Ana-Voica Bojar

Cretaceous to Cenozoic thermal evolution of the southwestern South Carpathians: evidence from fission-track thermochronology

Abstract The southwestern South Carpathian orogen is composed of various nappe complexes which were assembled during the Cretaceous–Cenozoic orogeny. These are from footwall to hangingwall: (1) the Danubian nappe complex including a Cadomian/Variscan basement; (2) the Arjana and Severin units with Jurassic to Early Cretaceous rift and oceanic sequences; and (3) the Getic nappe complex with Variscan continental basement. Fission track (FT) thermochronology on apatite, zircon and sphene from samples collected from various units of the South Carpathians, in conjunction with field constraints and previous geochronology is used to characterise the Alpine tectonic events and to restore the pattern and amount of exhumation since the Cretaceous. Zircon from the flysch unit and the Danubian Liassic cover sequence yields FT ages around 200 Ma suggesting cooling of the rift flanks prior to the opening of the Severin rift. Zircon and sphene from the Getic and Danubian basement units yield FT ages averaging 110 Ma and indicating cooling under 240°C of the basement contemporaneous with, or postdating thrusting. Apatite FT ages display a decreasing age trend from the hangingwall (65 Ma) to the footwall units (30 Ma). The age data and corresponding horizontal confined track length distributions suggest that exhumation of the nappe pile occurred in two stages: the first is related to the Late Cretaceous nappe stacking and the second one to the final thrusting of the South Carpathians onto the top of the Moesian platform. Apatite FT ages along major brittle wrench faults indicate reheating above ca. 120°C during fluid flow associated with fault (re)activation during Oligocene and Neogene times. Thus, shear zone rocks experienced a higher temperature overprint during Cenozoic time than rocks of the unaffected nappe pile. Temperatures of hydrothermal flow along these zones decreased below 100°C progressively starting with the Late Oligocene–Early Miocene when the area began to override the Moesian platform.

Middle Miocene seasonal temperature changes in the Styrian basin, Austria, as recorded by the isotopic composition of pectinid and brachiopod shells

Abstract Using the isotope record on pectinid and brachiopod shells, we reconstructed Middle Miocene palaeotemperatures for a shelf environment in the Styrian basin, Austria. Bivalve shells of Macrochlamys sp., collected from different horizons of the Lower Badenian Leitha limestone, show δ 18 O values in the range of −3 to 0‰ (PeeDee Belemnite). The large intrashell variability of 3‰ indicates significant seawater temperature fluctuation. The results suggest that despite the high geographical latitude (ca. 40°N), the climate in southern Austria was warm (subtropical) with a pronounced seasonality at that time. In contrast, the δ 18 O isotopic composition of a pectinid ( Flabellipecten sp.) and an indeterminate terebratulid brachiopod, from the siliciclastic deposits overlying the Leitha limestone, ranges between 0 and −1‰. This indicates cooler mean annual temperatures and reduced seasonal variations. A 39 Ar/ 40 Ar age on fresh volcanic biotites shows a value of 14.2±0.1 Ma. The tuffs containing the biotites are located below the layer containing the terebratulid and the Flabellipecten sp. The stable isotope and radiogenic data suggest that the drop in temperature is in direct relationship with the East Antarctic Ice Sheet expansion, which started at ca. 14 Ma. Due to the variation in temperature, the subtropical fauna in the Retznei quarry disappeared during the Early Miocene and the biogenic limestones were replaced by clastic sediments. The δ 18 O isotopic profiles from three pectinids collected from the Leitha limestone indicate that these grew within ca. 1.5 years. Growth interruptions occurred during the warm season because during this period the pectinids very likely used their energy for the growth and maturation of gonads. Carbon isotopic compositions vary between 0 and 2‰. Statistical tests show that for some of the analyzed shells, the cyclicity of the δ 13 C profiles may be explained by the temperature-dependent fractionation between air CO 2 and dissolved bicarbonate.

