Radiometric ages of the Gorgona Island (Colombia) komatiitic ophiolite

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Radiometric ages of the Gorgona Island (Colombia) komatiitic ophiolite

ESPINOSA, Armando, DELALOYE, Michel, WAGNER, Jean-Jacques

Abstract

A preliminary attempt to date the Gorgona Island komatiitic ophiolite with K-Ar has been made. Three gabbros give ages ranging from 86.2 ± 8.5 to 66.7 ± 8.0 m.y. which places the ophiolite in the Senonian (Upper Cretaceous).

ESPINOSA, Armando, DELALOYE, Michel, WAGNER, Jean-Jacques. Radiometric ages of the Gorgona Island (Colombia) komatiitic ophiolite. Ofioliti , 1982, no. 2/3, p. 237-238

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237 Ofioliti, 1982, (2/3), 237-238

RADIOMETRIC AGES OF THE GORGONA ISLAND (COLOMBIA) KOMATIITIC OPHIOLITE

Annando~"'INOSA

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0 INGE AS, Cali, Colombia

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0 0 Departement de Mineralogie de l 'Universite, Geneve, Suisse

Key words: komatiitic ophiolite, K-Ar dating, Senonian. Gorgona Island.

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Abstract

A preliminary attempt to date the Gorgona Island komatiitic ophiolite with K-Ar has been made. Three gabbros give ages ranging from 86 .2 t 8.5 to 66.7

±

which places the ophiolite in the Senonian (Upper Cretaceous).

Introduction

Gorgona Island (78°12'W, 2°8'N) 8 km long and 2,5 km wide is flanked by a smaller one called Gorgonilla. They are located W of the Tertiary Atrato-San Juan sedimenta- ry basin. The Islands seem to be a part of the continental shelf as defined by the 1000 metres contour.

Gorgona and Gorgonilla could geologically be the southern remnant of the Coastal Cordi 11 era of Serrani a de Baudo found north of the town of Buenaventura. We were able to visit the Islands during a field trip in September 1981. The geology was first described and mapped by Gansser (1950); he noticed the peculiar aspect of some olivine rocks. It is only later that Gansser et al. (1979) related them to ko- matiic lavas.

More recently Echeverria and Paris (1978) remapped in detail both islands. A good description of the field relations is given by Echeverria (1980). The main island Gorgona is block faulted, the rock sequence from bottom to top being: peridotites more or less serpentinised exhibiting undeformed dunites and wehrlites. Overlying these are coarse gabbros with a poikilitic texture and some troctolitic gabbros.

Upwards the grain size diminishes, forming microgabbros. On top of this rock type appear the basalts in massive flows and pillows. The komatiitic flows with spinifex texture are interlayered with the basalts . These intrusives and volcanics are in some places covered with tuff and tuff breccia.

This ophiolite type sequence, although a sheeted dyke complex is not really seen, is covered with sediments going from Upper Eocene possibly to Lower Oligocene. In a few places the lower Miocene transgresses the Eocene (Gansser, 1950).

Age problem

The presence of fossils like Lepidoeyc..llna. PeJtuviana. and Ci bieide-6 TuxpameYL6.U..

238

(Gansser, 1950) in the sediments shows that the ophiolite is older than Upper Eo- cene. We attempted to apply the K-Ar method (Delaloye et al., 1974) to five gabbros and to two massive lava samples. All but three samples have a very low K content

(less than 0,04%); we must use the isotopic dilution method for them. Two micro- gabbros (samples number 55 and 56) from Gorgonilla and one melagabbro (sample num- ber 70) from Gorgona were suitable for dating (K content ranging from D.1 to 0.89%).

The preliminary ages obtained are in the Senonian and are most likely to be in the Maestrichtian (Table I).

In conclusion, based on K-Ar dating, the Gorgona komatiitic ophiolite complex has to be placed in the Upper Cretaceous.

Acknowledgements

J.-J. Wagner thanks the Swiss Federal Grant Commission, M. Delaloye thanks the Daniel Jenny Foundation of the Swiss National Science Academy for financial support.

The authors are very grateful to the Colombian Government and especially to the Instituto Nacional de Investigaciones Geologico Mineras (INGEOMINAS) for field support and to the General Direction of Prisons for permission to work on Gorgona Island. Marie-Louise SchUrch from Ingeominas helped in the rock sampling.

Table I

Sample number Location and type or rock % K 40Ar rad. moles/g Age in M.a.

Cl 70 Gorgona Isl and non oriented mela-gabbro 0.10 l .209 . 10-11 70.5 ±: 9.5 Cl 55 Gorgonilla Island Fine grained oriented gabbro 0.82 9.717 . 10 -11 66.7 ± 8.0 Cl 56 Gorgonilla Island Fine grained oriented gabbro 0.89 1 .360 . 10 -10 86.2 ^±" 8.5

References

Delaloye M. and Wagner J.-J., 1974. Potassium Argon dating: an automatic equipment with digital output for computer processing. R.C. Soc. Phys. Hist. Nat. Geneve, 9: 66-74.

Echeverria L.N., 1980. Tertiary or Mesozoic komatiites from Gorgona Island, Colom- bia: Field relations and Geochemistry. Contr. Mineral. Petrol., 73: 253-266.

Gansser A., 1950. Geological and petrographical notes on Gorgona Island in relation to North-Western S. America. Schweiz. Min. Petrogr. Mitt., 30: 219-237.

Gansser A., Dietrich V.J. and Cameron W.E." 1979. Paleogene komatiites from Gorgo- na Island. Nature, 278: 545-546.

Received, December 19, 1981 Accepted, August 3, 1982

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