New petrological, geochemical, and geochronological perspectives on andesite-dacite magma genesis at Ruapehu volcano, New Zealand

Conway, Chris E.; Gamble, John A.; Wilson, Colin J. N.; Leonard, Graham S.; Townsend, Dougal B.; Calvert, Andrew T.

New petrological, geochemical, and geochronological perspectives on andesite-dacite magma genesis at Ruapehu volcano, New Zealand

dc.contributor.author	Conway, Chris E.
dc.contributor.author	Gamble, John A.
dc.contributor.author	Wilson, Colin J. N.
dc.contributor.author	Leonard, Graham S.
dc.contributor.author	Townsend, Dougal B.
dc.contributor.author	Calvert, Andrew T.
dc.contributor.funder	Department of Conservation, New Zealand	en
dc.contributor.funder	Victoria University of Wellington	en
dc.contributor.funder	Japan Society for the Promotion of Science	en
dc.date.accessioned	2018-10-22T11:55:57Z
dc.date.available	2018-10-22T11:55:57Z
dc.date.issued	2018-04-01
dc.date.updated	2018-10-22T11:44:23Z
dc.description.abstract	Time–composition relationships in eruptive sequences at composite volcanoes can show how the ongoing intrusion of magmas progressively affects the lithosphere at continental convergent margins. Here, new whole-rock and microanalytical major and trace element data from andesite-dacite lava flows are integrated with previous studies and existing isotopic data, and placed within the framework of a high-resolution chronostratigraphy for Ruapehu volcano (southern Taupo Volcanic Zone, New Zealand). The geochemical evolution of lavas erupted over the ∼200 kyr lifetime of the exposed edifice reflects variable degrees of fractionation and systematic changes in the type of crustal assimilation in the Ruapehu magma system. Lavas erupted from ∼200–150 ka have previously been distinguished from those erupted <150 ka based on Sr-Nd isotopic characteristics, which indicate that the oldest lavas were sourced from magmas that assimilated oceanic crust. Such source rocks underlie the regionally widespread Mesozoic meta-sedimentary greywacke-argillite basement, which was conversely assimilated by <150 ka magmas. New results from this work reveal that since 150 ka, an upper limit of magma differentiation occurred from ∼50–35 ka. High K2O (∼6 wt%) and Rb contents (∼270 ppm) in melt inclusions, interstitial glass, and glass from in situ quenched melts of partially fused crustal xenoliths are reported for andesite-dacite lavas erupted during this period. In addition to crystal fractionation, selective partial melting and assimilation of K- and Rb-rich mineral phases (e.g., biotite, K-feldspar) that are significant components of the meta-sedimentary basement rocks is inferred to explain these geochemical characteristics. These processes coincided also with the effusion of high-MgO andesitedacite lavas that display petrological evidence for mixing between andesite-dacite and more mafic magmas. An influx of hotter mafic magma into the system explains why the extent of crustal assimilation recorded by Ruapehu lavas peaked during the ∼50–35 ka eruptive period. From 26 ka to the present, andesite lavas have reverted to more mafic compositions with less potassic melt inclusion and whole-rock compositions when compared to the ∼50–35 ka lavas. We suggest that the younger lavas assimilated less-enriched melts because fertile phases had been preferentially extracted from the crustal column during earlier magmatism. This scenario of bottom-up heating of the lithosphere and exhaustion of fertile phases due to the progressive intrusion of magma explains the geochemical evolution of Ruapehu lavas. This model may be applicable to other long-lived composite volcanoes of the circum-Pacific continental arcs.	en
dc.description.sponsorship	Department of Conservation, New Zealand (DOCDM-593774); Victoria University of Wellington (DVC Research Grant 13311); Japan Society for the Promotion of Science (Postdoctoral Research Fellowship JSPS P16788)	en
dc.description.status	Peer reviewed	en
dc.description.version	Accepted Version	en
dc.format.mimetype	application/pdf	en
dc.identifier.citation	Conway, C. E., Gamble, J. A., Wilson, C. J. N., Leonard, G. S., Townsend, D. B. and Calvert, A. T. (2018) 'New petrological, geochemical, and geochronological perspectives on andesite-dacite magma genesis at Ruapehu volcano, New Zealand', American Mineralogist, 103(4), pp. 565-581. doi:10.2138/am-2018-6199	en
dc.identifier.doi	10.2138/am-2018-6199
dc.identifier.endpage	581	en
dc.identifier.issn	0003-004X
dc.identifier.issued	4	en
dc.identifier.journaltitle	American Mineralogist	en
dc.identifier.startpage	565	en
dc.identifier.uri	https://hdl.handle.net/10468/7029
dc.identifier.volume	103	en
dc.language.iso	en	en
dc.publisher	Mineralogical Society of America	en
dc.relation.uri	http://www.minsocam.org/MSA/AmMin/TOC/2018/Apr2018_data/Apr2018_data.html
dc.rights	© 2018, the Mineralogical Society of America. All rights reserved.	en
dc.subject	Ruapehu	en
dc.subject	Andesite	en
dc.subject	Dacite	en
dc.subject	Petrogenesis	en
dc.subject	Arc magma	en
dc.subject	Crustal contamination	en
dc.subject	High-Mg andesite	en
dc.subject	Dynamics of Magmatic Processes	en
dc.title	New petrological, geochemical, and geochronological perspectives on andesite-dacite magma genesis at Ruapehu volcano, New Zealand	en
dc.type	Article (peer-reviewed)	en