ABSTRACT 1 The felsic plutonic core of the western Talkeetna island arc crustal section, Alaska: Its formation and implications for crustal growth along continental margins Michael Johnsen, Department of Geology, WWU, Bellingham, WA Susan DeBari, Department of Geology, WWU, Bellingham, WA Matthew Rioux, Department of Earth Science, UCSB, Santa Barbara, CA Paper No The accretion of island arc crust is believed to be a major contributor to the growth of continents. A particularly important question in arc evolution is the origin of felsic plutonic rocks in island arcs. Felsic rocks represent the nucleus of continents, yet there is no clear consensus on how these rocks originate. The Jurassic Talkeetna island arc in south-central Alaska is an exhumed and titled arc section where middle and upper crust lithologies are now exposed at the surface, providing us with the rare opportunity to directly model processes responsible for the formation of the felsic core of an island arc. Studies in the eastern Talkeetna arc (Talkeetna and Chugach Mountains) indicate that the arc crustal section comprises a calc-alkaline suite where all lithologies display consistent major and trace element trends, each group of rocks forming by processes of fractional crystallization and accumulation. In contrast, two chemically distinct groups can be defined in the western Talkeetna arc (Lower Cook Inlet region). Compositional group I (n = 75; wt % SiO 2 ) shares many of the same trends as rocks from the eastern arc: relatively low K at a given SiO 2 and flat REE patterns ([La/Yb] N 5; average 7.5), where REE abundances decrease with increasing SiO 2. The most silicic members of this group exhibit concave-up patterns of HREE depletion. Of particular interest is the formation of the voluminous felsic core of the western arc, exposed over more than 4800 km 2 of the region. Major element and REE modeling indicate the majority of these rocks formed through fractional crystallization and accumulation (compositional group I). In addition, modeling results for compositional group II rocks indicate that the observed range of intermediate-felsic compositions can be produced by variable mixing of an andesitic parent liquid (presumably formed by fractional crystallization) with a felsic end-member magma (formed by ~15-25% partial melting of mafic crust). This study provides important insights into the range of processes responsible for the formation of the felsic core of island arcs and potentially the nucleus of continental crust. GEOLOGIC SETTING MINERALOGY AND PETROGRAPHY RESEARCH QUESTIONS EXPOSED ARC CRUSTAL SECTION TWO COMPOSITIONAL GROUPS WHOLE-ROCK CHEMISTRY The Talkeetna island arc is an exhumed and tilted section where deep levels of arc lower crust as well as middle and upper crust arc lithologies are now exposed at the surface. Lower Cook Inlet region study area and other Talkeetna island arc plutonic and volcanic exposures. Western arc exposures from Detterman & Reed (1980) and Riehle et al. (1993). Terranes from Barker et al. (1994), Siberling et al. (1994), and Wilson et al. (1998). 3 2 Research presented here addresses three main questions yet to be resolved for the intermediate-felsic core of the western Talkeetna arc: Along strike compositional variation: is there significant geochemical variability along arc strike (compared to the eastern arc)? Processes of magma differentiation: what magmatic processes are responsible for the formation of the exposed intermediate- felsic plutonic and volcanic rocks in this region? Continental Growth: what implications do the chemical trends and magmatic formation of western arc rocks have on models for the growth of continental crust at convergent margins? 