3 Metamorphic Processes in Cordilleran Arc Systems

Tuesday, 12 May 2020
8:00 AM - 12:00 PM
Madera Room

GEOCHRONOLOGY IN THE BOWELS OF MAGMATIC ARCS: A TALE OF TWO MINERALS, FIORDLAND NEW ZEALAND

Paper number: 3-3
Program: Theme Sessions
Time: 8:30 AM – 8:50 AM

Authors

Abstract

Precise and meaningful ages are essential for understanding thermal and mass fluxes within magmatic arcs and between arc crust and adjacent mantle. Isotopic dates are readily determined; however, direct ties between ages, magmatism, and metamorphism may not be apparent. SHRIMP-RG and CA-TIMS zircon, and ID-TIMS garnet ages outline the possibilities and limitations in rocks from the lower crust of the Fiordland magmatic arc where metamorphic temperatures were >850°C at ~14 kbar.

We present data for 3 migmatitic orthogneiss samples containing melt veins, garnet selvages, and garnet reaction zones (GRZ): 09NZ22-Crooked Arm, 15NZ25-Bligh, & 13NZ34b-Acheron. In 09NZ22, U-Pb zircon ages from the host and GRZ range from c. 121 to c. 116 Ma and do not define distinct populations based on age, morphology, or CL. Ten CA-TIMS zircon ages ranging from 118.3±.13 to 115.7±.18 Ma record episodic magmatism. The larger range of SHRIMP-RG ages record magma emplacement, metamorphic dissolution-reprecipitation, and local Pb loss. Garnet Sm-Nd ages range from 113.1±2.3 to 103.6±2.3 Ma and display systematic correlation with grain size. Ages for c. 1 cm garnet record growth during metamorphism, while the 0.1-0.3 cm grains with younger ages reflect intracrystalline diffusion and Sm-Nd closure. In 15NZ25, LA-ICPMS U-Pb zircon ages from the matrix and inclusions in large garnet porphyroblasts are 121.4±1.5 Ma (N=11) and 114.7±1.1 Ma (N=25/26), respectively. We interpret older zircon to record magma emplacement and younger zircon to record metamorphism. Sm-Nd ages for >1 cm garnet record growth during metamorphism at 112.3±2.0 (core-mantle, N=4) followed by overgrowth and/or diffusion at 106.9±3.1 Ma (rim, N=2). In 13NZ34, Sm-Nd ages for 1 cm garnet record growth from 115.9±1.1 Ma (core, N=5) to 111.3±1.8 Ma (rim, N=4), while c. 0.2 cm grains record closure of Sm-Nd to diffusion at 103.9±1.8 Ma (N=7).

The 3 migmatitic orthogneiss samples illustrate that: 1) large age uncertainties for SHRIMP-RG U-Pb zircon give estimates for magma emplacement but cannot resolve protracted magmatic processes, 2) CA-TIMS U-Pb zircon dates document timescales of protracted magmatism, 3) Sm-Nd ages for ca. 1 cm garnet grains reflect garnet growth at peak temperature, and 4) Sm-Nd ages for 0.1 to 0.3 cm garnet grains reflect cooling through c. 750°C.

Geological Society of America Abstracts with Programs. Vol. 52, No. 4

doi: 10.1130/abs/2020CD-347006

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