Very Low- to Low-grade Metamorphic Processes Related to the Collisional Assembly of Avalonia in SE Cape Breton Island (Nova Scotia, Canada)

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Abstract

To better understand geodynamic processes related to the assembly of various belts of volcanic–sedimentary rocks in the Avalonian Mira terrane of SE Cape Breton Island (protolith ages 680–560 Ma) we investigated metamorphic processes in 20 white-mica-bearing mafic and felsic metavolcanic rocks. The felsic metavolcanic rocks are foliated and partly sheared with blastoporphyritic relict fabric and contain the assemblage phengite–epidote–chlorite–albite–K-feldspar–quartz–titanite ± stilpnomelane ± calcite ± ilmenite. In contrast, many mafic metavolcanic rocks are relatively undeformed and display relict porphyritic and amygdaloidal textures. They contain the assemblages epidote–chlorite–albite–quartz–titanite ± phengite ± pumpellyite ± prehnite ± calcite ± K-feldspar and actinolite–epidote–chlorite–albite–quartz–titanite ± phengite ± calcite ± K-feldspar. Heterogeneous metamorphic overprinting is indicated by local relicts of magmatic clinopyroxene, magnetite and plagioclase. Metamorphic minerals formed by local precipitation in clusters and are due to continuous nucleation of very low-grade phases during pulses of variably pervasive fluids, which were released during intense dehydration at 250–300°C. Nucleaction rate dominated over growth rate at these conditions. Potassic white mica in both mafic and felsic rocks is mostly phengite with a wide compositional range (3·11–3·41 Si a.p.f.u.). Maximum Si contents are typically between 3·30 and 3·41 a.p.f.u. P–T pseudosections were calculated for the range 200–400°C and 1–7 kbar. The peak metamorphic assemblages occupy P–T fields consistent with the position of isopleths for corresponding maximum Si contents in white mica. Peak P–T conditions in the Mira terrane samples lie within a narrow range of 3·5 ± 0·4 kbar and 280 ± 30°C in samples representing all of the assembled volcanic–sedimentary belts. The derived peak metamorphic conditions suggest syn-collisional burial to 11–14 km depth and a low metamorphic geotherm of 20–25°C km–1. Under these conditions subsequent strike-slip deformation is attributed to the final assembly of magmatic arc slices to form the crust of the Mira terrane.

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