6 4 The western arc section is composed predominantly of intermediate-felsic plutonic rocks (diorites and tonalites), minor amounts of gabbroic rock, and a significant component of overlying Talkeetna Formation volcanic and volcaniclastic rocks. The western arc is a small sub-section of the entire arc crustal section exposed in the east. The Talkeetna magmatic arc was created by northward dipping subduction (present-day coordinates) of the oceanic Farallon plate beneath the Peninsular terrane (part of the WCT) (Plafker et al., 1994; Clift et al., 2005). Plutonic and volcanic exposures in the western Talkeetna island arc and the locations of rock samples used in this study. Western arc exposures are from Detterman & Reed (1980), Riehle et al. (1993), and Reed et al. (1982). Volcanic and volcaniclastic rocks Intermediate- felsic plutonic rocks Mafic plutonic rocks Upper- mid crust Upper crust ~7 km Schematic section showing rocks exposed in the western arc (Lower Cook Inlet region) 5 Samples include fine-grained massive volcanic rocks and rounded volcanic clasts within volcaniclastic sediment. Predominantly porphyritic, samples range from basaltic-andesite to rhyolite. Phenocrysts are mostly subhedral plagioclase and cpx (when present). Plagioclase is commonly concentrically zoned and intergrown, many contain seive textures. Amphibole phenocrysts are largely broken up and surrounded by reaction rims of Fe-Ti oxides + pyroxene. Trace minerals: quartz. Alteration minerals: chlorite. Subhedral to anhedral granular texture. Samples include diorite, quartz diorite, granodiorite, tonalite, and trondhjemite. Subhedral to anhedral plagioclase, commonly corroded and concentrically zoned. Poikilitic anhedral to subhedral amphibole containing plagioclase and/or biotite. Consertal and interstitial anhedral quartz. Some Mg-rich samples from the suite contain trace amounts of clinopyroxene and/or orthopyroxene (Mg-no ). Trace minerals: K-felspar, apatite, titanite, and zircon. Alteration minerals: chlorite, epidote, calcite, and pyrite. Anhedral granular texture dominant. Samples are predominately gabbroic. Anhedral to subhedral plagioclase, faint concentric zoning and corroded cores common. Anhedral poikilitic amphibole containing plagioclase. Clinopyroxene and orthopyroxene commonly cored in amphibole. Trace minerals: apatite and titanite. Alteration minerals: chlorite. Petrographic Characteristics Volcanic and volcaniclastic rocks; Talkeetna Formation ( wt % SiO 2 ) Intermediate to felsic plutonic rocks ( wt % SiO 2 ) Mafic plutonic rocks ( wt % SiO 2 ) 40-70% Matrix (plag dominated) Phenocrysts: 50-95% Plagioclase 0-20% Cpx (Mg-no ) 0-5% Opx (Mg-no ) 0-5% Amphibole (Mg-no ) 0-5% Fe-Ti oxide 40-70% Plagioclase (An 51-75) 10-30% Amphibole (Mg-no ) 5-20% Quartz 5-15% Biotite 1-5% Fe-Ti oxide 45-65% Plagioclase (An 73-92) 25-45% Amphibole (Mg-no ) 0-15% Cpx (Mg-no ) 0-10% Opx (Mg-no ) 0-5% Biotite 2-5% Fe-Ti oxide Zoned cpx phenocryst (volcanic sample 5713J05) Cpx cores in amphibole (mafic sample 2729M10A) Typical magnesio-hornblende (intermediate-felsic sample 2727M01) LithologyMineral Modes MAJOR ELEMENTS RARE EARTH ELEMENTS The arc crustal section studied in the east comprises a calc-alkaline suite where all lithologies display consistent major and trace element trends. In contrast, two compositionally distinct groups can be defined in the western arc using geochemistry of plutonic and volcanic rocks. Compositionally similar to rocks from the eastern arc: relatively low K at a given SiO 2 and flat REE patterns ([La/Yb] N < 5; average 2.6) where REE abundances increase with increasing SiO 2. Rocks cover a wide range of compositions (43.0 – 76.1 wt % SiO 2 ), including mafic and intermediate-felsic plutonic as well as volcanic samples. Show an entirely different trend of LREE enrichment and HREE depletion ([La/Yb] N > 5; average 7.3) where REE abundances decrease with increasing SiO 2. Samples exhibit higher Al 2 O 3, Sr, and Ba, and lower TiO 2, FeO, and Y compared to group I at the same silica contents. Define a more restricted range of compositions (56.1 – 73.6 wt % SiO 2 ), including intermediate- felsic plutonic and volcanic samples. 