A Neoproterozoic age for the chromitite and gabbro of the Tapo ultramafic Massif, Eastern Cordillera, Central Peru and its tectonic implications Colombo CC. Tassinari ,Ricardo Castroviejo -, Jose F. Rodrigues ,Jorge Acosta ,Eurico Pereira a Instituto de Geodendas da Universidade de sao Paulo, Rua do Lago 562, Cidade Universitaria, sao Paulo, CEP 05508-080, Brazil b Universidad Politecnica de Madrid, E.T.S. Ing. Minas, e/Ríos Rosas 21,28003 Madrid, Spain e Laboratorio Nadonal de Energia e Geologia, Rua da Amieira, Apartado 1089, 4466-901 sao Mamede de Infesta, Portugal d Instituto Geológico Minero y Metalúrgico, Direcdón de Recursos Minerales y Energéticos, Av. Canadá 1470, Lima 41, Peru ABSTRACT The ultramafic-mafic rocks of the Tapo Complex are exposed in the Eastern Cordillera of the Central Peruvian Andes. This complex is composed of serpentinised peridotites and metabasites with some podiform chromitite lenses and chromite disseminations and overlies the sandstones, conglomerates, and tuffs of the Carboniferous Ambo Group. The metagabbros and amphibolites showa tholeiitic affil­ iation 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. Sm-Nd isotopic analyses were performed on chromite as wel! as on whole rock from the gabbro. Al! samples yielded an 143 Sm-Nd isochrone age of718 ± 47 Ma with an initial Ndjl44Nd of0.51213 ± 0.00005. The ENd (718 Ma) values calculated for both chromite and gabbro are in close agreement, around 8.0, implying that they were formed at the same time from the same mantelic magma source. Furthermore a K-Ar age on amphibole of 448 ± 26 Ma was obtained, interpreted as the cooling age of a younger orogenic evento These rocks represent slices of oceanic crust (from a dismembered ophiolitic complex), metamorphosed and later overthrust on upper Palaeozoic continental formations. 1. Introduction a Pre-Andean age for the emplacement of the massif, The rocks of the Tapo Massif are overprinted by metamorphism reaching the A highly dismembered assemblage ofultramafic and mafic rocks lower amphibolites facies, but no metamorphism is observed in the is exposed in the Eastern Cordillera of the Central Peruvian Andes, sediments of the footwalL extending along a discontinuous NW-SE belt over some 250 km One of the most difficult tasks in geochronology is to date ser­ between 12° and 9° S of latitude (between Huancapallac and Tapo, pentinised ultramafic-mafic rocks, This is due to the fact that most s, inset in Fig, 1), One ofthe most important occurrences is the Tapo geochronological techniques are not adequate to date ultramafic Mafic-Ultramafic Complex, which occurs at 3750-4200 m aboye rocks and, in addition, there is the opening of the radiometric sea level, 2 km west of the village of Tapo, in the Tarma province, systems during the superimposed serpentinisation episodes, This about 200 km west of Lima, work intends to constrain the protolith age of the Tapo Complex, The Tapo Complex is a lens-shaped body, 5 km long and 1-2 km using Sm-Nd technique direct dating ofchromites and whole-rock wide, that consists mainly ofstrongly serpentinised peridotites and sample from the host gabbro. In addition K-Ar age determination some gabbros, Several small open pits won chromite from podiform on amphibole from the gabbro is presented to estimate the time of chromitite lenses (2:60% chromite) and from disseminated chro­ the metamorphic overprint. These data are very important to mite in serpentinite, but mining is no longer active, improve the constraints about the tectonic events and the recon­ The main structure trends of the Tapo Complex is NW-SE and struction of the Gondwana margin of the Peruvian Andes, the massif is tectonically emplaced upon Lüwer Carboniferous sedimentary rocks, There were no radiometric ages available for the ultramafic and mafic rocks, but Castroviejo et aL (2009b), proposed 2. Geological setting The Pre-Andean basement is extensively exposed in the Cordillera Oriental (Fig, 2), Some first order stratigraphic and metamorphic discontinuities are briefly outlined following § A N 0.5 km § - ....~ '" PERU HUANCAPALLA • 12' OCEANO PAcIFICO 18 Lithostratigraphic Legend Structural symbols D Cover deposits (Holocene) - Gravels, sands and muds I~I bedding Tapo ultramafic massif - Serpentinites with amphibolite lenses 1-.z...1 normal bedding ---------- Thrust I~I reverse bedding l.••• ·1 Pucará Group (Upper Triasic • Lower Jurassic) Micritic limestones with chert nodules l'<l...l serpentinitic main foliation ----------------- erosive su rface ---------- - - - - - minerallineation l. y. y ·1 Mitu Group (Upper Permian) y y Continental rhythmic molasse, with conglomerates El stratigraphic contact ----------------- erosive surface _ E:;] thrust plane Ambo Group ( Lower Carboniferous) o • chromite pit I sample - Conglomerates, sandstones and tuffs Fig.l. Geological map ofthe Tapo uItramafic Massif, showing Iocation of dated sampIes (n° 30 ~ TP140607-030; 33 ~ TP150607-033; 26 ~ 260607-(; and 3 ~ 090606-003, TabIes 3 and 4). 