Martin Engi

Metamorphic rates in collisional orogeny from in situ allanite and monazite dating

The prograde sequence of rare earth minerals recorded in metapelites during regional metamorphism reveals a series of irreversible reactions among silicates and phosphates. In individual samples from the northern Lepontine (Central Alps), allanite is partly replaced by monazite at 560–580 °C. Relic allanite retains its characteristic growth zoning acquired at greenschist facies conditions (430–450 °C). Coexisting monazite and allanite were dated in situ to delimit in time successive stages of the Barrovian metamorphism. In situ sensitive high-resolution ion microprobe (SHRIMP) U-Th-Pb dating of allanite (31.5 ± 1.3 and 29.2 ± 1.0 Ma) and monazite (18.0 ± 0.3 and 19.1 ± 0.3 Ma) constrains the time elapsed between 430–450 °C and 560–580 °C, which implies an average heating rate of 8–15 °C/m.y. Combined with new fission track ages (zircon, 10–9 Ma; apatite, 7.5–6.5 Ma), metamorphic rates of the entire orogenic cycle, from prograde to final cooling, can be reconstructed.

Role of the tectonic accretion channel in collisional orogeny

High-pressure relics studied in many collisional mountain belts are overprinted by subsequent Barrovian metamorphism that may reach migmatite grade in the central parts of such orogens. We propose that this evolution is linked to the development of a narrow tectonic accretion channel (TAC) at the subducting plate boundary. Geologic evidence from the “Southern Steep Belt” of the Central Alps and the tectono-metamorphic record of this orogen guided us in constructing numerical models of a TAC. Simulations indicate that the accretion of crust enriched in radioactive elements to mantle depth provides a mechanism to obtain P - T - t (pressure-temperature-time) trajectories in reasonable agreement with observations in the Alps. Similarly, the generation of granitoid melts predicted by the model during late-orogenic exhumation of the TAC is in line with the Alpine record. This case study suggests that accretion of upper-crust fragments to mantle depth, by underplating along a subduction fault, and subsequent extrusion of parts of the TAC along that same fault, may be fundamental processes in the dynamic evolution of many collisional orogens.

Metamorphism of metasediments at the scale of an orogen: a key to the Tertiary geodynamic evolution of the Alps*

Abstract Major discoveries in metamorphic petrology, as well as other geological disciplines, have been made in the Alps. The regional distribution of Late Cretaceous–Tertiary metamorphic conditions, documented in post-Hercynian metasediments across the entire Alpine belt from Corsica–Tuscany in the west to Vienna in the east, is presented in this paper. In view of the uneven distribution of information, we concentrate on type and grade of metamorphism; and we elected to distinguish between metamorphic paths where either pressure and temperature peaked simultaneously, or where the maximum temperature was reached at lower pressures, after a significant temperature increase on the decompression path. The results show which types of process caused the main metamorphic imprint: a subduction process in the western Alps, a collision process in the central Alps, and complex metamorphic structures in the eastern Alps, owing to a complex geodynamic and metamorphic history involving the succession of the two types of process. The western Alps clearly show a relatively simple picture, with an internal (high-pressure dominated) part thrust over an external greenschist to low-grade domain, although both metamorphic domains are structurally very complex. Such a metamorphic pattern is generally produced by subduction followed by exhumation along a cool decompression path. In contrast, the central Alps document conditions typical of subduction (and partial accretion), followed by an intensely evolved collision process, often resulting in a heating event during the decompression path of the early-subducted units. Subduction-related relics and (collisional/decompressional) heating phenomena in different tectonic edifices characterize the Tertiary evolution of the Eastern Alps. The Tuscan and Corsica terrains show two different kinds of evolution, with Corsica resembling the western Alps, whereas the metamorphic history in the Tuscan domain is complex owing to the late evolution of the Apennines. This study confirms that careful analysis of the metamorphic evolution of metasediments at the scale of an entire orogen may change the geodynamic interpretation of mountain belts.

