Santa Marta Massif Research Articles

Lithologic, geomorphic, and climatic controls on sand generation from volcanic rocks in the Sierra Nevada de Santa Marta massif (NE Colombia)

Tectonics, climate, and lithology are the three main factors controlling the generation and transfer of sediments as well as their textural and compositional signatures. In this paper, we investigate the generation of sands from volcanic rocks in nine drainages with mixed lithological assemblage in a high-relief – subtropical climate setting (Sierra Nevada de Santa Marta, NE Colombia). The integration of geomorphological analysis with sand petrography, and bulk-rock and clay geochemistry allows us to constrain the dynamics regulating the generation of sediments derived from felsic and mafic-intermediate volcanic basement rocks in relation to plutonic and metamorphic lithologies. The petrographic composition of modern fluvial sands from the area is extremely variable (litho-feldspatho-quartzose, feldspatho-quartzose, and quartzose-feldspatho-lithic), but it reveals a strong dilution of (especially mafic) volcanic compositional signals, and a consequent overrepresentation of plutonic and metamorphic lithologies. Traditional chemical weathering indices fail to represent the extent of chemical alteration, but instead seem to favour preservation of provenance signals. Chemical weathering primarily affects feldspars and the ferromagnesian groundmass of mafic volcanic rock fragments. Enrichment of ferromagnesian elements in the clay fraction supports the notion that mafic volcanic rock fragments are unstable relative to plutonic and metamorphic rocks. Drainage features (particularly river network and relief) control the abundance of volcanic lithic fragments and the rates and extent of chemical weathering. A linear log-ratio model permits us to quantify sediment generation from volcanic rocks in this mixed-lithology setting. The results indicate that even in the case where volcanic grains are practically absent in the sand fraction, there could still be up to 20% of volcanic rocks in the drainage basin, and in the case that 50% of the sand grains are volcanic, around 75% of drained lithologies are expected to be volcanic. Our results demonstrate that the contribution of volcanic detritus to modern sands systematically underrepresents the areal proportion of volcanics rocks in mixed source areas. Post-depositional diagenetic modifications are likely to alter and/or dissolve volcanic lithic fragments, making accurate reconstruction of sediment provenance even more difficult. Sediment-generation models based on a range of mixed lithologies in various tectonic and climatic settings will be needed to improve estimates of the abundance of volcanic rocks in drainage basins of ancient sediment routing systems.

Quantifying crustal thickness and magmatic temperatures of the Jurassic to Early Cretaceous North-Andean arc

ABSTRACT We characterized the crustal thickness variations and magmatic saturation temperatures of the Early Jurassic to Early Cretaceous (194–130 Ma) plutonic and volcanic rocks of the Northern Andes of Colombia and Ecuador, combining trace elements signatures and zircon and apatite saturation temperatures. Also, we analysed the implications of the emplacement conditions during the last stage of the magmatism using Al-in-hornblende thermobarometry. Moderate rare earth elements (REE) slopes and depleted heavy REE patterns show that the primary residual magma source was amphibole, but plagioclase and pyroxene were also significant residual phases indicating that the magma source was formed in a crust that varied in thickness from 35 to 50 km. According to the La/Yb and Sr/Y crustal quantifications and the REE ratios, two phases in the arc evolution are characterized by a thick crust. The first is in Santa Marta Massif, Upper Magdalena Valley and Garzón Massif (190–180 Ma), which could be associated with shortening or basaltic underplating due to magmatic events. The second one recorded in the Ibagué Batholith (160–154 Ma) is related to the crustal thickening on an oblique subduction setting. During the latter period, the magmatic volumes progressively decreased, as is recorded by saturation temperatures increase. Finally, a thin crust emplaced the last stage of the arc magmatism during the Late Jurassic to Early Cretaceous (154–130 Ma).

