Klaus Wemmer

Geochronological constraints on the evolution of the southern Dom Feliciano Belt (Uruguay)

Abstract: New U–Pb sensitive high-resolution ion microprobe, K–Ar and Ar–Ar data from the southernmost Dom Feliciano Belt allow the identification of four major events. Orthogneisses from the Punta del Este Terrane indicate a magmatic episode at c. 770 Ma and high-grade metamorphism at 641 ± 17 Ma. Granitoid emplacement at 627 ± 23 Ma was roughly coeval with peak metamorphism. Volcaniclastic rocks of the Las Ventanas Formation dated at 573 ± 11 Ma can be correlated with the peripheral foreland basin (571 ± 8 Ma). Transpression and coeval high-K calc-alkaline magmatism is recorded in the Maldonado granite dated at 564 ± 7 Ma. The following events are postulated: (1) magmatism at 850–750 Ma related to rifting; (2) metamorphism and granite emplacement at 650–600 Ma; (3) molasse sequences and foreland basins at c. 573 Ma; (4) late magmatism at 580–560 Ma associated with transpression. The data indicate that (1) the Punta del Este Terrane could be a portion of the Coastal Terrane of the Kaoko Belt, (2) granitoid emplacement at 650–600 Ma in the Punta del Este and Nico Pérez terranes favours westward subduction, and (3) widespread post-collisional synkinematic magmatism occurred in the Dom Feliciano and Kaoko belts between 580 and 550 Ma. Supplementary material: Analytical methods and data are available at http://www.geolsoc.org.uk/SUP18369.

Neoproterozoic to Early Palaeozoic events in the Sierra de San Luis: implications for the Famatinian geodynamics in the Eastern Sierras Pampeanas (Argentina)

The application of the sensitive high-resolution ion microprobe (SHRIMP) U/Pb dating technique to zircon and monazites of different rock types of the Sierra de San Luis provides an important insight into the provenance and timing of deposition of the sedimentary precursors as well as the metamorphic and igneous history of the various basement domains. Additional constraints on the Famatinian metamorphic episode are provided by Pb/Pb stepwise leaching experiments on one staurolite and two garnet separates. The results indicate that the sedimentary precursors of the Conlara Metamorphic Complex have a maximum age of c. 590 Ma, whereas the Pringles Metamorphic Complex metasediments appear to be sourced from the Pampean orogen in the Early Cambrian. Folded xenoliths within the c. 496 Ma El Peñón pluton suggest that the host Conlara Metamorphic Complex underwent a Pampean compression. From a 208Pb/232Th monazite age of 478 Ma for a migmatite from the Nogolí Metamorphic Complex, the structural evolution of this basement complex appears to be entirely post-Pampean. Onset of the Famatinian high-grade metamorphism, between c. 500 Ma and c. 450 Ma, follows a period of crustal extension on the western outboard of Gondwana and might not be related directly to a Mid-Ordovician accretion of the Cuyania Terrane.

Origin of illite in the lower Paleozoic of the Illinois basin: Evidence for brine migrations

In the lower Paleozoic of the Illinois Basin, three illite polytypes are found: 2M 1 of detrital origin, and 1M d and 1M of diagenetic origin. Illite polytype quantification of detrital 2M 1 illite and diagenetic 1M d and 1M illite, combined with K-Ar age dating, allows extrapolation to apparent detrital and diagenetic illite ages. Kinetic modeling of smectite illitization, combined with the calculated age of illitization, can evaluate different origins of illite. The diagenetic illite in the lower Paleozoic of the Illinois Basin is interpreted not to have formed solely by burial diagenesis but mainly during multiple brine events. The Upper Ordovician Maquoketa Group contains diagenetic illite (dominantly 1M d with minor 1M ) with an extrapolated age of ∼360 m.y. (356–377 m.y.) and formed from smectite at temperatures of 50–100 °C. This age falls within the span of dates for illite/smectite (I/S) in K-bentonites from the Upper Mississippi Valley and is interpreted to be a combined result of illitization by burial diagenesis and either a hydrothermal brine from the southern and deeper part of the basin or a K-rich brine from the Michigan Basin, Upper Mississippi Valley area, or Forest City Basin. In Ordovician and Cambrian shale partings and sandstone older than the Maquoketa Group, the diagenetic illite ( 1M d in shale and 1M in sandstone) has an age of ∼300 m.y. and formed at temperatures <140 °C. This late Paleozoic age falls within the range of illites from sandstone in the Upper Mississippi Valley and K-bentonites of the Appalachian Basin; it coincides with the Alleghany orogeny and is interpreted as having formed by gravity-driven flow from the uplifted Alleghanian-Ouachita orogenic belt that drove hot (<140 °C) fluids through the Illinois Basin.

