Abstract
The Khzama ophiolite is a highly dismembered complex located in the Siroua inlier of the Moroccan Anti-Atlas Belt. It consists of ultramafic rocks, cumulate gabbros, sheeted dikes, pillow lavas, and an overlying volcano-sedimentary sequence. Three main tectonic slices of sheeted dike complexes are studied in detail along three rivers, exposing well preserved outcrops where individual dikes are clearly distinguishable from the intruded host rock (Assif n’Tinzla, Assif n’Tasriwine, and Assif n’Iriri). Sheeted dikes of the Khzama ophiolitic complex are basaltic to andesitic in composition, displaying a clear sub-alkaline nature. We identify two sets of dikes that originate from lower High-Ti series (HTS) lavas and overlying upper Low-Ti series (LTS) lava. The immobile trace-element signatures of these rocks point to a genesis on a backarc environment with magmas sourced in a supra-subduction zone (SSZ) at the spinel peridotite zone. The obtained SHRIMP U-Pb data of the gabbro represent the first radiometric age of zircon extracted from the mafic rocks that were intruded by the sheeted dike complex of the Khzama ophiolite. These grains yield a concordia age of 763 ± 5 Ma, which is consistent with the 761.1 + 1.9/−1.6 and 762 + 1/−2 Ma U-Pb zircon ages of plagiogranites of Siroua. Based on their mineralogy, modal proportions, and major element chemistry, the felsic dikes are classified as high silica–low alumina trondhjemites or plagiogranites. These plagiogranites were likely formed by the partial melting of mafic rocks rather than by extreme fractional crystallization. A plagiogranite dated at 777 ± 4.7 Ma (U-Pb on zircon) is significantly older than the ca. 762 Ma plagiogranites previously recorded for the Khzama locality, suggesting a long-lived supra-subduction zone (SSZ) with conditions for the hydrous melting of mafic rocks.
Highlights
Ophiolites are distinctive assemblages of ultramafic, mafic, and felsic igneous rocks, which are often associated with pelagic sediments and they have long been recognized as important components of Neoproterozoic and younger orogenic belts [1]
Three main tectonic slices of sheeted dike complexes were studied in detail along three rivers, providing the least weathered outcrops where individual dikes are clearly visible (Assif n’Tinzla, Assif n’Tasriwine, and Assif n’Iriri, Figures 2 and 3)
The main conclusions arising from this study are: (1) The tectonomagmatic setting of the mafic and the felsic rocks associated with the sheeted dike complex and the tectonomagmatic setting of Khzama ophiolite is deciphered by using the immobile elements diagrams combined with the interpretation of REE spectra
Summary
Ophiolites are distinctive assemblages of ultramafic, mafic, and felsic igneous rocks, which are often associated with pelagic sediments and they have long been recognized as important components of Neoproterozoic and younger orogenic belts [1]. In the Neoproterozoic Anti-Atlas Belt of central Morocco (Figure 1A,B), the relics of oceanic crust (i.e., Bou-Azzer Group sensu [9]), are preserved in the Bou-Azzer (Aït Ahmane ophiolite) and Siroua (Khzama ophiolite) inliers as highly sheared allochthonous ophiolite complexes [10,11,12]. These ophiolitic assemblages are mainly made up of ultramafic rocks (serpentinites with chromite pods), and include mafic meta-cumulates, meta-basaltic sheeted dikes, and, to a smaller extent, pillow lavas [13,14,15]. Note that the northern boundary of the WAC is the Anti-Atlas Major Faults or the South Atlas Fault [26]
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