Western Cordillera Of Colombia Research Articles

Geology of the Frontino-Morrogacho Gold Mining District and Metallogeny of the El Cerro Igneous Complex

The Frontino-Morrogacho gold district is located on the western flank of the Western Cordillera, NW of Antioquia Province. Gold mineralizations in the area are spatially and genetically associated with the cooling of three mid- to late-Miocene age intrusive centers in the form of stocks and dikes (12-9 Ma): Cerro Frontino, La Horqueta and Morrogacho (El Cerro Igneous Complex). These composite magmatic pulses, with ultramafic to intermediate compositions, vary into diorite-, gabbro- and monzonitic-bearing phases. Mineralization in the complex is present as several structurally controlled fault veins, shear-related veins, sheeted quartz extension veins and quartz-carbonate tabular extension veins, with the development of swarms and nests of veins-veinlets, breccias and stockworks. Structures range from centimeter-wide individual veinlets to several meter-wide swarms of veins developed within broad mineralized structural corridors, with a metallic signature that consists of Au + Ag + Cu + Zn + Pb + As (± Te ± Bi ± Sb ± Hg ± W) assemblages. Veins are composed of multiple stages of mineralization, and the formation of these structures is enhanced by the presence of a local regime of extension and E-trending structures, including evidence of faults and shear zones with right-lateral displacement, which are likely involved in pluton emplacement and cooling. The ore mineralogy is composed of pyrrhotite, pyrite, chalcopyrite, sphalerite, galena and arsenopyrite assemblages formed in two or more mineralization stages, with complex Bi, Te, Sb and Hg mineral specimens associated with Au and Ag. Mineralized structures of the district present a preferential E-strike with dominant vertical to subvertical and occasional subhorizontal S-dips and secondary N- and NW-strikes that are steep to vertically E-dipping.
 The Frontino-Morrogacho Gold district presents characteristics related to the architecture, mineralogy and alteration of reduced (ilmenite-series) intrusion-related gold systems but is genetically associated with a parental oxidized magma source. The gold content is associated with three different families involving electrum, tellurides and alloys: gold rich (66 to 78% Au, 22 to 34% Ag), average (50 to 60% Au, 40 to 50% Ag) and silver rich (32 to 40% Au, 60 to 68% Ag). The formation of these bodies is associated with an N-S magmatic-metallogenic trend of Au-Ag-Cu deposits, which extend for more than 300 km along the Western Cordillera of Colombia. Similar plutonic suites span from the south of Chocó Province to the north of Antioquia Province, which indicates that the Frontino-Farallones-Botón arc can be proposed as an individual metallogenic belt.

Emplacement of Paleocene-Eocene magmatism under transtensional regime and its evolution to a dynamic equilibrium on the western edge of Colombia

The Cretaceous and Paleogene magmatic arcs of the Central and Western Cordilleras of Colombia have been attributed to the evolution of a subduction system in the Colombian Pacific coast. In this work the distribution and crystallization ages of plutons emplaced between 60 Ma and 53 Ma in the Central and Western Cordilleras are analyzed. From 53 Ma the magmatic arc migrates towards the west of Colombia, installing magmas in a plate edge transitional crust. The crystallization ages analyzed in this work suggest that, within the study area, the plutonic belt is continuous throughout the Western Cordillera. From 40±5 Ma to 26 Ma there was a significant reduction in the convergence velocity of the Farallon plate; as it decreases, also the tectonic loading diminishes resulting in a process of regional stress relaxation. The process of relaxation of the regional stress also occurred in the intra-continental environments producing peneplanization process in the topographic highs of northern Colombia, the reactivation of the piedmont with westwards progradation of sedimentation and the development of a middle- to late-Eocene regional unconformity. In continental shelf environments, the relaxation of the tectonic stress is evidenced by the distribution of reef limestone sequences throughout the Colombian Pacific margin and the Caribbean of Colombia, Ecuador, Panama and Costa Rica, and by a magmatic gap from 33 Ma to 26 Ma. The Paleocene-Eocene magmatic event distributed in the Central and Western Cordilleras took place under a transtensional regime, with the maximum horizontal compressive stress (σ1) oriented SW-NE, product of the oblique convergence between the Farallon and South American plates.

