Origin of biotite-apatite-rich enclaves, Achala batholith, Argentina |
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Authors: | Michael J Dorais Raul Lira Yadong Chen David Tingey |
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Institution: | (1) Department of Geological Sciences, Indiana University, Bloomington, IN 47405, USA, IN;(2) Consejo Nacional de Investigaciones Cientificas y Tecnicas (CONICET) I.G.A. – Celso Barrios 1665, 5000 Córdoba, Argentina, AR;(3) Royal Ontario Museum, 100 Queen's Park, Toronto, Ontario, M5S 2C6 Canada, CA;(4) Department of Geology, Brigham Young University, Prove, UT 84602, USA, US |
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Abstract: | The Achala batholith of Argentina contains very unusual layered enclaves containing up to 30% apatite and 50% biotite in
some layers. This modal mineralogy produces bulk-rock compositions that cannot represent liquids, having as little as 29%
SiO2 and up to 8% P2O5. Nor can the enclaves represent metasedimentary xenoliths because: (1) none of the Precambrian wall rocks has these compositions;
(2) none of the metasedimentary xenoliths present within the batholith shows any degree of transition to the mica-apatite-rich
enclaves; (3) the compositions and textures in the enclaves are inconsistent with metasediments; (4) a geochronological study
of zircon from an enclave gives an age of 368 ± 2 Ma, the exact age of zircons in the granitic host rocks. For these reasons,
we conclude that the enclaves are neither xenoliths of Precambrian wall rocks nor restite of a Precambrian source. The identical
age of the enclave and the host granites, coupled with textural, mineralogical, and bulk-rock characteristics of the enclaves,
indicates that the enclaves are magmatic segregations, i.e., cumulates. The F-rich nature of the stubby-shaped apatites and
biotites indicates a high F content of the magma parental to the enclaves. We infer that the viscosity of the melt was lowered
sufficiently to allow cumulates to form in spite of the granitic composition of the melt.
Received: 12 December 1996 / Accepted: 11 August 1997 |
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