Abstract
A petrologic, geochemical and geophysical study of two Late Cretaceous plutons of the Fuegian Andes is carried out: the Jeu-Jepén Monzonite (JJM) and the Mt. Kranck Pluton (MKP). The plutons show a wide lithological spectrum from ultramafic lithologies (clinopyroxenites and hornblendites), gabbros , diorites to late stage K-feldspar syenite veins. They represent the top of magmatic chambers exposed at or close to the roof level. Mode and chemistry indicate a monzonitic-mildly alkaline trend with high K 2 O content, equivalent to shoshonitic series. Trace element composition is typical of arc magmas . These features plus their high LILE content (Ba 320–1600 ppm, Sr 475–1560 ppm), high LILE/HFSE ratio (Rb/Zr 0.6–1.6) and age 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 petrographical and chemical characteristics of the plutons suggest the lithological spectrum is the result of magmatic differentiation . 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 convective currents 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-Fagnano fault system), on opposite blocks. However, lithological, chemical and geophysical subtle differences between these two plutons suggest they evolved in individual magmatic chambers and therefore the distance between them cannot be used as an estimation of total offset along the fault, as was indicated in previous studies. ► Late Cretaceous slab flattening caused rear-arc magmatism in the Fuegian Andes. ► Rear-arc plutons have monzonitic to mildly alkaline composition. ► Petrography and chemistry vary systematically away from paleo-trench. ► Subtle differences among plutons reflect magma evolution in individual chambers. ► Jeu-Jepén and Kranck plutons do not bring information about offset along Fagnano fault.
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