Some Isotopic and Geochemical Constraints on the Origin and Evolution of the Central Andean Basement (19°–24°S)

  • Klaus-Werner Damm
  • Russell S. Harmon
  • Shari Kelley


The Central Andes of northern Chile and northwestern Argentina developed in a largely autochthonous, intracontinental setting during Proterozoic and Palaeozoic times through a recurrent sequence of extensional and compressional tectonic regimes. Exposed pre-Mesozoic rocks comprise an intricate collage of crustal complexes consisting of metamorphosed basement, intrusive rocks, and weakly metamorphosed volcanic and sedimentary strata that rest upon and are intruded by various plutonic lithologies. U-Pb Proterozoic protolith ages range from 1460 to 1210 Ma and are broadly supported by Nd-and Sr- isotope model ages. Three Preandean orogenic cycles are recognized: (1) from c. 560 to 520 Ma, (2) from c. 505 to 405 Ma, and (3) from c. 350 to 240 Ma. Apatite and zircon fission track ages indicate the earliest crustal thickening and slowest cooling within the Precordillera south of c. 23°S, in the Limon Verde complex, and in the Cordon de Lila and Sierra Almeida intrusives. Further to the north, fission track ages for zircon reflect partial annealing during the Eocene magmatic event in this region, whereas apatite ages range from 40 to 25 Ma throughout the entire Precordillera. Preandean intrusive suites vary from dioritic to granitic in bulk composition. A combination of structural and field evidence, age relations, geochemical data, and isotopic data permit discrimination between anorogenic (A-type) and synorogenic (S-type) intrusives, and point to a predominantly crustal origin for all parental magmas. No I-type granitoids are observed in the Preandean basement. Pb-Pb and Sr-Nd isotopie systematics suggest a pedogenesis for the plutonic rocks involving variable extents of mixing between lower crustal sources and lower/upper crustal assimilants. During Phanerozoic orogenic cycles, anorogenic intrusions and related continental tholeiites, generated during extensional episodes, were followed by synorogenic rocks produced when the tectonic regime changed to one of compression and crustal thickening.


Intrusive Rock Plutonic Rock Contrib Mineral Petrol Radiogenic Isotope Apatite Fission Track 
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Copyright information

© Springer-Verlag Berlin Heidelberg 1994

Authors and Affiliations

  • Klaus-Werner Damm
  • Russell S. Harmon
    • 1
  • Shari Kelley
  1. 1.Isotopie Geosciences LaboratoryBritish Geological SurveyKeyworth, NottinghamUK

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