Annamaria Fornelli

Petrological evidence for crustal thickening and extension in the Serre granulite terrane (Calabria, southern Italy)

The paper presents the metamorphic trajectory recorded by metapelitic migmatites of the upper part of the Hercynian lower continental crust of the Serre (southern Calabria, Italy). The relict minerals, reaction textures and phase equilibria define a clockwise P-T path. The prograde metamorphism from temperature of about 500 ◦ C and pressure of 4-5 kbar to T < 700 ◦ Ca nd P ∼ 8 kbar stabilized the assemblage Grt + Ky + Bt + Ms(Si/11ox =3.26-3.29) in the uppermost metapelites of the profile. Progressive heating led to H2O-fluxed and dehydration melting first of Ms, then of Bt at T < 700 ◦ C in the stability field of sillimanite. This process was followed by nearly isothermal decompression producing additional melt with a transition from Grt to a Grt + Crd stability field. Further decompression caused the formation of Crd-corona around garnet. Nearly isobaric cooling led to rehydration and retrogression across the stability field of andalusite up to the stability field of kyanite. The lowermost metapelites of the studied profile have lost most of the memory of the prograde P-T path; they record decompression and cooling. High-temperature mylonites occur in which boudinage, elongation and pull-aparts characterize the porphyroclasts. The pull-aparts in the high-T mylonites are filled with low-P minerals (Crd + Spl). The Hercynian metamorphic trajectory and the microtextures are consistent with crustal thickening and subsequent extensional regime. During extension, an important tectonic denudation probably caused the isothermal decompression. Extension also occurred in post-Hercynian times as documented by pull-aparts in sillimanite porphyroclasts filled with chloritoid within a low-grade mylonite.

Multi-stage dehydration–decompression in the metagabbros from the lower crustal rocks of the Serre (southern Calabria, Italy)

Porphyroblastic garnet-bearing metagabbros from the base of the lower crust section of the Serre (southern Italy) exhibit multi-stage dehydration and decompression after the Panafrican emplacement of their protoliths. The first dehydration event produced Am–Opx–Cpx–Pl–Grt as the peak assemblage. Two decompression stages are documented by: (1) coronas of Opx–Pl and Opx–Am, and symplectites of Opx–Am–Pl around clinopyroxene within the porphyroblastic garnet as well as in the matrix and (2) symplectites of Pl–Am–Opx–Grt having different textures around the porphyroblastic garnet. During the second decompression stage, a new local, somewhat intense, dehydration occurred and produced rims of Opx + Pl around the porphyroblastic amphibole, or lenses of Pl–Opx–Am–Spl ± Bt between layers of dominant amphibole. A deformation stage separates older from younger reaction textures. The porphyroblastic garnet, its inclusions and the matrix are affected by fractures, which have been overgrown by coronas and symplectites around the porphyroblastic garnet and the amphibole of the matrix. Preferred P–T estimates are: ~ 900 °C and ~ 1.1 GPa at the metamorphic peak; ~ 850 °C and 0.8–0.9 GPa during the formation of corona around clinopyroxene; 750–650 °C and 0.7–0.8 GPa during the formation of corona around garnet. All these textures formed under granulite-facies conditions. The subsequent metamorphic evolution consists of rehydration under amphibolite-facies conditions. The P–T–t path agrees with the path shown by the uppermost migmatites of the Serre section, and the P–T estimates at the top and the bottom of the section are consistent with the thickness (7–8 km) of the lower crustal segment. A contractional regime, which caused a crustal thickening of about 35 km, was followed by an extensional one producing significant crustal thinning; the change of tectonic regime probably occurred about 300 Ma ago when the emplacement of voluminous granitoids and the initial stages of exhumation of the lower crustal section had taken place.

Provenance and tectonic implications of heavy minerals in Pliocene-Pleistocene siliciclastic sediments of the southern Apennines, Italy

Abstract During the late Tortonian-Pleistocene, rifting in the Tyrrhenian region and compression in the Apennines coexisted, with an eastward migration of the rift basin-thrust belt-foredeep systems. In a northwest-southeast area of the southern Apennine Pliocene deposits, accumulated in piggy-back basins, and Pliocene-Pleistocene sediments deposited in the foredeep basins, are contiguous. Here our attention is focused on the heavy minerals, which are sensitive indicators of source rocks. Garnet, blue amphibole, staurolite, clinopyroxene, epidote have been analysed and compared with the minerals of the likely protoliths. Blue amphibole and staurolite derive from Early Miocene ‘Tufiti di Tusa e Arenarie di Corleto’ Formation; pyroxenes derive from Pleistocene Vulture volcanics and, possibly from Pliocene volcaniclastic strata cropping out in Basilicata. Interestingly, blue amphibole and staurolite, present in the Early Miocene volcanolithic and quartzofeldspathic sandstones of the ‘Tufiti di Tusa e Arenarie di Corleto’ Formation, are absent in Tortonian siliciclastic strata, but appear again in Pliocene-Pleistocene siliciclastic sediments. The space-time distribution of these minerals was controlled by important morphostructural modifications affecting the southern Apennine thrust belt.

