Converging P-T paths of Mesozoic HP-LT metamorphic units (Diego de Almagro Island, Southern Chile): evidence for juxtaposition during late shortening of an active continental margin

摘要

On Diego de Almagro Island in Chilean Patagonia (51 deg 30 min S), a convergent strike slip zone, the Seno Arcabuz shear zone, separates .the Diego de Almagro Metamorphic Complex from very low grade metagreywackes in the east, which were intruded by Jurassic granitoids. The Diego de Almagro Metamorphic Complex is composed of a metapsammopelitic sequence containing blueschist intercalations in the west and (garnet) amphibolite lenses in the east. Peak metamorphic conditions (stage I) at 9.5-13.5kbar, 380-450 deg C in the blueschist and at 11.2-13.2, 460-565 deg C in the amphibolite indicate subduction and accretion at different positions within the deepest part of the accretionary wedge. A K-Ar age of 117 + - 28 Ma of amphibole approximately dates the peak of metamorphism in the amphibolite. The early retrograde stage of metamorphism occurred under static conditions and resulted in localized equilibration (stage II) at 6.3-9.6kbar, 320-385 deg C in the blueschist and 6.1-8.4kbar, 310-504 deg C in the amphibolite. Both P-T paths converge within a midcrustal level. In contrast, an orthogneiss of trondhjemitic composition occurring within the Seno Arcabuz shear zone is associated with a garnet mica-schist containing a high temperature/intermediate pressure assemblage formed at 4.9-6.5kbar, 580-690 deg C. A muscovite K-Ar age of 122.2 + - 4.6 Ma dates cooling after this event which is related to a concomitant magmatic arc. These rocks were overprinted by a mylonitic deformation, which is caused by convergent strike slip shearing and ends during formation of a retrograde phengite-chlorite-stilpnomelane assemblage at a minimum pressure of approximately 5.7kbar (at 300 deg C). Zircon fission track ages from rocks of the Seno Arcabuz shear zone are 64.9 + - 2.7 and 64.9 + - 2.7 Ma; they record the end of shearing in the Seno Arcabuz shear zone that juxtaposed all rocks in the middle crust. Zircon fission track ages ranging from 78 to 105 Ma in the South Patagonian batholith to the east indicate earlier cooling through 280 deg C. The rocks of the Diego de Almagro Metamorphic Complex were initially slowly exhumed and resided at a midcrustal level before being emplaced via shearing in the Seno Arcabuz shear zone. Apatite fission track ages (54 + - 8 Ma) from the Seno Arcabuz shear zone show that exhumation and cooling rates increased after this event. The incorporation of continental crust within the subduction system was a late process, which modified the Cretaceous accretionary wedge, resulting in considerable shortening of the convergent margin.