Simon Inger

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.

Dating deformation using Rb-Sr in white mica: Greenschist facies deformation ages from the Entrelor shear zone, Italian Alps

Ages of deformation have been obtained by Rb-Sr analysis of white micas whose microstructural and chemical characteristics indicate that they crystallized or recrystallized during shear fabric formation. Since white micas commonly define deformation fabrics in medium-grade metamorphic rocks, these ages can be directly related to structural geometries with regional context. This direct method contrasts with estimates of midcrustal deformation ages derived from cooling histories because it does not rely on assumptions about the thermal structure of the crust. It does require that the dated minerals attained isotopic equilibrium with the dominant Sr reservoir at temperatures lower than the closure temperature. This resetting was apparently achieved during dynamic recrystallization of white micas in greenschist-facies metasediments and metagranitoid units in the western Alps. The results suggest that the Sr isotopic composition of the new mica is buffered by the coexisting high-Sr phases (calcite, feldspar or epidote) via the grain boundary network. High-strain rocks from the Entrelor shear zone system of the western Alps have yielded indistinguishable white mica Rb-Sr ages along 30 km of individual and kinematically linked shear zones. The age of the back-thrusting event is constrained at 34±1Ma, the age yielded by the younger generation of synkinematically crystallized white micas. This event was short-lived, involving at least 20 km of shortening in ∼1 m.y. or less. An earlier, variably overprinted component, dating from 38 to 37 Ma, has been identified in the mica fabric, but its kinematic significance is uncertain. This method of dating strain fabrics offers a powerful tool for tectonic studies, since isotopic resetting can be directly linked to structural geometries, microstructural textures, and PT conditions. It allows testing of kinematic models in orogens and can provide important information on the rates of geological processes in the crust.

The role of fluids in the formation of High Himalayan leucogranites

Abstract During fractional melting of the crust, the availability of fluids is a critical variable that influences the melt fraction obtained, the capacity of the melt to migrate from its source and the relationship between melting and uplift. For the High Himalayan leucogranites, isotope systematics and metamorphic phase equilibria have identified micaceous metasediments as the most likely source, and the incongruent melting of muscovite as the appropriate melting reaction. High-field strength trace elements are controlled by dissolution kinetics and incomplete crystal/liquid segregation of accessory phases during crustal anatexis. In contrast, Rb and Sr are controlled by major phases contributing to the melting reaction. Hence the Rb/Sr ratios in primary melts can be correlated with the availability of fluids during partial melting. Geochemical modelling of melts resulting from anatexis of a wide range of metasedimentary compositions (plagioclase 5–30%, muscovite 5–30%, biotite 5–20%, quartz 30–85%) indicates that Rb/Sr ratios will vary between 4 and 10 during equilibrium melting and vapour-absent conditions, but are less than 3.5 during vapour-present melting. In either case, Rb/Sr ratios in melts are reduced under disequilibrium conditions. In general Rb/Sr ratios of Himalayan leucogranites vary between 3 and 6. Such high ratios could not have formed under vapour-present conditions unless feldspar fractionation has increased the Rb/Sr ratios of primary melts. Trace-element systematics and petrographic evidence do not support significant fractionation of feldspar. Hence for most, if not all, Himalayan granites, vapour-absent conditions prevailed during fractional melting, irrespective of either source composition or the kinetics of the melting reaction. This is consistent with the observation that the melts have migrated from their source, and allows decompression melting to play a role in their genesis.

Timing of an extensional detachment during convergent orogeny: New Rb-Sr geochronological data from the Zanskar shear zone, northwestern Himalaya

The Zanskar shear zone of northwest India forms the western segment of the South Tibetan detachment system, a north-dipping normal fault and shear zone that unroofed high-grade metamorphic rocks during contraction of the Himalayan orogen. New Rb-Sr mineral ages from the shear zone and its footwall show that ductile deformation was ongoing at 26 Ma, and continued to 16 Ma in some sections. The nature of the deformation varies along strike; displacement at 25 Ma or earlier in the western section did not result in a large thermal offset. By contrast, the sections to the east display substantial ductile thinning at midcrustal levels that was superseded by brittle detachments as the system continued to exhume rocks of the High Himalayan crystalline sequence. The implication is that the Zanskar shear zone may have had a smaller offset in the west and cut down-section to the east. The time of the bulk of the deformation of this structure is determined to be similar to that of the equivalent structure in most of the eastern and central Himalaya.