7 Compositional Group I (75 samples) Compositional Group II (21 samples) Compositional Group I rocks show trends similar to eastern arc samples. Compositional Group II samples show an increase in [La/Yb] N and Sr/Y with increasing SiO 2. Western ArcEastern Arc Compositional Group I REE patterns are relatively flat like the eastern arc, while Compositional Group II samples exhibit trends of concave-up HREE depletion. Int-Felsic Plutonic avg. [La/Yb] N = 4.0 Volcanic avg. [La/Yb] N = 4.6 avg. [La/Yb] N = 2.1 Mafic Plutonic< 54 wt % SiO 2 avg. [La/Yb] N = 2.0 avg. [La/Yb] N = 1.7 > 54 wt % SiO 2 Compositional Group I (green) Compositional Group II (blue) REE normalized to C1 chondrite (Sun & McDonough, 1989) Whole-rock major element variation diagrams for western arc rocks with published data from the eastern arc (Clift et al., 2005; Greene et al., 2006). 8 MAGMATIC PROCESSES Magma mingling of intermediate and felsic compositions, north shore Lake Grosvenor (5710J05) 12.6 % 700 km % 4,860 km % 24 km 2 Volcanic Int-felsic Mafic Unit Exposed Area Total = 5,584 km 2 What magmatic processes are responsible for the formation of intermediate-felsic plutonic and volcanic rocks exposed in the western arc? Were these rocks formed by simple fractional crystallization or did assimilation of existing crust and/or magma mixing play an important role? Greene et al. (2006) modeled fractional crystallization from basalt to andesite compositions in the eastern arc. The research presented here attempts to model the formation of more felsic compositions starting with an andesitic composition. Fractional Crystallization Mixing Intermediate and Felsic End-member Magmas Partial Melting of Mafic Compositions Compositional Group I Compositional Group II ~20-30% xtal cpx, opx, plag, hbl, mag ~15-25% xtal plag, hbl, mag, ilm 58.2 wt % SiO Increasing REE abundances with increasing SiO 2 (proxy for fractionation) and REE pattern shape are matched well by modeling fractional crystallization from andesitic to dacitic compositions wt % SiO wt % SiO wt % SiO wt % SiO wt % SiO 2 melting assemblage cpx, opx, plag, hbl, mag Simple fractional crystallization (FC) and assimilation fractional crystallization (EC- AFC developed by Bohrson & Spera, 2001) do not explain REE trends for compositional group II rocks, specifically decreasing REE abundances with increasing SiO 2 (fractionation) and HREE depletion. So what processes formed these rocks? Several intermediate and felsic end- member pairs from the group II suite seem to predict the observed range of observed samples. Presumably, the intermediate liquid formed via fractional crystallization, but how did the felsic liquid form? Several gabbroic samples collected from the western arc can be used to produce felsic liquids. On average, ~15-20% melting of gabbroic compositions can produce the observed felsic, HREE- depleted samples from group II. Age Correlation 9 ARC SECTION DEVELOPMENT Arc Maturation (compositional group II) Younger plutons (~ Ma) intrude into mature, thicker arc crust Conductive heating of gabbroic lower crust produces felsic partial melts Mixing of felsic and intermediate magmas produces the observed range of compositions Early Stages (compositional group I) Arc volcanism (starting ~198 Ma) Older plutons (~ Ma) emplaced in relatively thin, immature arc crust Fractional crystallization and accumulation dominant Calculated concentration (Cc) Measured (observed) concentration (Cm) Cc/Cm = 1 is a perfect match Group I rocks are older (~185 – 175 Ma) Group II rocks are younger (~170 – 160 Ma) How does western arc chemistry, petrologic modeling, and age correlation relate to the development of the arc crustal section?  Fractional crystallization, partial melting, and magma mixing may all have contributed to the formation of the western Talkeetna island arc crustal section.  Both compositional groups are components of the buoyant intermediate-felsic nucleus of the western arc that accreted to the continental margin.