Structural characterization and K–Ar illite dating of reactivated, complex and heterogeneous fault zones: lessons from the Zuccale Fault, Northern Apennines

We studied the Zuccale Fault (ZF) on Elba, part of the Northern Apennines, to unravel the complex deformation history that is responsible for the remarkable architectural complexity of the fault. The ZF is characterized by a patchwork of at least six distinct, now tightly juxtaposed brittle structural facies (BSF), i.e. volumes of deformed rock characterized by a given fault rock type, texture, colour, composition, and age of formation. ZF fault rocks vary from massive cataclasite to foliated ultracataclasite, from clay-rich gouge to highly sheared talc phyllonite. Understanding the current spatial juxtaposition of these BSFs requires tight constraints on their age of formation during the ZF lifespan to integrate current fault geometries and characteristics over the time dimension of faulting. We present new K–Ar gouge dates obtained from three samples from two different BSFs. Two top-to-the-east foliated gouge and talc phyllonite samples document faulting in the Aquitanian (ca. 22 Ma), constraining east-vergent shearing along the ZF already in the earliest Miocene. A third sample constrains later faulting along the exclusively brittle, flat-lying principal slip surface t

A New Albite Microanalytical Reference Material from Piz Beverin for Na, Al and Si Determination, and the Potential for New K-Feldspar Reference Materials

Determination of alkali elements is important to Earth scientists, yet suitable and reliable microanalytical reference materials are lacking. This paper proposes a new albite reference material and evaluates the potential for future K-feldspar reference materials. The proposed Piz Beverin albite reference material from Switzerland yields a homogeneous composition at the centimetre- to micrometre-scale for Si, Al and Na with \u3c 2000 μg g-1 total trace elements (mostly heterogeneously distributed Ca, K and Sr). EPMA and LA-ICP-MS measurements confirm a composition of 99.5(2)% albite component, which is supported further by bulk XRF measurements. A round robin evaluation involving nine independent EPMA laboratories confirms its composition and homogeneity for Si, Al and Na. In addition, a set of five distinct clear K-feldspar samples was evaluated as possible reference materials. The first two crystals of adular and orthoclase yield unacceptable inhomogeneities with \u3e 2% relative local variations of Na, K and Ba contents. The three other investigated sets of K-feldspar crystals are yellow sanidine crystals from Itrongay (Madagascar). Despite distinct compositions, EPMA confirms they are each homogeneous at the centimetre to micrometre scale for Si, Al and K and have no apparent inclusions; further investigation to find larger amounts of these materials is therefore justified

Construction and evolution of a granitic complex in the upper crust: geometrical and geochemical constraints on the emplacement of the Valle Mosso pluton (Ivrea Zone, Southern Alps, Italy)

The focus of this study is on the magmatic evolution of the Valle Mosso pluton, a granitic body intruded at intermediate to upper crustal levels into the rocks of the Alpine basement of the Ivrea-Verbano Zone and Serie dei Laghi, shortly after the end of Variscan orogeny. The Valle Mosso pluton has been recognized as an integral part of a magmatic system (Quick et al., 2009), which during the lower Permian developed through the continental crust up to the surface and caused explosive rhyolitic volcanism that eventually led to the formation of a >15 km diameter caldera. This granitic body was tilted as a consequence of Alpine orogeny and thus exposes both its deepest and shallower portions. This peculiar feature, shared worldwide by just a few other similar intrusions, if combined together with its characteristic geological framework makes the Valle Mosso pluton a rather unique case study. In fact, the opportunity to study a continuous vertical section of a granitic intrusion genetically linked to coeval volcanic deposits has substantial implications, both in the understanding of the mechanisms that rule granitic bodies’ emplacement in the upper crust and in the comprehension of the volcanic-plutonic connection. By means of detailed geological mapping, the internal structure of the pluton and the relationship with its host rocks have been determined. The pluton has been divided into six granitic petrofacies, based on textural criteria derived from up-to-date scientific literature on the mapping of granite terranes. The combination of field data with thin section examination have led to the compilation of a geological map of the Pluton at 1:15.000 scale, which is equipped with cross section and schemes that help the reader focus on the geometry and internal variability of the intrusive body. Laboratory analyses have been carried out alongside fieldwork, with the aim of defining the intrusion’s geochemical features, which are essential to the compilation of a petrogenetic model that could attest for the heterogeneity of the granitic body. Whole-rock analyses of over 100 granitic rock samples have been performed trough X-ray fluorescence (XRF) to determine major-element compositions, at least 25 selected samples have been analysed by means of ICPMS for their trace element contents and 5 more underwent mass spectrometry (TIMS) to determine their Sr isotopic ratio. In addition to whole-rock analyses, selected minerals (Qz, Plg, Ho, Bt) belonging to the deepest facies of the Valle Mosso pluton have been analysed by means of microprobe. These data obtained through ion microprobe analyses have been used to perform thermometric and barometric calculations, which purpose was to set constraints on the intrusion crystallization conditions. Starting from the data collected in both field and laboratory, an incremental growth of the Valle Mosso pluton is hypothesized through the emplacement of several sill-like (high aspect-ratio) granitic bodies inside the upper crust; they partially interacted and gradually evolved by means of fractional crystallization processes. Based on field evidence, an intrusion emplacement mechanism that considers both floor subsiding and roof lifting (together with the detachment of roof pendants) is favoured for the composite Valle Mosso pluton. In addition to discussing the processes that led to the generation and emplacement of the granitic magma, based on the collected data, there is also room to speculate on a possible plutonic record of rejuvenation of largely solidified magma chamber preserved inside the Valle Mosso intrusion. In fact, a particular porphyry granite found inside the pluton discloses mineral phenocrysts (Qz, Plg, Kf) that show clear petrographic and geochemical evidence for thermal and compositional disequilibrium, which are similarly found in crystals belonging to the coeval volcanic deposits of the Permian caldera. The collected data seem to indicate a possible link between reheating evidence in granitic rocks and volcanic events

