The Amlah terrane, Yemen – a missing link in the Neoproterozoic tectonic collage of the Arabian Shield

The Amlah terrane, NW Yemen, comprises a suite of calc-alkaline tonalitic to granodioritic gneisses bounded by mafic volcanic and probable ophiolites. Occupying a key location between the largely juvenile arc terranes of the Saudi Arabian Shield and their correlatives in NW Yemen to the west and continentally influenced terranes of central Yemen to the east, it allows evaluation of models for the final assembly of the Arabian Shield. A field, geochemical, isotopic and geochronological study was undertaken in order to constrain the evolution of the Amlah terrane and its tectonic setting. Calc-alkaline gneisses dated between c. 760 and 670 Ma have Nd- and Pb-isotopic signatures indicative of minor involvement of older continental crust in their petrogenesis. Syn- and post-tectonic intrusions dated between c. 635 and 605 Ma show that suturing of the Amlah terrane was complete by around 640 Ma. These data are used to infer that the Amlah terrane developed as an Andean-style marginal arc on an extended western margin of the Paleoproterozoic continental Abas terrane. The western margin of the Amlah terrane is interpreted as the continuation into Yemen of the Nabitah suture Zone, which separates eastern (oceanic) and western (continent-influenced) arc terranes in Saudi Arabia.