Alfons Berger

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.

Observations from the floor of a granitoid pluton: Inferences on the driving force of final emplacement

An east-west profile across the tilted Bergell pluton exposes a 10-km-thick interval in terms of crustal depth. Consequently, the floor as well as the root and “side” of the main intrusive body of the pluton crop out at the surface and a tentative three-dimensional geometry is constructed. At the highest crustal level, the geometry and deformation features at the margin of the pluton indicate ballooning, whereas the folded floor of the main intrusive body indicates synmagmatic shortening related to regional deformation. These contrasting features are best explained by shortening of the base of the pluton which caused an expansion at a higher crustal level. Final emplacement of the pluton into higher crustal levels was, therefore, not driven primarily by buoyancy, but rather by regional deformation within deeper levels of the crust.

DEFORMATION MECHANISMS AND REACTION OF HORNBLENDE : EXAMPLES FROM THE BERGELL TONALITE (CENTRAL ALPS)

Abstract Hornblendes in the Bergell tonalite begin to crystallize relatively early during the crystallization history of the pluton and become resorbed during the late stages of crystallization. During the crystallization of the magma deformation commences by magmatic flow. In this stage the hornblendes behave as rigid particles in a viscous matrix (melt). The rotation and alignment of hornblende as elongate, rigid particles have produced a strong preferred orientation of both, particle long axes and crystallographic directions. With progressive solidification of the melt, there is a gradual transition from magmatic flow to solid-state deformation in the Bergell tonalite. The crystallographic and shape preferred orientation that have developed during solid-state deformation are identical to those of magmatic flow. This identical fabric development during solid-state and magmatic flow deformation can be explained by the same deformation mechanism that has prevailed in hornblends during both deformation periods, i.e., hornblendes have always acted as rigid particles in a viscous matrix. TEM observations show no evidence for intracrystalline plasticity in hornblende grains. The modal abundance of hornblende decreases progressively with increasing solid-state deformation to yield more biotite, epidote and quartz. Small hornblende matrix grains form by cataclastic processes. The observed changes in grain size, shape and abundance of hornblende occur mainly by fracturing, dissolution and reaction, so that the solid-state deformation of the tonalite is a complex process, to which fracturing, dissolution of hornblende and metamorphic reactions all contribute. The relationship between deformation and reaction represents an example of incongruous pressure solution or diffusive mass transfer involving reaction. It is inferred that generally, hornblende does not appear to deform significantly by intracrystalline plasticity at temperatures below 650–750°C in the presence of an aqueous fluid. Crystal plasticity could become dominant at higher temperatures and/or lower aqueous fluid activities.

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.

Subduction-related metamorphism in the Alps: review of isotopic ages based on petrology and their geodynamic consequences

Abstract We summarize ages of the high‐pressure/low‐temperature (HP/LT) metamorphic evolution of the central and the western Alps. The individual isotopic mineral ages are interpreted to represent either: (1) early growth of metamorphic minerals on the prograde path; (2) timing close to peak metamorphism; or (3) retrograde resetting of the chronometers at still‐elevated pressures. Therefore, each individual age cannot easily be transferred to a geodynamic setting at a certain time. These different data indicate a subduction‐related metamorphism between 62 and 35 Ma in different units (e.g. Voltri Massif, Schistes Lustrés of the western Alps, Tauern Window). Oceanic and continental basement units show isotope ages related to eclogitic or blueschist facies metamorphism between 75 and 40 Ma. Most of these ages may record equilibration along the retrograde path, except of some Lu/Hf garnet ages and some zircon SHRIMP ages, which provide information on the prograde path. These different isotope ages are interpreted as different steps along pressure–time paths and so may provide some information on the geodynamic evolution. The data record a continuous subduction, which is ongoing for several tens of millions years. In a large‐scale picture, we have to assume fragmentation of the downgoing plate in order to explain the available P–T and t data. This interpretation questions the ongoing driving force for subduction during the disappearance of the Alpine Tethys.

Grain coarsening maps: A new tool to predict microfabric evolution of polymineralic rocks

Polymineralic rocks undergo grain coarsening with increasing temperature in both static and deformational environments, as long as no mineral reactions occur. The grain coarsening in such rocks is complex because the different phases influence each other, and it is this interaction that controls the rate of grain coarsening of the entire aggregate. We present a mathematical approach to investigate coupled grain coarsening using a set of microstructural parameters, including grain size and volume fraction of both second phases and matrix mineral in combination with temperature information. Based on samples from polymineralic carbonate mylonites that were deformed at different temperatures, we demonstrate how the mathematical relation can be calibrated for this natural system. Using such data sets for other lithologies, grain coarsening maps can be generated, which allow the prediction of microstructural evolution in polymineralic rocks. Such predictions are crucial for all subdisciplines in the earth sciences that require fundamental knowledge about microstructural changes and rheology of an orogen at different depths, such as structural geology, geophysics, geodynamics, and metamorphic petrology.

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.

Growth related zonations in authigenic and hydrothermal quartz characterized by SIMS-,EPMA-,SEM-CL- and SEM-CC imaging

Abstract Authigenic quartz overgrowths and hydrothermal quartz crystals from locations in Oman and Switzerland have been investigated with SIMS, EPMA, SEM-CL and SEM-CC. All techniques reveal similar zonation patterns with SEM-CL having the best resolution followed by SEM-CC, EPMA and finally SIMS. The observed zonations reflect chemical and/or physical changes during growth in the precipitation environment or disequilibrium precipitation at the crystal surface (i.e. sectoral and intrasectoral zonation). Based on the total Al content, two types of authigenic quartz are distinguishable. When the Al concentration is <500 μg g-1, the predominant CL emission is at ~630 nm; in such quartzes, SEM-CL and SEM-CC are directly correlated, and signal intensities drop as a function of increasing Al concentration. In contrast, authigenic quartz with Al concentrations between 500 μg g-1 and 1000 μg g-1 has CL emission maxima at both ~630 nm and ~380-400 nm, at which point the panchromatic SEM-CL and SEM-CC intensities become decoupled.

Syntectonic melt pathways in granitic gneisses, and melt-induced transitions in deformation mechanisms

Abstract Partial melting of granodioritic gneisses in the contact aureole of the Bergell Pluton (Central Alps) occurred during regional deformation. Melting occurred in the presence of water, which was released from the pluton. The presence of melt is evidenced by local segregations of granite in shear zones, veins and dykes, and by grain-scale interstitial films of K-feldspar and quartz that do not occur in the unmelted protolith. These films are oriented parallel, as well as perpendicular to the foliation plane. In contrast, on the outcrop scale, cm-wide leucosome veins are oriented almost exclusively parallel to the foliation plane, indicating foliation-parallel flow. The partially molten granitic rocks contain dm-long clasts of restitic, well-foliated gneiss, showing a higher competence than their granitic matrix. K-feldspar is lacking in these clasts, the microstructure of which is characterized by elongate aggregates of quartz and feldspar, both dynamically recrystallized. In contrast, the granitic matrix is characterised by a random distribution of minerals, whith a shape preferred orientation defining a weak foliation. These microstructures are indicative of granular flow, whereas the microstructures of the clasts indicate dislocation creep involving dynamic recrystallization. The presence of K-feldspar controls the onset of melting and thus the transition from dislocation creep to granular flow. The weakening resulting from this transition is indicated by the formation of strong clasts in a weaker matrix.