0000000000060052
AUTHOR
Thomas Reischmann
Further evidence for an early Carboniferous (∼340 Ma) age of high-grade metamorphism in the Saxonian granulite complex
U–Pb and Pb–Pb zircon ages for metamorphic zircons from granulites in the Saxonian granulite complex are reported, using the SHRIMP ion microprobe, conventional multigrain and single-gain techniques and the evaporation method. This is complemented by a Pb–Pb evaporation age for a post-granulite granite emplaced into the schist mantle around the granulites during uplift of the complex. We also demonstrate that zircon ages are not reset during high-grade metamorphism, as commonly argued, but have a very high closure temperature and usually preserve the isotopic composition reflecting the time of their formation. Multifaceted zircons from four granulite samples that probably grew close to the …
Miocene high-pressure metamorphism in the Cyclades and Crete, Aegean Sea, Greece: Evidence for large-magnitude displacement on the Cretan detachment
The Cyclades in the backarc region of the present Hellenic subduction zone are known for widespread Late Cretaceous to Eocene high-pressure metamorphism in the Cycladic blueschist unit. We report 40 Ar/ 39 Ar and Rb/Sr phengite ages of 24–21 Ma for high- pressure metamorphism (8–10 kbar, 350–400 °C) in the lowest tectonic unit in the Cyclades, the Basal unit, which structurally underlies the Cycladic blueschist unit. The Basal unit is correlated with the Tripolitza unit of the External Hellenides in the forearc region of the Hellenic subduction zone. The Tripolitza unit is unmetamorphosed on Crete, where it is separated from the underlying high-pressure (8–10 kbar, 300–400 °C) Plattenkalk a…
The evolution of the southern Menderes Massif in SW Turkey as revealed by zircon dating
This geochronological study concentrates on the evolution of the southern part of the Menderes Massif, a metamorphic core complex in W Turkey, by dating single zircons with the Pb–Pb evaporation and the U–Pb methods. We have analysed zircons from granitic augen gneisses of the core as well as from quartzites of the metasedimentary envelope. Zircons from the granitic core gneisses are euhedral with typical igneous morphologies. The single zircon ages of these gneisses indicate a time span for magmatic activity from 520 to 570 Ma with a major event at about 550 Ma. Some gneisses contain older zircons of c. 660 Ma which are interpreted as inherited grains. Zircons from the quartzites are detri…
Age and origin of the Böllsteiner Odenwald
The granitoid protoliths of the gneisses of the Bollsteiner Odenwald, a part of the Mid German Crystalline Rise, intruded at 405 ± 3 Ma. This age was obtained by single zircon 207Pb/206Pb evaporation measurements. It is supported by an upper discordia intercept age of 410 ± 11 Ma from single zircon U/Pb data. These granitoids were derived from hybrid magmas according to their geochemical characteristics. Inherited zircon grains with apparent ages of 882 and 1138 Ma further constrain the source heterogeneity. The isotope data with Nd model ages for the granitoid gneisses between 1.3 and 1.7 Ga also underline the involvement of Proterozoic material into the magma sources. Since the geochemica…
Provenance of sediments during subduction of Palaeotethys: Detrital zircon ages and olistolith analysis in Palaeozoic sediments from Chios Island, Greece
Abstract Detrital zircon geochronology and analysis of fossiliferous olistoliths from Chios Island, Greece, are used here to constrain terrane accretion processes and the provenance of crustal sources for sediments during the subduction of (a branch of) the Palaeotethys Ocean. U/Pb ages obtained by ion microprobe (SHRIMP-II) analyses of detrital zircons from a Carboniferous greywacke belonging to the tectonostratigraphic Lower Unit of Chios gave major age groups of 2150–1890 Ma, 640–540 Ma, 505–475 Ma and 365–322 Ma. Detrital zircons from a Permian–Triassic sandstone yielded prominent age clusters of 2200–1840 Ma, 1100–910 Ma, 625–560 Ma and 385–370 Ma. The lack of zircon ages between 1.8 a…
