| 1. |
|
|
| 2. |
|
|
| 3. |
- Hansman, Reuben J., et al.
(författare)
-
Absolute ages of multiple generations of brittle structures by U-Pb dating of calcite
- 2018
-
Ingår i: Geology. - 0091-7613 .- 1943-2682. ; 46:3, s. 207-210
-
Tidskriftsartikel (refereegranskat)abstract
- Direct dating of brittle structures is challenging, especially absolute dating of diagenesis followed by a series of superimposed brittle deformation events. We report 22 calcite U-Pb ages from tectonites and carbonate host rocks that date 3 diagenetic and 6 brittle deformation events. Results show that U-Pb dating of calcite fibers from these structures is compatible with overprinting relationships. Ages indicate that diagenesis occurred between 147 +/- 6 Ma and 103 +/- 34 Ma, and was followed by top-to-the-south, layer-parallel shearing due to ophiolite obduction at 84 +/- 5 Ma (2 sigma errors). Sheared top-to-the- northeast, layer-parallel veins were dated as 64 +/- 4 Ma and are interpreted to have developed during postobduction exhumation. After this event, a series of strike-slip structures, which crosscut and reactivated older faults due to northwest-southeast horizontal shortening, were dated as 55 +/- 22 Ma and 43 +/- 6 Ma. Eight ages from strike-slip faults and thrusts resulting from northeast-southwest shortening range from 40.6 +/- 0.5 Ma to 16.1 +/- 0.2 Ma. The youngest ages are from minor overprinting fibers ranging in age between 7.5 +/- 0.9 Ma and 1.6 +/- 0.6 Ma. Our results show that U-Pb dating of calcite fibers can be successfully used to constrain a complicated succession of brittle deformation structures that encompasses two orogenies and an intervening extension period.
|
|
| 4. |
- Kalsbeek, Feiko, et al.
(författare)
-
Enigmatic 1146 ± 4 Ma old granite in the southeastern rim of the West African craton, now part of the Dahomeyan orogenic belt in Ghana
- 2020
-
Ingår i: Journal of African Earth Sciences. - : Elsevier BV. - 1464-343X. ; 167
-
Tidskriftsartikel (refereegranskat)abstract
- A unique exposure of a strongly deformed late Mesoproterozoic augen-granite (U–Pb zircon age 1146 ± 4 Ma) was detected within rocks on the eastern rim of the West African Craton. These rocks, together with their sedimentary cover, were reworked during the Neoproterozoic Pan-African Dahomeyan orogeny, and are now incorporated in the Dahomeyan orogenic belt of southeast Ghana. To the authors’ knowledge no similar ages have been reported from elsewhere in the West African Craton or in the Pan-African orogenic belts farther north or their continuation in northeastern Brazil. Most other granitoid rocks in the Dahomeyan belt are of Palaeoproterozoic age. Two of them were dated for this study and yielded Concordia ages of 2067 ± 15 Ma and 2119 ± 13 Ma. Lower intercepts around 600 Ma in Wetherill discordia diagrams demonstrate disturbance of the U–Pb systems during the Pan-African orogenic cycle. Chemical data indicate that the Palaeoproterozoic granitoid rocks are arc-type granites (sensu lato), while the Mesoproterozoic augen-granite has the chemical composition of an alkaline within-plate granite. Chemical, whole rock Sm–Nd and zircon Lu–Hf data suggest that the magma that yielded the augen-granite was formed by melting within a mafic Palaeoproterozoic source at moderate crustal depth. The occurrence of a Mesoproterozoic granite in the (now) eastern margin of the West African Craton is evidence of a previously unknown igneous event within this terrain.
|
|
| 5. |
- Peillod, Alexandre, et al.
