ABSTRACT 1 Petrologic processes that generate the intermediate to felsic plutonic core of island arcs Susan DeBari & Michael Johnsen, Department of Geology, Western Washington University, Bellingham, WA In exhumed arc sections worldwide, the upper mid-crust is composed dominantly of hornblende-bearing tonalite, quartz diorite, diorite, and gabbro (49-76 wt.% SiO 2 ) whose compositions would correspond to an in situ Vp in the range of km/s. This is in contrast to a more mafic, cumulate lower crust composed dominantly of two-pyroxene gabbro (±hornblende, ±garnet) and pyroxenite, (43-52 wt.% SiO 2 ) whose compositions would correspond to an in situ Vp ~7.0 km/s. This grossly simplified crustal structure is surprisingly similar to many modern arcs whose seismic velocity structures have been determined (IBM, Tonga, Kurile, Aleutians, North Honshu, Cascades). In all of these modern arcs, an upper mid-crust with Vp km/s is present, corresponding to velocities calculated for exhumed arc upper mid-crust lithologies. If we presume that modern arcs and exhumed arcs all contain upper mid-crust with intermediate to felsic plutonic rocks (an unsubductable nucleus), we must be able to model how these rocks are generated. In general, we have discerned two chemically distinct groups of tonalite/diorite in the exhumed arc sections. The first compositional group (Type I) typically has flat to slightly LREE enriched rare earth element patterns where REE abundances increase with increasing SiO2. The second compositional group (Type II) shows trends of LREE enrichment and HREE depletion, where both LREE and HREE abundances decrease with increasing SiO 2. They are also depleted in Y and enriched in Sr. The more felsic members of this group generally exhibit concave-up patterns of HREE depletion. Most exhumed arcs show one or the other of these trends, but some, including Talkeetna, show both, but at different times in the arc’s history. In the Talkeetna arc, least squares calculations and REE Rayleigh fractionation modeling indicate that Type I tonalite/diorite (55-76 wt.% SiO 2 ) form via fractional crystallization from basalt to dacite. Type II tonalite/diorite (56-74 wt.% SiO 2 ) must be produced by more complicated means that involve some component of cannibalization of lower crust, either by partial melting, or by assimilation. Type II tonalites in the Talkeetna arc can be effectively modeled as a result of magma mixing between an andesitic parental liquid (presumably formed by fractional crystallization) and felsic partial melts of hornblende-bearing mafic rock (amphibolite, hornblende gabbro cumulates). In the Talkeentna arc, these Type II rocks post-date the Type I rocks, and were formed after the arc had matured and (presumably) thickened. These mechanisms provide a testable hypothesis for modern arcs. If the arcs are relatively young and thin, then tonalite/diorite should have geochemical characteristics of Type I (fractionation only). If the arcs are more mature and thicker, then tonalite/diorite may have geochemical characteristics of Type II (some component of lower crustal melting) REE normalized to C1 chondrite (Sun & McDonough, 1989) Talkeetna arc (Alaska) Jurassic x-section Bonanza arc (Vancouver Island) Jurassic x-section S. Coast Plutonic Cplx (Washington) Cretaceous x-section Kohistan arc (Pakistan) Cretaceous x-section Volcanic and volcaniclastic rocks Intermediate-felsic plutonic rocks Mafic plutonic rocks Upper- mid crust Upper crust 5-~7 km Intermediate plutonic layer No obvious exposure of crustal melting No oceanic basement Mafic bulk composition Intermediate plutonic layer Crustal melting Older oceanic basement Unknown bulk composition Intermediate plutonic layer Crustal melting Older oceanic basement Unknown bulk composition Intermediate to >30 km (10 kbar)! Crustal melting Older oceanic basement Int. bulk composition to ~30 km depth Questionable stratigraphy…… (bottom and top may actually be two sections based on ages) Common processes observed in the mid crust of exposed arc sections How is the intermediate-felsic mid crust generated? Using geochemical signatures to discern magmatic processes: The intermediate to felsic plutonic rocks of the mid crust display distinct REE patterns that fall into two categories Type 1 - REE abundances increase with increasing SiO 2 content (and decreasing Mg#). This can be easily modeled as fractional crystallization, typically involving cpx + plag + amphibole + Fe-Ti oxide (see Johnsen et al. poster for Talkeetna detailed example). Type 2 - REE abundances decrease with increasing SiO 2 content (and decreasing Mg#). This cannot be modeled as fractional crystallization (even taking into account observed quantities of apatite). This is best modeled as melting of a distinct low LREE source (cumulates?) coupled with mixing. Seismic stratigraphy of modern arcs - more similarities than differences? Modern arcs in a gross sense have similar seismic velocity structures Even Izu Bonin and Aleutians are not that different in the upper crust. What lithologies make up this upper crust? Can we make generalizations based on arc crustal sections? Shillington et al (2004) Suyehiro et al. (1996) Crawford et al. (2003) Iwasaki et al. (2001) Nakanishi et al. (2007) Parsons et al. (1998) The crustal sections These sections have been color-coded to their expected seismic velocities based on lithology (velocities calculated using formulation of Behn and Kelemen, 2003) 1. Magma mingling (and mixing) is pervasive in the middle crust in all arc sections. Extreme heterogeneity at all scales. 2. Crustal melting - snapshots of this process observed in the Bonanza arc and the Kohistan arc in the mid-crust to upper parts of the lower crust Close to homogenization? person for scale Fine-scale mingling Physical mixing of crystals Mingling on the large scale Break-up and mingling of basaltic sill Kohistan arc migmatite Bonanza arc migmatite Western Talkeetna Arc >54 wt.% SiO2 (diorite/tonalite) Type 2 Type 1 Increasing SiO 2 In the Talkeetna Arc, the older plutonic rocks are Type 1. The youngest rocks are Type 2 (thicker, more mature crust?) Mixing of the crustal melts with mantle-derived magmas can produce the Type 2 trend of decreasing REE with increasing SiO 2 Produce a Si-rich, Type 2 magma by crustal melting Concluding hypothesis: Type 1: Increasing REE with increasing SiO 2. Process is fractionation (young thin arcs?) Type 2: Decreasing REE with increasing SiO 2. Process is crustal melting and is often coupled with mixing with mantle- derived magmas and fractionation (older thicker arcs?) Both processes produce the non- subductable nucleus of continental crust Bonanza Arc Type 2 also have high Sr/Y with A Cascade volcano with a similar pattern: Glacier Peak volcano Residual Cpx 10% Opx 5% Plag 45% Amphibole 35% Magnetite 5% ~20% fractional melting Example 1: Talkeetna arc (see Johnsen et al poster) 44.8 wt% SiO wt% SiO 2 Example 2: Bonanza arc (field exposure of leucosome & melanosome) Type 1 Type 2 Increasing SiO 2 Decreasing SiO wt % SiO wt % SiO wt % SiO 2 Example: Talkeetna arc (see Johnsen et al poster) Type 2 (dacite) Mafic lavas that backmix with Type 2)