Description: The age of spreading of the Liguro–Provençal Basin is still poorly constrained due to the lack of boreholes penetrating the whole sedimentary sequence above the oceanic crust and the lack of a clear magnetic anomaly pattern. In the past, a consensus developed over a fast (20.5–19 Ma) spreading event, relying on old paleomagnetic data from Oligo–Miocene Sardinian volcanics showing a drift-related 30° counterclockwise (CCW) rotation. Here we report new paleomagnetic data from a 10-mthick lower–middle Miocene marine sedimentary sequence from southwestern Sardinia. Ar/Ar dating of two volcanoclastic levels in the lower part of the sequence yields ages of 18.94±0.13 and 19.20±0.12 Ma (lower–mid Burdigalian). Sedimentary strata below the upper volcanic level document a 23.3±4.6° CCW rotation with respect to Europe, while younger strata rapidly evolve to null rotation values. A recent magnetic overprint can be excluded by several lines of evidence, particularly by the significant difference between the in situ paleomagnetic and geocentric axial dipole (GAD) field directions. In both the rotated and unrotated part of the section, only normal polarity directions were obtained. As the global magnetic polarity time scale (MPTS) documents several geomagnetic reversals in the Burdigalian, a continuous sedimentary record would imply that (unrealistically) the whole documented rotation occurred in few thousands years only. We conclude that the section contains one (or more) hiatus(es), and that the minimum age of the unrotated sediments above the volcanic levels is unconstrained. Typical back-arc basin spreading rates translate to a duration ≥3 Ma for the opening of the Liguro–Provençal Basin. Thus, spreading and rotation of Corsica–Sardinia ended no earlier than 16 Ma (early Langhian). A 16–19 Ma, spreading is corroborated by other evidences, such as the age of the breakup unconformity in Sardinia, the age of igneous rocks dredged west of Corsica, the heat flow in the Liguro–Provençal Basin, and recent paleomagnetic data from Sardinian sediments and volcanics. Since Corsica was still rotating/drifting eastward at 16 Ma, it presumably induced significant shortening to the east, in the Apennine belt. Therefore, the lower Miocene extensional basins in the northern Tyrrhenian Sea and margins can be interpreted as synorogenic "intra-wedge" basins due to the thickening and collapse of the northern Apennine wedge.

Description: Published

Description: 231-251

Description: 2.2. Laboratorio di paleomagnetismo

Description: JCR Journal

Description: reserved

Description: The effects of metamorphic reactions occurring during decompression were explored to understand their influence on the 40 Ar– 39 Ar ages of micas. Monometamorphic metasediments from the Lepontine Alps (Switzerland) reached lower amphibolite facies during the Barrovian metamorphism related to the collision between European and African (Adria) continental plates. Mineral assemblages typically composed of garnet, plagioclase, biotite, muscovite and paragonite (or margarite) were screened for petrological equilibrium, to focus on samples that record a minimum degree of retrogression. X-ray diffraction data indicate that some mineral separates prepared for 40 Ar– 39 Ar stepwise heating analysis are monomineralic, whereas others are composed of two white micas (muscovite with paragonite or margarite), or biotite and chlorite. In monomineralic samples 37 Ar/ 39 Ar and 38 Ar/ 39 Ar (proportional to Ca/K and Cl/K ratios) did not change and the resulting ages can be interpreted unambiguously. In mineral separates containing two white micas, Ca/K and Cl/K ratios were variable, reflecting non-simultaneous laboratory degassing of the two heterochemical Ar reservoirs. These ratios were used to identify each Ar reservoir and to unravel the age. In a chlorite–margarite–biotite calcschist equilibrated near 560°C and 0·65 GPa, biotite, margarite, and muscovite all yield ages around 18 Ma. At slightly higher grade (560–580°C, 0·8–0·9 GPa), the assemblage muscovite–paragonite–plagioclase is in equilibrium and remains stable during retrogression. In this case, muscovite and paragonite yield indistinguishable ages around 16·5 Ma. Above 590°C, paragonite was mostly consumed to form plagioclase 〉590°C, whereby the relict mica yields an age up to 5·6 Ma younger than muscovite. This partial or total resetting of the Ar clock in paragonite is interpreted to reflect plagioclase growth during decompression. Where biotite is present within this same assemblage, it systematically yields a younger age than muscovite, by 0·5–2 Ma. However, these biotites all show small amounts of retrograde chlorite formation. We conclude that even very minor chloritization of biotite is apparently a more effective process than temperature in resetting the Ar clock, as is the formation of plagioclase from paragonite decomposition. Multi-equilibrium thermobarometry is an excellent means to ensure that equilibrium in investigated samples is preserved, and this helps to obtain geologically meaningful metamorphic ages. However, even samples passing such equilibrium tests may still show retrograde effects that affect the Ar retention of micas. A more robust interpretation of such 40 Ar– 39 Ar results may require use of a second geochronometer, such as U–Pb on monazite.

