Isotope disequilibrium during anatexis: a case study of contact melting, Sierra Nevada, California

https://doi.org/10.1016/S0012-821X(97)00031-9Get rights and content

Abstract

The geochemical consequences of contact melting were investigated in the Sierra Nevada Batholith, where trachyandesite magmas have intruded and melted the host granitic rocks. Partial melting of the granite varies from 10–20% at contact regions, to 50–70% in a granite block within the trachyandesite. Thermal modelling suggests that the temperature of the granite block exceeded its solidus within 3 months of trachyandesite intrusion, reached a maximum of ∼ 1000°C after ca. 1.5 yr, and remained above 500°C for ca. 40 yr.

Sr, Nd and Pb isotope data of granitic melts record marked Sr isotope, and to a lesser extent Pb isotope, disequilibrium both within different glass fractions and between glasses and their source. Rb/Sr isochron calculations on residual mineral-glass pairs fail to yield the age of melting obtained by40Ar-39Ar dating (11.9 Ma). Sr and Pb isotope data of glasses establish that major rock-forming minerals of the granite had not attained isotope equilibrium before the onset of melting. Consequently, the isotopic composition of the successive liquid fractions originating from granite melting depended on the relative contribution of each mineral to the elemental budget of the melts.

The extrapolation of these results to the petrogenesis of crustal melts and more generally to studies of crustal evolution, suggests that isotope disequilibrium during anatexis and high-grade metamorphism may be a common characteristic of many geologic settings. If future studies substantiate these results, some geochemical models of crustal melting and assimilation may need revision. In addition, marked Sr isotope disequilibrium in the residual mineral phases within the partially melted granite raises concerns about the reliability of radiometric dating in metamorphic terranes.

Referencess (45)

  • KroghT.E.

    Improved accuracy of U-Pb zircon ages by the creation of more concordant systems using an air abrasion technique

    Geochim. Cosmochim. Acta

    (1982)
  • ZhangL.S. et al.

    Inherited Pb components in magmatic titanite and their consequences for the interpretation of U-Pb ages

    Earth Planet. Sci. Lett.

    (1996)
  • ClemensJ.D. et al.

    Constraints on melting and magma production in the crust

    Earth Planet. Sci. Lett.

    (1987)
  • HoganJ.P. et al.

    The effect of accessory minerals on the redistribution of lead isotopes during crustal anatexis: A model

    Geochim. Cosmochim. Acta

    (1991)
  • PetfordN. et al.

    The ascent of felsic magmas in dykes

    Lithos

    (1994)
  • ArziA.A.

    Critical phenomena in the rheology of parthy melted rocks

    Tectonophysics

    (1978)
  • DePaoloD.J.

    Trace element and isotopic effects of combined wallrock assimilation and fractional crystallisation

    Earth Planet. Sci. Lett.

    (1981)
  • ThöniM. et al.

    Some new aspects of dating eclogites in orogenic belts: Sm-Nd, Rb-Sr, and Pb-Pb isotopic results from the Austroalpine Saualpe and Koralpe type-locality (Carinthia/Styria, southeastern Austria)

    Geochim. Cosmochim. Acta

    (1992)
  • BradyJ.B.

    Diffusion data for silicate minerals, glasses, and liquids

  • CherniackD.J. et al.

    A study of strontium diffusion in plagioclase using Rutherford backscattering spectroscopy

    Geochim. Cosmochim. Acta

    (1994)
  • HammoudaT. et al.

    Isotopic equilibration during partial melting: an experimental test of the behaviour of Sr

    Earth Planet. Sci. Lett.

    (1996)
  • KneselK.M. et al.

    Isotopic disequilibrium during melting of granite and implications for crustal contamination of magmas

    Geology

    (1996)
  • Cited by (55)

    • The role of melting on the geochemical evolution and isotopic variability of an anatectic complex in the Iberian Variscides

      2020, Lithos
      Citation Excerpt :

      Indeed, residue/melt isotopic disequilibrium can take place due to a) insufficient temperature to reach the closure temperature of some of the accessory phases, with which P and/or D elements are compatible, resulting in a significant fractionation (up to >20 εHf units; Tang et al. 2014); and b) chemical diffusivity being sluggish compared with the time frame necessary for melt extraction from the residue (Ayres and Harris 1997; Farina and Stevens 2011; Zeng et al., 2005a, b; Tang et al. 2014; Wolf et al. 2019). These factors suggest that isotopic disequilibrium during anatexis is ubiquitous (e.g. Tommasini and Davies 1997). However, in some cases, even for situations specifically investigated for isotopic disequilibrium, no significant evidence was found (e.g. Wolf et al. 2019).

    • Geochemistry of high-pressure to ultrahigh-pressure granitic melts produced by decompressional melting of deeply subducted continental crust in the Sulu orogen, east-central China

      2020, Geochimica et Cosmochimica Acta
      Citation Excerpt :

      To further constrain this issue, it needs to decipher the behavior of accessory minerals during the anatectic processes. Isotope disequilibria between residual, metamorphic, peritectic and anatectic minerals during crustal anatexis has been documented in many studies, especially for radiogenic isotope systems like Rb–Sr and Sm–Nd isotope systems (e.g., Barbero et al., 1995; Ayres and Harris, 1997; Tommasini and Davies, 1997; Zeng et al., 2005a, 2005b; Gao et al., 2017). Unlike stable isotopes, the radiogenic isotopes are not only controlled by element distribution between melt and mineral, but also determined by heterogeneous isotope compositions of protolith minerals that underwent incongruent melting.

    View all citing articles on Scopus
    View full text