10 How are Plutons Made? Physical and Chemical Records of Pluton Construction and Evolution

This session integrates structural and petrologic views of plutonic systems, exploring the crustal response to pluton formation, and processes that control the physical and chemical characteristics of pluton-host rock systems from initial emplacement to solidification.

Sunday, 19 October 2025
8:00 AM - 12:00 PM
HBGCC, 216AB

Mafic Recharge and Magma Diversification in Lower-Arc Crust, Fiordland, New Zealand

Paper number: 10-5
Program: Technical Sessions
Time: 9:10 AM – 9:25 AM

Authors

Abstract

We investigate the root of an Early Cretaceous continental magmatic arc (Fiordland, New Zealand) where field and petrographic observations show physical evidence for cyclic mafic recharge and the rejuvenation of crystal-rich magma reservoirs through time. We observe that ultramafic sheets were syn-magmatically emplaced into a voluminous sequence of host hornblende and two-pyroxene diorites at 25-50 km depth. These sheets range from meter- to kilometer-thick bodies and are primarily composed of hornblendites, with lesser amounts of websterites and hornblende peridotites. Websterites and hornblende peridotites are always enveloped and/or intruded by hornblendite. Contacts between ultramafic rocks and host diorite also show mutually intrusive cross-cutting relationships, evidence for crystal exchange, and lobate and cuspate margins, all of which suggest physical mixing in the presence of melt. In addition, granitic segregations are commonly concentrated near contacts with ultramafic sheets and are locally associated with peritectic garnets, likely signaling changing thermal and/or chemical compositions of the melt at the margins of the ultramafic sheets.

 

Petrographic investigation of the ultramafic rocks shows that hornblendites are dominated by pargasitic amphibole, with minor ilmenite and/or ortho- and clinopyroxene. Websterites display adcumulate textures with relict banding composed of varying modal concentrations of ortho- and clinopyroxene. In all cases, pyroxenes in websterites display reaction replacement textures with pargasite. In weakly reacted samples, pargasite forms micron-scale films along pyroxene grain boundaries. In more reacted samples, pargasite is concentrated in mm- to cm-thick veins where pyroxenes only occur as relict inclusions within large euhedral pargasite grains. In the hornblende peridotites, olivine grains are enveloped by orthopyroxene and pargasite. Rare clinopyroxene grains also display reaction replacement textures with pargasite.

 

We interpret field and petrographic observations to indicate that hydrous mafic melts intruded as sheets into dioritic mush. These mafic melts underwent extensive fractional crystallization forming peridotite and pyroxenite cumulates. Interaction between partially crystallized mafic sheets and the host dioritic melts led to two key reactions involving: 1) olivine + silicic melt -> orthopyroxene, and 2) clinopyroxene + silicic melt -> pargasite. We conclude that hornblendites and associated ultramafic rocks in the lower crust reflect repeated pulses of mafic recharge and open-system chemical interactions between mafic melts and dioritic mush leading to chemical hybridization, granitic melt segregation, and mobilization.

Geological Society of America Abstracts with Programs. Vol. 57, No. 6

doi: 10.1130/abs/2025AM-414460

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