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Chapter 8Volume 1
https://doi.org/10.32685/pub.esp.35.2019.08
Citation: Restrepo, J.J., Martens, U. & Giraldo–Ramírez, W.E. 2020. The Anacona Terrane: A small early Paleozoic peri–Gondwanan terrane in the Cauca–Romeral Fault System. In: Gómez, J. & Mateus–Zabala, D. (editors), The Geology of Colombia, Volume 1 Proterozoic – Paleozoic. Servi-cio Geológico Colombiano, Publicaciones Geológicas Especiales 35, p. 149–165. Bogotá. https://doi.org/10.32685/pub.esp.35.2019.08
Published online 21 May 2020
1 [email protected] Universidad Nacional de Colombia Sede Medellín GEMMA Research Group Medellín, Colombia
2 [email protected] S2SGeo 1315 Alma Ave Ste 134, Walnut Creek, CA 94596, USA
3 [email protected] Alcaldía de Marinilla Calle 30 n.° 30–13 Marinilla, Antioquia, Colombia
* Corresponding author
The Anacona Terrane: A Small Early Paleozoic Peri–Gondwanan Terrane in the Cauca–Romeral Fault System
Jorge Julián RESTREPO1* , Uwe MARTENS2 , and Wilmer E. GIRALDO–RAMÍREZ3
Abstract The Anacona Terrane is a small terrane south of Medellín that underwent a geologic history dissimilar to that of the adjacent Tahamí Terrane to the east and the Quebradagrande (Ebéjico) Terrane to the west. The metamorphic basement of the Anacona Terrane is relatively old, comprising amphibolites and metasedimentary rocks, with probable late Neoproterozoic depositional ages, and granitic orthogneisses, with Ordovician magmatic ages. The age of the last metamorphic event to affect the Ana-cona Terrane is constrained to the Devonian or earliest Carboniferous, while Triassic metamorphism, which is widespread in the Tahamí Terrane, has not been documented in the Anacona Terrane, indicating that the terranes were amalgamated during or after the Triassic. Correlatives of the terrane are the Acatlán Complex in southern México and the Marañón Complex and coastal islands in Perú; we surmise that the Anacona Terrane may have originated in a southerly position and migrated northwards, similar to the motion of the Caribbean Plate relative to the South American margin.Keywords: Anacona, tectonostratigraphic terranes, Colombian Andes, Proterozoic sedimentation, garnet amphibolites.
Resumen El Terreno Anacona es un pequeño terreno localizado al sur de Medellín que presenta una historia geológica diferente a la de los terrenos adyacentes, el Tahamí al este y el Quebradagrande (Ebéjico) al oeste. El basamento metamórfico del Terreno Anacona es relativamente antiguo, comprende anfibolitas y rocas metasedimentarias, con probable edad de depositación neoproterozoica tardía, y ortogneises graníticos con edades magmáticas ordovícicas. La edad del último evento metamórfico que afectó al Terreno Anacona está restringida al Devónico o Carbonífero temprano, mientras que el metamorfismo triásico, presente en el Terreno Tahamí, no ha sido registrado en el Terre-no Anacona. Lo anterior indica que estos terrenos se amalgamaron durante o después del Triásico. El Complejo Acatlán en el sur de México y el Complejo Marañón y las islas costeras en Perú son equivalentes del Terreno Anacona; proponemos que el Terreno Anacona podría haberse originado en una posición al sur y migrado al norte, siguiendo el desplazamiento de la Placa del Caribe con relación al margen suramericano.Palabras clave: Anacona, terrenos tectonoestratigráficos, Andes colombianos, sedimentación proterozoica, anfibolitas granatíferas.
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1. Introduction
The basement of the Central Cordillera of Colombia com-prises low– to high–grade metamorphic rocks of diverse age locally intruded by Triassic, Cretaceous, and Paleogene plu-tons. Initial studies regarded the metamorphic basement as a single unit, the Ayurá–Montebello Group (Botero, 1963), but it soon became apparent that several metamorphic events had affected this group of rocks. This led to the proposal that the metamorphic basement of the Central Cordillera was a poly-metamorphic unit (Restrepo & Toussaint, 1984) and that this metamorphic basement formed the “backbone” of the Tahamí Terrane (Toussaint & Restrepo, 1989; see also Restrepo & Toussaint, 2020).
Mapping conducted in the 1970s and 1980s revealed in detail the nature of the metamorphic basement exposed around the county of Caldas, ca. 20 km south of Medellín (Echeve- rría, 1973; Sepúlveda & Saldarriaga, 1980; Patiño & Noreña, 1984; Maya & Escobar, 1985). Unlike the majority of the met-amorphic basement in the Central Cordillera, which is char-acterized by low–P assemblages (e.g., Echeverría, 1973), the rocks near Caldas include garnet–bearing amphibolites and kyanite–bearing metapelites indicative of medium–pressure metamorphism (Restrepo & Toussaint, 1977). Furthermore, geochronologic work consistently yielded older ages than the Permian – Triassic metamorphic events detected in the rest of the Tahamí Terrane (Restrepo & Toussaint, 1978; Restrepo et al., 1991). The first Precambrian K–Ar hornblende age of 1650 ± 500 Ma (Restrepo & Toussaint, 1978) was discarded after the same sample was dated again by the same meth-od, yielding ages between 254 ± 9 Ma and 319 ± 48 Ma; in contrast, a K–Ar muscovite age from the granitic orthogneiss yielded an age 343 ± 12 Ma, older than all the other mica ages from gneisses in the Central Cordillera (Restrepo et al., 1991). Recently, more robust U–Pb and 40Ar–39Ar geochronology has confirmed that the metamorphic basement in the Caldas area underwent a different geological evolution compared with the rest of the metamorphic basement of the Tahamí Terrane. It was therefore separated as an independent unit termed the Anacona Terrane (Figure 1; Martens et al., 2014; Restrepo et al., 2009).
