Petrology of an unusual orthopyroxene-bearing minette suite from southeastern Peru, Eastern Andean Cordillera: Al-rich lamproites contaminated by peraluminous granites

The Puno minettes (Eastern Andean Cordillera, southern Peru) are phlogopite–phyric rocks with a groundmass consisting mainly of phlogopite, alkali feldspar and apatite microphenocrysts and/or microlites. They display the strong large-ion-lithophile-element enrichments that characterize potassic to ultrapotassic rocks of the lamprophyre clan (Rb, Ba, Sr up to 103× Primordial Mantle, REE up to 6.102× CI-chondrites). In addition to their close association with granitoids, the rocks of this series show many features of minettes occurring in arc-related collisional zones, i.e., absence of mantle xenoliths, up to 30 vol.% felsic xenocrysts and strong negative Nb and Ti anomalies. However, they are distinguished by unusual phenocryst phases, whole-rock compositions and groundmass mineralogy. Some minettes contain only a few per cent of biotite±quartz±plagioclase xenocrysts immersed in a typical minette groundmass whereas those richer in xenocrysts display the complete liquidus paragenesis of the coexisting peraluminous granite (biotite+quartz+plagioclase+alkali feldspar+cordierite+zircon+pyrite) in a high-K latitic to trachydacitic groundmass. Field relations, radiometric ages, whole-rock data and felsic xenocrysts all indicate that the Puno minettes result from a mixing process involving Al-rich lamproites and peraluminous monzogranites. The most common phenocryst assemblage (highly magnesian phlogopite+olivine±titanian Cr-spinel±Ca-rich enstatite±magnesian ilmenite±Al- and Ti-poor diopside) is similar to that reported in Al-rich phlogopite lamproites from southeastern Spain. Likewise, the host rocks have diagnostic features of Spanish phlogopite lamproites (e.g., K2O/Al2O3 ratios >9, CaO contents <2 wt.%, negative Ba and Sr anomalies in primitive mantle-normalized multi-element plots, LaN/NdN<1.5). Graphite in the groundmass and Cr-spinel and ilmenite compositions devoid of Fe3+ suggest strongly reduced conditions of formation. Another phenocryst assemblage (Ca–Al-rich enstatite+Ca-deficient highly magnesian augite+magnesian phlogopite±ilmenite±discrete olivine) is observed in one sample which derives from an Al-rich leucite lamproite.