TTE77

Petro logía e implicancias tectónicas de las rocas ígneas del antearco…

ABSTRACT

Fore- arc magmatism in Chile between 39˚ and 43˚S is reported since the late 1960s

by the first studies carried out on rocks from the provinces of Llanquihue, Chiloé and

Palena. Studies of the last two decades have addressed the presence of magmatic bodies in

Chiloe Insular with emphasis on the Miocene-Pleistocene period, leaving aside the Eocene

tectonic regime that gave rise to the only three magmatic units of this age between Valdivia

and Chiloe: Metalqui Granodiorite (39 Ma), Gamboa Dacitic Porphyry (37 Ma) and Punta

Gaviota Andesite (35 Ma).

In order to establish the genesis and tectonic regime of emplacement of the units

under study, transparent sections were made for petrographic description, geochemical

analysis of major and trace elements for compositional variations and anomalies graphs,

and geothermobarometric analysis to establish an estimated depth range based on the

feldspar-liquid thermobarometer, ideally used in the Gamboa Dacitic Porphyry.

The textures observed in the Metalqui Granodiorite rocks show a slow process of

fractional crystallization and a rise possibly in diapirs affected by two stages of hydration.

Petrography and chemical analyses in the Gamboa Dacitic Porphyry also show two

hydration stages but with an ascent favored by an extensive structure of regional type that

would also affect the Punta Gaviota Andesite. The successive hydration stages of the

Metalqui Granodiorite and Gamboa Dacitic Porphyry, together with the rapid ascent of

Punta Gaviota Andesite and the low concentrations of Ce/Pb and Nb/Yb are evidence of

changes in the tectonic regime of the area, where the subduction of the Aluk-Farallon Ridge

allowed the opening of an asthenospheric window in the upper Eocene that caused a series

of asthenospheric disturbances favoring the magmatic ascent of low volume in the crust.

The thermobarometric data worked in this report corroborate that the ascent occurred in a

relatively thin crust that was affected by a fluctuating convergence later stabilized in the

lower Eocene.

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