Mechanisms of mass and heat transport during Barrovian metamorphism: A discussion based on field evidence from the Central Alps (Switzerland/northern Italy)

[1] Tectonic and metamorphic data for the Central Alps (Switzerland/Italy) are used to discuss this classic example of a Barrovian metamorphic terrain, notably the evolution of its thermal structure in space and time. Available P‐T‐t data indicate variable contributions of advective and conductive heat transport during collision and subsequent cooling and exhumation. Some areas experienced a prolonged period of partial melting while other areas, at the same time, show but moderate heating. The Barrow‐type metamorphic field gradient observed in the final orogen is the result of two distinct tectonic processes, with their related advective and conductive heat transport processes. The two tectonic processes are (1) accretion of material within a subduction channel related to decompression and emplacement of high‐pressure units in the middle crust and (2) wedging and related nappe formation in the continental lower plate. The second process postdates the first one. Wedging and underthrusting of continental lower plate material produces heat input into lower crustal levels, and this process is responsible for predominantly conductive heat transport in the overlying units. The interacting processes lead to different maximum temperatures at different times, producing the final Barrovian metamorphic field gradient. The south experienced rapid cooling, whereas the north shows moderate cooling rates. This discrepancy principally reflects differences in the temperature distribution in the deeper crust prior to cooling. Differences in the local thermal gradient that prevailed before the cooling also determined the relationships between cooling rate and exhumation rate in the different areas. Citation: Berger, A., S. M. Schmid, M. Engi, R. Bousquet, and M. Wiederkehr (2011), Mechanisms of mass and heat transport during Barrovian metamorphism: A discussion based on field evidence from the Central Alps (Switzerland/northern Italy), Tectonics, 30, TC1007, doi:10.1029/2009TC002622.

The tectonometamorphic evolution of the Sesia–Dent Blanche nappes (internal Western Alps): review and synthesis

AbstractThis study reviews and synthesizes the present knowledge on the Sesia–Dent Blanche nappes, the highest tectonic elements in the Western Alps (Switzerland and Italy), which comprise pieces of pre-Alpine basement and Mesozoic cover. All of the available data are integrated in a crustal-scale kinematic model with the aim to reconstruct the Alpine tectono-metamorphic evolution of the Sesia–Dent Blanche nappes. Although major uncertainties remain in the pre-Alpine geometry, the basement and cover sequences of the Sesia–Dent Blanche nappes are seen as part of a thinned continental crust derived from the Adriatic margin. The earliest stages of the Alpine evolution are interpreted as recording late Cretaceous subduction of the Adria-derived Sesia–Dent Blanche nappes below the South-Alpine domain. During this subduction, several sheets of crustal material were stacked and separated by shear zones that rework remnants of their Mesozoic cover. The recently described Roisan-Cignana Shear Zone of the Dent Blanche Tectonic System represents such a shear zone, indicating that the Sesia–Dent Blanche nappes represent a stack of several individual nappes. During the subsequent subduction of the Piemonte–Liguria Ocean large-scale folding of the nappe stack (including the Roisan-Cignana Shear Zone) took place under greenschist facies conditions, which indicates partial exhumation of the Dent Blanche Tectonic System. The entrance of the Briançonnais micro-continent within the subduction zone led to a drastic change in the deformation pattern of the Alpine belt, with rapid exhumation of the eclogite-facies ophiolite-bearing units and thrust propagation towards the foreland. Slab breakoff probably was responsible for allowing partial melting in the mantle and Oligocene intrusions into the most internal parts of the Sesia–Dent Blanche nappes. Finally, indentation of the Adriatic plate into the orogenic wedge resulted in the formation of the Vanzone back-fold, which marks the end of the pervasive ductile deformation within the Sesia–Dent Blanche nappes during the earliest Miocene.