Phanerozoic tectonothermal history of the Arabian–Nubian shield in the Eastern Desert of Egypt: evidence from fission track and paleostress data

To constrain the post-Pan-African evolution of the Arabian–Nubian Shield, macro-scale tectonic studies, paleostress and fission track data were performed in the Eastern Desert of Egypt. The results provide insights into the processes driving late stage vertical motion and the timing of exhumation of a large shield area. Results of apatite, zircon and sphene fission track analyses from the Neoproterozoic basement indicate two major episodes of exhumation. Sphene and zircon fission track data range from 339 to 410 Ma and from 315 to 366 Ma, respectively. The data are interpreted to represent an intraplate thermotectonic episode during the Late Devonian–Early Carboniferous. At that time, the intraplate stresses responsible for deformation, uplift and erosion, were induced by the collision of Gondwana with Laurussia which started in Late Devonian times. Apatite fission track data indicate that the second cooling phase started in Oligocene and was related to extension, flank uplift and erosion along the actual margin of the Red Sea. Structural data collected from Neoproterozoic basement, Late Cretaceous and Tertiary sedimentary cover suggest two stages of rift formation. (1) Cretaceous strike-slip tectonics with sub-horizontal r1 (ENE/WSW) and r3 (NNW/SSE), and sub-vertical r2 resulted in formation of small pull-apart basins. Basin axes are parallel to the trend of Pan-African structural elements which acted as stress guides. (2) During Oligocene to Miocene the stress field changed towards horizontal NE–SW extension (r3), and subvertical r1. Relations between structures, depositional ages of sediments and apatite fission track data indicate that the initiation of rift flank uplift, erosion and plate deformation occurred nearly simultaneously. 2002 Elsevier Science Ltd. All rights reserved.

Geology, age and tectonic evolution of the Sierra Maestra Mountains, southeastern Cuba

We summarize the available geological information on the Sierra Maestra Mountains in southeastern Cuba and report new zircon fission track and biotite Ar-Ar ages for this region. Two different and genetically unrelated volcanic arc sequences occur in the Sierra Maestra, one Cretaceous in age (pre-Maastrichtian) and restricted to a few outcrops on the southern coast, and the other Palaeogene in age, forming the main expression of the mountain range. These two sequences are overlain by middle to late Eocene siliciclastic, carbonatic and terrigenous rocks as well as by late Miocene to Quaternary deposits exposed on the southern flank of the mountain range. These rocks are britle deformed and contain extension gashes filled with calcite and karst material. The Palaeogene volcanic arc successions were intruded by calc-alkaline, low- to medium-K tonalites and trondhjemites during the final stages of subduction and subsequent collision of the Caribbean oceanic plate with the North American continental plate. U-Pb SHRIMP single zircon dating of five granitoid plutons yielded 206Pb/238U emplacement ages between 60.5 ± 2.2 and 48.3 ± 0.5 Ma. These granitoids were emplaced at pressures ranging from 1.8 to 3.0 kbar, corresponding to depths of ca. 4.5-8 km. 40Ar/39Ar dating of two biotite concentrates yielded ages of 50 ± 2 and 54 ± 4 Ma, indicating cooling through ca. 300 oC. Zircon and apatite fission track ages range from 32 ± 3 to 46 ± 4 Ma and 31 ± 10 to 44 ± 13 Ma, respectively, and date cooling through 250 ± 50 oC and 110 ± 20 oC. The granitoids are the result of subduction-related magmatism and have geochemical characteristics similar to those of magmas from intra-oceanic island-arcs such as the Izu Bonin-Mariana arc and the New Britain island arc. Major and trace element patterns suggest evolution of these rocks from a single magmatic source. Geochemical features characterize these rocks as typical subduction-related granitoids as found worldwide in intra-oceanic arcs, and they probably formed through fractional crystallization of mantlederived low- to medium-K basalts. Several distinct phases of deformation were recognized in the Sierra Maestra, labelled D1 to D6, which define the transition from collision of the Palaeogene island arc to the formation of the Oriente Transform Wrench Corridor south of Cuba and later movement of the Caribbean plate against the North American plate. The first phase (D1) is related to the intrusion of a set of extensive subparallel, N-trending subvertical basalt-andesite dykes, probably during the early to middle Eocene. Between the late-middle Eocene and early Oligocene (D2), rocks of the Sierra Maestra were deformed by approximately east-west trending folds and north-vergent thrust faults. This deformation (D2) was linked to a shift in the stress regime of the Caribbean plate from mainly NNE-SSW to E-W. This shift in plate motion caused the abandonment of the Nipe-Guacanayabo fault system in the early Oligocene and initiation of a deformation front to the south where the Oriente Transform Wrench corridor is now located. Compressive structures were overprinted by widespread extensional structures (D3), mainly faults with southward-directed normal displacement in the Oligocene to early Miocene. During this period the plate boundary jumped to the Oriente fault. This event was followed by transpressive and transtensive structures (D4–D6) due to further development of the sinistral E-trending Oriente Transform wrench corridor. These structures are consistent with oblique convergence in a wide zone of left-lateral shear along an E-W-oriented transform fault.

