Francesca Tecce

Syn- versus post-orogenic extension: the case study of Giglio Island (Northern Tyrrhenian Sea, Italy)

Abstract Combined structural, petrographic and fluid inclusion data from Giglio Island (Northern Tyrrhenian Sea, Italy) are used here to constrain kinematics, timing and thermal regimes associated with the post-thickening tectonic evolution of the innermost sector of the Northern Apennine belt. Giglio Island is composed of Pliocene intrusives in tectonic contact with a poorly retrograded HP/LT, Fe-carpholite bearing nappe pile. Peak pressure estimates for the carpholite bearing Verrucano metasediments range between 10 and 14 kbar for temperatures lower than 350°C. Data from this study indicate a nearly isothermal exhumation of the Giglio HP metamorphic complex, which occurred during the Early–Middle Miocene as a consequence of a top-to-the-east syn-orogenic extensional shear. This resulted in the transposition of the HP fabric while the lithostatic pressure was decreasing, with a final re-equilibration in the greenschist facies along a cool geothermal-gradient regime. This relatively cold regime is referred to the continuous underthrusting of the cool continental Adria plate during exhumation. Afterwards, during the Late Miocene–Early Pliocene, the onset of the post-orogenic crustal thinning also showed an eastward asymmetry in an HT/LP back-arc environment, leading to partial melting of previously exhumed deep crustal levels and genesis and emplacement of the Giglio intrusives. A comparison with other peri-Tyrrhenian regions allows us to propose that the post-thickening Miocene extension in the Northern Apennines was driven by the collapse of the previously thickened Northern Apennine orogenic wedge, combined with the progressive eastward retreat of the Adria subduction boundary.

Leaching procedure used in dating young potassic volcanic rocks by the 226Ra/230Th method

A method for determining the ages of shoshonitic volcanics erupted within the last 5000 years is presented and assessed. This method is based on 226Ra/230Th radioactive disequilibrium and on a leaching procedure and γ-counting of volcanics containing Th-enriched, Rainitial-depleted mineral phases detected by microprobe analyses. K is used as a Ra-stable chemical analogue. The data are plotted on (226Ra/228Ra)− K(228Ra) Osmond-type diagrams in order to evaluate the 226Raradiogenic230Th activity ratio of the Th-enriched, Rainitial-depleted phases. Four samples of volcanics from Vulcano Island (southern Italy) gave ages consistent with historical reports andK/Ar chronology.

Fluid flow within the damage zone of the Boccheggiano extensional fault (Larderello–Travale geothermal field, central Italy): structures, alteration and implications for hydrothermal mineralization in extensional settings

The Neogene extensional province of southern Tuscany in central Italy provides an outstanding example of fossil and active structurally controlled fluid flow and epithermal ore mineralization associated with post-orogenic silicic magmatism. Characterization of the hydrodynamic regime leading to the genesis of the polysulphide deposit (known as Filone di Boccheggiano) hosted within the damage zone of the Boccheggiano Fault is a key target to assess modes of fossil hydrothermal fluid circulation in the region and, more generally, to provide inferences on fault-controlled hydrothermal fluid flow in extensional settings. We provide a detailed description of the fault zone architecture and alteration/mineralization associated with the Boccheggiano ore deposit and report the results of fluid inclusion and stable oxygen isotope studies. This investigation shows that the Boccheggiano ore consists of an adularia/illite-type epithermal deposit and that sulphide ore deposition was controlled by channelling of hydrothermal fluids of dominantly meteoric origin within the highly anisotropic permeability structure of the Boccheggiano Fault. The low permeability structure of the fault core compartmentalized the fluid outflow preventing substantial cross-fault flow, with focused fluid flow occurring at the hangingwall of the fault controlled by fracture permeability. Fluid inclusion characteristics indicate that ore minerals were deposited between 280° and 350°C in the upper levels of the brittle extending crust (lithostatic pressure in the order of 0.1 GPa). Abundant vapour-rich inclusions in ore-stage quartz are consistent with fluid immiscibility and boiling, and quartz ore vein textures suggest that mineralization in the Boccheggiano ore deposit occurred during cyclic fluid flow in a deformation regime regulated by transient and fluctuating fluid pressure conditions. Results from this study (i) predict a strongly anisotropic permeability structure of the fault damage zone during crustal extension, and (ii) indicate the rate of secondary (structural) permeability creation and maintenance by active deformation in the hangingwall of extensional faults as the major factor leading to effective hydraulic transmissivity in extensional terranes. These features intimately link ore-grade mineralization in extensional settings to telescoping of hydrothermal flow along the hangingwall block(s) of major extensional fault zones.

Microthermometric study of fluid inclusions in Neogene shallow intrusions from the Inner Carpathian arc (Romania)

Abstract Fluid inclusions have been observed in quartz phenocrystals of the most acid facies (rhyolite and dacite) of the subvolcanicarea (Rodna-Bârgu Mountains) in the East Carpathians. All identified fluid inclusions are aqueous (H 2 O NaC1 system). They occur either as primary or pseudosecondary and secondary fluid inclusions. Homogenisation, both to liquid and vapor, occurs between 120 and 540 °C. The general evolution of aqueous fluids is to decreasing salinity with decreasing temperature. Relatively low salinities (from 6 to 1.4 wt.% NaCl eq.) could indicate the presence of meteoric water. The occurrence of exploded fluid inclusions indicates strong decompression regime during magma uplift.