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Earth and Planetary Science L
Cretaceous alkaline intra-plate magmatism in the Ecuadorian
Oriente Basin: Geochemical, geochronological and
tectonic evidence
Roberto Barragana,*, Patrice Babya, Robert Duncanb
aInstitut de Recherche pour le Developpement (ex-ORSTOM), Unite de Recherche 104, Universite Paul Sabatier, Toulouse III,
38 rue des Trente-Six Ponts, 31400 Toulouse, FrancebCollege of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, Or 97331, USA
Received 27 September 2004; received in revised form 15 March 2005; accepted 31 March 2005
Editor: E. Bard
Abstract
Small volumes of Cretaceous alkaline basaltic magmas have been identified in the sedimentary infill of the Ecuadorian Oriente
foreland basin. They are characterized by a restricted range of compositional variation, low LILE/HFSE ratios and Sr–Nd isotope
values within the range of oceanic island basalts (OIB). Reflection seismic data show that a pre-existing NNE–SSW Triassic and
Jurassic rift controls the location and occurrence of these alkaline eruptive sites. Radiometric ages (40Ar–39Ar, incremental heating
method) and the biostratigraphic record of their surrounding sediments indicate a NNE–SSW systematic age variation for the
emplacement of this alkaline volcanism: from Albian (110F5.2 Ma) in the northern part of the Oriente Basin, to Campanian
(82.2F2.0 Ma) in the west-central part. The geochemical, geochronological and tectonic evidences suggest that asthenospheric
mantle has upwelled and migrated to the SSW, into the region underlying the pre-existing Triassic and Jurassic rift (thin-spot?).
We propose that subduction was abandoned, subsequent to the accretion of allochthonous terranes onto the Ecuadorian and
Colombian margin in the latest Jurassic–earliest Cretaceous, causing the relict slab material, corresponding to the eastwards-
directed leading plate, to roll-back. Unmodified asthenospheric mantle migrated into the region previously occupied by the slab.
This resulted in partial melting and the release of magmatic material to the surface in the northern part of the Oriente Basin since
at least Aptian times. Then, magmatism migrated along the SSW-trending Central Wrench Corridor of the Oriente Basin during
the Upper Cretaceous, probably as a consequence of the lateral propagation of the transpressive inversion of the Triassic–
Jurassic rift. Eventually, the Late Cretaceous east-dipping Andean subduction system was renewed farther west, and the
development of the compressional retro-foreland Oriente Basin system halted the Cretaceous alkaline magmatic activity.
D 2005 Elsevier B.V. All rights reserved.
Keywords: Northern Andes; Ecuador; Oriente Basin; alkaline magmatism; Cretaceous; roll-back; transpressive inversion
* Corresponding author. OXY Ecuador, Ave. Naciones Unidas E7-95 y Shyris, Edificio Banco del Pacıfico, Quito-Ecuador. Tel.: +593 2 299
0012-821X/$ - s
doi:10.1016/j.ep
3700; fax: +593
E-mail addre
etters 236 (2005) 670–690
ee front matter D 2005 Elsevier B.V. All rights reserved.
sl.2005.03.016
2 299 3701.
ss: [email protected] (R. Barragan).
R. Barragan et al. / Earth and Planetary Science Letters 236 (2005) 670–690 671
1. Introduction
Intra-continental plate alkaline magmatism is
reported from numerous locations in Mesozoic and
Cenozoic strata along the Pacific margin of the
Americas and the Antarctic Peninsula [1–11]. Dif-
ferent models, each related to a tectonic setting, have
been proposed to explain the generation and occur-
rence of alkaline magma. Possible mechanisms
include: (a) mantle plumes with abnormally high
asthenospheric temperatures beneath continental
crust [12–14] (e.g., Columbia River Basalts in the
NW United States [15]); (b) upwelling and decom-
pressional melting due to lithospheric extension [16]
and rift propagation (e.g., Andean Jurassic back-arc
ECUADOR
Guayaquil
Quito
Talara
ORIENTEBASIN
MARAÑONBASIN
PUTUMAYO BASIN
Real Cordillera
Subandean Syste
Central Corridor
CUTUCU UPLIFT
NAPO UPLIFT
PASTAZADEPRESSION
JAG
Y
DA
SUMACO
REVENTADOR
ANTISANA
CAYAMBE
SUB-ANDEAN FORELAND BASIN
GU
AY
AN
AS
HIE
LD
PUNGARAYACU
Fig. 1. Regional location map. Synthetic tectonic map of
basin between 258 S and 08 [1–3]); (c) development of
slab windows associated with the ending of subduc-
tion processes along active continental margins fol-
lowing ridge crest–trench collision [17–19] (e.g., Baja
California [4]; Southern Patagonia [5–7]; Antarctic
Peninsula [8 9]; British Columbia [10]); and (d) a
combination of slab roll-back associated with lateral
and vertical asthenospheric migration into the locus
of a pre-existing area of lithospheric thinning or
bthin-spotsQ (e.g., The Antarctic James Ross Islands
[11]).
Here we present evidence of active intra-plate alka-
line magmatism from the Oriente Basin of Ecuador
associated with the evolution of the northwestern
margin of the South American plate during Cretaceous
m
Eastern Inverted System
Cretaceous Igneous Bodies
Active volcanoes
0 100 Km
MARAÑON BASIN (PERU)
ORIENTE BASIN (ECUADOR)
PUTUMAYO BASIN (COLOMBIA)
TAPI
VISTA
LAGUNA
AUCAUAR
URALPA
YUNO
77 00' W
0 00'o
o
the Ecuadorian Oriente Basin (modified from [20]).
CHAMBIRA/CURARAY
Foreland Isostatic Rebound
TRANSPRESSIVE INVERSION
CRATONIC BASIN
TECTONIC EVENT
Initiation of Foreland
ARAJUNO continental
ORTEGUAZA
UP. TIYUYACU
continental
shallow marine
CHALCANA
MERA-MESA continental
LOW. TENA
LOW.TIYUYACU
UP. TENA continental
continental
UP. NAPO
LOW. NAPO
shallow marine
shallow marine
HOLLIN
Basal Tena
shallow marinecontinental
continentalTRANSPRESSION
PALEOCENE
0
10
20
30
40
50
60
70
80
90
100
110
NE
OG
EN
EP
ALE
OG
EN
EC
RE
TA
CE
OU
S
PLIOCENE
MIOCENE
OLIGOCENE
EOCENE
LATE
EARLY
CHRONO- STRATIGRAPHY
ROCK UNIT PALEO-
ENVIRONMENTLITHOLOGY
82.2 2.0 (40Ar/39Ar)
PR
E-O
RO
GE
NIC
M
EG
AS
EQ
UE
NC
EF
OR
ELA
ND
ME
GA
SE
QU
EN
CE
106 5 (40Ar/40K)
v
v
v
v
v
v
v
v
v
continental/shall marine
continental
91.2 4.6 (40Ar/39K)
v
v
R. Barragan et al. / Earth and Planetary Science Letters 236 (2005) 670–690672
times. The Oriente Basin is part of the Amazonian
retro-foreland basin system of the present-day north-
ern Andes (Fig. 1). The magmatism reported here
developed during deposition of a Mesozoic sedimen-
tary series that was deposited within relatively stable
shallow marine conditions, corresponding to the
Aptian to Campanian, Hollın and Napo formations
[21,22]. Furthermore, the geographic distribution of
this alkalic igneous activity is largely confined to a
major transpressive NNE–SSW corridor, in the central
part of the Oriente Basin, which formed by tectonic
inversion of an upper Triassic–lower Jurassic rift,
starting in the Late Cretaceous [20].
This paper presents geochemical, geochronological
and tectonic data aimed at understanding the origin
and evolution of this intra-plate magmatic activity. The
focus is not only on the origin of this small volume,
mafic igneous province, here termed bOriente Basin
Basaltic volcanismQ (OBB), but also on its relation-
ship with the geodynamic evolution of north-western
South America.
