Stratigraphy and geochemistry of accreted fragments of the ancestral pacific floor in southern south America

Abstract

The outer Pacific archipelago of southern Chile has allochthonous fore arc rocks that were accreted prior to the Late Jurassic along the ancestral Pacific margin of the southern South American sector of Gondwana. The fore arc rocks in the Archipielago madre de Dios (50°-50° 47′S) are divisible into three assemblages: the Tarlton Limestone, a sequence of massively bedded Upper Carboniferous to Lower Permian limestone; the Duque de York Complex, a sequence of flysch-like sedimentary rocks; and the Denaro Complex, an association of pillow basalt, bedded chert, detrital calcareous rocks, and shale. The Denaro Complex is interpreted as a sequence of the late Paleozoic ancestral Pacific floor. The Tarlton Limestone may represent a shallow water carbonate platform also built on a basaltic substrate located within the ancestral Pacific realm. The basalt units of the Denaro Complex have tholeiitic major element patterns similar to those of modern day “MOR” and “within plate” basalts. Chemical data for the Denaro Complex allow for the partial discrimination of biogenic, hydrothermal, hydrogenous, and detrital components within the sedimentary units, and facilitate comparison with modern day sediments as well as with other ophiolite sequences. Chemical data together with stratigraphic and structural field relations suggest that the evolution of the Denaro Complex and Tarlton Limestone involved seafloor spreading, hydrothermal activity, seamount volcanism, carbonate reef development, and convergence of the oceanic realm with the margin of Gondwana.