Upper Cretaceous volcanoclastic deposits from the Haţeg basin, south Carpathians (Romania): K-Ar ages and intrabasinal correlation

In order to constrain the age of the Upper Cretaceous continental Densuş-Ciula Formation from the Haţeg basin, South Carpathians, and correlate it with the other continental unit that occurs in the region, the Sânpetru Formation, we separated and dated by the K-Ar method biotites and amphiboles from volcanoclastic deposits. The mineral phases analysed are from two tuff layers and volcanic bombs cropping out near Rachitova village. Two tuff layers from the Densuş-Ciula Formation give early Maastrichtian ages of 69.8±1.3 and 71.3±1.6 Ma, respectively. The ages determined for the tuff layers constrain the age of deposition for the Densuş-Ciula Formation and enable further correlations with the available palaeomagnetic data from the deposits occurring along the Sibişel Valley that belong to the Sânpetru Formation. The volcanic bombs collected near to Răchitova village are andesites and dacites. The age determined by K-Ar method on hornblende separated from a volcanic bomb is 82.7±1.5 Ma, which is older than the underlying Campanian marine deposits in turbidite facies. This suggests that the volcanic bombs were re-deposited during the early Maastrichtian. Thus, the volcanics found at Răchitova have at least two origins: one type is related to an explosive synsedimentary volcanic activity, and the other type is represented by older andesitic/dacitic bombs, which most probably originate from a volcanic centre situated in the Haţeg region.

Evolution of veins and sub-economic ore at Strassegg, Paleozoic of Graz, Eastern Alps, Austria: evidence for local fluid transport during metamorphism

The north-eastern part of the Graz Paleozoic thrust sheet at Strassegg, Austria is characterised by metamorphic assemblages that reflect increasing temperature conditions from 3008C in hangingwall to around 5508C in footwall units. The main quartz andror carbonate vein generation cuts the schistosity at moderate to low angles is folded with the schistosity and is commonly boudinaged. There is a correlation between vein- and the host-rock mineralogy. Additionally the d 18 O and 13 Ž. d C signatures of vein minerals quartz andror carbonates vary systematically with the isotopic signature of the host rock and its minerals. Within individual units, d 18 O of vein quartz has a narrow range of values in contrast with a larger variation in both with d 18 O and d 13 C of carbonates. At Strassegg, the greenschist unit hosts an As-Au mineralisation. Arsenopyrite, Ž.

Geochemical record of Late Devonian to Early Carboniferous events, Palaeozoic of Graz, Eastern Alps, Austria

