SHRIMP U-Pb zircon geochronology and Sr-Nd isotopic systematic of the Neoproterozoic Ghimbi-Nedjo mafic to intermediate intrusions of Western Ethiopia: a record of passive margin magmatism at 855 Ma?

摘要

The reworked Pre-Neoproterozoic and juvenile Neoproterozoic terrane of the Western Ethiopian Shield (WES) consists of three N-S trending terranes. These are the western migmatitic gneissic terrane, the central meta-volcano sedimentary terrane (CVST) and the eastern migmatitic gneissic terrane. The eastern part of the CVST mostly consists of suture-related ultramafic-metasedimen-tary complexes, whereas metavolcanics predominate in the western part. Gabbroic to granitic intrusions frequently occur in the CVST and in adjacent areas. New zircon SHRIMP U-Pb ages for two gabbros and three diorites in the Ghimbi-Nedjo region of the WES indicate magmatic crystallization ages. Two pulses of magmatism, at 860-850 and 795-785 Ma, are documented with the former for the first time. The tholeiitic Kemashi diorite and Bikilal-Ghimbi gabbros have oceanic affinities and yield U/Pb zircon ages of 856.3 ± 9.8 and 846.0 ± 7.6 Ma, respectively. The calc-alkaline Gebeya Kemisa pyroxene diorite, and the Senbet Dura hornblende diorite plus the tholeiitic Wayu Meni gabbro, which collectively have arc-back arc characteristics are indistinguishable at ages of 794.3 ± 9.4, 787.7 ± 8.8 and 778.1 ± 6.3 Ma, respectively. Positive sNd (4.5-7.0) and low initial ~(87)Sr/~(86)Sr (0.7029 ± 0.0002) and a mean T_(DM) model age of 0.95 Ga for the Ghimbi-Nedjo region (mean T_(DM) model age of 0.95 Ga for the WES overall) indicate that the magmas were generated from juvenile Neoproterozoic depleted mantle sources, with no discernable involvement of pre-Neoproterozoic continental crust. The occurrence of gabbros and diorites with oceanic tholeiite affinities combined with the new ages suggests that the intrusions were emplaced in the earliest stages of the rifting of Rodinia. This event in the WES led to the development of a passive margin and associated plume-type magmatism at ~855 Ma. The two intrusive groups with differing magma chemistry and ages suggest that the earliest magmatism was tholeiitic and associated with the passive margin system followed by continental breakup to form the Mozambique Ocean. The combination of tholeiitic and calc-alkaline magmatism was related to arc and back-arc basin formation and later terrane accretion (~ 830-690 Ma).