Three types of peraluminous volcanic rocks from the Pairique complex, northern Puna: crustal melting and magma contamination in the central Andes.

CAFFE, P.J., TRUMBULL, R.B., SIEBEL, W.

Foz do Iguaçu

Congreso; AGU Meeting of the Americas; 2010

Strongly peraluminous (SP) volcanic rocks are rare in the Cenozoic magmatic record of the Argentine Puna. The origin of these rocks is typically related to crustal melting, or to a large proportion of crustal melts in hybrid magma genesis.Consequently, they may be useful for understanding the role of the Puna crust during Miocene magmatism. In the Pairique area (S22°55’ - W66°49’) three different types of SP rocks were recognised, based on mineral, geochemical and isotopiccomposition. These rocks are intercalated with high-K calcalkaline andesite to rhyolite volcanic deposits. The first SP type is represented by the Coyaguayma ignimbrite (~11.3 Ma), a high-SiO2 biotite-garnet rhyolite compositionally similar toperaluminous leucogranites (A/CNK > 1.2; low sumREE and, LREE, very low Zr, Th, Sr, La/Yb and Eu/Eu*). The contents of Rb, Sn, Nb, Y and U are high. Initial 87Sr/86Sr ratios are only moderately enriched (~0.713), whereas Nd (eNd -8 to -9) and Pbisotopes overlap the isotopic ratios of the other SP magmatic rocks from the area. The Corral Negro biotite-cordierite rhyolite (~10.8 Ma) represents another SP type. Compared with Coyaguayma, this has lower A/CNK (1.10-1.15) and SiO2 (71-72 %),lower Rb, Nb, Y and HREE, and higher MgO, FeO, CaO, Na2O, Sr, Ba, Zr, La/Yb (~35) and Eu/Eu*. These characteristics are similar to other central Andean cordierite rhyolites (e.g. Revancha dyke, Morococala). 87Sr/86Sr ratios (~0.725) are veryradiogenic, and eNd values are rather low (-11). The third SP magma type is represented by the Pairique domes, the youngest (<10.8 to 10.4 Ma) member of the Pairique volcanic complex. These biotite-orthopyroxene rhyodacites (67-70 % SiO2) havecordierite crystals as microlites or microphenocrysts with inclusions of fibrolite. Rock compositions follow the same chemical trends as those of the Corral Negro rhyolite, but some chemical features (e.g., A/CNK, Eu/Eu*, La/Yb) and isotope ratios(87Sr/86Sr ~0.716; eNd -9 to -10) are intermediate between the other two SP types. Melt inclusions in quartz are metaluminous, but matrix glasses are highly peraluminous. Mineralogical and geochemical characteristics, as well as estimatedcrystallisation conditions suggest that the three SP groups have related but different origins. The Coyaguayma ignimbrite is best explained by dehydration melting of a mid to upper crustal metasedimentary protholith, likely under disequilibrium conditions,as suggested by trace element and isotopic modelling. The Pairique domes are modelled by late stage (upper crustal) assimilation of metapelitic rocks or partial melts thereof into mildly peraluminous dacitic magmas, similar to calcalkaline rocks from thePairique complex. The latter probably acquired most of their geochemical and isotopic characteristics previously, by extensive assimilation of mid- to lower crust into mantle derived magmas. The Corral Negro rhyolites probably derived from equilibriumdehydration melting of crustal metasedimentary sources, possibly with minor contamination with mantle derived magmas.