Accretion of first Gondwana-derived terranes at the margin of Baltica

Abstract In central Europe, three crustal units, i.e. the Małopolska, the Łysogóry and the Bruno-Silesia, can be recognized by basement data, faunas and provenance of clastic material in the Cambrian clastic rocks. They are now situated within the Trans-European Suture Zone, a tectonic collage of continental terranes bordering the Tornquist margin of the palaeocontinent of Baltica, but during the Cambrian their position in relation to each other and to Baltica was different from today. These units are exotic terranes in respect to Baltica and are interpreted as having been derived from the Cadomian margin of Gondwana. Their detachment is probably related to the final break-up of the supercontinent Rodinia at c. 550–590 Ma. New detrital zircon and muscovite age data provide evidence that Małopolska was derived from the segment of the Cadomian orogen that bordered the Amazonian Craton. It must have already separated from Gondwana in Early Cambrian time (some 40–50 Ma before Avalonia became detached and began its rapid drift). The accretion of Małopolska to Baltica occurred between late mid-Cambrian and Tremadocian times. Both palaeontological and provenance evidence demonstrate that Małopolska and not Avalonia was the first terrane to join the Baltica palaeocontinent. This event initiated the progressive crustal growth of the European lithosphere, which continued during Phanerozoic times and led to the formation of modern Europe.

The Baltica-Gondwana suture in central Europe: evidence from K-Ar ages of detrital muscovites and biogeographical data

Abstract The Lysogory Unit, the Malopolska Massif and the Upper Silesian Massif in southern Poland are parts of a mosaic of contrasting crustal fragments separating the old Precambrian crust of the East European Platform (EEP) from the Phanerozoic mobile belts of western Europe. The geological histories of these blocks are markedly different. They have been regarded as integral parts of the palaeocontinent of Baltica (that is, the EEP), mostly because of presence of fossils typical for the Baltic realm, although geophysical and geological data and some faunal elements rather suggest linkages to the Peri-Gondwana plates. To provide additional constraints for the plate tectonic affinity of these blocks detrital muscovite grains extracted from Cambrian and Devonian clastic rocks were dated by the K-Ar method. The K-Ar cooling ages show a very complex provenance pattern for clastic material in Cambrian time. Combined with the biogeographical constraints, the new provenance data apparently show that the blocks of Lysogory, Malopolska, and Upper Silesia are in fact crustal fragments derived from the Gondwana margin, not displaced parts of the East European Craton. Thus, the Teisseyre-Tornquist Line (that is, the edge of the EEP) is the Baltica-Gondwana suture in central Europe. The combined data reveal an accretionary scenario in which the Malopolska Block was the first Gondwana-derived microplate that accreted to the margin of Baltica.

