LARROVERE mariano Alexis
congresos y reuniones científicas
Igneous architecture at the top of the Famatinian arc, NW Argentina
MEMETI, VALBONE; PATERSON, SCOTT R.; RATSCHBACHER, BARBARA; ALASINO, PABLO H.; LARROVERE, MARIANO A.; ARDILL, KATIE; CAWOOD, TARRYN; LUSK, ALEXANDER; INSIXIENGMAY, LESLIE
Conferencia; GSA Annual Meeting; 2017
Understanding the igneous architecture at the tops of arcs is an important aspect in studies of ancient magmatic arcs.Typically, the volcanic veneers, thin in comparison to overall arc crust thicknesses, are removed by erosion. In theOrdovician Famatinian arc, NW Argentina, a rare, well exposed example of the top of an arc occurs in the Sierra deNarváez and Sierra de Las Planchadas.After retro-deformation of Andean tectonics, Ordovician units consist of (1) gently folded clastic volcanics, lava flows andinterbedded marine sediments and turbidites, (2) shallow rhyolitic plugs, sills and dikes, and few mafic dikes, and (3) ahypabyssal pluton. The volcanics are composed of dominantly submarine to subaerial clastic volcanic rocks deposited bymass gravity flows, and welded ignimbrites. Volcanic lithics in the clastic rocks are reworked, mm- to dm-size volcanicfragments. They are interbedded with fine-grained marine sediments with Arenigian to Llanvirn brachiopods, gastropods,Rusophycus trace fossils, and ripple marks. Locally, these bedded units form a structural dome cored by multiple smallrhyolitic intrusions and sills likely related to subvolcanic resurgence and/or feeders. The high silica, aphanitic rhyolite plugsare 100s of meters to 1 km in diameter, discordantly intrude bedding, are flow-banded, and rich in mm to cm-scalelithophysae. Quartz and feldspar phenocrysts are minor. The hypabyssal, medium- to coarse-grained, K-feldsparporphyritic, enclave- and xenolith-bearing Los Angosturas monzogranite is exposed at the deepest levels in the faultboundedSierra de Narváez block.The volcanics, feeders and pluton are metaluminous to peraluminous and mostly calc-alkaline series with arc tholeiite andhigh K calc-alkaline outliers. Rock types include high silica rhyolites, dacites, and basalt and basaltic andesite ranging inSiO 47-77 wt% with a gap at 52-62 wt%, indicating a lack of andesites. MgO, FeO, TiO and CaO decrease withincreasing SiO , while Al O , Na O and K O show large scatter. Th/Yb and Nb/Yb ratios range 1-5, indicating acontinental arc signature. Based on our field observations and element geochemistry, we suggest that the top of theFamatinian arc consisted of volcanic centers, mafic and felsic feeders and plutons built into continental crust in a shallowmarine arc setting.