Description: The Yanque nonsulfide Zn-Pb deposit (26,491 kilotonnes of indicated resources at 2.37% Zn and 2.18% Pb [1.67% ZnEq cutoff]; 8,701 kilotonnes of indicated resources at 4.05% Zn and 3.32% Pb [4.00% ZnEq cutoff]) is located in the Andahuaylas-Yauri metallogenic province, southern Peru. The deposit occurs in a base metal district, with the Dolores porphyry copper deposit at its center. Sedimentary and igneous rocks predominate in the Yanque-Dolores area. The sedientary rocks belong to the Soraya, Mara, and Ferrobamba formations (Upper Jurassic-Upper Cretaceous). Igneous rocks are mainly intrusive products associated with the Andahuaylas-Yauri batholith (diorite/tonalite, tonalite porphyry, and quartz diorite/monzonite porphyry). The Yanque deposit is hosted by two different sedimentary facies consisting ofa siliciclastic conglomerate and a sedimentary dolomite breccia at the transition between the Mara and Ferrobamba formations. Hypogene mineralization consisted of galena, pyrite, and sphalerite, associated with a strong muscovite/illite hydrothermal alteration of the host rock (similar to alteration observed in porphyry copper deposits) and minor kaolinite, dolomite, and quartz. Minor element geochemistry, characterized by Sb, As, Mn, Ag, and Cu, suggests a magmatic-hydrothermal source for primary mineralization at Yanque. The Pb isotope compositions of Yanque and Dolores sulfides (pyrite and galena) are similar: 206 Pb/ 204 Pb ratios are between 18.545 ± 0.003 and 18.656 ± 0.002, 207 Pb/ 204 Pb ratios between 15.632 ± 0.003 and 15.653 ± 0.006, and 208 Pb/ 204 Pb ratios between 38.531 ± 0.009 and 38.649 ± 0.007. This indicates that the Yanque primary Zn-Pb mineralization likely formed from the same hydrothermal system that produced the Dolores porphyry copper deposit. The Pb isotope values of sulfides in both deposits are typical of the Tertiary magmatically derived ores in this part of Peru. Economic mineralization at Yanque consists of oxidized minerals resulting fromweathering and alteration of hypogene sulfide mineralization. Sauconite (Zn smectite clay) and cerussite (Pb carbonate) with minor hemimorphite (Zn hydrosilicate) and smithsonite (Zn carbonate) predominate. Sauconite formed as replacements of detrital feldspar and hydrothermal muscovite/illite and kaolinite. In contrast to carbonate-hosted nonsulfide Zn-Pb deposits, sauconite at Yanque occurs in comparable amounts as in the world-class Skorpion deposit (Namibia). Cerussite replaced galena and precipitated as cement in the host rock. Hemimorphite and smithsonite mainly crystallized in late-forming veins. Unlike most supergene Zn-Pb deposits, the Yanque orebody shows comparable ore rades for both Pb and Zn with an inverse supergene chemical zoning. Zinc is most abundant at the top of the deposit, whereas lead increases with depth. These two elements have different mobility in solution, which implies that the primary source of zinc in the Yanque nonsulfide mineralization was originally located far from the primary galena mineralization, the remnants of which are present in situ. Supergene zinc mineralization was derived from the dissolution of primary sphalerite hosted in the now-eroded rocks above the Yanque mine site. Zinc-rich solution migrated through the sedimentary host rocks and precipitated Zn minerals, mainly sauconite, in pore spaces and replaced detrital and hydrothermal alteration minerals.

