A megacryst- and xenolith-rich camptonite basalt from Ushuaia, Tierra del Fuego, Argentina

Abstract

Isolated pods of volcanic breccias with alkalinecomposition are hosted in the metaturbidites of the Yahgan Formation to west ofUshuaia, Isla Grande de Tierra del Fuego, Argentina. The briefly exposed outcropsare almost partially covered by bushy vegetation, whence identification of its scarceapparitions is hampered. Field features display sinuous chilled margins withpinch and swell phenomena or show uneven shape with irregular branching andsegmentation into discrete blobs. Pods of breccias, possibly components of apipe-cluster, seem to represent incipient explosion of the melt that wouldgiven rise to a fully developed vent. Unique horned ends may occur, whichtotally preclude any tectonic explanation for the off-set, even in an Andeanfolded environment. Country rocks are not apparently altered by contact around theigneous boulders. The former may perhaps have acted as a long-lives feeder tolavas, so allowing the hostess to heat up graduatelly. The rock is constituted by macrocrysts of cognatedkaersutite (146 ±5 Ma) and K-feldspar (127 ±4 Ma), phenochrysts of olivine andTi-augite in a groundmass of the same minerals, with plagioclase andfeldspathoids. Texture is mostly aphyric and richly porphyritic both, withfine-grained black matrix. Carbonation is common as secondary alteration. Alkalimetasomatism is marked by developing of kaersutite, feldspathoids and zeolites. The presence of xenoliths, mainly of lherzolitic,pyroxenitic and wehrlitic composition, is the most prominent characteristic ofthese eruptive bodies (Acevedo, 2016). The geochemical plotting of Ce/Yb ratio vs Sm contentsshown these rocks not belong to a specified field in the lamprophyre branches. On the other hand, from a petrographic and petrologic outlook,the mineral arrangement places this rock as a broad volatile-enriched alkalibasalt or camptonite that reflects the mantle upwelling during the stage ofhorizontal extension of the back-arc basin in middle times of the Jurassic-Cretaceoustransition in Tierra del Fuego.