Mauricio Belmar

Significance of K-Ar dating of very low-grade metamorphism in Triassic-Jurassic pelites from the Coastal Range of central Chile

Abstract K-Ar isotopic dating of very low-grade metamorphism affecting Triassic-Jurassic rocks in the Coastal Range of central Chile was carried out on whole rocks and their <2 μm size fractions. In the study area, a regional-burial low-grade metamorphism at anchizone conditions (T ≤ 190°C) and low-pressure conditions (P ≥ 1.3 kbar) has been described. The highest observed temperatures are related to a contact metamorphism produced by nearby Jurassic intrusions, with a P-T estimate at the immediate contact zone of ~650-690°C and 4 kbar. The whole-rock K-Ar age of 174 ± 5 Ma is interpreted as belonging to the contact metamorphism due to the intrusion of Jurassic plutons (165 ± 5 Ma to 175 ± 5 Ma). A time-interval of ~20 Ma between the diagenesis (206 Ma) and the anchizonal very low-grade metamorphism (181-184 Ma) is obtained, and a rate of subsidence of ~120 m/Ma is proposed for these Triassic-Jurassic basins. A thermal influence on the burial, very low-grade, regional metamorphism is invoked.

Geochronology of very low-grade Mesozoic Andean metabasites; an approach through the K–Ar, 40Ar/39Ar and U–Pb LA-MC-ICP-MS methods

Multiple geochronological methods using different metamorphic minerals were combined to date the regional, very low-grade metamorphism affecting Upper Jurassic–Lower Cretaceous volcano-sedimentary successions in the Andes of central Chile. Early Late Cretaceous metamorphic ages (between 82 and 108 Ma) were obtained by the K–Ar and U–Pb methods for celadonite and titanite. A much younger thermal event is responsible for actinolite formation at 8 Ma, most probably related to the intrusion of proximal Miocene granitoids. Previous models for the metamorphism should be reinterpreted taking into account the absence of a greenschist-facies event. The combination of different metamorphic minerals and chronometers is regarded as a powerful analytical tool to date the very low-grade metamorphism associated with the Mesozoic extensional regime developed within the Andes.

Geochronology of the Lower Cretaceous volcanism from the Coastal Range (29°20'-30°S), Chile

Ar/ 39 Ar age data (laser and furnace step heating) on plagioclase from Lower Cretaceous volcanic sequences from the Arqueros Formation in two sections of the Coastal Range at the latitude of La Serena (≈29°S) have been obtained. Due to the partial alteration of plagioclase crystals, disturbed age spectra in the furnace experiments have been observed, whereas laser heating determinations involving a much smaller quantity of grains carefully selected, could display plateau ages corresponding to pure plagioclase, as demonstrated by a constant 37

Mineralogical and chemical features of gangue phases in relation to hydrothermal mineralization and their host rocks

Gangue minerals from hydrothermal deposits (apatite and carbonates) and their host rocks from three different volcanic areas (SE Spain, La Serena and Melipilla in Coastal Range of Chile) have been studied for broad elemental compositions. Carbonate units at the Au-Cu Palai-Islica epithermal deposit are Fe-Mn-bearing, with a slightly higher concentration of these elements in the orebody than in adjoining hydrothermal alteration zones. Apatite has a composition that correlates with its origin and with hydrothermal processes. Thus, volcanic apatite is Cl-rich, whereas apatite from the associated orebody is almost pure fluorapatite. Furthermore, apatite from hydrothermally altered volcanic rocks has a transitional composition between volcanic and ore-related apatite. Samples of carbonate from Mn, Cu(-Ag) and Ba-Ag deposits in the La Serena area are all Mn-bearing calcite. In addition, Mn(Fe)-poor and Mg-rich calcite is common in low-grade Mn areas. Similar features have been found at the Melipilla Cu (Cu-Ag) deposit where epithermal calcite is also enriched in Mn(+Fe) with respect to non-mineralized veins, carbonate host rock, and metamorphic mineralization. In summary, F in apatite and Mn have been introduced in to carbonate from the La Serena area by hydrothermal fluids and could provide an index of hydrothermal ore-forming activity.