Despite its relatively small size, approximately 45 km2, the Anacona Terrane satisfies the definition of tectonostratigraphic terrane (Coney et al., 1980; Jones et al., 1983); it is a fault–bounded geologic entity or fragment that is characterized by a distinctive geologic history that differs markedly from that of adjacent terranes. Detailed mapping has shown that the Anacona–Tahamí Terrane boundary is a regional north–south trending ductile fault zone, the Santa Isabel Fault (Figure 2; Giraldo–Ramírez, 2013), which is characterized by mylonites and tectonic breccias.
2. Lithology of Units
The two main rock units of the Anacona Terrane are the Cal-das Amphibolite and La Miel Orthogneiss (Figure 3a, 3b). The amphibolite is a polyphase metamorphic rock composed main-ly of blue–green hornblende + almandine–rich garnet + pla-gioclase. Unlike the nearby amphibolites of the Tahamí Terrane, the amphibolites in the Anacona Terrane are rich in garnet, up to 30% by volume (Figure 3a). The garnet porphyroblasts are characterized by coronas of hornblende + plagioclase + quartz. Peak amphibolite facies pressure and temperature (PT) condi-tions were estimated at 1.35 GPa, 630 °C (Bustamante, 2003). These PT conditions are consistent with those established for the mineral assemblages in the metasedimentary schists that are locally interbedded with the amphibolite. These assemblages contain kyanite, garnet, and staurolite, indicating medium–pres-sure, lower–amphibolite facies conditions. The schists are also polyphase, showing evidence for at least three metamorphic phases (Restrepo, 1986).
Some features are suggestive of the Caldas garnet am- phibolite being a retrogressed eclogite: the high garnet con-tent is more typical of eclogite than amphibolite; symplectites of hornblende and plagioclase are common; corona textures of garnet producing amphibole + plagioclase are suggestive of retrogression by decompression (Figure 4). However, de-spite detailed thin section petrography, it has not been possible to identify relict sodic clinopyroxene (omphacite) in this unit (Martens et al., 2014).
Geochemical analyses of the Caldas Amphibolite (Giraldo–Ramírez, 2013) show a predominance of basaltic protoliths. The rare earth element (REE) patterns (Figure 5) are slight-ly enriched in light REE, with Ta and Nb anomalies suggest-ing the presence of subduction–related fluids in the melted source. The REE patterns are transitional between continental arc and E–MORB or continental tholeiite. The trace element geochemistry is inconsistent with the generation of the am- phibolite protolith in a typical mid–ocean ridge; instead, it has been proposed that the basaltic protoliths formed in a continen-tal arc (Giraldo–Ramírez, 2013). These features contrast with the geochemical character of Tahamí Terrane amphibolites, which are predominantly MORB tholeiites (Correa–Martínez et al., 2005; Vinasco et al., 2006; Restrepo, 2008; Giraldo, 2010). Sm–Nd isotopic analyses of Caldas Amphibolite are presented in Table 1, showing positive εNd ranging from 2.6 and 5.8 and model ages between 0.89 and 1.19 Ga.
The second major geologic unit in the Anacona Terrane is the granitic, S–type La Miel Orthogneiss, which is composed of quartz + plagioclase + K–feldspar + muscovite + biotite ± garnet. The K–feldspar is mainly orthoclase that has been in-verted to microcline. The foliation of the rock is defined by the muscovite and biotite, and locally, a primary mineral tabular
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The Anacona Terrane: A Small Early Paleozoic Peri–Gondwanan Terrane in the Cauca–Romeral Fault System
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Yalcón Terrane (Y)
Tahamí Terrane (Ta)
Kogi Terrane (K)
Calima Terrane (Ca)
Tumaco Terrane (Tu)
Tairona Terrane (Tai)
Cuna Terrane (Cu)
Cauca–RomeralUndifferentiatedTerranes (CRUT)
Legend
Chibcha Terrane (Ch)
Andaquí Terrane (An)
Amazonian Craton
Town
Nechí SuspectTerrane (Ne)
Anacona Terrane
0 100 200 300 km50Subduction zone
Medellín
Figure 1. Tectonostratigraphic terranes of Colombia (after Restrepo & Toussaint, 2020).
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reviR níllede
M
kL ea e rCl Cara
reeC l keiM aL
La Romera Cre ek
San Jerónim
o Fault
Tablacita Fault
Caldas
5°58' N
6° 00' N
6° 02' N
6° 04' N
6° 06' N
6° 08' N
75°
38' W
75°
36' W
20 1 3 4 km0.5
Legend
Muscovite–Plagioclase–Quartz Schist
La Miel Orthogneiss
Caldas Amphibolite
Anacona Terrane
Tahamí Terrane
Alluvial and colluvial deposits
Ebéjico Terrane
Cretaceous? Tonalite
Probable Fault
Fault
River
Town
Medellín
Colombia
Venezuela
Ecuador
Perú
Brasil
Panamá
Cali
Bogotá
Medellín
Barranquilla
PacicOcean
Caribbean Sea
Figure 2. Geologic map of the Anacona Terrane, modified from Giraldo–Ramírez (2013).
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The Anacona Terrane: A Small Early Paleozoic Peri–Gondwanan Terrane in the Cauca–Romeral Fault System
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0 1 mm0 1 mm
a
b
Sympl
GrtHbl + Pl + Op
0 2 mm
Figure 3. (a) Photographs of a polished slab of the Caldas Amphi-bolite (b) and La Miel Granitic Orthogneiss.
Figure 4. (a) Garnet replaced by a symplectite of plagioclase and hornblende; photomicrograph in plane light (Grt) garnet, (Sym-pl) symplectite, (Hbl) hornblende, (Pl) plagioclase, (Op) Opaque mineral. (b) Garnet with rotational structure and rims replaced by plagioclase and hornblende; photomicrograph in crossed–polar-ized light (from Restrepo, 1986).
orientation of feldspar is preserved (Figure 3b). Field relations clearly show that La Miel granitic protolith intruded the amphi-bolites and the associated metamorphic units.
