Geochronology, petrogenesis and tectonic implications of Late Neoproterozoic gabbroic intrusions from the Taba-Nuweiba district, Sinai, Egypt

A number of Late Neoproterozoic gabbroic intrusions are exposed in the basement rocks of the Sinai Peninsula, which represents the extreme northern tip of the Arabian-Nubian Shield (ANS). To clarify the ages and geochemical characteristics of such mafic intrusions we selected two localities in the Taba-Nuweiba district: the Tweiba and El-Mahash intrusions. The Tweiba mafic intrusion is a composite body ranging in composition from gabbro to diorite and regionally metamorphosed to amphibolite grade. It intruded into Taba schists and orthogneisses and is truncated by a group of batholithic calc-alkaline granites. The El-Mahash mafic intrusion is a small, undeformed and unmetamorphosed, gabbroic body intruding the metamorphic rocks and truncated by post-collisional alkali granite. New SIMS U-Pb dating of zircons from two samples for the Tweiba mafic intrusion yielded ages of 618.2±4.5 (sample TA, a metadiorite) and 641.2±8.3 Ma (TB, an amphibolite metagabbro), while zircons from one sample (T48) of the El-Mahash mafic intrusion yielded an age of 606.7±8.3 Ma. We interpret all these ages as crystallization ages. The zircon ages and geologic context enable us to recognize two phases of mafic magmatism in the East-Central Sinai, at 614-650 Ma and at ~607 Ma. Geochemically, the gabbroic rocks of both intrusions are characterized by enrichment in LILE relative to HFSE and LREE relative to HREE. The gabbroic rocks of the Tweiba intrusion show a prominent negative trough at Ta-Nb in their normalized multi-element patterns, whereas this feature is less pronounced in the El-Mahash gabbro. Samples of the Tweiba intrusion have notable positive Eu anomalies [Eu/Eu* = 1.3-1.6], in contrast to the negligible Eu anomalies of El-Mahash samples [Eu/Eu* = 0.95–1.1]. Hf isotope measurements of dated zircons from both intrusions reveal consistently positive eHf(0), when age corrected to the likely igneous age of each sample: 4.0±0.5 for metagabbro TB, 5.0±0.8 for metadiorite TA, and 5.5±0.5 for gabbro T48. The older mafic body sampled at Tweiba, truncated by subduction-related calk-alkaline granites, represents a subduction-related tectonic setting; it correlates with the Shahmon metagabbro-diorite of the Eilat area. It is related to the early calc-alkaline subduction-related batholithic stage. Its magma source was probably depleted lithospheric mantle with minor crustal components. On the other hand, the younger mafic body sampled at El-Mahash, undeformed and truncated only by post-collisional alkaline granite, shows a geochemical signature similar to that of post-collisional mafic intrusions in the ANS. The parental magma of the El-Mahash intrusion was likely generated during the post-collisional lithospheric delamination event, where upwelling asthenosphere partially melted and the segregated mafic melts infiltrated the base of the ANS crust. The geochronologic data presented here constrain the end of subduction and the passage to post-collisional conditions in this portion of the northern ANS to ~610±10 Ma.