Comment on “Emplazamiento del magmatismo Paleoceno-Eoceno bajo un régimen transtensional y su evolución a un equilibrio dinámico en el borde occidental de Colombia” by Grajales et al., Rev. Mex. Cienc. Geol. (2020), 37(3), 250-268

Grajales et al. (2020) reviewed geochronological and geochemical data from Paleogene volcanic and plutonic rocks outcropping in the Panama-Choco Block (north western Cordillera) and southern Western Cordillera, as well as the Central Cordillera of Colombia. These data were used to support a model of continuous Paleogene arc magmatism along the Colombian continental margin, and to propose a paleogeographic model for the arc. The authors did not discuss previously published paleomagnetic, geochemical, geochronological, thermochronological and provenance constraints from Cretaceous to Miocene rocks of western and northern Colombia, Panama, and Ecuador that support a more plausible model of a double subduction system controlled by the convergence of the Caribbean and Farallon plates beneath the north Andean block during Paleogene. In this comment, we discuss shortcomings in the data and model proposed by Grajales et al. (2020) and present an alternative interpretation for contemporaneous arc-like magmatism during the Paleogene in the Northern Andes. We conclude that the double subduction system is the more plausible explanation for the contemporaneous arc-like magmatism during the Paleogene, currently exposed in the northern and southern portions of the Northern Andes.

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.

Impact of the Southern Ecuadorian Andes on Oxygen and Hydrogen Isotopes in Precipitation

Determining how the elevation of the Northern Andes has evolved over time is of paramount importance for understanding the response of the Northern Andes to deformational and geodynamic processes and its role as an orographic barrier for atmospheric vapor transport over geologic time. However, a fundamental requirement when using stable isotope data for paleoaltimetry reconstructions is knowledge about the present-day changes of δ18O and δD with elevation (isotopic lapse rate). This study defines the present-day river isotopic lapse rate near the Equator (∼3°S) based on analysis of δ18O and δD of surface waters collected from streams across the Western Cordillera and the Inter-Andean depression in Southern Ecuador. The results for the two domains show a decrease of δ18O with elevation which fits a linear regression with a slope of −0.18‰/100 m (R2 = 0.73, n = 83). However, we establish a present-day lapse rate of −0.15‰/100 m for δ18O (R2 = 0.88, n = 19) and -1.4‰/100 m for δD (R2 = 0.93, n = 19) from water samples collected along the west facing slopes of the Western Ecuadorian Cordillera which is mainly subject to moisture transport from the Pacific. We argue that this empirical relationship, consistent with those obtained in different tropical areas of the world, can inform stable isotope paleoaltimetry reconstructions in tropical latitudes.

Onset of Carnegie Ridge subduction from low-temperature thermochronology

<p>Cenozoic growth of the Andes has been strongly influenced by subduction dynamics, inherited crustal heterogeneities, and the superposed effects of climate. Subduction of the Carnegie Ridge in Ecuador has impacted late Cenozoic magmatism and tectonic activity, including the formation of a crustal sliver escaping northward. However, the relationship between ridge subduction and topographic growth has remained unclear. We present new thermochronological data from the Western Cordillera of Ecuador to (1) pinpoint the timing of ridge subduction, and (2) evaluate the role of ridge subduction in prompting growth of the Ecuadorian Andes. Time-temperature inverse modeling of our results shows two phases of cooling separated by tectonic quiescence. The first cooling phase immediately post-dates magmatism in the Western Cordillera, and hence we attribute it to magmatic cooling. The second cooling phase starts at ~6-5 Ma. This we associate with onset of enhanced exhumation at this time in the Western Cordillera, synchronous with the last cooling phase in the Eastern Cordillera. Based on our thermal modeling and thermochronological age patterns along geological cross-sections we propose that recent crustal shortening and rock uplift triggered exhumation of Western Ecuadorian Andes starting at ~6-5 Ma. We suggest that the onset of Carnegie Ridge subduction in the latest Miocene increased the coupling at the subduction interface and promoted shortening and regional rock uplift in the northern Andes. Overall, our new thermochronological results highlight the pivotal role of bathymetric anomalies in distinct upper-plate deformation processes at non-collisional convergent plate margins.</p>

Influence of pre-existing structure on pluton emplacement and geomorphology: The Merrimac plutons, northern Sierra Nevada, California, USA

In much of the western Cordillera of North America, the geologic frame­work of crustal structure generated in the Mesozoic leaves an imprint on later plutonic emplacement, subsequent structural setting, and present landscape morphology. The Merrimac plutons in the northern Sierra Nevada (California, USA) are a good example of the influence of pre-existing structure at a larger scale. This paper updates and refines earlier studies of the Merrimac plutons, with the addition of analysis of gravity and magnetic data and new 206Pb/238U zircon dates. The gravity and magnetic data not only confirm the presence of two different neighboring plutons, but also (1) support the presence of a third pluton, (2) refine the nature of the contact between the Merrimac plutons as being structurally controlled, and (3) estimate the depth extent of the plutons to be ~4–5 km. The zircon 206Pb/238U dates indicate that the two main plutons have statistically different crystallization ages nearly 4 m.y. apart. Geomorphic analyses, including estimates of relief, roughness and drainage density and generation of chi plots, indicate that the two main plutons are characterized by different elevations with large longitudinal channel knickpoints that we speculatively attribute to possible reactivation of pre-existing structure in addition to lithologic variations influencing relative erosion susceptibility in response to prior accelerated surface uplift.

