Co-existing low-Ti and high-Ti dolerites in two large dykes in the Gap Dyke swarm, southeastern Karoo Basin (South Africa)

Abstract

AbstractThis paper presents major, trace element and Sr-Nd isotope data on two large, east-west trending dolerite dykes in the southeastern part of the Karoo Basin, the South Gap (155 km long, ≤275 m wide) and the North Gap (150 km long, ≤190 m wide) dykes. The Gap dykes represent a rare case in the Karoo Large Igneous Province (LIP) where low-Ti (Gap1: <1.7 wt% TiO2, <130 ppm Zr, 200 to 330 ppm Sr, 6 to 17 ppm La, 87Sr/86Sr183: 0.7045 to 0.7075, ɛNd183: +0.31 to -7.5, ΔNb: +0.4 to -0.2) and high-Ti rocks (Gap2: 2.4 to 3.1 wt% TiO2, 110 to 240 ppm Zr, 260 to 390 ppm Sr, 12 to 24 ppm La, 87Sr/86Sr183: 0.7066 to 0.7074, ɛNd183: -3.2 to -4.1, ΔNb: +0.2 to -0.4) alternate along the same dykes. The aim of the study is to unravel the origin and petrogenesis of the two rock groups. The sample groups show the chemical traces of fractional crystallization in increasing concentrations of incompatible trace elements with decreasing MgO, and pivot points where elements such as Ti, Ca, P, and Sc change from trends of increasing to decreasing concentrations with decreasing MgO. Trends of increasing 87Sr/86Sr183 with decreasing ɛNd183 ratios indicate crustal contamination. However, three Gap1 samples from the South Gap dyke with the least enriched Sr-Nd isotope ratios and highest ΔNb may be derived from uncontaminated plume-type mantle melts. Modelling of Sr-Nd isotopes indicate that the Gap1 samples were subjected to assimilation of crustal melts (≤8%) and fractional crystallization in deep crustal magma chambers. There are no restrictions on the mantle source for the Gap2 rocks. However, because both geochemical signatures occur at different localities along the two Gap dykes, we favor a SA (South African) type mantle source for the two groups. The Gap2 rocks appear to have a two-stage assimilation history with one stage in the SCLM (subcontinental lithospheric mantle) or deep crust (decreasing the ɛNd183, increasing the 87Sr/86Sr183 ratios, and inducing the high-Ti character), followed by a second stage of contamination and fractional crystallization in the upper crust (increasing Sr isotope and (Sm/Yb)N ratios, with only minor decrease in ɛNd183).