Geology of a Portion of the North-Central Venezuelan Andes

The stratigraphy, petrology, metamorphism, and structural geology of a portion of the glaciated central Venezuelan Andes were studied in an attempt to decipher the metamorphic and tectonic history of the region. Three distinct rock sequences were recorded: (1) the amphibolite grade metamorphics of the Sierra Nevada Formation, (2) the greenschist grade metasediments of the El Aguila Formation, and (3) the sedimentary rocks of the Cretaceous sequence of western Venezuela.

The principal rock types of the Sierra Nevada Formation are: (a) quartzo-feldspathic mica schists, sillimanite-bearing assemblages, and rare staurolite; (b) micaceous quartzo-feldspathic gneisses with rare sillimanite and garnet; and (c) hornblende-epidote amphibolites. All three major rock types (a,b,c) are believed to represent metamorphosed sedimentary rock. The formation is divided into an upper and a lower member on the basis of the presence or absence of amphibolite in the section. The unit is at least 5 km thick.

The El Aguila Formation is divided into three members: (d) the Gavilán Quartzite Member, a thinly laminated, fine-grained meta-quartzite; (e) the El Balcón Member, a unit of interbedded phyllites and metasiltstones; and (f) the Cebollata Limestone Member, amassive unit of thinly laminated siliceous limestones. Staurolite and andalusite are locally prominent minerals in the pelitic layers. The total thickness of the formation is about 1.8 km.

Formations of the well-known Cretaceous sequence of western Venezuela are seen in the northern portions of the area.

Two major granitic bodies intrude the area: (g) the El Carmen Granodiorite, characterized by a quartz-plagioclase-microcline-(mica) assemblage; and (h) the La Culata Adamellite, a leucocratic plagioclase-microcline-quartz-(mica) rock. Minor granitic and pegmatitic bodies are common.

Three deformational periods are recorded, as shown by cross-folding and by lineation and S-surface stereoplot analysis. The two major faults in the area are the high-angle reverse Gavilán fault and the high-angle (strike-slip ?) Boconó fault.

Comparison of observed mineral assemblages with experimental data on silicate minerals allows an estimation of the boundary values of the physical conditions of metamorphism. For the Sierra Nevada metamorphism, T_max is estimated as 620° to 710°C at pressures estimated from 4.3 to 7.0 kbars. For the El Aguila Formation, metamorphism T_max may have reached 400°C; T_min exceeded 210°C and probably 310°C at pressures which probably did not exceed 1kbar. Local modification of the general El Aguila metamorphism permitted the appearance of staurolite and andalusite.

The inferred sequence of events in the area is: (1) the deposition of the Sierra Nevada Formation in the (late ?) Precambrian; (2) D₁ deformation in (probably) the late Precam-brian; (3) intrusion of the La Culata batholith in the early Paleozoic; (4) deposition of the El Aguila Formation in Permo-Carboniferous time; (5) D₂ deformation and the intrusion of the El Carmen Granodiorite (and perhaps the La Culata batholith) about 200 m.y. ago; (6) deposition of the Cretaceous units; and (7) the post-Eocene D₃ deformation, involving largely uplift and tilting.