T21 Petrochronology: Advances and Applications to Petrogenesis and Tectonics I

Sunday, 22 October 2017
8:00 AM - 12:00 PM
Skagit 4

CAMBRIAN GRANULITE-FACIES METAMORPHISM OF THE LARSEMANN HILLS, PRYDZ BAY, ANTARCTICA: USING THERMODYNAMIC MODELING TO INFORM MONAZITE PETROCHRONOLOGY

Paper number: T21-15
Program: Technical Sessions
Time: 11:20 AM – 11:35 AM

Authors

Abstract

Rocks exposed along the southeast coast of Prydz Bay were affected by granulite-facies metamorphism, deformation, and plutonism associated with continental collisions at 900-1000 Ma and 500-530 Ma. Pseudosection modeling was used in conjunction with petrography, trace element mapping, and in-situ (U-Th)/Pb dating of monazite by electron microprobe to characterize the P-T-t history of Cambrian metamorphism in the Larsemann Hills. Age populations were identified by grouping domains based on petrographic context, and U, Th, Y, and Pb concentrations; these were linked to the P-T path by comparison with our modeling results. Two garnet-cordierite bearing metapelites and one garnet-orthopyroxene bearing metapsammite were selected for Perple_X modeling in the Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O-TiO2±Fe2O3 system. Peak metamorphic assemblages are predicted to occur at 5-7 kbar and 780-840 °C in all models, similar to past estimates of 6.5-7.5 kbar and 800-860 ° C. Less than 8 vol. % melt is stable at peak P-T, and both garnet and melt modes increase along possible prograde trajectories. Previous studies estimate two stages of retrogression: decompression to 3.0-4.5 kbar and 700-750 °C, followed by isobaric cooling at 3-3.5 kbar and 550-600 °C. Garnet is predicted to break down between 5.5 and 3 kbar, whereas cordierite begins to form and hercynite mode increases. These predictions reconcile early retrograde P-T estimates with corderite-quartz symplectite coronas on garnet and hercynite grains enclosing garnet and sillimanite. Low-moderate Y and Th monazite corresponds to garnet growth during prograde metamorphism from 561-527 Ma. High Th populations at 522-528 Ma correspond to melting at peak conditions. Retrograde monazite dated at 522-500 Ma is divided into high and low Y populations. Low-Y monazite is found as inclusions in non-symplectic cordierite, whereas high Y monazite is associated with cordierite symplectites rimming garnet. None of the monazite can be attributed to late andalusite-grade retrogression. In summary, Cambrian metamorphism followed a clockwise P-T path and lasted at least 40-60 Ma, i.e., synchronous with the assembly of Gondwana at c. 530 Ma. These results illustrate how pseudosection modeling may be used to inform the assignment of monazite age populations to reactions in P-T space.

Geological Society of America Abstracts with Programs. Vol. 49, No. 6

doi: 10.1130/abs/2017AM-302124

© Copyright 2017 The Geological Society of America (GSA), all rights reserved. Permission is hereby granted to the author(s) of this abstract to reproduce and distribute it freely, for noncommercial purposes. Permission is hereby granted to any individual scientist to download a single copy of this electronic file and reproduce up to 20 paper copies for noncommercial purposes advancing science and education, including classroom use, providing all reproductions include the complete content shown here, including the author information. All other forms of reproduction and/or transmittal are prohibited without written permission from GSA Copyright Permissions.