A highly dismembered assemblage of  ultramafic and mafic rocks is exposed in the Eastern Cordillera of the  Central Peruvian Andes, extending along a discontinuous NW-SE belt over some 250 km between 12° and  9° S of latitude. One of the most important occurrences is the Tapo Mafic-Ultramafic Complex, which  occurs at 3750 to 4200 m above sea level, 2 km to the west of Tapo locality, in the Tarma province, about  200 Km west of Lima.   The Tapo complex is a lens-shaped body, 5 km long and 1-2 km wide, that consists mainly of strongly  serpentinized peridotites and some gabbros. Several small open pits won chromite from podiform chromitite  lenses ( ≥60 chromite) and from disseminated chromite in serpentinite. The main structural trend of the  Tapo Complex is NW – SE and the massif is tectonically emplaced upon Lower Carboniferous sedimentary  rocks. The rocks of the Tapo massif are overprinted by metamorphism reaching amphibolite facies (see  Willner et al, 2010, for more information on the metamorphic conditions).  The main purpose of this work is to constrain the age determination of the Tapo Complex, using Sm-Nd  technique direct dating of chromites and, also, amphibole, plagioclase and whole-rock samples from the host  gabbro. In addition K-Ar age determination on amphibole is presented to date the metamorphic overprint

Acosta, Jorge

Castroviejo Bolibar, Ricardo

Pereira, Eurico

Rodrigues, Jose F.

Tassinari, Colombo G.

English

Servicio de Coordinación de Bibliotecas de la Universidad Politécnica de Madrid

NEOPROTEROZOIC ULTRAMAFIC AND MAFIC MAGMATISM IN THE EASTERN 
CORDILLERA OF THE CENTRAL PERUVIAN ANDES: THE TAPO MASSIF
 
Colombo C. G. Tassinari1; Ricardo Castroviejo2; Jose F. Rodrigues3
Jorge Acosta
, 
4 and Eurico Pereira
 
3 
1- Instituto de Geociências da Universidade de São Paulo, Rua do Lago 562, Cidade Universitaria, São Paulo, Brasil, CEP 
05508-080;  
2- Universidad Politecnica de Madrid. E.T.S. Ing. Minas, c/Ríos Rosas 21, 28003_Madrid, Spain; 
ccgtassi@usp.br 
3-
ricardo.castroviejo@upm.es 
Fac. Engenharia Univ. Porto (FEUP), r/ Dr. Roberto Frias s/n 4200-465 Porto, Portugal; , felic@fe.up.pt, 
euricosousap@gmail.com
4-
  
Instituto Geológico Minero y Metalúrgico. Dirección de Recursos Minerales y Energéticos. Av. Canadá 1470. Lima 41- 
Perú; jacosta@ingemmet.gob.pe. 
INTRODUCTION
A highly dismembered assemblage of  ultramafic and mafic rocks is exposed in the Eastern Cordillera of the 
Central Peruvian Andes, extending along a discontinuous NW-SE belt over some 250 km between 12° and 
9° S of latitude. One of the most important occurrences is the Tapo Mafic-Ultramafic Complex, which 
occurs at 3750 to 4200 m above sea level, 2 km to the west of Tapo locality, in the Tarma province, about 
200 Km west of Lima.  
The Tapo complex is a lens-shaped body, 5 km long and 1-2 km wide, that consists mainly of strongly 
serpentinized peridotites and some gabbros. Several small open pits won chromite from podiform chromitite 
lenses ( !"#$!%&'()*+,-! ./0!1'()!0*22,)*/.+,0!%&'()*+,! */! 2,'3,/+*/*+,4  The main structural trend of the 
Tapo Complex is NW – SE and the massif is tectonically emplaced upon Lower Carboniferous sedimentary 
rocks. The rocks of the Tapo massif are overprinted by metamorphism reaching amphibolite facies (see 
Willner et al, 2010, for more information on the metamorphic conditions). 
The main purpose of this work is to constrain the age determination of the Tapo Complex, using Sm-Nd 
technique direct dating of chromites and, also, amphibole, plagioclase and whole-rock samples from the host 
gabbro. In addition K-Ar age determination on amphibole is presented to date the metamorphic overprint. 
 
