Abstract

The reconstruction of the polymetamorphic history of basement rocks in orogens is crucial for deciphering past geodynamic evolution. However, the current petrographic features are usually interpreted as the results of the metamorphic recrystallization of primary sedimentary and/or magmatic features. In contrast, metamorphic rocks derived by protoliths affected by pre-metamorphic hydrothermal alterations are rarely recognized. This work reports textural, mineralogical and geochemical data of metasedimentary and metaigneous rocks from the Paleozoic succession of the Sant’Anna tectonic window (Alpi Apuane, Tuscany, Italy). These rocks were recrystallized and reworked during the Alpine tectono-metamorphic event, but the bulk composition and some refractory minerals (e.g., tourmaline) are largely preserved. Our data show that the Paleozoic rocks from the Alpi Apuane were locally altered by hydrothermal fluids prior to Alpine metamorphism, and that the Permian magmatic cycle was likely responsible for this hydrothermal alteration. Finally, the Ishikawa Alteration Index, initially developed for magmatic rocks, was applied to metasedimentary rocks, providing a useful geochemical tool for unravelling the hydrothermal history of Paleozoic rocks, as well as a potential guide to the localization of hidden ore deposits in metamorphic terranes.

Highlights

  • Polymetamorphic basement complexes may show a large variety of lithotypes that are usually interpreted as primary variations in sedimentary inputs and/or in the composition of magmas that fed igneous bodies, as well as metasomatic changes experienced during metamorphism/subduction (e.g., [1,2])

  • The age of the hydrothermal event is bounded between the age of the younger magmatic rocks and the occurrence of tourmalinite clasts in the upper Carnian-lower Norian metasedimentary rocks of the Alpi Apuane

  • The spatial distribution and geochemical features of hydrothermally altered rocks point to a genetic link with the ore bodies occurring in the Sant’Anna tectonic window, whose Permian age is hypothesized

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Summary

Introduction

Polymetamorphic basement complexes may show a large variety of lithotypes that are usually interpreted as primary variations in sedimentary inputs and/or in the composition of magmas that fed igneous bodies, as well as metasomatic changes experienced during metamorphism/subduction (e.g., [1,2]). The identification of hydrothermal alterations predating metamorphism is relevant for the reconstruction of old magmatic-hydrothermal systems and the correct interpretation of past geodynamic settings (e.g., [5]). They can be used as potential guides to the localization of hidden ore deposits. Hydrothermal alteration (triggered or not by magmatism) can produce very peculiar mineralogical assemblages and geochemical fingerprints, significantly deviating from the usual sedimentary and/or magmatic realms. These features can still be recognized after metamorphic recrystallization.

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