Palaeomagnetic study of the El Quemado complex and Marifil formation, Patagonian Jurassic igneous province, Argentina

Abstract

SUMMARY The upper Jurassic El Quemado Complex was sampled at 36 sites from five localities in the cordilleran foothills of southern Patagonia between Lago Argentino and Lago Posadas–Sierra Colorada, and the middle Jurassic Marifil Formation at 12 sites in the Somuncurá Massif near Camarones. The main lithology was ignimbrite, with minor tuff and lava. Petrographical and SEM observation show that the El Quemado rocks suffered an intense, high‐temperature alteration which resulted in transformation of most primary Ti‐magnetite in pseudobrookite, rutile and minor Ti‐haematite and Fe hydrated oxides. A similar, less pronounced alteration occurred in the Marifil rocks. 40Ar/39Ar dating of El Quemado was possible for one sample from Sierra Colorada and yielded an age of 156.5 ± 1.9 Ma. Magnetic mineralogy measurements (isothermal remanence, hysteresis loop, Curie balance) show that the remanent magnetization is dominated by PSD low‐Ti magnetite, often associated to a minor high‐coercive mineral (haematite). Secondary magnetization components are usually absent or weak at El Quemado sites, strong at Marifil. They were completely erased by thermal and AF demagnetization and a characteristic remanence (ChRM) stable up to temperatures higher than 550°C or peak‐field values of 100 mT was successfully isolated. The virtual geomagnetic pole (VGP) from the Marifil Formation (83°S, 138°E) is in agreement with the literature data for Jurassic rocks from stable South America. The El Quemado VGPs fall in two groups. The localities to the north of latitude 48°S (Lago Posadas, Sierra Colorada) yield a VGP (81°S, 172°E) close to that of Marifil, whereas those south of latitude 49°S (Lago San Martín, Lago Argentino) show a highly elongated VGP distribution consistent with counter‐clockwise block‐rotation about vertical axes. These rotations were likely caused by thrust sheets which were rotating counter‐clockwise at the same time they were advancing towards the foreland. The amount of rotation varies according to the location of the sampling sites in the thrust and fold belt.

Tracking the Late Jurassic apparent (or true) polar shift in U‐Pb‐dated kimberlites from cratonic North America (Superior Province of Canada)

“…The El Quemado igneous pole from Patagonia (South America), dated at 156.5 ± 1.9 Ma on the basis of 40 Ar/ 39 Ar analyses on one partially altered sample [ Iglesia Llanos et al ., ] and included in the 160 Ma mean pole of K&I, indeed plots with the constituents of that mean pole, for example, the 169 Ma Moat volcanics pole, rather than near either the Triple B or Peddie poles of ostensibly more similar ages (Figure ). The El Quemado Formation could be a few million years older with updated monitor standards [ Kuiper et al ., ], it may have suffered a relatively minor yet crucial resetting of its geochronological clock, or its magnetic directions were affected by local tectonic rotations [ Rapalini , ], emphasizing the tight age and structural control needed to resolve events during such an interval of rapid polar shift.…”

“…Also shown for reference are the accepted poles from North America nearest in age to the Triple B and Peddie kimberlite poles from the 169 Ma Moat volcanics [ Van Fossen and Kent , ] and the 146 Ma Ithaca kimberlite [ Van Fossen and Kent , ], which are included in the 160 and 145 Ma mean poles, respectively, that bracket the monster polar shift (dashed line). The pole from the El Quemado Formation of Patagonia [ Iglesia Llanos et al ., ] plots in North American coordinates (labeled EQ: 84.4°N 122.5°E) near the older end of the polar shift, suggesting that its age is somewhat older than cited (156.5 ± 1.9 Ma) or its direction may have been biased by local tectonic rotations [ Rapalini , ].…”

“…K&I did not use an intervening 150 Ma mean pole. This is because only the 157 Ma El Quemado igneous pole from Patagonia [ Iglesia Llanos et al ., ] was regarded as acceptable in the more than 20 Myr interval between the 167 Ma Chon Aike pole from Patagonia [ Vizan , ], the 168 Ma Prospect dolerite pole from Australia [ Schmidt , ], and the 169 Ma Moat volcanics pole from North America [ Van Fossen and Kent , ] on one hand (and already included in the 160 Ma mean) and on the other, the 147 Ma Swartruggens‐Bumbeni kimberlite pole from southern Africa [ Hargraves et al ., ; Heaman et al ., ], the 146 Ma Ithaca kimberlite pole from North America [ Van Fossen and Kent , ] (see below for analysis of U‐Pb data), and the 144 Ma Hinlopensretet sills pole from Svalbard [ Halvorsen , ]. The latter three poles form a coherent cluster at ∼145 Ma with a mean (61.2°N, 200.2°E A95 = 9.0°, K = 189) that is even more removed from the 160 Ma mean pole (Figure ) and was used by K&I to delimit the younger end of the monster shift.…”

Influence of inclination error in sedimentary rocks on the Triassic and Jurassic apparent pole wander path for North America and implications for Cordilleran tectonics

“…From elsewhere in South America, we exclude a venerable result for the Mendoza lavas from South Nihuil, Argentina [ Creer et al , 1970], their reported age (Triassic or Permian) being poorly constrained. On the other hand, comprehensive isotopic dating shows that the silicic volcanics of the Marifil Formation in Patagonia are essentially coeval with the Karroo‐Ferrar‐Tasmanian magmatic province at ∼183 Ma [ Pankhurst et al , 2000]; accordingly its pole [ Iglesia Llanos et al , 2003] is assigned that age. Although the Lepa and Osta Arena formations of Patagonia [ Vizán , 1998] are biostratigraphically dated as late Pliensbachian‐early Toarcian, and may therefore also belong to this magmatic province, we have listed the result separately.…”

“…Although the Lepa and Osta Arena formations of Patagonia [ Vizán , 1998] are biostratigraphically dated as late Pliensbachian‐early Toarcian, and may therefore also belong to this magmatic province, we have listed the result separately. Finally, we include poles for the 167 Ma Chon Aike Formation of Patagonia, summarized from the literature by Vizán [998], and for the 156.5 Ma El Quemado Formation of Patagonia [ Iglesia Llanos et al , 2003]. …”