Distribution of elements among minerals of a single (muscovite-) biotite granite sample – an optimal approach and general implications
Abstract: The petrography and mineral chemistry of the coarse-grained, weakly porphyritic (muscovite-) biotite Říčany granite (Variscan Central Bohemian Plutonic Complex, Bohemian Massif) were studied in order to assess the distribu- tion of major and trace elements among its minerals, with consequences for granite petrogenesis and availability of geochemical species during supergene processes. It is demonstrated that chemistry-based approaches are the best suited for modal analyses of granites, especially methods taking into account compositions of whole-rock samples as well as their mineral constituents, such as constrained least-squares algorithm. They smooth out any local variations (mineral zoning, presence of phenocrysts, schlieren…) and are robust in respect to the presence of phenocrysts or fabrics. The study confirms the notion that the accessory phases play a key role in incorporation of many elements during crystalli- zation of granitic magmas. Especially the REE seem of little value in petrogenetic modelling, unless the role of acces- sories is properly assessed and saturation models for apatite, zircon, monazite±rutile carefully considered. At the same time, the presence of several P-, Zr- and LREE-bearing phases may have some important consequences for saturation thermometry of apatite, zircon and monazite.
LA-ICP-MS U-Pb apatite dating of Lower Cretaceous rocks from teschenite-picrite association in the Silesian Unit (southern Poland)
Abstract: The main products of volcanic activity in the teschenite-picrite association (TPA) are shallow, sub-volcanic intrusions, which predominate over extrusive volcanic rocks. They comprise a wide range of intrusive rocks which fall into two main groups: alkaline (teschenite, picrite, syenite, lamprophyre) and subalkaline (dolerite). Previous 40Ar/39Ar and 40K/40Ar dating of these rocks in the Polish Outer Western Carpathians, performed on kaersutite, sub-silicic dio- pside, phlogopite/biotite as well as on whole rock samples has yielded Early Cretaceous ages. Fluorapatite crystals were dated by the U-Pb LA-ICP-MS method to obtain the age of selected magmatic rocks (teschenite, lamprophyre) from the Cieszyn igneous province. Apatite-bearing samples from Boguszowice, Puńców and Lipowa yield U-Pb ages of 103 ± 20 Ma, 119.6 ± 3.2 Ma and 126.5 ± 8.8 Ma, respectively. The weighted average age for all three samples is 117.8 ± 7.3 Ma (MSWD = 2.7). The considerably smaller dispersion in the apatite ages compared to the published amphibole and biotite ages is probably caused by the U-Pb system in apatite being less susceptible to the effects of hydrothermal alternation than the 40Ar/39Ar or 40K/40Ar system in amphibole and/or biotite. Available data suggest that volcanic activity in the Silesian Basin took place from 128 to 103 Ma with the the main magmatic phase constrained to 128—120 Ma.
New Lower Cretaceous basal mantodean (Insecta) from the Crato Formation (NE Brazil)
Abstract: Mantodea are very rare in the fossil record. 28 fossil species are reported since the earliest occurrence of mantodeans in the Upper Jurassic (Tithonian). Here, I describe Cretophotina santanensis n. sp. from the Aptian (Lower Cretaceous) Crato Formation of Chapada do Araripe (northeastern Brazil). This species is characterized by long an- tenna and primitive raptorial forelegs. Morphological characters shared with the living genus Chaeteessa would support its assignment to the family Chaeteessidae. The tropical occurrence of the Early Cretaceous genus Cretophotina in Gondwana, together with occurrences of the genus Chaetessa from subtropical and temperate zones of Laurasia, im- plies that members of the family Chaeteessidae achieved nearly cosmopolitan distribution during the Early Cretaceous.
Tectonic evolution of the Sicilian Maghrebian Chain inferred from stratigraphic and petrographic evidences of Lower Cretaceous and Oligocene flysch State of the art and objectives
Abstract: The occurrence of a Lower Cretaceous flysch group, cropping out from the Gibraltar Arc to the Balkans with a very similar structural setting and sedimentary provenance always linked to the dismantling of internal areas, suggests the existence of only one sedimentary basin (Alpine Tethys s.s.), subdivided into many other minor oceanic areas. The Maghrebian Basin, mainly developed on thinned continental crust, was probably located in the westernmost sector of the Alpine Tethys. Cretaceous re-organization of the plates triggered one (or more) tectonic phases, well recorded in almost all the sectors of the Alpine Tethys. However, the Maghrebian Basin seems to have been deformed by Late- or post-Cretaceous tectonics, connected with a “meso-Alpine” phase (pre-Oligocene), already hypothesized since the be- ginning of the nineties. Field geological evidence and recent biostratigraphic data also support this important meso- Alpine tectonic phase in the Sicilian segment of the Maghrebian Chain, indicated by the deformations of a Lower Cretaceous flysch sealed by Lower Oligocene turbidite deposits. This tectonic development is emphasized here because it was probably connected with the onset of rifting in the southern paleomargin of the European plate, the detaching of the so-called AlKaPeCa block (Auct.; i.e. Alboran + Kabylian + Calabria and Peloritani terranes) and its fragmentation into several microplates. The subsequent early Oligocene drifting of these microplates led to the progressive closure of the Maghrebian Basin and the opening of new back-arc oceanic basins, strongly controlled by extensional processes, in the western Mediterranean (i.e. Gulf of Lion, Valencia Trough, Provençal Basin and Alboran Sea).
Cretaceous—Quaternary tectonic evolution of the Tatra Mts (Western Carpathians): constraints from structural, sedimentary, geomorphological, and fission track data
Abstract: The Tatra Mts area, located in the northernmost part of Central Western Carpathians on the border between Slovakia and Poland, underwent a complex Alpine tectonic evolution. This study integrates structural, sedimentary, and geomorphological data combined with fission track data from the Variscan granite rocks to discuss the Cretaceous to Quater- nary tectonic and landscape evolution of the Tatra Mts. The presented data can be correlated with five principal tectonic stages (TS), including neotectonics. TS-1 ( ~ 95—80 Ma) is related to mid-Cretaceous nappe stacking when the Tatric Unit was overlain by Mesozoic sequences of the Fatric and Hronic Nappes. After nappe stacking the Tatric crystalline basement was exhumed (and cooled) in response to the Late Cretaceous/Paleogene orogenic collapse followed by orogen-parallel extension. This is supported by 70 to 60 Ma old zircon fission track ages. Extensional tectonics were replaced by transpression to transtension during the Late Paleocene to Eocene (TS-2; ~ 80—45 Ma). TS-3 ( ~ 45—20 Ma) is documented by thick Oligocene—lowermost Miocene sediments of the Central Carpathian Paleogene Basin which kept the underlying Tatric crystalline basement at elevated temperatures (ca. > 120 °C and < 200 °C). The TS-4 ( ~ 20—7 Ma) is linked to slow Mio- cene exhumation rate of the Tatric crystalline basement, as it is indicated by apatite fission track data of 9—12 Ma. The final shaping of the Tatra Mts has been linked to accelerated tectonic activity since the Pliocene (TS-5; ~ 7—0 Ma).