CONSTRAINTS ON THE EVOLUTION AND EMPLACEMENT OF THE BUCK CREEK COMPLEX AND OTHER BLUE RIDGE ULTRAMAFIC ROCKS, WITH IMPLICATIONS FOR OPHIOLITE EMPLACEMENT IN COLLISIONAL OROGENS

The Buck Creek mafic-ultramafic complex, among the largest alpine ultramafic bodies in the southern Blue Ridge, originated as a mid-ocean ridge cumulate massif, with subsequent deep emplacement in a subduction zone setting. Within the ultramafic body, dunite, troctolite, anorthosite, and spinel display centimeter- to meter-scale interlayering. Troctolitic rocks locally grade into the amphibolites that enclose the body. Meta-troctolite and metadunite compositions define almost continuous mixing between olivine and plagioclase end-members on an MgO vs. Al2O3 diagram, with some troctolites and amphibolites extending toward augitic clinopyroxene to define a “cumulate triangle” of cumulate compositions. Low-Ti amphibolites fall within the triangle, while high-Ti amphibolites are geochemically consistent with gabbroic protoliths. Relatively abundant troctolite cumulates, scarce pyroxene in the ultramafic rocks, and high Al2O3/TiO2 with low Cr-numbers in spinels best match LOT- or L-type ophiolites, consistent with crystallization in a slow-spreading mid-ocean ridge setting.

Sapphirine-bearing, spinel symplectites in the meta-troctolites suggest that Buck Creek rocks were largely anhydrous to ~800°C and ~0.9-1.1 GPa, conditions atypical of ophiolite emplacement. Limited hydration, mostly along contacts, took place near peak pressure conditions, possibly facilitating ductile disaggregation and emplacement of the body. Comparisons to the Zermatt-Saas ophiolite suggest rapid subduction of Buck Creek rocks to depths of ~ 30 km, where partial hydration, facilitated by prograde dehydration of surrounding rocks, aided in its emplacement into the subduction zone hanging wall.

Similar lithologies and prograde metamorphic assemblages in the nearby Lake Chatuge and Carroll Knob complexes point to comparable igneous origins and metamorphic histories, suggesting deep emplacement of oceanic rocks in a pre-Taconic subduction setting. By contrast, ultramafic and amphibolite bodies to the northeast in the Central Blue Ridge Cartoogechaye and Mars Hill terranes, including the Webster-Addie complex, differ compositionally, and record different metamorphic histories.