Geochemistry, petrogenesis and tectonic setting of the Samothraki mafic suite, NE Greece: Trace-element, isotopic and zircon age constraints
Abstract The Samothraki mafic suite in the north-eastern Aegean Sea, Greece, is an ‘in situ’ magmatic complex comprising gabbros, sparse dykes and basalt flows and pillows cut by late dolerite dykes. We have determined the age of the complex by SHRIMP zircon geochronology of a gabbro as 159.9 ± 4.5 Ma (i.e. Oxfordian; early Late Jurassic), which precludes any correlation with the so-called Lesvos ophiolite further south (253.1 ± 5.6 Ma; Latest Permian). Six distinct, hitherto unrecognised, geochemical groups have been identified among the basalts and dolerites of the Samothraki mafic suite on the basis of trace-element and Nd–Sr isotopic characteristics. All groups show the presence of an e…
Late proterozoic island arc volcanics from Gebeit, Red Sea Hills, north-east Sudan
The area of Gebeit Mine in the northern Red Sea Hills, Sudan, is built up of voluminous volcanic rocks and minor volcaniclastic and clastic sequences. According to their chemical and modal compositions the Gebeit volcanics can be devided into four groups: (a) cpx-physic basalts with clinopyroxene and plagioclase as the dominant phenocrysts and minor opaques; (b) hbl-physic basalts with hornblende, clinopyroxene, plagioclase and subordinate magnetite including one rare dacite; (c) pl-phyric andesites with plagioclase phenocrysts in a matrix that is rich in magnetite; and (d) aphyric basalts. The compositional variation within the distinct volcanic groups can only partly be explained by fract…
Timing of granitoid magmatism in the eastern mid-German crystalline rise
Abstract The eastern parts of the mid-German crystalline rise (MGCR) are exposed in the Spessart, Ruhla and Kyffhauser crystalline complexes and known from boreholes in the region of Dessau. In this study we determined igneous formation ages of granitoid intrusives and orthogneisses from this part of the MGCR using the Pb/Pb single zircon dating technique. The intrusion ages of the granitoids range from ∼489 to ∼302 Ma (i.e. from lower Ordovician to upper Carboniferous). The grouping of these ages marks several distinct magmatic episodes during the Variscan orogeny, with a major peak at the end of the lower Carboniferous. We tentatively suggest the following geodynamic scenario for the orig…
Mineral chemical and geochronological constraints on the age and provenance of the eastern Circum-Rhodope Belt low-grade metasedimentary rocks, NE Greece
Abstract In north-eastern Greece the mid-greenschist facies Makri Unit and the anchizonal Melia Formation belong to the eastern Circum-Rhodope Belt that forms the uppermost tectonostratigraphic unit of the Rhodope metamorphic nappe pile. The two metasedimentary successions had different source areas, although they now lie in close proximity in the Rhodope Massif. The U–Pb isotopic ages of detrital zircons from a metasandstone of the Makri Unit analysed using LA–SF–ICP–MS and SHRIMP-II gave age clusters at ca. 310–290 Ma and at ca. 240 Ma for magmatic zircons, which may have been derived from Carboniferous–Permian basement rocks of the Thracia Terrane (Lower Tectonic Unit of the Rhodope Mass…
Triassic rift-related meta-granites in the Internal Hellenides, Greece
AbstractThe Serbo-Macedonian Massif is a basement complex in the Internal Hellenides of northern Greece, situated between the Vardar Zone to the west and the Rhodope Massif to the east. The Serbo-Macedonian Massif comprises several distinct basement units interpreted as terranes, the largest of which is the Gondwana-derived Vertiskos Terrane in the northwestern and central parts of the massif. A series of leucocratic meta-granites intrude the Silurian orthogneiss basement of the Vertiskos Terrane. No similar granites are found in any of the other units of the Internal Hellenides. The meta-granites have a pronounced crustal within-plate signature which is visible in lithology, major- and tra…
Serbo-Macedonian revisited: A Silurian basement terrane from northern Gondwana in the Internal Hellenides, Greece
