crustal melts derived largely from mid-lower crust of the Asian plate, exposed today as the Nyainqentanglha gneisses that underlie the Gangdese batholith. We infer that melting of the Asian lithosphere extended south of the surface trace of the suture, requiring southward propagation of anatectic Asian middle crustal material during the Miocene. The emplacement ages of the southern dike swarm (12–9 Ma) thus delimit the timing of active southward ductile flow of Asian material.

A laser-probe 40Ar/39Ar study of pseudotachylite from the Tambach Fault Zone, Kenya: direct isotopic dating of brittle faults

Understanding the tectonic evolution of orogenic belts and intracratonic areas depends on our ability to determine the age of tectonic features on a variety of scales. This study demonstrates the value of the laser-probe 40Ar/39Ar dating technique, which, if applied to fault-derived pseudotachylites, may be used to directly determine the age of brittle faults. The laser-probe technique affords high spatial resolution, enabling a greater opportunity for discriminating between pseudotachylite matrix, host-rock clasts and alteration products that are often present in varying proportions within pseudotachylites. The laser-probe 40Ar/39Ar technique has been applied to pseudotachylite samples from the Tambach Fault Zone (TFZ), a major NW–SE trending strike-slip fault within the Kenyan part of the Late Proterozoic/Early Palaeozoic Mozambique Belt. The pseudotachylites of the TFZ were previously thought to have formed either (i) at about 530–430 Ma, or (ii) during the Cenozoic evolution of the Kenya Rift. In the latter case, seismic slip on the rift-bounding normal fault would have generated the pseudotachylites, due to the reactivation of old NW–SE trending structures in the basement. Based on our new data, we interpret the pseudotachylite formation age to be 400 Ma. This rules out the possibility that the pseudotachylites are related to the formation of the Kenya Rift. Although the inherited basement faults may have been locally reactivated as transfer faults, reactivation of these structures during rifting did not occur beyond the margins of the Kenya Rift.

Grenville-age pseudotachylite in the Lewisian: laserprobe 40Ar/39Ar ages from the Gairloch region of Scotland (UK)

Infrared laserprobe 40Ar/39Ar dating has been used to date pseudotachylite and host-rock minerals from a crush belt in the Lewisian basement of Scotland. It has revealed complexity in the pseudotachylite data that is attributable to the presence of refractory host-rock clasts and mineral fragments in the pseudotachylite. In conjunction with the host-rock mineral laserprobe 40Ar/39Ar data it has been possible to simplify the pseudotachylite data for the samples, and the preferred ages for these are: 980 ± 39 Ma, 999 ± 31 Ma and 1024 ± 30 Ma (2σ). These ages are the first record of Grenville-aged brittle deformation in the Lewisian. Further, this study serves to illustrate the complexity of dating pseudotachylites, and the advantages and limitations of the IR laserprobe applied to such materials.

Dating fault reactivation by Ar/Ar laserprobe: an alternative view of apparently cogenetic mylonite–pseudotachylite assemblages

Apparently cogenetic mylonite–pseudotachylite assemblages are commonly reported in major fault zones. They represent two very incompatible modes of deformation, and complex formation mechanisms have been proposed to explain this paradox. We report here one such assemblage from Central Norway in which apparently synchronous mylonite and pseudotachylite formation is separated by 100 Ma-mean muscovite Ar/Ar laserprobe spot ages from mylonite, and pseudotachylite matrix, are 406 ± 11 Ma and 290 ± 10 Ma respectively. In preference to a complex cogenetic model we invoke fault reactivation as a viable alternative, and suggest that assumed contemporaneity in such assemblages may be invalid in many cases.