U–Pb zircon geochronology of La Miel Orthogneiss has yielded Ordovician crystallization ages of approximately 445 Ma and 480 Ma in two different samples (Martens et al., 2014), implying an even older protolith age for the Caldas Amphibo-lite and their associated metasedimentites. 40Ar–39Ar white mica ages of the orthogneiss yielded an age of ca. 345 Ma (Vinasco et al., 2006), which is the best available age constraint for the timing of metamorphism in the Anacona Terrane. This age is consistent with the maximum age of ca. 360 Ma obtained from a U–shaped 40Ar–39Ar hornblende spectrum, which may reflect the incorporation of excess argon (Restrepo et al., 2008). Im-portantly, none of the 40Ar–39Ar step–heating experiments have revealed any Triassic component, indicating that the Anacona Terrane basement has not experienced the ubiquitous Triassic
1 cm
a
b
metamorphism characteristic of the Tahamí Terrane. It therefore seems unlikely that the Anacona Terrane was adjacent to the Tahamí Terrane during high–grade Triassic metamorphism or that it forms the basement to the Tahamí Terrane (Cajamarca Complex), as proposed by Villagómez et al. (2011).
Although less abundant, quartz + muscovite + garnet–bear-ing quartzites and mica schists are also present in the Anacona Terrane. They occur as metapelitic layers interbedded with the
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Sample (143Nd/144Nd) 0 (147Sm/144Nd) 0 (143Nd/144Nd) εNd (700 Ma) T dm (Ga)
MIG 1 0.513 0.152 0.512 2.612 1.192
MIG 2 0.513 0.172 0.512 5.799 0.891
WG 0.513 0.158 0.512 3.428 1.135
Source: Samples MIG 1 and 2 from Giraldo (2010); sample WG from Giraldo–Ramírez (2013). Note: Assumed values: (143Nd/144Nd)CHUR = 0.512638, (147Sm/144Nd)CHUR = 0.1967 (De Paolo, 1981). Present–day composition of the DM: 147Sm/144Nd = 0.222 and 143Nd/144Nd = 0.513114 (Michard et al., 1985).
Table 1. εNd and model ages for three samples of Caldas Amphibolite
Figure 5. (a) N–MORB (Hofmann, 1988) normalized analyses of the Caldas Amphibolite. Modified from Giraldo–Ramírez (2013). Samples CMK–50–B, JJ–316, JJ–1339 and JJ–1403 from Giraldo–Ramírez (2013) and COLC 35 and COLC 43 from Giraldo (2010). (b) Chondrite–normal-ized Caldas Amphibolite (Boynton, 1984; blue) in comparison with N–MORB, E–MORB and OIB (Sun & McDonough, 1989). Modified from Giraldo–Ramírez (2013).
100
10
1
Sam
ple
/Chondrite
N–MORB E–MORB OIB
b
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu
a100
10
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0.1
Sam
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/MO
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JJ–1403
COLC 35
JJ–1339
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COLC 43
Th Ta Nb Ce P Zr Hf Sm Ti Y Yb
Caldas Amphibolite samples
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The Anacona Terrane: A Small Early Paleozoic Peri–Gondwanan Terrane in the Cauca–Romeral Fault System
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amphibolites, and both are intruded by La Miel Orthogneiss. The paragenesis of the pelitic schist includes staurolite, kyanite, garnet, and chloritized biotite (Figure 6). This amphi-bolite–pelite association likely reflects a volcano–sedimentary sequence metamorphosed under medium pressure, lower–am-phibolite facies conditions. The microstructures of the pelitic schist more clearly show the polyphase metamorphic character of the unit (Figure 4b), which underwent at least three tectonic phases (Restrepo, 1986).
Porphyritic dikes of an intermediate composition locally in-trude the metamorphic basement of the Anacona Terrane, and range in thickness from 0.5–40 m; a tonalitic pluton also intrudes the metamorphic rocks on the western side. Their ages have yet to be determined. In fact, establishing whether the intrusions are related to magmatism in the Central Cordillera or to magmatic units east of the San Jerónimo Fault would be important in further constraining the geologic evolution of the Anacona Terrane.
3. Terrane Boundaries
To the east, the Santa Isabel Fault separates the Anacona Ter-rane from the Tahamí Terrane, whereas to the west, the San Jerónimo Fault, the easternmost fault of the Romeral Fault Sys-tem, separates the Anacona Terrane from the low–grade volca-nosedimentary rocks of the Quebradagrande Complex (Figure 2). Both faults run predominantly N–S in the region and define a narrow, near rhombic block at least 20 km long and only 4 km wide at its maximum extent. Mapping by González (1980) sug-gests that the Anacona Terrane may extend 60 km southwards. The NW–trending Tablacita Fault cuts the Santa Isabel Fault along the northern terrane boundary.
The San Jerónimo Fault, the easternmost strand of the Romeral Fault System, was initially described as an east–dip-ping reverse fault (Grosse, 1926). At present, most authors
regard the San Jerónimo as a dextral strike–slip fault with a reverse component (e.g., Maya & González, 1995). The San Jerónimo Fault exhibits fault–gouge zones up to 3 meters thick (Patiño & Noreña, 1984) along the extent of the Anacona Ter-rane and a variable trend ranging from N25ºW in the south-ern segment to N–S in the central and northern segments. The age of movement on the San Jerónimo Fault is constrained by the accretion of the Quebradagrande Complex to the Tahamí Terrane, which is estimated to have occurred at 117–107 Ma (Villagómez et al, 2011), 73–65 Ma (Jaramillo et al., 2017) or 70–58 Ma (Zapata & Cardona, 2017). The timing of accretion of the Anacona Terrane to the Tahamí Terrane is post–Triassic in age, probably occurring in the Late Cretaceous – Paleocene as a consequence of the northward displacement of the Ebéjico and Caribbean Terranes. The Santa Isabel Fault was first mapped by Sepúlveda & Saldarriaga (1980) and Patiño & Noreña (1984). The fault is a strike–slip, mainly ductile structure with a N–S trend and a vertical to 80°W dip. It can be traced from the Ver-salles–Montebello road in the south (Patiño & Noreña, 1984) to the north, where it ends against the Tablacita Fault. Locally, the fault is characterized by a brittle–ductile tectonic breccia with rock fragments ranging from 1–15 mm in size and embedded in a dark–gray schistose matrix. The breccia fragments include rocks typical of both the Anacona and Tahamí Terranes; some clasts belong to La Miel Orthogneiss, while others are andalu-site–bearing schist fragments that are likely derived from the Tahamí Terrane (Giraldo–Ramírez, 2013).