Abstract New geochronological and geochemical data on basement orthogneisses from the Vertiskos Unit of the Serbo-Macedonian Massif (SMM), Internal Hellenides, northern Greece, are used in order to constrain the pre-Alpine tectonic history of the basement units in the metamorphic hinterland of the Hellenides. The prevailing rock types in the Vertiskos crystalline basement are coarse-grained biotite augengneisses with subordinate leucocratic muscovite gneisses and two-mica gneisses. Zircon Pb–Pb and U–Pb ages on 20 samples range from 425.9 ± 4.2 Ma to 443.4 ± 5.5 Ma with a mean of 432.2 ± 3.2 Ma and are interpreted as primary crystallisation ages of the basement granites on the basis of the …
Geochronology of the mid-German crystalline rise west of the River Rhine
The mid-German crystalline rise has its westernmost exposures at the western margin of the Rhine graben in the southern Pfalz and the northern Alsace. The outcrops are made up of granitoid rocks and minor volcano-sedimentary sequences. Radiometric ages obtained by U/Pb, Pb/Pb, Sm/Nd and Rb/Sr analyses of the igneous rocks from this area range from ∼433 to ∼325 Ma thus covering a time span from the Silurian to the end of the lower Carboniferous. Because the investigated rocks are — according to their chemical composition — largely related to subduction zone environments, the following three geodynamical scenarios are postulated, always taking subduction of oceanic crust beneath the mid-Germa…
The weak and superfast Cretan detachment, Greece: exhumation at subduction rates in extruding wedges
Low-angle normal faults (detachments) are only efficient agents for bringing rocks from 40–>100 km depth back to the Earth’s surface if they operate with extreme slip rates exceeding 20 km Ma −1 . Here we propose a slip rate of ≧20–30 km Ma −1 for the Cretan detachment in the Aegean, Greece. The Cretan detachment and the subjacent subduction thrust bounded an extruding wedge above the Miocene Hellenic subduction zone. During exhumation the high-pressure rocks in the wedge were not significantly deformed. Very low shear coupling at the bounding faults (which are therefore weak) is needed to prevent significant deformation in the extruding wedge. The proposed slip rate of ≧20–30 km Ma −1 is s…
Geochemical constraints on the provenance and depositional setting of sedimentary rocks from the islands of Chios, Inousses and Psara, Aegean Sea, Greece: implications for the evolution of Palaeotethys
The provenance and depositional setting of Late Palaeozoic and Early Mesozoic clastic sediments from the eastern Aegean archipelago are examined here for the first time using whole-rock geochemistry and composition of detrital chrome spinel. Major- and trace-element data for Late Palaeozoic and Permo-Triassic clastic sediments from the Lower and Upper Units of Chios are compatible with an acidic to intermediate source, minor input of (ultra)mafic detritus and recycling of older sedimentary components. Chondrite-normalized REE profiles are uniform with light REE enrichments (La N /Yb N c . 7.7), negative Eu anomalies (Eu/Eu* c . 0.67) and flat heavy REE patterns (Gd N /Yb N c . 1.5), indicat…
The oldest rocks of Greece: first evidence for a Precambrian terrane within the Pelagonian Zone
The Pelagonian Zone in Greece represents the westernmost belt of the Hellenide hinterland (Internal Hellenides). Previous geochronological studies of basement rocks from the Pelagonian Zone have systematically yielded Permo-Carboniferous ages. In this study we demonstrate, for the first time, the existence of a Precambrian crustal unit within the crystalline basement of the Pelagonian Zone. The U–Pb single-zircon and SHRIMP ages of these orthogneisses vary from 699 ± 7 Ma to 713 ± 18 Ma, which identify them as the oldest rocks in Greece. These Late Proterozoic rocks, which today occupy an area of c. 20 × 100 km, are significantly different from the neighbouring rocks of the Pelagonian Zone.…
Age and origin of granitic rocks of the eastern Vardar Zone, Greece: new constraints on the evolution of the Internal Hellenides
