Constraining the timescales of mafic magmatism of the Central Karoo Large Igneous Province using high precision U-Pb zircon geochronology

Abstract

Recent U-Pb high-precision geochronological studies have shown rapid emplacement of the intrusive doleritic component of the Karoo Large Igneous Province (KLIP) in Southern Africa. However, these studies focused on a relatively small geographic and altitudinal region of the KLIP. Additionally, the timing of initiation of extrusive volcanism, preserved in the Drakensberg-Lesotho highlands and its relationship to the intrusive suite, has only been imprecisely constrained by Ar-Ar dates. Here, we present new high-resolution U-Pb zircon ages on dolerite sills and dykes from across the central eastern Karoo Basin (South Africa) at elevations between mean sea level and 1 560 m, as well as U-Pb detrital zircon data that can be used to estimate the maximum age of volcaniclastic deposition near the base of the extrusive component of the KLIP. Dolerite samples were taken across two areas: (1) thick dykes exposed along the coast of the Indian Ocean to ~1 600 m flanking the Drakensberg Escarpment in the Eastern Cape; and (2) sills between 20 and 220 m below surface, in a borehole core within the interior of the Karoo Basin, 400 km hinterland from the coastline. Our estimated dolerite emplacement ages span a range of ca. 80 thousand years (Kyr), between 183.122 ± 0.029/-0.061 and 183.042 ± 0.042/-0.072 million years ago (Ma), and fall within the 331 +60/-54 Kyr age range previously established for magmatism related to the KLIP, despite the marked increase in sampling coverage in terms of area and altitude in this study. Therefore, KLIP geochronology is consistent with other LIPS such as the Siberian and Deccan Traps that supports the hypothesis of rapid emplacement timescales (<1 Myr). Additionally, these data are consistent with, but better delineate that the KLIP in southern Africa appears to be ca. 500 Kyr older than the main phase of magmatism in the Ferrar LIP of Antarctica. Detrital zircons from the basal volcanic sequence of the Drakensberg Group exhibit age peaks at ca. 1 and 0.5 Ga, typical of the surrounding Namaqua-Natal and Pan-African basement rocks, as well as younger peaks at ca. 260 and 200 Ma that likely relate to source provenances from south-western Gondwana and reworking of the Karoo Supergroup sedimentary rocks. High-precision U-Pb dates of the youngest zircon grains result in a maximum depositional age for the basal pyroclastics of 185.25 ± 0.25 Ma, allowing for a ca. 2 Myr offset with the intrusive Karoo dolerite suite.