Intra-oceanic arc growth driven by magmatic and tectonic processes recorded in the Neoproterozoic Bougmane arc complex (Anti-Atlas, Morocco)

• Published in: Precambrian research 2018-01, Vol.304, p.39-63
• Main Authors: Triantafyllou, A., Berger, J., Baele, J-M., Bruguier, O., Diot, H., Ennih, N., Monnier, C., Plissart, G., Vandycke, S., Watlet, A.
• Format: Article
• Language: English
• Subjects: Arc growth; Dehydration melting; Earth Sciences; Garnet granulite; HP dehydration; Inttra-arc differentiation; Neoproterozoic; Pan-African orogeny; Petrography; Rutile dating; Sciences of the Universe
• ISSN: 0301-9268; 1872-7433
• DOI: 10.1016/j.precamres.2017.10.022

[Display omitted] •The Bougmane complex represents the lower crustal section of a Neoproterozoic oceanic arc.•Successive arc magmatic pulses triggered the ‘garnetisation’ of hydrous arc mafics and the in situ formation of tonalitic melt via dehydration and dehydration-melting reactions.•Moroccan oceanic arc relics reached a significant crustal thickness (>30–35 km) via a combination of intra-oceanic magmatic and tectonic processes. The Bougmane complex forms a Neoproterozoic tectonic unit located south of the Bou Azzer ophiolite in the Moroccan Anti-Atlas orogen. We show here that it corresponds to the lower crustal section of a Neoproterozoic intra-oceanic arc, which recorded 100–120 Ma of arc activity. The Bougmane complex is composed of two units: The 750–730 Ma gneissic unit is made of granodioritic gneiss-amphibolite which recorded a first magmatic pulse in the oceanic arc (IGN1). They are intruded during IGN2 event (710–690 Ma) by undeformed hydrous rocks consisting of hornblende gabbro (dated here at 706 ± 9 Ma, U-Pb protolithic zircons), hornblendite and minor tonalites. The high ƐNd values (+3.6 and +6.7) for all Bougmane rocks and their geochemical similarities ([La/Lu]N > 1.61; high LILE/HFSE ratio) with rocks in modern (Mesozoic to active) oceanic arcs, attest that the parental basic magmas were formed in a mantle wedge in an ocean-ocean subduction zone. The IGN2 event led to partial ‘garnetisation’ of the hornblende-gabbro along contacts with hornblendite bodies. Garnet formed by dehydration (sub-) and dehydration-melting (supra-solidus) of the hornblende-gabbro under HP conditions (>8 kbar). Field and geochemical modelling of Bougmane tonalites, showing low HREE ([La/Yb]N > 11) and high Sr/Y ratio (>1150), suggest that they formed by garnet-present disequilibrium melting of the hornblende-gabbro. U-Pb dating on rutiles in a hornblendite yielded an age of 686 ± 6 Ma, consistent with a post-IGN2 isobaric cooling. Rutiles from a garnet-hornblende gabbro yields a younger age of 658 ± 7 Ma suggesting that a high temperature event locally affected the Bougmane complex, likely related to a third igneous event (IGN3: 660–640 Ma) marked by intrusive diorites in the northern sector of the paleosuture. The building of the Anti-Atlas oceanic arc complexes occurred via three magmatic flare-ups (IGN1-2-3, centred at 750, 700 and 650 Ma respectively) interspersed with an early major tectonic episode (D1: 730–700 Ma). This suggests that the arc thickening in