Interpretations of the tectonic evolution of Northern Central America depend on a thorough knowledge of pre-Mesozoic rocks, because many of the major structures of the region probably are inherited from, or directly related to, older basement features. New data on these rocks clarify certain relations among Paleozoic metamorphic and sedimentary units.

Ultramafic rocks of the Central Guatemalan Cordillera have been interpreted as the lowermost and probably the oldest unit in the basement sequence, possibly uplifted mantle rocks. The large block of partly serpentinized peridotite north of the Sierra de Chuacus in Guatemala contains anorthositic dikes that seem to be products of partial melting. They may have been roots of larger granitic intrusions.

Isotopic data indicate that the Chuacus series may be of Precambrian age but is more likely younger, having inherited detrital zircon from a source 1,075 m.y. old. Apparently the Chuacus rocks were metamorphosed and the Rabinal granite was emplaced about 345 m.y. ago in Late Devonian time. The Matanzas granite, if correlative with the Rabinal, indicates one or more episodes of post-Devonian metamorphism. The latest was a heating event in latest Cretaceous or early Tertiary time.

A suite of mildly metamorphosed volcanic and related eugeosynclinal rocks south of the Motagua fault is named the "El Tambor Formation," and is interpreted tentatively as a more volcanic facies of the Chuacus series. A rubidium-strontium total-rock determination on an El Tambor metagraywacke allows, but does not suggest strongly, an early Paleozoic age. The structural and stratigraphic relations of the El Tambor Formation to the Chuacus series and younger phyllitic shale and limestone are difficult to interpret on the basis of present knowledge.

Previously proposed models for the evolution of the region are inadequate to explain the present distribution of Paleozoic rocks. Similarities between the Paleozoic rocks of Northern Central America and the southern Appalachian area of the United States may support a model based on continental drift.