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. 850 Ma at Belen to 1000-1100 Ma in Sierra de Moreno and Cordon de Lila. Late Proterozoic provenance throughout the region corresponds to a c.1 Ga igneous and metamorphic event. The main source region could have been the Proterozoic MARA block or Laurentia to the west. Early Ordovician plutonic rocks (465-485 Ma) correlated with the Famatinian magmatism of NW Argentina are recognised in all three outcrop areas, and contemporaneous volcanic rocks in Cordon 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 400 Ma 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.