Granite plutonism of the Sierras Pampeanas: An inner cordilleran Paleozoic arc in the Southern Andes

RAPELA, C.W.; TOSELLI, A.J.; HEAMAN, L.; SAAVEDRA, J.

Plutonism from Antarctica to Alaska

Geological Society of America

Año: 1990; p. 77 - 90

The eastem zone of the Sierras Pampeanas in central and northwestem Argentina is characterized by a Paleozoic granitoid series emplaced at dift´erent structurallevels in polymetamorphic terranes. Three main plutonic groups have been recognized on the basis of their ages and their relation to the deformational history. Gl (Late Precambrian lo Cambrian): Scattered concordant bodies of gabbros, amphibolitized norites, amphibolites, diorites, and tonalites were intruded during an early deformation and regional metamorphism (DI, MI). This group is the only one to show tholeütic affinities. G2 (Ordovician to Early Devonian): Syn-D2 (Early Ordovician) and late D2 (Middle to Late Ordovician) subgroups consist of small batholiths, plutons, and concordant bodies of tonalitic to granitic composition that were intruded during or shortly alter the D2 and M2 (amphibolite to granulite facies) episode. These granitoids, characterized by their peraluminous chemistry, contain primary muscovite, spessartite gamet, and magmatic epidote. Most initial87Sr/86Sr ratios are between 0.705 and 0.707. Some Upper Ordovician units have initial 87Sr/86Sr ratios greater than 0.710, suggesting an important upper-crustal contribution. Minimum crystallization pressures of 4 to 5 kbar are inferred from the granitoid paragenesis. The melts are inferred to be water- and volatile-rich as they produced ubiquitous tourmaline-bearing pegmatite swarms during the final stages of crystaUization. This plutonism seems to be related to the Famatinian magmatic arc that was located 400 to 800 km east of the present Peru-Chile trench. These syn-D2 and late D2 granitoids are thought to be part of an inner back-arc zone of this early Paleozoic arco Post-D2 (Late Ordovician to Early Devonian) granitoids consist of large batholiths (e.g., Achala and Velasco batholiths) and widespread smaller plutons that were emplaced during the later stages of the early Paleozoic (Famatinian) orogeny. Common features of the larger bodies include the presence of muscovite, aluminum silicates, and a dominant porphyritic phase characterized by microcline microperthite megacrysts in a groundmass of medium- to coarse-grained quartz, oligoclase, and biotite. Mioeralized pegmatites (Li, Be, Nb, Ta) and U deposits related to post-D2 granites have high levels of LIL (large ion lithophiles: K, Rb, LREE) and HFS (high field strength: Y, Nb) elements. The post-D2 group exhibits compositional features similar to alkali-calcic arc granitoids developed at a great distance from the subduction margin, in the later stages of arc evolution. G3 (Late Devonian to CarboniJe,ous): Homblende-biotite alkalic-like plutons culminate granitoid activity in the Sierras Pampeanas. These granitoids have high levels of HFS and LIL elements, and some have high initial 87Sr/86Sr ratios (0.710). The G3 granitoids are interpreted to be within-plate plutons indirectly related to the beginning of late Paleozoic subduction on the Gondwana margin.