250 Ma metagranitoid from Drangovo Village: a new discovery of Permo-Triassic magmatism in the Eastern Rhodopes, Bulgaria

This short communication reports a 251.4 ± 6.8 Ma age of a Permo-Triassic metagranitoid (augen gneiss) in the Bulgarian part of the Eastern Rhodopes. The rock is intruded by the early Eocene Drangovo pluton and represents part of the upper metamorphic unit of the Kessebir dome. The analyzed sample has slightly peraluminous (ASI = 1.11) granitic composition with SiO2 = 70.6 wt.%. It is enriched in LILE and LREE and depleted in HREE, with a deep Eu (Eu/Eu* = 0.49) anomaly consistent with garnet and plagioclase fractionation. The large number of xenocrystic zircons, along with the low (780 °C) crystallization temperature and petrochemical data, suggests significant assimilation of basement rocks by the granitic magma. The rock has a subduction-related signature.

An application of GIS analysis on structural data from metamorphic rocks in Santorini Island.

The Santorini volcanic island is located in the northern Cretan Sea and is part of the ecent subduction-related volcanic arc. The opening of Cretan Sea is the result of extension associated with a series of ductile and brittle detachment faults developed since the Middle Miocene. A detachment between two exhumed metamorphic units is exposed at Athinios, on Santorini Island. Two exhumed metamorphic units are identified that show evidence of similar brittle deformation, but distinctive ductile and ductile-brittle structures. Different thermal histories indicate that a Miocene metamorphic unit is juxtaposed structurally below an Eocene metamorphic unit. In this paper, a prediction map of structural observations in Athinios is generated with statistical and GIS software, and shows a spatial distribution consistent with the exposure of two metamorphic units. K-Mean Cluster analysis using SPSS software on lineation azimuths of the metamorphic rock units shows two populations with center values of 347.2o ± 0.73o degrees (NNW to N) and 003.4o ± 0.83o degrees (N to NNE). NNW-lineation (arc-parallel extension) population belongs to the lower Miocene metamorphic unit and the NNE-lineation represents the Eocene metamorphic unit that was affected by arc-normal extension. A geostatistical map of ordinary Kriging type displays the possible exposed tectonic contacts. This methodology provides a structural prediction map that after field verification facilitates efficient structural and thermochronological sampling.

The Ankara Mélange: an indicator of Tethyan evolution of Anatolia

The Ankara Mélange is a complex formed by imbricated slices of limestone block mélanges (Karakaya and Hisarlıkaya Formations), Neotethyan ophiolites (Eldivan, Ahlat and Edige ophiolites), post-ophiolitic cover units (Mart and Kavak formations) and Tectonic Mélange Unit (Hisarköy Formation or Dereköy Mélange). The Karakaya and Hisarlıkaya formations are roughly similar and consist mainly of limestone block mélange. Nevertheless, they represent some important geological differences indicating different geological evolution. Consequently, the Karakaya and Hisarlıkaya formations are interpreted as Eurasian and Gondwanian marginal units formed by fragmentation of the Gondwanian carbonate platform during the continental rifting of the Neotethys in the Middle Triassic time. During the latest Triassic, Neotethyan lithosphere began to subduct beneath the Eurasian continent and caused intense deformation of the marginal units. The Eldivan, Ahlat and Edige ophiolites represent different fragments of the Neotethyan oceanic lithosphere emplaced onto the Gondwanian margin during the Albian–Aptian, middle Turonian and middle Campanian, respectively. The Eldivan Ophiolite is a NE–SW trending and a nearly complete assemblage composed, from bottom to top, of a volcanic-sedimentary unit, a metamorphic unit, peridotite tectonites, cumulates and sheeted dykes. The Eldivan Ophiolite is unconformably covered by Cenomanian–Lower Turonian sedimentary unit. The Eldivan Ophiolite is overthrust by the Ahlat Ophiolite in the north and Edige Ophiolite in the west. The Ahlat ophiolite is an east–west oriented assemblage comprised of volcanic-sedimentary unit, metamorphic unit, peridotite tectonites and cumulates. The Edige Ophiolite consists of a volcanic-sedimentary unit, peridotite tectonites, dunite, wherlite, pyroxenite and gabbro cumulates. The Tectonic Mélange Unit is a chaotic formation of various blocks derived from ophiolites, from the Karakaya and Hisarlıkaya formations and from post-ophiolitic sedimentary units. It was formed during the collision between Anatolian Promontory and Eurasian Continent in the middle Campanian time.