Dracula irmelinae, a new species in the subtribe Pleurothallidinae (Orchidaceae) from the Western Andes of Colombia

A new species of Dracula (Pleurothallidinae: Orchidaceae) from Colombia is described, honoring Irmelin Indenbirken, mother of the actor and philanthropist Leonardo DiCaprio. D. irmelinae is phenotypically most similar to Dracula verticulosa, but differs in having larger and wider sepals that form a smoothly curved semicircular mentum behind the lip, an indumentum with dark red internal and external maculae and dense long pubescence on the internal sepal surfaces and bases of the tails. The geographic distributions of the two species also differ, with the new species being restricted to a small area in the western Cordillera of Colombia. The conservation status of the new taxon is assessed as Endangered (EN) according to the International Union for Conservation of Nature (IUCN) criteria.
 Key words: Colombia, Dracula, endangered species, Pleurothallidinae, Western Andes

Origin of pre-mesozoic xenocrystic zircons in cretaceous sub-volcanic rocks of the northern Andes (Colombia): paleogeographic implications for the region

This paper reports the presence of xenocrystic zircons in sub-volcanic rocks (Guarco Andesite and Porphyritic Intrusives units) exposed in the northeast of the Western Cordillera of Colombia (WCC) and provides a new contribution to the understanding of the tectono-magmatic evolution of the area during the Upper Cretaceous associated with the geological history of the Caribbean Large Igneous Province (CLIP). The study area is located in an accretionary terrane constituted by a basement of oceanic affinity (Barroso Formation and San José de Urama Diabases unit), amalgamated against the northwestern South American margin and associated with the development of CLIP. The elemental and isotopic geochemical data (with an initial ratio of 87Sr/86Sr between 0.70334 and 0.70543 and εΝd between +6.25 and + 7.42) indicate an island arc environment for the formation of sub-volcanic rocks and a mantle wedge source area (and/or the arc itself), which presents subordinate levels of crustal material. U-Pb zircon ages by SHRIMP IIe and LA-ICP-MS show ages from the Mesoarchean to the Upper Cretaceous, with representative peaks of approximately 2700, 2100, 600 and 90 Ma. The younger age population is interpreted as the crystallization age of the studied rocks and indicates a spatial and temporal relationship with the basement rocks associated with the CLIP. The older zircons are interpreted as xenocrysts coming from terrigenous sediments that were dragged down into the subduction channel (reflecting crustal recycling processes through the mantle in subduction zones). Alternatively, the xenocrystic zircons could have come from the assimilation of crustal sediments that were previously deposited over the primitive arc. We suggest that the source areas of zircon xenocrysts probably are located close to the Arequipa Massif and the Marañon Complex inliers, and close to the ensialic Basement of the Central Cordillera of Colombia (CCCB). Using these source areas and paleogeographic/paleomagnetic models published by other authors, we propose two possible paleogeographic scenarios differentiated by the polarity of the subduction zone.

The vulnerable colombian weasel Mustela felipei (Carnivora): new record from Colombia and a review of its distribution in protected areas

The Colombian weasel Mustela felipei is considered the rarest Neotropical carnivore only known from four localities in Colombia and one in Ecuador. It is considered Vulnerable on the IUCN Red List along its distributional range. We present an unexpected photographic record of a living specimen from Colombia that shed lights on its distribution on the Western Cordillera of Colombia, where it was previously known from single a record obtained on 1986. This is the first confirmed record of the Colombian weasel during the 21st century. Finally, we discuss the species’ occurrence in protected areas of Colombia, to provide tools for the conservation of this rare species.