The Tablacita Fault was mapped by Maya & Escobar (1985) and was defined as a terrane boundary by Giraldo–Ramírez (2013). It is a ductile fault trending N55ºW. The fault sepa-rates units of the Anacona Terrane to the SW from those of the Tahamí Terrane to the NE. Kinematic indicators show sinistral strike–slip movement and possibly a younger phase of fault movement than that on the Santa Isabel Fault. The Santa Isabel
0 0.5 1 mm
GrtChl
St
Ky
Bt
0 0.5 mm
a b
Bt
Ky
Qtz
Pl
Figure 6. (a) Photomicrograph of pelitic schist interbedded within the Caldas Amphibolite, showing staurolite (St), kyanite (Ky), garnet (Grt), and chloritized (Chl) biotite (Bt). The garnet was partially replaced by biotite. (Mineral abbreviations following Siivola & Schmid, 2007). Taken from Restrepo (1986). (b) Kyanite (Ky) crystal with biotite (Bt), quartz (Qtz) and plagioclase (Pl).
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100 µm
1390
120015301535
1200
1310
525
1540
1660
12801130
1460
1240
914
1095
1230
1625
1210
1170
Figure 7. CL imaging of sample AC–1 zircon grains. The numbers are U–Pb ages in Ma. Arrows point to thin metamorphic rims.
and the Tablacita Faults are predominantly ductile in character. They likely formed at a depth greater than 10 km with minor brittle reactivations; there is no evidence for present–day seis-mic activity (Giraldo–Ramírez, 2013).
4. New Detrital Zircon Geochronology
U–Pb zircon ages were obtained from an Anacona Terrane quartzite collected at the junction between La Romera Creek and La Miel Creek (6° 5’ 54’’ N, 75° 36’ 52’’ W). Zircon grains in the sample are anhedral and rounded, and their sizes range from 50–200 μm along their longest dimension (Figure 7). The cathodoluminescence (CL) textures vary from grain to grain and include homogeneous luminescence, concentric zoning, sector zoning, and irregular zones of low luminescence, among others. The roundness and the variety in size and texture at-test to the variable nature of the detrital zircon population in the sample. Importantly, CL images reveal thin, luminous, ho-mogenous rims in many of the zircon grains (Figure 7). These rims were likely produced by Paleozoic metamorphism in the Anacona Terrane, but they were too thin to be dated by the method used.
U–Pb geochronology was conducted by LA–ICP–MS at the Laboratorio de Estudios Isotópicos, Centro de Geociencias, UNAM, following the methods described in Solari et al. (2010). Isotopic ratios were corrected for common Pb using the method of Andersen (2002). Ninety–eight grains were dated (Table 2), and they yielded Precambrian ages; hence, 207Pb/206Pb ages were
preferred. Eight analyses were disregarded because they yielded uncertainties greater than 5%, discordance greater than 4%, or reverse discordance greater than 2%. Hence, 90 analyses were used to construct the probability density diagram in Figure 8.
The detrital zircon signature of the quartzite sample is char-acterized by a spread in ages from 850 Ma to 1700 Ma. Most of the zircon ages range from 1575–1425 Ma and 1250–1125 Ma. The former population is mostly absent from Laurentian sources (Hoffman, 1989; Martens et al., 2010) but is abundant in Amazonia (Tassinari et al., 2000), a strong indication that the Anacona Terrane is of Gondwanan affinity. The mid–Me-soproterozoic population points to provenance from a Grenville source within Gondwana, such as the Oaxaquia microcontinent (Ortega–Gutiérrez et al., 1995), the Putumayo Orogen (Ibañez–Mejia et al., 2011), or the Arequipa Massif (Wasteneys et al., 1995). A similar feature has been previously observed in the xenocrystic zircon component of La Miel Orthogneiss (Martens et al., 2014, Figure 9).
An important constraint from the geochronology is the time of deposition of the sedimentary protolith of the quartzite. The youngest dated spot yielded an age of 540 ± 75 Ma. However, this age does not necessarily imply a Paleozoic depositional age, because it may correspond to a mixture between a detrital core and a Paleozoic metamorphic rim (see spot in Figure 7). Indeed, the analysis was conducted on an external zone involv-ing part of the detrital core, the metamorphic rim, and epoxy. Furthermore, the Th/U ratio is relatively low, suggestive of a metamorphic isotopic component. It is therefore plausible to in-
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The Anacona Terrane: A Small Early Paleozoic Peri–Gondwanan Terrane in the Cauca–Romeral Fault System
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023.
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021.
27E+
032.
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032.
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03
Zirc
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0643
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420.
0955
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1519
1215
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1.43
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1.27
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7.87
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7.09
E+01
8.58
E+01
9.06
E+00
2.65
E+02
4.69
E+02
8.20
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1.40
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2.09
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3.74
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Zirc
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Zirc
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3.30
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2.35
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4.05
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1.22
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3.14
E+01
4.27
E+02
6.95
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1.13
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1.80
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2.45
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3.80
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4.79
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Zirc
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1710
9218
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Zirc
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2.79
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Zirc
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3.72
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Zirc
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Zirc
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Zirc
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7.49
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Zirc
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Zirc
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Zirc
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Zirc
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Zirc
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Zirc
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Zirc
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Zirc
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Zirc
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1.19
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Zirc
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1.71
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2.91
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1.74
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Zirc
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1.19
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2.39
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Zirc
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2610
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Zirc
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2766
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Zirc
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Zirc
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Zirc
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Zirc
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Zirc
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Zirc
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Zirc
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Zirc
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Zirc
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Zirc
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Zirc
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3874
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210.
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3211
4989
3.76
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2.14
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1.56
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2.91
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9.39
E+01
5.68
E+01
1.79
E+02
3.16
E+02
5.73
E+02
8.75
E+02
1.34
E+03
2.88
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3.78
E+03
Zirc
on_0
3995
200.
210.
0832
0.00
212.
550
0.09
60.
2228
0.00
450.
0680
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0034
00.