SHORT MOBILIZATION AND ERUPTION TIMESCALES RECORDED IN THE QUARTZ CRYSTALS OF A FOSSIL CALDERA PLUMBING SYSTEM, SESIA MAGMATIC SYSTEM, SOUTHERN ALPS

Granitic intrusions are not considered the ideal target for the study of short-lived, transient processes associated with remobilization and eruption of highly crystalline silicic magmas. However, crystal zoning preserved in phenocrysts from fossil upper crustal crystal mushes can retain information on the timescales and reactivation dynamics of silicic magma chambers. In the Southern Alps, the plumbing system of a Permian rhyolitic caldera is exposed to a depth of about 25 Km in tilted crustal blocks. The mid- to upper-crustal segment of this magmatic system (a.k.a. Sesia Magmatic System) is represented by a monzogranitic intrusion ( 4867 to 77 wt% SiO ), the Valle Mosso pluton (VMP), which intrudes cogenetic rhyolitic products of the >15 km diameter Sesia caldera. Field and petrographic evidence suggest that a significant portion of the VMP (ca.15% of the intrusion volume) underwent one or more rejuvenation and mobilization episodes. Titanium (Ti) in quartz content in grains from granitic and volcanic units of the Sesia Magmatic System has been investigated through cathodoluminescence (CL) imaging and microprobe (EPMA) analyses. Sharp contrast in concentration between Ti-poor cores and Ti-rich rims is observed in most of the granitic and volcanic quartz grains. Application of TitaniQ thermometer indicates sharp temperature increase across core-rim boundaries (\u394T of min 50 \ub0C to max 100 \ub0C) assuming uniform a TiO2 and pressure at the time of crystallization. Furthermore, one-dimensional modeling of Ti diffusion core-rime interfaces indicate short elapsed time (10s of years) between crystallization of the high-T rim and cooling of the system below magmatic temperatures, with striking similar results obtained for quartz grains from rejuvenated portion of VMP and volcanic products. These results suggest that a short-lived thermal flare-up, possibly related to mixing with a batch of hotter, more mafic magma, interested the upper portion of the Sesia Magmatic System during its upper crustal residence as a crystal mush, triggering remobilization and eruption of portions of the magma chamber. Such short timescales, typical of explosive eruptive processes, have never been identified before in fossil magma chambers, making this discovery relevant in the framework of the ongoing volcano-plutonic connection debate

Structural characterization and K-Ar illite dating of reactivated, complex and heterogeneous fault zones: lessons from the Zuccale Fault, Northern Apennines

