Metamorphic rocks, thought to be representative of the basement underlying nuclear Central America, crop out in Guatemala in a long, narrow area between the east-trending Cuilco-Chixoy-Polochic and Motagua-Jalapa fault zones. These basement rocks are overlain by late Paleozoic Santa Rosa Group shales and phyllites and are probably early Paleozoic in age. Metasedimentary rocks in the western part of this exposed area, which are described here, appear to correlate with the Chuacús group, a previously studied area of basement rocks in central Guatemala and, accordingly, are referred to in this report as the western Chuacús group. The metasediments of the western Chuacús group can be divided into a muscovite-rich unit, a banded gneiss unit, and an undivided unit, all of which trend northwest and probably represent original sedimentary formations. The high quartz and potassium content of these metasediments suggest that the parent sediments were derived in part from a granitic, cratonic(?) source. These rocks, as well as several pre- or syntectonic granitic intrusive rocks, contain mineral assemblages characteristic of the greenschist facies. During metamorphism, these rocks were tightly folded into a southeast-plunging antiform (possibly an anticlinorium) trending N. 50° W. Subsequently, the rocks were less tightly folded about steeply dipping axial planes striking about N. 15° W.
The Pucal marble, a distinctive member of the banded gneiss unit in the western Chuacús group, is part of a discontinuous belt of marble that extends from the Mexico-Guatemala border to central Guatemala. Another, less well-defined marble belt trends parallel to this belt about 20 km to the south. The stratigraphy and structure of these two belts, where known, suggest that they are on opposite limbs of a regional synform (or synclinorium) and that sediment probably entered the pre-metamorphic basin from the north.