Castillo Gonzalez, Paula Alejandra
Description
A number of conflicting tectonic models have been proposed to
explain the geological relationships between southern South
America (Patagonia) and West Antarctica within the palaeo-Pacific
margin of Gondwana. Extensive fragmentation and isolation of the
various tectonic blocks during Gondwana break-up have complicated
interpretations and palaeogeographic reconstructions. In order to
explore and test the different tectonic models, I combine zircon
U-Pb, Lu-Hf...[Show more] and O isotopic data for samples from key locations
throughout the north and south of Patagonia, the Antarctic
Peninsula and the Ellsworth Mountains in West Antarctica. Zircon
is a robust refractory mineral that occurs in igneous and
metamorphic rocks and survives multiple sedimentary cycles with
little change to its isotopic composition. It therefore preserves
a perfect archive for testing tectonic correlations.
Igneous rocks from the Ellsworth Mountains were dated at ca. 680
Ma, older than previously reported. These zircons indicate that
rifting, which affected Mesoproterozoic crust, likely occurred in
the Cryogenian and supports a connection between the
Ellsworth-Whitmore Mountain block and East Antarctica before the
amalgamation of Gondwana. This agrees with the break-up of
Rodinia in the context of the southwest United States and East
Antarctica configuration. U-Pb zircon dating and O-Hf isotopic
compositions of detrital zircons from the Ellsworth Mountains
also support this connection, indicating a likely East Antarctic
provenance. A Cambrian magmatic event is recorded by zircon at
ca. 520 Ma, also related to an extensional setting – but in
this case with crustal recycling. I interpret this Cambrian
magmatism as a result of a tectonic escape after a collision
between the Australo-Antarctic and West Gondwana/Indo-Antarctic
plates.
In Tierra del Fuego, samples from drill cores indicate that
Cambrian magmatism occurred between ca. 540 and 520 Ma with
strong similarities to the Pampean Orogen of Argentina.
Metamorphism occurred at ca. 265 Ma, when zircon crystallised
from high temperature hydrous fluids that previously interacted
with Grenvillian rocks. Importantly, igneous rocks from Tierra
del Fuego record the first evidence of Permian magmatism at ca.
255 Ma, arising from melting of Cambrian rocks. This suggests
prolongation of Permian magmatism from the North Patagonian
Massif in northern Patagonia and also connections to the Eastern
Domain of the Antarctic Peninsula. Granitic rocks in northern
Patagonia record mantle-like O magmatic inputs at ca. 280 Ma and
255 Ma, but with reworking of upper crustal materials between
these two events. In northwestern Patagonia, early Permian
granites indicate continuity of the Permian magmatic belt along
the western margin of South America farther north. Further,
detrital Permian zircons in late Palaeozoic–early Mesozoic
accretionary complexes suggest a continuation of a slightly older
Permian subduction-related magmatic arc, partly located in
Patagonia and extending to the Antarctic Peninsula.
All this data, together with other geological considerations, are
in line with an autochthonous or parautochthonous origin for
northern Patagonia. It also confirms connections between southern
Patagonia and the Antarctic Peninsula from late Palaeozoic to
Jurassic times. I suggest that Patagonia rifted from the South
African-Ellsworth sector of the paleao-Pacific margin of Gondwana
to then collide with the same sector during the Guadalupian.
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