Precambrian evolution of the Sirwa Window, Anti-Atlas Orogen, Morocco

Abstract

We present the results of a field, geochemical and geochronological study of a ∼5000 km 2 area of the Sirwa Window of the Anti-Atlas Orogen of Morocco. The region includes the northern edge of the Palaeoproterozoic (Eburnean) West African Craton (Zenaga Complex) and the southern margin of the Neoproterozoic (Pan-African) Anti-Atlas Orogen. The Zenaga Complex comprises medium grade supracrustal schists and intrusive granitoid orthogneisses, three of which gave within-error U–Pb SHRIMP zircon dates of ∼2035 Ma. The Anti-Atlas Orogen contains a vast thickness of volcano-sedimentary rocks known collectively as the Anti-Atlas Supergroup. The oldest of these comprises three, probably coeval, sequences collectively known as the Bleı̈da Group. The Bleı̈da Group includes tectonic inliers of schists and orthogneisses which gave a SHRIMP date of 743±14 Ma; medium-grade ophiolitic rocks in the central part of the area (Khzama and Nqob fragments) and a low-grade clastic-chemical volcano-sedimentary sequence (Taghdout Subgroup) along the northern margin of the Zenaga Complex. These rocks are interpreted as representing island-arc, fore-arc basin ocean-floor, and rifted continental margin sequences, respectively. The rocks developed north of the West African Craton during Neoproterozoic subduction of oceanic crust and the development of an arc/fore-arc complex. The Bleı̈da Group is overlain by the thick flysch-like volcano-sedimentary rocks of the Sarhro Group which were deposited before 615 Ma, according to the SHRIMP dates obtained from the oldest granitic bodies intruding them. The presence of glaciogenic diamictite units suggests a possible depositional age of ∼700 Ma. It is thought that the Sarhro Group was deposited in the fore-arc basin which developed between the island arc and the cratonic continental margin to the south. A reversal of plate movement vectors during Sarhro Group times led to a change from turbidite to coarse clastic deltaic deposition. This culminated in closure of the fore-arc basin and collision of the island arc with the Craton margin, ophiolite emplacement and deformation of the Sarhro Group and older rocks during the Pan-African Orogeny, which is probably dated at ∼660 Ma, the SHRIMP age given by metamorphic overgrowths on zircons from the arc rocks. The Pan-African collision event was associated with widespread late- to post-orogenic magmatism, including granitoids intruded at ∼614 Ma (Mzil Granite, Ida-ou-Illoun batholith) and a huge gabbro-diorite-granodiorite-granite I-type granitoid batholith at ∼580 Ma (Askaoun batholith). The granitoids are post-dated by the extensive post-orogenic volcano-sedimentary molasse sequences of the Ouarzazate Group and coeval polyphase granite plutons between 575 and 560 Ma. The Ouarzazate Group comprises immature, coarse clastic sedimentary rocks (conglomerates, arkoses, reworked volcanic rocks) acid to intermediate volcaniclastic rocks (lapilli and crystal tuffs, volcanic breccias, ignimbrites, etc.) and lavas (minor basalt, andesite and voluminous rhyolite). The volcanic component of this extensive succession was extruded from at least five separate interfingering volcanic centres, each with characteristic stratigraphies (designated subgroups), typically in fault-bound settings. The volcanic centres include calderas containing coeval high-level granites and quartz porphry bodies, along with rhyolitic plugs, domes and dykes. Post-orogenic magmatism terminated with the emplacement of a number of potassic leucogranite bodies, one of which has been dated at ∼560 Ma. This plutonic phase is recorded in the Zenaga Complex by Rb–Sr mica ages of ∼580–525 Ma. The Ouarzazate Group is conformably to disconformably overlain by a typical foreland basin succession (Tata Group). A sporadically-developed basal conglomerate unit is overlain by two marine carbonate-clastic/shale cycles. Stable isotope studies have indicated that the base of the Cambrian Era (∼544 Ma) lies near the top of the lowermost dolomitic unit. The evolution of the Sirwa Window may serve as a model for the entire Anti-Atlas Orogen, recording a cycle of cratonic rifting and ocean basin formation (∼800 Ma), subduction and island-arc formation (∼750 Ma), flysch deposition and volcanism in a fore-arc basin. Plate movement reversal from extension to convergence in Sarhro Group times led to eventual arc-continent collision and ophiolite obduction (∼660 Ma), late- to post-orogenic plutonism (∼615–580 Ma), post-orogenic extension, collapse, exhumation and molasse sedimentation, volcanism and plutonism (580–560 Ma) and marine foreland basin deposition (550 Ma to Palaeozoic).