GEOCHEMISTRY, AGE, AND DEFORMATION OF EOCENE SLAB WINDOW INTRUSIONS IN A REMNANT FOREARC BASIN, MATANUSKA VALLEY, SOUTHERN ALASKA

The Matanuska Valley (MV) is a remnant Late Mesozoic-Paleocene forearc basin in southern Alaska. Eocene intrusions along the MV include gabbro sills (10’s-100’s m thick), rhyodacite intrusions (100’s m to km across), and basaltic andesite dikes (meters to 10’s m thick) that cross-cut the rhyodacite intrusions. The MV intrusions are by far the most geochemically depleted igneous rocks yet documented in southern Alaska and represent magmatism from a mid-ocean ridge (MORB)-like source.

The gabbro sills are tholeiitic, contain olivine and clinopyroxene, and are relatively primitive (Cr up to 570; Mg# up to 0.59). They are strongly depleted in light rare earth elements (La/Yb = 0.3 to 1.4) and exhibit normalized trace element patterns similar to Pacific N-MORB (with exception of some enrichment in fluid-mobile elements Cs, Ba, and Pb). Two new U-Pb dates on the sills yield ages of 45.4 +/- 0.5 and 46.0 +/- 1.2 Ma. The rhyodacite intrusions are younger than the gabbro sills (U-Pb date of 39.5 +/- 0.4 Ma) and are relatively depleted (La/Yb = 1.9 to 4.3) but exhibit some large ion lithophile element (LILE) enrichment. Also present is an adakite-like felsic intrusion (HREE depletion, Sr/Y = 110). The basaltic andesite dikes, similar to the gabbros, are strongly depleted in LREE (La/Yb = 0.6 to 1.3) and also have MORB-like geochemical trends, although the dikes are more evolved (absence of olivine, Cr up to 350, Mg# up to 0.38).

The MV intrusions are consistent with magmatism above a slab window that formed during Late Paleocene spreading ridge subduction beneath southern Alaska. Parental magmas originated from suboceanic depleted mantle and were intruded into the deformed remnant forearc to form the gabbro sills. Magmas evolved by AFC processes to form the rhyodacite intrusions, with minimal contamination by LILE and fluid-mobile elements from forearc crustal rocks. The basaltic andesite dikes were the last phase and indicate a renewed pulse of geochemically depleted mafic magmatism. The dikes have a consistent NW-SE trend. In addition, intrusions along the MV are cross-cut by NW-SE normal faults and NNW-SSE left-lateral faults. These data indicate NE-SW extension during the final phase of MV magmatism and are consistent with Late Eocene right-lateral simple shear along the Castle Mountain Fault system that extends through the MV.