MAGMA RESERVOIR CHARACTERIZATION AND TECTONIC TRANSITIONS DURING CONSTRUCTION OF THE COAST MOUNTAIN BATHOLITH, SE ALASKA, BRITISH COLUMBIA

The Coast Mountains of SE Alaska and British Columbia record a history of convergence, terrane accretion, and arc magmatism since the mid-Cretaceous. We use trace element and isotope geochemistry to evaluate changes in magma reservoir characteristics and to compare tectonic processes that formed two successive plutonic belts in the Coast Mountains. The two belts are separated by the Coast Shear Zone, a subvertical 5 km wide structure that offsets the Moho. On the west, 100-88 Ma plutons intrude a thick-skinned thrust belt that records shortening during accretion of the Alexander-Wrangellia terranes. These plutons exhibit positive εNd values (+5) that suggest little continental crust contributed to them. This supports previous interpretations that most of the western plutons were produced from hydrous mantle within an oceanic subduction zone setting with minimal crustal melting. On the east, 82-57 Ma plutons show an increase in the range of εNd values (-7 to +4) through time, suggesting thick continental crust increasingly contributed material as the arc migrated eastward following a brief late Cretaceous magmatic hiatus. This pattern of increasing crustal involvement in magma generation culminated in the Eocene (55-52 Ma) when shortening ceased, extension was widespread, and crustal melting significantly modified mantle-derived melts. Although most of these rocks show a strong crustal affinity, the youngest Eocene rocks have higher ¹⁴³Nd/¹⁴⁴Nd values and are less radiogenic in their ⁸⁷Sr/⁸⁶Sr composition, indicating a shift towards mantle-dominated sources. This trend is consistent with crustal thinning and high heat flow during extension. Despite the differences in the age and composition of the eastern and western belts, Sr and Nd isotope ratios from Miocene–Recent rocks in both belts show increasing ¹⁴³Nd/¹⁴⁴Nd and decreasing ⁸⁷Sr/⁸⁶Sr with time, indicating a shift toward an increasingly depleted mantle source. This pattern conforms to a shift in tectonic setting as the plate margin evolved to a regime of intraplate extension. This study shows that the changes in the geochemistry of Cretaceous–Miocene igneous rocks closely track the tectonic evolution of the Coast Mountains from collision and accretion of a primitive arc terrane, to crustal thickening, orogenic collapse, and intraplate extension.