Petrology of ophiolites from southwestern Colombian Andes

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Petrology of ophiolites from southwestern Colombian Andes

SPADEA, P., DELALOYE, Michel, ESPINOSA, A.

SPADEA, P., DELALOYE, Michel, ESPINOSA, A. Petrology of ophiolites from southwestern Colombian Andes. Ofioliti , 1986, vol. 11, no. 3, p. 357-359

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357 Ofioliti, 1986, 11 (3)

PETROLOGY OF OPHIOLITES FROM SOUTHWESTERN COLOMBIAN ANDES ⁰

+ tt . tt+

.P . Spadea , M. Delaloye , A. Esp1nosa

+ Istituto di Scienze della Terra, Udine, Italy.

++ Département de Minéralogi~. Genêve, Switzerland.

+++ Ingeominas, Cali, Colombia.

ABSTRACT

In southwestern Colombian Andes, Mesozoic ophiolitic {sensu lato) complexes occur in Western Cordillera and along the eastern flank of Central Cordillera. They con- sist mostly of basalts associated with cherts and with volcanigenic and terrigenous

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Fig. 1 - Variations of Ti vs Fe/Mg ratio for extrusive sequences from the South- western Colombian Andes.

Symbols: a= basalts from W Cordillera, NNW of Cali; b = basalts and meta- basalts from the Bolivar complex; c = basalts from western Central Cordil- lera, NNE of Cali; d = metabasalts from the Ginebra complex; e = Mg-rich basalt from Pradera; f = basalts from La Tetilla complex; g = Mg-rich ba- salts from El Encenillo; h

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from the El Tambo - El Penol section; j = basalts and basaltic andesites from the Ricaurte-Altaquer section.

Source of data : present authors {unpublished); Espinosa 1980 {These n.

1970, Faculté qes Sciences, Université de Genêve, 242 p.); Millward et al., 1984 {Jour. Geol. Soc. London, 141: 847-860); Aspden and McCourt, 1986 {Geology, 14: 415-418); Spadea et al., 1987 (Jour. Geol., in press).

0 Presented to: 10° Geowissenschaftliches Lateinamerika-Kolloquium, Berlin 19-21 November 1986.

358

Table 1 - Plutonic ophiolitic complexes from southwestern Colombian Andes . Complex Cumula tes Non-cumula tes Crystallization Dike rocks

sequence

Bolivar dun1te, lherzolite, gabbronorite sp- ol- opx+cpx- pegmat.gabbro, .:tol-websterite, we- (Fe-), gabbro pl- hbl plagiogranite,

hr11te,

±.

nor1te, hbl-gabbro- diabase

nor1te

Ginebra dunite, wehrlite, ?cpx-gabbro sp- ol- opx+cpx- plagiogran1te,

+ ol-websterite, pl doler1te,

Ïherzol i te, + ol- diabase

gabbronorite

La Tetilla wehrlite, cpx- cpx-gabbro sp- ol- opx- diabase

gabbro cpx+pl- hbl- bi

Los Azules

±.

wehrlite,

±.

gabbro basalt

sediments of Late Cretaceous age, and older (Late Jurassic to Early Cretaceous).

Mafic-ultramafic plutonic sequences, possibly representing the intrusive equivalent of some of the basaltic ones, occur either in huge massifs, or in minor blocks. They are exposed within the Romeral fault zone, a major suture zone separating the oceanic domain of Western Cordillera ^f~om the continental domain of Eastern Cor- dillera. Scattered bodies of high-pressure metabasites and associated metasediments also occur along the Romeral Zone in western Central Cordillera.

In recent studies it has been inferred on the basis most of radiometric and structural data that the ophiolites from Southwestern Colombia include an older, i.e. Late Jurassic-Early Cretaceous, group (western Central Cordillera) and a younger i.e. Late Cretaceous, group (Western Cordillera). Both groups are considered to represent fragments of ocean floor; the geochemical patterns of the basaltic lithotypes suggest various tectonic environments for magma genesis.

A petrological study has been made on representative extrusive and plutonic ophiolitic sequences from Southwestern Colombia. The primary features, including the mineral chemistry of the main phases, and the major- and trace elements geochemistry of the rocks have been investigated along with the mineralogical and textural cha- racteristics related to the metamorphism.

Petrologically different extrusive sequences have been recognized in Western Cor- dillera and the Romeral Zone. They are represented by: a) poorly to highly frac- tionated (Fe-rich) basalts with low-Ti tholeiite affinity (NNW of Cali, Ginebra com- plex); b) primitive basalts with low-Ti tholeiite affinity (Bolivar, La Tetilla, and Los Azules complexes, El Tambo-El Penol); c) primitive basalt s. with high-Ti tho- leiite affinity (La Tetilla complex); d) basalts and basaltic andesites with calc- alkaline affinity (Ricaurte-Altaquer, Policarpa); e) primitive basalts with low-Ti tholeiite affinity associated with few Mg-rich basalts of komatiitic type (Pradera, El Enc~nillo). The differences in chemistry among the studied sequences are shown in

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359 a Ti02 vs FeOtot/MgO diagram (Fig. 1).

The plutonic sequences consist mostly of cumulates; massive lithotypes and dif- ferent dike rocks, are also present. Sorne significant petrological features are sum- marized in Table 1. The following aspects are remarked as significant to interpret their origin: a) the cumulates can be related to similar, but variably evolved, ma- fic magmas and_ crystallization conditions; b) most plutonic cumulates are typified by hydrous magmatic minerals; c) the lithological assemblages and the crystalliz- ation sequences indicate comagmatic relationships of the plutonics with the basalts showing a low-Ti tholeiitic affinity.

On petrological ground, it can be concluded that most of the ophiolitic complexes from Southwestern tolombia can be referred to the primitive island-arc type of ophiolites whose generation is related to supra-subduction zone environment. The re- lations of the Mg-rich rocks and of the calc-alkaline sequence with the tholeiitic ones are not clear up to now, and further studies, particularly on structures and stratigraphy, are necessary.