nature and provenance of accreted oceanic terranes in western Ecuador: Geochemical and tectonic constraints, The
Kerr, Andrew C
Abstract: Western Ecuador consists of a complex tectonic melange 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 Companion to Eocene time, and these arcs accreted to the continental margin along with oceanic plateau material (represented by the Pinion 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 (Pinon and Pedernales-Esmeraldas) and in Colombia (Gorgona and Serrania de Baudo), 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 COP formed over the Galapagos 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.
Keywords: Ecuador, oceanic plateaux, basalts, island arcs, accretionary wedges.
The growth of continental crust by subduction-accretion processes involving oceanic island arcs, marginal basins and, in particular, oceanic plateaux has been recognized by many workers (e.g. Abbott & Mooney 1995; Stein & Goldstein 1996; Polat et al. 1998; Puchtel et al. 1998; Condie & Abbott 1999; Kerrich et al. 1999; White et al. 1999). One of the major regions of continental crust growth over the past 150 Ma has been the western margin of the Northern Andes of South America (McCourt et al. 1984). Although the geochemical nature of the accreted Cretaceous and Tertiary terranes of western Colombia has been characterized in some detail (Millward et al. 1984; Kerr et al. 1996; 1997), the relationship of these terranes to those that occur within the Ecuadorian segment of the Northern Andes remains equivocal.
Between 1995 and 2000, the British Geological Survey (BGS) in partnership with the Corporacion de Desarrollo e Investigacion Geologico-Minero-Metalurgica (CODIGEM) carried out systematic geological mapping and geochemical (baseline) surveys of the Ecuadorian Cordillera Occidental between 1 deg N and 4 deg S over an area of c. 36 000 km^sup 2^. On the basis of this work we present a comprehensive set of new, high-precision, whole-rock geochemical data for the Cretaceous and Tertiary oceanic igneous rocks of western Ecuador. These data when used in conjunction with field evidence collected by the BGS-CODIGEM teams allow us to explore the possible correlations between Colombian and Ecuadorian units, and thus construct a new geodynamic model for the Cretaceous-Tertiary evolution of this portion of the Northern Andes. Furthermore, this study has provided important new insights into the spatial extent of the Caribbean-Colombian Oceanic Plateau (COP), suggesting that it may be composed of two different plateaux, derived from two separate mantle plumes.
Regional geology of western Ecuador and previous work
Geographically, Ecuador can be divided into three main areas; a western coastal plain (the Costa); a central Andean region (the Sierra) and the Amazon Basin (the Oriente) in the east. The Oriente is an extensive sedimentary basin, overlying older cratonic basement. The Andean Sierra comprises two mountain chains that are separated by a central inter-Andean valley or depression (Litherland & Aspden 1992). To the east is the Cordillera Real, which comprises metamorphic rocks, intruded by early and mid-Mesozoic granitoids (Aspden & Litherland 1992; Aspden et al. 1992a, 1992b; Litherland et al. 1994). To the west of the inter-Andean valley, the Cordillera Occidental predominantly consists of fault-bounded Cretaceous-Tertiary accreted oceanic terranes, comprising basaltic volcanic and volcaniclastic rocks, which are the main focus of this study. The western margin of the inter-Andean depression is marked by a major structural feature: the Calacali-Pallatanga Fault Zone (Fig. 1) (Aspden et al. 1987a; Eguez & Aspden 1993). The Cordillera Occidental is intruded by Mid-Eocene to Late Tertiary granites and is overlain by post Late Eocene continental arc volcanic rocks. The geology of the Costa is less well known but it probably comprises a series of one or more marginal basins and is thought to be underlain by the basic oceanic crust of the Pinon Unit.
The term Pinon Formation was first introduced by Tschopp (1948) to refer to the mafic volcanic sequences that are present in the Ecuadorian Coastal Ranges, particularly in the area to the north of Guayaquil. Subsequent mapping by the French Petroleum Institute (see Faucher & Savoyat (1973) for a synthesis) in the 1960s led to an extension of Tschopp’s original definition to include similar rocks that occur within the Cordillera Occidental that had previously been referred to simply as `rocas verdes y porfidicas’ (Wolf 1892) or more formally as `Formation Diabasica-Porfiritica’ (Sauer 1965). Goosens & Rose (1973) proposed that both the Formation Pinon and Formation Diabasica– Porfiritica be renamed the Basic Igneous Complex and suggested a correlation with similar rocks from Costa Rica, Panama and Western Colombia.
