Magmatic enclaves and andesitic lavas from Mt. Lamington, Papua New Guinea: Implications for recycling of earlier-fractionated minerals through magma recharge

Zhange, J., Davidson, J.P., Humphreys, M.C.S., Macpherson, C.G. and Neill, I. (2015) Magmatic enclaves and andesitic lavas from Mt. Lamington, Papua New Guinea: Implications for recycling of earlier-fractionated minerals through magma recharge. Journal of Petrology, 56(11), pp. 2223-2256. (doi: 10.1093/petrology/egv071)

Abstract

Mt. Lamington is a composite, dome-forming volcano in Papua New Guinea, sitting on the Papuan Ultramafic Belt (PUB) ophiolite. The 1951 eruption produced andesitic dome lavas with numerous basaltic–andesitic enclaves and a few PUB ultramafic xenoliths. To understand the nature of the 1951 eruption, and to assess the effect of assimilating ophiolitic crust in modifying the geochemistry of arc magmas, we carried out petrological, mineralogical and geochemical studies on andesitic lavas as well as magmatic enclaves and ultramafic inclusions. The mineralogy of the enclaves is dominated by amphibole and plagioclase, similar to the andesitic lava hosts. The textures of the enclaves vary from fine-grained diktytaxitic to coarser-grained plutonic textured. We interpret this variation to result from variable cooling rates in the enclave-forming magma body when it invades the overlying andesite. The diktytaxitic enclaves contain variable proportions of host-derived amph + plag antecrysts and xenocrysts of ol + sp ± cpx ± amph with disequilibrium textures, indicating interaction with host lava and assimilation of foreign materials, respectively. A previous study argued that the olivine xenocrysts with chromian spinel inclusions are derived from the PUB, and thus that the PUB contaminated the Mt. Lamington magmas. We demonstrate that this is highly unlikely on the basis of morphological and compositional discrepancies between PUB ol + sp, sampled in nodules, and the xenocrysts. Mass balance indicates that the high whole-rock Ni contents of enclaves and andesitic hosts can be explained by olivine incorporation and do not require any PUB involvement. The olivines are considered to represent crystal mush fractionated from precursor(s) of andesitic and/or pre-1951 shoshonitic lavas. Their presence in enclaves represents recycling of earlier-fractionated components through magma recharge. We argue that this recycling is an important and underestimated process in shaping arc magmas.

• Item Type: Articles
• Status: Published
• Refereed: Yes
• Glasgow Author(s) Enlighten ID: Neill, Dr Iain
• Authors: Zhange, J., Davidson, J.P., Humphreys, M.C.S., Macpherson, C.G., and Neill, I.
• Subjects: Q Science > QE Geology
• College/School: College of Science and Engineering > School of Geographical and Earth Sciences > Earth Sciences
• Journal Name: Journal of Petrology
• Publisher: Oxford University Press
• ISSN: 0022-3530
• ISSN (Online): 1460-2415
• Copyright Holders: Copyright © 2015 Oxford University Press
• First Published: First published in Journal of Petrology 56(11):2223-2256
• Publisher Policy: Reproduced in accordance with the copyright policy of the publisher.