U-PB GEOCHRONOLOGY AND FIELD STUDIES IN THE SOUTHERN COAST MOUNTAINS BATHOLITH, BRITISH COLUMBIA

The Coast Mountains batholith (CMB), in British Columbia is a well-exposed example of a large (> 1700 km in arc-length), relatively youthful, and long-lived (from ca. 220 – 50 Ma) Cordilleran arc. Plutonic and metamorphic rocks of the CMB comprise a northwest-trending belt that straddles and obscures the boundary between two large Paleozoic-early Mesozoic composite terranes. The southern CMB is an ideal location for investigating the influence of upper plate tectonism on subduction-related magmatism through an integrated geochronological, geochemical, structural and petrologic approach.

Mapping and sampling of plutonic bodies in Lochbourough and Bute Inlets in June 2014 confirm that the area is dominated by large plutonic bodies with subordinate metasedimentary and metavolcanic pendants, as shown by Roddick and Woodsworth (2007). Compositions of plutonic bodies range from granitic to dioritic. Plutons are mostly homogeneous, un-foliated and medium to coarse-grained equigranular; these we split into map units by rock type. There are also heterogeneous bodies, the main ones being tonalite-diorite in composition that shows diorite seemingly intruded by massive outcrops of homogeneous tonalite. These bodies were mapped as tonalite-diorite complexes. Our objectives for fieldwork were to collect samples for zircon geochronology and Hf geochemistry analyses, as well as map and classify plutons. In order to categorize pluton bodies we sampled in three or more locations in each major pluton body, once for U-Pb geochronology and Hf geochemistry and the rest for petrologic studies. Metasedimentary and metavolcanic pendants consist largely of metavolcanics, fragmental volcanics and fine-grained metasedimentary rocks. Quartzites from the metasediment outcrops were collected in order to determine terrane relationship to those in the northern CMB.

Previous studies from the northern and central portion of the batholith display an eastward younging trend, We will compare the new results to similar data from the central batholith to evaluate the processes that influence batholith growth through episodic magmatism.