MANTLE LITHOSPHERE PROPERTIES BENEATH, AND VOLCANISM ALONG, THE COAST BELT—INTERMONTANE BELT BOUNDARY, BRITISH COLUMBIA: INSIGHTS FROM A MANTLE-XENOLITH-BEARING DIKE AT MT. PRESTON

We present a new mantle xenolith locality in the B.C. Cordillera, at Mt. Preston. This site is important because: (1) it has a unique position geographically and geologically along the western margin of the Intermontane Belt, within 40 km of Coast Belt rocks; (2) the site lies between four major volcanic districts in an area with no described volcanics, (3) the dike is young (~19 Ma), indicating sampling of "near-present-day" mantle lithosphere underlying this portion of British Columbia, and (4) the BATHOLITHS geophysical project is imaging the mantle lithosphere in the area of Mt. Preston during their investigation of the Coast Belt.

Extensional forces acting on the lithosphere have resulted in the development of numerous small Cenozoic volcanic centers in the BC Cordillera. Many of these centers feature peridotite xenoliths carried from the mantle lithosphere to the surface; geothermometric studies on individual xenoliths can determine the source depth (i.e. temperature) for each of them. Using the source depths and properties of individual xenoliths (e.g. chemistry and fabric), a “stratigraphic column” of the mantle lithosphere may be constructed, identifying changes with depth and their possible tectonic implications.

The Mt. Preston dike is vesicular, suggesting shallow intrusion, and the wall rocks are partly melted, suggesting sustained magmatic flow. Chemically the dike is a basanite, and appears similar in properties to the Miocene Cheslatta Lake volcanic suite described ~40 km to the east. Magmatic calcite with mantle-derived isotopic signatures is a common accessory phase that lines vesicles and occurs in the groundmass. The dike is rich in crustal xenoliths, some of which do not correlate with the surrounding country rock. Mantle xenoliths are concentrated in a 5 m interval where they comprise 50-80 % of the dike; the dike is narrowest (1 m width) in this interval. These xenoliths are primarily lherzolites, with some dunites and wehrlites; cm-scale banding and weak mineral foliations were observed in several samples. Ongoing work is focused on constraining the source depths, mineral modes and chemistry, density, and fabric of the peridotite xenoliths.