Burmeistera villosa (Campanulaceae, Lobelioideae), a new vestured species from the Western Cordillera of Colombia

Burmeistera villosa (Campanulaceae-Lobelioideae), a new vestured species from the Choco/Valle del Cauca border region, is described and illustrated, and its affinities are discussed. The species is a small, unbranched herb that stands out from other species by its densely villous indument that covers all its organs, and the narrow, nearly straight corolla lobes that point forward. The new species grows in seepage areas of cloud forests of the Western Cordillera of Colombia. Eleven additional species of Burmeistera grow in the type locality, one of the hotspots for the genus. The species is known only from the type collection. The type locality is a threatened environment that holds an outstanding diversity for Burmeistera and currently lacks active and effective environmental protection policies.

A remarkable new species of Cheilolejeunea (Marchantiophyta: Lejeuneaceae) from Colombia.

We describe the new liverwort species Cheilolejeunea schiavoneana M.E.Reiner & Gradst. from submontane rainforest in the Western Cordillera of Colombia. The new species is related to C. adnata (C. sect. Cheilolejeunea) but differs by larger plant, leaf and underleaf size, robust stems, a 5-6 cells wide ventral merophyte, abundance of microphyllous branches, and vegetative reproduction by caducous leaf lobes produced on flagelliform shoots. The latter feature is characteristic of the genus Rectolejeunea and its presence in Cheilolejeunea is apparently a case of parallel evolution. The discovery of C. schiavoneana adds a further endemic taxon to the rich flora of the Colombian Western Cordillera. Key

Detrital zircon fission-track thermochronology and magnetic fabric of the Amagá Formation (Colombia): Intracontinental deformation and exhumation events in the northwestern Andes

New detrital zircon-fission track (ZFT) and magnetic fabric data are presented to constrain the time of deposition, provenance and deformation of the of Lower and Upper members of the Amagá Formation in the Amagá Basin. The Amagá Basin is located in the northern Andes, between the Western and Central Cordilleras of Colombia. The Amagá Formation was deposited in a transpressive geodynamic context and is allegedly synchronous with tectonic events such as the Andean orogeny and the Panama-Choco Block collision with the northwestern South American Plate. Detrital ZFT data confirm an Oligocene age for the Lower Member and a middle-Miocene age for the Upper Member of the Amagá Formation. In addition to constraining the depositional age, the ZFT data presented in this study also reflect Paleocene-Eocene, late to early Oligocene and late to middle Miocene cooling in sediment source areas mainly located in the Central and Western Cordilleras of Colombia. These ages can be associated with regional exhumation events in the central and northern Andes of South America. Collisional stages of the Panama-Choco Block against northwestern South America, subduction of the Farallon-Nazca Plate and strike-slip reactivation periods of the Cauca-Romeral fault system, caused NW-SE compression and NE-SW simple shear in the Amagá Basin. This deformational regime, identified by magnetic fabric data, induces syn- and post-depositional deformation over the Amagá Formation.

A paleointensity study of Cretaceous volcanic rocks from the Western Cordillera, Colombia

The Cretaceous Normal Superchron is a period of great interest to investigate global scale variations of the geomagnetic field. Long periods of single polarity are still a matter of debate: up to now there are two contradicting theories, which try to relate geomagnetic field intensity and reversal rate. We aim to shed light on the geomagnetic field strength during the Cretaceous Normal Superchron because data are still scarce and of dissimilar quality. To obtain reliable, absolute paleointensity determinations we investigate volcanic rocks from the Western Cordillera of Colombia. Several age determinations allow relating the samples to an age of about 92.5 Ma. To characterize the samples, we investigate rock magnetic properties and determine the characteristic remanent magnetization behavior. To determine paleointensities, we use a multimethod approach: first, we apply the classic Thellier-Coe protocol, and then, the relatively new multispecimen method. Rock magnetic measurements indicate magnetite as the main ferrimagnetic mineral, a stable magnetization revealed by reversible and nearly reversible thermomagnetic curves, and grain sizes that are either in the pseudosingle domain range or a mixture of single and multidomain grains. Alternating field and thermal demagnetization are rather complex, although we observe a few vector diagrams with a single, essentially uni-vectorial component with a small viscous overprint. Paleointensity determination with the Thellier-Coe protocol was unsuccessful, while with the multispecimen protocol we obtained four successful determinations out of 20. The failure of the Thellier-Coe protocol can be attributed to multidomain grains, which were observed during demagnetization and in rock magnetic experiments, and to the inhomogeneity of the volcanic rocks. Our multispecimen paleointensity determinations support low field strength at around 90 Ma during the Cretaceous Normal Superchron.

