The Moroccan Anti-Atlas ophiolites: Timing and melting processes in an intra-oceanic arc-back-arc environment

Abstract

The Moroccan Anti-Atlas orogenic belt encloses several Precambrian inliers comprising two major Neoproterozoic ophiolitic complexes: the Sirwa and Bou Azzer ophiolites. These ophiolites expose crustal and mantle units, thrusting over fragments of a long-lived intra-oceanic arc system. We present a detailed geochronological and petrogeochemical study of three mafic/ultramafic units of these two ophiolites: the Khzama sequence (Sirwa ophiolite) and the Northern and Southern Aït Ahmane sequences (Bou Azzer ophiolite). The crystallization of layered metagabbros from the Bou Azzer ophiolite (North Aït Ahmane sequence) has been dated here at 759 ± 2 Ma (U-Pb on zircons). This new age for the Bou Azzer ophiolite is similar to the formation of the Sirwa ophiolite (762 Ma) and suggests that both units formed during the same spreading event. Metabasalts of the three units show tholeiitic signature but with variable subduction-related imprints marked by LILE enrichments, HFSE depletions and variable Ti contents, similar to modern back-arc basin basalts (BABB). Their back-arc origin is also supported by the geochemical signature of ultramafic units showing very low contents in major and trace incompatible elements (Al 2 O 3 : 0.12-1.53 wt%, Ti: 3.5-64.2 ppm and Nb: 0.004-0.10 ppm), attesting of a highly refractory protolith. This is in agreement with the high Cr# (0.44-0.81) and low to intermediate Mg# (0.25-0.73) of their constitutive Cr-spinels. Dynamic melting models suggest that these serpentinites experienced intense and polyphased hydrous melting events, strongly influenced by supra-subduction zone SSZ-fluid influx and subduction-related melt percolation. Being particularly affected by these SSZ-melt/rock interactions and closer to arc units to the south, the Sirwa ophiolite and the South Aït Ahmane unit of the Bou Azzer ophiolite likely represent an early stage of the arc-back-arc system, which has been more influenced by the magmatic products of the arc activity compared to the North Aït Ahmane unit of the Bou Azzer ophiolite.

Magnetic Properties of Ferritchromite and Cr‐Magnetite and Monitoring of Cr‐Spinels Alteration in Ultramafic and Mafic Rocks

“…Serpentinites from the southern ultramafic unit of the Aït Ahmane ophiolitic sequence (Bou Azzer ophiolite, Moroccan Anti‐Atlas, see the detailed petrographic and petrological study in Hodel, 2017 and Hodel et al., 2020). These serpentinites are very similar to those of the Khzama ophiolite.…”

“…These rocks are essentially antigorite and generally present a nonpseudomorphic texture characteristic of high‐temperature serpentinization ( T > 350°C, Evans, 2004; Schwartz et al., 2013). Lizardite/chrysotile assemblages, sometimes occurring in the form of nodules within the antigoritic matrix, seem to indicate a first episode of serpentinization at a lower temperature (<350°C, Hodel, 2017), prior to antigoritization which likely occurred during the obduction of the ophiolitic sequences (Hodel, 2017; Hodel et al., 2020). Despite this important metamorphism episode involving temperatures above 350°C, spinels of these serpentinites are only weakly altered (Figures 2c and 2d).…”

“…Serpentinites from the Khzama ophiolite are located in the Sirwa Inlier, in the Moroccan Anti‐Atlas (see the detailed petrographic and petrological study in Hodel, 2017 and Hodel et al., 2020). These rocks are essentially antigorite and generally present a nonpseudomorphic texture characteristic of high‐temperature serpentinization ( T > 350°C, Evans, 2004; Schwartz et al., 2013).…”

“…This study is based on extensive sampling of ultramafic units from Neoproterozoic ophiolites in Brazil (the Araguaia Belt, Hodel et al, 2019) and Morocco (Bou Azzer and Khzama, Hodel et al, 2017Hodel et al, , 2018Hodel et al, , 2020. All samples investigated are fully serpentinized peridotite (hydrated mantle) that have been differently affected by obduction-related metamorphism and, for some of them, by intense hydrothermal alteration (Hodel et al, 2017(Hodel et al, , 2018.…”

“…Indeed, the chemistry of spinel core reflects partial melting and refertilization processes while spinel alteration rims (when present) testify the metamorphic conditions endured by their host rock (e.g., Barnes, 2000;Barnes & Roeder, 2001;Dick & Bullen, 1984). Spinels are commonly the only pristine mineral preserved in fully serpentinized peridotites, making them one of the most investigated minerals in these rocks (e.g., Ahmed & Surour, 2016;Arai, 1994;Barnes, 2000;Barra et al, 2014;Dick & Bullen, 1984;González-Jiménez et al, 2009;Hodel et al, 2017Hodel et al, , 2019Hodel et al, , 2020Ishii et al, 1992;Marques et al, 2007;Mellini et al, 2005;Pearce et al, 2000;Saumur & Hattori, 2013).…”