Mineralogical constraints on magma storage conditions in ultramafic arc cumulates and the nature and role of cryptic fugitive melts: Tulameen Alaskan-type intrusion, North American Cordillera

Abstract

Alaskan-type ultramafic–mafic intrusions in convergent-margin settings provide valuable information on melt-cumulate petrogenetic processes operating at depth in the sub-arc crust. Here, we report the compositions and textural relationships of cumulus and postcumulus minerals in a suite of clinopyroxenites and hornblendites from the peripheral zone of the Tulameen Alaskan-type intrusion in British Columbia, Canada. Mineral chemistry is used to establish magma storage conditions (P, T, fO₂, H₂O_melt) and to reconstruct the composition of cryptic residual liquids that equilibrated with the mineral phases and subsequently escaped the local mush system. Residual liquids in equilibrium with clinopyroxene (diopside) are metaluminous calc-alkaline basalt to andesite; melts equilibrated with amphibole (magnesio-hastingsite) are metaluminous to peraluminous calc-alkaline dacite to low-silica rhyolite. Thermobarometry yields a robust estimate of storage pressure of 400 ± 50 MPa (~ 15 km paleodepth) for the Tulameen magma reservoir and equilibration temperatures of 1130–960 °C for clinopyroxene and 950–850 °C for amphibole. The large cooling interval between the early crystallization of clinopyroxene and late appearance and continued crystallization of peritectic amphibole facilitated progressive extraction of residual liquids from clinopyroxene-rich cumulates, consistent with textural relationships, mass balance calculations and experimental petrology. Peritectic dacitic melts are hydrous (~ 6–8.3 wt% H₂O_melt), oxidized (fO₂ ~ NNO + 1.6 to NNO + 3.6 log units) and buoyantly mobile with low density (~ 2200 kg/m³) and viscosity (~ 10³ poise). Lower water contents likely reflect degassing of peritectic melts driven by amphibole crystallization; relatively high redox conditions are attributed to precursor fractionation of olivine and clinopyroxene preserved as cumulates in the core of the Tulameen intrusion. Peritectic amphibole crystallized in response to migration of a thermally buffered reaction front marking the stability limit of amphibole (≤ 950 °C) and driven by near-isobaric cooling. Pervasive infiltration of reactive dacitic liquids through the clinopyroxene mush formed intergranular/poikilitic amphibole and channelized flow was captured in part by cm-scale hornblendite segregations; aggregated melts formed in situ bodies of replacive hornblendite. The absence of orthopyroxene and rarity of plagioclase in the evolved ultramafic cumulates of Alaskan-type intrusions and similar arc-related rocks is attributed primarily to high H₂O_melt and oxygen fugacity in differentiated arc magmas.