Crust-mantle coupling during continental convergence and break-up: Constraints from peridotite xenoliths from the Borborema Province, northeast Brazil

Shiran Liu,Andréa Tommasi,Alain Vauchez,Maurizio Mazzucchelli

doi:10.1016/j.tecto.2019.05.017

Abstract

We studied a suite of mantle xenoliths carried by Cenozoic volcanism in the Borborema Province, NE Brazil. These xenoliths sample a subcontinental lithospheric mantle affected by multiple continental convergence and rifting events since the Archean. Equilibrium temperatures indicate a rather hot geotherm, implying a ca. 80 km thick lithosphere. Most xenoliths have coarse-granular and coarse-porphyroclastic microstructures, recording variable degrees of annealing following deformation. The high annealing degree and equilibrated pyroxene shapes in coarse-granular peridotites equilibrated at ~900 °C indicate that the last deformation event that affected these peridotites is several hundreds of Ma old. Coarse-porphyroclastic peridotites equilibrated at 950–1100 °C probably record younger (Cretaceous?) deformation in the deep lithospheric mantle. In addition, a few xenoliths show fine-porphyroclastic microstructures and equilibrium temperatures ≥1200 °C, which imply recent deformation, probably related to the dykes that fed the Cenozoic volcanism. Chemical and microstructural evidence for reactive percolation of melts is widespread. Variation in textural and chemical equilibrium among samples implies multiple melt percolation events well spaced in time (from Neoproterozoic or older to Cenozoic). Crystal preferred orientations of olivine and pyroxenes point to deformation controlled by dislocation creep with dominant activation of the [100](010) and [001]{0kl} slip systems in olivine and pyroxenes, respectively, for all microstructures. Comparison of xenoliths' seismic properties to SKS splitting data in the nearby RCBR station together with the equilibrated microstructures in the low-temperature xenoliths point to coupled crust-mantle deformation in the Neoproterozoic (Brasiliano) continental-scale shear zones, which is still preserved in the shallow lithospheric mantle. This implies limited reworking of the lithospheric mantle in response to extension during the opening of the Equatorial Atlantic in the Cretaceous, which in the present sampling is restricted to the base of the lithosphere.

Highlights

If you want to link crustal and lithospheric deformation, it becomes important to make sure that the reader knows about the main events as known from the crust
The coarse-porphyroclastic microstructures that characterize the lower part of the lithospheric mantle are more difficult to relate to a given tectonic episode
Integrated analysis of microstructures, crystal preferred orientations, mineral chemical compositions, and equilibrium temperatures in a suite of 22 peridotite xenoliths reveals that the lithospheric mantle beneath the Northern Borborema Province preserves microstructures related to different deformation

Summary

Introduction

If you want to link crustal and lithospheric deformation, it becomes important to make sure that the reader knows about the main events as known from the crust. They are usually composed by crustal domains with varied tectonic ages. Major tectonic events such as convergence, collision, and rifting involve the entire plate, the crust. How the deformation is accommodated in the lithospheric mantle and the level of coupling between crust and mantle deformation during these major tectonic episodes are still matter of debate

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