Geochemistry of Mafic Enclaves and Host Granitoids from the Chilliwack Batholith, Washington: Chemical Exchange Processes between Coexisting Mafic and Felsic Magmas and Implications for the Interpretation of Enclave Chemical Traits

Mafic enclaves from three plutons in the Chilliwack batholith have been compared with contemporaneous mafic stocks in order to determine (1) the processes by which mafic and felsic magmas hybridize in the plutonic environment and (2) whether analysis of early-formed enclave minerals, particularly apatite, can provide a means of seeing through hybridization effects and deciphering the original trace element characteristics of enclave magmas. Whole rock and mineral chemistry data reveal a two-stage history of enclave hybridization. Stage 1, a diffusive exchange of trace elements between coexisting liquids, produced enclaves with distinctive concave-upward rare earth element patterns that parallel those of the host granitoids but had minimal impact on the major elements, whose transfer is rate limited by the slow diffusion of Si. This stage probably occurred at a mafic-felsic interface in a stratified magmatic system. Stage 2, a partial reequilibration of enclave minerals with a differentiated and hybridized interstitial melt, occurred after the enclaves were entrained in the host and partially crystallized. This process caused enclave and host minerals (amphibole, biotite, apatite) from each pluton to have similar major oxide chemistries but did not reequilibrate the trace elements. As a result of these hybridization processes, even early-formed apatite crystals do not preserve information about the original trace element characteristics of enclave magmas in this case. However, the results of this study illustrate the potential of using enclave chemistry to constrain the nature and timing of mafic magma inputs into felsic magma bodies.