Thermal and Tectonic History of the Ordos Basin, China: Evidence from Apatite Fission Track Analysis, Vitrinite Reflectance, and K-Ar Dating

Apatite fission track analysis, vitrinite reflectance data, and K-Ar dating of Permian-Carboniferous and Mesozoic core samples have been successfully integrated to reconstruct the thermal and tectonic history of the Ordos basin, China. Apatite fission track ages of Carboniferous-Jurassic sedimentary rocks range between 3 and 137 Ma, and are significantly younger than the stratigraphic ages. Confined fission track lengths demonstrate exclusively mixed length distribution, indicating complex thermal history. The data suggest that the samples must have all experienced higher paleotemperatures in the past. Mean vitrinite reflectance values (R0) of the Triassic rocks range from 0.61 to 1.06%, giving a high coalification gradient of 0.36%/km and suggesting a high paleothermal gradient of 57°C/km. Permian-Carboniferous rocks have Ro values on the order of 1.0-3.0%, and locally up to 4.0-6.0%. Some high R0 values coincide with positive gravity and magnetic anomalies. K-Ar dating on Permian-Triassi samples reveals distinct illitization at 170-160 Ma, during which a thermal event occurred due to subsurface magmatic intrusion related to the early Yanshanian movement. The petroleum source rocks of the Upper Triassic experienced peak temperatures ranging from 90 to 160°C, corresponding to the oil window, and Permian-Carboniferous source rocks were heated to more than 150°C, passing through and out of the gas window. Due to rapid uplift and erosion in response to the rise of the Qinghai-Tibet plateau associated with the Asia-India collision and the Himalayan orogeny, cooling has taken place at least since approximately 23 Ma. The difference in the rate and amount of uplift between the eastern and western parts of the basin resulted in differential uplift and the present-day s ructural pattern of the basin.

Hercynian deformation and metamorphism in the Cordillera Oriental of Southern Bolivia, Central Andes

Abstract In southern Bolivia the very thick Ordovician siliciclastic rocks of the Cordillera Oriental show folds and a slaty cleavage of pre-Cretaceous age. These deformations had previously been attributed to either the late Ordovician Ocloyic or to the late Devonian to early Carboniferous Eohercynian (Chanic) orogenies. Now, we present K/Ar age determinations from phyllosilicates of the Ordovician slates, which have been interpreted in relation to illite crystallinity data. High anchizonal to epizonal metamorphism is indicated for most of the investigated samples. The majority of the samples have provided ages within the 320–290 Ma interval (late Carboniferous to early Permian), indicating a late Hercynian orogeny. Traces of synchronous orogenic processes are known from different parts of the Central Andes. This points to late Hercynian orogenic activities in that region.

Timing of deformation in the Sarandí del Yí Shear Zone, Uruguay: implications for the amalgamation of Western Gondwana during the Neoproterozoic Brasiliano–Pan‐African Orogeny

U-Pb and Hf zircon (sensitive high-resolution ion microprobe -SHRIMP- and laser ablation-inductively coupled plasma-mass spectrometry -LA-ICP-MS-), Ar/Ar hornblende and muscovite, and Rb-Sr whole rock-muscovite isochron data from the mylonites of the Sarandi del Yi Shear Zone, Uruguay, were obtained in order to assess the tectonothermal evolution of this crustal-scale structure. Integration of these results with available kinematic, structural, and microstructural data of the shear zone as well as with geochronological data from the adjacent blocks allowed to constrain the onset of deformation along the shear zone at 630–625 Ma during the collision of the Nico Perez Terrane and the Rio de la Plata Craton. The shear zone underwent dextral shearing up to 596 Ma under upper to middle amphibolite facies conditions, which was succeeded by sinistral shearing under lower amphibolite to upper greenschist facies conditions until at least 584 Ma. After emplacement of the Cerro Caperuza granite at 570 Ma, the shear zone underwent only cataclastic deformation between the late Ediacaran and the Cambrian. The Sarandi del Yi Shear Zone is thus related to the syncollisional to postcollisional evolution of the amalgamation of the Rio de la Plata Craton and the Nico Perez Terrane. Furthermore, the obtained data reveal that strain partitioning and localization with time, magmatism emplacement, and fluid circulation are key processes affecting the isotopic systems in mylonitic belts, revealing the complexity in assessing the age of deformation of long-lived shear zones.