As a result of a preliminary mapping survey (carried out by BGS and the then Ecuadorian Geological Survey (DGGM)) that included much of the Cordillera Occidental, Henderson (1979) proposed that the basic volcanic rocks of the Cordillera and the Costa were of a different age and origin. The name Pinion was restricted to the oceanic rocks of the Costa, whereas the name Macuchi Formation was created to include all the ‘oceanic’ rocks of the Cordillera Occidental, which were interpreted as island-arc volcanic rocks. On the basis of geochemical studies, Lebrat et al. (1985. 1987) suggested that the Macuchi Formation, as defined by Henderson (1979), included `oceanic MORB [mid-ocean ridge basalt]’ and calc-alkaline basalts in addition to island-arc tholeiites. The MORB rocks were correlated with the Pinon Formation of the Costa, and Van Thournout et al. (1992) also recognized similar volcanic sequences in the northwestern part of the Cordillera Occidental.
Within the past 5 years, detailed analytical geochemical studies have been carried out on the basalts of several localized areas of the Costa and the Cordillera Occidental south of Quito (Cosma et al. 1998; Reynaud et al. 1999; Lapierre et al. 2000); these studies have helped to shed new light on the origins of accreted Ecuadorian terranes.
Geology, petrology and age of the accreted igneous units
(Stratigraphic columns for the Cordillera Occidental and the Costa can be obtained from the Society Library or the British Library Document Supply Centre, Boston Spa, Wetherby, West Yorkshire LS23 7BQ, UK as Supplementary Publication No. SUP 18172 (5 pages).)
It has been suggested by Hughes & Pilatasig (2002) that the accretion of the more southerly Macuchi Block occurred slightly later (in Late Eocene time) than the accretion of the Naranjal Arc. In this part of the Cordillera, the Upper Paleocene to Upper Eocene Angamarca Group, which is located immediately to the east of the Macuchi Block and consists of dominantly turbiditic, quartz-rich sediments, has been interpreted as representing a basin fill sequence that was deposited between the Macuchi Arc to the west and the then continental margin (Hughes & Bermudez 1997; McCourt et al. 1997). The progressive closure of this basin culminated in the accretion of the Macuchi Arc during Late Eocene time, and led to both the local deformation of the older (Eocene) volcanic rocks of the Saraguro Group in the south and also to a hiatus in Saraguro volcanism (Dunkley & Gaibor 1997; Hughes & Pilatasig 2002). The presence of the undeformed Balsapamba pluton, which intruded and contact metamorphosed the Macuchi Unit to the west, has yielded concordant K-Ar biotite-hornblende mineral ages ranging from 34 to 33 Ma (McCourt et al. 1997). All this evidence suggests that the accretion of the Macuchi Arc was completed by Late Eocene time.
Although more precise information is required (particularly on the age of the Macuchi arc and the nature of the pre-Eocene portion of the Angamarca Group), the current evidence for the timing of accretion of the Macuchi and Naranjal Arcs appears to suggest that this event was markedly diachronous. The accretion of the Naranjal Arc appears to be reasonably well constrained and most probably occurred during early to earliest mid-Eocene time, whereas in the south the final closure of the Macuchi basin may not have taken place until Late Eocene time. A protracted Eocene accretion event may also be supported by recently published white mica and biotite ^sup 40^Ar/^sup 39^Ar and zircon and apatite fission-track data (Spikings et al. 2001) from the Cordillera Real. In addition to the Late Cretaceous cooling event already discussed, these data also contain significant evidence for a rapid (post-accretion) cooling from 43 to 30 Ma, an age span that is consistent with the deductions made here.
Accretion of the Costa units
Angamarca Group are considered to be in part contemporaneous, the absence of proven Macuchi debris within the Angamarca sequence suggests that these units were physically separated at the time of their formation and that their present juxtaposition is the result of later tectonic transport along the Chimbo lineament.
In the 0-1 deg N sector of the Cordillera, the Canande Fault separates the more easterly lavas of the Naranjal Unit, which possess an island-arc signature, from those in the west, which are of oceanic plateau affinity (Fig. 1). It is therefore possible that this fault may represent the suture between the Naranjal Arc and an oceanic plateau. Thus, the cessation of magmatic activity in the Naranjal Arc in Early Maastrichtian time may have been caused by the docking of a plateau with the arc at this time. However, perhaps because the collision was oblique to the continental margin, the back-arc basin associated with the Naranjal Arc did not fully close until early to earliest mid– Eocene time.
The oceanic plateau sequences of the Costa are occasionally overlain by, and tectonically intercalated with, arc lavas. These arc lavas range in age from Cenomanian-Turonian for the Las Orquideas Unit through Late Campanian-Maastrichtian in the Manabi area to Maastrichtian and Paleocene time in the more northerly Pedernales region (Fig. 1). The intimate tectonic association of arc and plateau lavas implies that the arc volcanism occurred, at least in part, before the Eocene collision of the Pinon and Macuchi Blocks with the continental margin. The calc-alkaline nature of this arc volcanism and its association with thickened plateau suggests that, like the Rio Cala Arc that formed during accretion of the Pallatanga Unit, the San Lorenzo Arc magmas developed below, and erupted through, the thickened oceanic crust of the Pinon plateau. As the final accretion of the Pinion, Macuchi and Naranjal Blocks did not occur until Eocene time, the calc-alkaline Late Cretaceous San Lorenzo Arc must have formed in an oceanic environment.
