Contributions to Mineralogy and Petrology

, Volume 113, Issue 3, pp 333–351 | Cite as

Petrology of the Chilliwack batholith, North Cascades, Washington: generation of calc-alkaline granitoids by melting of mafic lower crust with variable water fugacity

  • Jeffrey H. Tepper
  • Bruce K. Nelson
  • George W. Bergantz
  • Anthony J. Irving


Calc-alkaline granitoid rocks of the Oligocene-Pliocene Chilliwack batholith, North Cascades, range from quartz diorites to granites (57–78% SiO2), and are coeval with small gabbroic stocks. Modeling of major element, trace element, and isotopic data for granitoid and mafic rocks suggests that: (1) the granitoids were derived from amphibolitic lower crust having REE (rare-earth-element) and Sr-Nd isotopic characteristics of the exposed gabbros; (2) lithologic diversity among the granitoids is primarily the result of variable water fugacity during melting. The main effect of fH2O variation is to change the relative proportions of plagioclase and amphibole in the residuum. The REE data for intermediate granitoids (quartz diorite-granodiorite; Eu/Eu*=0.84–0.50) are modeled by melting with fH2O<1 kbar, leaving a plagioclase + pyroxene residuum. In contrast, data for leucocratic granitoids (leuco-granodiorites and granites; Eu/Eu* =1.0–0.54) require residual amphibole in the source and are modeled by melting with fH2O=2–3 kbar. Consistent with this model, isotopic data for the granitoids show no systematic variation with rock type (87Sr/86Sri =0.7033–0.7043; εNd(0)=+3.3 to +5.5) and overlap significantly with data for the gabbroic rocks (87Sr/86Sri =0.7034–0.7040; εNd(0)=+3.3 to +6.9). The fH2O variations during melting may reflect additions of H2O to the lower crust from crystallizing basaltic magmas having a range of H2O contents; Chillwack gabbros document the existence of such basalts. One-dimensional conductive heat transfer calculations indicate that underplating of basaltic magmas can provide the heat required for large-scale melting of amphibolitic lower crust, provided that ambient wallrock temperatures exceed 800°C. Based on lithologic and geochemical similarities, this model may be applicable to other Cordilleran batholiths.


Lower Crust Mafic Rock Basaltic Magma Conductive Heat Transfer Quartz Diorite 
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Copyright information

© Springer-Verlag 1993

Authors and Affiliations

  • Jeffrey H. Tepper
    • 1
  • Bruce K. Nelson
    • 1
  • George W. Bergantz
    • 1
  • Anthony J. Irving
    • 1
  1. 1.Department of Geological Sciences, AJ-20University of WashingtonSeattleUSA

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