Proceso Unificado de Rational

The U-Pb geochronological data reported here confirms that the emplacement of plutonic bodies in the upper crust was coeval with tectonic inversion of the Abanico Basin. Plutons were emplaced preferentially at the intersections of conjugate arc- oblique faults, or at the intersections of arc-oblique faults and the arc-parallel basin- margin faults (Fig. 7.4). Individual plutons are commonly elongated following the NW-trending faults, or they present rhombic shapes with the margins being defined by intersecting conjugate structures (Fig. 7.4). This suggests arc-oblique faults controlled the location and orientation of magmatic feeders.

The oldest Tertiary plutons were emplaced during the early Miocene, and occur only in the Rio Blanco-Los Bronces and Maipo segments (Fig. 6.9), to the north of the Piuquencillo fault system (Fig. 5.8). The Rio Colorado granodiorite, the La Obra granodiorite and the earliest facies of the Rio Blanco-San Francisco batholith (Fig. 7.4) were emplaced coevally with the formation of progressive unconformities between the Abanico and Farellones formations and with the deposition of the oldest pyroclastic flows of the basal member of Farellones Formation. The Rio Colorado pluton was emplaced at 21.8 ± 0.5 Ma (this work, U-Pb LA-ICPMS in zircons, sample AN12JP011, Table 3.1), the La Obra pluton was emplaced and cooled between and 21.6 and 16.2 Ma according to 40Ar/39Ar analyses in different minerals (Kurtz et al., 1997) and the earliest plutons of the Rio Blanco-San Francisco batholith were emplaced at 20.1 ± 2.0 Ma (K-Ar in hornblende, Warnaars et al., 1985). At the Rio Blanco-San Francisco batholith, the ~20 Ma plutons are cut by younger

intrusions of diverse composition; the youngest intrusions of this suite of early Miocene plutons correspond to syenogranites emplaced at 16.4 ± 0.2 Ma (sample 153119, Table 3.1). Coeval with the emplacement of these major plutons, dioritic and gabbroic dikes were emplaced syn-tectonically in dilational jogs along a NW- trending, sinistral strike-slip fault (Fig. 3.13A). The dikes have been

Chapter 7 – Discussion

Figure 7.4: Main Miocene plutonic bodies and fault systems of the study area. Ages from Kurtz et al. (1997), Deckart et al. (2010) and this work.

dated at 22.2 ± 0.2 Ma (U-Pb in zircons, Montecinos et al., 2008), and the kinematics of the fault is consistent with emplacement under E-directed compression during the first pulse of basin inversion. Geochemically, these early Miocene intrusions are

characterized by LREE enrichment and HREE slopes slightly lower than the coeval Farellones Formation (Fig. 5.6; average La/Sm = 3.8 and Sm/Yb = 1.9).

Early Miocene plutons of the Rio Blanco and Maipo segments are cut by a set of middle Miocene, commonly medium-grained equigranular intrusions. They occur across the entire study area, and are the oldest plutonic units in the El Teniente segment (Fig. 6.9). Radiometric ages of middle Miocene plutons are strongly concentrated in the 13-10 Ma range, coeval with the transition between the Coya- Machali Formation and the Teniente Volcanic Complex in the El Teniente segment (Kurtz et al., 1997; Deckart et al., 2010; this work, samples AN12JP008,

AN12JP013, AN13JP003, AN12MB029, AN12MB038, AN12MB041,

AN12MB067 and AN12MB068, Tables 3.1 and 5.1). Major plutons of this age range include the Rio Blanco Granodiorite at the Rio Blanco-San Francisco Batholith, as well as the La Gloria, Meson Alto, San Gabriel, Alfalfalito, Pangal and Cortaderal plutons (Fig. 7.4). They commonly occur at the central part of the inverted Abanico Basin (Fig. 7.4). Middle Miocene plutons are cut by lower late Miocene andesitic and dacitic dikes and stocks of porphyritic texture, with ages between 11 and 9 Ma (Rivera and Navarro, 1996; Deckart et al., 2014; this work, samples AN12JP001 and AN13JP014, Tables 3.1 and 5.1).

