285-10 RAPID CONSTRUCTION AND EPISODIC REJUVENATION OF LOWER-CRUSTAL MUSH IN THE MEDIAN BATHOLITH, NEW ZEALAND

Abstract:

Lower-crustal magma systems beneath continental arcs are important for the chemical evolution of arc magmas, yet there is considerable controversy regarding the timescales over which deep-crustal arc magmas are stored, remobilized, and extracted. We investigate this problem through integrated field mapping, mineral thermometry, published garnet ages, and U-Pb zircon chemical abrasion thermal ionization mass spectrometry (CA-ID-TIMS) from ten samples across the lower-crustal Malaspina Pluton and adjacent Resolution Orthogneiss in the Mesozoic Median Batholith, Fiordland, New Zealand. The Malaspina Pluton represents a ~550 km² dioritic body which was assembled by numerous intrusions at 35-40 km depth. Magma was emplaced through two steep feeder zones along the Straight River shear zone, which acted as lithospheric-scale melt-transfer pathway. Most of the Malaspina Pluton was fed from a northwestern feeder zone, and dioritic magma spread eastward and southeastward into subhorizontal, isobaric sheets. A second feeder zone (Breaksea–Crooked Arm corridor) preserves a complex record of melt-reworked rejuvenated domains that are manifested by mafic recharge, leucocratic veins, peritectic garnet-bearing assemblages in melt-rich reaction zones, disrupted mush textures, and high-strain fabrics signaling open-system modification of lower-crustal mush.

New zircon geochronology reveals that low-strain dioritic sheets crystallized rapidly giving tightly clustered weighted-mean ²⁰⁶Pb/²³⁸U dates between 117.33 ± 0.04 and 117.05 ± 0.05 Ma consistent with crystallization in less than 0.3 Myr. In contrast, samples from rejuvenated domains along the southern Breaksea–Crooked Arm corridor preserve broader, overdispersed zircon populations with individual samples recording age spans of 0.4–5.6 Myr consistent with episodic rejuvenation extending locally to ca. 111.6 Ma. A sample from the Resolution Orthogneiss yields a tightly clustered zircon age of 115.85 ± 0.07 Ma, recording renewed dioritic intrusion in the Breaksea area while adjacent older mush was being rejuvenated. Published Sm-Nd and Lu-Hf garnet ages of 115.6 ± 2.6 to 110.6 ± 1.9 Ma date peritectic garnet growth in melt-rich reaction zones, independently linking the younger zircon record to melt-present mush rejuvenation. Our data show that lower-crustal magma systems comprise dynamically evolving mosaics of mush in which neighboring domains record distinct crystallization and rejuvenation histories. Thus, rapid batholith construction (10⁵ yr) and prolonged magmatic rejuvenation (10⁶ yr) are paired processes in the construction of lower-arc crust.

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