Field Trip Guidebook: Granitic pegmatites of the San Luis Ranges

GALLISKI M. A.; MÁRQUEZ-ZAVALÍA MARÍA FLORENCIA

Zeta Editores

Lugar: Mendoza; Año: 2011 p. 46

The Pampean Pegmatite Province The Pampean pegmatite province (PPP) was defined to include the economic fields of granitic pegmatites of central and northwestern Argentina (Galliski 1992, 1994a, 1994b, 2009). It contains more than 95% of the granitic pegmatites of the country, with mineral resources that had been mined during the past 80 years, producing the majority of the feldspar, quartz and mica plus Be-, Li-, Ta-, Bi- and Rb-bearing minerals. The PPP extends discontinuously for more that 800 km N-S and 200 km E-W, from 24º30´ to 33º30´S, mostly in the Sierras Pampeanas of northwestern and central Argentina (Fig. 1), but the three northern districts are in the southern part of Cordillera Oriental and the adjacent mountain block of Puna. Its possible extension to the South could be even more than 400 km as indicated by discontinuous outcrops under recent cover that reach the province of La Pampa. The Tectonic Setting The orogenic pegmatite fields of the Pampean pegmatite province developed mainly in the early Paleozoic, during the Pampean (~550-520 Ma) and especially Famatinian (~500-435 Ma) orogenic cycles, along a pericratonic mobile belt located in the western protomargin of Gondwana. The tectonic evolution of this mobile belt is complex and it includes the accretion of several terranes (Cf.  Collo et al. 2009). Significant facts in the geological evolution of the pegmatite province include: (1) the ensialic deposition of several thousand meters thick psammopelitic succession of sediments, represented by the Puncoviscana Formation and equivalent protoliths, during the Late Proterozoic - Early Cambrian, and (2) the Upper Ordovician collision of the Precordillera terrane. The present tectonic setting that exposes the pegmatite province in faulted blocks eastward of the Andean Cordillera, results from the flat subduction geometry of the Nazca plate under this segment of the South America plate. The Distribution of the Pegmatite Fields The orogenic pegmatite fields The orogenic pegmatite fields encompass pegmatites of muscovite, muscovite?rare-element and rare-element classes using the classification of Černý & Ercit (2005). The distribution of the economic pegmatite fields is plotted in Figure 1. The muscovite class pegmatites are dominant in three fields aligned in the western side of the province, and one in the eastern side. The different fields from the rare-element pegmatite belt belong to the LCT (Li, Cs, Ta) petrogenetic family. They are aligned N-S and mostly hosted in medium-grade metamorphic rocks, preferentially in amphibolite facies from an Abukuma-type metamorphic belt, that suffered polyphase deformation during the Pampean and Famatinian orogenic cycles. Along the rare-element belt the lithium bearing pegmatites crystallized under increasing pressure conditions from the petalite-subtype pegmatites of the El Quemado district in the north, passing through the spodumene-subtype pegmatites of Ancasti district in the middle, and finishing in the economic pegmatite fields of the San Luis ranges. The Conlara, Totoral and La Estanzuela pegmatite fields are located in the San Luis ranges and have an important past record of producing mica, beryl, spodumene, tantalite, and lastly, K-feldspar, albite and quartz. These districts are hosted in metamorphic complexes of medium grade, intruded by orogenic and postorogenic granites. The medium-sized pegmatites are representative of several types and subtypes as beryl-columbite-phosphate, spodumene, albite-spodumene and albite. The K-Ar ages of the pegmatites from this belt, except for the Córdoba districts, are grouped in the Lower Paleozoic, in good correlation with the granitic magmatic of the Famatinian orogenic cycle. They are genetically linked to a suite of two- mica, or muscovite-tourmaline leucogranite, which usually are small to medium size composite intrusive of variable fabric, within the aplite to pegmatite range. They are peraluminous, low-Ca calc-alkaline rocks that have high LILE and volatile contents, especially B and P. They have total REE depleted contents and high