Neoproterozoic dyke swarms from southern Sinai (Egypt): geochemistry and petrogenetic aspects

Abstract

Neoproterozoic ( ca 0.6 Ga) basic to acidic dyke swarms are widespread in southern Sinai (Egypt). Two mineralogically and geochemically distinct subalkaline basic-intermediate dyke suites are spatially closely associated in the northern portions of the Arabian-Nubian Shield. Augite bearing dykes (CpxD) correspond to basalts and basaltic trachyandesites, while the amphibole bearing dykes (AmpD) are basaltic andesites and andesites. The acidic dykes have rhyolitic compositions. CpxD define trends typical of tholeiitic suites and are characterised by high TiO 2 and incompatible elements (e.g. light REE and HFSE), and are similar to high TiO 2 continental basalts. In contrast, the AmpD define a calcalkaline suite. Modelling shows that differentiation of both CpxD and AmpD suites is compatible with fractional crystallisation occurring at relatively low and high fO 2 (QFM versus NNO buffer) and H 2O (0–0.5 versus ≥2 wt%) conditions, respectively. Major and trace element relationships ruled out that CpxD and AmpD were derived from a similar parental magma, but require distinct primary melts and different source mantle. For the AmpD, crustal contamination cannot be ruled out, but it is difficult to evaluate the role of crustal components derived from a subducting slab, from that which might be due to crystalline basement. The source mantle of the Amp magmas was H 2O enriched by fluids released from a subducting slab, and this allowed the genesis of melts with high SiO 2. The source of the Cpx magmas would instead correspond to lithospheric mantle portions poorly affected by fluid release but characterised by a relatively high content of TiO 2 and incompatible elements. A fractional crystallisation genesis for the acidic dykes starting from Cpx or Amp magmas is not supported by trace element Rayleigh's calculations and the SiO 2 gap (60–71 wt%). Therefore, the possibility that they formed by the melting of lower basic-intermediate continental crust or underplated basalts cannot be ruled out. On the whole, the close association of tholeiitic (CpxD) and talc-alkaline (AmpD) magmas in space and time suggests that the studied Sinai dykes can be related to late extensional stages of the Pan-African cycle.