THE EDIACARAN OUARZAZATE GROUP, A METACRATONIC SILICIC LARGE IGNEOUS PROVINCE (SLIP): NEW U-PB ZIRCON AGES FROM OUZELLARH-SIROUA SALIENT (TIFNOUTE VALLEY)

During late Ediacaran times (580-543 Ma), outstanding thermo-magmatic events mark the post-collisional metacratonic evolution of the Moroccan Anti-Atlas, which is located at the northwestern border of the West African craton (north Gondwana). This late Pan-African period is marked by producing a kilometer-thick high-K calc-alkaline volcanosedimentary rocks associated with intrusions with a similar geochemistry, the whole belonging to the Ouarzazate Group (OG). The latter is preceded by the deposition of the Bou Salda Group (610-580 Ma) with which it forms the huge Ouarzazate Supergroup (OSG: 610-543 Ma). The OG is conformably to disconformably overlain by a thick Cambrian sedimentary formation (Tata Group), which was deposited in a post-orogenic extensional environment that led to the Early Ordovican opening of the Rheic Ocean.

We present here five new Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICPMS) U-Pb zircon ages of the OG rocks from the Tifnoute Valley area (Ouzellarh-Siroua salient). These ages are ranging from c. 577 to c.550 Ma and represent the whole magmatic activity of the OG in the area.

The Askaoun granodiorite yielded an age of 575 ± 2 Ma. This huge batholith has previously been dated at c.575 Ma and c. 558 which indicates its polyphasic feature. Same ages within errors of 577 ± 3 Ma and 574 ± 3 Ma are respectively yielded by a rhyodacitic ignimbrite and a rhyolitic flow at the bottom of the volcanic series. This age can be attributed to the beginning of the deposition of the OG and the minimum age of the collision event. Above, an andesitic tuff is dated at c. 562 ± 1 Ma and contains inherited zircons exclusively from the OSG (<610 Ma).

Lastly, a sample from Ougougane granitic dyke, a late volcanic arrival channel, provided an age of c. 550 ± 1.5 Ma with inherited zircons exclusively from the OG (<580 Ma). This dyke marks the end of the OG activity in the area.

In the Tifnoute Valley, the OG magmatic event emplaced by multiple pulses at c. 575 Ma, at c. 560 Ma and at c. 550 Ma, which is typical of LIP (Large Igneous Provinces). We discuss the genesis of this SLIP (Silicic Large Igneous Province) in the context of transtensive movements along the northern metacratonic border of the WAC at the end of the Pan-African orogeny.