RIFTING
SUBDUCTION ONSET
BACK-ARC EXTENSION
120
130
MIDDLE-UPPER JURASSIC
MISAHUALLI / CHAPIZA
continental
LOWER JURASSIC
TO UPPER
TRIASSIC
coastal plain to shallow
marine platform
SANTIAGO
BAS
EMEN
T
228
volcanoclastic
PR
E-C
RE
TA
CE
OU
S
ME
GA
SE
QU
EN
CE
PALEOZO
IC
v v
vv
vv
vv
vv
vv
vv
v
175
vv
Fig. 2. Stratigraphic column for the Ecuadorian Oriente Basin
indicating major tectonic and magmatic events (modified from
[26,27]).
2. Geological setting
The Ecuadorian Oriente Basin [23] forms part of
the large present-day Maranon–Oriente–Putumayo
foreland system [24], developed between the Pre-
Cambrian Brazilian–Guyana basement shields to the
East, and the Andean Cordillera to the West (Fig. 1).
The Oriente Basin preserves a sedimentary infill, ran-
ging in age from Paleozoic to Quaternary, overlying a
Precambrian cratonic basement [22–25]. The strati-
graphic column (Fig. 2) can be divided into a pre-
Cretaceous series [1], which is unconformably over-
lain by a continental to shallow marine Cretaceous
sedimentary cycle [22] and by a Cenozoic foreland
molassic and shallow marine cover [28].
The pre-Cretaceous series comprises Paleozoic
marine sediments, Triassic and Lower Jurassic marine
to continental rift deposits, and Late Jurassic back-arc
volcanoclastic sediments [1,23]. The tectonic setting
was dominated by Late Triassic–Lower Jurassic rift-
ing [26], induced by the E–W trending Tethyan sys-
tem [29], followed by an Upper Jurassic back-arc
extensional regime initiated by the onset of Andean
subduction [1] and associated activity of the continen-
tal Misahualli–Colan volcanic arc [30]. At c 140–
,
120 Ma [31], a major change in the geological
setting of the Oriente Basin occurred, with subduc-
tion and active arc magmatism ceasing, interpreted
as the result of the accretion of allochthonous ter-
ranes onto the Ecuadorian and Colombian margin
[29,31–34].
After a major sedimentary hiatus (c 120–110
Ma [21]), the Cretaceous sedimentary series was
deposited. It comprises fluvial to shallow marine
Aptian to Campanian deposits of the Hollın and
Napo Formations [21,22,25,35,36]. The Hollın-Napo
megasequence is characterized by cyclic sequences
of limestones, shales and sandstones. Its deposition
and distribution on a stable platform along a NW–SE
depocentre were controlled by worldwide eustatic sea
level fluctuations during Cretaceous times [22,27].
R. Barragan et al. / Earth and Planetary Science Letters 236 (2005) 670–690 673
Finally, the Late Cretaceous–Cenozoic sedimentary
series was deposited after a major sedimentary hiatus
at the base of the Maastrichtian–lower Paleocene
sandstones and red beds of the Tena Formation, an
erosive event that may reflect the accretion of the
allochthonous Pallatanga, Macuchi Terranes and
coastal Pinon block [31,37–40]. This Late Cretac-
eous–Cenozoic cover represents the detrital section
associated with the development of the true Andean
foreland system [28].
Structurally, the Oriente Basin is characterized by
overprinted structures [41]. Pre-Cretaceous exten-
Vista
Pungarayacu
Yuralpa
Dayuno
Shu
Puma AucaOso
Jaguar
CononacoArmadillo
Villano-1
RIO PASTAZA
COLOMBIA
AN
DE
AN
CO
RD
ILLE
RA
1
1
0
2
78 77
La
Jivino
Central Corridor(Pre-existing Triassic-LowerJurassic rift structural limits)
Waponi
93±3.4 Ma(b)
82.2±2 Ma(b)
84±2 Ma(b)
91.2±
102.4±2.4 Ma(b)
106±5 Ma(a)
110±5 Ma(a)
RIO NAPO
93.±
Fig. 3. Geographic distribution and location of Cretaceous extrusive a
radiometrically analyzed sample sites are shown. Samples level with pane
sional systems, inherited from the NNE–SSW paleo-
rift structures [26], have been tectonically inverted
along three major oblique NNE–SSW right-lateral
transpressive wrench-fault zones (Fig. 1), which
have deformed the foreland basin system since the
Late Cretaceous [20]. From west to east, these tectonic
domains are: (i) the northern Sub-Andean system
(Napo Uplift), formed by an echelon NNW–SSE
positive flower structure, which was mainly active
during the Pliocene and Quaternary, but currently
still exhibits strong seismic and volcanic activity; (ii)
the Central Corridor, developed in Late Cretaceous
Ginta
shufindi
RIO AGUARICO
RIO NAPO
76
guna
Tapi
4.6 Ma(a)
0 10 20 30 40 50
KILOMETERS
EXTRUSIVE EVENTSbasaltic tuff cones
INTRUSIVE EVENTSgabbroic sills
Oil Wells
Sample Sites
Jaguar Seismic Section(Fig. 4)
main wrench zones
Radiometric ages from unpublished sources
3.4 Ma(b)
nd intrusive events along the Oriente Basin. Geochemically and
l (b) are 40Ar–39Ar and with panel (a) are 40Ar–40K.
Fig. 4. Reflection seismic sections showing the emplacement of extrusive magmatic rocks facies in the Ecuadorian Oriente Basin during
deposition of the Campanian upper Napo Formation. Interpretation is based on well and seismic data correlations. Reflection seismic data show
the structural control of pre-existing extensional features on the emplacement of alkaline eruptive sites: (a) Jaguar extrusive structure (tuff cone).
(b) Puma structure (tuff cones). See Fig. 3 for the location of seismic profiles.
R. Barragan et al. / Earth and Planetary Science Letters 236 (2005) 670–690674
Table 1
Summary of the extrusive and intrusive events recognized within the Cretaceous section of the Oriente Basin and found at different exploratory oil wells
Extrusive events
Basalticglassy tuffs
Intrusive events
Thickness (m) (**)
Biostratigraphicage
Stratigraphicrecord
Area (*)
NNE TapiVista Jivino LagunaIndillana−ItayaAucaArmadilloCononacoPumaJaguarPungarayacuWaponiYuralpaDayunoWSW
200−250180−200
3015−8040−8050−703090−13510
8070
Hollin FmUpper Hollin−Basal Napo
Lower−NapoLower−NapoUpper Napo FmUpper Napo FmUpper Napo FmUpper Napo FmUpper Napo FmUpper Napo Fm
Upper Napo FmUpper Napo Fm
Middle AlbianMiddle Albian
Upper Alb−lower Cenom.Upper Alb−lower Cenom. Turonian−Santonian Turonian−Santonian Turonian−Santonian Coniacian−SantonianConiacian−SantonianSantonian−Campanian
Santonian−CampanianSantonian−Campanian
Basalticdikes
Gabbroic.sills
Thickness(m) (**)
Radiometricage (Ma)
Radiometricmethod
1−2 1−2 1−25 50−60 20 0.1−1
50150 5−190 50
110.2 ± 5.2 106 ± 5 102.4 ± 2.4 92 ± 3.9
93.1 ± 0.7; 91.2 ± 4.6
93.0 ± 3.4 82.2 ± 2.0 84 ± 2
40Ar/40K (***)40Ar/40K (***)40Ar/39Ar40Ar/40K (***)
40Ar/39Ar;40Ar/40K (***)
40Ar/39Ar40Ar/39Ar40Ar/39Ar (***)
�
Biostratigraphic information based on [21,22]. New 40Ar–39Ar data are shown in Tables 3a and 3b. Bold ages in the right column indicate 40Ar–39Ar plateau ages determined in this
study.
(*) Related to exploratory oil wells along the Oriente Basin of Ecuador (source: Petroproduccion-Geolab).
