Second ISAG, Oxford (UK),21-231911993 531

GEODYNAMIC EVOLUTION OF THE PALMOZOIC GONDWWA MARGIN IN WESTERN ARGENTINA (FAJVWTINA-SYSTEM)

. Rolf Mannheim

Jnstitut fiir Allgemeine und Angewandte Geologie Luisenstr. 37,80333 München 2, Germany

Los caracteres geoquimicos de los basaltos ordovicicos del Sistema deFamatina (SF) son comparables a los de basaltos calcoalcalinos modernos de arcos de islas oceinicos. Observaciones sedimentoi6gicas prueban la existencia de una Cuenca de retroarco entre el arc0 voldnico (SF) al oeste y el craton (Sierras Pampeanas) al este en el Ordovicico (relacionado a coordenadas actuales). Durante la colision del arco con el continentese intruyeron granitoides anatécticos corticales en el SF (Orogénesis Famatiniana). Como indicadores de un estadio poscolisional aparecen leucogranitos peralcalinos (Devonico?). En el mismo ambiente geotecthico del SF se intruyeronbasaltos de subducci6n. Los caracteres geoquimicos de estos basaltos correspondena los de basaltos calcoalcalinos de un borde continental activo como delactual tipo Chile.

KEY WORDS: Famatina-Systern, island arc - back arc, collision, nlagmatism

INTRODUCTION

The Famatina System 0's) representsa lithostrati- graphically and structurally independent unit within the NW-argentinian Sierras (fjg.1). It is characterized by Lower Middleto Ordovician volcanosedimentary scquences, which areintruded by calcalkaline granitoids of Upper Ordovician to Devonian age and by syn- to postplutonic volcanic dykes. Accordingto Aceiiolaza and Toselli (1954) andToselli etal. (1991) the xnagnlatic characteristics of the FS advocate for a destructiv plate margin. In the concept of Willner et al. (1957) and Rapela et al. (1990) the Ordovician wcstem Gondwana margin included a volcanic arc in the region of the FS and a several.hundred km largezone of intrusion situated in the back of the arc. Dalla Salda et al. (1992 a,b) suggest an Ordovician collision between Laurentiaand Gondwana, which lead to the Famatina Orogenesis. Fig.1: Morphostructural units in NW-Argentina. Puna (l), Cordillera Oriental (Z),Sierras Subandinas (3), Cord. Frontal (4), Precordillera (5), Famatina-Systern (6),Sierras Pampeanas 0,Cord. Principal (8). 532 Second ISAG, Oxford (UK),21 -231911993

GEOCHEMICALAND SEDWNTOLOGICBLARGUME~S

The Ordovician synsedimentargr and discordant basic to acidic volcaniashow comgruent geochemical properties andare interpreted as alcalkaline magmas of a magmatic arc. In wnirasG a youngergeneration @evonian?) of discordane basic dykes revals typicaI intraplate characteristics(fig.2).

1ow 7 10 1w FIg.2: Intermediate to acid (+) and basic (O,.) volcanic rocks in the Ti vs. 2 diagram aftfter Pemce (1982).

The Ordovician basalts exhibit typical island arc patterns in MON3-nomalized spider diagrans. Analogues patterns are known from modemoceanic island arcs (cg. Mariana or Javaarc, see Pearce 1983, fig.3). The presence of a back arc basinlocated between the volcanic arc and the continent in the area of theFS is documented by the intimate association of olisthostromes, rich in volcaniclastic material, derived fmm the island arc, and distal cratonal turbidites. Intercalated calcareous turbidites are supposed to be introduced from a carbonateplatfom situated on thecontinent-ward sideof the basin (Clemens 1992, Mannheimand Clemem 1992).

YtNb ppm 1 - . , ...... : , , . , : , . . _.,- 10 100 1' IO

FIg.3 MORB-normalized spider diagrams of the Fig.4: Intermediate and acid volcanic rocks in the Rb famatinian basalts W. (Y+Nb)-diagramafter Pearce (1 984). CX-dovician volc. (+,*), Devenian (7) laucogranites (O)

The intexmediate to acid volcanics of the Ordovician show geochemical characteristics of an island arc in a compressional environmentwith clearsyncollisional affinities (fig.4). They,'are not directlyrelated by differentiationprocesses tothe basicmagmas, but are generated by crusfal meltingand magma mixing (Mannheim 1993).

Peralcaline A-type leucogranites (Devonian?) represent the late orogenic to postcollisional stage of the FS (fig.4). Subduction-relatedbasalts generated by partialmelting of a subcontinentalmantle enriched by metasomatic processes intruded this geotectonic scenario. Calcalkaline basalts of the modern continental margin in central Chile exhibit similar pattern (fg.3, compare Pearce 1983). Second ISAG, Oxford (UK),21-231911993 533

CONCLUSONS

During the Cambrian the western edgeof Gondwana changes from a passive to an active continental margin. This is reported by a subduction-related 1-type magmatism in the Pampean Range,' wich conlmenced in the ' Middle Cambrian @mer 1986, Willner etal. 1987, see fig.5 a).

In the bwer Ordovician amarginal basinopened due to mantle convectioncreating an extensional environmenf and the initial arc developed to a rifted arc. In the Pampean Range (remnant arc) the 1-type magmatism ceased as a result of the westward shift of the subductionzone. According to Rapela et al. (1990) the Lower Ordovician calcalkaline plutonic bodiesintruded the "inner backarc" zone. The FS represents theocean-wards drifting part of the magmatic arc. Primary basic magmas were generated in a "depleted", purely oceanic mantle environment. From Tremadocian to Llanvimian the basin received sediments from the continent and fmm the island arc (fig.5 b). In the FS the volcanic arc and the adjacent part of the back arc basin are actuallyexposed. The continent-ward part of the basin (carbonate platfontl?) has been probably overriden during the postordovician accretionof the island arc to thecontinent.

W E

LOWER TO MIDDLE CAMBAIAN

Fig5: Geodynarnic evolution of the FarnatinaSystem in the Palaeozic

The closure of this short-lived marginal basin started in the Middle Ordovician. At the Ordovician/Silurian boundary large amountsof peraluminous collisional granitic melts intruded the FS (fig.5 c).

Probably in the Lower Devonianthe island arc - continent collision was conchded, the FS was accreated to westernGondwana. The incessant subduction lead to theintrusion of basalticmagmas generated in a metasomatically exuiched mantle. The "collision-thickened" active continental margin was intruded by late to postorogenic leucogranites indicatingthe extinction of the Famatinian Orogenesis (fig.5 d).

Similar Lower Palaeozoic back arc basins are described by Bahlburg (1990) for the Puna region situated north of the FS and by Loske(1992 a,b) for the Precordillera in the south(compare fig.1). 534 Second ISAG, Oxford (UK), 21 -231911993

The autor wkhes to achowledge the scientific and logistie help of Dr. Hubert Miller (München) and Dr. Alejandro Toselli (l-i~~dn).The Deutsche Forschungsgemeinschaft @FG) supported field and lab work (grants Mi 120/20-1,2).

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