Proto-Elamite Chronology: On Susa

New Rb-Sr and K-Ar datings help to clarify the geologic history of the Spessart Crystalline Complex, Mid-German Crystalline Rise. The oldest dates, refined by new measurements, are recorded by whole-rock Rb-Sr analyses of the orthogneisses of the Rotgneiss Complex. These confirm a late Ordovician to Silurian age which is interpreted as the time of intrusion of the granitic precursors.

Developments in Precambrian Geology, 2009

– The study was performed in central-northern Anatolia (from Ankara to Amasya) to investigate the relationships of the Sakarya Zone units and the Izmir–Ankara–Erzincan suture (IAES) melange. It reveals that all the Sakarya Zone units are metamorphic and three main tectonostrati-graphic units have been distinguished for the first time: the BAA (metasiliciclastic rocks capped by metacarbonates and varicoloured phyllite), the BKC (poly-metamorphic garnet-bearing micaschist and metabasite with a well-preserved relict HP–LT amphibole in a low-amphibolitic to greenschist-facies framework) and the AMC (meta-arkose passing vertically to carbonate–phyllitic alternations and, then, to a thick succession of prevailing acidic to intermediate–basic metavolcanites and volcanic-rich metasediments). The BAA and AMC, whose metamorphic frameworks are of Cimmerian age, underlie the Mesozoic carbonate cover sequences (e.g. t 2-3, j 3 –k 1) that often show tectonic detachments and slicing. The piling up of the BAA above the HP–LT BKC can be correlated to the tectonic superposition of two similar units (i.e. the Cimmerian Çangalda˘ g Complex and the Alpine Middle– Upper Cretaceous Domuzda˘ g Complex, respectively) defined by previous authors in other sectors of the Central Pontides front. The ophiolitic melange generally underlies the Sakarya Zone, but locally (e.g. SE of Amasya) tectonically rests above the latter, probably owing to back-thrusting that occurred during the Tertiary syn-collisional shortenings and the later strike-slip tectonics. We hypothesize that, also in these areas, the Sakarya Zone–IAES consists of a complex tectonic stack of different units, belonging to different palaeogeographic domains and orogenic events (Cimmerian versus Alpine oro-genies), but originated within a single long-lived (since Late Triassic to Paleocene/Eocene times), prograding subduction–accretion system in front of the Laurasian continent.

Swiss Journal of Geosciences, 2015

The Penninic nappe stack in the Central and Western Alps was formed in a collision zone environment after the closure of the Penninic oceans in the Paleogene. This study reports Lu-Hf garnet-whole rock ages of 56.5 ± 2.7 and 58.2 ± 1.4 Ma for two eclogite samples from the Theodul Glacier Unit, which is inserted within the structurally uppermost parts of the ophiolitic Zermatt-Saas Zone. The distribution of major elements, Mn, Y, and Lu in garnet, and specifically an enrichment of Lu in the cores, indicate that the ages record prograde growth of garnet during pressure increase. They provide direct evidence for the continuation of subduction during the ''Paleocene restoration phase'', often regarded as a tectonically quiescent period due to a reduction in clastic sediment deposition, lack of folds and thrusts of this age, and a cessation of Africa-Europe convergence as derived from the magnetic anomaly pattern in the Atlantic Ocean. The evidence for ongoing subduction in the absence of Africa-Europe convergence suggests that the subduction system was driven by gravity acting on the downgoing slab in a rollback setting, and that subduction was balanced by extension of the upper plate. The overlap of the Lu-Hf ages of both samples from the Theodul Glacier Unit show that this tectonic element represents a coherent body. The difference with respect to the 48 Ma Lu-Hf age of the Lago di Cignana Unit, another element of the Zermatt-Saas Zone, shows that the Zermatt-Saas Zone consists of tectonic subunits, which reached their respective pressure peaks over a prolonged period of approximately 10 Ma.

