NEW DETRITAL AND METAMORPHIC ZIRCON AGES FROM COVER ROCKS OF THE MONASHEE COMPLEX, SOUTHEASTERN CANADIAN CORDILLERA

The Monashee Complex is an exposure of rocks from ancestral North America located within the Omineca Belt of the Canadian Cordillera. It comprises a Paleoproterozoic basement of orthogneiss and paragneiss and the cover consists of quartzite, orthogneiss, paragneiss, pelitic schist, calcsilicate gneiss and marble. The age of deposition and metamorphism of the cover sequence in northwest Thor-Odin, the southern dome of the Monashee Complex, is the subject of this study. Zircon from five samples has been dated using U/Pb laser ablation inductively coupled plasma mass spectrometer (LA ICP-MS) methods. Detrital cores and younger overgrowths within zircon were imaged, using optical microscopy and scanning electron microscopy (SEM). Data were collected from cores as well as from the younger metamorphic rims. Overgrowth ages are between 54 and 114 Ma and are interpreted as metamorphic ages. Ages from cores are 560-2600 Ma.

The ~2.6-1.8 Ga ages suggest detrital sources. The 1800-560 Ma ages may signify metamorphic events or distinct detrital sources. The latter interpretation is preferred, because (1) data come from detrital cores, (2) the ~2.6 Ga grains do not seem to be affected by a ~1.8-1.2 Ga event, which would be expected if the younger were a metamorphic age, and (3) three ~786-560 Ma detrital grains are present, which suggests the sediments were deposited after ~560 Ma and thus could not have experienced early Proterozoic metamorphism.

The current data indicate sediments near the base of the cover were deposited after 560 Ma. However, in Frenchman Cap, the northern dome of the Monashee Complex, the 724 Ma Mount Grace syenite intrudes part of the cover sequence. Therefore, either the age of the cover sequence in Thor-Odin is younger than the cover sequence in Frenchman Cap, or older rocks exist elsewhere in the cover sequence of Thor-Odin.

Detrital ages from the cover sequence in northwest Thor-Odin indicate that at least part of the Monashee Complex cover sequence was deposited after ~560 Ma. Detrital ages can be correlated with the 2.8-1.8 Ga Alberta basement, the 1.7-1.2 Ga Belt-Purcell, and the 780-570 Ma Windermere supergroups to the west of the Monashee Complex.