75'30­ 9'30' 21)..1 (Panao) 75'00' 10'00' ~'-"'oI.&-----''\---T-;,¡~~=o:::;';:;t----,;¡¡----------;;;;'~~=:;t-:S;''d;----''-~-----;2=-2_-:-m-;(o=-x--.7t.--m--p.~)10'30' 76'30' D Qualernarycover: alluvial, colluvial, glacial deposits, etc. D Andeancycle sediments: syn orogenic siliciclastic and carbonate deposits (UpperTriassicto Pleistocene). D Variscan mollasic sedimenls: mainly continental deposits and volcanics. Lower Carboniferous lo Lower Triassic. D Siliciclastic sequence wilh low grade metamorphism. Lower Ordovician to Upper Devonlan (?) (" Low grade siliciclastic metasediments and rare metavolcanics. Likely Upper Pre-Cambrian. (a) High grade metamorphic cores (gneisses,migmatites and granulites). D Upper paleozoic to mesozoic plutonic rocks. D Andean subduction-relaled cenozoic magmalism. Fig.2. Geological sketch ofthe Eastern Cordillera (9°30'-11°30'), with location ofknown ultramafic-mafic occurrences (marked with circles). Dalmayrac et al. (1980), whose scheme is adapted to the Tarma (Castroviejo et al., 2009b), are briefly reminded. The tectonic nature region. The oldest unit is a low grade metasedimentary sequence of of the contacts (always thrusts or faults), the existence of a strong terrigenous origin with some metavolcanic intercalations (Huácar shear deformation associated to the ultramafites, but not observed Group), for which a Precambrian age was first proposed (Megard in the siliciclastic sequences of the basement (either Huácar or et al., 1996), but recent geochronological data show that orogenic Ambo Groups), together with the absence of any evidence of events in the Peruvian Eastern Cordillera occurred during Lower thermal metamorphism in the latter, show clearly the allochtonous Palaeozoic times (Chew et al., 2007). This unit is overlain uncon­ emplacement ofthe ultramafic bodies in their present position, and formably by a Lower Palaeozoic marine sequence which is covered preclude the previously accepted hypothesis (Megard et al., 1996, by Late Palaeozoic to Lower Triassic, tardi- or post-tectonic sedi­ and references therein) of ultrabasic magma intrusion in these ments of marine and continental facies, including the Ambo and sequences. Relict internal features in the ultramafites witness the Mitu Groups. The sedimentary sequences of the Andean cycle a previous deformational history not found in the footwall rocks rest upon an erosional surface over the former units (from (Rodrigues et al., 2010a,b). Moreover, the chromite ores are of the Precambrian to Upper Palaeozoic). In the studied region the Andean podiform type, and do not correspond to the stratified concentra­ cycle begins with an Upper Triassic - Lower jurassic carbonate tions typical of intrusions (Castroviejo et al., 2010b) and finally, as sequence (Pucará Group). lntrusive rocks ofvarious ages and recent shown below, metabasite geochemistry suggests an ocean ridge or deposits complete the regional framework. an ocean island protolith. In the Tarma area, three ultramafic occurrences are known: Tapo, named after the near village E of Tarma, and two smaller 3. Petrology bodies occurring ~7 km NE of Acobamba (Megard et al., 1996; Castroviejo et al., 2009a). Only the main one, the Tapo massif, has The petrographic and geochemical characterisation of the been sampled for isotopic analysis. The extreme serpentinisation, ultramaficjmafic lithologies is briefly discussed. Most of the ultra­ and the absence ofchromite or other primary magmatic minerals in mafites are totally altered to serpentinites and extremely deformed. the Acobamba bodies preclude dating their magmatic crystalliza­ Sheared serpentinites and serpentine mylonites are the most tion. Nevertheless Castroviejo et al. (2009a) shows that their common lithology. Extreme metasomatism of the ultramafites geology, structure, emplacement, and therefore their age, are produces locally silica and carbonate-silica hydrothermalites, bir­ similar to those of the main body, Tapo. birites and listvaenites, due to hydrothermal fluid circulation The main body of ultramafic rocks, the Tapo Massif (Fig. 1) enhanced along thrusts and faults. comprises serpentinites with minor lenses of amphibolites which Peridotitic remnants are scarce and, when found, are usually lie over the Lower Carboniferous sandstones, conglomerates, and overprinted by serpentinisation. Olivine or pyroxene relics, sug­ tuffs ofthe Ambo Group; these sediments