Abstract
Three samples of meta-acidic rocks with pre-Alpine metamorphic relicts from the Sesia-Lanzo Zone eclogitic continental crust were investigated using stepwise controlled elemental maps by means of the Quantitative X-ray Maps Analyzer (Q-XRMA). Samples were chosen with the aim of analysing the reacting zones along the boundaries between the pre-Alpine and Alpine mineral phases, which developed in low chemically reactive systems. The quantitative data treatment of the X-ray images was based on a former multivariate statistical analytical stage followed by a sequential phase and sub-phase classification and permitted to isolate and to quantitatively investigate the local paragenetic equilibria. The parageneses thus observed were interpreted as related to the pre-Alpine metamorphic or magmatic stages as well as to local Alpine re-equilibrations. On the basis of electron microprobe analysis, specific compositional ranges were defined in micro-domains of the relict and new paragenetic equilibria. In this way calibrated compositional maps were obtained and used to contour different types of reacting boundaries between adjacent solid solution phases. The pre-Alpine and Alpine mineral parageneses thus obtained allowed to perform geothermobarometry on a statistically meaningful and reliable dataset. In general, metamorphic temperatures cluster at 600–700 °C and 450–550 °C, with lower temperatures referred to a retrograde metamorphic re-equilibration. In all the cases described, pre-Alpine parageneses were overprinted by an Alpine metamorphic mineral assemblage. Pressure-temperature estimates of the Alpine stage averagely range between 420 to 550 °C and 12 to 16.5 kbar. The PT constraints permitted to better define the pre-Alpine metamorphic scenario of the western Austroalpine sectors, as well as to better understand the influence of the pre-Alpine metamorphic inheritance on the forthcoming Alpine tectonic evolution.
Highlights
The recognition of metamorphic relicts followed by the identification of the thermodynamic equilibria affecting basements rocks is of paramount importance to quantitatively constrain the tectonics and geodynamics of continental crust at extensional as at convergent margins (e.g., [1,2,3,4,5])
Quantitative X-ray Maps Analyzer (Q-XRMA) is used to classify rock-forming minerals, starting from an initial stage that uses as input of the process an array of low-resolution major elements X-ray maps (i.e., Al, Ca, Fe, K, Mg, Mn, Na, Si, Ti) at thin section scale, to pass successively at a high-resolution array of wavelength-dispersive spectroscopy (WDS) X-ray maps at microstructural domain scale, acquired with a higher dwell time ranging from 130 ms to 160 ms at a resolution ranging from 450 × 350 to 770 × 600 for pixel sizes spanning from 5 to 2 μm
In the following parts of the work, we will use the obtained results to identify and chemically differentiate obtained local parageneses. These parageneses are often preserved in very limited volumes and are interpreted as representative of the pre-Alpine metamorphic relicts as well as of the reaction products developed during the high pressure Alpine metamorphic stage
Summary
The recognition of metamorphic relicts followed by the identification of the thermodynamic equilibria affecting basements rocks is of paramount importance to quantitatively constrain the tectonics and geodynamics of continental crust at extensional as at convergent margins (e.g., [1,2,3,4,5]). The CTM approach may prove unsuccessful where relicts are too small to be mapped This limitation, well known in most of the pre-Alpine relicts metamorphic complexes of western Alps, is normally overcome by means of a detailed local analysis that can be performed combining microstructural analysis with X-ray elemental maps, obtained by Electron Microprobe Analyzer (EMPA). The pre-Alpine evolution of the Sesia-Lanzo Zone was ascribed to the post-Variscan extension that produced, during Permian time, large high temperature/migmatitic terranes associated with magma sourced from the migmatitic crust or from continental mantle [22,23,24,25,26,27] Such lithosphere-scale extension was the precursor of the rifting and oceanization that started during the Triassic to Jurassic [26,27,28] that led to the Tethys Ocean. We present the petrological implications of the analysis (Section 5) and we discuss the results in light of the tectonic evolution of the Sesia-Lanzo Zone and the rock memory during tectonic cycles (Section 6)
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
