Silicic magmatism and continental break-up : the Frontal Cordillera Composite Batholith, Mendoza, Argentina
The Frontal Cordillera Composite Batholith (FCCB) is made up of a string of granitic plutons thought to be Permo-Triassic in age, that stretch with a N/S strike for the entire length of the Chilean and Argentine Frontal Cordillera (FC). Associated spatially and temporally with the batholith is a series of volcanic rocks called the Choiyoi Formation. Seven FC stocks were studied: the Boca del Rio, Cacheuta, Cerro Médanos, Cerro Arenales, Cerro Bayo, Punta Negra and Punta Blanca. They are high-K, I-type metaluminous, calc-alkaline granitic plutons, which range in composition from granodiorites to monzogranites to syenogranites. Contact relationships between the granitic stocks and the Choiyoi Formation, long thought to be their effusive equivalents, are intrusive, suggesting that the volcanic rocks are older. Chemically, the Choiyoi Formation rocks are also high K, alkalic and silica undersaturated, and similar to the granites. The stocks contain typical metaluminous modal mineralogies of plagioclase, quartz and alkali feldspar felsic phases, with amphibole and biotite as mafics, and magnetite, zircon, titanite, apatite and allanite as accessories. Field studies and P-T data from amphiboles suggest depths of emplacement of less than 10km. All the stocks have been affected by hydrothermal alteration, the main effects being sericitisation and/or argillisation of plagioclase cores, chloritisation of biotite and amphibole and oxidation of magnetite to haematite. The source of the hydrothermal fluids is thought to be the granites themselves. Four of the stocks were modelled with major, trace and rare earth elements. It was concluded that the Cacheuta stock evolved to aplitic compositions by ~35% fractionation of mainly plagioclase and alkali feldspar; the Cerro Arenales stock by ~30% fractionation of plagioclase, biotite and k-spar; and the Punta Negra stock by ~25% fractionation of plagioclase, alkali feldspar and quartz. The Punta Blanca stock was interpreted as consisting of more than one batch of magma. The granites from this segment of the FCCB differ from typical I-type granites in some crucial respects: 1) they are mainly high K, 2) alkali feldspar crystallised early and fractionated in the more evolved stocks, 3) the stocks contain high Ba and intermediate Sr, with U and Th enrichment in the leucocratic stocks, and 4) the stocks form part of a batholith in which granites predominate over granodiorites and tonalites. Typical I-type Cordilleran batholiths like the Coastal Batholith of Peru, are predominantly tonalitic to granodioritic. Three stocks from the study area were dated by the U-Pb zircon method. These are the first such dates for any plutons of the FCCB. The Punta Blanca stock was found to be 276 ± 1 Ma old, the Cerro Médanos stock 263 ± 1 Ma old, and the Cerro Bayo stock 262 ± 3 Ma old. All of these ages are in the Lower Permian. Sr-Nd isotopes suggest that the Choiyoi Formation and FCCB stocks were derived from different lower crustal sources which were variably enriched with [sup]87 Sr and unradiogenic Nd, extracted from the mantle at ~1.3 Ga and ~1.25 Ga, respectively. AFC processes are not thought to be important. The geodynamic circumstances which lead to the generation of the Choiyoi Formation and the FCCB are still not clear. The evidence collected during this study suggests that the Choiyoi phase of volcanism subsequent to the movements of the San Rafael Orogenic Phase at the end of the Carboniferous was possibly the result of extension, which caused extensive melting of a lower crustal source extracted from the mantle at ~1.3 Ga. This was followed by a switch in tectonic style in the Early Permian, possibly due to the intiation of oblique subduction, which lead to the sampling of a younger lower crustal source and the formation of the FCCB. This scenario is analogous to that recorded by Rapela et al., (1996) further to the south in the 'Gondwana' magmatism of Patagonia between the Late Triassic to Jurassic.