Mobilisation of rare earth elements in shear zones: insights from the Tabouchent granodioritic pluton (Jebilet massif, Variscan Belt, Morocco)

In the Jebilet massif (Variscan belt, Morocco), two peraluminous granodioritic plutons (the Eastern and Central Jebilet plutons) are spatially associated with a regional shear zone. The Central Jebilet pluton includes the Tabouchent, Bramram and Bamega intrusions, which represent variably eroded portions of an arcuate apical part of the pluton. The intrusions are composed of biotite ± cordierite-bearing granodiorite that is crosscut by tourmaline-bearing leucogranite, which is greisenized in the Bramram cupola. Fluorine, Sn-W (Au) mineralization is present in the pluton while Cu-Au and Pb-Zn-Ag-Au mineralizations are present in the contact metamorphic aureole around the intrusion. Deformation is more homogeneous in the Bramram and Bamega than in the Tabouchent granodioritic intrusion where deformation is very heterogeneous and concentrated within conjugate subvertical meter-scale ductile shear zones that crosscut the intrusion. In the shear zones the breakdown of feldspar and biotite resulted in the transformation of the granodiorite to phyllonites composed of quartz + muscovite ± chlorite. 40Ar/39Ar dating on white mica allowed us to date the regional shearing event to 305.9 ± 0.9 Ma. Fluid-rock interaction in the shear zones resulted in mobilization of rare earth elements (REE) and other major and trace elements. Na and Ca are typically leached from the shear zones while K and volatiles are gained. REE mobilisation was decoupled from that of the major elements and was accompanied by fractionation between light and heavy REE. It was mainly controlled by the alteration of magmatic accessory REE-bearing phases, predominantly monazite, followed by the reprecipitation of syntectonic monazite, xenotime, Th + Ca ± REE phosphate, Ce oxide and REE fluorocarbonates. Other typical changes in the shear zones are enrichments in Sn, W, As and Sb. The mineralogical and chemical changes recorded in the shear zones that crosscut the granodiorite are similar to those associated with the greisenization that affected the Bramram leucogranite. They indicate that the shear zones provide pathways for rising acidic magmatic fluids exsolved from S-type leucogranites.