Generation of Tonalitic and Dioritic magmas by Coupled Partial Melting of Gabbroic and Metasedimentary Rocks within the deep Crust of the Famatinian Magmatic Arc, Argentina

OTAMENDI, J.E., DUCEA, M., TIBALDI, A.M., BERGANTZ, G., DE LA ROSA, J. Y VUJOVICH, G.

JOURNAL OF PETROLOGY

Oxford

Año: 2009 vol. 50 p. 841 - 873

0022-3530

Thesource regions of dioritic and tonalitic magmas have been identified in a deepcrustal section of the Famatinian arc (Sierras Pampeanas of western Argentina). Thesource zones of intermediate igneous rocks are located at the transitionbetween a gabbroic-dominated mafic unit and a tonalitic-dominated intermediateunit. In the upper levels of the mafic unit, maficmagmas intruded intometasedimentary packages, crystallised mainly as amphibole gabbronorite, andcaused the partial melting of the metasedimentary country rocks. In turn,theleucogranitic melts sourced from the metasedimentary rocks intruded into thenewly crystallised but still hot mafic layers and catalysed the process ofpartial melting of gabbroicplutonic rocks. Gabbroic rocks became mafic migmatitesformed by amphibole-richpyroxene-bearing mesosomes and leucotonalitic veins.Significantly, most mafic migmatites have isotopic composition (87Sr/86Sr(T)< 0.7063; Nd(T) = -0.94 to +2.24) similar to those of gabbroic rocks andmore distinct than those of their complementary leucotonalitic veins(87Sr/86Sr(T) = 0.7075 to 0.7126 and epsilon Nd(T) < -2.65), providingevidence for the idea that melting of the mafic Rocks was triggered by theintrusion of leucogranitic anatectic melts(87Sr/86Sr(T) = 0.715;Nd(T) = -6.21).Mass balance calculations show that the model reaction: plagioclase + amphibole+ leucogranitic melt; leucotonalitic melt +clinopyroxene ± orthopyroxene canbetter explain partial melting of the gabbroic rocks. Based on fieldobservations, we argue that the coalescence of leucotonalitic veins in themafic migmatites led to breakdown of the solid matrix to form melt-dominatedleucotonalitic pools. However, the leucotonalitic veins that crystallisedbefore leaving behind the mafic migmatitic rock are chemically (elemental andisotopic) more evolved than the dioritic and tonalitic rocks. We envisage thatwhile detached from the source region, leucotonalitic magmas were able toreact, commingle and mix with entrained fragments of both mafic and metasedimentaryrocks. This process gave rise to leucotonalitic melts became tonalitic anddioritic magmas. This study concludes that the generation of intermediatemagmas is a multistage process with three critical steps, which are: 1) influxand emplacement of hydrous mafic magmas into a deep crust containingmetasedimentary country rocks; 2) physically and chemically coupled melting ofmafic and metasedimentary rocks, leading to the formation of aleucotonaliticvein and dyke system that coalesce to form leucotonalitic or tonalitic magmabodies; 3) retrogression of the leucotonalitic magmas by partially assimilatingentrained fragments of their mafic and metasedimentary precursors. Thedimensions of the source zone seem to be insufficient to generate crustal scalevolume of intermediate igneous rocks. However, the Famatinian paleo-arc crustwould only expose those magma source zones which were still active during thetectonic closure of the arc. Ultimately, a time-integrated perspectiveindicates that early active source zones were cannibalized during the downwardexpansion of the plutonic bodies already dominated by intermediate plutonicrocks