GEOLOGICAL SETTING AND SAMPLING 
The ultramafic–mafic rocks of the Tapo Complex comprise serpentinized peridotites and metabasites with 
some podiform chromitite lenses and chromite disseminations. These rocks lie over the lower-carboniferous 
sandstones, conglomerates, and tuffs of the Ambo Group; these sediments show no signs of thermal 
metamorphism. The contact shows clear evidences of cataclasis, both in the serpentinites of the hangingwall, 
and in the Ambo deposits of the footwall. This basal contact, as well as the overlying serpentinites, are 
folded together with the Ambo sediments by a NW-SE Andean folding phase. The internal deformation of 
the ultramafites shows a strongly non-coaxial character that is interpreted as the result of their pre-andean 
tectonic emplacement. Yet the main foliation transposes another one, observable in  
59
XV Congreso Peruano de Geología. Resúmenes Extendidos. 
Sociedad Geológica del Perú, Pub. Esp. N° 9 (2010), Cusco p. 59-62
 
 
Fig. 1.- Geological map of the Tapo massif, with location of samples analysed. 
 
microlithons that should correspond to an older episode of mantelic deformation (Castroviejo et al., 2009). 
Most of the ultramafites are totally altered to serpentinites and extremely deformed. Sheared serpentinites 
and serpentine mylonites are the most common lithology. Peridotitic remnants are scarce and, when found, 
60
are usually overprinted by serpentinisation; olivine or pyroxene relics, suggesting dunitic and harzburgitic or 
lherzolitic protoliths, are rarely seen. 
The metabasites are, when fresh, banded metagabbros or hornblende-plagioclase amphibolites with flaser or 
nematoblastic fabric, often brecciated and sometimes mylonitic. They represent basaltic to picrobasaltic 
protoliths, and show a tholeiitic affiliation. and a flat REE spider diagram, with a slight LREE depletion and 
a positive Eu anomaly suggesting magmatic accumulation of plagioclase, in an ocean ridge or ocean island 
environment. Resulting petrographic types are varied (chlorite or chlorite-serpentine schists, mylonites or 
blastomylonites, amphibole-chlorite-serpentine-titanite schists and breccias, garnet-epidote granofelses, etc.), 
and include rodingites or gabbro-derived metasomatic rocks with a peculiar calc-silicate mineralogy 
(grossular, zoisite, epidote, wollastonite, titanite, tremolite) as well as chlorite ± albite, ilmenite and relic 
pyroxene / plagioclase. Chromite ores occur as small podiform chromitite lenses and disseminated in 
chromite serpentinites throughout the massif. Both types show a relatively simple primary mineralogy 
(chromite, magnetite; traces of pyrite, as minute inclusions), later modified by metamorphism and 
metasomatism. 
Due to the widespread metamorphic and/or metasomatic overprint, enhanced by deformation, the 
identification of reliable rock samples to date the protoliths has been a difficult task, requiring several field 
campaigns. Most of the original olivine and pyroxene disappeared, but some relatively fresh samples of 
chromitite or chromite rich serpentinite and of meta-gabbro or amphibolite could be found which, after 
careful petrographic survey and mineral processing, proved adequate (location in Fig. 1). 
 
GEOCHRONOLOGICAL RESULTS
In order to characterize the age of the Tapo mafic-ultramafic complex, six Sm-Nd analyses were performed 
on chromite and on whole-rock, plagioclase and amphibole from the associated gabbro. The two groups 
(chromite and separated minerals from the gabbro) overlap in 147Sm/144Nd and 143Nd/144Nd ratios, and 
together both define a line on Sm-Nd isochronic diagram, with slope that corresponds to an age of 677 ± 59 
Ma with an initial 143Nd/144
The 5
Nd of 0.512185 ± 0.00008 and MSWD value of 1.5.  The defined age is 
interpreted as the age of the chromites and associated gabbro crystallization. 
Nd values, calculated for 670 Ma, for both chromite and gabbro whole-rock samples, are in close 
agreement and show a narrow range of values between + 7.7 and + 8.3.  67%&!.!3(2*+*8,!5Nd
The K-Ar analysis performed on amphibole sample from metamorphosed gabbro yield an age of 448 ± 26 
Ma. This result can be interpreted as the cooling age, indicating that temperature dropped below 500°C at 
upper ordovician times, just after the metamorphic peak of the overprint event that reach the amphibolite 
facies.  
 values suggest 
that Nd was derived from the depleted mantle and both, chromite and host gabbro, were formed at the same 
time from the same magma source, which indicate that the chromitite pods have a magmatic origin. 
 
