IN-SITU CHEMICAL FRACTIONATION IN THIN BASALTIC LAVA FLOWS - EXAMPLESFROM THE AUCKLAND VOLCANIC FIELD, NEW-ZEALAND, AND A GENERAL PHYSICALMODEL

Basaltic lava flows of the Auckland volcanic field, northern New Zeala nd, commonly contain near-vertical cylinders and sub-horizontal sheets of a rock more coarse grained and vesicular than the host. The cylind ers and sheets, referred to collectively as pegmatoid autoliths, are 1 -8 cm in diameter/thickness. The flows in which they occur are all les s than 10 m thick and of pahoehoe type. Pegmatoid autoliths are enrich ed in most elements except Cr, Ni, Mg and Ca which are depleted, and S i, Al, and Sr which remain unchanged with respect to the host flow. Pe gmatoid autoliths represent in situ chemical fractionation of a basalt ic lava flow. Mass-balance calculations show that pegmatoid autoliths have the same composition as the interstitial liquid after 36-50% crys tallisation of the host. The segregation process requires a rigid perm eable crystal framework through which the interstitial liquid moves by gas filter-pressing. A physical model of gas filter-pressing in a coo ling lava is developed and predicts that a combination of high permeab ility, low melt viscosity and thick lava should favour the segregation of interstitial melt. A review of geological observations shows that the occurrence of pegmatoid autoliths conforms with the model's predic tions.