Mantle petrology, mineralogy and major elements geochemistry of the northern Cache Creek terrane – Preliminary results

English Abstract:

The northern Cache Creek terrane extends for more than 500 km in north British Columbia and south Yukon. This composite terrane comprises mafic and ultramafic complexes and carbonate and chert assemblages that were previously interpreted as accreted seamounts, spreading centers and rifted arc complexes. Our observations show that mantle facies dominate and are often structurally capped by brecciated hypabyssal and volcanic rocks, with only rare occurrences of mafic and ultramafic cumulates. Geochemical signatures suggest an arc affinity for most volcanic and hypabyssal rocks. Variably serpentinized and metamorphosed mantle rocks were sampled at King Mountain, Dease Lake, Hardluck and Peridotite peaks, Mt. Barham, Atlin and Marsh Lake. Primary mineralogy and textures are often preserved. Foliated mantle harzburgite tectonite dominates, with about 25-35% of porphyroclastic orthopyroxene and subordinate chromite, often with holly-leaf morphology. Chromite shows a wide spectrum from translucent to opaque facies, suggesting a wide range of Cr/Al and variable melt depletion signatures. Rare orthopyroxenitic to websteritic layers are interpreted to be transposed dykes. Dunite dykes are common but subordinate in abundance and contain sub-euhedral chromite. Gabbroic dykes are also common. At Mt. Barham the peridotite shows spectacular anthophyllite dendrites near a late granitic intrusion. Detailed mantle petrology, mineralogy and major elements geochemistry will be used to establish a coherent regional geologic framework for the Cache Creek terrane and its tectonic reconstruction.