Successive geothermal, volcanic-hydrothermal and contact-metasomatic events in Cenozoic volcanic-arc basalts, South Shetland Islands, Antarctica

摘要

Hydrothermal alternation in volcanic arcs occurs in many settings and may involve magmatic, marine, lacustrine or groundwaters, driven bymagmatic, tectonic or thermal events. King George Island, part of the South Shetland Island Cenozoic volcanic arc, contains an 80 km long zone of propylitized volcanic rocks, with numerous occurrences of quartz veining, silicic, sericitic, argillic and advanced-argillic alteration. On Barton Peninsula, a basaltic lava sequence (49-44 Ma) intruded by a small, high-level granodiorite pluton (～42 Ma), contains these alteration types, previously interpreted as a single porphyry-copper system. In this study, we report three, possibly four, distinct fossil hydrothermal episodes. (1) Banded chalcedonic quartz, quartz-sericite and prpylitic alteration occurs along ESE faults and as reworked clasts in nearby tuffs. Drusy quartz + calcite veins with silicic/sericitic, argillic and propylitic wallrocks may represent feeders to the near-surface silicification. These characteristics, and anomalous Ag + Pb + Sb + Au plus Te + Se + Zn + As, sugges a neutral-pH geothermal system that was active during volcanism. (2) The lavas and banded-quartz rocks were brecciated, veined and replaced by alunite + native sulphur + pyrite, and pyrophyllite + quartz + pyrite + zunyite + diaspore assemblages with anomalous Hg + Se + As + Bi + Au + Tl + Sb + Cu. Such advanced-argillic alteration is diagnostic of degassing of a felsic magma into shallow ( < 500 m) meteoric groundwaters. Rhyolite tuffs, previously not reported on kIng George Island, may represent leakage of this magma to the surface. (3) Subsequent burial to ～3 km was followed by emplacement of a granodiorite pluton and formation of a silicic contact-metasomatic aureole containing muscovite, biotite, actinolite, magnetite, K-feldspar and tourmaline. Disseminated andalusite + corundum also formed in areas previously affected by the advanced-argillic alternation. Iron/copper-sulphide veinlets are locally abundant, but a porphyry-style geochemical signature is not present. Early Cretaceous Ar-AR ages near the intrusive contact indicate flow of an excess AR-bearing hydrothermal plume up the contact. Finally, isolated areas of propylitic alteration in the lavas nearby may be related either to quartz veins of episode 1 at depth or to (4) continued circulation of heated groundwaters around the cooling pluton.