Morphometric patterns in assemblages of Cricetid rodents from the Central and Western Cordilleras of Colombia

The mechanisms that mediate the formation and coexistence of species assemblages have been a historical issue in evolutionary ecology, raising the question of whether these assemblages are shaped at random or are influenced by biotic and abiotic processes. An increasingly popular hypothesis points out that certain patterns or rules determine coexistence patterns within assemblages at a regional or continental scale. Thus, morphological and ecological similarities could influence the degree and intensity of competition between species. Considering the morphology of organisms within an environmental framework, we could assess the morphology and understand the ecological role of a given species within the assemblage. This study assesses the similarities and differences of Andean rodents within local assemblages through a Principal Component Analysis (PCA), using craniodental characters of these species to establish whether the relationships between morphometry and coexistence provide evidence to explain assemblage structure patterns. We measured rodent specimens deposited in the Coleccion Teriologica of the University of Antioquia, collected from the Western and Central cordilleras of Colombia. We recorded 15 craniodental variables from all specimens, which were analyzed with a PCA to search for a general organization pattern within assemblages. Afterwards, we plotted the scores for principal components 1 and 2 to evidence the separation or clustering of specimens in an Euclidean space. We recorded 424 specimens from 10 genera and 18 species of the family Cricetidae, with the genera Nephelomys and Thomasomys showing the largest number of specimens. In the overall PCA, the first three components account for 85.1 % of the variation, with 63.9 % for PC1. The variables with the greatest contributions to PC1 were APZ, AA and AFM; to PC2, LFI, APZ and AFM; and to PC3, LFI, LO and LHM. We detected some overlapping and scattering patterns among species at the morphospace defined by PC1 and PC2. We recovered an arrangement of species that shows differences in size and shape between them (as a function of their morphology), as evidenced in the scatter plots for the regional and local analyses. Besides, we did not find a general pattern for the load of variables, although some of these accounted for a larger part of the variation in the overall and local analyses (APZ, AFM, LFI, LHM and AR), indicating the segregation among species. Although the local analyses revealed a similar pattern with these loads changing from site to site, five variables account for the largest variation, what we interpret as a key role to determine the morphometric and/or ecological segregation among species. We believe this first approximation shows the value of ecomorphological studies in understanding patterns of diversity and geographic replacement of species; these patterns should be integrated with analyses of ecological aspects to understand the coexistence of species within local assemblages.

Holocene climatic variations in the Western Cordillera of Colombia: A multiproxy high-resolution record unravels the dual influence of ENSO and ITCZ

