196,549 research outputs found
Clockwise and anticlockwise P-T path in the axial zone of the Variscan Sardinia-Corsica block
No full textRecent petrological studies on the Axial Zone of the Variscan chain have revealed the occurrence of anticlockwise P-T trajectories in metamorphic rocks from Corsica and Sardinia. These new findings, at first glance, appears to be in contrast with the clockwise P-T paths classically described for several high-pressure metamorphic rocks from the Variscan Sardinia - Corsica block in the last decades.
For the first time in Variscan Corsica, Massonne et al. (2018) described the anticlockwise P-T path of micaschists from the Porto Vecchio region. These rocks underwent prograde metamorphism starting at 3 kbar/600°C and then reached peak metamorphic conditions at 7 kbar/600-630°C. The Porto Vecchio micaschists are bordered to the north by the Solenzara-Fautea metamorphic rock unit that is, in turn, characterized by migmatites and granulites that underwent a clockwise P-T loop related to burial of continental crust to high-pressure (1.8–1.4 GPa) followed by high- to ultrahigh-temperature conditions (900–1000 °C; Giacomini et al., 2008). New, preliminary results on these rocks reveal that these latter peak temperatures could be probably significantly lower than those above mentioned, but there is no doubt about the clockwise shape of their P-T path. In the neighbouring Variscan terrains of north-eastern Sardinia, Scodina et al. (2019) reconstructed the P-T history of an amphibolite body whose burial path started at 0.8 GPa and was then followed by a slight increase in temperature up to peak metamorphic conditions of 1.3–1.4 GPa/690–740 °C. These amphibolites are hosted by migmatites for which a clockwise P-T path, related to a nearly isothermal exhumation, is widely accepted in the literature and confirmed by the most recent studies (Massonne et al., 2013 and references therein). In conclusion, it thus appears that during the Variscan collision, rock slices of different crustal levels were brought together and metamorphic rocks from upper and lower plates were involved in a particle path along an exhumation channel.
Giacomini F., Dallai L., Carminati E., Tiepolo M. & Ghezzo C. 2008. Exhumation of a Variscan orogenic complex: Insights into the composite granulitic–amphibolitic metamorphic basement of southeast Corsica (France).J. Metam. Geol., 26, 403–436.
Massonne H.-J., Cruciani G. & Franceschelli M. 2013. Geothermobarometry on anatectic melts - a high-pressure Variscan migmatite from northeast Sardinia. Int. Geol. Rev., 55, 1490–1505.
Massonne H.-J., Cruciani G., Franceschelli M. & Musumeci G. 2018. Anticlockwise pressure–temperature paths record Variscan upper-plate exhumation: example from micaschists of the Porto Vecchio region, Corsica. J. Metam. Geol., 36, 55–77.
Scodina M., Cruciani G., Franceschelli M. & Massonne H.-J. 2019. Anticlockwise P-T evolution of amphibolites from NE Sardinia, Italy: geodynamic implications for the tectonic evolution of the Variscan Corsica-Sardinia block. Lithos, 324–325, 763–775
Two stage of garnet growth in mylonitic micaschist from NE Sardinia: evidence from major and trace elements in garnet
No full textMylonitic micaschists in the south-eastern sector of the Posada-Asinara Shear Zone in the Axial Zone of the Sardinia Variscan consist of garnet porphyroblasts associated with plagioclase, quartz, biotite, staurolite, white mica, and chloritoid. The garnet porphyroblasts, enveloped by the S2 schistosity, preserve an internal foliation identified by the alignment of quartz, sometimes arranged into a sigmoidal pattern suggesting rotation of the garnet during growth. Major element compositional variation follows a bell-shaped zoning profile with Ca and Mn contents progressively decreasing, and Fe and Mg increasing, from the core (Alm45Grs25 Prp1Sps29) to the outer rim domain (Alm86Grs3Prp11Sps1; Cruciani et al., 2022). LA-ICPMS trace element mapping revealed
that the boundary between core and rim is marked by a thin annulus enriched in Y and HREE. The garnet core consists of an inner and an outer zone where the maximum concentration of elements from Lu to Tb progressively moves outwards with decreasing atomic number. This trend continues in the rim outside the annulus, where a further distinction between a Sm -, Eu-, Gd-, Tb-rich inner rim and a REE-poor outer rim was
observed (Franceschelli et al., 2023). The P-T path of the mylonitic micaschist, which was reconstructed from major element zoning in garnet, and from K-white mica composition and mineral assemblage preserved in garnet, is clockwise, subdivided into two different stages. The P-T trajectory was refined by the Compositional Zoning in Garnet and its Modification by diffusion software by Faryad and Ježek (2019). The garnet growth occurred into two stages, marked by partial resorption of the garnet core. The first stage of the P-T path is a prograde segment up to the peak pressure (T 430–490°C, P 1.3–1.4 GPa) whereas the second one reflects garnet rim growth and staurolite formation at peak metamorphism (560–630 °C/0.6–1.1 GPa) followed by exhumation
