Journal of Petrology Volume 43 Number 2 Pages 219-241 2002
© Oxford University Press 2002
Hornblende- and Phlogopite-Bearing Gabbroic Xenoliths from Volcán San Pedro (36°S), Chilean Andes: Evidence for Melt and Fluid Migration and Reactions in Subduction-Related Plutons
1SECTION DES SCIENCES DE LA TERRE, UNIVERSITÉ DE GENÈVE, 13 RUE DES MARAÎCHERS, 1211 GENEVA, SWITZERLAND
2DEPARTMENT OF GEOLOGY AND GEOPHYSICS, UNIVERSITY OF WISCONSIN–MADISON, 1215 W. DAYTON ST., MADISON, WI 53706, USA
Two groups of gabbroic xenoliths (I and II) containing large proportions of late-crystallized hornblende (up to 50 vol. %) and Na-rich phlogopite (up to 15 vol. %), were brought to the surface by a late Holocene eruption of Volcán San Pedro, the youngest edifice of the Tatara–San Pedro Volcanic Complex (36°S, Chilean Andes). Group I are inferred to be fragments of partially solidified Holocene plutons because they contain residual interstitial glass, whereas exsolution and deformation textures in Group II indicate that they are fragments of pre-Quaternary plutonic basement. On the basis of textural relations plus the mineral and whole-rock compositions of both groups of xenoliths, we suggest that hornblende and phlogopite with high mg-numbers and Cr contents have formed by reactions between refractory cumulus minerals (olivine, Cr-spinel, pyroxenes or plagioclase) and evolved melts ± aqueous fluids that migrated through partly solidified crystalline frameworks. Thus, the hydrous minerals are not early-crystallized phases in the basaltic magmas from which the cumulus minerals precipitated. The high proportions of hornblende in many subduction-related gabbroic plutons and xenolith suites compared with its paucity in basaltic or basaltic andesitic lavas may be partially explained by multistage plutonic crystallization histories involving reaction and migration of evolved melt ± aqueous fluids that either could have originated within the cumulus pile of the mafic intrusion or were derived externally, from broadly contemporaneous felsic magmas.
KEY WORDS: Tatara–San Pedro; gabbroic xenolith; hornblende; phlogopite; melt migration
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