Abstract The Paramo de Frontino (3460 m elevation) in Colombia is located approximately halfway between the Pacific and Atlantic oceans. It contains a 17 kyr long, stratigraphically continuous sedimentary sequence dated by 30 AMS 14C ages. Our study covers the last 11,500 cal yr and focuses on the biotic (pollen) and abiotic (microfluorescence-X or μXRF) components of this high mountain ecosystem. The pollen record provides a proxy for temperature and humidity with a resolution of 20–35 yr, and μXRF of Ti and Fe is a proxy for rainfall with a sub-annual (ca. 6-month) resolution. Temperature and humidity display rapid and significant changes over the Holocene. The rapid transition from a cold (mean annual temperature (MAT) 3.5 °C lower than today) and wet Younger Dryas to a warm and dry early Holocene is dated at 11,410 cal yr BP. During the Holocene, MAT varied from ca. 2.5 °C below to 3.5° above present-day temperature. Warm periods (11,410–10,700, 9700–6900, 4000–2400 cal yr BP) were separated by colder intervals. The last 2.4 kyr of the record is affected by human impact. The Holocene remained dry until 7500 cal yr BP. Then, precipitations increased to reach a maximum between 5000 and 4500 cal yr BP. A rapid decrease occurred until 3500 cal yr BP and the late Holocene was dry. Spectral analysis of μXRF data show rainfall cyclicity at millennial scale throughout the Holocene, and at centennial down to ENSO scale in more specific time intervals. The highest rainfall intervals correlate with the highest activity of ENSO. Variability in solar output is possibly the main cause for this millennial to decadal cyclicity. We interpret ENSO and ITCZ as the main climate change-driving mechanisms in Frontino. Comparison with high-resolution XRF data from the Caribbean Cariaco Basin (a proxy for rainfall in the coastal Venezuelian cordilleras) demonstrates that climate in Frontino was Pacific-driven (ENSO-dominated) during the YD and early Holocene, whereas it was Atlantic-driven in Cariaco (ITCZ-dominated). From ca. 8000 cal yr BP, climate in both areas was under the dual influence of ENSO and ITCZ, thereby showing existing teleconnections between the tropical Pacific and Atlantic oceans. The Frontino record is to date the highest-resolution Holocene study in NW Colombia. An implication of these results is that new records should be analyzed with multiproxy tools, in particular those providing high resolution time series, such as μXRF.

A new species of Mikania (Asteraceae, Eupatorieae) from the Western Cordillera of Colombia

A new species of Mikania from the northern area of the Western Cordillera of Colombia is described and illustrated. Taxonomic affinities with its closest relatives M. lloensis and M. flabellata are discussed. The diagnostic traits of the new species are: the long peduncle of the synflorescence; the size of the head; the presence of a midvein in the corolla lobes; the presence of moniliform, multicelular, uniseriate trichomes below the commissural sinuses of the corolla lobes; the insertion of the stamens at the mid-level of the hypocrateriform corolla; and the retuse apical appendages of the anthers. The latter trait is newly used here as diagnostic in the genus.

A taxonomic revision of Zamia montana and Zamia oligodonta, with notes on their conservation status

The taxonomy of Zamia montana and Z. oligodonta, two poorly understood species occurring at high elevations in the Western Cordillera of Colombia, is discussed. Zamia oligodonta was recently synonymized under Z. montana, but information derived from recent field studies is provided showing the two species are quite distinct. A treatment of both species is presented, including discussion of the major morphological differences between them and also between other South American species with prominently-veined leaflets. The conservation status of both species is evaluated.

Thermochronology and tectonics of the Central and Western Cordilleras of Colombia: Early Cretaceous–Tertiary evolution of the Northern Andes

New thermochronological data record a complex cooling history in the Central and Western Cordilleras of Colombia that is a function of Early Cretaceous to late Miocene tectonic events. Alkali-feldspar 40Ar/39Ar cooling ages of ~138–130Ma immediately post-date the cessation of Jurassic arc-magmatism and a major unconformity within the retro-foreland region of the northern Andes. We interpret these ages as cooling driven by exhumation in response to either compression driven by subduction of a seamount, or extension and oceanward migration of the slab during the earliest Cretaceous, giving rise to the Lower Cretaceous Quebradagrande arc sequence. Biotite and alkali-feldspar 40Ar/39Ar data from the palaeocontinental margin reveal the presence of a younger cooling event at 117–107Ma, which was contemporaneous with hornblende 40Ar/39Ar cooling ages obtained from medium–high P–T metamorphic relicts of a Late Jurassic–Early Cretaceous subduction channel. This cooling event is attributed to exhumation driven by the collision and accretion of a fringing arc against the continental margin, and obduction of the subduction channel onto the forearc. Inverse modelling of zircon and apatite fission track and (U–Th)/He data from throughout the Central and Western Cordilleras reveals three periods of rapid cooling since the Late Cretaceous. The earliest phase is recorded by Jurassic and Cretaceous granitoids that cooled rapidly during 75–65Ma. We attribute cooling to exhumation of the continental margin during ~75–70Ma (~1.6km/My), which was forced by the collision and accretion of the Caribbean Large Igneous Province in the Campanian. The Central Cordillera exhumed at moderate rates of ~0.3km/My during ~45–30Ma, which are also observed over widely dispersed regions along the Andean chain, and were probably caused by an increase in continent–ocean plate convergence rates. Exhumation rates drastically increased in the middle–late Miocene, with the greatest amount occurring in southern Colombia as a consequence of the collision and subduction of the buoyant Carnegie Ridge at 15Ma.