Abstract The late Famennian mass extinction has been associated with a worldwide anoxic event evidenced at many sites by the accumulation of organic-rich shales starting prior to the Devonian–Carboniferous (D/C) boundary. Environmental reconstructions are hampered by the fact that only a few D/C boundary sections exist in carbonate facies. One of these is the Palaeozoic Steinberg section, Eastern Alps, Austria. Based on detailed geochemical investigations and correlation with previously investigated sections, we propose a succession of events from the late Famennian to the Tournaisian. Both the low total rare earth element (REE) contents and the normalized REE distribution patterns indicate low terrigenous input from the praesulcata Zone to the typicus Zone. The δ13C, δ18O, δ34S and total organic content (TOC) show a shift towards higher values, which correlates well with the late Famennian isotopic trends that are related to a period of active ocean circulation, cooling and increased productivity. Two isotopic shifts are present in the section: the older isotopic shift of the δ13C values is situated at the level of the Lower to Middle praesulcata Zone, while the younger one is found at the level of the Upper praesulcata Zone. In the sulcata Zone, a sharp decrease in δ18O values and a shift in the Ce anomaly follow. Concomitantly, a drop in the δ13C and δ34S values, highly variable TOC values, and the main and secondary element concentrations are interpreted to be related to a drop in primary productivity, with the recovery continuing in the crenulata Zone. In this context, the variations towards lower CaO, Sr, Rb and Ba values probably correlate with biomass variations and a transition from cooler towards warmer conditions.

Isotopic Studies in Cretaceous Research

The Cretaceous was a period characterized by very warm climate, oceanic anoxic and oxic events and enhanced volcanic activity. The end of the Cretaceous is punctuated by a well-documented asteroid impact and the extinction of, among other groups, the dinosaurs. This volume elucidates various aspects of Cretaceous marine and continental environmental conditions. The articles in this book present a broad range of interdisciplinary contributions, which are grouped into sections on marine environments(including anoxic and oxic events, volcanism and the Cretaceous–Palaeocene boundary); mixed marine–freshwater environments and continental records. The isotopic data are combined with further geochemical, palaeontological, lithological and mineralogical proxies. The interdisciplinary approach offered here gives a solid investigation base for this fascinating period. There are examples from Europe, Asia, South and North America, and from the Early Cretaceous to the Cretaceous–Paleogene boundary.

The Cretaceous–Paleogene boundary in the East Carpathians, Romania: evidence from geochemistry, mineralogy and calcareous nannofossils

Abstract We performed detailed geochemical, mineralogical and calcareous nannofossils investigations on two Maastrichtian–Paleocene sections situated in the Eastern Carpathians, Romania. One of the sections is situated in the Carpathian bend area, in the Ialomiţa Valley and is composed of red marine hemipelagic sediments. The other section is located in the northern region of the Eastern Carpathians, along Varniţa Brook and consists mainly of turbidites. In both sections, the Cretaceous–Paleogene (K–Pg) boundary is characterized by nannoplankton mass extinction (over 90%) of the Cretaceous nannofloras, followed by blooms of survivor taxa, such as the calcareous dinoflagellate genus Thoracosphaera and the nannofloral species Braarudosphaera bigelowii. In the Ialomiţa section, the K–Pg boundary is marked by a drop in calcite and TOC content, as well as a negative excursion in the δ13C values of organic material. These trends are only partly present in the Varniţa Brook section, where both calcite and TOC content show a decrease, but no negative excursion is seen in the δ13C values of organic material.

Bison bonasus skull from the Bihor Mountains, Romania: Isotopic and morphological investigations

In this study, we investigated morphologically and geochemically a skull from an open pit situated on the Călineasa-Şesu Gârzii Plateau, Bihor Mountains. The study presents the first 14C dating and stable isotope composition of bison skeletal remains from the Romanian Carpathians. The result of 14C determination yielded two relevant maxima: one is around AD 1550 and the second around AD 1645. The data indicate that the investigated bison died during the ‘Little Ice Age’ (LIA), approximately 200 years before the last ones were hunted to extinction. Stable isotope composition of carbon from bison tooth enamel suggests that the bison diet consisted mainly of C3 grasses, compatible with a high-altitude habitat, vegetation distribution and low mobility. Oxygen isotope composition of both phosphate and carbonate groups of tooth enamel indicates that the bison drank from stagnant water sources, such as lakes or puddles formed from rain water. This further suggests low mobility of the bison as well, as the presence of such small stagnant water sources is characteristic even today for the plateau. Calculated oxygen isotope compositions of rain and drinking water at the time the bison lived indicate a lower relative humidity, around 60–70% at that period of LIA when the bison lived, which is lower than today’s humidity of 80%.