Petrological and Geochemical Considerations About the Lower Paleozoic Granitoids Ofthe Pampean Ranges, Argentina*

Revista Brasileira de Geociências 17(4):619-622, dezembro de 1987 PETROLOGICAL AND GEOCHEMICAL CONSIDERATIONS ABOUT THE LOWER PALEOZOIC GRANITOIDS OFTHE PAMPEAN RANGES, ARGENTINA* ALEJANDRO J. TOSELLI**, JUANA N. ROSSI DE TOSELLI** and JULIO SAAVEDRA*** I NTRODUCTlON In the Central and Northern Pampean characteristic, only muscovite and exceptionally cordierite Ranges of Argentina, a discontinuous belt of granitic plutons (Rapela 1982), but garnet is found in the aplite suite (Lira can be observed. These plutons (Fig. 1) extended from the 1985). southwestern part of the Province of Salta through East The granitoid of Loma Colorada shows cordierite, Catamarca and West Tucumán to the center of la Rioja, West sillimanite, and kyanite, and the granitoid of Los Alisos Córdoba, and Northeast San Luis. contains muscovite, topaz, and spodumene. The Lower Paleozoic plutons are of variable sizes, the In general, the granitoids are rather leucocratic; porphyric dominant rock types being granodiorites, tonalites and facies are common and have very often magmatic and granites. There are peraluminous granitoids with transition to metamorphic inclusions, with different states of assimilation. metaluminous and, exceptionally, to peraikaline characters Initial 87Sr/86Sr values are: 0.7109 for the Velasco (Shand 1927). They also show a variable range of initial batholith; 0.7146 for the Capillitas batholith; and 0.7048 for 8 7 ~ r/ 8 6 S r ratios. the Achala batholith (Rapela et aI. 1982); they suggest a subjacent sialic source. PETROGRAPHIC CHARACTERISTICS The dominant peraluminous features of the granitoids are defined PETROGENESIS OF THE GRANIT IC as well by the chemical (molecular proporcions) A1203/ MINERALOGY The following considerations refer to (CaO + Na20+ K 20 ) as by rnineralogy, for these granitoids the minerals present in each granite group and their contain muscovite, garnet, cordierite, topaz, andalusite and significance: sillimanite. Nevertheless, some peraluminous granitoids J. Epidote and hornblende-bearing granitoids - We have contain other minerals: observed epidote that has textural relations, which indicate J. Epidote and hornblende-bearing granitoids - They are -the magmatic origin in the granodioritic and granitic plutons. found in the big granitic batholiths, such as the ones of the The textural relations of the epidote against biotite, Sierra Norte de Córdoba, the Sierra de Los L1anos, Las plagioclase, hornblende, and interstitial epidote (Loma Pelada, Chacras and Cafayate, and they form huge masses, up to 100 Cafayate and Sierra de Ancasti granitoids) show that the km2 wide . Smaller bodies, -such as the granitoids of the epidote is really a magmatic mineral. Nevados de Cachi, the Megafault of Tafi, the Sierra de Naney's experimental'data (1983) indicate that the epidote Ancasti, and Guasayán have similar characteristics. is stable at 8 kbar, i,e. the hypersolidus phase in both the Mineralogically these granites consist of quartz, two granite and granodiorite compositions. This suggests that the feldspars, one or more characteristic phases such as biotite,' coexistence of epidote with silicate liquid on the high primary muscovite, hornblende, and epidote (Saavedra et ai. pressure, a water content greater than 6% and temperatures 1986). Exceptionally, they may contain garnet which are not higher than 70QOC (Naney op. cit., Zen & (almandine-spessartite) or hornblende. Hammarstrom '1984, Saavedra et ai. 1986). The relatively low initial 87Sr/86Sr ratio, i.e. 0.705 for the Thus, although more petrological studies are necessary to Cafayate batholith (Rapela et aI. 1982) and 0.705 to 0.715 in explain more precisely the pressure genesis of the primary the granitoids of the Sierra de Ancasti (Knüver 1983; Lottner epidote (see discussion by Moench 1986, Tulloch 1986), it is & Miller 1985), suggests a derivation from, or at least the likely that, in the case of the epidotic granitoids, this mineral contaminative influence, of a subjacent sialic source. crystallized directly from the magma at considerable pressure, 2. Cordierite and aluminium silicates-bearing granitoids - In pressures, though much lower than 8 kbar. Similarly the some cases they outcrop with the above mentioned group, as abundance of micas and tourmaline suggests that the magma occurs in the Nevados de Cachi (Gallisky 1983). Other places, could have contained notable quantities of halogenes which where they are present, are the big batholiths of the Sierra de could have displaced the solidus curve towards lower Velasco and Mazán, Capillitas and Achala. Smaller bodies, temperatures. such as the granitoids of Loma Colorada and Los Alisos, have 2. Cordierite and aluminium silicates-bearing granitoids similar