PEÑA DE MORA, Augengneiss

See PEÑA DE MORA, Formation

PEÑA DE MORA, Formation

MIDDLE OR UPPER MESOZOIC

Distrito Federal, Venezuela

Authors of name: S. E. Aguerrevere and G. Zuloaga, 1937.

Original reference: S. E. Aguerrevere and G. Zuloaga, 1937, p. 8.

Original description: ibid.

S. E. Aguerrevere and G. Zuloaga (1937, p. 8) named the augengneiss of Peña de Mora (p. 8) from the outcrops at the place known as Peña de Mora, on the old road from Caracas to La Guaira. It also outcrops at the peaks of La Silla, Naiguatá, etc., as well as between the kms. 9 and 10 of the highway from Maracay to Oeumare de la Costa, where the progressive variation from definite conglomeratic phases with scarce granitic injections into augen-gneiss facies with more abundant injections, can be observed. Such fact already suggested the inclusion of the Peña de Mora augengneiss as a metamorphic facies of the Las Brisas formation.

The same authors describe the augen-gneiss a metamorphic rock composed of injections, lit-par-lit, of a granitic magma into a laminar rock, locally conglomeratic. The "eye" structure of the augen-gneiss, up to 3 ems. in diameter or more, is essentially constituted of light feldspars on a greenish-gray basis of mica and pyroxene. To both sides of the anticline structure of the type locality which includes the augen-gneiss phase, garnetiferous schists, praphitic and micaceous schists and lenticular masses of metamorphic limestones which pertain, at least in part, to the Las Mercedes formation, are mentioned.

Dengo (1951, p. 59) uses the name Peña de Mora augen-gneiss or Peña de Mora formation (p. 61) indicating its statigraphic correlation with the quartz micaschist of the Las Brisas formation, but he enlarges the definition of that lithological unit, in order to include in it other types of paragneiss. The augen-gneiss is coarse-grained with feldspars eyes ("augen") up to 31 cms. in diameter; it occurs most typically in the lower part of the Pefia de Mora metamorphic phase, where it is found intercalated with quartzite beds and aplite veins (maximum thickness of 30 cm.), injected lit-par-lit. Typical augen-gneiss outcrops can be observed at Boquerón (Quebrada Tacagua). Higher in the section, the gneiss become more typically banded, containing biotite and epidote bands alternated with light bands of feldspars and quartz.

The upper gneiss, found at km. 19 of the old road from Caracas to La Guaira as well as in the northern slope of Galipan peak are typically garnetiferous. On the same road, 800 mts. south of Las Trincheras, the same type of gneiss contains marble lenses, up to 3 m. thick, with tremolite and diopside. Those localities represent the lateral transition of the Peña de Mora tectofacies into the Las Brisas formation.

H. G. Kugler (1953, p. 29-30) describes under his Dragón formation, a typical augen-gneiss which constitutes the basement of the Caribbean Series, in the eastern part of the Paria Península, State of Sucre.

Locally, at the foothill of the Avila Mountain, in the Caracas Valley, the augen-gneiss lies along a reverse fault dipping north over alluvial beds. These last form terraces uplifted at a 100 meters over the actual level of the valley. They are also found at the Baruta-El Hatillo valleys and they are considered probably as old as Upper Tertiary. (J. Royo y Gómez, oral communication to the VI Convention for the Advance of Science, Caracas, 1956.)

S. E Aguerrevere and J. M. Sellier de Civrieux