Glass-rich, Cordierite-Biotite Rhyodacite, Valle Ninahuisa, Puno, SE Peru: Petrological evidence for hybridization of 'Lachlan S-type' and potassic mafic magmas

Abstract

The Revancha dyke, a 24·1 Ma, ≤5 m wide body of cordierite-biotite rhyodacite exposed in the Central Andean Inner Arc of southeastern Peru, is interpreted as a pristine analogue of a Lachlan-type, strongly peraluminous, S-type monzogranite. The preservation of both glass and microphenocrysts, as well as the hypabyssal setting, provide evidence of crystallization processes that are largely disguised in the more widespread plutonic or the largely pyroclastic extrusive examples of such rocks. The dyke contains up to 74% undevitrified glass, phenocrysts of plagioclase, biotite, Fe-cordierite and rare quartz and sanidine, and accessory sillimanite, apatite, zircon, ilmenite and mozanite. Both plagioclase and bitite phenocrysts exhibit complex zonation and convincing evidence of magmatic dissolution, whereas groundmass microphenocrysts and magnesian compositions than the phenocrysts. The magma was highly reduced, the sparse ilmenite approaching stoichiometric FeTiO3. Althogh comparable in most salient aspects with S-type granitoids of the Lachlan Fold Belt of SE Australia, the rhyodacite is enriched in K2O, Na2O, Al2O3, P2O5, U and Rb, but depleted in MgO, CaO, FeOT, TiO2, V and Co. The Revancha rhyodacite, like many peraluminous S-type suites, exhibits mineralogical, textural and chemical features strongly supporting shallow-crusral hybridization with a mafic melt, or melts, of mantle derivation, but the geochemical relationship suggest that the latter were probably markedly potassic rather than basaltic. The Revancha rhyodacite is a unit of the Picolani Group, a 22-26 Ma suite of diverse mafic and felsic rocks emplaced along the foreland boundary of the Central Andean orogen during the Aymará tectonic event. High heat-flow and relatively thin orogenic crust at this time promoted anatexis of clastic rocks at shallow depths. This geotectonic setting contrasts markedly with that of the subsequently, 7 17 Ma, Macusani Formatin ash-flows, and other strongly peraluminous, 'Himalayan-type', tow-mica rhyolites and granites of the Quenamari Group, emplaced during the major crustal thickening of the Quechuan orogeny.