Adakitic-like granitoids at west Getic basement of the South Carpathians petrogenesis and thermotectonic events evidenced by zircon geochemistry
Anca Dobrescu
Geological Institute of Romania, Dept. Regional Geology, Bucharest, Romania; ancadobrescu2003@yahoo.com
Research on two granitoids from Buchin (BG) and Slatina-Timiş (STG) at west Getic Domain of the
Semenic Mountains (South Carpathians) identified trondhjemites, tonalites, granodiorites and granites of
I-S type and adakitic signature. High Na2O, Al2O3 and Sr, depleted Y (<18 ppm) and HREE (Yb< 1.8
ppm) contents , high Sr/Y (>40), (La/Yb)N (>10) ratios and low (+/-) to no Eu anomalies overlap the highsilica adakites (HSA) main characteristics, though differences like lower Mg# and Ni contents, slightly
increased K2O contents and 87Sr/86Sr ratios revealed. Comparison with HSA and results of experiments
indicates a mafic source melting at >12.5 kbar and 900o-800oC as main genetic process, leaving residues
of garnet granulite to garnet amphibolite type.
U-Pb age data on zircons acquired by LA-ICP-MS were interpreted exclusively on their internal
structure as Ordovician igneous crystallization time, Neoarchean-Neoproterozoic1 inheritance,
Neoproterozoic2-3 source material and Variscan recrystallization imprint (Dobrescu et al., 2010).
Considering the debate on a possible BG consanguinity with the Poniasca-Sicheviţa magmatism during
the post-collisional Variscan orogenic phase, the age data interpretation was reviewed by a geochemical
study on the dated BG and STG zircons analyzed by secondary-ion mass spectrometry (SIMS). U
contents of 114-305 ppm, high Th/U (~1) and low U/Yb (0.2-0.8) ratios for zones of 462 Ma and 434 Ma
on BG zircons, U contents of 234-396 ppm, reasonably high Th/U (~0.6) and low U/Yb (0.6-0.8) ratios
for zones of 493 Ma and 465 Ma on STG zircon argue for magmatic origin. Increased HREE contents,
positive Ce and negative Eu anomalies confirm the igneous crystallized zones at Ordovician time. By
contrast, the outer zircon rims with ages between 357-309 Ma have significantly higher U (553-3891
ppm) contents, low Th/U (0.032-0.200) ratios, increased Hf (9037-9952 ppm), lower Y (240-892 ppm)
and Yb (29-290 ppm) contents and high U/Yb (6-30) ratios, recording the recrystallized overgrowth effect
of a high temperature tectonothermal event. Ordovician magmatic ages are quite widespread in the SebeşLotru basement as in the protoliths of Căpâlna augen gneiss (459 Ma), Latoriţa orthogneiss (466 Ma) and
Tău metagranite (473 Ma), some surrounded by Variscan overgrowths as a consequence of partial or total
resetting during the high-grade metamorphic event (Medaris et al., 2003; Balintoni et al., 2010).
REFERENCES
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Dobrescu, A., Tiepolo, M., Negulescu, E., Dordea, D. 2010. U/Pb zircon geochronology on TTG rocks
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Acknowledgements
This work was financially supported by UEFISCDI grant PN-III-P1-1.1-MC-2017-2295.
Many thanks to Martin Whitehouse and Gavin Kenny for expert guidance and work in NORDSIM Stockholm.