ABSTRACT

1. Volcanic and volcaniclastic rocks Upper crust 5-~7 km Intermediate-felsic plutonic rocks Upper-mid crust Mafic plutonic rocks 60.4 wt % SiO2 71.2 wt % SiO2 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 1 2 Seismic stratigraphy of modern arcs - more similarities than differences? 3 The crustal sections ABSTRACT 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 In exhumed arc sections worldwide, the upper mid-crust is composed dominantly of hornblende-bearing tonalite, quartz diorite, diorite, and gabbro (49-76 wt.% SiO2) whose compositions would correspond to an in situ Vp in the range of 6.0-6.3 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.% SiO2) 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 6.0-6.5 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 SiO2. 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.% SiO2) form via fractional crystallization from basalt to dacite. Type II tonalite/diorite (56-74 wt.% SiO2) 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). Crawford et al. (2003) Suyehiro et al. (1996) Questionable stratigraphy…… (bottom and top may actually be two sections based on ages) Iwasaki et al. (2001) Nakanishi et al. (2007) Shillington et al (2004) Parsons et al. (1998) • 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? • 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 These sections have been color-coded to their expected seismic velocities based on lithology (velocities calculated using formulation of Behn and Kelemen, 2003) • 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 SiO2 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 SiO2 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. 7 4 5 6 Mixing of the crustal melts with mantle-derived magmas can produce the Type 2 trend of decreasing REE with increasing SiO2 Produce a Si-rich, Type 2 magma by crustal melting Common processes observed in the mid crust of exposed arc sections 1. Magma mingling (and mixing) is pervasive in the middle crust in all arc sections. Extreme heterogeneity at all scales. Example 1: Talkeetna arc (see Johnsen et al poster) Example: Talkeetna arc (see Johnsen et al poster) 44.8 wt% SiO2 Physical mixing of crystals Residual Cpx 10% Opx 5% Plag 45% Amphibole 35% Magnetite 5% ~20% fractional melting 71.2 wt% SiO2 Fine-scale mingling Break-up and mingling of basaltic sill person for scale Mingling on the large scale Close to homogenization? 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 Western Talkeetna Arc Bonanza Arc Type 1 Kohistan arc migmatite 65.8 wt % SiO2 Example 2: Bonanza arc (field exposure of leucosome & melanosome) A Cascade volcano with a similar pattern: Glacier Peak volcano Increasing SiO2 Type 1 In the Talkeetna Arc, the older plutonic rocks are Type 1. The youngest rocks are Type 2 (thicker, more mature crust?) Bonanza arc migmatite Increasing SiO2 Concluding hypothesis: Mafic lavas that backmix with Type 2) Type 1: Increasing REE with increasing SiO2. Process is fractionation (young thin arcs?) Type 2: Decreasing REE with increasing SiO2. 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 Type 2 Type 2 Decreasing SiO2 >54 wt.% SiO2 (diorite/tonalite) REE normalized to C1 chondrite (Sun & McDonough, 1989) Type 2 (dacite) Type 2 also have high Sr/Y with