A New Species of Psilochilus (Triphoreae, Orchidaceae) from Colombia

Abstract A new Psilochilus species from the Western Cordillera of Colombia, Psilochilus vallecaucanus is described and illustrated. The new species is similar to P. macrophyllus, from which it differs by a long-clawed lip with obscure, obtuse lateral lobes. The relevant morphological features are illustrated. A key to the species of Psilochilus is provided. The information about the ecology and distribution of the new species is provided.

The nature and provenance of accreted oceanic terranes in western Ecuador: geochemical and tectonic constraints

Western Ecuador consists of a complex tectonic mélange of oceanic terranes accreted to the continental margin from Late Cretaceous to Eocene time. New geochemical data from these accreted terranes (arising from a 5 year British Geological Survey mapping programme) indicate that they comprise rocks from a variety of oceanic tectonic settings: from thickened (and relatively unsubductable) oceanic plateau basalts, through island-arc tholeiites, with occasional more calc-alkaline lavas, to back-arc basin basalt sequences. This study has enabled us to construct a new geodynamic model for the Cretaceous–Tertiary evolution of the Northern Andes, and has placed important new constraints on the extent of oceanic plateau sequences in Colombia and around the Caribbean. The age and nature of sediments, combined with evidence for the age of peak metamorphism, suggests that a prolonged (15–20 Ma) accretionary event occurred in Late Cretaceous time and involved the collision of an oceanic plateau (represented by the Pallatanga Unit) with the continental margin. This accreted unit can be correlated with similar oceanic plateau sequences from the Western Cordillera of Colombia and those within and around the Caribbean region. The Naranjal and Macuchi island arcs and the associated La Portada back-arc basin developed along the accreted margin from Late Campanian to Eocene time, and these arcs accreted to the continental margin along with oceanic plateau material (represented by the Piñon Unit and Pedernales–Esmeraldas sequences) during Eocene time. The development of island arcs, which separate the two accretionary events, implies that the most westerly (coastal) oceanic plateau sequences, both in Ecuador (Piñon and Pedernales–Esmeraldas) and in Colombia (Gorgona and Serranía de Baudó), cannot belong to the Caribbean–Colombian Oceanic Plateau (CCOP). It therefore appears that at least two different oceanic plateaux are preserved within the accreted oceanic terranes of the Northern Andes. It is possible that the CCOP formed over the Galápagos hotspot, as previously proposed, but the more westerly Coastal plateau was derived from a more southerly hotspot source region, such as Sala y Gomez, in the SE Pacific.