(**) Apparent thickness defined from well log analysis and the associated seismic section.
(***) Unpublished radiometric age data (source: Petroproduccion Library facilities). Age determinations on whole rock samples were performed by request of Texaco Petroleum
Company at the Geochron Laboratories Division, Krueger Enterprises, Inc.
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Table 2
Representative analyses of major and trace elements by XRF. REE, Th, Ta, Hf, Sc, U and Cs were analyzed by ICP-MS
Jivino-1
DJGJ1
Auca-16
DJGA16
Auca-23
DJGA23
Auca-20
A FLD
Pungarayacu-9
DJGP9
Pungarayacu-16
P9BX16
Pungarayacu-39
P9BX39
Yuralpa-1
DJGY1
Waponi-1
RBTW1
Yuralpa-2
RBTY2
Pungarayacu-10
PUNGA
Yur
Centro-1
AL C1
Dayuno-1
ONI 2
SiO2 (wt.%) 45.62 43.02 43.47 43.25 42.31 42.04 41.83 46.29 41.69 46.22 43.01 45.23 41.70
Al2O3 11.28 12.58 9.51 12.33 10.85 10.31 10.91 9.53 9.22 12.76 10.75 9.14 9.42
TiO2 3.03 3.05 2.61 3.06 3.15 2.89 3.11 2.54 3.74 3.29 3.06 2.45 3.73
FeO 11.52 13.02 11.48 14.12 11.97 11.85 13.17 12.43 12.42 11.38 12.52 12.25 12.88
MnO 0.17 0.11 0.13 0.12 0.19 0.18 0.20 0.18 0.20 0.16 0.18 0.18 0.21
CaO 11.86 10.31 11.81 10.31 12.22 11.78 11.80 11.10 11.82 10.69 12.48 11.07 12.37
MgO 10.30 8.01 13.63 8.24 12.37 14.56 12.54 15.86 15.47 9.27 12.84 15.54 14.41
K2O 1.27 1.91 0.77 1.90 1.17 1.20 1.54 1.09 0.23 1.61 0.96 1.08 0.23
Na2O 2.62 2.42 1.85 2.19 3.13 2.98 2.64 2.57 2.14 2.85 2.77 2.56 2.66
P2O5 0.58 0.79 0.73 0.82 0.84 0.78 0.83 0.44 0.97 0.70 0.82 0.47 0.97
LOI 4.19 8.75 7.12 8.88 2.48 1.77 1.95 0.46 5.93 2.32 3.70 0.65 4.33
Total 102.44 103.96 103.11 105.22 100.68 100.35 100.52 102.49 103.83 101.26 103.09 100.60 102.90
Ni (ppm) 258.00 58.00 344.00 75.00 234.00 409.26 264.66 404.00 329.00 178.00 323.00 404.00 334.00
Cr 317.00 277.00 482.00 304.00 314.00 550.30 348.78 535.00 520.00 256.00 410.00 557.00 524.00
Sc 24.00 22.50 21.20 21.70 28.00 24.90 27.20 32.50 30.60 23.90 19.00 24.10 22.60
V 264.00 276.00 250.00 266.00 297.00 242.33 263.82 246.00 286.00 264.00 272.00 225.00 279.00
Rb 26.00 38.00 16.00 35.53 14.00 16.53 21.59 17.00 6.43 29.13 12.16 19.01 4.73
Sr 662.00 1008.00 460.00 916.66 782.00 873.67 740.36 383.00 726.12 740.55 860.44 519.80 706.27
Ba 518.00 697.00 380.00 605.38 559.00 478.02 507.61 301.00 621.77 460.91 514.21 317.22 601.66
Nb 60.50 84.00 60.00 73.76 81.00 76.29 79.11 93.28 100.04 62.71 73.78 40.04 99.44
Zr 194.00 246.00 187.00 223.17 233.00 207.95 219.38 168.00 290.15 252.50 209.41 150.74 281.61
Y 25.00 26.00 23.00 26.10 26.00 24.87 26.79 32.41 29.90 28.28 24.85 18.92 26.31
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La 36.93 47.57 40.77 48.49 47.08 45.91 47.51 57.51 57.13 41.61 46.95 26.60 57.26
Ce 70.41 88.99 77.11 90.03 89.20 84.47 88.16 107.03 107.72 79.56 86.61 51.32 107.15
Nd 34.85 41.64 38.49 42.78 43.39 40.22 42.28 52.46 51.50 38.99 41.32 26.77 51.70
Sm 8.16 9.65 8.60 9.81 9.67 9.24 9.76 12.25 11.62 9.22 9.52 6.69 11.49
Eu 2.73 3.11 2.81 3.16 3.20 2.97 3.12 3.97 3.61 2.99 3.04 2.20 3.62
Gd 7.61 8.12 7.49 8.49 8.55 8.20 8.60 10.53 9.98 8.27 8.45 6.07 9.66
Dy 5.90 6.12 5.57 6.12 6.36 5.84 6.23 7.69 6.89 6.28 5.98 4.52 6.42
Ho 1.04 1.06 0.95 1.05 1.06 0.98 1.05 1.32 1.16 1.09 1.01 0.77 1.05
Er 2.34 2.43 2.15 2.45 2.43 2.20 2.32 2.84 2.64 2.57 2.29 1.76 2.37
Tm 0.31 0.31 0.27 0.30 0.30 0.27 0.29 0.34 0.33 0.33 0.28 0.22 0.29
Tb 1.18 1.12 1.19 1.49 1.36 1.19 1.16 0.85 1.26
Yb 1.73 1.70 1.49 1.69 1.69 1.47 1.56 1.83 1.76 1.83 1.53 1.21 1.53
Lu 0.25 0.24 0.22 0.24 0.23 0.21 0.22 0.25 0.25 0.27 0.21 0.17 0.21
Hf 4.73 5.34 4.61 5.30 5.52 4.95 5.23 6.92 6.84 6.11 5.22 3.99 7.02
Ta 3.55 4.50 3.47 4.38 4.86 4.60 4.75 5.82 6.46 3.91 4.55 2.58 6.46
Pb 2.29 3.78 5.59 4.49 3.46 2.45 4.59 2.23 4.06 4.04 4.94 2.78 3.77
Pr 8.33 10.21 9.11 10.32 10.44 9.62 10.04 12.51 12.31 9.19 9.91 6.15 12.39
Th 3.57 4.87 3.81 5.59 4.81 5.71 5.89 7.10 7.65 5.64 5.40 2.89 6.65
U 1.10 1.57 1.21 1.58 1.64 1.62 1.71 1.58 2.19 1.51 1.55 0.82 1.92
Mg/Mg+Fe 66.00 57.00 72.00 56.00 69.00 72.50 67.00 74.00 73.00 64.00 69.00 73.00 71.0087Sr/86Sr 0.70519 0.70442 0.7035587Sr/86Sr (i) 0.70503 0.70428 0.70345143Nd/144Nd 0.51282 0.51284 0.51282143Nd/144Nd (i) 0.51272 0.51275 0.51274
Representative samples of diabasic dikes and gabbroic sills come from well cores and cuttings (see Fig. 3 for location). The Mg num er was calculated assuming FeO/Fe2O3=0.33.
FeO* is total iron as FeO. Sr and Nd isotopic ratios of three samples were calculated using international standards [43]. 87 Sr/86 Sr (i nd 143 Nd/144 Nd (i) are age corrected isotopic
data for approximately 100 Ma.
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b
) a
R. Barragan et al. / Earth and Planetary Science Letters 236 (2005) 670–690678
and Cenozoic times, resulting from the inversion of
the NNE–SSW Upper Triassic to Lower Jurassic rift,
which emerges and crops out in the Southern Sub-
Andean foothills [26]; (iii) the Eastern Inverted Sys-
tem, generated by the inversion of Upper Jurassic half
grabens, which developed along the eastern border of
the Oriente Basin [20].