Neoproterozoic to Jurassic tectono-metamorphic events in the Sierra Nevada de Santa Marta Massif, Colombia: insights from zircon U-Pb geochronology and trace element geochemistry

ABSTRACT The Sierra Nevada de Santa Marta Massif (SNSM) in the Northern Andes affords an insight into lower crustal section structural relations. This polymetamorphic basement comprises high-grade gneisses, which alternate with imbricate slivers of medium-to high-grade schists. New field observations coupled with LA-ICP-MS U-Pb zircon geochronology and trace element geochemistry reveal some hallmarks on the evolution of this strongly remobilized crustal section. The Proterozoic Rodinia-type basement (1100 − 880 Ma) recorded at least two separated tectono-metamorphic events (M0 and M1) and was subsequently affected by a magmatic episode during the Ordovician (~450 Ma; M2). During the Permian, subduction of Panthalassian oceanic crust beneath Pangaea led to the installation of a magmatic-arc and probably coeval metamorphism (~288-266 Ma; M3). Middle-Late Triassic roll-back extension of the upper plate led to anatexis and metamorphism (M4) reaching peak conditions at ca. 239 Ma with concurrent crystallization of garnet and sillimanite, followed by a decreasing geothermal gradient at ca. 202 Ma. Early Jurassic arc build-up under a rejuvenated convergence was contemporaneous to metamorphism on an accretionary wedge (M5) at ca. 185 Ma. The SNSM Massif basement units yield metamorphic ages that compare to similar tectono-metamorphic provinces in NW-Gondwana, including the Central Cordillera of Colombia, the Guajira Peninsula, the Mérida Andes, the Perijá Range, the Santander Massif, and the Central American and Mexican terranes to the south of the Ouachita-Marathon Suture (Oaxaquia, Mixteca, and Maya blocks). Age equivalence of these events is best explained by the existence of a conjugate margin with Laurentia until the Late Triassic ~216 Ma, which preceded the inception of a Colombian marginal seaway during the break-up of western Pangaea. Furthermore, the presence of Ordovician metamorphic rocks in the SNSM Massif confirms the extent of a Famatinian arc along NW-Gondwana during the Early Palaeozoic.

New distributional records and conservation importance of the San Salvador Valley, Sierra Nevada de Santa Marta, Colombia

This study presents new observations on the avifauna of humid tropical forest and humid premontane forest within the San Salvador valley on the northern slope of the Sierra Nevada de Santa Marta massif, La Guajira Department, northern Colombia. A total of 374 bird species were recorded within the study area, including new records for the region and extensions of altitudinal ranges. Important data were collected for the conservation of endemic and range-restricted bird species and for ten threatened bird species, as well as information on vertical distribution, habitat preferences and threats for bird populations. The study shows the importance of natural ecosystems and shade-grown coffee plantations on the northern slope of the Sierra Nevada de Santa Marta for 59 boreal migratory bird species. Based on the results, conservation priorities have been identified, a conservation corridor has been designed, and a habitat conservation strategy within the San Salvador valley was developed.

Middle-Late Triassic metamorphism of the Guajira Arch-basement: Insights from zircon U–Pb and Lu–Hf systematics

The Guajira Arch is located in the northernmost portion of South America and holds the record of several stages of magmatism and metamorphism. We performed LA-ICP-MS U–Pb zircon geochronology coupled with Lu–Hf isotope geochemistry on 11 samples from a high-grade gneissic basement hosting interleaved complexes of younger meta-granitoids and meta-sediments to unravel its tectono-metamorphic evolution. The oldest rocks correspond to a Rodinia-type basement at ca. 1100-880 Ma, affected by mafic magmatism during the Permian at ca. 272 Ma, anatexis during the Carnian at ca. 230 Ma, updoming and the formation of an extensional detachment fault during the Norian at ca. 224 Ma concluding with Jurassic magmatism since ca. 190 Ma. Trace element geochemistry shows that Triassic zircons are depleted in HREE and yield lower Th/U ratios than Jurassic zircons enriched both in HREE, Yb, and Y. Hf systematics indicates a Triassic to Jurassic evolution towards more radiogenic compositions. The lithological associations, structural setting, U–Pb ages, and Hf isotope geochemistry of the Triassic-Jurassic rocks at the Guajira Arch characterize an active margin setting of a hot orogenic belt in NW-Gondwana, comprising the Central Cordillera of Colombia, the Santa Marta Massif, the Merida Andes, the Perijá Range, the Santander Massif and the para-autochthonous terranes south of the Ouachita-Marathon Suture (Oaxaquia, Acatlán, and Maya), and thus vindicate the existence of a conjugate margin with Laurentia until the Late Triassic, preceding the breakup of western Pangaea. The Triassic anatectites and Jurassic granitoids from the Guajira Arch yield Proterozoic Hf TDMi ages which are indistinguishable from the autochthonous Proterozoic basement and thus render an allochthonous origin by terrane accretion highly improbable. Instead, these data point to melting of the continental crust by the incorporation of primitive material reflecting a two-stage process at the boundary of a large underlying mantle convection cell (i) Triassic slab steepening, subduction arrest, updoming anatexis at ca. 230-223 Ma and (ii) Early Jurassic arc-magmatism at ca. 190-165 Ma due renewed convergence. Furthermore, we propose that the structural setting, lithology, and isotope geochemistry of the Guajira Arch units, fit into the style of a cordilleran core-complex emplaced in a roll-back extensional setting preceding Pangaea break-up.