Zoning of chloritoid from kyanite facies rocks, Alpi Apuane, Italy
No full textChloritoid with significant Mg-Fe zoning occurs as lath-shaped porphyroblasts and as clusters of subradiating crystals in the Triassic Verrucano metapsammite (quartz + muscovite + chlorite + chloritoid ± kyanite) of the Massa Unit, Alpi Apuane. Two main types of chloritoid zoning profiles were found. The first type is characterised by gradual increase in Mg from core to rim of the porphyroblasts. In the second type the Mg content is constant from the core to the inner rim, but sharply decreases in the outer rim. Both types of zoning have been interpreted as the result of prograde growth during the Alpine metamorphism. The temperature, estimated using the chlorite-chloritoid thermometer, ranges from 467 to 560°C
Decentralized Centroid Estimation for Multi-Agent Systems in Absence of any Common Reference Frame
No full textIn this paper, a novel distributed algorithm to deal with the problem of estimating the network centroid in a multi-agent system is proposed. In this scenario, agents are assumed to be lacking any global reference frame or absolute position information. The proposed algorithm can be thought as a general tool to retrieve information about the centroid of a network of agents. Indeed, this allows to release several simplifying assumptions for a significant family of algorithms dealing with decentralized motion coordination. The convergence properties of the algorithm are carefully investigated in the case of a fully connected network for which a proof of convergence is provided. Successively, simulations to show the effectiveness of the algorithm also for arbitrary undirected connected graphs are given
Gossip-Based Centroid and Common Reference Frame Estimation in Multiagent Systems
No full textIn this study, the decentralized common reference frame estimation problem for multiagent systems in the absence of any common coordinate system is investigated. Each agent is deployed in a 2-D space and can only measure the relative distance of neighboring agents and the angle of their line of sight in its local reference frame; no relative attitude measurement is available. Only asynchronous and random pairwise communications are allowed between neighboring agents. The convergence properties of the proposed algorithm are characterized, and its sensitiveness against additive noise on the relative distance measurements is investigated. An experimental validation of the effectiveness of the proposed algorithm is provided
Anatectic amphibole and restitic garnet in Variscan migmatite from NE Sardinia, Italy: insights into partial melting from mineral trace elements
No full textWe report results of a laser-ICP-MS investigation of trace element contents in the main constituent minerals of an amphibolebearing
migmatite from the Variscan orogen in northeastern Sardinia. The migmatite is associated with migmatised orthogneiss and Alsilicate-
bearing pelitic migmatites. The protolith of the amphibole-bearing migmatite was a mid-Ordovician igneous rock of intermediate
composition characterised by a biotite þ plagioclase þ quartz assemblage. The migmatite consists of mesosomes and tonalitic (or, less
frequently, granodioritic) leucosomes, characterised by amphibole crystals (potassian ferropargasite) up to 2 cm in size. The tonalitic
leucosomes aremade up of quartz, plagioclase,K-feldspar, biotite,amphibole, garnet. The mesosomes are foliated rocks made up of the
same minerals with different modal proportions. In leucosomes, amphibole is the most abundant mafic mineral, occurring as euhedral
crystals rich in plagioclase, quartz, and small garnet inclusions. Garnet occurs as corroded and fractured grains in the matrix or within the
amphibole. Zircon forms euhedral bipyramidal grains up to a few hundreds of micrometres in size. Some amphibole rims have higher REE
and negative Eu anomalieswhereas cores exhibit lower REE and positive Eu anomalies. Garnet has strongly fractionated REE patterns with
chondrite-normalised abundances up to 2000 for HREE. Plagioclase has flat REE patterns with pronounced positive Eu anomalies. Zircon
displays fractionated REE patterns with HREE enrichment, LREE depletion, positive anomalies for Ce and negative ones for Eu.Monazite
shows high REE abundances, LREE enrichment, HREE depletion and negative Eu anomalies. Garnet is mostly a restitic phase, as indicated
by significant variation inHREEconcentrations between grains in themesosome, the absence of a noticeableEu anomaly, andYdepletion in
the leucosomes as compared to the mesosomes. In the leucosomes and mesosomes, the cores of zoned amphibole are characterised by
positive Eu anomalies: these crystallised from or in the presence of melt produced by anatexis of the original Bt þ Pl þ Qtz protolith.
