Complete Database for the following paper: "Regional trends and petrologic factors inhibit global interpretations of zircon trace element compositions"submitted to American Journal of Science, by: Nick M W Roberts, Christopher J Spencer, Stephen Puetz, C. Brenhin Keller, and Simon Tapster
Metamorphic core complexes are classically interpreted to have formed during crustal extension, although many also occur in compressional environments. New U−(Th)−Pb allanite and xenotime geochronologic data from the structurally highest Zas Unit (Cycladic Blueschist Unit) of the Naxos metamorphic core complex, Greece, integrated with pressure−temperature−time (P−T−t) histories, are incorporated into a thermal model to test the role of crustal thickening and extension in forming metamorphic core complexes. Metamorphism on Naxos is diachronous, with peak metamorphic conditions propagating down structural section over a ∼30−35 m.y. period, from ca. 50 Ma to 15 Ma. At the highest structural level, the Zas Unit records blueschist-facies metamorphism (∼14.5−19 kbar, 470−570 °C) at ca. 50 Ma, during northeast-directed subduction of the Adriatic continental margin. The Zas Unit was subsequently extruded toward the SW and thrust over more proximal continental margin and basement rocks (Koronos and Core units). This contractional episode resulted in crustal thickening and Barrovian metamorphism from ca. 40 Ma and reached peak kyanite-sillimanite−grade conditions of ∼10−5 kbar and 600−730 °C at 20−15 Ma. Model P−T−t paths, assuming conductive relaxation of isotherms following overthrusting, are consistent with the clockwise P−T−t evolution. In contrast, extension results in exhumation and cooling of the crust, which is inconsistent with key components of the thermal evolution. Barrovian metamorphism on Naxos is therefore interpreted to have resulted from crustal thickening over a ∼30−35 m.y. time period prior to extension, normal faulting, and rapid exhumation after a thermal climax at ca. 15 Ma.
Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) U-Pb geochronology for carbonate minerals, calcite in particular, is rapidly gaining popularity as an absolute dating method. In this study, we review the latest technical progress in LA-ICP-MS carbonate geochronology, including the pre-screening strategies (on-line spot selection with a threshold, image-guided approach, and image-based approach), preferred instrumentation (Q-ICP-MS, SF-ICP-MS and MC-ICP-MS), calibration methods, common Pb corrections and the development of reference materials, with the aim of further improving the precision and accuracy of this technique. We emphasized the characterization of two calcite reference materials (TLM and LSJ07) for micro-beam U-Pb geochronology and C, O isotope ratio measurements. The latest geological applications of LA-ICP-MS U-Pb carbonate geochronology in dating of diagenesis and hydrothermal activity were reviewed.
Structural and metamorphic pressure-temperature-time history data together provide insight into the tectonic mechanisms by which crustal rocks are buried, transformed and transported in plate collision zones. Rates of prograde and retrograde metamorphism, especially in high-grade metamorphic rocks, are commonly determined from differences in temperature and time recorded by minerals that grow at different stages in the metamorphic evolution and/or by minerals that have different closure temperatures to the diffusion of their daughter products.
The Greater Himalayan Sequence (GHS) forms the high-grade metamorphic core of the Himalayan orogen. The mechanisms by, rates at, and timescales over which these rocks were buried, transformed and exhumed within an overall compressional setting are still much debated, yet are important for providing insight into middle and lower crustal processes in continent-continent collision zones. New monazite U-Th-Pb and muscovite Ar/Ar data from the highest structural levels of the GHS Arunachal Pradesh, in conjunction with previously reported data from Sikkim and Bhutan, suggest that the timing of peak metamorphism and rates of exhumation-related cooling appear to get younger further eastwards in the orogen. Monazites associated with peak metamorphism and/or melting reactions yield ages of ca. 16-11 Ma in Arunachal, compared with 15-13 Ma in NW Bhutan and 26-23 Ma in N. Sikkim. Muscovite Ar/Ar ages mirror this younging trend, yielding ca. 7 Ma in Arunachal Pradesh, compared with 13-11 Ma in Bhutan and 13-12 Ma in Sikkim.