Fieldbook and geodatabases: tools for field data acquisition and analysis

Abstract We introduce FieldBook and GeoDatabase, 2 new and effective tools for geologic field data acquisition and analysis. FieldBook is an application for Apples Newton MessagePad. Geological data collected at the outcrop, including notes and drawings, can be entered directly and on-site. The formalization of the multiparameter information leads directly to a consistent database. This procedure results in a complete, up-to-date database where all information collected by different researchers in a project is available anytime, and no data are lost. GeoDatabase is an application based on FileMaker™ Pro, representing the FieldBook interface on PC/Macintosh. GeoDatabase provides extensive search possibilities and strong export features that are needed for field-data analysis, either in the field or in the office. It can be used as a central database within a local network with several users on either PC or a Macintosh. FieldBook and GeoDatabase both are simple to use, yet they satisfy the demands of field campaigns involving numerous scientists. Applications of field projects in the crystalline basement of the Salalah area and the Masirah ophiolite are given.

Geometry and kinematics of the Roisan-Cignana Shear Zone, and the orogenic evolution of the Dent Blanche Tectonic System (Western Alps)

The Dent Blanche Tectonic System (DBTS) is a composite thrust sheet derived from the previously thinned passive Adriatic continental margin. A kilometric high-strain zone, the Roisan-Cignana Shear Zone (RCSZ) defines the major tectonic boundary within the DBTS and separates it into two subunits, the Dent Blanche s.s. nappe to the northwest and the Mont Mary nappe to the southeast. Within this shear zone, tectonic slices of Mesozoic and pre-Alpine meta-sediments became amalgamated with continental basement rocks of the Adriatic margin. The occurrence of high pressure assemblages along the contact between these tectonic slices indicates that the amalgamation occurred prior to or during the subduction process, at an early stage of the Alpine orogenic cycle. Detailed mapping, petrographic and structural analysis show that the Roisan-Cignana Shear Zone results from several superimposed Alpine structural and metamorphic stages. Subduction of the continental fragments is recorded by blueschist-facies deformation, whereas the Alpine collision is reflected by a greenschist facies overprint associated with the development of large-scale open folds. The post-nappe evolution comprises the development of low-angle brittle faults, followed by large-scale folding (Vanzone phase) and finally brittle extensional faults. The RCSZ shows that fragments of continental crust had been torn off the passive continental margin prior to continental collision, thus recording the entire history of the orogenic cycle. The role of preceding Permo-Triassic lithospheric thinning, Jurassic rifting, and ablative subduction processes in controlling the removal of crustal fragments from the reactivated passive continental margin is discussed. Results of this study constrain the temporal sequence of the tectono-metamorphic processes involved in the assembly of the DBTS, but they also show limits on the interpretation. In particular it remains difficult to judge to what extent pre-collisional rifting at the Adriatic continental margin preconditioned the efficiency of convergent processes, i.e. accretion, subduction, and orogenic exhumation.

Nondestructive chemical dating of young monazite using XRF: 1. Design of a mini-probe, age data for samples from the Central Alps, and comparison to U–Pb (TIMS) data

The analytical capabilities of an improved mini-XRF instrument for trace element analysis are explored to obtain U–Th–Pb ages. For single monazite grains as small as 50 μm, the detection limits for U, Th and Pb, with counting times of 10 min, are ∼10 ppm; counts collected for 40 min are sufficient to date single monazite grains as young as 15 Ma. The precision of the age data depends essentially on the counting statistics of the main X-ray peak (NPb Lα1/2) used to analyze for lead; the age uncertainty is proportional to the age of the grain, the total contents of Th and U, and the (squared) integration time. The reliability of the chemical dating method is tested using monazite separates from amphibolite facies metamorphic monazite samples from the central Alps, for which the chemical ages are compared to mass spectrometric (U–Pb TIMS) ages. We show that populations of analyses obtained using the new instrument on single grains as young as 25–30 Ma agree within error with the TIMS data. For monazite separates with complex age patterns, such as polymetamorphic samples yielding discordant isotopic ages, the statistical analysis of chemical age data, obtained from populations of 26–50 grains, is advantageous in that it may provide additional insight into the reasons for the discordancy. Chemical Th–U–Pb dating of monazite by the XRF method offers considerably improved accuracy and precision, compared with chemical dating by electron microprobe, which typically becomes impractical for samples less than 100–200 Ma in age. Conversely, the minimum grain size required for the new instrument (Φ∼50 μm) is well above the spatial resolution of the electron microprobe (Φ∼2 μm). A companion paper [Scherrer et al., this volume] describes further developments towards microanalysis by XRF, with improved spatial resolution.