Mineralogy and geochemistry of Fe-Ti oxide ores from the Don Dieguito massif-type anorthosite suite, Sierra Nevada de Santa Marta, Colombia

Fe-Ti oxide ores are commonly associated with Proterozoic massif-type anorthosite bodies emplaced during the Grenville orogeny (~1.2-1.0 Ga). Some of these anorthositic bodies occur in the northernmost part of the Santa Marta Massif, Colombia. They locally contain crosscutting Fe-Ti(-V) ore bodies between the El Hierro creek and the Don Dieguito river. We have distinguished two types of Fe-Ti(-V) ores: i) oxide-apatite norite (fine grained ilmenite and magnetite disseminated in an assemblage of apatite, amphibole, chlorite, rutile and sericitized plagioclase) and ii) banded nelsonite (coarser grained ilmenite, magnetite and apatite distributed in bands, with minor baddeleyite, srilankite and högbomite). Ilmenite in the two ore types display distinct hematite exsolution features: a)needle-like in the oxide-apatite norite ore; and b) two generations of exsolutions in the banded ore. Magnetite shows similar Ti and V contents in both ore types (0.4 wt % TiO2 and 0.5 wt % V2O3 on average in the oxide-apatite norite ore, 0.5 wt % TiO2 and 0.4 wt % V2O3 in the banded ore), whereas ilmenite has higher hematite, pyrophanite and geikielite components in the banded ore. The ores might have formed from the combination of fractional crystallization and magma mixing, with exsolution occurring probably at 575-600°C. This first detailed mineralogical and textural study of Fe-Ti(-V) oxide ores in anorthosite massifs from Colombia suggests that the Santa Marta Massif is an interesting target for future research.

Late Triassic rift tectonics at the northernmost Andean margin (Sierra Nevada de Santa Marta)

Break-up of western Pangaea initiated a back-arc type tectonic setting along the northwestern margin of Gondwana, which resulted in the accumulation of sedimentary and volcanic deposits in extensional basins during the Middle-Late Triassic. The Sierra Nevada de Santa Marta massif, at the northwestern termination of the Andes, exposes Upper Triassic syn-rift successions beneath a voluminous Lower Jurassic volcanic cover. In this contribution, we present a sedimentological and stratigraphical analysis of the Upper Triassic Los Indios and Corual formations, by integrating whole-rock geochemistry, zircon U–Pb geochronology, and a biostratigraphic revaluation of the conchostracan fossil content. We divide the Los Indios Formation into two sandy to conglomeratic and two muddy members. First and third members were deposited in a coarse-grained delta environment. The second and fourth members are interpreted to represent offshore deposits resulting from the encroachment of gravity-flow lobes. Volcanoclastic and volcanic rocks interbedded with sedimentary deposits of the Los Indios and Corual formations record bimodal basic-acid volcanism with a variably alkaline tendency. Based on their alkaline and calc-alkaline signature, and rare earth and trace element patterns, volcanic rocks display a back-arc affinity. With respect to the age of the Los Indios Formation, we recovered conchostracans diagnostic of Upper Carnian and Lower Norian fauna, bearing similarity with faunal assemblages of other Upper Triassic deposits exposed in the northern Andes. The Los Indios and Corual formations are considered as syn-rift successions in a back-arc extension setting. The development of this back-arc basin coincided with the Late Triassic separation of the Oaxaquia, Acatlán, and Chortis terranes during Pangaea break-up. Finally, the Early to Middle Jurassic evolution of the NW Gondwana margin is marked by the inception of a magmatic arc, which characterizes the establishment of Andean-type subduction coeval to ignimbrite flare-up.