We studied the Zuccale Fault (ZF) on Elba, part of the Northern Apennines, to unravel the complex deformation history that is responsible for the remarkable architectural complexity of the fault. The ZF is characterized by a patchwork of at least six distinct, now tightly juxtaposed brittle structural facies (BSF), i.e. volumes of deformed rock characterized by a given fault rock type, texture, colour, composition, and age of formation. ZF fault rocks vary from massive cataclasite to foliated ultracataclasite, from clay-rich gouge to highly sheared talc phyllonite. Understanding the current spatial juxtaposition of these BSFs requires tight constraints on their age of formation during the ZF lifespan to integrate current fault geometries and characteristics over the time dimension of faulting. We present new K-Ar gouge dates obtained from three samples from two different BSFs. Two top-to-the-east foliated gouge and talc phyllonite samples document faulting in the Aquitanian (ca. 22 Ma), constraining east-vergent shearing along the ZF already in the earliest Miocene. A third sample constrains later faulting along the exclusively brittle, flat-lying principal slip surface to < ca. 5 Ma. The new structural and geochronological results reveal an unexpectedly long faulting history spanning a ca. 20 Myr time interval in the framework of the evolution of the Northern Apennines. The current fault architecture is highly heterogeneous as it formed at very different times under different conditions during this prolonged history. We propose that the ZF started as an Aquitanian thrust that then became selectively reactivated by early Pliocene out-of-sequence thrusting during the progressive structuring of the Northern Apennine wedge. These results require the critical analysis of existing geodynamic models and call for alternative scenarios of continuous convergence between the late Oligocene and the early Pliocene with a major intervening phase of extension in the middle Miocene allowing for the isostatic re-equilibration of the Northern Apennine wedge. Extension started again in the Pliocene and is still active in the innermost portion of the Northern Apennines. In general terms, long-lived, mature faults can be very architecturally complex. Their unravelling, including understanding the dynamic evolution of their mechanical properties, requires a multidisciplinary approach combining detailed structural analyses with dating the deformation events recorded by the complex internal architecture, which is a phenomenal archive of faulting and faulting conditions through time and space.ISSN:1869-9510ISSN:1869-952

Modeling Zircon growth during open-system crystallization

International audienceRecent advances in zircon geochronology by isotope dilution thermal-ionization mass spectrometry (ID-TIMS) techniques allow for age precision beyond the 0.02 % level for single zircon 206Pb/238U dates [1] and can resolve zircon crystallization heterogeneities in a dispersed age population. As zircon saturation is dependent on melt temperature and composition, quantitative analysis of dispersed zircon age populations can provide insight into the timing and magnitude of intensive parameter variations during the lifetime of magma reservoirs

AgeSpectraAnalyst: A MATLAB based package to model zircon age distributions in silicic magmatic systems

International audienceIn the last decade, improvements in the analytical precision achievable by zircon U-Pb geochronological techniques have allowed to resolve complexities of zircon crystallization histories in magmatic rocks to an unprecedented level. A number of studies have strived to link resolvable dispersion in zircon age spectra of samples from fossil magmatic systems to the physical parameters of their parent magma bodies. However, the methodologies developed have so far been limited to reproduce the effect of simple thermal histories on the final distribution of zircon ages. In this work we take a more nuanced approach, fine-tuning a thermodynamics-based zircon saturation model to predict the relative distribution of zircon ages in samples from silicic magma reservoirs experiencing open-system processes (e.g. heat/mass addition, mechanical mixing). Employing the MATLAB package (AgeSpectraAnalyst) presented in this contribution: • Users can forward model the effect that diverse thermal histories and mechanical mixing processes characteristic of silicic magma bodies have on zircon age distributions as measured by high-precision, chemical abrasion thermal ionization mass spectrometry (CA-ID-TIMS) U-Pb geochronology. • Zircon CA-ID-TIMS datasets from silicic magmatic systems can be easily compared with model output to gain semi-quantitative information on thermo-mechanical history of the system of interest. • We demonstrated (Tavazzani et al., in press) that distribution of high-precision zircon ages in crystallized remnants of shallow (∼ 250 MPa), silicic magma reservoirs can discriminate between systems that experienced catastrophic, caldera-forming eruptions and systems that underwent monotonic cooling histories

AgeSpectraAnalyst: A MATLAB based package to model zircon age distributions in silicic magmatic systems

In the last decade, improvements in the analytical precision achievable by zircon U-Pb geochronological techniques have allowed to resolve complexities of zircon crystallization histories in magmatic rocks to an unprecedented level. A number of studies have strived to link resolvable dispersion in zircon age spectra of samples from fossil magmatic systems to the physical parameters of their parent magma bodies. However, the methodologies developed have so far been limited to reproduce the effect of simple thermal histories on the final distribution of zircon ages. In this work we take a more nuanced approach, fine-tuning a thermodynamics-based zircon saturation model to predict the relative distribution of zircon ages in samples from silicic magma reservoirs experiencing open-system processes (e.g. heat/mass addition, mechanical mixing). Employing the MATLAB package (AgeSpectraAnalyst) presented in this contribution: • Users can forward model the effect that diverse thermal histories and mechanical mixing processes characteristic of silicic magma bodies have on zircon age distributions as measured by high-precision, chemical abrasion thermal ionization mass spectrometry (CA-ID-TIMS) U-Pb geochronology. • Zircon CA-ID-TIMS datasets from silicic magmatic systems can be easily compared with model output to gain semi-quantitative information on thermo-mechanical history of the system of interest. • We demonstrated (Tavazzani et al., in press) that distribution of high-precision zircon ages in crystallized remnants of shallow (∼ 250 MPa), silicic magma reservoirs can discriminate between systems that experienced catastrophic, caldera-forming eruptions and systems that underwent monotonic cooling histories.ISSN:2215-016