Petrology of rocks formed by interaction of mafic magmas and high-grade metamorphic rocks at Suya Taco area, northern Sierra de Comechingones, Argentina

Abstract

Interaction between mafic magmas and crustal metamorphic rocks at mid-crustal levels causes modification of mafic magma composition by assimilation or reaction with crustal materials. This petrologic process is studied at the Suya Taco Complex where a suite of mafic magmas interacted with metamorphic rocks at 6.6±1 kbar. The Suya Taco Complex consists of close-spaced bodies of biotite-bearing gabbronorites. These magmas intruded into a supracrustal sequence consisting of amphibolite– to granulite–facies gneisses and migmatites. The peak-metamorphic mineral assemblage Bt+Qtz+Pl(An 30)+Grt+Kfs±Crd±Sil of supracrustal rocks crystallised before igneous mafic magmatism. Interaction between mafic magmas and metamorphic rocks produces two major hybrid lithologies: diorites made up of Opx+Bt+Pl (An 38–50)+Ilm±Cpx±Qtz; and garnet- and orthopyroxene-bearing rocks characterised by the coexistence of Bt+Pl (An 30–45)+Grt+Opx+Qtz+Ilm±Spl±Kfs. The latter lithology may locally appear as a quartz-absent and high-aluminous mineral assemblage. The two hybrid rocks occur interlayered at the contact zone between igneous mafic rocks and high-grade gneisses. Individual layers range from centimeter to meter scales; moreover, dioritic and orthopyroxene- and garnet-bearing layers present a spectrum of mineral assemblages and mineral chemistry that spread among those that appear in igneous mafic and metamorphic rocks. The work shows that the changes of mineral assemblages and variations of mineral chemistry in the hybrid diorites and orthopyroxene- and garnet-bearing rocks are the result of interaction processes between mafic magmas and regional metamorphic rocks. Field relationships, petrographic features, mineral compositional changes and qualitative analyses of phase relations suggest that rocks formed by interaction between mafic magmas and metamorphic rocks results from: (1) assimilation of metamorphic rocks by mafic magmas at variable degrees and (2) ion diffusion among mineral assemblages of contrasting composition. The implication of these processes is that Ca is transferred from mafic magmas to metamorphic rocks, which forces the breakdown of garnet and quartz, and induces the crystallisation of plagioclase and orthopyroxene in the resulting hybrid rocks.