A comparative study of two rear-arc plutons and implications for the Fuegian Andes tectonic evolution: Mount Kranck Pluton and Jeu-Jepén Monzonite, Argentina

GONZÁLEZ GUILLOT, M.; PREZZI, C.; ACEVEDO, R.; ESCAYOLA, M.

JOURNAL OF SOUTH AMERICAN EARTH SCIENCES

PERGAMON-ELSEVIER SCIENCE LTD

Lugar: Amsterdam; Año: 2012 vol. 38 p. 71 - 88

0895-9811

A petrologic, geochemical and geophysical study of two Late Cretaceous plutons of the Fuegian Andes iscarried out: the Jeu-Jepén Monzonite (JJM) and the Mt. Kranck Pluton (MKP). The plutons show a widelithological spectrum from ultramafic lithologies (clinopyroxenites and hornblendites), gabbros, dioritesto late stage K-feldspar syenite veins. They represent the top of magmatic chambers exposed at or closeto the roof level. Mode and chemistry indicate a monzonitic-mildly alkaline trend with high K2O content,equivalent to shoshonitic series. Trace element composition is typical of arc magmas. These features plustheir high LILE content (Ba 320e1600 ppm, Sr 475e1560 ppm), high LILE/HFSE ratio (Rb/Zr 0.6e1.6) andage confirm they belong to a rear-arc, monzonitic to mildly alkaline suite (the Fuegian Potassic Magmatism).Therefore, they represent the farthest-from-the-trench plutons in the suite. The petrographicaland chemical characteristics of the plutons suggest the lithological spectrum is the result of magmaticdifferentiation. The process involved crystal accumulation and magma mingling at the early stages,favored either by injections of fresh magma from deeper reservoirs and/or the action of convectivecurrents within the chamber. Country rock assimilation also took part simultaneously with crystallization.The JJM and MKP lie along the trace of a Cenozoic left-lateral strike-slip fault (Magallanes-Fagnanofault system), on opposite blocks. However, lithological, chemical and geophysical subtle differencesbetween these two plutons suggest they evolved in individual magmatic chambers and therefore thedistance between them cannot be used as an estimation of total offset along the fault, as was indicated inprevious studie