Paragenese a faible variance dans les metapelites de la serie de Filali (Rif interne marocain); description, interpretation et consequence geodynamique

The metamorphic series of Filali and the Beni Bousera massif represent the most metamorphic unit in the inner part of the Moroccan Rif. The Filali series is composed of micaschists, gneisses, migmatites and granulites wrapped around the ultramafic body of the Beni Bousera. The foliation is broadly coherent all over the massif although differences in lineation and fold axis have been found between the granulites and the micaschists-gneisses. Metamorphic grade increases continuously towards the ultramafic body; however, seven metamorphic zones which account for this increase can be defined: - chlorite zone: muscovite and chlorite underline the S1 schistosity refolded by S2. Small garnet is wrapped by or cut across S2; - chloritoid zone: chloritoid displays the same textures as chlorite and muscovite, underlining S1 and S2 schistosity. Chlorite reacts out to biotite; - staurolite+andalusite zone: staurolite, andalusite, cordierite and garnet have pre-, syn- and post- kinematic features with respect to S2. Garnet reacts out to muscovite, biotite and plagioclase; - staurolite+andalusite+kyanite zone: same textural relationships as in the previous zone. Kyanite is an additional phase, often displaying epitaxial textures with staurolite and andalusite; - andalusite+kyanite+sillimanite zone: fibrolitic sillimanite generally associated with biotite crystallizes from staurolite. However, sillimanite is also directly formed from andalusite and kyanite. As in the previous zone garnet reacts out to biotite, muscovite and plagioclase; - sillimanite+k-feldspar zone: muscovite reacts out to k-feldspar. Modes of biotite and sillimanite increase; - migmatites zone are composed by biotite, sillimanite, garnet, cordierite quartz and feldspars. Garnets do not display the reaction textures seen in the previous zones; - granulites are composed of a primary paragenesis with kyanite+k-feldspar+garnet+ or -biotite+plagioclase+rutile, partially obliterated by a secondary paragenesis with cordierite, sillimanite and spinel.

The age and origin of the garnet amphibolite underlying the Thetford Mines ophiolite, Quebec

The Thetford Mines ophiolite in southern Quebec is partially underlain by an 800 m thick amphibolite aureole. Rocks in the metamorphic unit grade inward towards the base of the ophiolite from chlorite–muscovite schist, epidote amphibolite, and garnet amphibolite to clinopyroxene-bearing garnet amphibolite closest to the fault contact with the basal tectonized peridotite unit of the ophiolite. The polydeformed metamorphic unit pinches out north and south of Belmina Ridge and is discordant with the overlying tectonized peridotite. Analyses of nine samples show a range of compositions, but the low K2O, P2O5, Rb, Zr, and Sr suggest that the amphibolite protolith was tholeiitic basalt, probably midocean ridge tholeiite.Microprobe mineral compositions determined for coexisting clinopyroxene–garnet–amphibole and garnet–amphibole suggest that amphibolites near the peridotite contact equilibrated at about 850 °C and that the temperatures decreased to about 380 °C 700 m from the contact. Geobarometers based on coexisting clinopyroxene and garnet, and Al and Na partitioning in amphibole yield variable results, but suggest that the pressure during formation of the amphibolite was probably greater than 5 kbar (5 × 105 kPa).Eight conventional K–Ar ages average 488 ± 11 Ma but show some scatter due to inaccuracy of K2O determinations at very low concentrations and, in some samples, possible inherited argon. A single 40Ar/39Ar incremental heating experiment gives concordant plateau and isochron ages of 491 ± 4 and 494 ± 4 Ma, respectively. The best weighted age of 491 ± 3 Ma probably dates the decoupling of the Thetford Mines ophiolite during the initial phase of closing of the Proto-Atlantic Ocean.