A permo-carboniferous U-Pb age for part of the Guanta Unit of the Elqui-Limarí Batholith at Río del Tránsito, Northern Chile

A U-Pb zircon age of 285.7±1.5 Ma is presented for granodiorite from the Chanchoquín pluton of the Quanta Unit, Elqui Superunit of the Elqui-Limarí Batholith. This is the first U-Pb zircon age reported from this batholith and is interpreted as dating emplacement of part of its oldest unit, very close to the time of the Carboniferous-Permian boundary. The age is similar to published Rb-Sr ages for the Palaeozoic batholiths of the Coast Range in Central Chile, in the Algarrobo and Nahuelbuta areas, indicating a widespread episode of plutonic activity along the Gondwana margin of Northern and Central Chile. If the new age were applicable to the whole of the Quanta unit, the other units of the Elqui Superunit (Cochiguas and El Volcán units) would have been emplaced during Permian or later times. A new Rb-Sr whole-rock isochron of 256±10 Ma for the Cochiguas Unit in Río Los Molles, at the southern end of the batholith, is in agreement with this, although previous evidence for the Cochiguas Unit in its type section indicated a Carboniferous age. This suggests either that the Rb-Sr system in Los Molles has been reset, or that the mapped units of the batholith, although lithologically similar in different areas, are not contemporaneous. Further U-Pb dating is needed to resolve these questions.