The Vardar Zone is an integral part of the Internal Hellenides and has long been regarded as an ophiolite-decorated suture zone separating two distinct continental blocks, namely the Pelagonian Zone to the west and the Serbo-Macedonian Massif to the east. Several bodies of granites, gneisses and volcanic rocks are associated with the ophiolitic rocks and can provide additional constraints on the evolution of the suture. Single-zircon and monazite dating of felsic rocks yields accurate ages for the processes of accretion of the suture. The igneous formation ages obtained range from 155 to 164 Ma, suggesting an important magmatic phase in the Late Jurassic. The chemical and isotopic compositi…
Rutile chemistry and thermometry as provenance indicator: An example from Chios Island, Greece
Abstract In the present study we focused on detrital rutile separated from 12 psammitic samples that belong to three different sedimentary successions (Carboniferous, Permo-Carboniferous, Permo-Triassic) occurring on Chios Island, Greece. The Ti, Cr, Al, Fe, Nb, Zr, Si, and V contents of the rutiles were obtained by electron-microprobe analyses to trace their provenance. The Cr and Nb concentrations of the analysed rutile grains show a wide range and indicate that this mineral in the Carboniferous succession is mainly derived from metamafic rocks, whereas in the Permo-Carboniferous and Permo-Triassic successions stem from a metapelitic source. The calculated formation temperatures using the…
Geochemistry, provenance and stratigraphic age of metasedimentary rocks from the eastern Vardar suture zone, northern Greece
Abstract Uppermost Palaeozoic and Mesozoic sediments from the Vardar suture zone in northern Greece record a cycle of Neotethyan oceanic basin opening and closure and their analysis places tight constraints on the geotectonic evolution of the area. The oldest post-Carboniferous sedimentary unit in the study area is the Examili Formation, which comprises mainly metaarkoses and metaquartzites, and was deposited in an intracontinental rift-related sedimentary basin in proximity to the Vertiskos Terrane during the Permian–Triassic. The Melissochori Formation (former Svoula flysch) comprises predominantly metasandstones with significant carbonate content and was deposited in front of a Carbonife…
Mechanism and time of deformation and metamorphism of mylonitic orthogneisses from the Shagou Shear Zone, Qinling Belt, China
Abstract The Shagou Shear Zone south of Xian (Shaanxi Province) is a branch of the Shangdan Fault Zone in the Qinling Belt and documents ductile deformation that affected granitoid rocks. Mylonitization took place under conditions of the lower amphibolite facies at temperatures of ca. 530°C, followed by a phase of stagnation, still under the same metamorphic conditions. The last deformation was a weak event during retrogressive greenschist metamorphism. The emplacement of the granitoid precursors of the mylonite is dated at 211 ± 8 Ma by the U/Pb zircon method, much younger than Nd model ages of 1.2–1.3 Ga, which indicates the involvement of older continental crust, probably in an active co…
Geochemical constraints on the provenance and depositional setting of sedimentary rocks from the islands of Chios, Inousses and Psara, Aegean Sea, Greece: implications for the evolution of Palaeotethys
The provenance and depositional setting of Late Palaeozoic and Early Mesozoic clastic sediments from the eastern Aegean archipelago are examined here for the first time using whole-rock geochemistry and composition of detrital chrome spinel. Major- and trace-element data for Late Palaeozoic and Permo-Triassic clastic sediments from the Lower and Upper Units of Chios are compatible with an acidic to intermediate source, minor input of (ultra)mafic detritus and recycling of older sedimentary components. Chondrite-normalized REE profiles are uniform with light REE enrichments (LaN/YbN c. 7.7), negative Eu anomalies (Eu/Eu* c. 0.67) and flat heavy REE patterns (GdN/YbN c. 1.5), indicating an up…