PETROGENESIS OF TONALITES BASED ON 3-D ARCHITECTURE OF BELL ISLAND PLUTON, SE ALASKA

The mid-Cretaceous Bell Island pluton (~1300 km2) in SE Alaska provides a natural laboratory for investigating geochemical and isotopic signatures of tonalitic magmas as a function of emplacement depth. Based on contact aureole geobarometry, the pluton provides a 3-D profile across 6-8 km of structural relief. The pluton was emplaced at mid crustal levels (10.5 to 7 kb) and may have originated at depths in excess of 15 kb. Across the entire structural profile we recognize three distinct igneous assemblages: (1) Gar+Hbl+Plag+Bt+Q+Ep+Sp+Zr+Al, (2) Hbl+Plag+Bt+Q+Ep+Sp+Zr+Al+-Ksp, (3) Plag+Qtz porphyry with Hbl+Bt+Al+Sp+Zr. Although assemblage (2) is the dominant rock type, garnet bearing tonalites occur along the eastern margin, and an areally minor porphyritic facies occurs in the central part of the pluton. Geochemical data from the garnet free and garnet present regions show a restricted range in bulk composition with limited evidence of fractionation between mineralogically distinct facies or within a single facies. For example, REE contents show limited changes in (La/Yb)N ratios (6 to7) over a limited silica range (59-61 wt.%), and suggest minimal compositional diversity across the structural gradient. Similarly, incompatible element ratios (e.g. Th/Tb and Th/Ta) are co-linear and suggest derivation of the liquid from similar sources. However, initial strontium isotopic ratios are measurably distinct between a core facies (>0.7050 and elongated east -west) and the margins (<0.7049) of the complex, and are independent of mineral assemblages, i.e.+-garnet. The limited compositional diversity coupled with an isotopic zonation suggests the emplacement of at least two distinct liquids during the construction of the complex. We also recognize the requirement of in-situ crystallization without significant mineral fractionation, and provide field based documentation that the dominantly melt rich magmas straddled the garnet phase boundary, leading to crystallization of garnet at the ~10Kb end of the pluton. The Bell Island pluton thus provides estimates of liquid compositions associated with melting of a dominantly basaltic source with minimal crustal interaction during the primitive stages of arc development.