3. Characteristics and regional distribution of the
Oriente Basin basaltic volcanism (OBB)
The regional distribution of the Cretaceous extru-
sive and intrusive bodies recognized within the Hol-
lin-Napo sedimentary infill of the Ecuadorian Oriente
Basin is shown in Fig. 3. Although the volume of
eruptive products is relatively small, the OBB is a
NNE-trending system made up of at least 30 isolated
eruptive centers. They comprise tuff cones or mono-
genetic volcanic fields mainly characterized mostly by
hyaloclastites, the largest being only 200–250 m thick
and covering an area of 2–3 km2 (e.g., Fig. 4—Jaguar
and Puma areas, see also Fig. 3 for location). In
addition, several basaltic dykes, 1–10 m wide, and
at least four major shallow intrusive fine-grained gab-
broic to diabasic sills have been identified, the largest
being 80–200 m thick and covering an area of 15–20
Km2 (e.g., Yuralpa–Dayuno and Laguna areas, see
Fig. 3 for location).
The extrusive facies are mainly characterized by
tuff cones and tuff rings interpreted on several seismic
cross sections (Fig. 4) and corroborated by several
drill core samples, well logs and outcrops (Table 1,
see Fig. 3 for sample locations). Volcanoclastic com-
ponents of the volcanic piles consist of stratified, thin-
bedded, basaltic tuff units of well-to-poorly-sorted
fine ash. The fragments consist mainly of hypocrystal-
line basalt (hyaloclastites). Accretionary lapilli are
also very common, indicative of sub-aqueous deposi-
tional conditions. Much of the basaltic glass is altered
to palagonite. The volcanoclastic deposits which are
mainly altered tuffs and palagonitized hyaloclastites,
recognized at different locations within the shallow
marine Cretaceous section [21,22], suggest that the
Cretaceous extrusive bodies in the Oriente Basin
resulted from hydrovolcanic eruptions with a charac-
teristic Surtseyan eruptive style [42]. No basaltic lavas
have been identified.
The time-equivalent intrusive igneous bodies are
characterized by alkalic fine-grained gabbroic to dia-
basic sills and basaltic dykes, identified at different
locations from well-preserved cores and cutting sam-
ples (Table 1, see Fig. 3 for location). They were
emplaced stratigraphically at various horizons within
the Cretaceous sedimentary series. Despite the vari-
able emplacement, there remains a strong composi-
tional and mineralogical uniformity among the
gabbroic to diabasic sills and dikes. Petrographically,
the alkaline sills show a fine-grained phaneritic tex-
ture, and are primarily composed of ophitically inter-
grown olivine, labradorite and augite phenocrysts.
The diabase dykes typically contain olivine as the
dominant phenocryst phase with subordinate labrador-
ite and clinopyroxene. The intergranular groundmass
contains plagioclase microlites and granules of clin-
opyroxene, olivine and magnetite.
4. Analytical procedures
The abundance of major and trace elements in the
alkaline magmas from the Cretaceous Oriente Basin
was determined from 12, well-preserved, core samples
(Table 2). The samples are dykes or shallow intrusive
sills that microscopically do not show significant
alteration to palagonite, if any.
A relatively high LOI of some samples, with values
of up to 8 wt.%, appears to demonstrate a certain
degree of alteration. Nonetheless, based on the Che-
mical Index of Weathering (CIW [44]), the Chemical
Index of Alteration (CIA [45]), and certain elemental
ratios such as Th/Ta, La/Ta, La/Nb or Ba/Zr besides
many others, lead to the conclusion that even samples
with high LOI do not shift out of the general scheme
demonstrated by samples with usually low LOI which
are considered to be fresh or unaltered of the same
sample set. In particular, CIW values of 28–37 and
CIA values of 27–36, fall within known fields of
unaltered Mesozoic Basalts [46].
Fig. 3 shows the location of these samples and their
characteristics are listed in Table 1. The distribution of
the OBB was determined by reflection seismic sec-
tions provided by the Ecuadorian National Oil Com-
pany (Petroproduccion). However, the thick overlying
Cenozoic section severely limits the amount of Cre-
taceous sedimentary outcrops in the Oriente Basin,
A) Jivino-1 whole rock
B) WAPONI-1 whole rock
C) Yuralpa-1 whole rock
0
30
60
90
120
150
180
210
240
0 10 20 30 40 50 60 70 80 90 100
Age
(M
a)
MSWD = 1.93
102.41 ± 2.37 Ma
0.0000
0.0010
0.0020
0.0030
0.0040
0.000 0.008 0.016 0.024 0.032
36A
r / 4
0Ar 101.70 ± 2.41 Ma
MSWD = 0.18
0
20
40
60
80
100
120
140
160
0 10 20 30 40 50 60 70 80 90 100
Age
(M
a)
93.02 ± 3.42 Ma
MSWD = 0.940.0000
0.0010
0.0020
0.0030
0.0040
0.000 0.008 0.016 0.024 0.032
36A
r / 4
0Ar 91.44 ± 3.87 Ma
MSWD = 0.50
0
20
40
60
80
100
120
140
160
0 10 20 30 40 50 60 70 80 90 100
Cumulative 39Ar Released (%)
Age
(M
a)
82.15 ± 1.99 Ma
MSWD = 7.030.0000
0.0010
0.0020
0.0030
0.0040
0.000 0.010 0.020 0.030 0.040
39Ar / 40Ar
36A
r / 4
0Ar 81.47 ± 2.60 Ma
MSWD = 0.00
0.00.00.20.20.40.40.60.60.80.81.01.0
K/C
a
0.00
0.10
0.20
0.30
0.40
K/C
a
0.00.40.81.21.62.0
K/C
a
Fig. 5. Representative age and K/Ca spectra (left) and inverse isochron (right) plots for 40Ar–39Ar incremental heating experiments, performed at
Oregon State University on diabasic dikes (Waponi-1 and Jivino-1) and a gabbroic sill (Yuralpa-1) in drilled, whole rock, core samples of the
OBB. The vertical range of horizontal boxes indicates the estimated analytical error (F2j) for each step age. A plateau age (indicated) has been
determined from the weighted mean of contiguous, concordant step-ages. The 36Ar/40Ar vs. 39Ar/40Ar isotope correlation diagrams are
constructed from the step Ar-compositions measured. The isochron age (indicated) is calculated from the best-fitting line through collinear step
compositions. Tables 3a and 3b present full isotopic data for the new 40Ar/39Ar plateau and isochron ages from the Oriente Basin Basaltic
Volcanism (OBB). Analytical procedures are described in [47,50].
R. Barragan et al. / Earth and Planetary Science Letters 236 (2005) 670–690 679
R. Barragan et al. / Earth and Planetary Science Letters 236 (2005) 670–690680
concealing the true amount of magmatism generated
during the OBB igneous event.
Major-elements and Ni, Cr, Sc, V, Ba, Rb, Sr,
Zr, Y and Nb were analyzed by XRF procedures in
the Geoanalytical Laboratory at Washington State
University (Johnson et al., pers. comm., 1998).
The precision of the data and sample heterogeneity
were tested by multiple analyses of a single speci-
men. Major-element precision is b 2% of the abso-
lute abundance; trace-element precision is b 5%
except for Nb and Rb, which are 10% in low-abun-
dance samples. Rare-earth elements (REE) were ana-
lyzed by standard Inductive by Coupled Plasma Mass
Spectrometry (ICP-MS) techniques, at Washington
State University. In addition, Sr and Nd isotopic
data, listed in Table 2, were determined at Cornell
University using a VG Sector Mass Spectrometry.
They were calculated using the NBS 987 Sr-isotope
standard (Mean: 0.7102452F0.000023; 2j S.D.:
2.30391E-05) and the Ames Nd-standard (Mean:
0.51213433F0.000008; 2j S.D.: 8.3267 E-06).