Patterns of recent deformation of the western Maracaibo block, northern Colombia and western Venezuela, based on integration of geomorphic indices with regional geology

The Maracaibo block is a triangular, continental tectonic terrane that includes two isolated Andean ranges of northern Colombia and western Venezuela: the Sierra de Santa Marta Massif (SSMM; maximum elevation 5700 m) in the west and the Perija Range (PR; 3600 m) to the east. The Cesar-Rancheria Basin (CRB) is an intermontane basin that separates the two ranges. To establish patterns of recent deformation of this elevated region and to infer its tectonic mechanism, we have integrated the following results: (1) analysis of 350 stream profiles and calculations of geomorphic indices, including stream length-gradient (SL) index, ratio of valley-floor width to valley height (VF), and hysometric curves for 20 watersheds in both ranges and (2) interpretation of three seismic reflection profiles within the CRB and adjacent areas. We determine that the northeastern part of the SSMM is tectonically quiescent based on its concave stream profiles, low geomorphic indices, and few vertical-step knickpoints. In comparison, we find that the central, southern, and eastern parts of the SSMM show tectonic uplift and recent fault control based on slope-break knickpoints and values in steepness and geomorphic indices with possible additional controls from lithologies of varying erosional resistance. Correlations between steepness, SL indices, slope-break knickpoints, and topographic elevations of the SSMM and central PR all indicate recent deformation of these areas. We use seismic reflection profiles from the eastern part of the CRB to confirm the existence of late Quaternary faulting and folding in these geomorphologically active areas. We propose that active, southeastward shallow (approximately 10°–15°) subduction of the Caribbean plate along the base of the South American continental crust produces active crustal deformation within the southern and eastern SSMM. The central PR and eastern CRB are also being deformed by active strike-slip faults.

Tectono-magmatic implications of the Jurassic volcanic and volcanoclastic record of the Santa Marta Massif (Colombia)

The Lower-Middle Jurassic volcanic and volcanoclastic rocks of the Santa Marta Massif (SMM) record the magmatic evolution of a continental arc developed on the NW margin of Gondwana. However, the lack of a comprehensive description of these successions has made it difficult to understand the geologic processes that occurred during the onset, development, and termination of the volcanic activity. Our new U–Pb zircon ages, whole-rock geochemistry, and petrographic analyses suggest that volcanism started at ca. 191 Ma, similar to plutonic activity, likely after the slab rolled-back from a previous flat-slab subduction zone. During this initial stage, the volcanic deposits consisted of andesite-basaltic andesite and trachy-andesite rocks, of calc-alkaline to high-K calc-alkaline character and dominantly metaluminous affinity. At ca. 177–168 Ma, the geologic record included andesite-basaltic andesites, dacites and, rhyolites of high-K calc-alkaline to calc-alkaline character, and peraluminous to metaluminous affinity. Finally, before cessation, at ca. 167–163 Ma, the volcanism presented andesite-basaltic andesite, trachy-andesite, trachyte and rhyolite compositions, with dominantly high-K calc-alkaline character, and peraluminous to metaluminous affinity. The geochemical comparison with coeval volcanic units deposited in La Guajira, Middle Magdalena Valley, Perijá, and San Lucas ranges indicate a similar magmatic evolution in the north of Colombia. The causes of the cessation of magmatic activity are still unknown, however, these may be associated with the generalized extensional event that was triggered by the breakup of Pangea.

Extreme gravity: Geologic effects

The maximum positive combined effect on the earth's gravity field of all geologic sources present in a given area, including any isostatic compensation, is limited by the ability of the lithosphere to support load. Bouguer gravity is one possible proxy for this effect. Bouguer maxima close to +350 mGal have been measured on Chichijima Island and Penrhyn Atoll in the Pacific Ocean. These may be the highest values anywhere on the planet. Suggestions of even higher values in the northernmost segment of the Colombian Andes rely on extrapolation of trends established in areas surrounding the Santa Marta Massif and must be treated with caution.

