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Article: The origin of garnets in andesitic rocks from the Northland arc, New Zealand, and their implication for sub-arc processes

TitleThe origin of garnets in andesitic rocks from the Northland arc, New Zealand, and their implication for sub-arc processes
Authors
KeywordsGarnet Andesite
Magmatic Garnets
Northland Arc
Subduction Processes
Issue Date2012
PublisherOxford University Press. The Journal's web site is located at http://petrology.oxfordjournals.org/
Citation
Journal Of Petrology, 2012, v. 53 n. 6, p. 1169-1195 How to Cite?
AbstractAn unusual andesitic suite from the Miocene volcanic arc in Northland, New Zealand, comprises pyroxene andesite and garnet-bearing hornblende-pyroxene, hornblende and biotite-hornblende andesites. Garnet crystals occur as 1-10 mm single crystals or more commonly as two or more annealed crystals and as garnetite lenses. The andesitic rocks also contain enclaves of high-MgO pyroxenite, hornblendite, and pyroxene-hornblende gabbro as well as high-Al 2O 3 hornblende gabbro, garnet-hornblende gabbro, and anorthosite. Garnet crystals in the andesitic volcanic rocks and in the enclaves show comparable compositional ranges, zoning patterns and inclusions, which indicate that they share a common petrogenetic history. They can be grouped into four distinct types on the basis of mode of occurrence, chemical composition and zoning patterns, which leads to their interpretation as antecrysts rather than orthocrysts. The compositions of the garnets, as well as their included mineral assemblages, reflect a petrogenetic trend from high-temperature pyroxene-bearing high-Mg garnet to low-temperature Fe-rich garnet at relatively constant pressure. Well-preserved zoning patterns, in particular those of the Ca- and Mg-rich garnets, reflect processes within a deep crustal arc environment. Later assimilation is suggested by some zoning patterns that show decreasing Ca and increasing Fe and Mn contents. The garnets are interpreted as being derived by disintegration of discrete but closely related cumulate material that formed at pressures of 8-10 kbar. The host volcanic rocks and their garnet crystals together with the enclaves thus represent a consanguineous mixture of liquid and solid components that developed where subduction-related magmas ponded and interacted at or near the base of the crust. Together they represent a rare snapshot of the processes and components that produce arc-type rocks. © The Author 2012. Published by Oxford University Press. All rights reserved.
Persistent Identifierhttp://hdl.handle.net/10722/178247
ISSN
2015 Impact Factor: 3.768
2015 SCImago Journal Rankings: 2.992
ISI Accession Number ID
References

 

DC FieldValueLanguage
dc.contributor.authorBach, Pen_US
dc.contributor.authorSmith, IEMen_US
dc.contributor.authorMalpas, JGen_US
dc.date.accessioned2012-12-19T09:43:54Z-
dc.date.available2012-12-19T09:43:54Z-
dc.date.issued2012en_US
dc.identifier.citationJournal Of Petrology, 2012, v. 53 n. 6, p. 1169-1195en_US
dc.identifier.issn0022-3530en_US
dc.identifier.urihttp://hdl.handle.net/10722/178247-
dc.description.abstractAn unusual andesitic suite from the Miocene volcanic arc in Northland, New Zealand, comprises pyroxene andesite and garnet-bearing hornblende-pyroxene, hornblende and biotite-hornblende andesites. Garnet crystals occur as 1-10 mm single crystals or more commonly as two or more annealed crystals and as garnetite lenses. The andesitic rocks also contain enclaves of high-MgO pyroxenite, hornblendite, and pyroxene-hornblende gabbro as well as high-Al 2O 3 hornblende gabbro, garnet-hornblende gabbro, and anorthosite. Garnet crystals in the andesitic volcanic rocks and in the enclaves show comparable compositional ranges, zoning patterns and inclusions, which indicate that they share a common petrogenetic history. They can be grouped into four distinct types on the basis of mode of occurrence, chemical composition and zoning patterns, which leads to their interpretation as antecrysts rather than orthocrysts. The compositions of the garnets, as well as their included mineral assemblages, reflect a petrogenetic trend from high-temperature pyroxene-bearing high-Mg garnet to low-temperature Fe-rich garnet at relatively constant pressure. Well-preserved zoning patterns, in particular those of the Ca- and Mg-rich garnets, reflect processes within a deep crustal arc environment. Later assimilation is suggested by some zoning patterns that show decreasing Ca and increasing Fe and Mn contents. The garnets are interpreted as being derived by disintegration of discrete but closely related cumulate material that formed at pressures of 8-10 kbar. The host volcanic rocks and their garnet crystals together with the enclaves thus represent a consanguineous mixture of liquid and solid components that developed where subduction-related magmas ponded and interacted at or near the base of the crust. Together they represent a rare snapshot of the processes and components that produce arc-type rocks. © The Author 2012. Published by Oxford University Press. All rights reserved.en_US
dc.languageengen_US
dc.publisherOxford University Press. The Journal's web site is located at http://petrology.oxfordjournals.org/en_US
dc.relation.ispartofJournal of Petrologyen_US
dc.subjectGarnet Andesiteen_US
dc.subjectMagmatic Garnetsen_US
dc.subjectNorthland Arcen_US
dc.subjectSubduction Processesen_US
dc.titleThe origin of garnets in andesitic rocks from the Northland arc, New Zealand, and their implication for sub-arc processesen_US
dc.typeArticleen_US
dc.identifier.emailMalpas, JG: jgmalpas@hku.hken_US
dc.identifier.authorityMalpas, JG=rp00059en_US
dc.description.naturelink_to_subscribed_fulltexten_US
dc.identifier.doi10.1093/petrology/egs012en_US
dc.identifier.scopuseid_2-s2.0-84861827551en_US
dc.identifier.hkuros203667-
dc.relation.referenceshttp://www.scopus.com/mlt/select.url?eid=2-s2.0-84861827551&selection=ref&src=s&origin=recordpageen_US
dc.identifier.volume53en_US
dc.identifier.issue6en_US
dc.identifier.spage1169en_US
dc.identifier.epage1195en_US
dc.identifier.isiWOS:000304533000003-
dc.publisher.placeUnited Kingdomen_US
dc.identifier.scopusauthoridBach, P=55240496400en_US
dc.identifier.scopusauthoridSmith, IEM=7404425799en_US
dc.identifier.scopusauthoridMalpas, JG=7006136845en_US

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