CLINOPYROXENE FROM THE TULAMEEN ALASKAN-TYPE ULTRAMAFIC-MAFIC INTRUSION IN THE NORTH AMERICAN CORDILLERA RECORDS MID-CRUSTAL MAGMATIC CONDITIONS IN A MESOZOIC ISLAND ARC

Clinopyroxene from the Late Triassic (ca. 205 Ma) Tulameen Alaskan-type intrusion displays a wide range of textures and chemistry indicative of a dynamic crystallization environment in a transcrustal magmatic system. The Tulameen intrusion, an elongate sill-like body, is the largest Alaskan-type intrusion (>60 km²) in the North American Cordillera. It is coeval with a Late Triassic magmatic flare-up in the Quesnel accreted-arc terrane that produced the regionally extensive Nicola Group volcanic rocks and coeval Cu-Au porphyry deposits in British Columbia. Clinopyroxene is diopsidic and present in all major rock types (dunite, olivine clinopyroxenite, hornblende clinopyroxenite/hornblendite, gabbro-syenite) with Mg# ranging from 0.94 to 0.64. In dunite and olivine clinopyroxenite, weak to absent chemical zonation is related to high-temperature re-equilibration, whereas normal zoning in clinopyroxene hornblendite and gabbro-syenite preserves evidence of progressive crystallization from evolving melts. Sodium, Al, Ti, Mn, and V increase, and Cr and Ni decrease with decreasing Mg#, consistent with an early magmatic evolution dominated by the crystallization of olivine (+Cr-spinel) and clinopyroxene followed by peritectic crystallization of hornblende. Clinopyroxene does not define a single compositional trend, in support of field evidence for open-system behavior during the emplacement of the Tulameen intrusion. Some gabbroic to syenitic rocks contain clinopyroxene with different minor element chemistry compared to the ultramafic rocks suggesting that they crystallized from distinct parent magmas. Preliminary thermobarometry indicates that clinopyroxene crystallized over an extended temperature interval (~1200-950°C) at an average pressure of 4.3 ± 1.1(1s) kbar corresponding to a depth of ~16 ± 4 km. The primitive parent magmas to the Tulameen Alaskan-type intrusion ascended from their mantle source into the overlying Quesnel arc where they stalled and differentiated in a mid-crustal reservoir(s). Clinopyroxene from the Tulameen intrusion is similar in composition to clinopyroxene from mafic to intermediate volcanic rocks of the Nicola Group, establishing a possible petrogenetic link between magmatic evolution at depth and widespread volcanism and porphyry mineralization during the development of a Mesozoic island arc.