Early–Middle Jurassic Dolerite Dykes from Western Dronning Maud Land (Antarctica): Identifying Mantle Sources in the Karoo Large Igneous Province

Abstract

A suite of dolerite dykes from the Ahlmannryggen region of western Dronning Maud Land (Antarctica) forms part of the much more extensive Karoo igneous province of southern Africa. The dyke compositions include both low- and high-Ti magma types, including picrites and ferropicrites. New Ar-40/Ar-39 age determinations for the Ahlmannryggen intrusions indicate two ages of emplacement at similar to 178 and similar to 190 Ma. Four geochemical groups of dykes have been identified in the Ahlmannrygggen region based on analyses of similar to 60 dykes. ne groups are defined on the basis of whole-rock TiO2 and Zr contents, and reinforced by rare earth element (REE), Sr-87/Sr-86 and Nd-143/Nd-144 isotope data. Group I were intruded at similar to 190Ma and have low TiO2 and Zr contents and a significant Archaean crustal component, but also evidence of hydrothermal alteration. Group 2 dykes were intruded at similar to 178Ma; they have low to moderate TiO2 and Zr contents and are interpreted to be the result of mixing of melts derived from an isotopically depleted source with small melt fractions of an enriched lithospheric mantle source. Group 3 dyke were intruded at similar to 190Ma and form the most distinct magma group; these are largely picritic with superficially mid-ocean ridge basalt (MORB)-like chemistry (flat REE patterns, Sr-87/Sr-86, similar to 0.7035, is an element of Nd-i similar to 9). However, they have very high TiO2 (similar to 4 wt %) and Zr (similar to 500ppm) contents, which is not consistent with melting of MORB-source mantle. The Group 3 magmas are inferred to be derived by partial melting of a strongly depleted mantle source in the garnet stability field. This group includes several high Mg Fe dykes (ferropicrites), which are interpreted as high- temperature melts. Some Group 3 dykes also show evidence of contamination by continental crust. Group 4 dykes are low-K picrites intruded at similar to 178Ma; they have very high TiO2-Zr contents and are the most enriched magma group of the Karoo-Antarctic province, with ocean-island basalt (OIB)-like chemistry. Dykes of Group I and Group 3 are sub-parallel (ENE-WSW) and both groups were emplaced at similar to 190Ma in response to the same regional stress field, which had changed by similar to 178Ma, when Group 2 and Group 4 dykes were intruded along a dominantly NNE-SSW strike.