Metamorphic evolution of the Río de la Plata Craton in the Cinco Cerros area, Buenos Aires Province, Argentina

2019

The Blåhø Nappe on the island of Fjørtoft, which represents an isolated portion of the Seve Nappe Complex in the Western Gneiss Region, Norway, has been suggested to have experienced two deep burial cycles during the Caledonian orogeny. However, evidence on this multiple burial process by the derivation of a pressure–temperature–time (P–T–t) path has never been given in the literature. In this study, the ‘diamondiferous’ kyanite–garnet gneiss from the Blåhø Nappe on Fjørtoft was revisited to determine if such a process was correct. Two types of garnet, porphyroblastic garnet‐1 and fine‐grained garnet‐2, were recognized in the gneiss. The core of garnet‐1 is poor in Ca and documents P–T conditions of 1.2–1.3 GPa at c. 880°C based on pseudosection modelling. The inner rims of garnet‐1 and the core of garnet‐2 are both richer in Ca, recording P–T conditions of 1.35–1.45 GPa and 770–820°C. Application of conventional geothermobarometry on the outer rim of garnet‐1 and the rim of garnet‐2 yielded retrograde P–T conditions of 0.75–0.90 GPa and 610–685°C. These estimates define an anticlockwise P–T path at pressures below 1.5 GPa. Accessory monazite was dated with the electron microscope. Relicts of detrital monazite in the gneiss point to Sveconorwegian and possibly also Cryogenian provenance for the detritus of the sedimentary protolith. Metamorphic monazite in the gneiss records a wide age range from 460 to 380 Ma, with a peak c. 435 Ma and a shoulder at 395 Ma. These data suggest that the original (Ediacaran?) Baltica margin sediment (gneiss protolith) was transported to the base of an overlying plate during the early Caledonian (pre‐Scandian) orogeny. A long residence time of the metasedimentary rock at this base caused its heating to 880°C and homogenization of the early garnet chemistry. The late Caledonian (Scandian) collision between Baltica and Laurentia led to further burial, during which the studied gneiss was close to the former surface of the downgoing continental plate and, thus, cooled. The reconstructed P–T–t path confirms the multiple burial history of the Blåhø Nappe but contradicts previous ideas of deep burial of the Fjørtoft gneiss to more than 100 km.

Section: Methodsmentioning

confidence: 99%

An anticlockwise P–T–t path at high‐pressure, high‐temperature conditions for a migmatitic gneiss from the island of Fjørtoft, Western Gneiss Region, Norway, indicates two burial events during the Caledonian orogeny

Liu

2019

“…The analytical procedure to determine the composition and age of monazite with the EMP was described by Massonne et al. () and Massonne, Dristas, and Martínez (). This procedure resulted in a good agreement with monazite and zircon ages obtained by mass spectrometric methods (see Massonne, , ; Massonne et al., ; van Leeuwen et al., ; Waizenhöfer & Massonne, ).…”

Section: Methodsmentioning

confidence: 99%

Two Tertiary metamorphic events recognized in high‐pressure metapelites of the Nevado‐Filábride Complex (Betic Cordillera, S Spain)

2018

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Two phengite–garnet‐bearing metapelites from the Nevado‐Filábride Complex, Betic Cordillera, one from the Ragua unit (sample 23085) and the other (sample 23098) from the Calar Alto unit, were studied in detail to elucidate their metamorphic evolution. We calculated various P–T pseudosections for different O2 and H2O‐CO2 contents and Fe3+/Fe2+ ratios with PERPLE_X . On the basis of the compositions of the garnet core and the highest Si content in potassic white mica, different peak pressures of 12.7 ± 0.7 kbar at 525 ± 10°C (23085) and 17.4 ± 1.2 kbar at 526 ± 17°C (23098) resulted (errors refer to an estimated 1σ range). The suggested clockwise P–T loop was followed in both cases by another loop caused by a reheating process. On the basis of garnet rim compositions, peak temperatures of 596 ± 10°C at 10.7 ± 1.4 kbar (23085) and 629 ± 11°C at 8.5 ± 0.7 kbar (23098) resulted, supported by Zr‐in‐rutile thermometry (23085: 591 ± 11°C; 23098: 607 ± 10°C). In situ dating of monazite (23085) with the electron microprobe yielded ages between c. 48 and 14 Ma. On the basis of the monazite composition and histogram analysis, two age populations with mean ages of 40.2 ± 1.7 (1σ) and 24.1 ± 0.8 Ma could be defined. Low Y2O3 contents (<0.6 wt%) in monazite point to its formation after initial growth of garnet. The older generation was therefore assigned to the formation of the garnet core and the early P–T loop. The younger monazite population was related to the late P–T loop. A geodynamic scenario is hypothesized that assigns the Eocene metamorphism to subduction of a continental margin to depths of 50–65 km, followed by exhumation in an exhumation channel accompanied by mixing of slices of different rock types including mantle rocks. This process ended with stacking of such slices causing reheating of the studied rocks in the Early Miocene. The noted time gap of c. 15 Ma between subduction and nappe stacking is similar to the continent–continent collisional orogeny of the Himalaya.

“…Oxygen was calculated by stoichiometry. Similar to Massonne, Dristas, and Martínez (), a slight correction of the measured intensity at the U Mβ peak was applied because of a contribution from the Th Mγ line. In addition, a correction of the determined intensity of the Pb Mα radiation for interferences with Th and Y lines was achieved according to Jercinovic and Williams () and Pyle, Spear, Wark, Daniel, and Storm ().…”

Section: Analytical Proceduresmentioning

confidence: 99%

Anticlockwise pressure–temperature paths record Variscan upper‐plate exhumation: Example from micaschists of the Porto Vecchio region, Corsica

Cruciani

Franceschelli

et al. 2017

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To better understand the evolution of deep-seated crust of the Variscan orogen in the Sardinia-Corsica region, we studied garnet-bearing micaschists which were sampled 3 km east and 15 km northeast of Porto Vecchio, south-eastern Corsica.After a careful investigation of the textural relations and compositions of minerals, especially of zoned garnet, a P-T path was reconstructed using contoured P-T pseudosections. U-Th-Pb dating of monazite in the micaschists was undertaken with the electron microprobe. The micaschists from both localities were formed along similar anticlockwise P-T paths. The prograde branch of these paths starts at 3 kbar close to 600°C in the P-T field of sillimanite and reaches peak conditions at 7 kbar and 600 (15 km NE of Porto Vecchio) to 630°C (3 km E of Porto Vecchio). The metamorphism at peak P-T conditions happened c. 340 Ma based on low-Y (<0.65 wt% Y 2 O 3 ) monazite. Ages of monazite with high-Y contents (>2 wt% Y 2 O 3 ), which probably have formed before garnet, scatter around 362 Ma. The retrograde branch of the P-T paths passes through 4 kbar at 550°C. We conclude that the micaschists belong to a common metasedimentary sequence, which extends over the Porto Vecchio region and is separated from other metamorphic rock sequences in the north and the south by major tectonic boundaries. This sequence had experienced peak pressures which are lower than those determined for metamorphic rocks, such as micaschist and gneiss, from north-eastern Sardinia. At present, we favour a continent-continent collisional scenario with the studied metasedimentary sequence buried during the collisional event as part of the upper plate. The contemporaneous high-P metamorphic rocks from NE Sardinia were part of the upper portion of the lower plate. The addressed rocks from both plates were exhumed in an exhumation channel.