Generation of tonalitic and dioritic magmas by coupled partial melting of gabbroic and metasedimentary rocks within the deep crust of the Famatinian magmatic arc.

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

JOURNAL OF PETROLOGY

Elsevier

Año: 2009 vol. 50 p. 841 - 873

0022-3530

The source regions of dioritic and tonalitic magmas have been identified in a deep crustal section of the Famatinian arc (Sierras Pampeanas of western Argentina). The source zones of intermediate igneous rocks are located at the transition between a gabbroic-dominated mafic unit and a tonalitic-dominated intermediate unit. In the upper part of the mafic unit (on the western side of the geological section), mafic magmas intruded into metasedimentary supracrustal packages and crystallised mainly as amphibole-rich gabbronorites. We interpreted this relation to indicate that hydrous mafic magmas heated and caused the partial melting of the metasedimentary country rocks. In turn, field relationships show that the leucogranitic melts sourced from the metasedimentary rocks intruded into the still hot mafic layers and catalysed the process of partial melting of gabbroic plutonic rocks. Mafic plutonic rocks become mafic migmatites formed by leucotonalitic leucosomes and amphibole-rich pyroxene-bearing mesosomes. Significantly, the mafic migmatitic mesosomes have isotopic composition (87Sr/86Sr(T) < 0.7063 and .Nd(T) = -0.94 to +2.24) more primitive than their complementary leucotonalitic leucosomes (87Sr/86Sr(T) = 0.7075 to 0.7126 and .Nd(T) < -2.65), so providing evidence for the idea that melting of the mafic rocks is caused by the intrusion of leucogranitic anatectic melts (87Sr/86Sr(T) = 0.715 and .Nd(T) = -6.21). Reinforcing the hypothesis, mass balance with mineral and whole-rock compositions is used to show that the model reaction: plagioclase + amphibole + leucogranitic melt 9 leucotonalitic melt + clinopyroxene ± orthopyroxene, may better explain partial melting of the gabbroic rocks. Based on field observations, we show that the coalescence of leucotonalitic leucosomes in the mafic migmatites leads to breakdown of the solid matrix to form melt-dominated leucotonalitic pools. However, the leucotonalitic segregations that crystallised before leaving behind the mafic migmatitic rock are chemically (elemental and isotopic) more evolved than the dioritic and tonalitic rocks. We argue that, while detached from the source region, leucotonalitic magmas were able to react, commingle and mix with entrained fragments of both mafic and metasedimentary rocks. This process leads to leucotonalitic melts become tonalitic and then dioritic magmas. Our study concludes that the generation of intermediate magmas is a multistage process with three critical steps, which are: 1) influx and emplacement of hydrous mafic magmas into a deep crust containing metasedimentary country rocks; 2) physically and chemically coupled melting of mafic and metasedimentary rocks, leading to the formation of a leucotonalitic veining and dyking system which coalesce to form leucotonalitic or tonalitic magma bodies; 3) retrogression of the leucotonalitic magmas by partially assimilating entrained fragments of their mafic and metasedimentary precursors. The dimensions of the source zone seem to be insufficient to generate crustal scale volume of intermediate igneous rocks. However, the Famatinian paleoarc crust would only expose those magma source zones which were still active during the tectonic closure of the arc. Ultimately, a dynamic perspective would indicate that early active source zones were cannibalized during the down-ward expansion of the plutonic bodies dominated by intermediate plutonic rocks.