characteristics. Andalusite from the granitic Capillitas batholith is always free In ali these bodies, tonalite and granodiorite are the from inclusions, and water-clear, shows often colourless to dominant rock types, accompanied by lesser amounts of pink pleochroism, and subhedral to euhedral shapes are granites, especially in the larger masses. common. The granitic Capillitas batholith (Toselli & Indri 1984) The andalusite normally - shows a muscovite-fibrolite shows the same dominant rock types, but also variable mantle in the contact with plagioclase. The contact reaction mineralogical compositions and textures. It tends to be coarse has taken place at"4 kbar (H20) and at a temperature of grained, locally foliated and contains large areas of potassium approximately 675°C (Toselli & lndri 1984). Muscovite is feldspar megacrysts. A few minerals are characteristic, among found not on1y as a product of the andalusite reaction, but them muscovite, cordierite, topaz, andalusiteand sillimanite. more frequently as flakes associated with biotite, feldspar The granitic Sierra de Velasco and Mazán batholith, despite and quartzo of this complexity, has many common points with the above The cordierite, on the other hand, clearly indicates a mentioned batholith, where cordierite and garnet appear in primary peraluminosity. The shape of the grains is cornpatible the porphyritic granite. with a magmatic crystallization (Clemens & WaII 1981) and The granitic Achala batholith shows variable geochemical indicates pressures lower than 6 kbar, while the garnet compositions, textures and mineralogy. Few minerals are associated with cordierite (observed only in the granitic * Extended abstract ** Facultad de Ciencias Naturales. Miguel Lillo 205, Tucumán, Argentina *** Instituto Nacional de Edafología y Agróbiologfa. Apartado 257, Salarnanca, Espana 620 Revista Brasileira de Geociências, Volume 17, 1987 Mazán batholith) could form (according to Clemmens & Wall f ig. 1 o . .. • , op. cit.) at 2 kbar and approximately 7000C. 0 . ... .3 The gamet is relatively rich in spessartine members and it • • ·.· ·6 is also common in the leucocratic facies (e.g. Loma Pelada SALTA Granite) in which the deuteric activity and the muscovitization of biotite are evident (Saavedra et ai. 1984, . --_.... ~- ..,-\ 1985). The abundance of tounnaline, topaz , fluorite and mica :' suggests that the granitic magma contained amounts of B and F, in the Sierra de Ancasti granitoids, and that the primary muscovite possibly crystallized at lower pressures and temperatures ·than anhydrous magmas or than when there is only water (Pichavant 1981). The granitoid projection in the QAP-Streckeisen modal Q diagram is also significant. The granitoids with cordierite arid aluminium silicates appear in the fields determined by normal potassium tendencies or by calc-alkaline granodiorites (CAG), or in the fields containing even more potassium and sub-alkaline monzonites (SAM) (Bowden et ai. 1984). On the contrary, the epidote-bearing granitoids (e.g. Loma Pelada, Cafayate, etc .) show a tonalitic or trondhjemitic calc-alkaline A .P tendency, or even a compositional concentration which . indicates a very slight change in the magma like in the case of O the Nevados de Cachi granitoids. Concerning the granitic Capillitas batholith the values show a great dispersion, but three groupings could be admitted: one of calc-alkaline tendency and very poor in potassium; one between potassium ~ p normal and potassium rich, which could be included in the , CAG and SAM fields; and, finally, an alkali rich one (ALK). PETROLOGY AND GEOCHEMISTRY In the use of - , the geochernical diagrams of the granites, those taken into ~ v account are the diagrams which include the gre atest possible . ' 30° -- number of chemical compo nents, i.e. the most representative of the mineralogical composition. The R1·R 2 chemical variation diagram used (De La Roche et ai. 1980) includes all of the major cations, a mineralogical network, the degree of silica saturation and the combined changes in the Fe/(Fe+ Mg) o and (Ab+Or)/An ratios , etc . in which the basic factors of ,r , magmatic evolution are clearly defined. , In this diagram (Fig. 2), the granitoids are arranged , , according to tendencies which coincide with quantitative , contents of alkali feldspar and quartz, with variable quantities of micas, and according to linear changes, which are approximately parallel to the RI axis, for the granites with 3,5 7.0 195 Km aluminium silicates (Rossi de Toselli et ai. 1986); while the granites with epidote show tendencies which are tran sversal , to the previous ones, i.e. approximately parallel to R2 of , tonalitic tendencies, i.e., they have major contents of \ Alurdniun sí.Lícates plagioclase and minor contents of quartz and potassic / ~ ~ granitcdds

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