Abstract
Geochemical signatures throughout the layered Earth require significant mass transfer through the lower crust, yet geological pathways are under-recognized. Elongate bodies of basic to ultrabasic rocks are ubiquitous in exposures of the lower crust. Ultrabasic hornblendite bodies hosted within granulite facies gabbroic gneiss of the Pembroke Valley, Fiordland, New Zealand, are typical occurrences usually reported as igneous cumulate hornblendite. Their igneous features contrast with the metamorphic character of their host gabbroic gneiss. Both rock types have a common parent; field relationships are consistent with modification of host gabbroic gneiss into hornblendite. This precludes any interpretation involving cumulate processes in forming the hornblendite; these bodies are imposter cumulates. Instead, replacement of the host gabbroic gneiss formed hornblendite as a result of channeled high melt flux through the lower crust. High melt/rock ratios and disequilibrium between the migrating magma (granodiorite) and its host gabbroic gneiss induced dissolution (grain-scale magmatic assimilation) of gneiss and crystallization of mainly hornblende from the migrating magma. The extent of this reaction-replacement mechanism indicates that such hornblendite bodies delineate significant melt conduits. Accordingly, many of the ubiquitous basic to ultrabasic elongate bodies of the lower crust likely map the ‘missing’ mass transfer zones.
Highlights
Earth has a heterogeneous, layered crust that overlies a relatively homogeneous mantle
The igneous nature of the hornblendite unit is supported by the following features: (i) high mode of one or two minerals with unimodal grain size distribution and interlocking euhedral grain shapes; (ii) interstitial minor phases with low dihedral angles, representing pseudomorphs of former melt[10,11]; (iii) presence of domains exhibiting a shape preferred orientation of hornblende and clinozoisite grains while lacking microstructures typical of crystal plastic deformation; (iv) presence of small pockets rich in plagioclase and biotite, of approximate granodioritic composition[12]; and (v) minor pegmatitic domains with very coarse, randomly oriented, interlocking euhedral grains
The presence of a body with typical igneous character within a high-grade metamorphic gneiss, without a structural break, requires that the former is younger than its metamorphic host
Summary
Earth has a heterogeneous, layered crust that overlies a relatively homogeneous mantle. The continuity and progressive modification of the garnet granulite reaction zones in the host gabbroic gneiss, to garnetite stringers in the hornblendite, link the two contrasting rock types.
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