The metamorphic "basement" of Ecuador TO MAS FElNlNGER DPpartement de GPologie, Universitt Laval, Qutbec, QuPbec, Canada GI K 7 P4 ABSTRACT current knowledge of Ecuadorian metamorphic rocks and to depict their distribution on a small-scale map (Fig. 1). The characteristics of five of the principal metamorphic terrains of Ecuador are summarized in Table I. The metamorphic rocks of Ecuador, once thought to constitute a uniform and ancient basement, belong to several distinct units of widely differing ages, compositions, and metamorphic histories. The largest area of metamorphic rocks constitutes an unbroken belt on the eastern Andean slope from the Colombian to the Peruvian border. The bulk ofthe rocks in this belt are of Cretaceous age, and were metamorphosed in the greenschist facies under a barrovian facies series. Rocks in the extreme north, and those south of lat. 3' 15's are different, and may be of Paleozoic age. A smaller terrain, on the western Andean slope in southwestern Ecuador, is yet more varied. It includes four metamorphic terrains: the polymetamorphic Piedras Group of Precambrian age; the low-pressure Tahuin Group of Paleozoic age; the high-pressure Raspas Formation of Cretaceous age; and a low-pressure terrain of uncertain age north of the La Palma fault. Several isolated outcrops of metamorphic rocks in Ecuador are of uncertain significance. The present report is at best a preliminary compilation. I have resorted to many sources of information of a wide range of detail and quality. Aside from the cited sources, I have had access to the dozens of reconnaissance geologic maps at 1: 100,000 and 1:50,000 published since 1970 by the Direction General de Geologia y Minas, Quito (formerly the Servicio Nacional de Geologia y Mineria); unpublished reports of PREDESUR (an autonomous Ecuadorian development agency active in the southern part of the country); unpublished theses at the Escuela Politecnica Nacional, Quito; and personal observations made throughout much of Ecuador from 1970 to 1980. HISTORICAL BACKGROUND THE METAMORPHIC ROCKS OF ECUADOR Large areas of regionally metamorphosed rocks in Ecuador crop out in: (I) an unbroken belt, 645 km long and from 15 to 65 km wide, on the eastern Andean slope between the borders of Colombia and Peru and (2) a terrain about 2,500 km2in area on the western Andean slope to the south, in El Oro Province (Fig. 1). Traditionally, and until as recently as 1969 (Servicio Nacional de Geologia y Mineria), the metamorphic rocks of the country were treated as a more or less uniform and ancient basement, entirely of Paleozoic and Precambrian age (Sauer, 1965, 1971). An early suggestion by Liddle (Liddle and Palmer, 1941) that this interpretation was too simple, and that some metamorphic rocks on the eastern Andean slope are of Cretaceous age, was far ahead of its time and it was largely ignored. More recent studies in Loja and El Oro Provinces in southern Ecuador, based chiefly on photogeology, led Kennerley (1973) to distinguish the metamorphic rocks of the eastern Andean slope ("Zamora Series," supposedly of Paleozoic age), from those on the western Andean slope ("Tahuin Series," supposedly of Cretaceous age). Subsequent regional geologic mapping by Feininger (1979, 1980a) and K-Ar age determinations showed the "Tahuin Series" to be composed of no less than four metamorphic belts of unlike characteristics and ages. Nearly simultaneously, Bristow (1973) revived Liddle's suggestion, and was able to trace fossiliferous rocks of Late Cretaceous age laterally into low-grade schists on the eastern Andean slope, east of Cuenca. Modern mapping and topical studies have combined to show that regional metamorphic rocks in Ecuador belong to several distinct units that range widely in age, composition, and metamorphic history. The purpose of the present report is to summarize Perhaps the best-known metamorphic rocks in Ecuador are those on the western Andean slope in the south. The least-known rocks are at the extreme north and south ends of the belt on the eastern Andean slope. These areas are particularly inaccessible and are characterized by great local relief and dense forest. The remainder of the eastern belt has been studied in detail only along the three roads that cross it, near Quito, Ambato, and Cuenca. The "Zamora Series" is crossed by a road east from the city of Loja in the south. SOURCES OF INFORMATION Piedras Group and Other Precambrian Rocks The Piedras Group crops out in El Oro Province, on the western Andean slope in southern Ecuador. It constitutes an eaststriking belt as much as 12 km wide, from the Peruvian border in the west, at least to the town of Portovelo, 65 km to the east (Feininger, 1978; Bristow and Hoffsetetter, 1977). The Piedras Group constitutes the basement under the Paleozoic Tahuin Group to the south (Feininger, 19804, and is separated from unnamed metamorphic rocks to the north by the regional LaPalma fault. The Piedras Group is composed dominantly of such basic rocks as greenschist and amphibolite, with minor intercalations of quartzite and quartz-sericite schist. Amphibole from amphibolite at Portovelo has given a Late Precambrian (743 k 14 m.y.) K-Ar age (Kennerley, 1980). Intrusive igneous rocks are absent. The Piedras Group is polymetamorphic. High-grade rocks have a strong retrograde overprint, and amphibolite is characterized by feathery green hornblende and wholly saussuritized plagioclase. Criteria to Geological Society of America Bulletin, v. 93, p. 87-92, 1 fig., 1 table, January 1982. 87
