Evolución metamórfica del sector nororiental del basamento de Tandilia, Argentina: metamorfismo en facies granulita y anatexis cortical /
The northeastern portion of Tandilia Basement, differentiate mainly from the rest of the range by its lithological variability. This characteristic point to this region as a propitious place for the development of detailed petrological studies. Two localities were considered of critical interest to...
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| Formato: | Libro |
| Idioma: | Spanish |
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2000.
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| Sumario: | The northeastern portion of Tandilia Basement, differentiate mainly from the rest of the range by its lithological variability. This characteristic point to this region as a propitious place for the development of detailed petrological studies. Two localities were considered of critical interest to the knowledge of the metamorphic evolution of the area: The Sierra de Bachicha and the Cerro El Triunfo. These localities are composed mainly of gneisses, amphibolites, marbles and associated calcsilicates, granulites, migmatites and pegmatitic segregates. Detailed petrological studies were carried out, including mainly the following techniques: field and petrographic-microstructural studies, mineral and whole rock geochemical analysis and geothermobarometry (internally consistent methodology, classical geothermobarometry and fluid inclusion analysis). In addition to electron microprobe analyzer, several accessory techniques such as EDAX, x-ray difractometry, infrared spectroscopy, transmission electron microscopy, staining methods, etc. were also employed in the determination of mineral identities and characteristics. A voluminous number of previous author's data and the results obtained in this work, indicate that the present geologic constitution of the northeastern portion of Tandilia basement, is the consequence of an extended and complex history of petrologic and structural transformations. Petrologic and structural field relationships found in the present contribution, in addition to evidences attained from whole rock major and trace elements geochemical analysis, allow establishing that the development of a subsident sedimentary basin precedes the beginnning of the metamorphic evolution in the study area. Preserved evidences sustain the postulation of an original sedimentary pile, constituted in its bas bay iron-rich pelites and/or graywackes with increasing proportions of carbonates to the upper levels of the sequence, characterized by marly rocks with calcareous intercalations. This sedimentary sequence constituted the ore probable protolith from which complex metamorphic associations further develop. Mafic igneous rocks, emplaced dominantly in the form of sills folllowing So, intruded the sedimentary sequence. In the upper levels, mafic rocks intercalated with carbonates have the greater thickness. The metamorphic evolution in the area started in or previously to the lower Paleo-Proterozoic as can be inferred from radiometric ages exceeding 2200 Ma. registered in the older rocks from the studied region. Prograde regional metamorphism initially transforms the pelites and/or graywackes from the base of the sequence in tonalitic gneisses (Pl+Qtz+Bt1+Grt1,acc.Zr+Ap), and calcareous rocks in dolomitic marbles. To this stage of metamorphic evolution is assigned the development of a primitive foliation (S1, deformational phase F1), subparallel to the lithological contacts (So). As a result of extreme metamorphic condidtions, tonalitic gneisses were partially transformed to heterogeneous migmatites ' ' via incongruent reactions of biotite. Field, textural-petrographic and geochemical evidences support 'in situ' generation of neosomes. More often migmatites show gradual grain size increase to attain pegmatitic appearance, preserving the mineralogical composition of equivalent fine grained migmatites, but increasing neosome proportions over restitic palaesome. Areas of most intense partial melting generate pegmatitic segregates ' ' in the form of veins and lenses that either remain close to the source or migrate to the upper levels intruding the overlaying mafic-carbonatic sequence. Scarce two-pyroxene and orthopyroxene granulites ' ', always included in migmatites, remain as restites of the partial anatexis, possibly as a result of water avidity of the forming neosomes. Low-water activity during this stage is indicated by equilibrium mineral asssociations analyzed via internally consistent methodology, and by the presence of pure-CO2 fluid inclusions in the granulite rocks. Equivalent high-grade metamorphic conditions to those found in the felsic-intermediate rocks from the lower levels of the sequence are documented in the mafic-carbonatic rocks from the upper levels. Upper-amphibolite to granulite facies conditions, are recorded by mafic-marly rocks transformed to amphibolites ' '-granoblastites' ' and by carbonates converted to impure dolomitic marbles (Cc+Dol+Cpx+Fo+Spl). The prograde reactions, which led to the crystallization of marble associations, are decarbonation reactions that release CO2 to the fluid phase. Pure-CO2 primary fluid inclusions in a quartzitic band adjacent to a thick marble layer are assigned to this stage of the metamorphic evolution.//CALIFICACION DEPARTAMENTO DE GRADUADOS Calificación de la defensa oral: Sobresaliente - 10 (diez) Fecha: 5/7/00 |
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| Descripción Física: | 180 h. : ilus. ; 30,5 cm.. |
| Bibliografía: | Incluye referencias bibliográficas. |