Origin and emplacement of Tertiary ultramafic complexes in northwest Colombia: Evidence from geochemistry and KAr, SmNd and RbSr isotopes

Si-undersaturated high-Mg basalts with early fractionated ultramafic cumulates (Viravira complex) and a zoned ultramafic Alaskan-type intrusion (Condoto complex) are located in the upper Condoto River within the Western Cordillera of Colombia. These complexes are related in space and time and originated from a common parental magma. The isotopic characteristics of the complexes are similar and plot close together near the mantle array in the ϵ Nd vs. ϵ Sr diagram. It is argued that the compositions of the Viravira basalts and of geochemically comparable hornblende-plagioclase dykes in the zoned complex are close to the parental melt from which both complexes derived. This melt probably had a mafic tholeiitic composition and was supplied from two sources: high-grade partial melting (20%) of moderately depleted mantle above a subduction zone (major part), and modification by a metasomatic LILE- and slightly LREE-enriched fluid which could have originated by dewatering of the subducting slab. KAr dates of hornblende of the Condoto complex indicate an emplacement age of about 20 Ma. This represents the youngest age on the earth of a zoned ultramafic intrusion. Textural features demonstrate that the zonal arrangement with a dunitic core was the result of a series of crystal mush intrusions of deeper seated unconsolidated cumulate fractionations. The occurrence of these (and further) ultramafic complexes along a major NS strike-slip fault substantiates an exceptional tectonic activity on the northwestern edge of the South American continent during the Tertiary. Horizontal plate movements along the deeply penetrating fault system favoured a release of pressure, and the formation of Si-undersaturated Mg-rich melts in the mantle. High heat flow and the possibility of vertical migration allowed the intrusion of mantle-derived melts into shallow levels of the upper crust. In the Condoto region these conditions were particularly provided by the intersection of the NS fault system with WNW-ESE trending transverse and oblique faults.

The Guerrero suspect terrane (western Mexico) and coeval arc terranes (the Greater Antilles and the Western Cordillera of Colombia): a late Mesozoic intra-oceanic arc accreted to cratonal America during the Cretaceous

The Guerrero suspect terrane, composed of Late Jurassic-Early Cretaceous sequences, extends from Baja California to Acapulco and is considered to be coeval with the late Mesozoic igneous and sedimentary arc sequences of the Greater Antilles, the West Indies, Venezuela and the Western Cordillera of Colombia. These sequences represent the remnants of an arc which accreted to the North American and northern South American cratons at the end of the Cretaceous. In western Mexico, the arc sequences built on continental crust consist of high-K calc-alkaline basalts, andesites and rhyolites enriched in LREE with abundant siliceous pyroclastic rocks interbedded either with Aptian-Albian reefal limestones or red beds. They do not show magmatic changes during the arc development. In contrast, the arc sequences built on oceanic crust show an evolution with time. Arc activity began with the development of depleted low K-tholeiitic mafic suite (Guanajuato igneous sequence), followed first by mature tholeiitic basalts and then by calc-alkaline olivine basalts interbedded with micritic limestones and radiolarian oozes of Early Cretaceous age. At the end of the arc growth, during Aptian-Albian times, calc-alkaline pillow basalts and and esites poured out in the volcanic front while shoshonitic olivine basalts extruded in the back arc. The tholeiitic and shoshonitic mafic rocks as well as the calc-alkaline lavas are mildly enriched in LREE, Y and Nb and show high ϵ Nd ratios, typical of oceanic arcs. In contrast, the calc-alkaline mafic suite enriched in LREE, Y and Nb exhibits lower ϵ Nd ratios suggesting that it was derived by the partial melting of a mantle source contaminated either by Paleozoic subducted sediments or old source enrichments (OIB). The Cretaceous arc rocks of the Greater Antilles, interbedded with and/or capped by Aptian-Albian limestones, the Cretaceous andesites of northern Colombia, the Cretaceous tholeiitic and calc-alkaline volcanic rocks of Venezuela, and the Cretaceous volcano-plutonic arc assemblage of Tobago share a similar magmatic evolution with the western Mexican oceanic arc. The tholeiitic plutono-volcanic assemblage of Tobago, depleted in LREE and characterized by high ϵ Nd values is similar to the Guanajuato volcano-plutonic sequence of Mexico, considered to represent the pristine stage of the arc. The mature tholeiitic sequences exposed in the proto-Caribbean arc show flat to moderately enriched LREE patterns like those of the Guerrero terrane. However, felsic plutonic and volcanic rocks prevail in the Caribbean. Calc-alkaline suites, accompanied locally by shoshonitic lavas, characterize the end of arc magmatic activity in both places. Thus, the geochemical features of the Late Jurassic-Cretaceous arc series of the Guerrero terrane and the proto-Caribbean are consistent with the following plate tectonic model. The Guerrero terrane and the proto-Caribbean probably belonged to the same intra-paleo-Pacific arc system the development of which was related to the subduction of oceanic basins fringing the North and northern South American cratons. This subduction zone was WSW dipping. While subduction was going on, these magmatic arcs drifted, moved closer to the North and South American cratons, and finally collided with the American borderlands at different periods during the Cretaceous. The late Mesozoic Guerrero and proto-Caribbean arc sequences show striking similarities with the Miocene calc-alkaline lavas dredged from the Banda Ridges, the North Marianas Seamount Province, and the Halmahera and Philippine arcs. We suggest that the diverse but mostly submarine segments of this late Mesozoic intra-Pacific arc rimmed the North and South American cratons as much as these Tertiary arcs rim Southeast Asia.