Middle Miocene plutons emplaced in the Rio Blanco-Los Bronces and Maipo segments have a stronger LREE enrichment and HREE fractionation than any of the stratigraphic units (Fig. 5.6), with average La/Sm ratios of 5.8, Sm/Yb ratios of 3.5 and La/Yb ratios of 21.3. Those emplaced at the El Teniente segment, in contrast, show a REE chemistry similar to the Farellones Formation (average La/Sm = 3.9, Sm/Yb = 2.9 and La/Yb = 12.2; Fig. 5.6)

The youngest Tertiary rocks exposed in the Andes of central Chile correspond to late Miocene-early Pliocene porphyritic stocks and dikes, hydrothermal breccias and diatremes. They occur towards the central part of the inverted Abanico Basin in the Rio Blanco-Los Bronces and El Teniente segments. Rocks of this age range have not been documented in the Maipo segment. Some of these intrusions are temporally and genetically related with the Rio Blanco-Los Bronces and the El Teniente porphyry Cu-Mo deposits (Maksaev et al., 2004; Deckart et al., 2005). The age range of this

Chapter 7 – Discussion suite of porphyries and breccias is 7-4 Ma (Quirt et al., 1971; Warnaars et al., 1985; Maksaev et al., 2004; Deckart et al., 2005; this work, sample AN13JP013, Table 3.1). Late Miocene-early Pliocene rocks have a REE pattern similar to the middle Miocene plutons emplaced in the northern segments (Fig. 5.6): the average La/Sm ratio is 5.9, Sm/Yb ratio is 3.5 and La/Yb ratio is 21.0.

The oldest systems of hydrothermal alteration and mineralisation in the study area were developed during the middle Miocene at the Rio Blanco-Los Bronces segment, in the northern part of the study area. They produced the sub-economic Los Piches and El Plomo prospects (~13 Ma and ~10 Ma respectively, based on 40Ar/39Ar ages in sericite from tourmaline-cemented breccias, Toro et al., 2012) and the

hydrothermal alteration and veins in samples AN12JP008 and AN13JP014 (Fig. 3.1, Table 3.1). These two samples were collected from pre- and post-mineral intrusions, and their zircon U-Pb LA-ICPMS ages constrain the age of hydrothermal activity in the area to the 11-9 Ma range. Actinolite mineral fibres collected from a NE-striking fault plane in sample AN12JP008 (Fig. 6.1) yielded an 40Ar/39Ar age of 9.72±0.09, consistent with the inferred age of hydrothermal activity in the area. The actinolite fibres form steps indicating dextral strike-slip movement, confirming that

hydrothermal activity was syn-tectonic and, according to fault kinematics, associated with E-directed compression. Similar conclusions can be obtained from the other two samples in which late Miocene ages were obtained from syn-tectonic hydrothermal minerals (AN13JP007 and AN13JP012, Fig. 6.5, Table 6.1). These middle Miocene- lower late Miocene hydrothermal systems are coeval with the emplacement of the belt of middle Miocene plutons and with the transition from the Coya Machali Formation to the Teniente Volcanic Complex in the southern El Teniente segment. The economically relevant hydrothermal systems of the study area were formed in the late Miocene-early Pliocene, between 7 and 4 Ma (Maksaev et al., 2004; Deckart et al., 2005, 2013, 2014), associated with the porphyritic stocks and hydrothermal breccias of the same age. The age of these hydrothermal systems overlaps with the most important period of uplift and exhumation affecting the Tertiary rocks of central Chile, according to existing zircon and apatite fission track data (Maksaev et al., 2009) and to nine new (U-Th)/He ages in zircons and apatites (this work, Table 5.2).