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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.
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This content will become publicly available on April 16, 2026
Leaving no zircon unturned: quantifying the impact of handpicking sharply faceted detrital zircon on maximum depositional age analysis
ABSTRACT Using the youngest detrital-zircon date(s) of a sedimentary deposit to constrain its maximum depositional age (MDA) is a widespread and growing application of geochronology. Most MDA studies analyze zircon grains at random, but this strategy can be costly and inefficient in cases where the youngest age group is only a small fraction of the population. We propose that handpicking sharply faceted zircon grains will increase the likelihood of encountering first-cycle zircon that have not undergone significant sedimentary transport, thus producing MDA estimates that are closer to the depositional age. We evaluate this procedure by conducting intra-sample comparisons of randomly selected versus handpicked zircon separates from 30 samples analyzed via laser-ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS). Our results show that handpicking zircon produces an overall shift towards younger ages in comparison to their randomly analyzed counterparts by an average of ∼ 406 Myr. In randomly analyzed separates, only 1.6% of grains were within 5 Myr of an independent estimate of the MDA, while handpicked separates contained 14.2%, an approximately nine-fold increase. However, handpicking can also lead to selection of older grains if they have been minimally transported, as with one handpicked Mesozoic sample that yielded 81% of ∼ 1.1 Ga zircon interpreted to be derived from a local granitic source. Handpicking is most effective in samples where young, sharply faceted grains are diluted by older, rounded grains, as with one sample that exhibited an ∼ 18-fold increase in the proportion of near-depositional-age zircons relative to its counterpart where grain selection was random. Because handpicking zircon imparts a severe bias on the resulting U–Pb age distribution, we recommend that two separate aliquots be used for quantitative provenance characterization through random analysis and MDA analysis through handpicking.
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- Award ID(s):
- 1925915
- PAR ID:
- 10599924
- Publisher / Repository:
- Geological Society of America
- Date Published:
- Journal Name:
- Journal of Sedimentary Research
- Volume:
- 95
- Issue:
- 2
- ISSN:
- 1527-1404
- Page Range / eLocation ID:
- 434 to 443
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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