Developments in Precambrian Geology, 2009

The apparent absence of Mesoproterozoic rocks in the Rı ´o de la Plata Craton (RPC) was interpreted in the past as evidence of this block being unrelated to Rodinia (e.g. Cordani et al., 2003). Other authors, however, pointed out that geological similarities between the Amazonia and the Rı ´o de la Plata palaeocontinents suggested that they were linked in the Mesoproterozoic and were part of Rodinia (e.g. Meert and Torsvik, 2003). In Uruguay, the occurrence of Mesoproterozoic tectonomagmatic events in the RPC has been advocated by different authors in the last 15 years. While mapping in detail the Piedra Alta mafic dyke swarm, dated at 1,79075 Ma (U-Pb on beddeleyite: Halls et al., 2001; Ar-Ar: Teixeira et al., 1999), Bossi and Campal (1992) recognised and described a dextral megashear zone (Sarandı ´del Yı ´Shear Zone, SYSZ) with mylonites up to 8 km thick (Figure 4.6.1). The curvature of the eastern part of the dyke swarm is consistent with dextral shear, unlike all ''Brasiliano 2 Pan African'' structures in the area. The age of the SYSZ is probably late Mesoproterozoic as shown by post-emplacement thermal overprinting of the Piedra Alta mafic dike swarm (Bossi et al., 1993a) between 1,370 and 1,170 Ma, reflected in initial release portions of 40 Ar-39 Ar spectra and Rb-Sr mineral isochrons (Teixeira et al., 1999). Bossi and Navarro ( ) report an 40 Ar-39 Ar age of 1,24075 Ma for the same event (Figure 4.6.1). Cingolani, in Bossi et al. (1998), reported K-Ar ages of 1,253732 Ma for synkinematic muscovites that crystallised along southwest-vergent thrust planes in the Nico Pe ´rez Terrane (Figure .6.1), thus consistent with the above-mentioned shear sense and timing of this tectonic event. The SYSZ is a continental-scale megashear (Unrug, 1996), which suggests significant reworking of the RPC during the Grenvillian Orogeny. $

IN FELTEN, F., GAUSS, W. & SMETANA, R.( …, 2007

Paleomagnetic, magnetic anisotropy data and 40 Ar/ 39 Ar ages are presented for high-grade metamorphic rocks of the Jequié block, São Francisco Craton. Jequié charnockites and enderbites gneisses from the eastern border of the block present northern and steep-downward directions, carried by Ti-poor titanomagnetite with high unblocking temperatures (550-600 • C). A mean direction for 12 sites of enderbites from the eastern sector yielded a magnetic component A (D m = 47.0 • , I m = 75.7 • ,˛9 5 = 6.3 • , K = 48.8) with a corresponding paleomagnetic pole at 339.6 • E, 5.4 • N (A 95 = 11.2 • ). Sites sampled on other metamorphic rocks including granulite-facies, tonalites and dacites yield different magnetic components. Anisotropy of magnetic susceptibility measured for all sampling sites shows a high degree of anisotropy (P = 1.121-1.881), with NE-or NW-trending magnetic lineations and vertical magnetic foliations. These data were used to correct the mean site directions for all components. While the other components presented larger scatter after anisotropy correction, the component A shows a slightly tighter clustering of magnetic directions (D m = 61.2 • , I m = 76.5 • ,˛9 5 = 5.4 • , K = 66.2) giving a new, anisotropy corrected paleomagnetic pole at 342.1 • E, −0.5 • N (A 95 = 9.6 • ). 40 Ar-39 Ar plateau ages of 2035 ± 4 Ma (hornblende) and 1876 ± 4 Ma (biotite) obtained for one of the samples with component A imply a very low cooling-rate of 1.4 • C/Ma for these rocks. Based on these ages and corrected unblocking temperatures of the magnetic component A, we argue that the characteristic magnetization of the Jequié metamorphic rocks was acquired by high temperature thermo-chemical processes during regional cooling of the adjacent Itabuna-Salvador-Curaç á belt between 2089 and 1985 Ma (pole age likely at 1.99 Ga). Comparison of this pole with available paleomagnetic poles from South America and Africa suggests that neither Atlantica nor Ur have ever existed, and dispersed continental fragments dominated the paleogeography at ca. 2.0 Ga ago.

2019