6012
9624
1285
2812
7149
3.59
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7.72
E+00
1.19
E+00
2.60
E+00
1.66
E+01
3.61
E+00
6.73
E+01
1.15
E+02
2.17
E+02
3.62
E+02
5.73
E+02
1.06
E+03
1.37
E+03
Tabl
e 2.
Cor
rect
ed z
ircon
U–P
b is
otop
ic ra
tios,
age
s, a
nd c
hond
rite–
norm
aliz
ed R
EE c
once
ntra
tions
of s
ampl
e AC
–1 (a
n An
acon
a Te
rran
e qu
artz
ite).
158
RESTREPO et al.
Cor
rect
ed r
atio
s2C
orre
cted
Age
s (M
a)C
hond
rite
–nor
mal
ized
REE
con
cent
ratio
ns
U
(ppm
)1Th
(ppm
)1Th
/U20
7 Pb/
206 P
b±2
s abs
207 P
b/23
5 U±2
s abs
206 P
b/23
8 U±2
s abs
208 P
b/23
2 Th
±2s a
bsR
ho20
6 Pb/
238 U
±2s
207 P
b/23
5 U±2
s20
7 Pb/
206 P
b ±2
sLa
Ce
PrN
dSm
EuG
dTb
Dy
Ho
ErY
bLu
Zirc
on_0
4031
410
70.
340.
0928
0.00
133.
144
0.07
50.
2471
0.00
330.
0725
00.
0014
00.
8014
2317
1443
1914
9130
1.73
E-01
1.95
E+01
1.14
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0788
0.00
082.
267
0.04
80.
2067
0.00
260.
0609
00.
0019
00.
2812
1114
1202
1511
6719
1.39
E-01
2.88
E+01
1.06
E+00
4.20
E+00
2.76
E+01
2.01
E+00
1.28
E+02
2.62
E+02
4.67
E+02
8.14
E+02
1.24
E+03
2.42
E+03
3.01
E+03
Zirc
on_0
8060
513
80.
230.
0779
0.00
032.
103
0.01
50.
1953
0.00
180.
0585
00.
0017
00.
6611
509.
711
49.8
511
46.1
9.1
9.28
E-02
2.71
E+01
8.41
E-01
2.54
E+00
1.16
E+01
4.44
E+00
5.83
E+01
1.26
E+02
2.59
E+02
4.82
E+02
8.25
E+02
1.92
E+03
2.63
E+03
Tabl
e 2.
Cor
rect
ed z
ircon
U–P
b is
otop
ic ra
tios,
age
s, a
nd c
hond
rite–
norm
aliz
ed R
EE c
once
ntra
tions
of s
ampl
e AC
–1 (a
n An
acon
a Te
rran
e qu
artz
ite) (
cont
inue
d).
159
The Anacona Terrane: A Small Early Paleozoic Peri–Gondwanan Terrane in the Cauca–Romeral Fault System
Car
boni
fero
usD
evon
ian
Cor
rect
ed r
atio
s2C
orre
cted
Age
s (M
a)C
hond
rite
–nor
mal
ized
REE
con
cent
ratio
ns
U
(ppm
)1Th
(ppm
)1Th
/U20
7 Pb/
206 P
b±2
s abs
207 P
b/23
5 U±2
s abs
206 P
b/23
8 U±2
s abs
208 P
b/23
2 Th
±2s a
bsR
ho20
6 Pb/
238 U
±2s
207 P
b/23
5 U±2
s20
7 Pb/
206 P
b ±2
sLa
Ce
PrN
dSm
EuG
dTb
Dy
Ho
ErY
bLu
Zirc
on_0
8130
077
0.26
0.07
740.
0009
2.21
30.
043
0.20
470.
0027
0.05
530
0.00
160
0.18
1201
1411
8514
1132
225.
49E-
011.
45E+
011.
13E+
003.
17E+
001.
07E+
011.
79E+
005.
06E+
011.
05E+
022.
14E+
023.
97E+
026.
28E+
021.
30E+
031.
70E+
03
Zirc
on_0
8217
263
0.36
0.09
170.
0008
3.20
50.
057
0.25
210.
0042
0.07
480
0.00
230
0.70
1449
2214
6115
1461
161.
39E+
013.
03E+
012.
16E+
012.
69E+
013.
31E+
017.
82E+
001.
09E+
021.
95E+
023.
60E+
026.
20E+
029.
29E+
021.
74E+
032.
24E+
03
Zirc
on_0
8334
977
0.22
0.06
960.
0012
1.47
20.
030
0.15
290.
0017
0.04
642
0.00
059
0.37
917.
49.
591
912
914
357.
17E-
027.
83E+
004.
74E-
018.
75E-
013.
18E+
006.
20E+
001.
67E+
013.
57E+
016.
79E+
011.
33E+
022.
26E+
025.
78E+
029.
31E+
02
Zirc
on_0
8421
374
0.35
0.08
190.
0030
2.19
70.
066
0.19
860.
0038
0.05
928
0.00
086
0.28
1168
2111
8021
1238
702.
24E-
011.
90E+
011.
65E+
004.
81E+
002.
85E+
018.
35E+
001.
08E+
022.
14E+
023.
94E+
027.
01E+
021.
10E+
032.
17E+
032.
80E+
03
Zirc
on_0
8526
586
0.32
0.08
170.
0014
2.34
80.
063
0.20
820.
0024
0.06
220
0.00
250
0.43
1219
1312
2619
1237
331.
10E-
011.
58E+
011.
13E+
003.
33E+
001.
91E+
012.
42E+
008.
99E+
011.
87E+
023.
24E+
025.
51E+
028.
53E+
021.
64E+
032.
11E+
03
Zirc
on_0
8625
143
0.17
0.08
880.
0007
3.00
50.
060
0.24
480.
0029
0.08
060
0.00
390
0.06
1412
1514
0815
1399
163.
80E-
021.
76E+
019.
27E-
013.
54E+
002.
45E+
011.
49E+
019.
35E+
011.
82E+
023.
26E+
025.
71E+
029.
04E+
021.
86E+
032.
68E+
03
Zirc
on_0
8714
136
0.26
0.08
350.
