Sedimentary provenance of the Late Paleozoic metamorphic basement,south-central Chile: Implications for the evolution of the western margin of Gondwana

ABSTRACT

U-Pb detrital zircon age patterns are presented for nine samples of metapelites from the metamorphic basement of south-central Chile between 37° and 40°S, along with detrital zircon ages for a sample from the Piedra Santa metamorphic complex and a crystallization age of the Chachil plutonic complex, farther east in Argentina. Two distinct zircon age patterns are identified. One is a pattern with a dominant population of zircons at ca. 470 Ma (Ordovician), a widespread presence of Mesoproterozoic ages (1200 ? 1000 Ma), and a Carboniferous maximum deposition age indicated by the youngest zircon population. The second pattern is drastically different, with a main population of Permian zircons, a 290–250 Ma maximum deposition age, and a minor contribution of Pennsylvanian age zircons. Our results, coupled with previously reported metamorphic ages, show that the patterns reflect the presence of different tectonic blocks separated by the Mocha-Villarrica Fault Zone at 39°S. Metapelites north of this structure have the Ordovician-dominated pattern characteristic of the Western Series of the Paleozoic basement, and those south of the fault show the Permian-dominated pattern observed in the younger high-pressure metapelites from the Bahía Mansa metamorphic complex. The Piedra Santa metamorphic complex also shows the Ordovician-dominated pattern and was intruded by the Chachil plutonic complex dated here at ca. 303 Ma. Therefore, the Piedra Santa complex is interpreted as coeval with the Chilean metamorphic basement. The present dislocation of blocks with different ages is attributed to continental-scale dextral strike-slip tectonics along the Huincul Fault Zone, Argentina, which extends to the west as the Mocha-Villarrica Fault Zone in Chile.     

Alteration patterns of chromian spinels from La Cabaña peridotite,south-central Chile

La Cabaña peridotite is part of a dismembered ophiolite complex located within the metamorphic basement of the Coastal Cordillera of south-central Chile, and is the only location in Chile were Cr-spinels have been described so far. The La Cabaña peridotite is part of the Western Series unit, which comprises meta-sedimentary rocks, metabasites, and serpentinized ultramafic rocks. This unit has been affected by greenschist-facies metamorphism with reported peak PT conditions of 7.0–9.3 kbar and 380°–420 °C. Within La Cabaña peridotite Cr-spinels are present in two localities: Lavanderos and Centinela Bajo. In Lavanderos, Cr-spinel occurs in small chromitite pods and as accessory/disseminated grains with a porous or spongy texture in serpentinite, whereas in Centinela Bajo Cr-spinel is present as accessory zoned grains in partly serpentinized dunites, and in chromitite blocks. All Cr-spinels display variable degrees of alteration to Fe²⁺-rich chromite with a variation trend of major elements from chromite to Fe²⁺-rich chromite similar to those observed in other locations, i.e., an increase in Fe₂O₃ and FeO, a decrease in Al₂O₃ and MgO. Cr₂O₃ content increases from chromite to Fe²⁺-rich chromite in chromitite pods from Lavanderos and chromitite blocks from Centinela Bajo, but decreases in ferrian chromite zones in accessory grains from Centinela Bajo. The minor element (Ti, V, Zn, Ni) content is mostly low and does not exceed 0.4 wt.%, with the exception of MnO (<0.9 wt.%), which shows a correspondence with increasing degree of alteration. Cr# (Cr/Cr?+?Al) versus Mg# (Mg/Mg?+?Fe²⁺) and Fe³⁺/Fe³⁺+Fe²⁺ versus Mg# plots are used to illustrate the Cr-spinel alteration process. Overall, the Cr-spinels from Lavanderos (chromitite pods and disseminated grains) exhibit Cr# values ranging from 0.6 to 1.0, Mg# (Mg/Mg?+?Fe²⁺) below 0.5, and (Fe³⁺/Fe³⁺+Fe²⁺) <0.4. Cr-spinels from chromitites in Centinela Bajo have Cr# and Mg# values that range from 0.65 to 1.0, and 0.7-0.3, respectively, and (Fe³⁺/Fe³⁺+Fe²⁺)?3+/Fe³⁺+Fe²⁺) ratio is less than 0.4 in chromite cores and Fe²⁺-rich chromite, and >0.5 in ferrian chromite and Cr-magnetite. Interpretation of the data obtained and Cr-spinel textures indicate that the alteration of Cr-spinel is a progressive process that involves in its initial stages the reaction of chromite with olivine under water-saturated conditions to produce clinochlore and Fe²⁺-rich chromite. During this stage the chromite can also incorporate Ni, Mn, and/or Zn from the serpentinization fluids. As alteration progresses, Fe²⁺-rich chromite loses mass resulting in the development of a spongy texture. In a later stage and under more oxidizing conditions Fe³⁺ is incorporated in chromite/Fe²⁺-rich chromite shifting its composition to an Fe³⁺-rich chromite (i.e., ferrian chromite). Depending on the fluid/rock and Cr-spinel/silicate ratios, Cr-magnetite can also form over Fe²⁺-rich chromite and/or ferrian chromite as a secondary overgrowth. The compositional changes observed in Cr-spinels from La Cabaña reflect the initial stages of alteration under serpentinization conditions. Results from this study show that the alteration of Cr-spinels is dependent on temperature. The degree and extent of alteration (formation of Fe²⁺-rich and/or ferrian chromite) are controlled by the redox nature of the fluids, the Cr-spinel/silicate and the fluid/rock ratios.