Further procedural details can be found in [43]. In
Table 3a40Ar–39Ar incremental heating results for sills from the Oriente Basin (Ec
Sample, material Heating step
(8C)
36Ar 37Ar 39A
Yuralpa-1, whole rock 550 0.00035 0.02881 0.07
( J =0.001651) 750 0.00121 0.95230 0.17
950 0.00100 0.05468 0.21
1150 0.00011 0.07595 0.18
1250 0.00001 0.98607 0.16
1300 0.00007 0.57876 0.01
Weighted plateau age (steps 2–5): 82.15F1.99
Waponi-1, whole rock 550 0.00783 0.02832 0.00
( J =0.001440) 750 0.00209 0.01204 0.00
950 0.00315 0.01639 0.00
1150 0.00055 0.01382 0.00
1250 0.00031 0.04887 0.00
1300 0.00016 0.14883 0.00
1400 0.00026 0.65012 0.00
Weighted plateau age (steps 1–7): 93.02F3.42
Jivino-1, whole rock 550 0.00044 0.03429 0.01
( J =0.001534) 750 0.00167 0.10707 0.09
950 0.00270 0.14306 0.18
1150 0.00007 0.08652 0.15
1250 0.00014 0.12646 0.11
1300 0.00021 0.96509 0.08
Weighted plateau age (steps 2–5): 102.41F2.3
37Ar has been corrected for decay since irradiation. K/Ca ratios are d
addition, the 87Sr/86Sr (i) and 143Nd/144Nd (i)
values (Table 2) are age corrected isotopic data for
approximately 100 Ma.40Ar/39Ar incremental heating experiments were
performed for five drill core gabbroic to diabasic
samples from the Oriente Basin (Figs. 3 and 5; Tables
3a and 3b). Age determinations on whole rock sam-
ples were performed at Oregon State University using
standard 40Ar–39Ar incremental heating techniques
[47]. Because of the aphyric nature of the samples,
no preparation other than coring, cutting and washing
~ 100 mg mini-disks of rock from hand specimens
was performed prior to neutron irradiation in the OSU
TRIGA reactor for 6 h at 1 MW power. Individual
ages for each 40Ar–39Ar temperature step were calcu-
lated (using the ArArCALC software, [48]) after cor-
rection for background, mass discrimination, isotopic
interference and atmospheric argon content. All ages
are calibrated to the FCT-3 biotite standard (28.04 Ma,
[49]) and uncertainties are 2 S.D., based on errors in
both peak height fitting and the interpolation of neu-
tron fluence measurements ( J-values). Age spectra
uador)
r 40Ar
(r)
AgeF2s
(Ma)
%40
Ar (r)
%39
Ar
K/Ca
585 2.08924 80.23F1.16 95.3 9.2 1.132
775 5.18536 84.86F2.11 93.5 21.5 0.803
133 6.08387 83.77F0.93 95.3 25.6 1.662
560 5.21339 81.78F1.12 99.3 22.5 1.051
004 4.50298 81.92F0.33 99.9 19.4 0.070
561 0.30627 57.50F6.79 93.6 1.9 0.012
MSWD: 7.03
501 0.20698 104.22F19.59 8.2 10.6 0.076
820 0.29889 92.33F5.96 32.7 17.3 0.293
862 0.34621 101.41F8.75 27.1 18.2 0.226
589 0.21375 91.84F8.09 56.6 12.4 0.183
673 0.24391 91.77F6.42 72.4 14.2 0.059
534 0.19296 91.56F8.03 80.7 11.2 0.015
769 0.27829 91.70F5.29 78.3 16.2 0.005
MSWD: 0.94
956 0.79728 109.42F5.52 86.0 3.0 0.245
928 3.80170 102.98F0.85 88.5 15.0 0.399
253 6.97685 102.79F1.05 89.7 27.6 0.549
655 5.90993 101.56F0.94 99.7 23.7 0.778
576 4.39505 102.12F1.21 99.0 17.5 0.394
736 5.33395 161.51F1.74 98.9 13.2 0.039
7 MSWD: 1.93
etermined from measured 39Ar/ 37Ar. 40Ar (r) is radiogenic 40Ar.
Table 3b
Summary of radiometric dating results for five samples from the Oriente Basin basalts
Sample, material Total
fusion age
(Ma)
2jerror
Plateau
age
(Ma)
2jerror
N MSWD Isochron
age
(Ma)
2j MSWD 40Ar/36Ar
initial
2j error J
Auca-16, whole rock 165.63 3.64 None
developed
None
developed
0.001477
Pungarayacu,
whole rock
231.7 4.98 None
developed
None
developed
0.001402
Yuralpa-1, whole rock 82.38 1.93 82.15 1.99 4/6 7.03 81.47 2.60 0.002 469.0 166.9 0.001651
Waponi-1, whole rock 94.91 3.96 93.02 3.42 7/7 0.94 91.44 3.87 0.50 299.8 4.86 0.001440
Jivino-1, whole rock 110.48 2.50 102.41 2.37 4/6 1.93 101.7 2.41 0.18 328.8 28.8 0.001534
R. Barragan et al. / Earth and Planetary Science Letters 236 (2005) 670–690 681
(age vs. % 39Ar released) and 36Ar/40Ar vs. 39Ar/40Ar
isotope correlation (isochron) diagrams are shown in
Fig. 5. Plateau ages were calculated where appropri-
ate, using the procedure described in [47,50].
Two samples produced ideal gas release patterns,
from which reliable crystallization ages can be calcu-
lated. The Jivino-1 sample exhibits a 4-step plateau
with a weighted (by 1/variance) mean age of
102.4F2.4 Ma. The corresponding isochron age is
concordant, at 101.5F2.4 Ma and yields a 40Ar/36Ar
intercept (329F29) very close to the atmospheric
value (296). The statistical measure of significance,
MSWD or mean square of weighted deviations, is
appropriately low (below the ~ 2.5 critical value)
that the plateau age can be accepted as a reliable
estimate of the crystallization age. Similarly, a 7-step
plateau age of 93.0F3.4 Ma for the Waponi-1 sample
passes all tests of reliability as a crystallization age.
The Yuralpa-1 sample produced a pattern that approx-
imates a plateau at 82.2F2.0 Ma, but with a slightly
decreasing age with increasing temperature trend that
reflects 39Ar recoil within the sample during irradia-
tion. In this case, the MSWD is larger than the critical
value, indicating dispersion of the step ages beyond
that expected from the step age errors. Because the
near-concordant trend in the central four step ages
accounts for 89% of the total gas released, the calcu-
lated age is probably close to the crystallization age;
however, we cannot discount some Ar-loss. Reliable
crystallization ages were determined despite signifi-
cant groundmass alteration (smectitic clay) in Jivino-1
and Waponi-1, indicated by LOI of 4–6%. We con-
clude that our standard 400 8C pre-heating was suc-
cessful in removing much of the loosely-held Ar from
non-retentive alteration phases. The lowest tempera-
ture step in each of these experiments reveals the
effect of greater proportions of atmospheric Ar (larger
age uncertainties) and Ar-loss (sample Yuralpa-1).
The remaining two samples contained significant
amounts of excess Ar (undegassed at cooling) that pro-
duced old and variable step ages. The isochron plots for
these samples do not reveal any common component of
initial Ar than can be subtracted from the step ages,
hence the only useful age estimates are the ages of the
youngest steps (least excess Ar), which are 93.1F0.7
Ma (Auca-16) and 123.8F0.9 Ma (Pungarayacu). We
note that these are maximum age estimates only.
Finally, other 40Ar/39Ar and 40Ar/39K radiometric
ages were obtained from unpublished sources (Petro-
produccion National Oil Company, pers. comm.,
1998). The values are shown in Table 1 (see Fig. 3
for location) for comparison purposes.
5. Geochemical character and petrogenesis
The composition and abundance of major and trace
elements from representative rock samples of the Cre-
taceous OBB are summarized in Table 2. In spite of
their different geographic location and age distribution
(110–80 Ma), they exhibit a restricted range of com-
positional variation, both in major and trace elements.
