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Mineria); unpublished reports of PREDESUR (an autonomous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country); unpublished theses at the Escuela Politecnica Nacional,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.
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