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GEOLOGY OF CENTRAL ARAGUA, VENEZUELA

1960; Geological Society of America; Volume: 71; Issue: 3 Linguagem: Inglês

10.1130/0016-7606(1960)71[249

1943-2674

R. Shagam,

Geological and Tectonic Studies in Latin America

The Central Aragua area is on the southern flank of the mountain ranges of north-central Venezuela. The south boundary of the map area separates the mountainous metamorphic zone from the low hills and plains underlain by unmetamorphosed rocks to the south. Most of the area is underlain by a sequence of metamorphosed volcanic rocks more than 3000 m thick, the Villa de Cura group. This sequence has been subdivided into four conformable formations. The lowest three formations (El Cano, El Chino, and El Car-men) consist of metamorphosed spilitic basalts and associated volcanic sedimentary rocks. These rocks are now greenstones, some of which contain glaucophane. A striking feature of the metalavas is the presence of unaltered augite. The uppermost (Santa Isabel) formation is considerably more siliceous than the lowest three formations and consists of chlorite-quartz-albite schists and granulites; many rocks contain glaucophane. The Santa Isabel rocks closely approximate keratophyre lavas in chemical composition but are clearly of sedimentary origin. Dioritic rocks intruded into the Santa Isabel formation predate the main metamorphism. North of the Villa de Cura group a sequence of carbonaceous phyllitic schists and metamorphosed lithic arenite sandstones and conglomerates and a distinctive limestone with large clastic calcite grains crop out. The sequence, the Tucutunemo formation, is considered to be the uppermost member of the Caracas group of metasedimentary rocks and to overlie conformably the Las Mercedes formation. The volcanic rocks of the Villa de Cura group are considered to have been erupted after the deposition of the Tucutunemo formation. Possibly the volcanic rocks were erupted in a geographically separate area. Scanty paleontologic evidence suggests an Early Cretaceous age for the Caracas group. In mid-Cretaceous times the main crustal deformation took place, accompanied by the intrusion of ultramafic rocks. The grade of metamorphism attained was not greater than that represented by the albite-epidote-amphibolite facies of regional metamorphism. After the deformation another series of spilitic basalts and tuffs, the Tiara volcanic rocks, was extruded. These extrusions are texturally and mineralogically distinct from those of the Villa de Cura group. The Arrayanes formation on the southern boundary of the map area consists of lithic wacke siltstones, sandstones, and conglomerates composed largely of Tiara pebbles. The formation is dated by a Coniacian ammonite found near San Juan de los Morros. Neither the top nor the bottom of the formation have been observed. In Maestrichtian to Paleocene times deposition of calcareous lithic wacke siltstones and sandstones and aphanitic foraminiferal limestones was continuous. The Maestrichtian sequence in the present mountain belt is referred to as the Paracotos formation. A second weaker orogeny began after the deposition of the Paracotos formation. The Tiara and Paracotos formations were caught within the deforming mountain belt and underwent mild dynamic metamorphism. The Arrayanes and Maestrichtian rocks to the south were merely carried northward on the south limb of this belt and were unaffected by the metamorphism. During Paleocene time, deposition was conformable with the Maestrichtian in the south but unconformable in the north. A third weak deformation took place in late- or post-Paleocene time. The dominant structural trend of N.75°E. is explained by north-northwest—south–southeast compression. This compression could have resulted from an east–west shear couple produced by the movement of the Caribbean block eastward into the deforming arcuate tectogene. Normal faulting, possibly in response to isostatic adjustment, took place after the deformations. This faulting was accompanied by solid reinjection of the ultramafic rocks.