Two end-member models have been proposed for the Paleogene Andean foreland: a simple W-E migrating foreland model and a broken-foreland model. We present new stratigraphic, sedimentological and structural data from the Paleogene Quebrada de los Colorados (QLC) Formation, in the Eastern Cordillera, with which to test these two different models. Basin-wide unconformities, growthstrata and changes in provenance indicate deposition of the QLC Formation in a tectonically active basin. Both west-and east-vergent structures, rooted in the basement, controlled the deposition and distribution of the QLC Formation from the Middle Eocene to the Early Miocene. The provenance analysis indicates that the main source areas were basement blocks, like the Paleozoic Oire Eruptive Complex, uplifted during Paleogene shortening, and that delimits the eastern boundary of the present-day intraorogenic Puna plateau. A comparison of the QLC sedimentary basin-fill pattern with those of adjacent Paleogene basins in the Puna plateau and in the Santa B arbara System highlights the presence of discrete depozones. These reflect the early compartmentalization of the foreland, rather than a stepwise advance of the deformation front of a thrust belt. The early Tertiary foreland of the southern central Andes is represented by a ca. 250-km-wide area comprising several deformation zones (Arizaro, Mac on, Copalayo and Calchaqu ı) in which doubly vergent or asymmetric structures, rooted in the basement, were generated. Hence, classical foreland model is difficult to apply in this Paleogene basin; and our data and interpretation agree with a broken-foreland model.

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The Palaeogene Maíz Gordo Formation is one of the main lacustrine events recorded in northwestern Argentina. It consists of sandstone, mudstone, and limestone beds 200 m thick, deposited in a brackish-alkaline lake and braided alluvial systems. The Maíz Gordo Lake evolved mainly as a closed system, with brief periods as an open one. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to study samples from seven sites, corresponding respectively to proximal, intermediate, and transitional positions of the fluvial environment and marginal and inner-lake environment, focusing on the clay mineralogy and analcime formation. The basinward zonation of diagenetic minerals identified in the Maíz Gordo Lake was: mordenite → analcime → K-feldspar. Although not a typical zonation of saline-alkaline lakes, it does indicate an increase in salinity and alkalinity towards the centre.In proximal fluvial settings, smectite predominates at the base of the sequence, with scarce kaolinite. Towards the top, a striking increase in kaolinite content suggests a change from a relatively arid climate with alternating humid and dry seasons, towards a warm and humid climate. Kaolinite content clearly decreases in a basinward direction. Such a variation is attributable to changes in hydro-geochemistry, denoting the progressive influence of the brackish and alkaline lake water on interstitial pores. SEM images of intermediate fluvial samples reveal authigenesis of illite at the expense of kaolinite booklets.In littoral and inner-lake settings the clay fraction is composed of muscovite, sometimes with subordinate smectite. Analcime occurs in variable amounts in all sedimentary facies, in rock pores or filling veins. It forms subhedral square to hexagonal, or anhedral rounded crystals, denoting that they coarsened at low to moderate degrees of supersaturation. Although the mordenite identified in a fluvial level would have been the precursor of analcime in the Maíz Gordo Basin, no textural evidence of analcime formation through replacement of mordenite or other precursor zeolite was found. Hence it is more probable that analcime formation took place by direct authigenic precipitation or through the reaction between interstitial brines and clay minerals or plagioclase.

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The continental Paleocene-Eocene sequence investigated in this study belongs to the Salta Group, deposited in an intracontinental rift, the Salta Basin (NW Argentina), that evolved from the lower Cretaceous to the middle Paleogene, and is subdivided into the Pirgua, the Balbuena and the Santa Barbara Subgroups. The Maíz Gordo Formation (200 m thick) is the middle unit of the Santa Bárbara Subgroup, deposited during late post-rift sedimentation. We studied the mineralogy of fine-grained horizons of this formation by X-ray diffraction and Scanning Electron Microscopy (SEM) in order to examine the connection between vertical changes in clay mineralogy in alluvial sediments and paleosols, and global paleoclimatic changes registered during the Paleogene.

