SUBDUCTED CONTINENTAL CRUST: METAMORPHIC CONTROL OF DECOMPRESSION IN THE HIND PORTION OF THE AIURUOCA-ANDRELÂNDIA NAPPE, SOUTH-SOUTHWEST BORDER OF SÃO FRANCISCO CRATON, MINAS GERAIS* * Supported by FAPESP 00/11389-6 and 98/15624-8.
LUCIANA P. SANTOS AND MARIO C. CAMPOS NETO
Instituto de Geociências, USP, São Paulo, SP.
Presented by ANTONIO C. ROCHA-CAMPOS
The Nappe system in south-southwest São Francisco Craton represents the southern extension of the Brazilian belt and describes an inverted metamorphic pile (stack) of green schist facies (bottom) toward amphibolite facies (Carmo da Cachoeira e Aiuruoca-Andrelândia nappes) and high pressure granulites in the top, under a high temperature allochtonous (Socorro-Guaxupé nappe).
The hind portion of the Aiuruoca-Andrelândia nappe, south of Caxambú and Aiuruoca (MG), consists of a structural-metamorphic domain with extensional structures in normal shear zones and compressive structures exhibiting E-NE transport. Tourmaline leucogranites in stratiform bodies and diatexitic gneisses occur frequently. There is a metamorphic transition, north to south, from the kyanite zone (and metabasic rocks in eclogite facies) to kyanite and sillimanite coexistence, until kyanite broken and exclusive sillimanite presence.
The S2 texture foliation on mineral assemblage, suggests a decompressive path, where metabasics boudins preserve paragenese eclogite facies (Omp-Pl-Grt-Rt-Qtz) with simplectite textures between Cpx-Pl-Qtz. Externally, corroded garnets wrapped by plagioclase coronas, in amphibole matrix, represent the amphibolite facies reequilibration (MgHbl-Pl-Grt-Zo-Ilm-Ttn). Metapelites (Rt-Ky-Grt-Ms-Bt-Pl-Qtz) may show Sil-Pl-Qtz coronitic intergrowths around garnets.
The maximum temperatures for the distinguished metamorphic domains (14 samples), vary from 710 ± 30ºC (Ky domain) to 730 ± 50ºC (Sil domain). Although the pression vary at 17 ± 0.5 to 6 ± 0.5kbar. P-T conditions at 720 ± 50ºC and 11.5 ± 2.5kbar marks the kyanite assemblage re-equilibration (amphibole facies). P-T conditions 625 ± 55ºC and 6.8 ± 2kbar represent the cooling path (Ky-Sil fields).
The high pression decompressive path suggests an extrusional process, immediately after buried at about 60km deep. Fast exhumation, process controlled by convergent events, derived by the São Francisco plate subductions and tectonic erosion take these units, isothermally, to upper levels (20-33 km). Later, the metamorphic path shift toward a near-isobaric cooling though in a slower extrusional process.
P-T path continuity through three metamorphic domains suggests a sin-metamorphic process (S2 foliation) as a continuous effect S1x S2, without diacronism or ensuing event superposition.
However, large amount of leucogranites denotes a regional thermal event, subsequent and superimposed, responsible for quartz static polygonization in schists/gneisses and for Ms breakdown with microcline crystallization (730oC/6kbar) in the vicinity of large granite plutons. (December 14, 2001) .
Publication Dates
-
Publication in this collection
09 Oct 2002 -
Date of issue
Sept 2002