Intra-oceanic settings of the western Mexico Late Jurassic-Early Cretaceous arc sequences. Implications for the Pacific-Tethys geodynamic relationships during the Cretaceous

The Guerrero suspect terrane composed of Late Jurassic-Early Cretaceous sequences, extends from Baja California up to Acapulco and is considered to be coeval with the Late Mesozoic igneous and sedimentary arc sequences of the Greater Antilles, Venezuela and Western Cordillera of Colombia. New geological, petrological and geochemical data from central and southern Mexico, led us to propose a new model for the building of the Alisitos-Teloloapan arc. This arc, partly built on the Pacific oceanic lithosphere and partly on continental fragments, could be related to the subduction of an oceanic basin - the Arperos basin - under the Paleo-Pacific plate. This subduction was dipping southwest. At the beginning of the magmatic activity of the oceanic segment of this arc, depleted tholeiitic basalts were emitted in a submarine environnement below the CCD. While subduction was going on, the arc magmas evolved from LREE depleted tholeiites to slightly LREE enriched tholeiites and then, to calc-alkaline basalts and andesites enriched in LREE and HFSE. Concurrently, the arc sedimentary environment changed from deep oceanic to neritic with the deposition of Aptian-Albian reefal limestones, at the end of the arc building. In the continent-based segment, the arc magmas are exclusively differentiated calc-alkaline suites depleted in HREE and Y, formed of predominantly siliceous lavas and pyroclastic rocks, emitted in a sub-aerial or shallow marine environment. Thus, taking into account this above mentioned model, the Cretaceous volcanic series, accreted to the margins of cratonal America, in Colombia, Venezuela, Greater Antilles and Mexico, could be related to the same west-south-west dipping subduction of oceanic basins, fringing the North and South American continental cratons and connected directly with the inter-American Tethys. While the subduction was proceeding, this magmatic arc drifted towards the North and South American cratons and finally, collided with the continental margins at different periods during the Cretaceous.

The Genus Geissanthus (Myrsinaceae) in the Chocó Floristic Province

Studies on the plant diversity of Las Orquideas National Park in Antioquia, Colombia, along with preparation of a treatment of the Myr- sinaceae for Flora de Colombia, resulted in the discovery of four new taxa and two new combina- tions in Geissanthus. Examination of species known from the entire Western Cordillera of Colombia and the Choco Floristic Province, from Panama to Ec- uador, resulted in the discovery of 17 species from the area. An artificial key to the species of Geis- santhus from the Choco and Western Cordillera is presented, along with descriptions of four new spe- cies, G. callejasii, G. betancurii, G. francoae, and G. cogolloi. Three new combinations are made, G. longistamineus, G. perpuncticulosus, and G. scrob- iculatus, and revised descriptions based on complete material are presented for G. longistamineus and G. perpuncticulosus.