The East African Orogen formed as a result of collision between portions of East and West Gondwanaland as the Mozambique Ocean closed in Late Neoproterozoic time, but it is not known exactly when. We use distinctive chemical and isotopic composition of deformed 'schistose' dykes in southern Israel to argue that this collision occurred after about 630 Ma, when the dykes were emplaced. These magmas had compositions of basaltic andesites and andesites but had high Mg# (100 Mg/Mg+Fe; 55-70 ppm), Ni (70-240 ppm), and Cr (100-400 ppm) indicating that the most primitive samples were in equilibrium with mantle peridotite; evolved samples suffered modest fractionation. The schistose dykes are a medium-K, calc-alkaline suite, strongly enriched in light rare earth elements and depleted in heavy rare earth elements. They are high-magnesium andesites and are similar to low-Ca type 2 boninites; similar magmas today only form over active subduction zones. The schistose dykes have non-radiogenic initial 87Sr/86Sr (0.7026-0.7033) and radiogenic 143Nd/144Nd, with ε Nd(630 Ma) of +2.7 to +4.9 and TDM=0.77-0.94 Ga. Hf isotopic compositions (εHf(630 Ma)=+6.8 to +8.8) confirm the juvenile nature of these magmas. The dykes are closely related to a nearby quartz diorite, 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 high-Mg andesite magma at 630 Ma involved reactive porous flow of a slab-derived melt through the mantle, requiring an active subduction zone, and strongly suggesting that young, hot seafloor - perhaps the spreading ridge of the Mozambique Ocean - was subducted. This indicates that collision between components of E. and W. Gondwana to form the East African Orogen must have occurred more recently than 630 Ma.
Bibliographical noteFunding Information:
This paper is dedicated to the memory of Yaacov Bentor, who pioneered the study of Precambrian basement of southern Israel, and who first studied the schistose dykes. L. Halicz (GSI) is thanked for the chemical analyses. This work was partly supported by GSI project 40082 and by the US–Israel BSF grant #2002337. Reviews by Joe Meert and Mark Reagan are gratefully acknowledged. [KF]
- Arabian-Nubian Shield
- East African Orogen
- High-magnesium andesite