Deformation migration in an orogen-scale shear zone array: an example from the Basal Briançonnais Thrust, internal Franco-Italian Alps

Combined structural, geochemical and isotopic studies have allowed an understanding of the timing and nature of an orogen-scale fault array. The results indicate that the deformation loci within the internal western Alps, during the Alpine collision, occurred as a foreland propagating thrust sequence. The east to west deformation migration within the internal zones is apparently in-sequence in relation to the external zones. Rb–Sr white mica dating of syn-kinematic greenschist-facies mineral assemblages from the Basal Brianconnais Thrust indicate that thrusting ceased between 27 and 32 Ma, several million years after shearing in the hinterland and several million years prior to shearing in the foreland. The Brianconnais Domain, which constitutes the hanging wall to the Basal Brianconnais Thrust, preserves two major shearing episodes. The first, with a top-to-the-northwest overshear, has been tentatively dated at 45 Ma. The second, a very pervasive, east–west orientated, greenschist-facies event was previously dated at 34 Ma on the hinterland margin of the Brianconnais Domain and has now been dated at 27–32 Ma on the foreland margin of the Brianconnais Domain. The period between 34 and 27 Ma apparently dates the migration of deformation through the relict European passive margin, represented by the Brianconnais Domain. This is believed to be in response to overthrusting of Adria/Africa and its associated subduction complex. Structural mapping indicates that the present Basal Brianconnais Thrust in the Col du Petit St Bernard region, Franco-Italian Alps, is a break-back thrust which cuts through an already imbricated pile. Geochronological evidence suggests that the early imbrication of the Brianconnais stratigraphy occurred prior to full interaction of the European and Adria/African plates, that is, during subduction, docking and escape from the subduction complex under Adria. Therefore, although the present Basal Brianconnais Thrust is a break-back thrust in terms of local structural geometries, it is an in-sequence foreland-propagating structure. Geochronological, micro-structural and micro-chemical data indicate that the Brianconnais Domain in the Col du Petit St Bernard zone is formed from granitoid material which intruded and cooled at approximately 320 Ma. During the Alpine event, deformation and metamorphism were insufficient to affect the Sr isotopic system. This suggests that this portion of the Brianconnais Domain was probably subducted to much shallower depths and underwent much less pervasive deformation than the other internal European basement material.

Magmagenesis associated with extension in orogenic belts: examples from the Himalaya and Tibet

Abstract Structures in mountain belts which accommodate layer extension may be synchronous with either ongoing thickening or reduction in crustal thickness. Either case may be associated with magmagenesis because they contribute to the thermal regime in the lithosphere, and the chemistry of magmas produced can reveal source compositions and thereby conditions of melting. In the High Himalaya, exhumation of upper-amphibolite facies metamorphic rocks in the footwall of a major normal shear zone occurred during crustal thickening. This rapid decompression enhanced fluid-absent melting reactions in micaceous pelites and produced the High Himalayan leucogranite suites which were subsequently intruded into the zone of normal shear. Similar leucogranites are seen in the Cretaceous North Lhasa Terrane (NLT), in association with a calc-alkaline series. These suites are also crustally derived but their chemistry indicates higher melting temperatures than in the Himalaya, suggesting that heat flow into the base of the crust was enhanced. This thermal anomaly was most probably associated with lithosphere attenuation following an episode of thickening. The Tibetan Plateau is currently a topographically elevated region of thick crust that appears to be undergoing orogenic collapse—active structures are stretching and thinning the crust. The plateau is underlain by a thin, hot mantle lithosphere, supporting theoretical models that suggest that a lithospheric root of a thick orogen would become unstable and be removed by convection. Consequent thermal perturbations in the mantle lithosphere cause melting in suitably fertile horizons, leading to widespread volcanism. Comparisons have been drawn between the Tibetan model and the late stages of the Variscan belt in Europe. Magmatic rocks of diverse composition occur throughout the late Variscan, many of them indicating derivation from a previously enriched lithospheric source, suggesting thermal anomalies in the subcontinental mantle akin to those inferred for Tibet. Taken with structural and sedimentary evidence, these indications strongly support the hypothesis that the Variscan orogen underwent orogenic collapse in the Tibetan style.