⁴⁰ Ar- ³⁹ Ar ages for dykes from the Falkland Islands with implications for the break-up of southern Gondwanaland

Abstract

40 Ar- 39 incremental-heating spectra analyses were made of five microgabbro dykes from the Falkland Islands, two of which yielded an acceptable plateau, and a third a close maximum estimate. All three indicate an age of dyke intrusion of about 190 Ma, supporting a single, published K-Ar date. This provides further support for the geological continuity of the Falkland Islands and the eastern Karoo Basin where dyke swarms of similar petrological affinities were intruded at about the same time. The age constrains the previously published palaeoreconstruction of the Falkland Islands, based on palaeomagnetic data, which places the Falklands immediately to the east of southern Africa, to be valid until at least the late Early Jurassic.

Rotation of the Falklands microplate reassessed after recognition of discrete Jurassic and Cretaceous dyke swarms

“…1) have given K-Ar dates of 192±10 Ma (Cingolani & Varela, 1976), and 176±7 and 162±6 Ma (Thistlewood et al, 1997); the latter two from "north-south" trending dykes that we assign to our NE-SW swarm. More precise Ar-Ar dates were reported by Mussett & Taylor (1994): a dyke from the Cape Orford swarm gave 190±4 Ma, an "east-west" dyke (from our radial swarm) gave 188±2 Ma, and a "north-south" dyke (from our NE-SW swarm) gave a maximum age of 193±4 Ma. These three Ar-Ar dates were regarded as indistinguishable within error, and have been taken to prove intrusion of all of the Falklands dykes at about 190 Ma.…”

“…This proposal was negated by the photo-interpretation of Greenway (1972) who identified up to 400 individual dykes on West Falkland, separated into three distinct but intersecting swarms: NNE-SSW dykes occurred mostly in the north and centre of the island whereas WNW-ESE dykes were confined to the south and the southwestern outlying islands; a more variable, localized pattern of dykes was identified in the extreme southwest, around Cape Orford. Subsequent work appeared to affirm Greenway's distinctions, with the two more widely seen 3 trends being usually, if inaccurately, referred to as "north-south" and "east-west" dyke swarms (Taylor & Shaw 1989;Mussett & Taylor 1994;Mitchell et al 1999).…”

“…Until recently the dykes were regarded as exclusively of Jurassic age (Mussett & Taylor 1994), but are now known to have been intruded during separate Jurassic and Cretaceous episodes of magmatism (Stone et al 2008). Palaeomagnetic results from the Jurassic dykes (Taylor & Shaw 1989) have been taken to support the proposition that the Falkland Islands form part of a microplate that during the break-up of Gondwana was rotated through 180 o from an original position adjacent to the east coast of South Africa (Adie 1952).…”

“…Even where dykes with different polarities intersect, no consistent sense of the intrusion sequence emerges. The possibility that dykes of different orientation 5 formed parts of a conjugate set (Mussett & Taylor 1994) can be discounted in view of their significantly different radiometric ages (discussed below) and compositions. The Jurassic radial dyke swarm in the south of West Falkland consists mostly of olivineclinopyroxene dolerites, the NE-SW Jurassic dykes are composed of dolerite containing both clino-and orthopyroxene, whilst the Cretaceous N-S dykes consist of glassy dolerite with only clinopyroxene in the ferromagnesian phase.…”

The three-dimensional lithospheric structure of the Falkland Plateau region based on gravity modelling

“…It is this consideration that makes the hypothesis more likely than one involving serpentinisation of the upper mantle. Karoo age igneous activity in the region is demonstrated by the dykes of this age exposed on the Falkland Islands (Mussett & Taylor 1994). Where underplating is present, the average crustal density will be higher than normal so the crustal thickness indicated by the 3D model may be an underestimate, although the effect may be offset to some extent by reduction in the density of the underlying mantle as a result of melt extraction (Kimbell et al 2004).…”