Calc-alkaline and tholeiitic dyke swarms of Tandilia, Rio de la Plata craton, Argentina: U–Pb, Sm–Nd, and Rb–Sr 40Ar/ 39Ar data provide new clues for intraplate rifting shortly after the Trans-Amazonian orogeny

Abstract

The Tandilia system, Argentina, southernmost part of Rio de la Plata craton (RLPC), is intruded by two Paleoproterozoic unmetamorphosed dyke swarms which are: (i) intermediate (I) and acid (A); and (ii) basic (B 1 and B 2) in composition. The latter dykes have tholeiitic characteristics, while for both I and A dykes major, minor, and trace elements, including REE (e.g. the higher values of SiO 2, K 2O and Ba compared to tholeiitic dykes) are characteristic of calc-alkaline suites. The calc-alkaline dykes (I and A) yielded 40Ar/ 39Ar step-heating plateau ages of emplacement of 2020±24 and 2007±24 Ma. These ages are within error in agreement with a Rb–Sr errorchron of 1956±110 Ma (1σ) [initial 87Sr/ 86Sr=0.7038±0.0025 (MSWD=19)]. An upper intercept U–Pb age on two baddeleyites from a tholeiitic (B 1) dyke places the intrusion at 1588±11 Ma. The tholeiitic dykes (B 1 and B 2) have K–Ar whole-rock ages from 803±14 to 1193±18 Ma and a 40Ar– 39Ar plateau age of 811±36 Ma (2σ) on plagioclase. These discordant apparent ages suggest variable Ar loss of the mineral systems. Calc-alkaline dykes mainly trend E–W, and were emplaced during the transtensional stage of the Trans-Amazonian orogeny during which the plutonic rocks of the Tandilia system were formed. Such a scenario has similarities with the Eburnean evolution of the Richtersveld plutonic arc complex of the southern African subcontinent (Namaqualand) that faces the RLPC in the West Gondwana reconstruction. The significantly younger tholeiitic dykes of Tandilia (1.59 Ga) trend mainly NW-SE. Their presence constrains the time of crustal extension at the Paleo-Mesoproterozoic boundary during which basin-formation tectonics and anorogenic magmatism took place worldwide within a stabilized Paleoproterozoic lithosphere. Such an intraplate regime for the emplacement the youngest dykes of Tandilia is consistent with transcontinental scale, diachronous extensional episodes within the South American continent which initiated shortly after the Trans-Amazonian orogeny, as illustrated by the 1.73 Ga Florida tholeiitic dyke swarm and coeval, anorogenic granitoids scattered across the RLPC (Uruguay and Tandilia). Paleoproterozoic geologic features of the RLPC—namely, the occurrence of plutonic arc rocks (2.14–2.07 Ga) succeeded by emplacement of anorogenic granitoids and mafic dykes (1.73–1.59 Ga)—allow direct correlation with the postulated Gondwana counterpart, mirrored by broadly contemporary plutonic rocks of the Richtersveld and Bushmanland subprovinces of Namaqualand. The broad picture reinforces the idea that the Trans-Amazonian/Eburnean orogenies played an important role for juvenile crustal accretion within the southern South America and its southern Africa counterpart, which was followed by a tendency to dispersion of the stabilized continental fragments during the Mesoproterozoic, preceding the assembly of the Rodinia supercontinent.