Model of tectonic evolution
In an attempt to illustrate and help explain the observed variation in both the geology and the timing of geological events along the length of the Cordillera Occidental a series of schematic crosssections across the Ecuadorian margin are presented in Fig. 6, and these are discussed below.
Starting in the north, we propose the following sequence of events.
(1) The Pallatanga oceanic plateau collided with the continental margin of Ecuador in Campanian time (83-74 Ma) (Fig. 6a). A subduction zone initiated at the trailing edge of the accreted plateau and gave rise to the calc-alkaline lavas of the Rio Cala Arc that were erupted through the accreted Pallatanga plateau (Fig. 6b).
(2) During Late Campanian-Mid-Maastrichtian time (76– 70 Ma), activity in the Rio Cala Arc ceased as a new arc initiated (Naranjal) and a back-arc basin (represented by the La Portada Unit) opened between the developing Naranjal Arc and the remnants of the Rio Cala Arc (Fig. 6c).
(3) Activity in the Naranjal Arc was short lived, and is believed to have been terminated by the approach and docking of an oceanic plateau in early Maastrichtian time. However, perhaps because the collision was oblique to the continental margin, the La Portada back-arc basin did not fully close until early to early mid-Eocene time (Fig. 6c).
(4) The docking of this plateau with the Naranjal plateau resulted in a back-stepping of the subduction zone to behind the trailing edge of the Pinon plateau, leading to the eruption of tale-alkaline lavas through the oceanic plateau and the formation of the San Lorenzo Arc (Fig. 6d).
(5) As a result of sustained compressive (probably dextral) stress, throughout Paleocene and early Eocene time, the La Portada back-arc basin gradually closed (Fig. 6e and f). The final closure in northern Ecuador was completed at the latest by early mid-Eocene time, and the closure is represented by the Mulaute Shear Zone.
In the more southerly parts of the Cordillera we propose that the sequence of tectonic events was as follows.
(1) Perhaps as a result of oblique collision of the plateau, the Pallatanga plateau probably commenced accretion to the southern continental margin in Late Campanian-Early Maastrichtian time (76-70 Ma), slightly later than it did in the north (Early Campanian time; 83-80 Ma) (Fig. 6a and b).
(2) Maastrichtian arc rocks, intruding or overlying the Pallatanga Unit, are absent in the southern sector of the Cordillera. However, arc rocks of this age are found in the San Lorenzo Arc, which intrudes an oceanic plateau sequence. This suggests that, following the accretion of the Pallatanga Block, subduction was initially located some distance from the continental margin (Fig. 6c).
(3) The initiation of subduction between the Pinon Plateau and the continental margin (to form the Macuchi Arc) and the cessation of activity in the southern portion of the San Lorenzo Arc, possibly in Early Paleocene time, were accompanied by the formation of a back-arc basin, in which the Angamarca sediments were deposited (Fig. 6d).
(4) The collision of the Pinon plateau with the Macuchi Arc may not have commenced until Mid-Eocene time (Fig. 6e) and resulted in the cessation of arc activity and the gradual closure of the Angamarca back-arc basin, with final accretion being completed by Late Eocene time (Fig. 6f).
Correlation with the Caribbean-Colombian Oceanic Plateau
One of the most significant aspects of this study is the recognition that the Macuchi and Naranjal island arcs separate two different oceanic plateau sequences in western Ecuador. Although these two plateaux appear to have formed at broadly the same time, their different accretion ages (Late Cretaceous and Eocene time) suggest that they did not form at the same hotspot. Although it has been maintained that the accreted Ecuadorian plateau sequences cannot be easily correlated with the Colombian sequences (Reynaud et al. 1999), we do not share this view. Currently available age dates of 92-84 Ma (Kerr et al. 1997a, 2002) place the oceanic plateau sequences in the Western and Central Cordillera of Colombia within the broad late Cretaceous time frame of the Pallatanga Unit. This, combined with the similar structural position of the Colombian and Ecuadorian (Pallatanga) terranes, makes it probable that they were originally part of the same oceanic plateau, which accreted to the Northern Andean continental margin at 85-65 Ma. A significant proportion of the oceanic plateau now preserved in and around the Caribbean basin is of the same age (93-85 Ma; Sinton et al. 1998), and this material appears to have begun to move into the Proto-Caribbean seaway from the Pacific from about 85 Ma onwards (e.g. White et al. 1999). Taken together, these lines of evidence from the Caribbean and its margins suggest that the accreted Late Cretaceous oceanic plateau sequences in the Cordillera of the Northern Andes may well belong to the same oceanic plateau, the Caribbean-Colombian Oceanic Plateau (CCOP).
