Amphibole exerts a fundamental control on arc magma petrogenesis, differentiation, and the long-term evolution of the arc crust. This study identifies two texturally distinct amphibole populations within andesitic lavas and entrained hornblendite cumulates at the Quillacas monogenetic volcanic center in the Eastern Altiplano, Bolivia. Within the hornblendites, all amphiboles are tschermakitic, large (≤800 μm) with thick, granular reaction rims (avg. 27 μm thickness). In the host andesites, tschermakites are also the dominant amphibole species but are smaller (250–400 μm) with thin, symplectic reaction rims (avg. 7–9 μm thickness). An intergrowth of symplectic and granular reaction rims is also observed in this population. Collectively, the amphibole populations within the Quillacas magmatic system also record irregular volumetric decomposition where amphibole is replaced by mineral aggregates of plagioclase, pyroxene, and oxide within the crystal. This supports a relatively slow reaction of amphibole with melt trapped in fractures and cleavages during decompression-induced degassing. Geothermobarometry suggests that the hornblendite cumulate tschermakites crystallized at P-T conditions ranging from 453 to 598 ± 12 % MPa and 928–1004 ± 22 °C. The host andesite tschermakites crystallized at P-T conditions ranging from 430 to 617 ± 12 % MPa and 928–1004 ± 22 °C. These geothermobarometric constraints correspond to depths of 16–24 km which, within this region of the Central Andean crust, also corresponds to a regionally extensive low-seismic velocity zone. The texturally distinct amphibole populations imply a multi-stage trans-crustal magmatic system likely exists beneath the Quillacas volcanic center. In this scenario, a crystal mush zone exists at upper crustal depths where cumulate amphiboles initially crystallized. Magma recharge into this mush zone initiated a reaction between hornblendite cumulates and the melt which formed the amphibole granular rims. This recharge event also transported the host andesite amphiboles that subsequently developed symplectic rims due to heating and ascent-driven decompression. This study supports the presence of amphibole-dominated mush filters in the upper crust of the Central Andean arc and advances our understanding of amphibole's role in the evolution of arc magmatic systems.

Sheared peridotite xenoliths have been entrained by kimberlites in most global cratons but typically constitute only a minor proportion of mantle xenolith suites. Nevertheless, sheared peridotites are important because they record the short-lived cycles of metasomatism, deformation and rheological weakening that occur in Earth's ancient cratonic mantle immediately prior to their entrainment. Our work is focused on a highly-deformed ilmenite-dunite entrained from the south-east margin of the Kaapvaal craton by a Late Cretaceous kimberlite from Thaba Putsoa (northern Lesotho). The compositions of olivines and ilmenites show that this dunite is enriched in Fe and Ti and not simply a fragment of highly-refractory mantle. Two populations of olivine porphyroclasts are present: (i) Olivine porphyroclasts with moderate forsterite (Fo) contents (Fo85–86) and minor and trace element concentrations comparable to those in many other sheared peridotites and also Cr- and Fe-poor olivine megacrysts (Fo83–88) found in kimberlites from northern Lesotho and adjacent South Africa; and (ii) olivine porphyroclasts (Fo78–79) with compositions that are enriched in Mn, Zn and Ge and depleted in Ni, Ca, Cr, Al, V and Cu, and overlap with Cr-poor, Fe-rich olivine megacrysts (Fo78–82). Olivine neoblasts in the sheared ilmenite-dunite xenolith reveal the full range of Fo contents (Fo79–86) exhibited by the porphyroclasts whereas the orthopyroxene neoblasts have Mg# of 86.5, CaO contents of ~1 wt% CaO and variable Ti, Al and Cr contents. Ilmenite neoblasts have variable concentrations of TiO2 (45–55 wt%), MgO (9–12 wt%), and calculated Fe2O3 (2–12 wt%).This sheared fragment of mantle material is relatively unique in that it experienced multiple oxidizing magmatic/metasomatic events accompanied by deformation, giving important insights into a highly-dynamic environment within the Kaapvaal craton 90 Ma ago. The olivine megacrysts are thought to have crystallized in the Kaapvaal mantle from percolating proto-kimberlite melts and their chemical similarities with the porphyroclasts in the ilmenite-dunite imply that they both crystallized from a similar melt, but the two populations of porphyroclasts were subsequently mechanically mixed during deformation and contemporaneous oxidizing, Ti-rich metasomatism. Oxidation is implied by heterogeneous V/Sc ratios in olivine porphyroclasts and neoblasts and heterogeneous Fe2O3 contents in ilmenite neoblasts. We propose that multiple proto-kimberlitic pulses were widespread in northern Lesotho during the Late Cretaceous and led to the crystallization of megacrysts, metasomatism, oxidation and deformation of the surrounding mantle wall-rock. This short-lived metasomatism-deformation cycle caused mechanical, thermal and chemical perturbations in the lower Kaapvaal lithosphere, which had several consequences: (i) chemical and physical pre-conditioning of the lithosphere that facilitated subsequent kimberlite pulses to reach the surface; (ii) an overall mechanical weakening and destabilization of the lower lithosphere; and (iii) resorption of any diamonds present due to interaction with oxidizing melts.