PETROLOGY OF AN UNUSUAL ORTHO-PYROXENE-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 phlog opite-phyric rocks with a groundmass consisting mainly of phlogopite, alkali feldspar and apatite microphenocrysts and/or microlites. They d isplay the strong large-ion-lithophile-element enrichments that charac terize potassic to ultrapotassic rocks of the lamprophyre clan (Rb, Ba , Sr up to 10(3) X Primordial Mantle, REE up to 6.10(2) X CI-chondrite s). In addition to their close association with granitoids, the rocks of this series show many features of minettes occurring in are-related collisional zones, i.e., absence of mantle xenoliths, up to 30 vol.% felsic xenocrysts and strong negative Nb and Ti anomalies. However, th ey are distinguished by unusual phenocryst phases, whole-rock composit ions and groundmass mineralogy. Some minettes contain only a few per c ent of biotite +/- quartz +/- plagioclase xenocrysts immersed in a typ ical minette groundmass whereas those richer in xenocrysts display the complete liquidus paragenesis of the coexisting peraluminous granite (biotite + quartz + plagioclase + alkali feldspar + cordierite + zirco n + pyrite) in a high-K latitic to trachydacitic groundmass. Field rel ations, radiometric ages, whole-rock data and felsic xenocrysts all in dicate that the Puno minettes result from a mixing process involving A l-rich lamproites and peraluminous monzogranites. The most common phen ocryst 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 lampr oites from southeastern Spain. Likewise, the host rocks have diagnosti c features of Spanish phlogopite lamproites (e.g., K2O/Al2O3 ratios >9 , CaO contents <2 wt.%, negative Ba and Sr anomalies in primitive mant le-normalized multi-element plots, La-N/Nd-N <1.5). Graphite in the gr oundmass and Cr-spinel and ilmenite compositions devoid of Fe3+ sugges t strongly reduced conditions of formation. Another phenocryst assembl age (Ca-Al-rich enstatite + Ca-deficient highly magnesian augite + mag nesian phlogopite +/- ilmenite +/- discrete olivine) is observed in on e sample which derives from an Al-rich leucite lamproite.