The pre-Mesozoic rocks of northern Chile: U–Pb ages, and Hf and O isotopes

Abstract

Supposed pre-Mesozoic igneous and metamorphic rocks from northern Chile are reviewed in the light of twenty-one new SHRIMP U–Pb zircon age determinations. Metamorphic rocks from the precordillera upthrust belt mostly show a wide spectrum of zircon ages, indicating derivation from sedimentary protoliths. Youngest detrital zircon ages (i.e., maximum depositional ages) range from c. 850Ma at Belén to 1000–1100Ma in Sierra de Moreno and Cordón de Lila. Late Proterozoic provenance throughout the region corresponds to a c. 1Ga igneous and metamorphic event. The main source region could have been the Proterozoic MARA block or Laurentia to the west. Early Ordovician plutonic rocks (465–485Ma) correlated with the Famatinian magmatism of NW Argentina are recognised in all three outcrop areas, and contemporaneous volcanic rocks in Cordón de Lila. Hf- and O-isotope data for Ordovician zircon in these rocks, and for c. 1450 detrital zircon in the metasedimentary rocks, are consistent with ultimate derivation from Early Mesoproterozoic to Paleoproterozoic lithosphere. A depositional age younger than 400Ma is determined for the Quebrada Aroma metamorphic complex, indicating post-Early Silurian metamorphism and folding. Carboniferous igneous and deformational events occur in the Coast Range, where metasedimentary complexes are mostly related to Late Paleozoic subduction–accretion; deformation and metamorphism continued near the present Pacific shore line until Triassic and earliest Jurassic times. The underlying crust of much of Norte Grande is considered to be Proterozoic (Arequipa–Antofalla block or MARA), although there are no igneous rock outcrops of this age.