Automated petrography analysis by QEMSCAN® of a garnet-staurolite schist of the San Lorenzo Formation, Sierra Nevada de Santa Marta massif

El análisis petrográfico automatizado integra hardware de microscopía electrónica de barrido y espectroscopía de rayos X con software experto para generar mapas de composición de rocas a escala de micras. Si bien las soluciones de petrografía automatizada, como QEMSCAN®, se usan ampliamente en las industrias de minería, procesamiento de minerales y petróleo para caracterizar los depósitos minerales y las formaciones rocosas subsuperficiales, no se ha utilizado en petrología metamórfica. Este estudio aplica el análisis petrográfico automatizado utilizando QEMSCAN® a un esquisto con granate y estaurolita de la Formación San Lorenzo, provincia geológica de Sevilla (macizo de la Sierra Nevada de Santa Marta), y demuestra que esta técnica analítica tiene una clara aplicación potencial en estudios petrológicos.

Basin Evolution and Shale Tectonics on an Obliquely Convergent Margin: The Bahia Basin, Offshore Colombian Caribbean

AbstractOblique convergent margins accumulate strike‐slip deformation that controls basin formation and evolution. The Bahia Basin is located offshore, proximal to major strike‐slip fault systems that affect northern Colombia. It lies behind the toe of the modern accretionary prism, where the Caribbean Plate is being subducted obliquely beneath South America. This is the first attempt using 3‐D seismic reflection data to interpret a complex strike‐slip basin at the western end of the southern margin of the Caribbean Plate. Detailed 2‐D and 3‐D seismic mapping of regional unconformities and faults is used to describe the structural geometry, timing, and evolution of extensional and strike‐slip faults, which controlled the formation of the basin. Analysis of the fault zones is coupled with a description of the seismic‐stratigraphic units observed within the Bahia Basin to reconstruct the spatial and temporal evolution of deformation and to evaluate the influence of the pervasive shale tectonics observed in the area. The results, presented as a series of structural‐paleogeographic maps, illustrate an initial stage of transtension that controlled the formation of shale‐withdrawal minibasins from late Oligocene to late Miocene times. The continuous deformation and northward expulsion of the Santa Marta Massif resulted in transpression during Pliocene times, leading to basin inversion and ultimate closure of the basin. Basin evolution along the southern Caribbean oblique, convergent margin, shows the occurrence of a complex interaction between subduction and major‐onshore strike‐slip fault systems and illustrates how strain‐partitioning led to the break‐up and lateral displacement of early accretionary prisms formed along the margin.

The geochemistry and geochronology of Early Jurassic igneous rocks from the Sierra Nevada de Santa Marta, NW Colombia, and tectono-magmatic implications

The Sierra Nevada de Santa Marta in NW Colombia is an isolated massif at the northernmost end of the Andes chain near the boundary with the Caribbean plate. Previous geologic mapping and K-Ar dating have shown that Jurassic plutonic and volcanic units make up a large part of the Santa Marta Massif (SMM). These rocks have been considered to be part of a Jurassic magmatic arc extending from NW Colombia to northern Chile, but without any geochemical basis for comparison. This paper reports on a geochemical and Sr-Nd-Pb isotope study of the Jurassic rocks in the SMM and provides 12 new U-Pb zircon ages from in-situ laser ICP-MS dating. The plutonic and volcanic units span a range from 45 to 78 wt.% SiO2, with a dominance of intermediate to felsic compositions with SiO2 > 57 wt.%. They classify as calc-alkaline, medium to high-K, metaluminous rocks with trace-element features typical for arc-derived magma series. In terms of their major and trace-element compositions, the SMM Jurassic units overlap with contemporary plutonic and volcanic rocks from other regions of the Central and Eastern Cordilleras of Colombia, and confirm an arc affinity. The new U-Pb ages range from 176 ± 1 Ma to 192 ± 2 Ma (n = 12), with most between 180 and 188 Ma (n = 7). The initial Sr isotope ratios (at 180 Ma) are between 0.7012 and 0.7071 (n = 29), with 3 outliers attributed to mobilization of Rb and/or Sr. Nearly all samples have negative εNd(180) values between −10.3 and 0.0 (n = 30), the two exceptions being only slightly positive (1.1 and 1.9). Measured Pb isotope ratios fall in a narrow range, with 206Pb/204Pb from 18.02 to 19.95, 207Pb/204Pb from 15.56 to 15.67 and 208Pb/204Pb from 37.76 to 39.04 (n = 28).In the regional context of previous studies, these results confirm early Jurassic ages and an arc affinity for the widespread magmatism exposed in the eastern and northeastern Colombian Andes. We also note patterns in the distribution and composition of magmas. The magmatic activity in the Central Cordillera tends to be younger than in the Eastern Cordillera and is spatially more restricted to the vicinity of regional fault systems. In terms of composition, Jurassic igneous rocks in the Eastern Cordillera have systematically lower εNd(180) values than those from the Central Cordillera, whereas the Pb isotope ratios overlap. We ascribe the Nd isotope variations to heterogeneity in the mantle source and/or degree of crustal contamination, whereas the Pb isotope ratios are crust-dominated and similar throughout the region. The spatio-temporal and compositional evolution of Jurassic magmatic rocks in the Northern Andes reflect the major plate kinematic readjustment between the Triassic and the Early Jurassic in the proto-Andean margin.

