Variability of Metamorphic P-T-t Paths in Compressional Magmatic Arcs and the Relationship Between Magmatism, Metamorphism, and Crustal Loading: Example from the North Cascades, WA

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Proposed research will provide critical constraints on the nature of loading/high pressure metamorphism in contractional magmatic arcs, and greatly enhance data available for deciphering the tectonic history of the North Cascades. Research will integrate new high precision geochronologic, thermobarometric, and metamorphic phase equilibria data from theWenatchee Block (WB), Cascades Crystalline Core (CCC) of Washington. Published thermobarometric, geochronologic, and structural data for the WB have been used to propose tectonic models; however, the metamorphic data are largely inadequate to resolve some of theimportant problems. For example, tectonic interpretations hinge on whether the entire WB experienced synchronous loading and high-pressure metamorphism, and the timing of high-pressure metamorphism with respect to igneous intrusions in the central WB. Yet, there are few reliable ages for peak metamorphism and few P-T-t path determinations for the central and northern WB. Available data comprise P-T estimates based on mineral rim chemistry, P-T estimates for growth of garnet cores based on zoning, and P estimates for pluton emplacement based on Al in hornblende (Ague & Brandon, 1996). Detailed data are available for the NE margin of the Mt. Stuart batholith (Stowell & Tinkham, 1999; Tinkham & Stowell, 2000a, 2000b; Tinkham in prep. 2). The PI's preliminary quantitative P-T-t paths for metamorphism indicate that regional garnet growth postdates the Mt. Stuart. Geochronologic and petrologic data are compatible with post intrusion metamorphism likely involving a small P increase. Research will integrate garnet Sm-Nd ages and garnet growth conditions north of the Mt. Stuart batholith. These data will be used to develop quantitative P-T-t paths that will complement existing data for the NE edge of the Batholith. Metamorphic P-T-t paths and new U-Pb zircon ages for orthogneisses at Pear Lake and within the Banded Gneiss on Wenatchee Ridge will be used to constrain the heat source for metamorphism. Ages for garnet will be estimated from garnet - rock Sm-Nd isochrons. P and T of initial garnet growth will be estimated from mineral compositions and pseudosections, supplemented with P and T from equilibrium thermobarometry. U-Pb zircon geochronology will provide critical ages for magmatic bodies in the Banded Gneiss and the Pear Lake orthogneiss that may have provided significant heat during metamorphism. Geochronologicand thermobarometric data will be interpreted in the context of structural data to place constraints on the conditions and timing of deformation. Integration of these data will help define the WB metamorphic history, and elucidate how metamorphism and subsequent cooling relates to magmatic arc plutonism and deformation. It will also document how detailed garnet thermobarometric and chronologic techniques can be used to extract complex tectonic histories. Tectonic models describing metamorphic/orogenic processes in a magmatic arc can only be critically evaluated after details of metamorphic P-T-t paths have been described, and these paths spatially and temporally related to plutonism and deformation. The results of this study will help to distinguish between metamorphism resulting from advective heat transfer by plutons and conductive heat transfer after crustal thickening.