THE SOUTH PATAGONIAN BATHOLITH: 150 MY OF MAGMATISM AT A PLATE EDGE

A new database of 70 U-Pb zircon ages indicates that the South Patagonian batholith resulted from amalgamation of subduction-related plutons from the Late Jurassic to the Neogene. Construction began with a bimodal Late Jurassic body mainly composed of leucogranite and some gabbro, emplaced along its present eastern margin within a restricted time span (157 to 145 Ma). This episode is coeval with rhyolitic ignimbrites of the Tobífera Formation, deposited in the Rocas Verdes Basin east of the batholith, the last of silicic volcanic episodes in Patagonia that commenced during the Early Jurassic. The quasi-oceanic mafic floor of the basin was also contemporaneous with this Late Jurassic batholithic event.

Changes in subduction parameters then triggered the generation of earliest Cretaceous plutons (144 - 137 Ma) westwards of the Late Jurassic ones, a westward shift that culminated at 136 - 111 Ma along the present western margin of the batholith. After the opening of the southern Atlantic Ocean . Most mid to Late Cretaceous (122 - 77 Ma) and Paleogene (64 - 40 Ma) granitoids are geographically restricted plutons, emplaced between the previously established margins of the batholith, and mostly in the far south; no associated volcanic rocks of similar age are known . During Neogene plutonism (22 - 16 Ma) a recurrence of coeval volcanism is recognized within and east of the batholith. Typical Ndt values for the granitoids vary from strongly negative (-5.0) in the Late Jurassic, to progressively higher values for the earliest Cretaceous (-3.6), the late Early Cretaceous (+0.3) to the Late Cretaceous (+2.3) and the Paleogene (+5.0), followed by lower and more variable ones in the Miocene (+2.5). These variations may reflect different modes of pluton emplacement: large crustal magma chambers developed in the early stages (Late Jurassic to earliest Cretaceous), leading to widespread emplacement of plutons with a crustal signature, whereas in the mid to Late Cretaceous and Palaeogene parts of the batholith resulted from incremental assembly of small plutons generated at greater depths and with higher Ndt. This does not in itself justify the idea of a reduction in crustal character due to progressive exhaustion of fusible material in the crust through which the magmas passed.