Paleogene Sedimentation and Eurekan Deformation in the Stenkul Fiord Area of Southeastern Ellesmere Island (Canadian Arctic): Evidence for a Polyphase History

Field studies and interpretative mapping of the area southeast of Stenkul Fiord (Ellesmere Island) revealed that the Margaret Formation clastic deposits consist of at least four sedimentary units (Units 1–4) separated by unconformities. Several centimeter-thick volcanic ash layers, identified within coal layers and preserved as crandallite group minerals (Ca-bearing goyazite), suggest an intense volcanic ash fall activity. Based on new U-Pb zircon dating (ID-TIMS) of three ash samples from one layer, this activity took place at 53.7 Ma in the early Eocene, i.e., within the period of the Eocene Thermal Maximum 2 hyperthermal. This age further suggests that the lowermost Unit 1 can be assigned to the late Paleocene–earliest Eocene, Unit 2 to the early Eocene, whereas Units 3 and 4 might be early to middle Eocene in age. Sedimentation was followed and partly accompanied by compressive Eurekan deformation after ~53.7 Ma, which led to the formation of fold and fault structures. Several pulses of deformation caused uplift and erosion and were followed by sedimentation of the next unit above an unconformity. Deformation presumably ended before the middle Eocene. An earlier phase of probably extensional Eurekan deformation in Unit 1 can be assigned to the latest Paleocene–earliest Eocene. These results show that Paleocene/Eocene sedimentation and Eurekan deformation represent a protracted history comprising several phases of ongoing clastic sedimentation, deformation, uplift, and erosion. This suggests that the Eurekan deformation on Ellesmere Island cannot be assigned to a single fixed time in the Paleogene only

Geochronology of igneous rocks in the Sierra Norte de Córdoba (Argentina): Implications for the Pampean evolution at the western Gondwana margin

U-Pb zircon data (secondary ion mass spectrometry [SIMS] and thermal ionization mass spectrometry [TIMS] analyses) from igneous rocks with differing structural fabrics in the Sierra Norte de Córdoba, western Argentina, suggest that the sedimentary, tectonic, and magmatic history in this part of the Eastern Sierras Pampeanas spans the late Neoproterozoic–Early Cambrian. A deformed metarhyolite layer in metaclastic sedimentary rocks gives a crystallization age of 535 ± 5 Ma, providing a limit on the timing of the onset of D1 deformation and metamorphism. The new data coupled with published Neoproterozoic zircon dates from a rhyolite beneath the metaclastic section and detrital zircon ages from the section indicate a late Neoproterozoic to Early Cambrian depositional age, making this section time equivalent with the Puncoviscana Formation (sensu lato) of northwest Argentina. A synkinematic granite porphyry gives a crystallization age of 534 ± 5 Ma, providing a limit on the age of dextral mylonitization in the Sierra Norte area (D2 event). The new age is consistent with ages of 533 ± 4 Ma from a mylonitic granite with dextral sense-of-shear fabrics and 531 ± 4 Ma from a late-synkinematic dacitic porphyry, which broadly indicates the final age of dextral deformation. A crystallization age of zircons from the postkinematic, high-level El Tío granite (530 ± 4 Ma) suggests that both stages of Pampean deformation and regional metamorphism, accompanied by synkinematic intrusions, were followed by uplift and took place during a very short time span in the Early Cambrian. This is supported by zircon dates of 523 ± 5 Ma from a rhyolite to dacite in the western part of the Rodeito area and dates from the undeformed El Escondido rhyolite and granite of 519 ± 4 Ma and 521 ± 4 Ma, respectively. These three crystallization ages also indicate that ductile dextral shearing and mylonitization associated with the Pampean D2 event terminated in the Early Cambrian. Both stages of Pampean deformation in this segment of the western pre-Andean Gondwana margin seem to represent a continuous event that can be related to oblique dextral convergence between the overriding plate in the east and the subducting and finally colliding plate in the west. The postkinematic intrusions and extrusions are related to the late stage of the Pampean magmatic history, which terminated before Early Ordovician (Famatinian) time.Fil: Von Gosen, W.. Friedrich-Alexander-Universität; AlemaniaFil: McClelland, W. C.. University of Iowa; Estados UnidosFil: Loske, W.. Ludwig-Maximilians-UNiversität München; AlemaniaFil: Martinez, Juan Cruz. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Bahía Blanca. Instituto Geológico del Sur; Argentina. Universidad Nacional del Sur; ArgentinaFil: Prozzi, Carlos. Universidad Nacional del Sur. Departamento de Geologia. Catedra de Petrologia; Argentin