Coupled U-Hf isotopes and trace elements of detrital zircon grains from the lower Ediacaran turbidites in the Skoura inlier (Central High Atlas, Morocco): Implications for crustal evolution of the north-western Gondwana margin

Abstract

A thick succession of marine turbidites from the Skoura inlier (Central High Atlas) are studied here for the first time, using zircon LA-ICP-MS U-Pb dating, trace elements and Hf isotope geochemistry, to constrain the crustal evolution of the northwestern Gondwana margin, close to the West African Craton, during the late Neoproterozoic time. Zircon grains were extracted from four samples of lower Ediacaran siliciclastic rocks from the Skoura inlier (Central High Atlas, Morocco) in order to characterize the maximum depositional age and the provenance of the turbidites, as well as to analyse the composition of the magmatic sources where the zircon grains were formed.The maximum depositional ages of the four samples define a range of ca. 652–612 Ma (652.1 ± 5.7 Ma in sample KS, 621.2 ± 14.0 Ma in sample TA, 619.3 ± 10.0 Ma in sample AG and 611.7 ± 8.2 Ma in sample AS). The age distribution of detrital zircon grains suggests the existence of several sources comprising several populations dated at ca. 3200–2500 Ma, 2300–1800 Ma, 1600–1000 Ma, 950–650 Ma and 630–610 Ma. The detrital zircon grains show notable Mesoproterozoic ages and abundant peaks in the range of ca. 2200–2500 Ma that are considered absent in the West African Craton, implying contribution of other sources. Zircon REE patterns display prominent positive Ce anomalies, negative Eu anomalies and HREE enrichment, which are typical characteristics of a magmatic source as indicated by their high Th/U ratios and oscillatory zoning. The εHf(t) varies from + 0.6 to + 17.0 with a notable cluster between + 5 and + 15 for the Neoproterozoic zircon grains (ca. 720–611 Ma), indicating juvenile sources with slight mixing of old crust. The pre-Neoproterozoic populations indicate that juvenile sources and reworking of old crust played a role in the continental growth of the north-western margin of the West African Craton. The Skoura turbidites were deposited at ca. 650–610 Ma synchronous with orogenic processes in the northern West African Craton margin during the assembly of Gondwana.