On a total alkali vs. silica plot (Fig. 6), the OBB
igneous rocks fall into the alkaline basaltic field.
They are also characterized by high contents of TiO2
(2.5–3.7 wt.%), K2O (0.23–1.91 wt.%), Na2O (1.85–
3.13 wt.%) and P2O5 (0.444–0.967 wt.%), with most
values being greater than 1.0, 1.2, 2.4 and 0.7 wt.%,
respectively. They yield a high abundance of MgO
(8.01–15.8 wt.%), Ni (58–404 ppm) and Cr (277–557
ppm) suggesting a primitive nature. Mg numbers
(100�Mg/(Mg+Fe2+)) range from 56 to 73 and
1
2
3
4
5
41 42 43 44 45 46 47 48 49 50 51 52 53 54
MORB
Alkalin
e
Tholeite
SiO2 (wt%)
K2O
+N
aO
(%
wt)
OBB
Fig. 6. Plot of Na2O+K2O vs. SiO2 of 13 samples from the OBB,
showing the divide between alkaline and tholeiitic basalts [51]. Mid-
ocean ridge basalt (MORB) composition from [51].
0.1
1
10
100
1000
Rb Ba Th U Ta Nb K La Ce Sr N
OBB Galapa
Antartica Pen. San Q
Ro
ck/M
OR
B
0.1
1
10
100
1000
Rb BaTh U Ta Nb K La Ce Sr Nd P
Ro
ck/M
OR
B
Fig. 7. MORB normalized incompatible trace element distribution from the
variation, and (b) a comparison of the OBB rocks (representative sample) w
alkaline basalts (CAB). Data sources are shown in Tables 3a and 3b. MO
R. Barragan et al. / Earth and Planetary Science Letters 236 (2005) 670–690682
most values are greater than 64 (Table 2). These
primitive compositions suggest the samples crystal-
lized from a magma, which had experienced minor
fractionation since its origin in its mantle source
region, assuming a typical Mg number of 88–90 for
the mantle [52].
The range of trace-element compositions for the
Cretaceous OBB relative to average mid-ocean ridge
basalts (MORB) is shown in Fig. 7a. With the excep-
tion of two samples (RBTW-1 and ONI-2) that show
low contents of K and Rb, all the incompatible ele-
ment concentrations in the OBB lavas are elevated in
comparison with normal MORB. However, K/Rb
d P Sm Zr Hf Eu Ti Tb Y Yb Lu
gos (OIB) Patagonia (CAB)uintin Reventador (CALC-ALKALINE)
SmZr Hf Eu Ti GdTbDy Y Er TmYbLu
P9BX16
P9BX39
DJGJ1 DJGA16 DJGA23DJGP9 DJGY1 RBTW1RBTY2 PUNGAALC1 ONI2 AFLD
Cretaceous OBB, showing (a) the restricted range in compositional
ith selected oceanic island basalt (OIB), calc-alkaline and continental
RB average data are taken from [53].
10 204 300
200
400
600
MORB
OIB
Ba/La=10
Ba/La=20
continentalmagmatic arc
La/Ta
Ba/
Ta
OBB
Fig. 8. (a) Plot of Ba/Ta vs. La/Ta ratios (modified from [14]
showing the similarity between OBB magmas and ocean island
basalts (field labelled OIB). The field for Andean continental mag
matic arcs is also shown [5].
R. Barragan et al. / Earth and Planetary Science Letters 236 (2005) 670–690 683
ratios in all the OBB samples are relatively constant
(~ 500). Table 4 shows selected incompatible ratios
for the Cretaceous OBB rocks and from several other
basaltic tectonic settings. Overall, the Cretaceous
OBB samples show trace-element abundances similar
to alkaline magmas from several continental (slab
window-related basalts) and intra-oceanic settings
(OIB), with low large-ion-lithophile element (LILE)/
high-field-strength element ratios (HFSE) (e.g., Ba/
Nbc6.33–8.56; Th/Tac1.00–1.44; La/Tac8.84–
11.75; La/Nbc0.57–0.67; Ba/Zrc1.79–2.83).
For comparison, primitive-mantle normalized
incompatible element abundance patterns for alkaline
basalts from San Quintin, Baja California [55], basalts
from the Antarctic Peninsula [8,9] and Patagonia
[6,7], and for oceanic island basalts from Galapagos
Islands [51,56] are given in Fig. 7b. In general, the
OBB rocks exhibit similar trace element abundance
patterns to OIB and some continental basalts. Low Th/
Ta (0.98–1.44), La/Ta (8.84–11.75) and Ba/Ta (52–
155) ratios are typical of intra-plate alkaline magmas
that have not interacted with enriched lithosphere and/
or continental crust (Fig. 8), as is suggested for the
Antarctic Peninsula basalts [8]. The high absolute
HFSE abundance in the OBB lavas, coupled with
the lack of depletion of Nb and Ta that is characteristic
of subduction-related magmas [5], corroborate this
observation. For comparison, Fig. 7b shows typical pri-
Table 4
Selected incompatible-element ratios for basaltic rocks of the Cretaceous Oriente Basin and for basalts from several other tectonic settings
OBB Subduction-related basalts MORB OIB CAB
Oriente Basin IAT HAB and CA Mantle plume origin Slab window Roll-back
San Quintin Cameroon (Antarctic Penins) James Ross Island
K/Zr 35.5–67.8 147 216 12 44 57.5–60.08
Rb/Zr 0.06–0.15 0.21 0.35 0.01 0.1 0.128–0.138 0.129 0.038–0.119 0.09–0.125
Ba/Zr 1.79–2.83 5 7.5 0.1 1.7 1.54–1.83 1.93 0.399–1.188 0.877–1.43
Ba/Nb 6.33–8.56 157 214 4 7 7.14–9.19 8.09 2.69–8.21 4.73–6.14
Ba/Ce 4.93–7.83 30 13 1 5 5.52–6.86 5.25 1.81–5.4 7.16–8.52
La/Nb 0.57–0.67 1.86 7.14 0.97 0.66 0.66–0.74 0.8056 0.61–0.881 1.22–3.47
Zr/Nb 2.63–3.89 31 29 27 4 4.5–5.37 4.194 4.657–7.66 5.87–10.6
Zr/Y 7.76–9.46 1.8 2.7 2.9 7.3 9.64–7.807 10.34 5.43–18.57 4.81–10.46
Ce/Yb 40–60 – – – – 24.61–35.29 – 15.67–44.978 10.97–45.33
Sm/Yb 4.71–6.86 – – – – 2.69–3.72 – 2.606–4.387 2.61–4.6
CeN/YN 6.51–8.35 1.2 3.5 0.7 4.73–9.1 4.88–7.825 8.78 2.58–9.15
OBB—Oriente Basin basalts (Fig. 3). Subduction-related basalts: IAT—island-arc tholeiite; HAB—high-alumina basalt; CA—calc-alkaline
basalt (data source: [54,57]). OIB—ocean island basalt. Data from the Galapagos Islands ([51,56] CAB—continental alkaline basalts; mantle
plume origin. Data from the Cameron line [58], San Quintin [4,55]; slab window-Antarctic Peninsula [8,9], Southern Patagonia [5–7]; Roll
back–James Ross Island [11]).
)
-
mitive-mantle normalized incompatible element abun-
dance patterns of a calc-alkaline lava (Reventador
volcano) from the sub-Andean zone of Ecuador [57].
The relatively consistent abundances of the HREE,
the typical profile of alkali basalts and the geochemical
signature of this alkaline magma (LaN/YbNc15–27,
Sr/Yc12.4–32.9, Ce/Ybc40.7–62.3 and Sm/Ybc4.7–6.9) indicate low degrees of partial melting from a
predominantly asthenosphericmantle source region, and
suggest that residual garnet was present during partial
melting yielding unevolved basalts [58]. Such a garnet-
bearing mantle source requires melt generation at high
pressure, typically at depths of z 60–80 km [59].