Metamorphic xenoliths and microgranular enclaves in the Serre granodiorites (Southern Calabria-Italy): Their connection with granitoid genesis

SummaryLate-hercynian granodiorites in the eastern Serre are calc-alkaline and peraluminous in composition; actinolitic-hornblende typically occurs in the mafic types, whereas muscovite occurs in the felsic ones. Magmatic microgranular enclaves and small metamorphic xenoliths are present in these granitoids. A petrographical study and microprobe analyses on plagioclase, biotite, amphibole, cordierite and cummingtonite in xenoliths and enclaves and in the host rocks were performed. The presence of sillimanitecordierite-biotite-hercynite and of cummingtonite-biotite-bearing xenoliths suggests that partial melting of a heterogeneous deep crust played an important role in the genesis of the granitoids. In contrast, the occurrence of microgranular enclaves suggests that a sub-crustal component might have been involved. Two alternative mechanisms are proposed to explain the magma genesis: i) mixing between subcrustal and crustal melts; ii) direct crustal genesis.ZusammenfassungSpätherzynische Granodiorite des östlichen Serre-Gebietes zeigen kalk-alkalische und peraluminöse Zusammensetzung. Aktinolithische Hornblende tritt typischerweise in den mafischen, Muskowit in den sauren Gesteinen auf. In den Granitoiden vorkommende, magmatische, mikrogranulare Einschlüsse und kleine metamorphe Xenolithe wurden petrographisch und mittels Mikrosondenanalytik an Plagioklas, Biotit, Amphibol, Cordierit und Cummingtonit untersucht. Sillimanit-Cordierit-Biotit-Hercynit-und Cummingtonit-Biotit- führende Xenolithe weisen darauf hin, daß der partiellen Aufschmelzung einer heterogenen Unterkruste eine bedeutende Rolle für die Genese der Granitoide zukommt. Mikrogranulare Einschlüsse weisen dagegen auf die Beteiligung einer subkrustalen Komponente hin. Zwei alternative Mechanismen werden für die Magmengenese diskutiert. i) Mischung von subkrustalen und Krustalen Schmelzen; ii) direkte krustale Genese.

REE partition among zircon, orthopyroxene, amphibole and garnet in a high-grade metabasic system

A mafic amphibole-bearing granulite with porphyroblastic garnet was investigated to evaluate: (1) the rare earth element (REE) partition among garnet, zircon, orthopyroxene and amphibole during the metamorphic evolution; (2) the significance of the REE distribution along lobes and bights of reabsorbed garnet rim; and (3) REE distribution coefficient values (DREE) suggestive of chemical equilibrium, assuming garnet as a reference. The results have been compared with those deriving from an intermediate granulite containing porphyroblastic garnet, without amphibole. Porphyroblastic garnet from both samples is rimmed by a continuous corona formed during post-peak decompression characterized by REE-enriched lobes and REE-poor bights. The amphiboles from corona have various REE abundances, reflecting a different dissolution rate of original garnet rim. The initial slow rate of garnet dissolution caused high REE concentration in the new garnet rim due to intra-crystalline diffusion, leading to the formation of REE-poorer amphiboles in corona. Subsequently, under an increasing geothermal gradient and fluid-present conditions, the faster dissolution of garnet determined the formation of bights and the transfer of REEs towards the corona. The timing of garnet growth and its dissolution were checked by U–Pb zircon ages. The zircons dated from 339 Ma to 303 Ma in two rock types combined with the garnet domains (core, outer core, rim) show similar distribution of patterns relative to heavy rare earth elements for zircon and garnet (DHREE zrn/grt ), suggesting chemical equilibrium. Zircons dated at c. 300 Ma do not appear in equilibrium with REE-rich garnet lobes, and younger zircons (278 Ma) show a new equilibrium with REE-poor garnet bights. On this basis, the DHREE amph/grt values obtained in specific textural sites might be interpreted as suggestive of equilibrium under granulite conditions.

Erratum to: Diachronic and different metamorphic evolution in the fossil Variscan lower crust of Calabria

0.069 0.145 – 0.109 0.048 0.019 – 0.128 16.53Mg 1.364 0.826 – 3.192 1.172 1.432 – 3.348 –Mn 0.046 0.003 – 0.021 0.084 0.006 – 0.012 –Ca 0.506 0.885 0.900 1.874 0.487 0.012 0.949 1.833 –Na – 0.082 0.127 0.814 – – 0.080 0.956 –K – – 0.001 0.110 – – 0.001 0.107 –Tot cat. 8.022 4.000 5.018 15.793 8.000 4.006 4.998 15.899 23.904Fe/(Fe ? Mg) 100 44.3 2.3 21.9 51.5 24.9 18.0 100.0An 87.5 92.1Al