Adjacent rimswith negative Eu-anomalies developed in coexistencewith a Eu-depleted melt that had experienced plagioclase fractionatio
P-T conditions of garnet-staurolite-bearing schists from Variscan NE Sardinia (Italy)
No full textIn north-eastern Sardinia, in the southern side of the Posada-Valley which is a km-wide shear zone of regional relevance, we collected garnet-staurolite schists with plurimillimetric garnet and staurolite porphyroblasts. At the outcrop scale the schists appears as silver-coloured, strongly foliated rocks rich in mica characterized by the occurrence of reddish to brownish garnet porphyroblasts up to 0.8-1 cm in size. Garnet crystals are enveloped by the main S2 regional schistosity which is identified by the orientation of white mica and chlorite. The garnet porphyroblasts are characterized by an inclusion-rich nucleus (garnet core) surrounded by a garnet mantle almost free of inclusions. The
inclusions within the garnet crystals, which mainly consist of quartz, rutile, ilmenite, chloritoid, mica and zircon, often follow an oriented spiral-shaped and/or sigmoidal arrangement recording an earlier S1 schistosity. Some smaller (less than 0.5 cm) and/or anhedral garnet grains lack the inclusion rich nucleus. The yellowish staurolite porphyroblasts (up to 0.5 cm in size) are often characterized by opaque minerals oriented inclusions. Chloritoid is found as oriented and isolated single crystals within the rock matrix. Paragonite locally grows on chloritoid, when this latter is included in the
garnet. Accessory minerals in the schists are apatite and tourmaline. EMP X-ray mapping and rim-core-rim garnet compositional profiles reveal a strong compositional zoning of the garnet components and the occurrence of a very thin (only a few dozens of μm in size) outern garnet rim, not detectable with the polarizing microscope. The garnet core composition ranges between XCa (Ca/(Ca+Mg+Fe+Mn): 0.21-0.27; XMg (Mg/(Ca+Mg+Fe+Mn): 0.01-0.02; XFe (Fe/(Ca+Mg+Fe+Mn): 0.45-0.50; XMn (Mn/(Ca+Mg+Fe+Mn): 0.25-0.30. The garnet mantle is enriched in almandine (XFe
0.55-85), pyrope (XMg 0.018-0.075) and depleted in grossular (XCa 0.06-0.18) and spessartine (XMn 0.05-0.24) components as compared to the garnet core. The narrow outern rim shows an abrupt decrease in grossular (XCa 0.02-0.03) and increase in pyrope content (XMg 0.11-0.13). Plagioclase single crystals in the matrix are oligoclase/albite (Ab ca. 90 mol.%). Staurolite porphyroblasts show XMg ratio of 0.14-0.13 in the core and 0.11-0.10 in the rim. Ti and Mn
contents in staurolite is 0.1 and 0.02-0.05 a.p.f.u, respectively. Chloritoid, independently from its microstructural position (i.e. inside the garnet or in the rock matrix) shows XMg ratio of 0.12-0.13. Biotite in the matrix has XMg ratio of 0.47. White mica shows different composition based on the microstructural position: white mica included in garnet has Si (a.p.f.u.) 6.38 and XMg 0.54, white mica single crystals in the matrix shows Si 6.11 a.p.f.u. and XMg 0.47, white mica in the micaceous layers has Si 6.04 a.p.f.u. and XMg 0.40. Chlorite shows XMg 0.4. Monazite in situ dating with
EMP yielded ages comprised between 417 and 322 Ma (average 377 Ma) for monazite included in garnet and 402-330 Ma (average 357 Ma) for monazite in the rock matrix. Small rutile grains preserved in garnet contain up to 0.02 wt.% zirconium, whereas the rare rutile grains in the rock matrix have undetectable zirconium content. Application of the Zr-in-rutile thermometer, following the calibration after [1], to 35 rutile grains included in garnet gives an average temperature of 557°C. Preliminary results on thermodynamic modelling suggest that the schist underwent a clockwise P-T path with a prograde portion that reached ca. 1.0 GPa peak pressure and temperature up to 600°C. The rock underwent later low-grade re-equilibration and mylonitization.
References:
[1] Tomkins et al. (2007) J Metamorphic Geol 25:703–71
Decentralized Centroid Estimation for Multi-Agent Systems in Absence of a Common Reference Frame: A Convergence Analysis
No full textIn this paper a novel algorithm for the decentralized estimation of the network centroid for a multi-agent system deployed in a d-dimensional space is presented. It is assumed that the agents are static, do not share a common coordinate system and are only able to estimate relative distance and angle of sight with respect to themselves during pairwise asynchronous communications. It is shown that if the graph representing the network topology is rigid, then the proposed algorithm solves the network centroid estimation problem
- …