initial 87Sr/86Sr ratios (≥ 0.71). Most of these S-type leucogranite and associated pegmatites are syn- to late-kinematic and were slightly or strongly deformed during the late-stage phases of the Famatinian deformation. The most favored model for the origin of this suite comprises episodic crustal anatexis, produced by muscovite ± biotite dehydration melting, of dominant Puncoviscana protoliths by shear-heating during a collisional orogeny, as proposed by Nabelek & Liu (2004) for other regions (Galliski 2009). This collisional leucogranite suite is space-related to another suite of TTG dominant lithology, which includes some more basic intrusives; it is also calc-alkaline, peraluminous, and has lower 87Sr/86Sri ratios (~0.706). This last suite has been considered of I-type and related to a subduction setting, with variable lower crust component and possible assimilation of supra-crustal protoliths (Pankhurst et al. 2000). The post-orogenic pegmatite fields The granitic pegmatites of this kind form distinctive units in the El Portezuelo (Papachacra) and Potrerillos groups, and Velasco and Punilla districts. The pegmatites of the El Portezuelo group belong to the miarolitic class, miarolitic-rare-element subclass, possibly of gadolinite-fergusonite type and NYF (Nb, Y, F) petrogenetic family (Colombo 2006). The pegmatites from the Velasco district are units of beryl-columbite-phosphate subtype contained in the Huaco sieno- to monzogranite, dated at 350-358 Ma (Grosse et al. 2008). The pegmatites of the Punilla field are rounded bodies, similar to the Velasco ones, but generally larger, with more varied mineralogy, and contained in porphyritic granites of the Achala batholith, dated at 368± 2 Ma (Dorais et al. 1997). Except for the El Portezuelo miarolitic pegmatites that have a diagnostic NYF mineralogy, and the Potrerillos pegmatites that locally have primary fluorite, monazite-(Ce) and ilmenorutile suggesting the same signature, the pegmatites of the other districts are difficult to classify because they lack typical diagnostic paragenesis. In general, the parental granites of these pegmatites form composite batholiths or stocks lacking post-emplacement deformation, that were intruded in upper- to medium upper crust levels (P 150-250 MPa), mostly during the Lower Carboniferous. Petro-graphically, they are generally biotite porphyritic granites to monzo- orsienogranites, locally showing K-feldspars with incipient rapakivi-like textures. They are usually high silica, meta- to mildly peraluminous, high-K calc-alkaline granites, with Fe-, Mg-, and Ca-contents higher than the orogenic leucogranites. Commonly, these granites are moderately LIL and HFSE enriched. The initial 87Sr/86Sr ratios, if not disturbed, are generally low to medium (0.703-0.706); the contents and normalized REE patterns, and δ 18O values are most likely comparable to those of the NYF petrogenetic family. These attributes suggest that this is a post-orogenic suite probably of aluminous A-type granites, intruded in an intraplate tectonic setting. In the El Portezuelo and Huaco granites the genetic interpretation favor a mixed source with melting of crustal I-type granite protoliths, previously metasomatized by a mantle fluid component (Colombo 2006, Grosse et al. 2008). Concluding Remarks The framework for the geological evolution of at least the Famatinian representatives of the Pampean pegmatite province comprises basically three major episodes: (1) intrusion of muscovite class pegmatites, poorly or non-connected with parental granites, in a MP-MT metamorphic environment generally westward of, (2) a LCT rare-element pegmatite belt and the parental collisional leucogranites placed at the axis of a LP-MT metamorphic belt developed in the Upper Ordovician during the Famatinian tectonic cycle, and (3) occurrence of a suite of rare-element pegmatites, locally with definite NYF signature, mostly contained in post-orogenic granites that form major composite batholiths or minor plutons, possibly of aluminous A-type granites intruded during the Lower Carboniferous in an intraplate tectonic setting. Comparing with the Eastern Pegmatite Province of Brazil, the PPP has a pericratonic tectonic setting and its radiometric ages are slightly younger.