0012
2.64
00.
063
0.22
850.
0031
0.06
680
0.00
230
0.23
1326
1613
1117
1279
293.
29E+
001.
47E+
013.
23E+
004.
16E+
001.
01E+
011.
08E+
003.
77E+
017.
98E+
011.
61E+
023.
16E+
025.
39E+
021.
19E+
031.
67E+
03
Zirc
on_0
8830
717
70.
580.
1020
0.00
173.
412
0.06
90.
2442
0.00
400.
0712
00.
0016
00.
7114
0821
1507
1616
5930
1.73
E+00
3.37
E+01
4.59
E+00
9.02
E+00
2.77
E+01
1.71
E+01
8.59
E+01
1.45
E+02
2.78
E+02
5.11
E+02
8.99
E+02
2.25
E+03
3.43
E+03
Zirc
on_0
8911
459
0.51
0.09
050.
0012
3.16
10.
061
0.25
100.
0031
0.07
340
0.00
210
0.41
1443
1614
4715
1435
242.
57E-
011.
08E+
011.
06E+
004.
70E+
002.
78E+
015.
63E+
001.
21E+
022.
26E+
024.
00E+
027.
07E+
021.
06E+
031.
92E+
032.
56E+
03
Zirc
on_0
9017
066
0.39
0.09
640.
0007
3.61
30.
041
0.27
240.
0027
0.08
130
0.00
250
0.38
1553
1415
52.1
9.1
1556
131.
90E-
011.
77E+
011.
37E+
003.
94E+
002.
15E+
016.
77E+
008.
67E+
011.
70E+
023.
09E+
025.
26E+
028.
14E+
021.
49E+
031.
93E+
03
Zirc
on_0
9155
171
0.13
0.09
560.
0011
3.58
30.
045
0.27
120.
0026
0.09
340
0.00
520
0.59
1547
1315
46.9
9.6
1543
212.
36E-
021.
52E+
008.
84E-
012.
67E+
002.
68E+
011.
28E+
001.
62E+
024.
18E+
028.
38E+
021.
55E+
032.
43E+
034.
52E+
035.
72E+
03
Zirc
on_0
9229
098
0.34
0.07
920.
0006
2.21
80.
039
0.20
290.
0025
0.06
170
0.00
160
0.70
1191
1311
8612
1177
165.
44E-
012.
15E+
011.
70E+
005.
73E+
002.
96E+
016.
16E+
001.
27E+
022.
34E+
024.
10E+
027.
10E+
021.
08E+
032.
02E+
032.
57E+
03
Zirc
on_0
9374
519
90.
270.
0919
0.00
253.
150
0.13
00.
2489
0.00
460.
0730
00.
0014
00.
8314
3324
1443
3114
6451
3.76
E-01
1.18
E+01
1.52
E+00
3.61
E+00
2.09
E+01
1.01
E+01
1.26
E+02
2.54
E+02
5.00
E+02
8.81
E+02
1.42
E+03
2.77
E+03
3.69
E+03
Zirc
on_0
9461
664
01.
040.
1024
0.00
044.
051
0.04
20.
2885
0.00
280.
0839
00.
0025
00.
7816
3414
1644
.38.
316
67.3
6.7
7.51
E+00
1.16
E+02
1.77
E+01
3.06
E+01
1.09
E+02
5.81
E+01
3.77
E+02
6.68
E+02
1.20
E+03
2.05
E+03
3.08
E+03
5.98
E+03
7.98
E+03
Zirc
on_0
9570
711
80.
170.
0787
0.00
042.
134
0.02
50.
1973
0.00
200.
0618
00.
0022
00.
7311
6111
1159
.78.
111
63.3
9.3
-6.2
1E-0
59.
89E+
004.
42E-
017.
88E-
013.
85E+
004.
83E+
001.
85E+
013.
82E+
018.
50E+
011.
70E+
023.
25E+
029.
53E+
021.
59E+
03
Zirc
on_0
9628
913
90.
480.
0883
0.00
092.
935
0.05
20.
2400
0.00
250.
0693
00.
0021
00.
4613
8713
1391
1313
8919
3.92
E-01
1.76
E+01
4.09
E+00
1.54
E+01
8.63
E+01
2.63
E+01
3.81
E+02
7.03
E+02
1.24
E+03
2.14
E+03
3.15
E+03
5.29
E+03
6.62
E+03
Zirc
on_0
9752
423
30.
440.
0951
0.00
202.
845
0.05
30.
2170
0.00
240.
0637
10.
0007
60.
7312
6613
1367
1415
2939
2.83
E+00
3.94
E+01
7.87
E+00
1.37
E+01
4.39
E+01
2.82
E+01
1.63
E+02
2.76
E+02
5.18
E+02
8.53
E+02
1.35
E+03
2.64
E+03
3.44
E+03
Zirc
on_0
9822
754
0.24
0.07
880.
0008
2.16
10.
064
0.20
070.
0037
0.06
320
0.00
320
0.40
1179
2011
6820
1166
191.
39E-
011.
49E+
011.
23E+
004.
25E+
001.
68E+
014.
80E+
009.
10E+
011.
80E+
023.
38E+
025.
99E+
028.
69E+
021.
78E+
032.
31E+
03
Zirc
on_0
9917
536
0.21
0.08
530.
0014
2.63
70.
077
0.22
560.
0040
0.06
880
0.00
370
0.85
1311
2113
1421
1319
321.
90E-
016.
36E+
005.
06E-
012.
54E+
001.
22E+
013.
52E+
006.
48E+
011.
24E+
022.
33E+
024.
21E+
026.
49E+
021.
25E+
031.
69E+
03
Zirc
on_1
0099
272
20.
730.
0882
0.00
332.
840
0.11
00.
2335
0.00
360.
0691
00.
0013
00.
3913
5319
1366
2813
8772
1.52
E-01
1.70
E+02
3.57
E+00
1.20
E+01
7.50
E+01
5.88
E+01
2.43
E+02
4.38
E+02
8.20
E+02
1.39
E+03
2.29
E+03
5.18
E+03
7.07
E+03
Tabl
e 2.
