Petrochronology of high-grade blocks of the Franciscan subduction-accretion complex
Abstract
We investigate a suite of garnet amphibolite blocks from serpentinite matrix mélanges of the Califonia Coast Ranges (Jenner, Ring Mountain, and Berkeley Hills). Other blocks in these mélanges are eclogites, blueschists, and blueschist-facies metasediments. The garnet amphibolites show variable eclogite and blueschist-facies overprints, previously interpreted as recording a polymetamorphic (reburial) P-T-thistory. The Franciscan mélanges are prime sites in the controversy of the respective relevance of i) buoyancy-driven serpentine diapirism and ii) return-flow in a subduction channel for the exhumation of high-grade rocks during subduction. We are currently acquiring a dataset consisting of 40Ar/39Ar dates of calcic amphibole and white mica dating cooling and post-peak P-T overprints. Combined with published prograde Lu-Hf garnet ages, they allow quantifying the maximum residence time of the blocks at high-grade conditions as a function of peak grade, and exploring the variation of timing of blueschist-facies overprint. We also analyze U-Pb and trace-element systematics of rare euhedral and nodular zircons from the garnet amphibolites using LASS-ICP-MS depth profiling of their surfaces and conventional ablation of their cores. We aim to contribute to the understanding of initiation of Franciscan subduction, the mass transfer of serpentinite in subduction zones, and the exhumation of high-grade rocks during subduction.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2017
- Bibcode:
- 2017AGUFM.V31D..07R
- Keywords:
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- 3653 Fluid flow;
- MINERALOGY AND PETROLOGY;
- 3660 Metamorphic petrology;
- MINERALOGY AND PETROLOGY;
- 8110 Continental tectonics: general;
- TECTONOPHYSICS;
- 8130 Heat generation and transport;
- TECTONOPHYSICS