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

Abstract

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 oceanic settings can be driven by magma intrusions at several levels of the crust but it can also be enhanced by intra-oceanic crustal shortening (hence, back-arc on arc stacking) – a still poorly documented geodynamic context in modern oceanic arc settings.