Petrogenesis of the flood basalts forming the northern Kerguelen Archipelago: Implications for the Kerguelen Plume

The thick, >20 km, crust of the Kerguelen Archipelago formed as the tectonic setting of the Kerguelen Plume changed from an oceanic ridge-centered location at 43 Ma to its present location beneath the Antarctic plate. The uppermost crust is dominantly flood basalt with a thickness of up to 10 km. Inverse isochron ⁴⁰Ar/³⁹Ar ages for upper and lower lavas in a 630 m section of basalt flows from Mont Bureau are 30.4 and 29.0 Ma; Re-Os isotopic systematics are consistent with this age. Most of the lavas in two stratigraphic sections (Mont Bureau and Mont Rabouillère) from the northern part of the archipelago have Sr, Nd and Pb isotopic characteristics similar to the youngest (Upper Miocene to Pleistocene) lavas erupted in the southeast part of the archipelago, i.e. initial ⁸⁷Sr/⁸⁶Sr >0.7050, ¹⁴³Nd/¹⁴⁴Nd <0.5127 and ²⁰⁶Pb/²⁰⁴Pb <18.3. The dominance of this isotopic signature in archipelago lavas for 30 my and its presence in ∼40 Ma gabbros is consistent with the previous interpretation that these are isotopic characteristics of the Kerguelen Plum. Although this component occurs in high (>10%) MgO alkalic lavas in the Southeast Province of the archipelago, in these northern sections it is confined to transitional lavas with <6% MgO. A low plume flux and extensive crustal processing are inferred. In contrast to the plume-derived lavas, ∼15% of the flood basalts in these sections have lower initial ⁸⁷Sr/⁸⁶Sr (to 0.70396), higher ¹⁴³Nd/¹⁴⁴Nd (to 0.51289), and they have some compositional characteristics of plagioclase-rich cumulates, i.e. high Sr/Nd and Ba/Th and positive Eu anomalies. However, plagioclase phenocrysts are absent in most of these lavas; therefore a plagioclase-rich component is required in their source. A plausible interpretation is that plagioclase-rich cumulates formed in the lower oceanic crust when the Southeast Indian Ridge was coincident with the plume at ∼43 Ma; subsequently these cumulates were melted by the plume and the melts contributed to a small proportion of the flood basalts. Previously it was proposed that as the distance between the archipelago and Southeast Indian Ridge increased, there was a systematic decrease in the proportion of mid-ocean ridge basalt (MORB)-related component in the source of archipelago lavas. The new data show that: (1) there is no systematic temporal trend in the proportion of MORB to plume source components and (2) the MORB component was derived from cumulate rocks in the oceanic crust rather than as melts derived directly from the asthenosphere. Finally, there is no evidence of a continental lithosphere component in the source of Kerguelen Archipelago lavas.