Primer registro de la tortuga de río Kinosternon leucostomum (DUMERIL & BIBRON, 1851), para las estribaciones de la Sierra Nevada de Santa Marta en la Guajira colombiana

La tortuga de río (K. leucostomum), fue registrada por primera vez en 2011 para el departamento de Guajira, dentro de la franja baja de la Serranía de Perijá (GALVIS-PEÑUELA et al., 2011). Nuestras observaciones constituyen el primer registro de esta especie para las estribaciones de la Sierra nevada de Santa Marta, y el segundo registro para el departamento de la Guajira, Caribe colombiano.

Curie point depth in the SW Caribbean using the radially averaged spectra of magnetic anomalies

We have estimated the Curie Point Depth (CPD) using the average radial power spectrum in a tectonically complex area located in the SW Caribbean basin. Data analyzed came from the World Digital Magnetic Anomaly Map, and three methods have been used to compare results and evaluate uncertainties: Centroid, Spectral Peak, and Forward Modeling. Results show a match along the three methods, suggesting that the CPD values in the area ranging between 6km and 50km. The results share the following characteristics: A) High values (>30km) are in continental regions; B) There is a trend of maximum CPD values along the SW-NE direction, starting from the Central Cordillera in Colombia to the Maracaibo Lake in Venezuela; C) There is a maximum CPD at the Sierra Nevada de Santa Marta (Colombia) as well as between Costa Rica - Nicaragua and Nicaragua - Honduras borders. The lowest CPD values (<20km) are associated with the coastal regions and offshore. We also tested results by estimating the geothermal gradient and comparing measured observations of the study area. Our results suggest at least five thermal terrains in the SW Caribbean Basin: A) The area that is comprising the Venezuela Basin, the Beata Ridge and the Colombia Basin up to longitude parallel to the Providencia Throat. B) The area that includes zones to the north of the Cocos Ridge and Panam Basin up to the trench. C) The orogenic region of the northern Andes and including areas of the Santa Marta Massif. D) The continental sector that encompasses Nicaragua, northern Costa Rica and eastern of Honduras. E) Corresponds to areas of the northern Venezuela and Colombia, NW of Colombia, the Panamanian territory and the transition zones between the Upper and Lower Nicaragua Rise.

