‘Boninitic’ clasts from the Mesozoic olistostromes and turbidites of Angelokastron (Argolis, Greece)

Abstract

The Lower Unit of the ophiolitic sequence of Northern Argolis comprises turbiditic sediments and olistostromes. both containing ophiolitic clasts. mainly crystal fragments (clinopyroxene. plagioclase. Cr-spinel, amphibole) in the turbidites and cumulitic intrusives (quartz noritic amphibole-bearing gabbros). subvolcanic rocks (dolerites) and various effusive lithologies (mainly Si-rich basalts to basaltic andesites) in the olistostromes. The volcanic rocks belong to three groups. In rare cases the lavas are niineralogically and chemically comparable with MORB; most of them. and the subvolcanic rocks, contain primary quartz and amphibole, orthopyroxene, Ca-rich plagioclase and clino-pyroxene_+ Cr-spinels. All rocks are Si-and Mg-rich and have high concentrations of 'compatible' and very low concentrations of 'incompatible' elements. The REE profiles are characteristically U-shaped. Many of the observed features are comparable with those of suhduction-related lavas and, in particular, with present day boninites and ophiolitic boninitic rocks. The gabbroic rocks have mineralogical and chemical analogies with the dolerites and lavas, thus it may be argued that the gabbros represent the intrusive counterparts of the 'boninitic' volcanic clasts. The mineral clasts occurring in the turbidites are chemically comparable with those analysed in the ophiolitic clasts of the overlying olistostrome. It may be concluded that the ophiolitic clasts of both olistostromes and turbidites were derived from a subduction-related sequence. An island arc back-arc system might explain the occurrence of both boninitic and MORB-type lithologies in the olistostrome of Angelokastron. This may support the hypothesis of the onset of compressive tectonics along the Pindos Ocean during the Jurassic. X; 1996 No. of Tables: 6 . No. of Refs: fvl 3-4 vol.%) intergranular phase of most samples, whereas biotite forms rare intergranular flakes. The opaque minerals are rare accessories crystallizing both as early phases included in the cumulus pyroxenes and in the intercumulus. Dolerites and granophjwsThe dolerites (samples 1495, 1503, 1512 and A33a) are non-cumulus. ophitic to subophitic or intergranular in texture; most samples are porphyritic, with orthopyroxene, clinopyroxene, plagioclase and opaque minerals as possible phenocrysts. Orthopyroxene and/or plagioclase were the first minerals to separate, followed by clinopyroxene. The same phases compose the matrix of the porphyritic rocks and the aphyric samples, in addition to the following minerals: light green --light brown amphibole (up to 30% modal content), frequently poikilitic with pyroxenes and plagioclase; primary interstitial quartz, variable from a few vol.% up to 20 vol.%; very rare biotite; opaque minerals locally abundant; and rare apatite. Sample 1499 is a medium-grained quartz-rich differentiate, resembling granophyre, which contains 70% modal granophyric intergrowths of primary quartz and andesine, in addition to green amphibole, apatite and ores. Group I effusivesThe rocks of this gr...

Triassic mid-ocean ridge basalts from the Argolis Peninsula (Greece): new constraints for the early oceanization phases of the Neo-Tethyan Pindos basin

“…Remnants of the Pindos basin are widely preserved in late Mesozoic-Cenozoic accretionary complexes in the Mirdita-Subpelagonian zone of the Dinaride-Hellenide belt and are mainly represented by complex tectonosedimentary associations of Triassic rift-related volcanic rocks (Pc-Piper 1998, and references therein), Middle Jurassic mid-ocean ridge (MOR) and suprasubduction-zone (SSZ) ophiolitic sequences Capedri et al 1996), Permian-Jurassic marginal and platform-related sedimentary rocks and Juras-sic-Tertiary trench-type sedimentary rocks (Clift & Robertson 1989;Robertson 1994;Degnan & Robertson 1998). The nature and composition of these various rock types suggest a general geodynamic evolution of the Pindos basin characterized by Late Permian-Triassic rifting phases between the Apulian and Pelagonian microplates, followed by Jurassic spreading of the Pindos Neo-Tethyan oceanic basin and subsequent development of intra-oceanic convergent zones, as well as Mid-Late Jurassic generation of oceanic lithosphere in a suprasubduction setting (Jones & Robertson proposed by some workers (Ferri~re 1982;Robertson et al 1991;Robertson et al 1996;Pe-Piper 1998).…”

Geological Contribution to the Tectono- Stratigraphy of the Nafplion Area (Nw Argolis, Greece)

“…The obduction of Argolis ophiolitic nappe would originated from Pindos zone (Robertson & Dixon, 1984;Clift, 1996) or from Vardar zone (Dercourt et al, 1993;Bortolotti et al, 2003;Bortolotti & Principi, 2005) in late Upper Jurassic time and was manifested by the deposition of ophiolite-derived debris flows and turbidites generated by the advancing thrust sheet of pillow lavas nappe (Baumgartner, 1985;Photiades, 1986;Photiades, 1989;Dostal et al, 1991;Capedri et al, 1996) with T-MOR and N-MOR basalts affinities of Triassic to Jurassic ages (Bortolotti et al, 2002(Bortolotti et al, , 2003Saccani et al, 2004).…”

“…This sequence is followed upward by ophiolitic olistostromes (5-50m in thickness) containing rounded fragments (up to 50 cm in diameter) of boninitic lavas and coarse-grained boninitic-type rocks set in arenitic matrix of various ophiolitic clasts and fragments originated from the underlying limestones and radiolarian cherts. The boninitic lavas and coarse-grained boninitic-type considered to be generated in supra-subduction setting (Photiades 1986;Dostal et al 1991;Capedri et al 1996). The presence of such deposit records the paroxysmal tectono-sedimentary events due to early compressional late Jurassic tectonic phase, affecting synchronously the oceanic basin and the Subpelagonian continental margin (Photiades, 1986;Dostal et al 1991;Capedri et al 1996;Bortolotti et al, 2003).…”

“…3A-B) emplaced tectonically over the previous ones and synchronously thrust over the flyschoidal melange, and is composed, at the base, by a schistose serpentinite tectonic sole (up to 100m thick) bearing meter-sized, lens-shaped blocks of basalts, boninitic rocks (Capedri et al, 1996) and various exotic blocks (Photiades, 1986) and, at the top, by an unconformably Cretaceous carbonate sequence cover (up to 350m thick). Furthermore, several Cretaceous limestone (Dercourt, 1964;Decrouez, 1977a, b) outliers such as Palamidi, covered with apparently post-Upper Cretaceous flysch, Akronafplia, Profitis Ilias and Aria with serpentinite tectonic sole, as well as tectonosomes and/or megablocks of Triassic-Liassic carbonates detached from the shallow-water carbonate sequence of the Subpelagonian Lower Unit and are overthrust the flyschoidal melange (Fig.…”

Geochemical characteristics of the Cretaceous ophiolitic rocks of Ikaria island, Greece

“…Fields for mid-ocean ridge basalt (MORB), island-arc tholeiite (IAT) and boninite from other Greek ophiolites based on analyses inDostal et al (1991),Capedri et al (1996), Stolz, Engi & Rickli (1997, and Pe-Piper,Tsikouras & Hatzipanagiotou (2004).…”