Petrographic, Geochemical and Zircon Typology of Neoproterozoic Granitoids: Evidence of Continental Thickening in Saghro Arc (Eastern Anti-Atlas, Morocco)

Abstract

Saghro is considered as a volcanic arc of the Neoproterozoic. The magmatic activity during this period is characterized by the emplacment of granitic plutons North-East from Bou-Azzer El Graara's suture zone. Sidi Flah-Bouskour buttonhole represents the near part of Saghro to this zone and Ougnat hills the far part. The buttonhole of Sidi Flah-Bouskour is cut by several successive granitic intrusions of Neoproterozoic. The earliest suite, composed of gabbro, quartz diorite and amphibole granodiorite constitute the Pan-African B1 phase related to pre- collision domain, and shows a calc-alkaline chemistry. The second phase includes the biotite granite of Bouskour which belong to the potassic calc-alkali suite emplaced during the Pan- African B2 phase in a syn-collisional environment. The granodioritic and granitic trends show separate evolution in CaO-Na,O-K,O diagram. Quartz diorite and amphibole granodiorite have moderate content of Sr (420-40ppm), Rb (133-88ppm), Cr (177-21ppm), Zr (131-67ppm) and V(470- 30ppm) and hight content of Ba (1156-368ppm). Garnet and biotite have low content of Sr (3lppm), Rb (104ppm), Cr (42ppm), V (93ppm), Zr (93ppm) and Ba (450ppm). REE normalised to MORB show that the Sidi Flah-Bouskour granitoids are enriched in LREE and LILE and relatively depleted in HFSE and Nb.The zircon populations of the quartz diorite and amphibole granodiorite show a strong frequency in the sub-type S17-18 with low @ index and high T index implying a temperature of 750 - 800°C. In addition to the T.E.T, the average points (0, T) placed in the (LA, 1.T) typological diagram permit to classify these rock into the hybrid granites domain which belong to the calc-alkali granodioritic affinity. The biotite granite shows zircon populations with dominance of the S19 sub-type implying high 0 index with apparition of the types P, D and J, and high T index. The average points (0, T) and the T.E.T. permit to classify the biotite granite among the hybrid granites implying a alkali-rich calc-alkali trend. Quartz diorite, amphibole granodiorite and garnet granite are emplaced in Ougnat hill, far separated from the suture zone. Petrographic, geochemical and zircon typology study of these rocks shows existence of two magmatic groups. The quartz diorite and the amphibole granodiorite represents a medium-K calc-alkaline trend and shows alkali enrichment (Bl). CaO-Na,O- KzO relationship shows that quartz diorite and granodiorite evolved separately from that of the garnet granite. Petrographic, mineralogical (zircon typology) and geochemical (overlap of garnet granite REE pattern on calculated crustal partial melt pattern) features of garnet granite are similar to those of crustal anatectic granites (B2). The occurrence of granodiorite very rich in biotite and mantled garnet grains identical to those in the crustal garnet granite indicates processes of assimilation and magma contamination during the emplacement of Ougnat granitoids. Biotite granite emplaced in Sidi Flah-Bouskour, adjacent to Saghro near the suture zone shows strong crustal participation and granitoids in Iknioun show high-K affinities. Away from the suture zone' in Ougnat' garnet granites of crustal origin occur. Isotopic data show variation of ENdi between biotite granite in Sidi Flah-Boukour (ENd= -1.8-2.4) and garnet granite in Ougnat (ENdi= -4.5). This is interpreted to indicate an increase in incorporation of crustal component while moving North-East. The contrast in isotopic data agree well with the spatial zonation observed in Saghro. This zonation might have evolved by crustal thickening toward the axis of Bou-Azzer El Graara's suture which is considered as a paleo-subduction zone during Neoproterozoic according to the model of Meen (1987).