Dike systems and their volcanic host rocks on King George Island, Antarctica: Implications on the geodynamic history based on a multidisciplinary approach

Abstract

A magmatic arc was active along the Antarctic Peninsula's western margin during the Mesozoic and most of the Cenozoic. We report new constraints on the geodynamic history of King George Island (South Shetland Islands, northern Antarctic Peninsula), based on a multidisciplinary approach combining structural, petrological, geochemical, geochronological and paleomagnetic data from magmatic dike swarms and their host rocks. The deformation of the dikes is expressed as jointing and faulting. Joint systems present in the host rocks occur also within the dikes, indicating that the stress field outlasted dike intrusion. No folding axis could be determined for the investigated areas, but the orientation of the folding axis evident on nearby Livingston Island shows that the corresponding tectonic stress field there was almost identical to the one on King George Island. This suggests a uniform tectonic pattern throughout the area during the time of dike intrusion. Dikes from King George Island yielded Thanetian to Lutetian 40Ar/ 39Ar ages. A pronounced peak of intrusive activity is evident at 47–45 Ma and coincides well with a dramatic change in subduction zone parameters that caused a change from a compressional to an extensional tectonic regime, as also reflected by the uplift of the Smith Island metamorphic rocks at the same time. The dikes comprise a subalkaline series of predominantly calc-alkaline affinity. Tholeiites occur also, apparently related to the initial stages of intrusive activity in the respective areas and thus a time of strongly increasing velocity of the subducting plate. Crustal contamination is not evident from the dikes on King George Island and the main intrusive phase during the Lutetian is characterized by increasing contribution of subducted sediments to arc magma genesis. Enrichment of the depleted mantle source by fluids derived from the subducted slab as reflected by elevated Ba/Th ratios is not uniform in the studied areas, indicating heterogeneities in the mantle wedge that are also confirmed by other HFSE systematics. Paleomagnetic studies undertaken in order to check for Cenozoic tectonic rotations indicate high between-site dispersion probably associated with strong secular variation during Paleocene/Eocene times. However, an interpretation in combination with previously published data suggests that no post-formational vertical-axis rotations and latitudinal displacement occurred.