TIARA, Volcanic Rocks

UPPER CRETACEOUS (?)

State of Miranda, Venezuela

Author of name: R. J. Smith.

Original reference: R. J. Smith, 1952, p. 380.

Original description: ibid.

The name Tiara volcanic rocks was introduced by R. J. Smith (1952, p. 380) to designate a series of basic, volcanic and tufaceous rocks outcropping to the southern part of the area studied by the author at the Cordillera de la Costa (see Geological sheet of Los Teques-Cua, in Smith, 1952), Such unit overlies and intrudes the metamorphic rocks of the area. The thickness estimated by Smith is about 900 meters.

The lower part of the sequence is represented by coarse-grained gabbroid rocks composed of pyroxene and plagioclase in approximately equal amounts; many pyroxene and plagioclase crystals are well developed, attaining diameters of about 10 mm.; the texture is ophitic. The borders of some of the pyroxenes are altered to hornblende and chlorite; the plagioclases to clay minerals. The hydrothermal alteration shows a particular preference for the plagioclase, where it is so pervasive that it suggests deuteric action. At some places, serpentine, probably pseudomorphus after olivine, forms 2 to 3 percent of the rock. Such rocks show up as sills or might form dikes up to 2 meters in thickness; they are the hypabyssal magmatic equivalents of the fine grained, upper basaltic portions.

Such fine grained basaltic portions overlie the gabbroid rocks; they are dense, light gray to black, and are of subophitic texture. Some varieties are porphyritic with large plagioclase phenocrystals in a fine-grained matrix, hydrothermally altered; most of the plagioclases are altered to clay minerals and the pyroxene to hornblende and chlorite. Calcite, pyrite, epidote, phrenite and ilmenite are also found, frequently altered to leucoxene. These rocks show a poorly developed bedding and very little shearing.

Locally, a greenish black, sedimentary section made-up of volcanic agglomerates, tuffs, and a small amount of graywacke is associated with the igneous rocks. The agglomerates are constituted by the underlying basaltic rocks, and contain plagioclases, pyroxenes, chlorite, and hornblende.

The tufaceous rocks are commonly dense, black, and its matrix is very fine; show little optical reaction. They are made-up of fragments of volcanic rocks, plagioclase, quartz and chert. Preferred orientation of such materials suggests clastic deposition. Faulting has fractured the rocks slightly, allowing the infiltration of solutions and secondary deposition of quartz and calcite.

Lithologically, this section of pyroclastic rocks closely resembles the Arrayanes formation described by Mencher (see). Such section, although included by Smith in his Tiara volcanic rocks, might well represent a younger stratigraphic unit. López (1942, p. 49-53) mentions the presence of a series of igneous rocks at the Boca Chica-La Puerta region, north of San Juan de los Morros. They are largely of porphyric texture and show-up generally as dikes and sills. The most common types are: augite porphyry and augite-diorite diorite porphyry; López also mentions irregular masses of rocks of equigranular texture and basic and ultrabasic composition. These rocks intruding the metamorphic sequence of such area are possibly equivalent, at least in part, to the Tiara volcanic rocks.

Alirio Bellizzia