Geochemical characterization and emplacement of the Early Triassic felsic dyke swarms, Menucos- La Esperanza, North Patagonian Massif

MARTINEZ DOPICO, C.I.; LÓPEZ DE LUCHI, MÓNICA; MARQUEZ, EMILIA; FRANCESCHINIS, PABLO R.; RAPALINI, AUGUSTO E.; MIGUEZ, MAXIMILIANO; COLAVITTO, BRUNO

Congreso; XXI Congreso Geológico Argentino; 2022

Acidic dyke swarms are an exceptional expression of crustal extension as feeding channels for silicarichmagmas in their way from the magmatic reservoirs throughout the upper crust. Evidence of extensionalconditions from the Late Permian to Late Triassic can be tracked in the centre of the North PatagonianMassif, such as large shallow plutonic-volcanic complexes and volcanoclastic deposits (i.e. La Esperanzaplutonic- volcanic complex, Martínez Dopico et al. 2019, Los Menucos basin, Luppo et al. 2018) and largeW-E dextral strike-slip fault systems with kilometric displacements (i.e. Giacosa et al. 2007 Luppo et al.2018). In the area of Estancia Cerro Piche, 45 km to the NW of Los Menucos (40º51´S, 68º5´E), three setsof rhyolitic dyke swarms (10-50 m-length; 0.5 to 3 m-width) were recognized, mainly W-E to WNW-ESE andNW-SE, and minor NE-SW. The distribution of dykes is parallel to the Jurassic (?) E-W-trending transcurrentfaults (Giacosa et al. 2007). In Los Menucos area, the dyke swarm crosscuts Palaeozoic units, such as theCambrian metaclastic Colo Niyeu Formation (Martínez Dopico et al. 2017b), the andesitic and dacitic couleescorrelated with the Collinao Dacite (Luppo et al. 2019), and sections of the rhyolitic to andesitic volcanicand volcaniclastic sequences of the Los Menucos Complex (Lema et al. 2008) which were dated at 257 ± 2and 252 ± 2 Ma, respectively (Luppo et al. 2018). Similarly, 20 km to the north, in La Esperanza area, anotherE-W to minor SW-NE rhyolite dyke swarm intrudes the Collinao Dacite (253 ± 2 Ma; Luppo et al. 2019) andthe epizonal leucogranites of the Calvo Granite (250 ± 2 Ma; Pankhurst et al. 2006) and older units of theLa Esperanza plutonic-volcanic Complex (Martínez Dopico et al. 2019). The rhyolite dyke swarm was datedin La Esperanza area at 246 ± 2 and 244 ± 2 Ma (Pankhurst et al. 2006; Luppo et al. 2019).Textural di erences were observed both along and perpendicular to the strike, highlighting signi cantgrain size variations between the centre and the lateral sectors. On fresh surfaces, dykes exhibit coloursranging from shades of pink to dark red. Although all are classi ed as rhyolites, two types of dykes aredistinguished among the swarms: porphyritic and aphyric, either  ne-grained to microgranular rhyolites. Inthe porphyritic type, the phenocrysts (>5 mm; 35-50%) are composed of quartz, k-feldspar and plagioclaseimmersed in a microgranular groundmass together with oxides and a few biotite, apatite (and amphibole?);whereas in the aphyric and microgranular rhyolites the amount of microphenocrysts/matrix is variable,between 35/65% to 55/45%, and the relative proportion of quartz as microphenocryst and the k-feldspar/plagioclase ratio diminishes, altered ma c minerals are also present up to 15%. The textures of the groundmassof the aphyric rhyolites are mostly microgranular to felsitic and, to a lesser extent, granophyric and,locally, spherulitic. A similar mineralogical composition is shown by the coarse-grained equigranular leucograniteplugs of the Calvo Granite (around La Esperanza). Although the rhyolite dykes intrude and seem tosurround and radiate from the N-S trending plugs of the Calvo Granite, the relative chronology between theswarms in La Esperanza is not clearly established. Most E-W-trending dykes are concentrated close to W-Edextral transcurrent faults (synchronous/ late with the E-W dyke swarm?) and crosscut and displace manyESE-WNW and ENE-WSW-trending aphyric rhyolite dykes (possibly opened as Riedel´s synthetic systems). Inestancia Cerro Piche, the NE-SW rhyolite dykes are parallel to a shear-zone which is apparently crosscut byE-W dextral transcurrent faults and associated with E-W porphyritic and aphyric rhyolite dykes.The dykes with porphyritic textures have moderate to high SiO2 (69.9-76.75 wt.% on anhydrous basis)and K2O+Na2O (6.2-8.3 wt.%) contents, and relatively high contents of MgO (0.3-1.2 wt.%; 15