Mafic dike swarms in the South Shetland Islands volcanic arc: Unravelling multiepisodic magmatism related to subduction and continental rifting

Eight groups of mafic dikes and related high-level stocks cut Triassic accr etionary complex and Mesozoic magmatic are formations on Livingston Island. Some are affected by silicic/sericitic alteration, related to Cretaceous h ydrothermal activity, and propylitic/epidosite alteration, analogous to tha t in ocean floor sheeted dikes. Alteration was accompanied by major and tra ce element metasomatism. Ar-Ar analysis of the freshest rocks indicates fiv e intrusive events, some of which are unexpectedly young. Groups 1-3 were i ntruded in the mid to late Cretaceous (similar to 108-74 Ma) and were coeva l with the calc-alkaline are. Between 70 and 50 Ma, relatively rapid and ob lique plate convergence led to strike-slip tectonism and a pause in magmati sm. At similar to 52 Ma, orthogonal, slow convergence resulted in extension al faulting and emplacement of calc-alkaline (group 2) and primitive tholei itic dikes (groups 4-6) between 51 and 45 Ma. Extension of Antarctic Penins ula-southern South American crust culminated in emplacement of mafic to int ermediate, medium-grained plutons and group C porphyries between 44 and 36 Ma. Localized hydrothermal flow along fault zones resulted in partial to co mplete argon loss from nearby Cretaceous lavas and Ar-Ar reset ages of simi lar to 40 Ma in mid-Cretaceous hydrothermal K-feldspar. Primitive olivine b asalts (group D) and epithermal carbonate veins (31-29 Ma) were emplaced du ring along-are extension accompanying the opening of Drake Passage and Powe ll Basin. Excess argon occurs in two forms: strongly held in melt? inclusio ns in the primitive tholeiites and weakly held in some secondary alteration . There is no radiometric evidence, in the area studied, for magmatism rela ted to late Cenozoic subduction, nor to the Pleistocene-Recent opening of t he back are Bransfield rift.