GEOCHEMISTRY AND TECTONO-MAGMATIC SIGNIFICANCE OF HP/LT METAOPHIOLITES OF THE ATTIC-CYCLADIC ZONE IN THE LAVRION AREA (ATTICA, GREECE)

Abstract

The Lavrion area corresponds to the northwestern end of the Attic-Cycladic Complex and mainly consists of metamorphic rocks formed during the Eocene high-pressure/low-temperature (HP/LT) event and the Upper Oligocene-Lower Miocene medium-pressure metamorphic event. These metamorphic rocks are found in two superimposed tectonic units: the Kamariza Unit, which includes metavolcanic rocks and the overlying Lavrion blueschist Unit, which is largely represented by metaophiolites. Protoliths of metavolcanic rocks in the Kamariza Unit are calc-alkaline basalts displaying a marked enrichment in Th, U, and LREE and depletion in Ta, Nb, Hf and Ti, which point to a genesis from a depleted mantle source further enriched by subduction components. The Lavrion blueschist Unit mainly includes metavolcanic rocks with tholeiitic composition, as well as subordinate metagranites and metavolcanic rocks with calc-alkaline affinity. The tholeiitic metavolcanic rocks are mainly represented by enriched-type (E-) mid-ocean ridge basalts (MORB) and subordinately by normal-type (N-) MORB. E-MORB chemistry implies a genesis from a depleted asthenospheric source modified by an OIB component, whereas the N-MORB has chemical features typical for rocks generated in a mid-ocean ridge setting from a primitive asthenospheric source. Previous works suggested that the magmatic protoliths of similar HP/LT metamorphic rocks from elsewhere in the Cyclades reflect an arc-back-arc tectonic setting, which developed during the Cretaceous closure of the Pindos oceanic basin. However, recent geological studies have shown that the Lavrion metamorphic Units, unlike similar units from the Cycladic zone, represent Triassic Pelagonian sequences metamorphosed under HP/LT conditions typical of the Cycladic zone. The geochemical and petrological characteristics of the Lavrion metamorphic rocks support this conclusion. In particular, calc-alkaline protoliths display many similarities with the Triassic calc-alkaline rocks associated with the rift of Gondwana, whereas MORB-type protoliths are similar to the Triassic MORB found in the Subpelagonian ophiolitic mélanges. The magmatic protoliths of the Lavrion HP/LT metamorphic rocks are thus compatible with a paleotectonic evolution which encompasses the Triassic continental rift, followed by the early oceanization stage of the Pindos ocean, and emplaced on the border between the Pelagonian continental margin and the Pindos basin. These rocks were probably included into mélanges during the Jurassic closure of the Pindos basin, and finally they were involved in the Eocene and Upper Oligocene-Lower Miocene metamorphic events that affected the Cycladic zone.