Published online by Cambridge University Press: 17 February 2023
The ultimate origin or provenance of the masses of orthopyroxene primocrysts is of paramount interest. Many lines of reasoning lead to the conclusion that these were entrained in magma ascending from deeper in the mantle, generated by the continental breakup of Gondwana, somewhere below the local continental crust. The crystals themselves look old, showing signs of long-term annealing, and the crystals are in strong isotopic contrast (Sr 87/86 and O-18) with the basaltic magma itself, with the Opx assemblage being much more radiogenic that the basalt. This is especially marked in the dais rocks in going from Opx dominated layers to more basaltic layers. Moreover, the basaltic magma itself, even when it carries no large primocrysts, is highly heterogeneous isotopically. A profile through the Peneplain Sill at Solitary Rocks, near Pandora’s Spire, in an otherwise thick (330 m) featureless sill, shows strong variations in Sr-87/86. This reflects what others previous show, that the Ferrar are isotopically "noisy" and, remarkably, exhibit isotope patterns similar to the local crust. The obvious answer that this is all from local contamination and weather cannot be true, but instead this comes from the uppermost mantle underlying local crust that has had a long physical association.
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