10-5 The physical mechanisms that move melts out of lower-crustal reservoirs to form granitic plutons in the middle-upper crust: field evidence from the Median Batholith, New Zealand

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

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The Median Batholith in western New Zealand exposes a nearly complete crustal cross section through a Mesozoic, Cordilleran-style magmatic arc that once extended from Gondwana to Laurentia. One of the last pulses of magmatism in the batholith formed the ~15 km thick Misty pluton, which was constructed during a brief ≤5 Ma period from 120-115 Ma at 25-40 km depth. For this study, we used the Misty pluton to determine how granitic melts that formed and resided within deep, crystal-rich reservoirs were mobilized and extracted from the base of the lower crust to shallower depths where they formed granitic plutons and batholiths.

Transects across the Misty pluton, including its upper and lower boundaries, reveal that granitic melts moved through and out of lower-crustal reservoirs by the following processes: (1) the cyclical injection of meter- to kilometer-thick, mafic to ultramafic sheets into the lower crust where they reheated and partially melted crystal-rich mushes of dioritic composition; (2) dike intrusion combined with viscous compaction and vertical coaxial shortening mobilized and segregated trapped melts, which formed outcrop-scale pools and lattices; and (3) melt escape out of crystal-rich, high-strain zones occurred by porous flow through multiscale fracture networks and in steep, melt-rich channels and dikes. The dominance of compaction and vertical coaxial shortening at the base of the pluton is indicated by changes in deformation geometry (using folds, foliations, and boudinage) across mesoscopic and macroscopic strain gradients. Melt mobilization through an increasingly viscous, crystallizing magma mush is indicated by increases in the frequency of mush fracturing and changes in dike geometry and texture through time. Melt segregations progressed from relatively small, disorganized patches and pools in the deepest part of the arc to organized veins, dikes, and channels higher up in the pluton. Non-coaxial flow in steep, melt-rich channels is recorded by asymmetric structures, cross-cutting dikes and foliations, and complex melt-enhanced folding.

This natural example provides direct, field-based evidence of the mechanisms that mobilize, segregate, and extract trapped melts from the deep, lower crustal roots of continental arcs to where they form shallow-level plutons.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-9014

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