DISCUSSION AND CONCLUSIONS
Based on the regional geochronological pattern established for the Eastern Cordillera (Chew et. al. 2007; 
Cardona et. al. 2006 and Dalmayrac et. al. 1980) and on our geochemical results, the Sm-Nd isochronic age 
of 677 ± 59 Ma defined for the gabbro and associated chromite of the Tapo Complex could be interpreted as 
the age of production of oceanic lithosphere, near the NW margin of Amazonia. This interpretation appears 
as the only reasonable to explain the geochemical character of the Tapo protoliths as oceanic crust and, also, 
the allochtonous character of the massif as well. It implies local rifting related to some regional extensional 
event and a subsequent closure and accretion of the newly generated crust to the continental margin, as is 
typical of ophiolites. The new K-Ar age of 448 ± 26 Ma could be related to the cooling of the medium to 
high grade metamorphic episode dated back at 484 – 474 Ma by Chew et. al. (2007) in the Eastern Cordillera 
or represent the cooling at 500 °C, very close to metamorphic peak, of another younger orogenic event. 
Tp conclude, our new Sm-Nd data indicate that the Tapo Mafic-Ultramafic magmas seem to have 
crystallized at about 680 Ma. and it is suggested that both gabbro and chromite are contemporaneous and 
they were derived from the same mantelic source. The K-Ar geochronology shows that the Tapo Complex 
was affected by an Ordovician metamorphic episode, which reached the amphibolite facies; cooling below 
500º C occurred at ca. 450 Ma (late Ordovician). 
61
Despite of the lack of more complete geochronological pattern, some fundamental facts related to the 
evolution of the central Peruvian Eastern Cordillera can already be established:  
 as a consequence of pre-cambrian rifting probably related to the early stages of  rodinian break-up, 
ca. 680 Ma, oceanic lithosphere was generated near the NW Amazonian craton, pre-andean margin;  
 some slices of this newly formed crust including podiform chromitite bodies were accreted as 
ophiolites, involved in the Famatinian or Terra Australis Orogen and subject to metamorphism in middle 
ordovician times (up to 450 Ma); 
 fragments of this oceanic lithosphere were subsequently overthrust on upper paleozoic sequences of 
the pre-andean amazonian margin, and incorporated to the andean orogen. 
 
REFERENCES
Cardona, A., Cordani, U.G., Ruiz, J., Valencia, V., Nutman, A.P. and Sanchez, A., 2006. U/Pb detrital zircon 
geochronology and Nd isotopes from Paleozoic metasedimentary rocks of Marañon Complex: Insights on the proto-
Andean tectonic evolution of the Eastern Peruvian Andes: Fifth South Amerdican Symposium on Isotope Geology, 
short papers, volume, pp. 208-211. 
Castroviejo, R., Pereira, E., Rodrigues, J.F. and Acosta, J., 2009. Pre-andean serpentinite-chromite orebodies in the 
Eastern Cordillera of Central Peru, Tarma province.  Proc. 10th Biennial SGA Meeting, Townsville, Australia, p. 927-
929. 
Chew, D.M., Schaltegger, U., Kosler, J., Whitehouse, M.J., Gutjahr, M., Spikings, R.A. and Miskovic, A., 2007. U-Pb 
geochronology evidence for the evolution of the Gondwana margin of the north-central Andes, GSA Bulletin 119 
pp.697-711. 
Dalmayrac, B., Laubacher, G. and Marocco, R.,1980. Geologie des Andes Peruviénnes. Caractéres Generaux de 
l’Evolucion Geologique des Andes Péruviennes: Paris, Travaux ET Document de L’Office de La Recherche 
Scientifique ET Technique Outre-mer, 122, pp. 501. 
Willner A. et al. (2010) High pressure metamorphic conditions in garnet amphibolite from a collisional shear zone 
related to the Tapo ultramafic body, Eastern Cordillera of Central Perú. (This vol.). 
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Neoproterozoic ultramafic and mafic magmatism in the Eastern Cordillera of the central peruvian Andes: the Tapo Massif

Archivo Digital UPM

Geologie des Andes Peruviénnes. Caractéres Generaux de l’Evolucion Geologique des Andes Péruviennes: Paris, Travaux ET Document de L’Office de La Recherche Scientifique

High pressure metamorphic conditions in garnet amphibolite from a collisional shear zone related to the Tapo ultramafic body, Eastern Cordillera of Central Perú. (This vol.).

Pre-andean serpentinite-chromite orebodies in the Eastern Cordillera of Central Peru, Tarma province.

U-Pb geochronology evidence for the evolution of the Gondwana margin of the north-central Andes,

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Neoproterozoic ultramafic and mafic magmatism in the Eastern Cordillera of the central peruvian Andes: the Tapo Massif

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