ROLE OF MELT SEGREGATION STYLE ON GRANITE CHEMISTRY AND MULTIPLE MELTING EVENTS IN THE SIERRA DE QUILMES, NW ARGENTINA

WOLFRAM, L; WEINBERG, ROBERTO; BECCHIO, RAÚL

Rimini

Conferencia; European Minerlogical Conference; 2016

European Minerlogical Union

Compositional variability of crustal-derived granites has been attributed to a multitude of processes. Though there has been much discussion on the entrainment of residuum and its effect on granite magma chemistry, the exact nature of what is entrained and when entrainment is efficient remains unclear. We describe the relationship between granulite facies migmatites and granites in the Sierra de Quilmes, NW Argentina, focussing on how the style of melt segregation impacts its ability to disaggregate the source and carry residual minerals, and therefore control granite chemistry.The north-south trending mountains that define the region known as the Sierras Pampeanas are characterised by variably metamorphosed turbidites of the Neoproterozoic?Cambrian Puncoviscana Formation. Early Ordovician subduction on the western Gondwana margin produced widespread high-T, low-P metamorphism of theturbidites during the Famatinian orogeny (~500?440 Ma), resulting in extensive anatexis and granite plutonism. The Tolombón complex of the northern Sierra de Quilmes is a tilted metamorphic sequence in the northern Sierras Pampeanas, providing near-complete exposure from granulite-facies magma source rocks in the southwest to granite emplacement levels in amphibolite and greenschist facies rocks in the northeast.Anatexis is associated with fluid-absent incongruent breakdown of biotite in granulite facies rocks, evidenced by the presence of peritectic Grt±Crd±Opx. The western Tolombón complex is separated from the Ovejeria complex by a shear zone. The Ovejeria complexis distinguished by a generally higher melt fraction, and is dominated by gradational contacts from metatexites to diatexites and granitesthat remain in or close to their source. In contrast, in the Tolombón complex,with few examples of diatexites, magmas are more commonly extracted from metatexites, and go on to feed stocks and plutons at higher crustal levels. Granites derived from diatexites tend to remain close to the source, and retain strong compositional similarities to the Puncoviscana Formation protolith, indicating significant mobilisation of residuum, defining the Ovejeria style of granite formation. Granites derived dominantly from melt extraction from metatexites tend to be leucogranites with compositions approaching those of experimental melts, defining the Tolombón style of granite formation. Magma derived through either mechanism undergoes further fractionation, giving rise to the compositionallydiversesuite of rockscommon to most anatectic terranes.Magmas derived through the Tolombón style of melt extraction are leucocratic, and impoverished in LREE, Th, and Zr compared to both the sedimentary source rocks and residuum-rich magmas derived through the Ovejeria style of granite formation. The low solubility of zircon and monazite in relatively dry and peraluminous leucogranite meltsguarantees that Zr, Th, REE behave as compatible elements during dehydration melting of metasedimentary packages.Therefore, neither style of granite formation contributes to the transfer of many of the typical trace elements enriched in the upper crust. Instead, biotite dehydration melting in the Sierra de Quilmes had the opposite effect of typical crustal differentiation, concentrating these trace elements in the residual source.