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Metamorphism of the Crystalline Basement of Central Chile

1971; Oxford University Press; Volume: 12; Issue: 1 Linguagem: Inglês

10.1093/petrology/12.1.149

1460-2415

F. GONALEZ-BONORINO,

Geology and Paleoclimatology Research

The metamorphic basement of the Cordillera de la Costa, in Central Chile, consists mainly of slate, meta-sandstone, phyllite, and mica schist but contains small quantities of greenschist. The greater part of the basement is comprised in the Curepto series, and is characterized by dynamo-metamorphism whose intensity increases westward. East of this is the smaller Nirivilo series, characterized by contact metamorphism whose intensity increases eastward toward a granodiorite batholith. At the northern end of the metamorphic belt lies the Pichilemu series, in which the metamorphism is dynamo-thermal and increases in intensity to the east. The rocks in the first two areas are divisible into zones that trend north-north-east, essentially parallel to the margin of the batholith and to the Pacific coast; but in the Pichilemu area the zoning trends north-west. In the Curepto series three zones were distinguished; these are, in order from east to west: (1) a muscovite-chlorite(-albite) slate zone, (2) a muscovite-biotite(-albite) phyllite zone, and (3) a muscovite-chlorite-albite (±garnet) schist zone. The rocks in all three zones belong to the greenschist facies; the chlorite in the third zone is believed to be a product of H metasomatism. In the much narrower Nirivilo area no zones were mapped, but the following eastward succession of critical assemblages was recognized: (1) muscovite-biotite-chlorite-albite; (2) muscovite-biotite-chlorite-andalusite-albite; (3) biotite-muscovite-andalusite-oligoclase; (4) biotite(± muscovite)-andalusite-sillimanite (or cordierite). The muscovite in the rocks that have undergone highgrade metamorphism is largely of metasomatic origin. Part of the sillimanite has been formed at the expense of biotite and andalusite. In most of the Nirivilo area the most strongly metamorphosed rocks are of the hornblende-hornfels facies, but small parts of that area may contain rocks of the pyroxene-hornfels facies. The Pichilemu area comprises the following zones and subzones: (1) a biotite zone, which includes (a) a muscovite-chlorite-biotite-albite subzone and (b) a muscovite-biotite-albite subzone; (2) a garnet-oligoclase zone; (3) a staurolite-andalusite zone; (4) a muscovite-sillimanite zone; and (5) a sillimanite-orthoclase zone which includes (a) a sillimanite-orthoclase subzone and (b) a sillimanite-orthoclase-cordierite-almandine subzone. Some thermal metamorphism is superimposed on the regional metamorphism. The facies grade from the greenschist to the granulite or intermediate between the granulite and the amphibolite facies. The three series represent different pressure conditions: the Nirivilo series corresponds to a low-pressure (contact) type, Pichilemu represents a low-pressure intermediate, dynamo-thermal type, and Curepto is a dynamothermal, high-pressure intermediate type series. The Curepto area was affected first, the Pichilemu area next, and the Nirivilo area last. The metamorphism in all three series is thought to have occurred during a single tectonic cycle within Late Paleozoic time, but it was probably interrupted by periods of erosional unloading. The width of the contact aureole is thought to have been determined by the irregular expansion of the batholith. The analogy between the metamorphic series of the Cordillera de la Costa and Miyashiro's circum-Pacific paired belts is briefly discussed.