(författare)
-
Disruption of a high‐pressure unit during exhumation: Example of the Cycladic Blueschist unit (Thera, Ios and Naxos islands, Greece)
- 2023
-
Ingår i: Journal of Metamorphic Geology. - : John Wiley & Sons. - 0263-4929 .- 1525-1314. ; 42:2, s. 225-255
-
Tidskriftsartikel (refereegranskat)abstract
- Reconstructing the original geometry of a high-pressure tectonic unit is challenging but important to understand the mechanisms of mountain building. While a single nappe is subducted and exhumed, nappe-internal thrusts may disrupt it into several subunits. The Middle-CBU nappe of the Cycladic Blueschist Unit (Hellenide subduction orogen, Greece) shows evidence of such disruption along a Trans-Cycladic-Thrust (TCT), however, the timing of this thrusting is unknown. Here, we report multi-petrological and geochronological data from the Middle-CBU nappe from the Thera and Ios islands (Greece). Using Zr-in-rutile thermometry coupled with quartz-in-garnet elastic barometry, average P–T and phase equilibrium thermodynamic modelling, we show that garnet growth in Ios occurred during prograde metamorphism at 6.7 ± 1.4 kbar to 13.0 ± 1.6 kbar and 326 ± 20°C to 506 ± 13°C (2σ uncertainty) followed by early exhumation to 10.1 ± 0.6 kbar and 484 ± 14°C and a greenschist facies overprint at 5.7 ± 1.2 kbar and 416 ± 14°C. For Thera, we constrain peak HP conditions of 7.6 ± 1.8 kbar and 331 ± 18°C, followed by exhumation and equilibration at ~2 kbar and ~275°C using average P–T and phase equilibrium thermodynamic modelling. For Ios, Uranium-Pb garnet geochronology provides ages of 55.7 ± 5.0 Ma (2σ uncertainties) for prograde and 40.1 ± 1.4 Ma for peak HP metamorphism. Combining our new P–T–t data from Thera and Ios islands with existing data from Naxos island, we conclude that the studied nappe segments represent remnants of a former coherent nappe. The P–T–t data define an Eocene subduction rate of 2.1 ± 1.0 km/Ma, which is distinctly slower than the current subduction rate of 40–45 km/Ma. After subduction, the exhumation of the Middle-CBU nappe occurred during the Oligocene at different rates for different localities. The Middle-CBU nappe of Naxos was exhumed at a rate of ~6 km/Ma, contrasting with the exhumation rate of ~3 km/Ma calculated for Ios. This result suggests that the Middle-CBU nappe of Naxos rocks was thrust on the Ios one during the Oligocene. Using P–T–t data and assuming realistic subduction angles during the Eocene and the Oligocene, we present a 2D structural reconstruction of the Middle-CBU nappe of these islands. This reconstruction helps to understand the mechanisms of subduction of a continental margin and its disruption during exhumation.
|
|
| 6. |
- Petersson, Andreas, et al.
(författare)
-
Extensive reworking of Archaean crust within the Birimian terrane in Ghana as revealed by combined zircon U-Pb and Lu-Hf isotopes
- 2018
-
Ingår i: Geoscience Frontiers. - : Elsevier BV. - 1674-9871. ; 9:1, s. 173-189
-
Tidskriftsartikel (refereegranskat)abstract
- Located in the southeastern parts of the Baoulé Mossi domain of the West African Craton, the Birimian terrane in Ghana is known to consist of predominantly juvenile crust emplaced during the Eburnean orogeny. Here we present novel coupled zircon U-Pb and Lu-Hf isotope data from nine Birimian granitoids intruding different domains from northwestern, western and southeastern Ghana. Sub-chondritic e(open) Hf-values indicating reworking of Archaean crust are recorded in zircon from both northwestern and southeastern Ghana. As a conservative estimate 71% of all analysed zircon grains spanning in age from 2220 Ma to 2130 Ma require contribution from a reworked Archaean source, contradicting the common belief that the Birimian terrane consist of predominantly juvenile crust. A minimum e(open) Hf-value of -10.5 at 2139 Ma suggests a Palaeoarchaean to late Mseoarchaean component as the contributing ancient source. Combined with previously reported heavy zircon δ 18O signatures from Birimian river zircon in Ghana, our new data suggests reworking of Archaean aged sediments during subduction initiated crustal growth in the vicinity of one, or possibly in between two Archaean aged cratons.
|
|
| 7. |
- Petersson, Andreas, et al.
(författare)
-
Mesoproterozoic continental growth: U-Pb-Hf-O zircon record in the Idefjorden Terrane, Sveconorwegian Orogen
- 2015
-
Ingår i: Precambrian Research. - : Elsevier BV. - 0301-9268 .- 1872-7433. ; 261, s. 75-95
-
Tidskriftsartikel (refereegranskat)abstract
- The Idefjorden Terrane of the Sveconorwegian Orogen, Fennoscandia, is known to be an area of comparatively juvenile Mesoproterozoic continental growth. Here we provide an improved model of crustal growth based on new coupled zircon U-Pb-O-Lu-Hf isotopic data on thirteen samples of mafic to intermediate plutonic rocks from different domains of the Idefjorden Terrane. The new data support a retreating volcanic arc system, with shorter pulses of accretion. A gradual increase of radiogenic Hf (mean slif from 3.5 to 5.4) in plutonic rocks intruded between ca. 1630 Ma and 1560 Ma reflects an increase in juvenile mantle-derived magma in the genesis of the plutonic suites. This trend is consistent with development of an extensional back-arc rift geotectonic setting, accommodating deposition of the Stora Le-Marstrand greywacke dominated metasediment sequence. Combined isotopic information and the detrital zircon record of the Stora Le-Marstrand Formation support the interpretation that the Idefjorden Terrane was separated from the Fennoscandian Shield before the Sveconorwegian Orogeny. (C) 2015 Elsevier B.V. All rights reserved.