We have combined recent advances in petrogenetic modeling, trace element fingerprinting techniques, and diffusion modelling to yield robust insights into the evolution and cooling history of the GHS in the Eastern Himalaya. These data suggest a higher degree of complexity in the architecture and mechanisms of formation and exhumation of the eastern Himalayan GHS than is reported for the central portions of the orogen, with corresponding implications for formation and exhumation mechanisms.
The tectonic setting and mechanisms and duration of emplacement of Proterozoic massif-type anorthosites and the significance of typically associated ultrahigh-temperature (UHT) host rocks have been debated for decades. This is particularly true of the Rogaland Anorthosite Province (RAP) in the SW Sveconorwegian Orogen. Earlier studies suggest that the RAP was emplaced over 1–3 Myr around 930 Ma towards the end of orogenesis, resulting in an up to 15–20 km-wide contact metamorphic aureole. However, our structural observations show that the RAP is located in the footwall of a 15 km-wide extensional detachment (Rogaland Extensional Detachment, RED), separating the intrusions and their UHT host rocks from weakly metamorphosed rocks in the hanging wall. U–Pb zircon dating of leucosome in extensional pull-aparts associated with the RED yields ages of 950–935 Ma, consistent with Re–Os molybdenite ages from brittle extensional structures in the hanging-wall block that range between 980 and 930 Ma. A metapelite in the immediate vicinity of the RAP yields a 950 Ma U–Pb age of matrix-hosted monazite, and part of the RAP was intruded by the Storgangen norite dike at ca. 950 Ma, providing a minimum age of emplacement. These ages are consistent with Ar–Ar hornblende and biotite ages that show rapid cooling of the footwall before 930 Ma, but slow cooling of the hanging wall. Field and geochronologic data suggest that the RAP formed and was emplaced over a long period of time, up to 100 Myr, with different emplacement mechanisms reflecting an evolving regional stress regime. The distribution of UHT rocks around the RAP reflects differential extensional exhumation between 980 and 930 Ma, not contact metamorphism. The duration and style of orogenic activity and externally (as opposed to gravitationally) driven extension suggest that the RAP formed in a continental back-arc setting.
Quantitative constraints on the rates of tectonic processes underpin our understanding of the mechanisms that form mountains.In the Sikkim Himalaya, late structural doming has revealed time-transgressive evidence of metamorphism and thrusting that permit calculation of the minimum rate of movement on a major ductile fault zone, the Main Central Thrust (MCT), by a novel methodology.U-Th-Pb monazite ages, compositions, and metamorphic pressure-temperature determinations from rocks directly beneath the MCT reveal that samples from ~50 km along the transport direction of the thrust experienced similar prograde, peak, and retrograde metamorphic conditions at different times.In the southern, frontal edge of the thrust zone, the rocks were buried to conditions of ~550°C and 0.8 GPa between ~21 and 18 Ma along the prograde path.Peak metamorphic conditions of ~650°C and 0.8-1.0GPa were subsequently reached as this footwall material was underplated to the hanging wall at ~17-14 Ma.This same process occurred at analogous metamorphic conditions between ~18-16 Ma and 14.5-13 Ma in the midsection of the thrust zone and between ~13 Ma and 12 Ma in the northern, rear edge of the thrust zone.Northward younging muscovite 40 Ar/ 39 Ar ages are consistently ~4 Ma younger than the youngest monazite ages for equivalent samples.By combining the geochronological data with the >50 km minimum distance separating samples along the transport axis, a minimum average thrusting rate of 10 ± 3 mm yr À1 can be calculated.This provides a minimum constraint on the amount of Miocene India-Asia convergence that was accommodated along the MCT.
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