-
R. Barragan et al. / Earth and Planetary Science Letters 236 (2005) 670–690684
Sr and Nd isotopic ratios of three OBB samples are
shown in Table 2. The OBB magmas exhibit values
within the range of those observed for the OIB main
array [9]. They show a wide range of 87Sr/86Sr values
(0.703450–0.705027) and restricted 143Nd/144Nd
(0.512724–0.512751) ratios, similar to values yielded
by cratonic continental alkali basalts from Patagonia
[6] and close to those fields occupied by OIB basalts
(e.g., Azores Island [4]) and the James Ross Island–
Antarctic Peninsula [11].
6. Geochronology: timing of the OBB
Three new 40Ar/39Ar radiometric ages from drill
core basaltic samples combined with unpublished40Ar/39K and 40Ar/39Ar radiometric ages from differ-
ent locations within the Oriente Basin are summarized
in Tables 1 and 3b (locations shown in Fig. 3). These
ages, combined with biostratigraphic ages of the sur-
rounding Hollin and Napo sedimentary rocks [e.g.,
20,21,23,25,34,35], indicate a systematic age progres-
sion of the mafic magmas with geographic position
for the emplacement from Albian through Campanian
times (i110–80 Ma) (Table 1). However, no evidence
of basaltic volcanism has been found in younger
Tertiary sediments.
The oldest volcanic episode, a basaltic tuff cone,
was emplaced in the Middle Albian Hollin Formation
within the north–central part of the Oriente Basin
(Vista and Tapi areas, Fig. 3). Radiometric ages of
diabase dikes intersected by the Tapi-5 and Vista-1
exploratory wells (40Ar/39Ki110.2F5.2 Ma and
106F5 Ma, respectively [27]), corroborate the bios-
tratigraphic record at these locations.
Evidence of diachronous volcanism is found dur-
ing deposition of the Albian–Campanian Napo Fm.
with volcanism advancing towards the SSW. Thus,
alkaline basaltic volcanism was coeval with sedimen-
tation of the upper Albian–Cenomanian lower Napo
section in the central part of the basin (Laguna and
Jivino areas), the Turonian middle Napo Fm (Auca-
Armadillo and Waponi areas) and the Campanian
upper Napo Fm in the western–central part of the
Oriente Basin (Yuralpa and Dayuno areas). Radio-
metric ages of gabbroic to diabasic sills and dikes
recovered from several exploratory wells at the corre-
sponding geographic locations confirm the biostrati-
graphic record for these volcanic events, respectively:40Ar/39Ari102.4F2.4 Ma (Jivino-1), 40Ar/39Ki91F4.6 Ma (Auca-16, Petroproduccion, pers.comm)
and 40Ar/39Ari93.1F0.7 Ma maximum age (see
above), 93.0F3.4 Ma (Waponi-1) in the central part
of the basin; 40Ar/39Ari84F2 Ma (Dayuno-1, Pet-
roproduccion, pers.comm.); and 82.2F2.0 Ma (Yur-
alpa Centro-1), in the western–central part of the basin
(Table 1, locations shown in Fig. 3).
Therefore, the systematic age progression from
Albian (~110 Ma), in the northern part of the Ecua-
dorian Oriente Basin, to lower Campanian (~82 Ma),
in the west central part of the Ecuadorian Oriente
Basin, suggests that magmatism migrated towards
the SSW along the active Central Corridor (Fig. 3).
7. Structural control of cretaceous basaltic
magmatism in the Oriente Basin
The geographic distribution of the Cretaceous alka-
line extrusive and intrusive bodies, in the Ecuadorian
Oriente Basin, is mainly restricted to the NNE–SSW
Central Corridor (Fig. 3). Reflection seismic data
evidence that major pre-Cretaceous extensional struc-
tures largely control the location and occurrence of
these alkaline eruptive sites, particularly along deep
NNE normal faults bordering Triassic–Jurassic gra-
bens and half-grabens (Fig. 4). Some of these normal
faults were reactivated as transpressive, right lateral
strike-slip structures related to major tectonic inver-
sion. Seismic evidence of some compressional synse-
dimentary deformation suggests that this regional
tectonic event occurred from Turonian to Maastrich-
tian times in the Oriente Basin [20,27]; however, it
started probably in Middle Albian, as it is evident in
the eastern Maranon Basin of Peru [60], where this
period is marked by compressional inverted struc-
tures. Nonetheless, it can be related to the Peruvian
contractional phase defined in the northern Central
Andes, corresponding to a period of high convergence
rate between the Farallon and the South American
plates [61]. The structural association with volcanism
suggests that pre-existing extensional structures
exerted fundamental control over the generation of
alkalic magmatism during the evolution of the Cretac-
eous Oriente Basin. Magma ascent was channeled by
pre-existing structural discontinuities, corresponding
R. Barragan et al. / Earth and Planetary Science Letters 236 (2005) 670–690 685
to areas of previously thinned lithosphere that were
related to the Triassic–Jurassic rift system and Late
Jurassic back-arc extensional features. As suggested
by [15], these areas of continental weakness may
produce asthenospheric upwelling, which eventually
triggers partial melting.
During the Upper Cretaceous, magmatism
migrated to the SSW, along the Central Corridor of
the Oriente Basin, probably as a result of the lateral
propagation of the transpressive inversion of the
Triassic–Jurassic rift. Given the strong dextral
wrenching component of northern Andean deforma-
tion during this period and the NNW–SSE orientation
of the paleo-rift, the inversion was probably a strike-
slip dominant inversion with reactivation of NE–SW
to E–W extensional faults, which permitted ascent of
magma.
8. Discussion: relations between OBB and the
geodynamic evolution of north-western South
America
The Ecuadorian Oriente Basin records drastic geo-
dynamic changes associated with major tectonic plate
reorganization within the northwestern margin of
South America during Late Jurassic and Early Cretac-
eous times. This period was marked by significant
geodynamic changes, with the end of subduction
and the generation of the consequently active arc
magmatism (Misahualli–Colan arc) [29,30]. This
magmatic gap is considered a result of the oblique
accretion of oceanic and continental exotic terranes
onto the Ecuadorian and Colombian margin [29–34].
The eastern boundary of these accreted terranes is
marked by the occurrence of tectonic slides of HP
metamorphic rocks [34], which yield Early Cretac-
eous cooling ages (i.e., 132 Ma K/Ar age [62], 130–
115 Ma Ar/Ar [63,64]) and are thought to represent a
suture zone [33,38,40,62,65–67].
Independently of the nature of these allochthonous
terranes, during collisional events of continental and/
or oceanic fragments, the margin is uplifted and sub-
mitted to hiatus and/or erosions [68]. This may be a
consequence of the subduction blocking and tectonic
underplating of oceanic material beneath the continen-
tal margin [69]. Along the Oriente Basin, these pro-
cesses are clearly evident, resulting in important
paleogeographic changes [70]: uplift and erosion of
the pre-Cretaceous section, as shown by a major
sedimentary hiatus and regional unconformity at the
base of the Hollin Formation; installation of stable
platform conditions corresponding to the Cretaceous
shallow marine Oriente Basin system [21,22]; and
development of intra-continental alkali basaltic mag-
matism oriented parallel to the previous subduction
zone and located coincident to the major Triassic–
Jurassic rift system in the central part of the Basin
(Central Corridor [20]). Unfortunately, the geophysi-
cal record does not constrain the timing and geometry
of subduction and plate collision. Consequently, one
can only speculate on the relationship between the
alkaline magmatism and subduction along the Ecua-
dorian margin during Cretaceous times.
