G. Piccarreta’s research while affiliated with University of Bari Aldo Moro and other places

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.

The recognition of the coeval growth of zircon, orthopyroxene and garnet domains formed during the same metamorphic cycle has been attempted with detailed microanalyses coupled with textural analyses. A coronitic garnet-bearing granulite from the lower crust of Calabria has been considered. U–Pb zircon data and zircon, garnet and orthopyroxene chemistries, at different textural sites, on a thin section of the considered granulite have been used to test possible equilibrium and better constrain the geological significance of the U–Pb ages related to zircon separates from other rocks of the same structural level. The garnet is very rich in REE and is characterised by a decrease in HREE from core to outer core and an increase in the margin. Zircons show core–overgrowth structures showing different chemistries, likely reflecting episodic metamorphic new growth. Zircon grains in matrix, corona around garnet and within the inner rim of garnet, are decidedly poorer in HREE up to Ho than garnet interior. Orthopyroxene in matrix and corona is homogeneously poor in REE. Thus, the outer core of garnet and the analysed zircon grains grew or equilibrated in a REE depleted system due to the former growth of garnet core. Zircon ages ranging from 357 to 333 Ma have been determined in the matrix, whereas ages 327–320 Ma and around 300 Ma have been determined, respectively, on cores and overgrowths of zircons from matrix, corona and inner rim of garnet. The calculated DREEzrn/grt and DREEopx/grt are largely different from the equilibrium values of literature due to strong depletion up to Ho in zircon and orthopyroxene with respect to garnet. On the other hand, the literature data show large variability. In the case study, (1) the D zrn/grt values define positive and linear trends from Gd to Lu as many examples from literature do and the values from Er to Lu approach the experimental results at about 900 °C in the combination zircon dated from 339 to 305 Ma with garnet outer core, and (2) D opx/grt values define positive trends reaching values considered as suggestive of equilibrium from Er to Lu only with respect to the outer core of garnet. The presence of a zircon core dated 320 Ma in the inner rim of garnet suggests that it, as well as those dated at 325–320 Ma in the other textural sites and, probably, those dated at 339–336 Ma showing depletion of HREE, grew after the garnet core, which sequestered a lot of HREE and earlier than the HREE rich margin of garnet. The quite uniform REE contents in orthopyroxene from matrix and corona and the low and uniform contents of HREE in the zircon overgrowths dated at about 300 Ma allow to think that homogenisation occurred during or after the corona formation around this age. The domains dated around 325–320 Ma would approximate the stages of decompression, whereas the metamorphic peak probably occurred earlier than 339 Ma.

Different grain-size fractions of samples collected from shales were investigated to determine the role of late diagenetic grade and mineralogy on the rare earth elements (REE) distribution. The samples were collected from Upper Cretaceous varicoloured shales of the Sicilide Unit near the Corleto Perticara villages, Southern Apennines (Italy). The mineralogical and chemical composition of the sample's five fractions (>63, 32-63, 2-32, 0.1-2, and <0.1 μm) was studied. The data indicate that certain accessory minerals are more important than clay minerals in controlling the REE distribution. In particular, zircon controls the distribution of earth elements in the 0.1-2 μm fraction and is an efficient mechanism in determining the concentration and distribution of REEs in the studied shales.

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Quartz-monzodioritic dykes have intruded the Hercynian continental lower crust of Calabria at 323 ± 5 Ma. Pseudosection and average PT calculations have been performed using THERMOCALC. The obtained P (aver-age PT) and T (pseudosection and average PT) values indicate that the quartz-monzodioritic dykes have in-truded the 750 °C hot metapelitic–migmatitic country-rocks at a depth of about 32 km, with a temperature of ca. 850 °C. Microstructural analysis suggests that deformation of the dykes started at hypersolidus conditions during emplacement and continued after cooling at sub-solidus granulite facies conditions together with the country rocks. During this deformation event, which started not later than 323 ± 5 Ma ago, the main S n + 1 schistosity evolved. Vorticity analysis of the quartz-metadioritic dykes and of the metapelitic country rocks shows that the S n + 1 schistosity evolved during a general shear deformation characterized by 60 to 70% of pure shear. This high percentage of pure shear suggests that the main S n + 1 schistosity evolved in the hot lower crust undergoing extension during vertical shortening. A horizontal attitude of this extensional schis-tosity is confirmed by retro-deformation of the today obliquely SE-ward dipping S n + 1 schistosity. Extension continued with time and propagated upwards within the crust, affecting the middle crust in a time interval of 306 ± 1 to ca. 300 Ma when huge masses of granitoid rocks intruded it. The deepest batches of these granitoid bodies show a gneissic schistosity which developed under ductile conditions, while granitoids at higher levels became deformed in a brittle fashion. Our whole data set is consistent with a late Hercynian evolution in Calabria characterized by a progressively crust overlying a mantle that gradually rose during lithospheric extension. This extension was asynchronous, starting at 323 ± 5 Ma in the lower crust and affecting higher crustal levels at progressively later times.