Zircon Geochronology of Subducted Crust: Interpreting Zircon-Garnet Equilibrium and Disequilibrium, North Qaidam Terrane, Western China
Abstract
Zircon geochronology is a powerful tool to constrain subduction and exhumation processes, but interpretation of results can be challenging. Here we combine trace element and inclusion analysis of Zrn with phase equilibrium modeling to link U-Pb ages to the P-T path. The North Qaidam terrane near Dulan exposes UHP eclogite across >100 km2 and a smaller exposure of HP granulite west of the eclogite outcrops. Zrn U-Pb ages are ca. 463-422 Ma from eclogites and 449-410 Ma from HP granulites, but the connection between these ages and the samples' P-T paths are not always clear. A UHP phengite eclogite records P-T conditions of 675°C, 32 kbar, near the lawsonite stability limit in this rock composition. Zrn (429 ± 2 Ma) shows flat HREE slopes (Yb/Gd = 0.52-4.2), and small to absent negative Eu anomalies (Eu/Eu* = 0.65-2.1). Grt shows little zoning, and REE patterns show flat HREE slopes (Yb/Gd = 0.3-1.5) without negative Eu anomalies (Eu/Eu* = 1.2-1.7). HREE patterns and concentrations (10-30x chondrite) are similar in both Zrn and Grt, and Zrn contains inclusions of Grt and Omp. We therefore interpret the Zrn age to record UHP conditions; fluid release from lawsonite breakdown likely promoted both Zrn growth and equilibration of the major minerals that record the P-T conditions. An intermediate HP granulite records P-T conditions of 830-900°C, ~22 kbar, and field evidence suggests significant partial melting; lower-P conditions are defined by Pl-equilibria (775-875°C, 14-17 kbar). Zrn (418 ± 2 Ma) shows flat HREE slopes (Yb/Gd = 1.4-5.6) without negative Eu anomalies (Eu/Eu* = 0.98-1.2), and contains multiphase inclusions of Pl + Kfs + SiO2 ± Bt. Grt is zoned (Prp12-Prp40) with highest Mg at the rim. Grt HREE slope and curvature increase from cores (Yb/Gd ~ 5) to mantles and rims (Yb/Gd ~ 30-40) without negative Eu anomalies (Eu/Eu* ~ 1.5-1.8). All Grt zones contain much lower HREE concentrations than Zrn (Yb = 10-40x vs. 100-1000x chondrite), and Zrn lacks the curved HREE pattern shown by Grt, suggesting Zrn did not equilibrate with any of the analyzed Grt zones. Multiphase inclusions suggest Zrn grew in the presence of melt, but lack of Eu anomalies suggests Pl was minor or absent, so Zrn likely dates post-peak melt-present decompression before significant Pl growth, between ~22 and 15 kbar, where Grt is predicted to resorb rather than grow.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.V43F0150M
- Keywords:
-
- 1031 Subduction zone processes;
- GEOCHEMISTRY;
- 8031 Rheology: crust and lithosphere;
- STRUCTURAL GEOLOGY;
- 8150 Plate boundary: general;
- TECTONOPHYSICS;
- 8178 Tectonics and magmatism;
- TECTONOPHYSICS