Inglés

The first occurrence of two-step growth of textural sector- and sigmoidal-zoned garnet of metapelitic rocks of the San Lorenzo Schists in the Sierra Nevada de Santa Marta massif is documented here. This textural type of zoned garnets occurs in the staurolite metamorphic zone of the San Lorenzo Schists. They are generally coarse-grained (up to 2.5 mm in diameter), and occur in quartz-rich domains. Together with staurolite, garnet is a porphyroblastic mineral in these rocks. Two periods of garnet growth have been observed: (1) a xenoblastic core, with a sector-zoned garnet; and (2) a subidioblastic rim with a sigmoidal-zoned garnet. Garnet preserves chemical zoning, with decreasing XMn and increasing XMg from core to rim (a patch trend), suggesting that garnet grew during increasing temperature. XCa and XFe follow a radial trend. Study of this type of garnet indicates that it has grown during the late emplacement of gabbroic rocks of the Santa Marta Batholith close to thepeak regional of metamorphism of the San Lorenzo Schists. During the first steps of garnet growth, it develops a textural sector-zoning, characterized by the cleavage domes, pressure shadows and shearing away cleavage domes, whereas during the last steps a textural sigmoidal-zoned garnet overgrowth occurred after textural sector-zoned garnet. This type of textural sector- and sigmoidal-zoned garnet from the San Lorenzo Schists is very important as a key piece for interpretation of thetectono-metamorphic history of this metamorphic unit.Keywords: San Lorenzo Schists, two-step growth, sector- and sigmoidal-zoned, garnet, metamorphism.

Occurrence of a skarn-type mineralogy found in Ciénaga Marbles, located in the NW foothills of the Santa Marta Massif (Colombia)

&lt;p&gt;The early Cretaceous Ciénaga Marbles that crop out in the NW foothills of the Santa Marta Massif (Colombian Caribbean region) present an epigenetic mineral assemblage (skarn-type), overprinting the metamorphic mineral assemblage previously developed along the regional metamorphic history that affected this unit. The skarn-type mineralogy allows at least three paragenetic contexts to be distinguished, which are represented by the following neoformed minerals: (a) garnet, forsterite, diopside, titanite, wollastonite and calcite (early anhydrous metamorphic stage), (b) actinolite, tremolite, allanite and clinohumite (metasomatic or hydrated stage), and (c) chlorite, serpentine, sepiolite and quartz (late low temperature retrograde stage, probably due to infiltration of descending meteoric waters). The skarn-type mineralogy is observed as alteration halos developed around porphyritic granodiorites emplaced as sills between anisotropy planes related to metamorphic regional foliation of rock that are considered to be the causative bodies of the skarntype mineralogy. Zircon U-Pb ages obtained from granodioritic bodies yielded an age of 55.5±0.7 Ma (Ypresian, Early Eocene). The formation of the skarn-type mineralogy in the Ciénaga Marbles is temporarily related to the formation and emplacement of hydrated silicate masses that were generated at the beginning of the subduction polarity change (i.e. when the Caribbean oceanic plate began to subduct beneath South American continental plate).&lt;/p&gt;

Structural analysis of the Tabaco anticline, Cerrejón open-cast coal mine, Colombia, South America

The Tabaco anticline is a 15 km long, south plunging, east-vergent anticline in northern Colombia, close to the transpressional collisional margin between the Caribbean and South American plates. In the Cerrejón open-cast coal mine, systematic mapping of coal seams in the middle to upper Paleocene Cerrejón Formation has yielded an exceptional dataset consisting of 10 horizontal slices (sea level to 90 m elevation, regularly spaced at 10 m intervals) through the anticline. Coal seams and fault traces in these slices are used to construct a 3D model of the anticline. This 3D model shows tighter folds within lower coal seams, NW-vergent thrusts and related folds on the gentler western limb, and strike-slip faults on the steeper eastern limb. Fault slip-tendency analysis is used to infer that these two faulting styles resulted from two different stress fields: an earlier one consistent with thrusting and uplift of the Perijá range, and a later one consistent with strike-slip faulting (Oca, Ranchería and Samán faults). Our preferred interpretation is that the anticline developed its eastern vergence during the early stages (late Paleocene-early Eocene) of tilting of the Santa Marta massif. Later NW-vergent thrusting on the western limb (early to middle Eocene) was related to western propagation of the Perijá thrust system. These results contribute to the understanding of the structural evolution of the area. They are also a good example of the complex interplay between detachment folding, thrusting, and strike-slip faulting during the growth of a km-size fold in a transpressive setting.

