John A. Aspden

New geological and geochronological data from the Colombian Andes: continental growth by multiple accretion

The regional geology of Colombia consists of a composite Palaeozoic to Cenozoic sedimentary basin overlying Precambrian shield rocks, which are exposed in the Eastern Cordillera, Magdalena Valley and at least part of the extensive Llanos Plains. A Palaeozoic metamorphic schist belt with abundant post-metamorphic igneous rocks comprises the Central Cordillera. A Mesozoic volcanic suite extends from the western flank of the Central Cordillera and accounts for the bulk of the Western Cordillera. A Cenozoic sedimentary sequence, probably overlying a Late Cretaceous oceanic crust and fore-arc complex, forms the Pacific Coastal Plain. The continental edge of central and southern Colombia is interpreted as a composite margin made up of successively-accreted oceanic island arc related sequences of Palaeozoic, Early Mesozoic and Late Cretaceous ages. The present-day cycle, which began in the Miocene, is not associated with island arc volcanism.

Geometrical control of subduction-related magmatism: the Mesozoic and Cenozoic plutonic history of Western Colombia

Radiometric age data from Western Colombia, combined with geological evidence, allow five main plutonic episodes, ranging in age from Triassic to Tertiary, to be recognized. It is suggested that variation in the convergence angle of the oceanic plate relative to the continental edge was an important factor affecting the timing and spatial distribution of plutonic activity. On a regional scale, major breaks in activity are probably best attributed to either low-angle/parallel convergence or periods of accretion along the convergent margin.

The geology and Mesozoic collisional history of the Cordillera Real, Ecuador

Abstract The geology of the metamorphic rocks of the Cordillera Real of Ecuador is described in terms of five informal lithotectonic divisions. We deduce that during the Mesozoic repeated accretionary events occurred and that dextral transpression has been of fundamental importance in determining the tectonic evolution of this part of the Northern Andes. The oldest event recognised, of probable Late Triassic age, may be related to the break-up of western Gondwana and generated a regional belt of ‘S-type’ plutons. During the Jurassic, major calc-alkaline batholiths were intruded. Following this, in latest Jurassic to Early Cretaceous time, a volcano-sedimentary terrane, of possible oceanic or marginal basin origin (the Alao division), and the most westerly, gneissic Chaucha-Arenillas terrane, were accreted to continental South America. The accretion of the oceanic Western Cordillera took place in latest Cretaceous to earliest Tertiary time. This latter event coincided with widespread thermal disturbance, as evidenced by the large number of young K-Ar mineral ages recorded from the Cordillera Real.

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.

Regional geochemical reconnaissance of the Cordillera Occidental of Ecuador: economic and environmental applications

A regional geochemical reconnaissance survey of the Cordillera Occidental of Ecuador was initiated in 1995 as a sub-component of a wider Mining Development and Environmental Control Technical Assistance Project (PRODEMINCA) in Ecuador. The 36,000 km2 survey area encompasses oceanic and continental-margin volcano-sedimentary terranes with known occurrences of porphyry-style Cu-Mo, exhalative massive sulphide, epithermal Au and mesothermal polymetallic mineralisation. A survey sample medium of <177 μm stream sediments was selected following an orientation study in the vicinity of known porphyry Cu mineralisation. In the 2–4°S sector of the Cordillera Occidental for which data are presented, 4850 drainage samples were collected at an average density of 1 per 2.57 km2. All were analysed for 36 major and trace elements (Au, Ag, Cu, Pb, Zn, Mo, Ni, Co, Cd, Bi, As, Sb, Fe, Hg, Mn, Te, Ba, Cr, V, Sn, W, La, Al, Mg, Ca, Na, K, Sr, Y, Ga, Li, Nb, Sc, Ta, Ti, Zr). A stringent quality-control procedure included the systematic analysis of certified reference samples, field duplicates and replicates, data for which were used to calculate analytical precision, temporal drift and practical detection limits. Results for this part of the cordillera highlight the contrasting lithogeochemical signatures of the ocean-floor basalt terrane (Pallatanga Unit), the island-arc terrane of the Macuchi Unit, the continental volcanics of the Saraguro Group and the acid and intermediate lavas extruded from the Late Miocene to the Quaternary. Regional geochemical images for Au and associated pathfinder elements are dominated by anomalies relating to known mines and prospects. New exploration targets, often inconspicuous at the regional scale, have however been identified through the normalisation of data for individual lithological units against their respective geochemical backgrounds. In addition to mineral exploration, the drainage geochemical dataset for the Cordillera Occidental provides an unparalleled environmental baseline against which the impacts of future anthropogenic activities (including mining) may be assessed. A basis for the formulation of pragmatic sediment quality criteria and for the identification of natural geochemical hazards is also provided.

New U–Pb and Rb–Sr constraints on pre-Acadian tectonism in North Wales

A new U–Pb date of 615.2 ± 1.3 (2σ) Ma for the Twt Hill Granite, North Wales, contrasts with an Rb–Sr isochron age of 491 ± 12 (2σ) Ma from the same body. The latter age is thought to result from isotope resetting during regional low-grade metamorphism or fault reactivation. The Rb–Sr age also coincides with the onset of latest Cambrian to Early Tremadoc regression and is taken to reflect tectonic uplift prior to the Arenig overstep at around 478 Ma. It is proposed that this in turn reflects plate-scale processes along the contemporary peri-Gondwanan continental margin.