Cor
rect
ed z
ircon
U–P
b is
otop
ic ra
tios,
age
s, a
nd c
hond
rite–
norm
aliz
ed R
EE c
once
ntra
tions
of s
ampl
e AC
–1 (a
n An
acon
a Te
rran
e qu
artz
ite) (
cont
inue
d).
Note
: 207 P
b/20
6 Pb
ratio
s, a
ges,
and
err
ors
are
calc
ulat
ed a
ccor
ding
to P
etru
s &
Kam
ber (
2012
). An
alyz
ed s
pots
wer
e 23
mic
rom
eter
s, u
sing
an
anal
ytic
al p
roto
col m
odifi
ed fr
om S
olar
i et a
l. Da
ta m
easu
red
empl
oyin
g a
Ther
mo
Xser
ies
QIC
PMS
coup
led
to a
Res
onet
ics,
Res
olut
ion
M05
0 ex
cim
er la
ser w
orks
tatio
n.1 U
and
Th
conc
entr
atio
ns a
re c
alcu
late
d em
ploy
ing
an e
xter
nal s
tand
ard
zirc
on a
s in
Pat
on e
t al.
(201
0).
2 2 s
igm
a un
cert
aint
ies
prop
agat
ed a
ccor
ding
to P
aton
et a
l., 2
010.
160
RESTREPO et al.
Sam
ple/
Cho
ndrit
e
0.01
0.1
1
10
500 750 1000 1250 1500 1750 2000
Age (Ma)
Th
/U
d
10000
1000
100
10
1
0.1
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Yb Lu
aM 698
aM4 98
aM 139
c
0
2
4
6
8
10
12
14
16
18
20
250 500 750 1000 1250 1500 1750 2000
Age (Ma)
Re
lative
pro
ba
bili
ty
Nu
mb
er
b
0.02
0.06
0.10
0.14
0.18
0.22
0.26
0.30
0 1 2 3 4 5207 235Pb/ U
20
62
38
Pb
/U
200
600
1000
1400
da
ta–
po
int
err
or
elli
pse
s a
re 2sa
Figure 8. (a) Wetherill concordia diagram of dated zircon spots; dark, filled ellipses correspond to analyses selected for the probability density diagram. (b) Probability density plot. (c) Chondrite–normalized REE patterns of youngest zircon grains, used to constrain the protolith age. (d) Th/U versus age (Ma) diagram; filled symbols correspond to analyses selected for the probability density diagram.
terpret this analysis as reflecting an isotopic mixture between an inherited core and the Paleozoic metamorphism of the sample. A more robust constraint on the depositional age is the youngest population of zircon, which includes three grains that yielded near–identical ages with a mean of 894 ± 8 Ma (MSWD = 0.01). It is therefore likely that the sedimentary protolith of the quartzites and the basic protolith of the interbedded amphib-olites of the Anacona Terrane may be Neoproterozoic in age.
5. Comparison with Adjacent Terranes
The main characteristics that allow differentiating the Anacona Terrane from the neighboring Ebéjico and Tahamí Terranes are presented in Table 3. In terms of the basement, the Anacona Ter-rane is characterized by pre–Carboniferous metamorphic rocks (e.g., the Caldas Amphibolite and its associated metasedimen-
tites); no rocks as old as these are present in the predominantly Permian – Triassic basement of the Tahamí Terrane, while the volcano–sedimentary Ebéjico Terrane lacks medium– or high–grade metamorphic basement. The Ordovician granites present in the Anacona Terrane (e.g., La Miel Orthogneiss) are also ab-sent in the Tahamí and Ebéjico Terranes (Restrepo et al., 2009).
Triassic metamorphic rocks are widespread in the Tahamí Terrane (e.g., Las Palmas Gneiss) but are unknown or not pres-ent in either the Cretaceous–aged Ebéjico Terrane or Anacona Terrane. The Tahamí Terrane has well–documented Cretaceous sedimentary cover units (e.g., the Abejorral, San Luis, and San Pablo Formations), and the Ebéjico Terrane is composed of Cretaceous volcano–sedimentary successions. In contrast, no such sedimentary sequences are present in the Anacona Ter-rane. Finally, the presence of spilites and other low–grade mafic rocks is only reported in the Ebéjico Terrane (Table 3).
161
The Anacona Terrane: A Small Early Paleozoic Peri–Gondwanan Terrane in the Cauca–Romeral Fault System
Car
boni
fero
usD
evon
ian
Figure 9. Comparison of probability density plots of quartzite AC–1 and the xenocrystic component in granites of the Acatlán Complex in México, the Marañón Complex in Perú, and the Anacona Terrane in Colombia (modified from Martens et al., 2014 and references therein).
2
4
6
8
10
12
14
16
2
4
6
8
10
12
2
4
6
8
10
2
4
6
8
10
12
14
16
18
Acatlán Complex, México
Northern Marañón, Perú
La Miel OrthogneissAnacona Terrane, Colombia
Caldas QuartziteAnacona Terrane, Colombia
500 750 1000 1250 1500 1750 2000
Age (Ma)
Rela
tive p
robability
6. Correlatives of the Anacona Terrane in Perú and MéxicoPrevious work has correlated the Anacona Terrane with peri–Gondwanan terranes containing relics of an Ordovician mag-matic belt that fringed Gondwana (Martens et al., 2014). This Famatinian Orogen is present in South America from Argenti-na to Venezuela (Ramos, 2018). Potential correlatives are the Mixteca Terrane and the Acatlán Complex in southern México, which initially made part of Gondwana, the early Paleozoic
component of the western Marañón Complex in Perú, and the Famatinian magmatic rocks found on the islands off the coast of Perú (Romero et al., 2013). This correlation is supported by the similarity in the ages of xenocrystic zircons from these Ordovician granites in each of these terranes (Figure 9). In the case of the Mexican terranes, the Gondwanan origin has been shown paleontologically (Robison & Pantoja–Alor, 1968; Sán-chez–Zavala et al., 1999; Stewart et al., 1999).
Given that no basement rocks similar to those in the Anacona Terrane are known from Ecuador and that the general
162
RESTREPO et al.
