DECIPHERING MAFIC ENCLAVES – WHAT CAN THEY TELL US ABOUT THE TIMING AND NATURE OF BASALTIC INPUTS INTO GRANITOID MAGMA SYSTEMS?

Arc batholiths commonly contain mafic enclaves that represent globules of basaltic magma incorporated into, and hybridized with, a host granitoid that was still partially molten. We have analyzed enclave and host rock samples from three epizonal plutons in the 36 Ma – 2 Ma Chilliwack batholith (North Cascades, WA) in order to understand the nature of the hybridization processes. Based on whole rock and mineral chemistry we recognize two types of hybridization, each associated with a different style of mafic-felsic magma interaction. Type 1, a diffusive exchange of trace elements (particularly HFSE) between coexisting mafic and felsic liquids, produced enclaves with distinctive concave-upward REE patterns that parallel those of the host granitoids. This process had minimal impact on major elements (whose diffusion rate is limited by slow Si diffusion) and resulted in a decoupling of major element abundances (and mode) from trace element abundances. Type 1 hybridization occurred prior to significant enclave crystallization, probably at the mafic-felsic interface of a stratified magmatic system. Type 2 exchange, a partial re-equilibration of enclave minerals with hybridized and differentiated interstitial melt, occurred after enclave dispersal within the host granitoid. This process resulted in enclave and host minerals (apatite, biotite, amphibole) having similar major oxide compositions, but did not fully re-equilibrate the trace elements. The results of this study suggest the timing and style of mafic-felsic magma interactions can be estimated based on the type and extent of enclave-host hybridization. Long-lived stratified magmatic systems, likely at lower- to mid-crustal depths, will lead to enclaves with Type 1 hybridization, whereas more turbulent basalt injections will be associated with Type 2 hybridization. In both cases the extent of hybridization will depend upon the temperature and cooling rate (depth) of the host granitoid. Better knowledge of intracrystalline diffusion rates for enclave minerals would permit a more quantitative estimation of the time scales of mafic-felsic magma interactions and the longevity of granitic magma systems.