The occurrence and generation of the OBB alkalic
intra-plate magmatism in the Oriente Basin could be
explained either by the interaction of a deep mantle
plume or hot-spot [12–14], by rifting [16], slab win-
dow formation [17,18] or a droll-backT [11] mechan-
ism. Any model for the magmatic and tectonic
evolution of the Ecuadorian Cretaceous Oriente
Basin has to fit with the following observations: (i)
the volume of eruptive products generated by the
Cretaceous magmatic event is small; (ii) the geochem-
ical and isotopic OIB-type signature is consistent with
an asthenospheric source with no evidence of a com-
ponent from the subducting slab; (iii) paleo-Triassic
and Jurassic rift structures, inverted in a transpressive
regime during Cretaceous times, exerted a tectonic
control on the location and occurrence of major erup-
tive sites; and (iv) magmatism migrated to the SSW
along the active Central Corridor, starting in the
Albian (~ 110 Ma), in the northern part of the Ecua-
dorian Oriente Basin and finishing during the Campa-
nian (~ 80 Ma), in the west-central part.
The present-day back-arc position of OBB vol-
canism suggests that it erupted in an extensional
regime. However, as there is no evidence of there
having been enough lithospheric extension to cause
decompressional melting during Cretaceous times in
the Oriente Basin, it seems unlikely that the alkaline
magmatism is associated with rifting. In contrast, the
major alkaline eruptive sites are controlled by trans-
pressive inversion of a rift [20]. In such system,
strike-slip dominant inversion, only few antithetic
faults can be reactivated in extension. The mantle–
R. Barragan et al. / Earth and Planetary Science Letters 236 (2005) 670–690686
plume model can explain the SSW migration of mag-
matism over time, as suggested by radiometric data
combined with biostratigraphy. However, the rela-
tively small volume of volcanism generated by the
OBB magmatic event (50–100 Km3) is not consistent
with this model.
Is it possible then that the OBB alkaline event is an
indicator of slab-window formation in the Ecuadorian
margin during Cretaceous times? The slab-window
A TRIASSIC-MIDDLE JURASSICMisahualli
Arc
Allochthonous Plate
beginning ofSlab Roll-Back
Slab Roll-Back
Possible Slabbreak-off ??
LATE JURASSICEARLY CRETACEOUS (130-120 Ma)
CRETACEOUS (110-80 Ma) Alkaline volcanism
Collision, end of subductionand active arc magmatism
B
C
subduction is abandoned
en
Fig. 9. Schematic model of the geodynamic origin and evolution of the intr
Jurassic rift system and post rift back-arc basin associated with the Jurassi
Chaucha–Arenillas) ([29,31–34]) and cessation of the active subduction sy
80 Ma), progressive slab roll-back and migration of unmodified asthenos
beneath the Central Corridor.
model would explain the generation of small volumes
of alkalic melts in an area not affected by major
lithospheric extension after the cessation of active
subduction processes. However, the main element of
the slab window model is missing in the Cretaceous
Oriente Basin. Slab windows are formed consequently
to ridge–trench collision [17] and there is no geologi-
cal evidence of this interaction during the Latest Jur-
assic–Earliest Cretaceous. In fact, during the
(Syn-Rift ) Triassic-Lower Jurassicand Jurassic Back-Arc Basin
Oriente Cratonic Basin
Thin-Spot(previous lithospheric thinning)
AsthenosphericMigration
d of Jurassic Back-Arc Basin
TectonicInversion
a-continental thin-spot along the Oriente Basin. (A) Triassic–Middle
c Misahualli arc [32]. (B) Collision of an allochthonous terrane (i.e.,
stem ([29,30,60]). Beginning of slab roll-back. (C) Cretaceous (110–
pheric material towards a region of previously thinned lithosphere
R. Barragan et al. / Earth and Planetary Science Letters 236 (2005) 670–690 687
collisional event (~ 140–120 Ma) [29,31–34], subduc-
tion was merely blocked and abandoned along the
Ecuadorian margin. Therefore, the Cretaceous OBB
alkalic event is not likely to be related to slab-window
formation.
We propose that slab roll-back, associated with
asthenospheric migration towards areas of continental
weakness (previously thinned lithosphere), can
account for the characteristics described for the origin
of the Cretaceous intra-continental alkalic magmatism
in the Oriente Basin. Similar examples include the
cratonic Patagonia Plateau lavas [6] and basalts from
the James Ross Island–Antarctic Peninsula [9]. Fig. 9
illustrates this possible mechanism. After subduction
ceased (~ 140–120 Ma), either the relict slab material,
corresponding to the eastwards-directed leading plate,
was simply rolled-back or detached and sunk as a
result of the large density contrast between the sub-
ducted part of the lithosphere and the surrounding
asthenospheric mantle. In both cases, this resulted in
lateral and vertical migration of unmodified astheno-
sphere, which moved into the region previously
occupied by subduction-modified mantle wedge. Con-
sequently, considerable slab roll-back has occurred
since Early Cretaceous times and may have started
in the north allowing unmodified asthenospheric
material to upwell into the region underlying the
pre-existing Triassic–Jurassic paleo-rift or litho-
spheric pre-existing thin-spot [15]. This resulted in
partial melting and the subsequent input of magmatic
material into the northern part of the Oriente Basin.
Major tectonic inversion occurred during Upper Cre-
taceous times [20,27,60], reactivating pre-existing
extensional features within a dextral transpressive
strike-slip system. As a consequence of the lateral pro-
pagation of the transpressive inversion of the Triassic–
Jurassic rift, magmatism migrated to the SSW along
the Central Wrench–fault Corridor during the Late
Cretaceous.
Pre-existing structures clearly excerted a funda-
mental control on the generation of alkalic magmas
during the evolution of the Cretaceous Oriente Basin,
and focused partial melting by facilitating astheno-
spheric upwelling. Eventually, the Cretaceous alka-
line magmatic event was halted by the progressive
effects of east-dipping subduction that was renewed
further west in the Late Cretaceous after the accre-
tion of allochthonous terranes currently exposed in
the Cordillera Occidental (i.e., Pallatanga [38,39])
and the fore-arc [31,37], as well as the development
of the subsequent compressional Oriente foreland
basin system.
9. Conclusions
The Oriente Basin of Ecuador offers new evidence
of a Cretaceous intra-plate alkaline magmatic activity
associated with the evolution of the northwestern
margin of South America. The geochemical and iso-
topic compositions of these basaltic rocks are consis-
tent with an asthenospheric source unmodified by
subduction, similar to an OIB-type mantle source. A
slab roll-back mechanism, associated with lateral and
vertical asthenospheric migration, is thought to have
facilitated the generation of alkalic magma when sub-
duction was abandoned possibly after the accretion of
oceanic and continental allochthonous terranes
between the Late Jurassic and Early Cretaceous.
Pre-Cretaceous rift and back-arc extensional struc-
tures influenced the generation and location of magma
emplacement, by acting as pre-existing lithospheric
thin-spots, promoting asthenospheric upwelling and
partial melting. Cretaceous tectonic inversion in the
Oriente Basin resulted in the reactivation of exten-
sional features within a dextral strike-slip transpres-
sive system causing a south–southwestward migration
of magmatism within the Upper Cretaceous section
following the trend of the Central Corridor. East-dip-
ping subduction resumed in the Late Cretaceous and
placed the Oriente Basin into a compressional regime,
which resulted in a cessation of OBB magmatism.
Acknowledgments
We thank D. Geist, T. Toulkeridis, R. Fleming, D.
Quirk, M. Weber, R. Spikings and anonymous for
stimulating comments and informal reviews. We are
grateful to John Huard (Oregon State University) and
Bill White (Cornell University) for assistance in40Ar/39Ar radiometric dating and Sr and Nd isotopic
analyses, respectively. We thank the University Paul
Sabatier of Toulouse (France) and Petroproduccion,
especially Jorge Toro for valuable help and discus-
sion. This research was supported by Kerr McGee Oil
R. Barragan et al. / Earth and Planetary Science Letters 236 (2005) 670–690688
and Gas Corporation and by the Institut Francais de
Recherche pour le Developpement. We want to ded-
icate this study to our friend and colleague Michel
Monzier.
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