Abstract

High-K calcalkaline metaluminous granitoids are abundant in many 'orogenic' to 'post-orogenic' tectonic settings. Models for their origins include: assimilation of crustal rocks by basaltic magmas, partial melting of enriched mantle, reactive assimilation of wall rocks by normal-K magmas, magma mixing, and melting of interlayered pelitic and amphibolitic source rocks. In many cases, multiple source and mixing origins can be ruled out on geological and geochemical grounds, and some writers prefer single-stage crustal melting as a model. Published experimental data show that metabasaltic rocks are not suitable sources; nor are any studied greywackes, metatonalites or metadacites. Roberts and Clemens (1993) hypothesized that the suitable protoliths would be high-K andesites - common rock types in magmatic arcs. Subsequently it has been shown that certain metaluminous Bt-Pl-Qtz gneisses can partially melt to produce peraluminous high-K melts. The present preliminary experiments investigated the Roberts and Clemens hypothesis, using two unmetamorphosed high-K andesites from the Miocene (12 - 5 Ma) Cordillera Blanca batholith, northern Peru. Both contain around 55 wt% SiO2 and 2 wt% K2O. One has 10% Bt + 40% Hbl and the other 7% Bt + 20% Hbl. Experiments were at 1 GPa, fluid-absent, carried out in Au capsules, in a piston-cylinder apparatus with NaCl ± Pyrex cells. Oxygen fugacity was ~ FMQ. Runs at 890 and 910°C were for 70 to 125 h duration respectively. Products include glass (quenched melt) + Pl + Bt + Hbl ± Qtz, and new Hbl + Cpx ± Opx. Electron probe (EDS) analyses of glasses show that melts are generally high-K, mostly monzogranitic, with one granodiorite. High-K granitoid melts can indeed be produced by partial melting of potassic arc andesites, but the experiments produced only relatively small melt proportions. If high-K magmas are formed in this way, crustal temperatures must be > 900°C. The single-source model is attractive because it relies only on the presence of rocks typical of arc settings, and does not demand that they be hydrated and metamorphosed as a prelude to partial melting. The main problem is to heat the crust sufficiently to induce partial melting. This can probably be accomplished through multiple injection of mafic magma. Given that high-K rocks are usually emplaced into the continental sides of arcs, in extensional settings, hot mantle itself may dome into the thinned crust. The preliminary conclusion is that neither the protoliths nor the conditions involved in the genesis of the high-K series need be special or complicated. Single-stage melting of typical old arc crust may be all that is required to impart the observed chemical and isotopic signatures.