A LATE NEOPROTEROZOIC (~630MA) BONINITIC SUITE FROM SOUTHERN ISRAEL: IMPLICATIONS FOR THE EVOLUTION OF THE EAST AFRICAN OROGEN

The East African Orogen formed as a result of collision between E and W. Gondwanaland in Late Neoproterozoic time, but it is not known exactly when . It is particularly important to understand when accretion of juvenile arc terranes ended and when terminal collision began. The composition of well-dated magmatic rocks with distinctive compositions is a powerful tool in the effort to resolve this. We report on the chemical and isotopic composition of a suite of nine deformed dykes in southern Israel, known as the ‘schistose dykes’. Emplacement of the dykes can be constrained to have occurred about 630 Ma. These magmas had silica contents corresponding to basaltic andesites and andesites but had high Mg# (55 to 70) as well as Ni (70 to 225 ppm) and Cr (110-450 ppm) contents indicating that the most primitive samples were in equilibrium with mantle peridotite, while fractionated samples suffered modest fractionation. They are a medium-K, calc-alkaline suite, strongly enriched in LREE and depleted in HREE. They are clearly high-magnesium andesites and are very similar to Low-Ca Type 2 boninites; the latter magma in particular only forms in primitive arc environments. They have non-radiogenic initial 87Sr/86Sr (0.7026 to 0.7033) and radiogenic 143Nd/144Nd, with eNd(630) of +2.6 to +5.2 and TDM=0.75 to 0.95 Ga. An intimate relationship with a nearby quartz diorite is indicated, although it is not clear whether the dykes represent magma that fed into a magma body now filled with the quartz diorite, or issued from it. The generation of boninitic magma at 630 Ma requires an active and probably quite juvenile subduction zone. This further indicates that collision between E. and W. Gondwana to destroy the Mozambique Ocean and to create the end-Neoproterozoic supercontinent of Greater Gondwanaland or Pannotia must have occurred more recently than 630 Ma.