Bulletin 94: Origin and Tectonic Setting of Ophiolitic Ultramafic
and Related Rocks in the Atlin Area, Northwestern British Columbia
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Bulletin 94 indicates that Mesothermal gold-bearing quartz veins throughout the Atlin placer camp of northwestern British Columbia are contained within or marginal to carbonate-altered ultramafic rocks (listwanites). These ultramafic rocks comprise individual thrust slices which form part of an imbricated package of late Paleozoic oceanic crust and upper mantle lithologies referred to as the Atlin ophiolitic assemblage. This ophiolitic assemblage was obducted onto a subduction-related accretionary complex, the Atlin accretionary complex, during the Middle Jurassic collision of Stikinia with North America.
Ultramafic rocks include both foliated harzburgite with subordinate dunite pods and peridotite cumulates, most likely wehrlite, that form individual thrust slices. The harzburgite is characterized by a medium to coarse-grained porphyroclastic texture, a well-developed primary foliation fabric (S1), and compositional banding with isoclinal folding of both banding and pyroxenite dikes. These features are widely accepted as indicating ductile deformation at subsolidus to hypersolidus conditions during and subsequent to mantle partial melting.
Primary silicate phase chemistry of the harzburgite indicates that the unit is highly refractory with a very limited compositional range (Ens87.8-90.2, F089.5-90.7). Chrome spinels in the harzburgite form highly irregular amoeboid shapes and compositionally display a large reciprocal variation of chrome and alumina, which defines a partial melting trend. The texture, structure and the primary phase chemical composition of the harzburgites indicate that they represent metamorphic mantle material formed by partial melting below a paleo-oceanic spreading centre. More specifically, the unit may be characterized as an abyssal peridotite formed within a mid-ocean ridge (MOR) paleotectonic setting as the chrome spinels have low Cr#[(Crx100/(Cr+Al); 23-47] and clinopyroxene contents are locally greater than 5%, indicating relatively fertile depleted mantle.
Whole-rock major, trace and rare-earth elemental (REE) data obtained for the metabasaltic rocks emplaced tectonically with the ultramafics, indicate that they are subalkaline tholeiitic in composition and also suggest a MOR paleo-tectonic eruptive setting for their origin. Petrogenetic modeling of the mantle source region using the basalt chemistry indicates that the basaltic liquids were derived by limited degrees of partial melting (10 to 15%). The ultramafic and metabasaltic rocks are therefore considered to be cogenetic.
The mantle section contains S2 emplacement-related fabrics that are defined by retrograde metamorphic assemblages after primary olivine and orthopyroxene. The distribution and variation in intensity of development of the S2 fabrics indicate that the ultramafic body has been affected by inhomogeneous bulk strain during tectonic emplacement in the solid state. Overprinting of these ductile fabrics by listwanite alteration along the basal thrust fault of the ultramafic body indicated subsequent influx of carbon-dioxide rich fluids.
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