Description: Multichronometric analyses were performed on samples from a transect in the French–Italian Western Alps crossing nappes derived from the Briançonnais terrane and the Piemonte–Liguria Ocean, in an endeavour to date both high-pressure (HP) metamorphism and retrogression history. Twelve samples of white mica were analysed by 39 Ar– 40 Ar stepwise heating, complemented by two samples from the Monte Rosa nappe 100 km to the NE and also attributed to the Briançonnais terrane. One Sm–Nd and three Lu–Hf garnet ages from eclogites were also obtained. White mica ages decrease from c. 300 Ma in the westernmost samples (Zone Houillère), reaching c. 300°C during Alpine metamorphism, to 〈48 Ma in the internal units to the east, which reached c. 500°C during the Alpine orogeny. The spatial pattern of Eocene K–Ar ages demonstrates that Si-rich HP white mica records the age of crystallization at 47–48 Ma and retains Ar at temperatures of around 500°C. Paleocene–early Eocene Lu–Hf and Sm–Nd ages, recording prograde garnet growth before the HP peak, confirm eclogitization in Eocene times. Petrological and microstructural features reveal important mineralogical differences along the transect. All samples contain mixtures of detrital, syn-D 1 and syn-D 2 mica, and retrogression phases (D 3 ) in greatly varying proportions according to local variations in the evolution of pressure–temperature–fluid activity–deformation ( P–T–a–D ) conditions. Samples from the Zone Houillère mostly contain detrital mica. The abundance of white mica with Si 〉 6·45 atoms per formula unit increases eastward. Across the whole traverse, phengitic mica grown during HP metamorphism defines the D 1 foliation. Syn-D 2 mica is more Si-poor and associated with nappe stacking, exhumation, and hydrous retrogression under greenschist-facies conditions. Syn-D 1 phengite is very often corroded, overgrown by, or intergrown with, syn-D 2 muscovite. Most importantly, syn-D 2 recrystallization is not limited to S 2 schistosity domains; micrometre-scale chemical fingerprinting reveals muscovite pseudomorphs after phengite crystals, which could be mistaken for syn-D 1 mica based on microstructural arguments alone. The Cl/K ratio in white mica is a useful discriminator, as D 2 retrogression was associated with a less saline fluid than eclogitization. As petrology exerts the main control on the isotope record, constraining the petrological and microstructural framework is necessary to correctly interpret the geochronological data, described in both the present study and the literature. Our approach, which ties geochronology to detailed geochemical, petrological and microstructural investigations, identifies 47–48 Ma as the age of HP formation of syn-D 1 mica along the studied transect and in the Monte Rosa area. Cretaceous apparent mica ages, which were proposed to date eclogitization by earlier studies based on conventional ‘thermochronology’, are due to Ar inheritance in incompletely recrystallized detrital mica grains. The inferred age of the probably locally diachronous, greenschist-facies, low-Si, syn-D 2 mica ranges from 39 to 43 Ma. Coexistence of D 1 and D 2 ages, and the constancy of non-reset D 1 ages along the entire transect, provides strong evidence that the D 1 white mica ages closely approximate formation ages. Volume diffusion of Ar in white mica (activation energy E = 250 kJ mol –1 ; pressure-adjusted diffusion coefficient D' 0 〈 0·03 cm 2 s –1 ) has a subordinate effect on mineral ages compared with both prograde and retrograde recrystallization in most samples.