The Porter Puddle Complex, petrology and geochemistry of the Marmot Formation, northern Canadian Cordilleran miogeocline

Abstract

The Porter Puddle Complex (PPC) of the Ordovician–Silurian Marmot Formation volcanics (MFV) is located in the northeastern Canadian Cordillera (NTS map sheet 106B/SE). The PPC has previously been interpreted as a rift-related submarine volcanic edifice within a carbonate platform and its seaward shale equivalent. It consists of potassic-ultrapotassic and alkalic volcanic rocks. Previous geochemical and mineralogical studies of these rocks are limited. New mapping by the Northwest Territory Geoscience Office in 2009-2012 applied newer stratigraphy and better delineated the PPC. Samples were collected from the PPC for mineralogical and geochemical study to better understand the petrogenesis of the volcanic rocks. All samples have experienced considerable alteration under hydrothermal conditions at the seafloor and during burial diagenesis. Primary mineral assemblages include pseudomorphs after olivine, dominant clinopyroxene, phlogopite, K-feldspar, albite and Ba-feldspars of the celsian–hyalophane series. These minerals suggest a potassic-ultrapotassic, low-silica basaltic magma. Early crystal fractionation involved clinopyroxene, olivine, and spinel at pressures well above 30 kbar. REE patterns indicate a lack of plagioclase fractionation and presence of residual garnet in the source, also implying a predominant mantle origin for MFV magmas. Ocelli textures suggest an immiscible phase in a silica-undersaturated, high-volatile basalt. Estimates for crystallization pressure for oxyhornblende in the ocelli suggest that later stage petrogenesis involved immiscible phases, K-feldspar fractionation, and crystallization of Ba-feldspar. Phlogopite associated with early clinopyroxene suggests magma evolved from a more hydrous system into a later stage of ocelli crystallization, further suggesting a hydrous-silica-undersaturated immiscibility. These multiple controls on petrogenesis may be responsible for the geochemical variability in the Marmot Formation volcanic rocks described in previous literature.