Exhumation and deformation history of the lower crustal section of the Valstrona di Omegna in the Ivrea Zone, southern Alps

Abstract The Ivrea Zone (southern Alps) is one of the key regions interpreted as exposing a section of the lower continental crust and was the subject of several review-type articles. The Ivrea–Verbano Zone was rotated into an upright position along the Insubric mylonite belt. In the southeast, this unit is in contact with the Strona Ceneri Zone, which is interpreted as upper continental crust crossing the Permian Cossato–Mergozzo–Brissagio Line (CMB Line). The CMB mylonites are locally overprinted by the mylonites and cataclasites of the Pogallo Line, which was active during the Jurassic. In addition, the sinistral, steeply inclined Rosarolo shear zone was active over a long time span from the ductile into the brittle field, i.e. from the Early Permian (high-temperature ultra-mylonites) to the Neo-Alpine basic dykes and pseudotachylites. The high-temperature mylonites accommodated crustal extension and may be related to normal faults generated by magmatic underplating. The reactivation at different crustal levels during exhumation and tilting is documented by strain increments at decreasing P/T conditions. Its present subvertical orientation was attained during the Neo-Alpine deformation. Constraints on its exhumation history are based on new 40Ar/39Ar hornblende ages, K–Ar biotite ages and zircon fission-track data along the NE–SW trending Valstrona section. A re-interpretation of existing U–Pb monazite ages is included, based on a higher closure temperature for monazite. The oldest monazite ages are observed in proximity to the Pogallo Line (c. 292 Ma). Heat input by mafic intrusions was sufficient to reset the U–Pb monazite system, as is evidenced by the youngest ages in the vicinity of the Insubric Line. The re-interpretation favours the hypothesis that the oldest monazite ages are the result of complete resetting by a Permian thermal event. The 40Ar/39Ar hornblende ages and K–Ar biotite ages document the cooling after Permian heating. Roughly parallel age progressions decrease from the Pogallo Line (hornblende: 271 Ma vs. biotite: 227 Ma) towards the Insubric Line (hornblende: 201 Ma vs. biotite: 156 Ma). Zircon fission-track ages run parallel to the biotite ages in the upper part of the profile, whereas towards the Insubric Line a significant deviation from the biotite age progression is attributed to tilting of the basement during the Oligocene. Zircon fission-track ages around 38 Ma are found close to the Insubric Line. No age offset, neither at the CMB nor at the Pogallo Line, is observed. This confirms the hypothesis that the Pogallo Line is an oblique normal fault, and that the CMB Line has accommodated only minor vertical displacement. The capture of the different cooling ages confirms the tilting of the Ivrea–Verbano Zone during the Neo-Alpine deformation and contradicts the tilting of the Ivrea–Verbano Zone during the Permian.

Metamorphic evolution of the Tethyan Himalayan flysch in SE Tibet

Abstract The metamorphic conditions and the age of thermal overprint were determined in metapelites, metaarenites and metabasites of the Tethyan Himalayan Sequence (THS) in SE Tibet using Kübler Index and vitrinite reflectance data and applying thermobarometrical (Thermocalc and PERPLEX) and geochronological methods (illite/muscovite K–Ar and zircon and apatite (U–Th)/He chronology). The multiple folded thrust pile experienced a thermal overprint reaching locally peak conditions between the diagenetic stage (c. 170 °C) and the amphibolite facies (c. 600 °C at 10 kbar). Burial diagenesis and heating due to Early Cretaceous dyke emplacement triggered the growth of illite in the metapelites. Eocene collision-related peak metamorphic conditions have been reached at c. 44 Ma. During collision the different tectonic blocks of the THS were tectonically buried to different structural levels so that they experienced maximum greenschist to amphibolite facies metamorphism. Later, during Oligocene to Miocene times the entire THS underwent anchi- to epizonal metamorphic conditions, probably associated to continuous deformation in the flysch fold-thrust-system. This period terminated at c. 24–22 Ma. Adjacent to the north Himalayan metamorphic domes, the base of the THS was metamorphosed during Miocene times (c. 13 Ma). Post-metamorphic cooling below c. 180 °C lasted until Late Miocene and took place at different times.