MAGMA MIXING AND GARNET FORMATION IN THE MIDDLE CRUST AS AN ANALOG FOR DEEP-CRUSTAL MAGMA MIXING IN CONTINENTAL ARCS

The mid-crustal Svarthopen pluton in north-central Norway was emplaced into metasedimentary rocks of the Helgeland Nappe Complex at ~465 Ma. Along with homogeneous gabbro through quartz diorite, tonalite, and megacrystic granite, the pluton is characterized by mingling and mixing of diorite with peraluminous, garnet-bearing biotite granite. Variation of bulk-rock Sr and Nd isotopes suggests simple mixing, however enrichments of Zr and the rare earth elements (REE) suggest that individual magma batches underwent post-mixing fractionation. Hybrid intermediate rocks typically contain Ca-rich garnet, yet Ca-rich garnet is absent in mafic end members, and garnet in felsic end members is Ca-poor. The Ca-rich garnets are interpreted to be igneous and to have formed by mixing of a mafic, Fe-rich end member with a peraluminous felsic end member. The high-Ca-rich garnets have low Zr contents, positive light REE slopes, flat to negative heavy REE slopes, and lower total REE abundances than typical igneous garnet. These data and garnet habits suggest near-solidus garnet stability that followed fractionation of zircon and allanite. Similar hybrid rocks with Ca-rich garnet occur elsewhere in the area and all are adjacent to dioritic plutons.

In many arc settings, petrologic models indicate deep-seated mixing of mafic magmas with peraluminous crust-derived magmas. It is possible that the garnet-bearing hybrid rocks in the Svarthopen pluton are mid-crustal analogs for such mixing. If so, then widespread mixing of evolved, Fe-rich mafic magmas (evolved tholeiite) with peraluminous felsic end members should result in significant precipitation and probable accumulation of magmatic garnet. One consequence of this type of magma hybridization in continental arc settings is development of a ‘garnet signature’ in magmas fractionated from the hybrid. Variations in mixing end members and the proportion of cumulus garnet would then result in isotopic signatures consistent with simple mixing but widely diverse major and trace element variations. A second consequence is formation of garnet-rich cumulates. If hybridization were of sufficient volume and extent in the lower crust, the density contrast between the garnet-rich lower crust and upper mantle would permit crustal delamination.