IGNEOUS ARCHITECTURE AT THE TOP OF THE FAMATINIAN ARC, NW ARGENTINA

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 the Ordovician Famatinian arc, NW Argentina, a rare, well exposed example of the top of an arc occurs in the Sierra de Narváez and Sierra de Las Planchadas.

After retro-deformation of Andean tectonics, Ordovician units consist of (1) gently folded clastic volcanics, lava flows and interbedded marine sediments and turbidites, (2) shallow rhyolitic plugs, sills and dikes, and few mafic dikes, and (3) a hypabyssal pluton. The volcanics are composed of dominantly submarine to subaerial clastic volcanic rocks deposited by mass gravity flows, and welded ignimbrites. Volcanic lithics in the clastic rocks are reworked, mm- to dm-size volcanic fragments. 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 small rhyolitic intrusions and sills likely related to subvolcanic resurgence and/or feeders. The high silica, aphanitic rhyolite plugs are 100s of meters to 1 km in diameter, discordantly intrude bedding, are flow-banded, and rich in mm to cm-scale lithophysae. Quartz and feldspar phenocrysts are minor. The hypabyssal, medium- to coarse-grained, K-feldspar porphyritic, enclave- and xenolith-bearing Los Angosturas monzogranite is exposed at the deepest levels in the fault-bounded Sierra de Narváez block.

The volcanics, feeders and pluton are metaluminous to peraluminous and mostly calc-alkaline series with arc tholeiite and high K calc-alkaline outliers. Rock types include high silica rhyolites, dacites, and basalt and basaltic andesite ranging in SiO₂ 47-77 wt% with a gap at 52-62 wt%, indicating a lack of andesites. MgO, FeO, TiO₂ and CaO decrease with increasing SiO₂, while Al₂O₃, Na₂O and K₂O show large scatter. Th/Yb and Nb/Yb ratios range 1-5, indicating a continental arc signature. Based on our field observations and element geochemistry, we suggest that the top of the Famatinian arc consisted of volcanic centers, mafic and felsic feeders and plutons built into continental crust in a shallow marine arc setting.