However, the evidence from this study sheds new light on the interpretation of the accreted oceanic plateaux preserved in the coastal regions of Colombia and Ecuador. The oceanic plateau sequences in question are those of the Pinon and Esmeraldas– Pedernales regions combined with the basalts of the Serrania de Baudo in Northern Colombia (Kerr et al. 1997a), and the komatiites, picrites and basalts of Gorgona Island, 50 km west of the Colombian coast at c. 3 deg N (Kerr et al. 1996). We contend that these sequences are not part of the COOP, sensu stricto, as they are in large part separated from the Cordilleran oceanic sequences by Late Cretaceous-Early Tertiary island-arc terranes. Additionally, these sequences are characterized by generally more depleted trace element and radiogenic isotopic signatures than those of the Cordillera and the Caribbean (Kerr et al. 1997a, 1997b). Further support for the proposal of a different plateau is provided by new Hf isotope ratios from the komatiites of Gorgona Island (Thompson 2002), which are markedly different from those of the CCOP. Given the potentially older age of the Pinon Unit, combined with the likelihood that it was accreted from a more southerly direction and the fact that it is separated from the Esmeraldas-Pedernales sequences by the Puerto Cayo– Canande Fault Zone, it is possible that Pinon represents a different plateau from the Esmeraldas-Pedernales, Serrania de Baudo and Gorgona sequences.
A corollary of the evidence for two separate oceanic plateau events with similar igneous emplacement ages but different tectonic accretion ages, is that these two plateaux cannot have formed from the same mantle plume or hotspot. It has been long been proposed that the COP formed at the start-up phase of the Galapagos hotspot (Richards et al. 1991; Storey et al. 1991); however, it has been suggested by Revillon et al. (2000) that at least some of the Caribbean and Northern Andean Cretaceous oceanic plateau sequences have been derived from the Sala y Gomez hotspot (currently located 3000 km SW of the Caribbean at 33 deg S, 110 deg W). In addition, palaeomagnetic evidence from Gorgona Island (Estrada & MacDonald 1994) has suggested that the lavas cannot have formed at the Galapagos hotspot, but rather originate from a much more southerly latitude.
(1) A wide variety of accreted oceanic igneous terranes of Late Cretaceous to Early Tertiary age are preserved in Western Ecuador; these include oceanic plateau material, tale-alkaline arc rocks erupted through thickened oceanic plateau, island-arc sequences and their associated back-arc basins.
(2) Two main accretionary phases can be recognized in western Ecuador. The first appears to have been a relatively prolonged (10-20 Ma), Late Cretaceous event and corresponds to the accretion of the Pallatanga (oceanic plateau) Unit. The second accretion event spans early to late Eocene time and marks the accretion of the Naranjal and Macuchi Units (island arcs) along with the Pinion and Pedernales-Esmeraldas oceanic plateau sequences.
(3) The oceanic plateau sequences of the Cordilleran Pallatanga and the coastal Pinon-Pedernales-Esmeraldas are separated by the Late Cretaceous to Early Tertiary Naranjal and Macuchi island arcs.
(4) As the two plateau sequences of very similar ages are separated by accreted island arcs they cannot be part of the same oceanic plateau or have formed at the same hotspot. The Pallatanga Unit can be correlated with, and probably belongs to, the same oceanic plateau as those sequences exposed both in the Western and possibly the Central Cordillera of Colombia and within and around the Caribbean region. The Pinon-PedernalesEsmeraldas- Units represent a part of one (and possibly two) different oceanic plateau(x) which are also preserved on Gorgona Island and the Serrania de Baudo in Colombia. These plateau sequences may have been generated by the Sala y Gomez plume.
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Received 8 November 2001; revised typescript accepted 28 March 2002. Scientific editing by Sally Gibson
ANDREW C. KERR1, JOHN A. ASPDEN2, JOHN TARNEY3 & LUIS F. PILATASIG4
1Department of Earth Sciences, Cardiff University, Main Building, PO Box 914, Cardiff CF10 3YE, UK
2British Geological Survey, Keyworth, Nottingham NG12 5GG, UK
3Department of Geology, University of Leicester, University Road, Leicester LE1 7RH, UK
4Direccion Nacional de Geologia, Casilla, 17-03-23, Quito, Ecuador
Copyright Geological Society Publishing House Sep 2002
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