Middle Jurassic dyke swarms in the North Patagonian Massif: the Lonco Trapial Formation in the Sierra de Mamil Choique, Rı́o Negro province, Argentina

Middle Jurassic volcanism is a major magmatic event in the evolution of Patagonia. In the southwestern corner of the North Patagonian Massif at Sierra de Mamil Choique, basic to intermediate, ca. 170 Ma dykes are widespread. In this article, new chemical data from the dykes are presented, analyzed, and compared with regional information about Jurassic volcanism. The dyke swarms are composed of an alkaline and a subalkaline series. Fractionating phases that controlled the magmatic evolution were plagioclase (andesine)+clinopyroxene with clinoamphibole (for compositions of SiO2>60%) in the subalkaline series and clinoamphibole+plagioclase (oligoclase)±magnetite in the alkaline series. The source of both series could be an enriched lithospheric mantle or lower crust. Although crustal contamination or enrichment by subduction-derived fluids cannot be demonstrated from the available data, a subduction-enrichment process of the magma source is suggested for the subalkaline rocks. Different volatile composition, water content and perhaps temperature at the melting site would control the higher Zr content of the alkaline series and would allow low melt fractions to leave their sources. Absolute values for Ba/Nb (60–160), La/Nb (4–5), and K/Nb (>2000) in both series indicate a significantly LIL-enriched crustal component. Nd/Th of approximately 4 for the alkaline rocks argues against a primitive mantle, supported by the low MgO, Cr, and Ni and high LILE contents. In contrast, Nd/Th between 15 and 27 in the subalkaline rocks is similar to a mantle source. Crustal components are indicated by the higher La/Yb and Na2O and lower CaO in the alkaline rocks. Regional, structural, and, to a certain extent, petrological evidence for the Middle Jurassic dyke swarms of the Sierra de Mamil Choique point to an extensional intracontinental tectonic setting in which older structures controlled the development of the volcanism.