In situ U-Pb dating of zircons from five samples of Calabrian augen gneisses shows that their protoliths are Latest Precambrian to Early Cambrian in age (562 ± 15, 547 ± 7, 540 ± 4, 539 ± 16 and 526 ± 10 Ma), and contain Archaean (3.1 Ga), Palaeoproterozoic (1.7-2.4 Ga) and Neoproterozoic (0.6-0.9 Ga) inheritance. Geochemical signature of augen gneisses is typical of high-K calc-alkaline post-collisional magmatism. Their Sr-Nd isotopic compositions [0.7093 < ( 87 Sr/ 86 Sr) i < 0.7139; -3.2 < e Nd(t) < -5.4; 1.5 < T DM < 1.7 Ga] indicate the involvement of a crustal component in significant proportions. The Calabrian augen gneisses have, therefore, to be distinguished from the orthogneisses of Sardinia and northern Algeria, and from the porphyroids of Sicily, which are Middle Ordovician. By contrast, the Calabrian augen gneisses show a close similarity to the Pan-African post-collisional granitoids of the northern edge of the West African craton (e.g. the Moroccan Anti-Atlas). This suggests a periGondwana origin and corroborates previous palaeogeodynamic reconstructions attributing the Alboran microplate to the northern margin of the West African craton.
The lower crust of the Serre massif (Calabria,\ud
southern Italy) provides a window into the mid- to lower\ud
crust of the south European Variscan orogenic belt.\ud
Previously, zircon U-Pb ages were employed to date\ud
high-temperature processes affecting this portion of the\ud
Variscan crust. The present paper reports new LAICP-\ud
MS U-Pb data on the zircon of a deformed quartzmonzodiorite\ud
dike and of three mafic granulites sampled\ud
at the base of the lower crust section. Determination of\ud
trace elements on zircon, including rare earth elements\ud
(REE), has been also performed. The end of the Variscan\ud
exhumation, dated by anatectic zircon from migmatitic\ud
metapelites, and the growth-modification of zircon with\ud
respect to the growth of Variscan metamorphic garnet\ud
have been assumed as “time markers”. The concordant\ud
zircon ages of the metamorphic basic rocks cover a\ud
range from 744±20 Ma to 231±10 Ma with high age\ud
density from 357±11 Ma to 279±10 Ma, a few ages\ud
comprised between 418±14 Ma and 483±12 Ma and\ud
between 505±11 Ma and 593±14 Ma. Zircon from the\ud
deformed quartz-monzodiorite dike evidences a minimum age of emplacement of 323±5 Ma. Most of the analysed\ud
zircon domains dated between 357±11 Ma to 279±10 Ma\ud
from garnet-bearing metabasic rocks show flat patterns of\ud
heavy rare earth elements (HREE), as expected in the case\ud
of their simultaneous growth with garnet. This allows to\ud
consider (1) zircon domains giving Variscan ages as\ud
“metamorphic” with specific geological significance,\ud
and (2) zircon domains with ages ranging from 564±17\ud
Ma to 593±14 Ma as dating the emplacement of the\ud
magmatic protoliths as shown by internal microtextures,\ud
fractionated patterns of HREE and Th/U ratios (0.16–\ud
0.19). The Variscan zircon ages (357–279 Ma) reflect\ud
effects of crustal thickening, peak metamorphism and\ud
subsequent multistage Variscan decompression documented\ud
by the statistically significant clusters of ages\ud
around 347–340 Ma, 323–318 Ma, 300–294 Ma and\ud
279 Ma. The U-Pb zircon ages of the metabasic rocks\ud
suggest a period of about 60–70 Ma for granulite facies\ud
metamorphism and anatectic conditions. Literature data\ud
indicate that the migmatitic metapelites of the upper part\ud
of the Serre crust section also underwent a long period,\ud
about 40 Ma, of granulite facies metamorphism and anatectic\ud
conditions. A diachronism emerges through the time comparison\ud
of the Variscan evolution between the upper and the\ud
lower portions of the Serre deep crust. The duration of the\ud
Variscan processes defined in Calabria is comparable to that of\ud
other south European Variscan blocks
The paper deals with the U-Pb data of zircon separated from three samples representative of mylonitic leucogranites, trondhjemites and pegmatites occurring along the Alpine tectonic zone between the Castagna and Sila Units in northern Calabria. These mylonites are associated to Variscan granitic-granodioritic biotiterich augen gneisses derived from Neo-Proterozoic-Early Cambrian protoliths. Apparent ages ranging from Early Cambrian to post-Variscan have been obtained. Th, U and rare earth elements have been determined in two zircon domains of mylonitic leucogranite and trondhjemite giving different ages in order to get information relative to their geological significance. The pegmatite preserves intrusive contact with the augen gneisses and with the other mylonites; it turns out to be emplaced at 290-300 Ma, like the Variscan plutonites of the Castagna Unit. The deformation masks the original contacts of the mylonitic leucogranite and trondhjemite with the biotite-rich augen gneisses. The age-group averaging 540 Ma is interpreted as indicative of the emplacement of the protoliths and it coincides with the age previously determined for the emplacement of the protoliths of the biotite-rich augen gneisses. Zircon from the mylonitic pegmatite includes domains showing concordant and discordant ages younger than 290 Ma, thus reflecting various degrees of partial resetting and Pb-loss caused by post-Variscan events. Zircon from the mylonitic leucogranite and trondhjemite includes apparent ages between 300 and 280 Ma as well as ages younger than 250 Ma. Perturbation of U-Pb system by Alpine shearing appears evident; however, possibile effects caused by thermal input and hydrothermal fluid infiltration from the Variscan plutonites cannot be excluded.
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