Thinning of the Antarctic Peninsula lithosphere through the Mesozoic: evidence from Middle Jurassic basaltic lavas

Rare basaltic volcanic rocks from the northern Antarctic Peninsula (Jason Peninsula) are established as marginally predating a silicic large igneous province, which developed along the proto-Pacific margin of Gondwana. Ar-40-Ar-39 geochronology reported here has dated the basalts in the interval, 175-168 Ma, which overlaps with the dates previously obtained on the silicic volcanic rocks, 171-168 Ma. The basalts are evolved with low Mg#, Cr and Ni, indicating they have undergone significant fractional crystallisation from mantle-derived melts. Their trace elements (high Th/Yb) and isotope ratios (positive epsilonSr and low epsilonNd) indicate that they are derived from lithospheric mantle, which has been significantly modified by subduction-derived fluids and sediments. Cretaceous age (126-106 Ma) primitive mafic dykes from the Antarctic Peninsula are also interpreted as partial melts of the subduction-modified mantle, and together with the Middle Jurassic basalts are used to monitor the Antarctic Peninsula lithosphere. The shift from lithosphere-derived (Jurassic) to asthenosphere-derived (Cretaceous) partial melts in the northern Antarctic Peninsula is attributed to thinning of the lithosphere, which was a consequence of the voluminous silicic volcanism of the Middle Jurassic, coupled with regional extension. The chemistry, chronology and rarity of the basalts are consistent with the model proposed for the voluminous silicic volcanism.