INSTITUTO DE INVESTIGACIONES EN ENERGIA NO CONVENCIONAL
Unidad Ejecutora - UE
congresos y reuniones científicas
New U-Pb ages in the Diablillos Intrusive Complex, Southern Puna, Argentina: A long magmatic event in the Paleozoic Arc, SW Gondwana.
ORTIZ, AGUSTÍN; HAUSER, NATALIA; BECCHIO, RAÚL; NIEVES, ALEXIS; SUZAÑO, NESTOR
Congreso; XV Congresso Brasileiro de Geoquímica.; 2015
Sociedad Brasilera de Geologìa.
1.Introduction and Geological settingThe Puna geological region comprises Salta, Jujuy and Catamarca provinces, northwestern Argentina. This 4000 meter above sea level high-plateau region lies between the Central Argentinian Andes. The Puna basement in the central Andes consists of Proterozoic?Paleozoic metamorphic rocks and granitoids. Diverse authors, proposed differents models to explain the origin of the basement, where two orogenic events are recognized: the Pampean (Upper Precambrian?Lower Cambrian) and Famatinian (Upper Cambrian?Lower Silurian) (e.g. Ramos et al., 1986; Ramos, 1988; Loewy et al., 2004; for opposite points of view see Becchio et al., 1999; Bock et al., 2000; Buttner et al., 2005). Hence, Lucassen et al. (2000) proposed for the Central Andean basement, an evolution in a mobile belt, where the Pampean and Famatinian cycles are not distinct events but, they are one single, non-differentiable event from 600 to 400 Ma. The mobile belt culminated in low-P/ high-T metamorphism at approximately 525-500 Ma. Then, these were followed by a long-lasting high-thermal gradient regime in the mid-crust until Silurian times. Becchio et al., (2011) defined the Diablillos Intrusive Complex (CID, by its Spanish name), emplaced in the Inca Viejo Range. This range splits the Salares Ratones-Centenario with the Salar Diablillos (Fig.1). This Complex is located in the Eastern Magmatic Belt, Southern Puna, Argentina. Here we present new zircons U-Pb ages by LA-MC-ICPMS in the Diablillos Intrusive Complex, contributing to understanding the magmatic event in the lower Paleozoic arc, SW Gondwana.2. Zircon U/Pb ages2.1 Age determination of the Diablillos Intrusive Complex Three samples were selected to study and register the magmatic event on the Diablillos Intrusive Complex. Each sample corresponds to the Diorite, Granodiorite and Monzogranite facies units of this complex. Zircon grains from the Monzogranite (Sample D-13-01) gave a weighted average U/Pb age of 517.1±3.5 Ma (Fig.2a). The U/Pb analyses of the Diorite (Sample D-13-40) displayed a weighted average age of 514.9±5.8 Ma (Fig.2b). And zircon grains from the Granodiorite (Sample D-13-02) yielded a Concordia age of 520.6±4.5 Ma (Fig.2c). Since the ages in all three samples overlap with each other within error, we cannot define a unique emplacement age for the complex. After the reduction of the data, Pb loss was disregarded because there was not an evident loss. Since we can see a continuous age distribution in the Concordia diagrams (from ~540 Ma to 490 Ma, Fig.2) where there is not a marked age difference between core and rims single grain analyses and single spot grain analyses; a combination of Pb-loss and inheritance is possible. To confirm or disregard accurately the unseen Pb loss in this complex, a more precise analyses method should be done.2.2Age variation in the Diablillos Intrusive Complex In the Probability Density Plot (PDP, Fig.2 insets) the three samples display various zircons populations; a small population of ~490 Ma, a peak population of ~515-520 Ma, a frequent population of ~530-540 Ma, other of ~600-630 Ma and, an older population of ~1000-1100 to 1300 Ma. Clearly, the zircon population of ~490 Ma is present in all three samples. Since the data available is very little for this age group, we cannot define a certain geological meaning to this group, although, this age group could indicate the last magmatic activity in the complex; further geochronology analyses should be done to confirm this. The most frequent population of ~515-520 Ma would represent the peak magmatic activity in the Complex; whereas the population of ~530-540 Ma, symbolize zircon grains antecryst representing early magmatic activity in the CID. Finally, the oldest zircon populations of ~600-630 to 1000-1300 Ma, signify inherited xenocryst from the source rock, indicating Neopreoterozoic and Mesoproterozoic magmatic activity respectively.2.3Diablillos Intrusive Complex: A long magmatic eventWe propose that the Diablillos Intrusive Complex was emplaced between the 540-490 Ma (~50Ma magmatic event). Where the CID would represent early stages of the Paleozoic arc, formed in a long lived magmatic event. Supporting the idea presented by Lucassen et al. (2000), where they proposed for the Central Andean basement, an evolution in a mobile belt, where the Pampean and Famatinian cycles are not distinct events but, they are one single, non-differentiable event from 600 to 400 Ma.