Journal of Petrology, Volume 40, Issue 6: June 1999.

Stable Isotopic Evidence for Limited Fluid Flow through Dolomitic Marble in the Adamello Contact Aureole, Cima Uzza, Italy

MARTHA L. GERDES¹, LUKAS P. BAUMGARTNER² AND JOHN W. VALLEY

DEPARTMENT OF GEOLOGY AND GEOPHYSICS, UNIVERSITY OF WISCONSIN, MADISON, WI 53706, USA

At Cima Uzza, the tonalitic Adamello batholith and volumetrically minor gabbroic stocks and dikes were emplaced into a thick sequence of silica-poor dolostone. Stable isotopic data from dolomite in dolomitic marble in the contact aureole [[delta]¹⁸O = 29·7 ± 1·3%° (1[sgr]), [delta]¹³C = 2·2 ± 1·3%°] and from equivalent dolostone outside the aureole ([delta]¹⁸O = 28·8 ± 1·1%°, [delta]¹³C = 2·5 ± 0·2%°) are indistinguishable. These high and uniform [delta]¹⁸O values in the dominant lithology suggest metamorphism in a closed system, as does a closed-system reaction model that is consistent with observed isograd development. Localized fluid infiltration is reflected by low [delta]¹⁸O (13·2-18·5%°) and [delta]¹³C (-2·6 to -6·7%°) calcite values in siliceous marble xenoliths entrained in small gabbroic stocks at the tonalite margin. Such siliceous marble probably formed by Si and Al metasomatism of dolomitic marble during infiltration of magmatic fluids because no equivalent lithology occurs outside the aureole. Brucite marble, which occurs in xenoliths and as reaction rims around mafic dikes, displays a wide range of [delta]¹⁸O and [delta]¹³C values which are generally intermediate between those for siliceous marble and dolomitic marble. This range of isotopic values in brucite marble is consistent with a hydrothermal formation from a silica-poor dolostone protolith. In contrast, high [delta]¹⁸O values from centimeter-scale calc-silicate nodules and layers, which are irregularly distributed in the dolomitic contact aureole, indicate a sedimentary origin for such rocks. Hydrothermal alteration of dolomitic marble at Cima Uzza is limited to small areas near gabbroic stocks and dikes, whereas negligible country rock alteration is associated with the larger tonalite. The limited infiltration of magmatic fluids possibly reflects a low volatile content in the tonalitic magma. In addition, low permeability in the thick sequence of dolomitic marble could explain the small amount of fluid flow relative to other carbonate contact aureoles.

Keywords: Adamello;contact metamorphism;dolomitic marble; fluid flow; stable isotopes

Pages 853-872