Neoproterozoic granitoids associated with the Bou-Azzer ophiolitic melange (Anti-Atlas, Morocco): evidence of adakitic magmatism in an arc segment at the NW edge of the West-African craton

摘要

The Neoproterozoic intrusions of the Bou-Azzer El Graara inlier consist of metaluminous, medium to high-K, I-type granitoids. Two groups of granitoids can be distinguished based on chemistry and isotopic signature: (1) the early (≈ 670 Ma) medium-K calc-alkaline, pre-collisional diorites of Ousdrat, Bou-Azzer, Bou-Izbane, and Ait-Hmane, with less fractionated REE patterns (2.6 < (La/Yb)_N < 12.1); (2) the late (≈ 615Ma) high-K calc-alkaline, post-collisional granodiorites of Bleida characterized by relatively more fractionated REE patterns (8.9 < (La/Yb)_N < 12.6). These rocks show characteristics typical of arc magmatic rocks (depleted in Ti, Nb and Ta) and display, to various degrees, an adakitic signature (Al_2O_3 > 15%, 3 < %Na_2O < 6.4, Yb < 1.8 ppm, Y < 20 ppm and isotopic ratios of Sr and Nd similar to the ophiolitic rocks). However their La/Yb and Sr/Y are relatively low in most of the samples. The origin of these arc magmas is not completely understood. In this paper we argue that some of these rocks probably contain components of adakitic melts. The early group was produced by partial melting of subducted oceanic crust followed by interaction of the melt with the overlying mantle wedge, and the late group by dehydration melting of underplated basalts in the lower crust in the garnet stability field.