Transient Cenozoic tectonic stages in the southern margin of the Caribbean plate: U-Th/He thermochronological constraints from Eocene plutonic rocks in the Santa Marta massif and Serranía de Jarara, northern Colombia

We use U-Th/(He) zircon and apatite thermochronology and Al in hornblende geobarometry from Eocene granitoids of the Sierra Nevada de Santa Marta and Guajira uplifted massifs in northern Colombia to elucidate the exhumation history of the northern South America continental margin and its bearing to Cenozoic Caribbean– South American plate interactions. Aluminium in hornblende geobarometry from the Eocene Santa Marta batholith yields pressures between 4.9±0.6kbar and 6.4±0.6kbar, which indicate that at least, 14.7-19.2km of unroofing took place since 56-50Ma in the northwestern Sierra Nevada de Santa Marta. In the Guajira Peninsula, calculated pressures for the Eocene Parashi stock are 2.3±0.6kbar and 3±0.6kbar. Stratigraphic considerations pertaining to Oligocene conglomerates from the Guajira area suggest that 6.9-9km of crust was lost between 50Ma and ca. 26Ma. U-Th/He zircon and apatite thermochronology from granitoids in the Sierra Nevada de Santa Marta shows the existence of major exhumation events in the Late Eocene (ca. 45-40Ma), Late Oligocene (ca. 25Ma) and Miocene (ca. 15Ma). The Guajira region records the Late Eocene to Early Oligocene (35-25Ma) event, but it lacks evidence for the Miocene exhumation phase. These differences reflect isolation of the Guajira region from the Sierra Nevada de Santa Marta and the Andean chain due to extensive block rotation and transtensional tectonics that affected the region during post-Eocene times. The post-Eocene events correlate in time with an increased convergence rate and the frontal approach of North and South America. It is suggested that the two major tectonic mechanisms that govern exhumation in these Caribbean massifs are: 1) subduction of the Caribbean plate, and 2) post Eocene changes in plate convergence obliquity and rates that caused the South American continental margin blocks to override the Caribbean plate. Temporal correlation with other Caribbean and Northern Andean events allows to resolve the regional Cenozoic plate tectonic reorganizations experienced by the South American, Caribbean and Pacific plates at a regional scale

The paleogene synorogenic succession in the northwestern Maracaibo block: Tracking intraplate uplifts and changes in sediment delivery systems

The integration of sandstone petrography, detrital zircon U-Pb ages, and sedimentological data was carried out for lower Paleogene rocks in four sections of the western Maracaibo Block, allowing for the documentation of a shift from regional to localized fluvial drainage systems associated with intraplate uplifts. The lower to middle Paleocene units have similar thicknesses, and show a depositional profile varying northward from fluvial-estuarine environments to shallow marine carbonates. Sandstones show high quartz percentages (up to 80%) and detrital zircon age populations are dominantly older than 0.9 Ga (with peaks in 1.55 and 1.8 Ga), with minor populations in the range of 400–600 Ma. In contrast, the upper Paleocene units were deposited in marginal, coal-rich environments, and have strong variations in thickness among the four studied areas. These sandstones show quartz percentages between 40 and 70%, and have a significant increase in metamorphic fragments (approximately 13% of the total framework) as compared to the lower Paleocene sandstones (5–7% of metamorphic lithic fragments). The lower Eocene sandstones, on the other hand, show an increase in k-feldspars and quartz content. The detrital zircon age populations for the upper Paleocene and lower Eocene sandstones in the western sections show a strong decrease in ages from 1.3 to 2.5 Ga and an increase in ages from 55 to 360 Ma; in contrast, age populations older than 0.9 Ga persist in the southern section. The lower-middle Paleocene rocks suggest a regional, basin-wide drainage system fed by the Cretaceous sedimentary cover exposed in low-amplitude localized uplifts and developed a mixed siliciclastic-carbonatic platform in the shelf areas. On the other hand, the upper Paleocene-lower Eocene synorogenic succession accumulated in several basin compartments separated by more pronounced source areas and recorded the introduction of new ones. These sandstones contain basement rock fragments from marginal uplifts of the Santa Marta Massif and the Central Cordillera, as well as fragments from emerging intraplate ranges, such as the Perijá Range and the Santander massif. Caribbean subduction along the northwestern margin of South America induced tectonic changes inside the Maracaibo Block, modifing sedimentary depocenters from a regional basin (ca. 300 km width) to isolated intermontane basins, which have been separated since the late Paleocene to present.