The Yerba Loca Formation (Middle-Upper Ordovician), exposed in the Western Precordillera, San Juan, Argentina, is made up of clastic-carbonate turbidites, and basic-ultrabasic rocks. It is affected by regional Siluro-Devonian very low-grade metamorphism that locally reaches greenschist facies. At Ancaucha creek, 45 conodont samples were taken from two sections that include 30 to 50 m thick sills. In order to analyse the thermal alteration patterns produced by these intrusive bodies, conodont Colour Alteration Index (CAI) is contrasted with optical petrography and X-ray diffraction analyses of clay minerals. The intrusions are dated as post-Darriwilian (Da2), as determined by conodont biostratigraphy of the host rock, which indicates the Paroistodus horridus Subzone of the Lenodus variabilis Zone. The distribution of CAI values defines a thermal aureole of about 2.5 times intrusion thickness that prevailed over the later very low-grade metamorphism. Metasomatism at Ancaucha creek is recorded by CAI values of 4 to 7, particularly restricted to a few layers close to the intrusions, as indicated by conodont textures and rock fabric. One-dimensional thermal computer simulation conforms to empirical data indicating temperatures greater than 600 • C for the contact zone, although it points out slightly narrower thermal aureoles. The clay mineral assemblage of most of the analysed samples (chlorite, illite, smectite and I/S mixed-layers) is complex and probably derives from several superimposed processes, thus representing non-equilibrium assemblages. In turn, KI values (0.27-0.32) indicate anchizone metamorphism, in agreement with regional CAI values of 4; consequently, the occurrence of smectite and I/S probably resulted from retrograde diagenesis processes.

A B S T R A C T :The Puncoviscana Formation, largely cropping out in NW Argentina, is mainly composed of a pelite-greywacke turbidite sequence affected by incipient regional metamorphism and polyphase deformation. Metapelites, composed mainly of quartz, albite, dioctahedral mica and chlorite, were sampled in the Lules-Puncoviscana and Choromoro belts. Lattice-fringe images, selected area electron diffraction and analytical electron microscopy analyses, coupled with previous data from white mica crystallinity index, indicate a state of reaction progress for Puncoviscana slates consistent with medium anchizone-to epizone-grade metamorphism. The 2M polytype prevails in dioctahedral micas, coexisting in a few cases with the 1Md polytype as a consequence of lack of equilibrium. The 2M polytype coexists with 3T in two slates and long-range four-layer and ten-layer stacking sequences were identified in another sample. Samples with 3T and long-range stacking sequences present b values characteristic of intermediate-high pressure metamorphism and ordered chlorites (1L, 2L, 3L and 7L) prevail.Based on the Si contents of dioctahedral micas and considering peak temperatures of 350 -400ºC, pressures from 5 kbar and 5 -7 kbar were derived for metapelites from the LulesPuncoviscana and Choromoro belts, respectively. These values agree with facies series derived from the b values. Micas with a wide range of phengitic substitution, as evidenced by Fe + Mg and Si contents, coexist. These variations could not arise from the disturbing effect of detrital white K-mica because TEM evidence indicates that they are absent or represent <10% of the mica population. Thus, compositional variations suggest that dioctahedral micas of individual slates crystallized at different pressure conditions in response to the P-T path of the metamorphism. Moreover, in several biotite-free slates the illite crystallinity (IC) values lead to an underestimation of the metamorphic grade attained in these rocks.The coexistence of IC corresponding to anchizone and the occurrence of biotite in some slates and felsic metavolcanic rocks intercalated in the Puncoviscana metasediments are interpreted to be the result of a metamorphic path including a relatively high-pressure/ low-temperature (HP/LT) event, followed by a lower-pressure overprint possibly at higher temperatures than the HP/LT event. Small micas formed during the high-pressure stage would prevail in the <2 mm fraction, producing anchizone IC.