Western Canadian freshwater availability: current and future vulnerabilities

The western cordillera supplies freshwater across much of western Canada mainly through meltwater from snow and ice. This “alpine water tower” has been, and is projected to be, associated with changes in the seasonality and amount of freshwater availability, which are critical in supporting the societal and environmental flow needs of the region. This study incorporates existing information to synthesize and evaluate current and future freshwater supplies and demands across major north-, west-, and east-flowing sub-basins of the Canadian western cordillera. The assessment of supply indicators reveals several historical changes that are projected to continue, and be exacerbated, particularly by the end of this century and under a high emission scenario. The greatest and most widespread impact is the seasonality of streamflow characterized by earlier spring freshets, increased winter, and decreased summer flow. Future winter and spring warming over all basins will result in decreases in end of season snow and glacier mass balance with greatest declines in more southern regions. In many areas, there will be a greater likelihood of summer freshwater shortages. All sub-basins have environmental and economic freshwater demands and pressures, especially in more southern watersheds where population and infrastructure are more prevalent and industrial, agricultural, and water energy needs are higher. Concerns regarding the continued ability to maintain suitable aquatic habitats and adequate water quality are issues across all regions. These water supply changes along with continued and increasing demands will combine to create a variety of freshwater vulnerabilities across all regions of western Canada. Southern basins including the South Saskatchewan and Okanagan are likely to experience the greatest vulnerabilities due to future summer freshwater supply shortages and increasing economic demands. In more northern areas, vulnerabilities primarily relate to how the rapidly changing landscape (mainly associated with permafrost thaw) impacts freshwater quantity and quality. These vulnerabilities will require various adaptation measures in response to alterations in the timing and amount of future freshwater supplies and demands.

New mineralogical data on platinum-group minerals from the Río Santiago alluvial placer, Esmeraldas province, Ecuador

Mineralogical studies on platinum-group minerals found in placer deposits from the Río Santiago (Ecuador) are scarce. In this investigation, one sample collected from the Río Santiago alluvial placer was studied via a multi-disciplinary approach, including optical microscopy, scanning electron microscopy, electron microprobe, and Raman spectroscopy. Whole-rock geochemistry data of the sample confirms elevated Au and platinum-group elements contents and the chondrite-normalized pattern reveals pronounced positive Ir and Pt anomalies. Free grains of platinum-group minerals were separated via hydroseparation techniques and identified as: i) Pt-Fe alloy (Pt3Fe), ii) tulameenite (Pt2FeCu) and iii) hongshiite (PtCu). The most abundant platinum-group mineral is Pt-Fe alloy (85%) that occasionally hosts cuprorhodsite (CuRh2S4) inclusions. Although the primary source remains unknown, the geochemical and mineralogical data suggests that the source of platinum-group minerals in the Río Santiago alluvial placer is a mafic-ultramafic Ural-Alaska type complex. Possible primary sources are the mafic and ultramafic rocks found in the mafic basement of the coastal region and the Western Cordillera (Piñón, San Juan and Pallatanga units), which derive from the Late Cretaceous Caribbean Colombia Oceanic Plateau (CCOP).

Sedimentology, Provenance and Radiometric Dating of the Silante Formation: Implications for the Cenozoic Evolution of the Western Andes of Ecuador

The Silante Formation is a thick series of continental deposits, exposed along a trench-parallel distance of approximately 300 km within the Western Cordillera of Ecuador. The origin, tectonic setting, age and stratigraphic relationships are poorly known, although these are key to understand the Cenozoic evolution of the Ecuadorian Andes. We present new sedimentological, stratigraphic, petrographic, radiometric and provenance data from the Silante Formation and underlying rocks. The detailed stratigraphic analysis shows that the Silante Formation unconformably overlies Paleocene submarine fan deposits of the Pilalo Formation, which was coeval with submarine tholeiitic volcanism. The lithofacies of the Silante Formation suggest that the sediments were deposited in a debris flow dominated alluvial fan. Provenance analysis including heavy mineral assemblages and detrital zircon U-Pb ages indicate that sediments of the Silante Formation were derived from the erosion of a continental, calc-alkaline volcanic arc, pointing to the Oligocene to Miocene San Juan de Lachas volcanic arc. Thermochronological data and regional correlations suggest that deposition of the Silante Formation was coeval with regional rock and surface uplift of the Andean margin that deposited alluvial fans in intermontane and back-arc domains.