New time constraints from 40Ar/39Ar geochronology on andesitic-dacitic lavas and acidic dyke rocks: An attempt to date the associated mineralization in the Western Thrace supra-detachment basin (Kirki, NE Greece)

Εpithermal and porphyry-type mineralization is genetically associated with acidic dyke rocks in a part of the supra-detachment Western Thrace Basin. ⁴⁰Ar/³⁹Ar ages on biotite of an andesitic lava dome and on K-feldspar of quartz-feldspar porphyritic dykes were determined and thus, new temporal constraints on the age of volcanism and mineralization were obtained.           

Biotite of an andesitic lava dome yields a ⁴⁰Ar/³⁹Ar plateau age of 33.05 ± 0.07 Ma (P=0.12). The dated andesite is considered as representative of the andesitic-dacitic rocks of large volcanic and subvolcanic bodies in the Western Thrace basin (Mavropetra Formation, Kirki area). Andesitic rocks indicate affinities of calc-alkaline to high-K calc-alkaline series magmatism. They are coeval to the high-K calc-alkaline magmatic suite of Leptokarya – Kirki, which forms an ENE-WSW 30 km long magmatic dome, developed between the Rhodope metamorphics extending northwards and the overlying detached Melia non-metamorphic formations and Middle-Upper Eocene molassic clastics, extending southwards.

Smaller bodies of acidic dyke rocks (rhyolite and quartz-feldspar porphyry), crosscut the overall dome structure with the andesitic-dacitic volcanics, the Middle-Upper Eocene clastic sediments, the mafic rocks of the Melia unit, the metamorphics of the Kechros Unit of Rhodope and the Leptokarya - Kirki granitoids. They appear with planar subvertical boundaries following a general NNW-SSE trend, perpendicular to the main ENE-WSW dome structure. They are concentrated along a major fault zone  (Ag. Filippos fault), with high- to intermediate sulfidation epithermal polymetallic sulfide mineralization, as well as in a roughly 8 km long and 1 km wide fracture zone to the east and northeast of Aisymi village with porphyry-type mineralization. Structural observations document the mega-tension gashes nature of the dykes with pronounced sinistral strike-slip kinematic indicators of the Kirki mineralized tectonic zone. K-feldspars from quartz-feldspar porphyritic dykes at Kirki yield a ⁴⁰Ar/³⁹Ar plateau age of a 31.89 ± 0.12 Ma (P=0.08).  The acidic dyke rocks contain calc-alkaline to high-K calc-alkaline differentiation trends. They exhibit marked enrichment of LREE relative to the HREE, flat HREE pattern, negative Eu anomaly and Eu/Eu* values ranging between 0.32 and 0.82.

In conclusion, the ENE-SSW Leptokarya - Kirki granitic dome was developed contemporaneously with the andesitic-dacitic volcanics at the contact between the Rhodope metamorphics and the detached Melia formations and Middle-Upper Eocene clastics at about 33 Ma, followed by the NNW-SSE transverse faults and acidic dykes with epithermal and porphyry-type mineralization at about 32 Ma.