Daniel A. Gregori

Provenance and Evolution of the Guarguariiz Complex, Cordillera Frontal, Argentina

Abstract The Guarguardz Complex, basement of the Cordillera Frontal, included in the proposed Chilenia Terrane, consists of metasedimentary rocks deposited in clastic and carbonatic platforms. Turbiditic sequences point out to slope or external platform environments. According to geochemical data, the sedimentary protoliths derived through erosion of a mature cratonic continental basement. Volcanic and subvolcanic rocks with N and E-MORB signature were interbeded in the metasedimentary rocks during basin development. A compressional stage, starting with progressive deformation and metamorphism, followed this extensional stage. Continuing deformation led to the emplacement of slices of oceanic crust, conforming an accretionary prism during Late Devonian. The Guarguardz Complex and equivalent units in western Precordillera and also in the Chilean Coastal Cordillera share common evolutional stages, widely represented along the western Gondwana margin. These evidences imply that Chilenia is not an allochthonous terrane to Gondwana, but a portion of its Early Paleozoic margin. Regional configuration indicates that the Guarguardz Complex and equivalent units represent the accretionary prism of the Famatinian arc (Middle Ordovician-Late Devonian).

New evidence on the nature of the Frontal Cordillera ophiolitic belt — Argentina

Abstract New petrographic and chemical data of volcanic rocks from the Metales Belt, Rio de las Tunas area, in the Frontal Cordillera of Argentina are presented. The rocks display a broadly basaltic composition. Two varieties of basaltic dikes can be differentiated. One group, whose mineralogy comprise essentially tremolite-actinolite, plagioclase, pyroxene and limited amounts of olivine as the main phases, display metamorphic textures. The other group show porphyritic textures consisting of zoned plagioclase and K-feldspar. Zoned orthopyroxenes are scarce. Chemically, the rocks are similar to the Caradocian basaltic rocks (the ofiolitas famatinianas) from the Precordillera, San Juan Province. REE chondrite-normalised patterns and overall abundances, along with other petrologic and geochemical similarities, suggest a common mode of origin for both groups of basaltic rocks. Their geochemical and petrographical characateristics are consistent with a continental-scale extension environment during Eopalaeozoic times that could include an oceanic rift in an early stage of development, a transitional ridge segment or oceanic intraplate magmas, similar to those erupted in oceanic islands.

Mössbauer study of some Argentinian chlorites

Three chlorite samples obtained from mining areas in Mendoza, Argentina, have been studied by wet chemical analysis, X-ray diffraction and57Fe Mössbauer spectroscopy. The total Fe contents and the Fe2+/Fe3+ atomic ratio are used to characterize the samples and are discussed in connection to the likely genesis of the minerals.