Units Ebéjico Anacona Tahamí
Pre–Carboniferous metamorphic rocks
Absent Present Absent
Ordovician granites Absent Present Absent
Triassic metamorphic rocks Absent Absent Present
Cretaceous sedimentary rocks Present Absent Present
Spilites and other mafic rocks Present Absent Absent
Table 3. Comparison between the main characteristics of the Anacona, Ebéjico, and Tahamí Terranes.
Tahamí Terranes is thought to have occurred during the Late Cretaceous – Paleogene, dated at 73–65 Ma by Jaramillo et al. (2017) and 70–58 Ma by Zapata & Cardona (2017).
8. Conclusion
The recognition of the Anacona Terrane is significant, despite its relatively small size. It lies along the eastern Cauca–Romeral Fault Zone, a major tectonic boundary in the Colombian Andes that separates a domain of predominantly oceanic affinity in the west from a continental–dominated domain in the east. The terrane is unlike others in the Western or Central Cordilleras, comprising basement rocks with a geologic history spanning the Neoproterozoic – Ordovician. Its medium–pressure meta-morphism, xenocrystic and detrital zircon age spectra, and early Paleozoic metamorphism contrast with the low–pressure, Tri-assic metamorphic basement of the adjacent and much larger Tahamí Terrane. The closest correlative Gondwanan terranes of the Anacona Terrane are in México and Perú. The initial accretion of the Anacona and the Tahamí Terranes occurred in the latest Triassic or later, and we surmise that from a southerly position, the Anacona Terrane was pushed northwards along the Cauca–Romeral Fault by the oblique convergence of the Caribbean Plate located to the northwest.
Acknowledgments
We are grateful to David M. CHEW and Victor A. RAMOS, who improved the manuscript considerably with their sugges-tions and corrections as reviewers.
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trend of tectonic transport by the collision of the Caribbean with the South American margin was northward, it is likely that the Anacona Terrane formed further south (in present–day Perú) during the Ordovician, being transported by the north–moving transcurrent faults related to the Cauca–Romeral Fault System. Amalgamation with the Tahamí Terrane occurred at some time between the Jurassic and the end of the Cretaceous (Martens et al., 2014). Given its allochthonous or parautochthonous nature in relation to the Central and Eastern Colombian Andes, all the blocks west of the Anacona Terrane are also necessarily alloch-thonous or parautochthonous.
7. History of Accretion
The Anacona Terrane does not record a Triassic thermal per-turbation of the 40Ar–39Ar hornblende or biotite systems. This result indicates that during the main stage of Triassic orogenesis that substantially reworked the Tahamí Terrane, the Anacona Terrane was not nearby. This constraint provides an upper tem-poral limit on the juxtaposition of the two terranes. Based on the geochronological and field data, we conclude that the Ordovi-cian intrusion of La Miel granite occurred when the amphibolite and biotite schists had already undergone a phase of regional metamorphism as shown by xenoliths of foliated amphibolite within the gneiss. A second phase of metamorphism resulted in the formation of a gneissic foliation in La Miel unit. The timing of this second metamorphic phase has not yet been constrained by U–Pb geochronology, but a 40Ar–39Ar hornblende age yields a maximum age constraint of 360 Ma (Restrepo et al., 2008), along with an 40Ar–39Ar white mica age of ca. 345 Ma (Vinasco et al., 2006); these mineral ages are currently the best constraints for the onset of cooling following this second metamorphic event. These ages imply that the Anacona Block was not adja-cent to the Tahamí Terrane during regional Permian – Triassic metamorphism, and so the terrane docking is post–Triassic.
The Ebéjico Terrane is formed by mafic volcanic rocks in-terbedded with sedimentary rocks (González, 2001; Jaramillo et al., 2017). The accretion of this block to the Anacona and
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Explanation of Acronyms, Abbreviations, and Symbols:
CL CathodoluminescenceE–MORB Enriched mid–ocean ridge basaltLA–ICP–MS Laser ablation inductively coupled plasma mass spectrometryLow–P Low pressureMORB Mid–ocean ridge basalt
MSWD Mean square weighted deviationN–MORB Normal mid–ocean ridge basaltOIB Ocean island basaltPT Pressure and temperature REE Rare earth elementUNAM Universidad Nacional Autónoma de México
Authors’ Biographical Notes
Jorge Julián RESTREPO obtained a degree in mining engineering and met-allurgy at the Universidad Nacional de Colombia in 1968 and a Master of Sci-ence degree in geology at the Colorado School of Mines in 1973. He was a fac-ulty member of the Universidad Nacio-nal de Colombia Sede Medellín, for over 40 years and currently holds the titles of Emeritus Professor and “Maestro Uni-
versitario”. He taught Mineralogy, Metamorphic Petrology, Regional Geology, Field Geology, and Geochronology. His research focused on plate tectonics applied to the geology of Colombia, tectonostratigraph-ic terranes, geochronology, and the geologic evolution of metamorphic and mafic/ultramafic complexes of the Central Cordillera. Other inter-ests are photography, genealogy, and the study of passifloras.
Uwe MARTENS graduated with a de-gree in geological engineering from the Universidad Nacional de Colombia, Sede Medellín, and a PhD in geological and environmental science from Stan-ford University. He has taught at the Universidad Nacional de Colombia, San Carlos National University in Guatema-la, and Stanford University. He currently is a visiting researcher at Centro de Geo-
ciencias, UNAM, México and works as an independent consultant in the field of source–to–sink analysis.
Wilmer E. GIRALDO–RAMÍREZ is a geological engineer who graduated from the Universidad Nacional de Colombia Sede Medellín (2014) and has a Master of Science degree in basins and mobile belt analysis from the Universidade do Estado do Rio de Janeiro (2017). He has worked at institutions such as the Uni-versidad Nacional de Colombia, Uni-
versidad Católica de Oriente, Corporación Autónoma Regional de las Cuencas de los Ríos Negro y Nare “Cornare” and city of Marinilla. His main interests are the geodynamic evolution of the northern Andes, and he has secondary interests in the land use planning of eastern An-tioquia, biology of Passiflora and sports including rugby and Brazilian jiu–jitsu.