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Low-grade metamorphism of the Brook Street Volcanics, D'Urville Island, New Zealand
| Content Provider | Scilit |
|---|---|
| Author | Sivell, Warwick J. |
| Copyright Year | 1984 |
| Abstract | Basic lavas, pyroclastics, and labile sediments, together with ultramafic and gabbroic intrusives, comprising the Lower Permian volcanic arc sequence of the Brook Street Volcanics at west D'Urville Island, New Zealand, have undergone variable mineralogical adjustment during low-grade metamorphism. Two metamorphic zones are recognised. Zone 1 is characterised by pumpellyite, pistacitic epidote, albite, chlorite, and rare grandite garnet. Assemblages in Zone 1 are correlated with the prehnite-pumpellyite facies, although prehnite is generally absent. Zone 2 is characterised by actinolite and pistacite-poor epidote, as well as albite and diminishing abundance of chlorite. Assemblages in Zone 2 are typical of greenschist facies metabasites and are often associated with minor sulphide mineralisation. Additional metamorphic phases in both zones include phengitic muscovite, orthoclase, calcite, quartz, magnetite, hematite, and sphene. The metabasalts in Zone 1 are only poorly recrystallised. Relict metastable phenocrysts persisted during alteration and bulk-rock equilibrium was not reached. However, the secondary mineralogy of these rocks is consistent with calculated residual metabasalt compositions after removal of unreacted mafic phenocryst components. The metabasalts in Zone 2 underwent more extensive recrystallisation. Decomposition of relict clinopyroxene added considerable mafic components to the reacting system and changed its chemistry toward the bulk-rock composition. The degree of metamorphism is chiefly related to permeability of the rocks to migrating fluids. In Zone 1, pumpellyite-bearing assemblages developed in vesicles and cavities in permeable tuffs, tuffbreccias, agglomerates, and amygdaloidal flow tops. These hydrous assemblages were promoted by high (i.e., lithostatic pressure) in essentially closed systems well removed from joints and fracture systems. In Zone 2, less hydrous actinolite-bearing assemblages formed in massive lava flows, dikes, and tuffs adjacent to a thick ultramafic-gab-broic sill. In these rocks, fracture systems related to the intrusive body persisted due to enhanced wall-rock strength, and maintained reduced hydrostatic pressure) that facilitated extensive recrystallisation. Impermeable strata in both zones show little metamorphic reconstitution. The direct transition from prehnite-pumpellyite to greenschist facies assemblages in the Brook Street rocks indicates that contemporaneous, shallow level (comagmatic) intrusive activity locally elevated geothermal gradients in the rapidly buried volcanic pile and caused recrystallisation under short-lived, low P/T conditions. |
| Related Links | http://www.tandfonline.com/doi/pdf/10.1080/00288306.1984.10422525?needAccess=true |
| Ending Page | 190 |
| Page Count | 24 |
| Starting Page | 167 |
| ISSN | 00288306 |
| e-ISSN | 11758791 |
| DOI | 10.1080/00288306.1984.10422525 |
| Journal | New Zealand Journal of Geology and Geophysics |
| Issue Number | 2 |
| Volume Number | 27 |
| Language | English |
| Publisher | Informa UK Limited |
| Publisher Date | 1984-04-01 |
| Access Restriction | Open |
| Subject Keyword | Mineralogy Low-grade Metamorphism Brook Street Volcanics Metabasalt Prehnite-pumpellyite Greenschist |
| Content Type | Text |
| Resource Type | Article |
| Subject | Geology Geophysics |