|
|
| 8. |
- Petersson, Andreas, 1980-
(författare)
-
Tracing Proterozoic arc mantle Hf isotope depletion of southern Fennoscandia through coupled zircon U–Pb and Lu–Hf isotopes
- 2017
-
Ingår i: Lithos. - : Elsevier BV. - 0024-4937 .- 1872-6143. ; 284–285, s. 122-131
-
Tidskriftsartikel (refereegranskat)abstract
- Constraints on the composition of the depleted mantle Sm–Nd and Lu–Hf crust formation ages have a long history of scientific debate. When calculating mantle extraction ages, and constructing crustal growth models, a linear evolution of incompatible trace elements in a depleted mantle since > 4 Ga is routinely used. Mantle depletion however varies regionally and over time and subduction of sediments and oceanic crust renders a mantle-wedge variously enriched relative to a modelled depleted mantle.Here we show that primitive mantle-derived subduction related gabbroic intrusions from southern Fennoscandia have Hf isotope compositions that are enriched relative to a MORB-like linear depleted mantle evolution curve. Extrapolation of primitive Paleoproterozoic gabbro suites enables the construction of a regional mantle evolution curve, providing improved constraints on model ages, crustal residence times and the fraction of juvenile versus reworked continental crust. Convergent margins are assumed to be one of the main sites of continental crust growth, and using an overly depleted mantle source yield model ages that are too old, and hence cumulative crustal growth models show too much crust generation early in the Earth's history. The approach of using the Hf isotope composition of zircon from primitive subduction related gabbroic intrusions as a proxy for mantle Hf isotope composition, piloted in this study, can be applied to other convergent margins.
|
|
| 9. |
- Ring, Uwe, et al.
(författare)
-
Kinematics of the Alpenrhein-Bodensee graben system in the Central Alps : Oligocene/Miocene transtension due to formation of the Western Alps arc
- 2016
-
Ingår i: Tectonics. - 0278-7407 .- 1944-9194. ; 35:6, s. 1367-1391
-
Tidskriftsartikel (refereegranskat)abstract
- We report fault slip data from exhumed fault surfaces along the NNE trending Alpenrhein valley and its intersection with the WNW striking Bodensee Graben near the Alpine thrust front of the Central Alps in the Swiss-Austrian-German border region. This conjugated graben system straddles the boundary between the Alps and its foreland and allows comparing the kinematics of graben formation between the two different tectonic domains. Our data show sinistral transtension along the Alpenrhein Graben and dextral transtension along the Bodensee Graben. Both transtensional graben systems resulted from the same kinematic regime of NW directed shortening and NE oriented extension. The graben faults are not older than NW striking tear faults associated with NW directed Oligocene nappe emplacement in the Helvetics starting at 35-30 Ma and ending by 25-20 Ma. Compatible with this are six U-Pb ages of calcite fibers from four samples yielding consistent ages ranging from 25.3 +/- 5.6 Ma to 21.8 +/- 3.4 Ma (2 sigma errors). Earthquake data since 1996 show that kinematic directions persisted until the Recent. Our data broadly fit with the kinematic evolution of the Oberrhein Graben, which shows significant E-W extension in the Oligocene. We suggest that Oligocene extension in the Alps and its foreland resulted from the increased curvature of the Western Alps arc and associated moderate tangential stretching in the internal parts of the Central Alps. We discuss a tectonic model of eastward rollback of the west Mediterranean subduction zone associated with counterclockwise rotation of Adria, the latter of which aided the formation of the Western Alps arc.
|
|
| 10. |
- Slama, Jiri, et al.
(författare)
-
Plesovice zircon : A new natural reference material for U-Pb and Hf isotopic microanalysis
- 2008
-
Ingår i: Chemical Geology. - : Elsevier BV. - 0009-2541 .- 1872-6836. ; 249:02-jan, s. 1-35
-
Tidskriftsartikel (refereegranskat)abstract
- Matrix-matched calibration by natural zircon standards and analysis of natural materials as a reference are the principle methods for achieving accurate results in inicrobeam U-Pb dating and Hf isotopic analysis. We describe a new potential zircon reference material for laser ablation ICP-MS that was extracted from a potassic granulite facies rock collected in the southern part of the Bohemian Massif (Plesovice, Czech Republic). Data from different techniques (ID-TIMS, SIMS and LA ICP-MS) and several laboratories suggest that this zircon has a concordant U-Pb age with a weighted mean Pb-206/U-238 date of 337.13 +/- 0.37 Ma (ID-TIMS, 95% confidence limits, including tracer calibration uncertainty) and U-Pb age homogeneity on the scale used in LA ICP-MS dating. Inhomogeneities in trace element composition due to primary growth zoning prevent its use as a calibration standard for trace element analysis. The content of U varies from 465 ppm in pristine parts of the grains to similar to 3000 ppm in actinide-rich sectors that correspond to pyramidal faces with a high degree of metamictization (present in ca. 30% of the grains). These domains are easily recognized from high intensities on BSE images and should be avoided during the analysis. Hf isotopic composition of the Plesovice zircon (>0.9 wt.% Hf) is homogenous within and between the grains with a mean Hf-176/Hf-177 value of 0.282492 +/- 0.000013 (2SD). The age and Hf isotopic homogeneity of the Plesovice zircon together with its relatively high U and Pb contents make it an ideal calibration and reference material for laser ablation ICP-MS measurements, especially when using low laser energies and/or small diameters of laser beam required for improved spatial resolution.
|
|