Nondestructive chemical dating of young monazite using XRF 2. Context sensitive microanalysis and comparison with Th-Pb laser-ablation mass spectrometric data

A newly developed XRF-microprobe at the Institute of Mineralogy and Petrology, University of Bern, Switzerland has been applied for precise chemical Th–U–Pb dating of individual monazite grains separated from Pb-free polished petrographic thin sections. The nondestructive nature of the XRF-measurement permitted a comparative study of dating methods by sequentially applying chemical dating by electron microprobe analysis (EMPA), chemical dating by XRF-microprobe analysis, and isotopic 208Pb/232Th dating by Laser Ablation Plasma Ionisation Multi-collector Mass Spectrometry (LA-PIMMS) analysis. As an example, the 2σ precision achieved with the XRF-microprobe for well characterised reference material, monazite FC-1 (TIMS age 54.3±1 Ma; μ-XRF age 55.3±2.6 Ma), doubly polished to 30 μm in thickness, is below 5% after 90 min integration time (50 kV; 30 mA) at a spatial resolution of 90 μm. At 38-μm spatial resolution, the uncertainty is 35% for the same integration time. The sample characteristics are 200–300 ppm of Pb (μ-XRF), 3.8–5.1 wt.% of Th (EMPA), and 0.4–1.4 wt.% U (EMPA). Combined with an electron microprobe and conventional optical microscopy, the XRF-microprobe is thus a competitive low-cost and nondestructive alternative to more costly isotopic methods. The XRF-microprobe is easy to use and maintain.

Redistribution of REE, Y, Th, and U at high pressure: Allanite-forming reactions in impure meta-quartzites (Sesia Zone, Western Italian Alps)

Abstract Accessory phases are important hosts of trace elements; allanite may contain >90% of the REE in a bulk rock. The mobility and redistribution of several trace elements, notably HREE, Th, U, and Y is thus controlled by reactions involving allanite and other REE phases, as well as several rock-forming minerals. As these elements are commonly concentrated in mature clastic sediments, a suite of impure quartzite was studied. Two eclogite facies samples from the Monometamorphic Cover Complex of the Sesia Zone (Western Italian Alps) are presented in some detail, as they reveal a remarkably rich spectrum of reaction relationships that involve REE phases. Two allanite-forming reactions were inferred from textures and phase compositions (1) monazite + Ca-silicate(?) + fluid → allanite + apatite + thorite; (2) monazite + thorite + Ca-silicate(?) + fluid → Th-rich allanite + auerlite ± apatite. Petrographic observations and thermodynamic models suggest that allanite entered the HP assemblage at ~530 °C and 17-18 kbar during prograde metamorphism. In one sample, allanite is rimmed by epidote rich in Y and HREE that grew at the expense of xenotime. Two net transfer reactions were derived (3) xenotime + allanite + fluid → Y-rich epidote + apatite + thorite; (4) xenotime + allanite + fluid → Y-rich epidote + aeschynite + thorite + (phosphate?). Textural relationships and trace element analyses of coexisting allanite/monazite and xenotime/ Y-rich epidote reveal systematic partitioning of the REE. Partition coefficients for the HREE are compatible with equilibrium fractionation, whereas those for the LREE show patterns that seem to be inherited from the precursor phases, in this case zircon with variable LREE composition.