Thesis
Timing of metamorphism in the main central thrust zone in south-central Bhutan: insights from garnet and monazite geochronology
Washington State University
Master of Science (MS), Washington State University
2018
Handle:
https://hdl.handle.net/2376/103942
Abstract
In the Himalayan orogen, major shear zones such as the south-vergent Main Central thrust (MCT) have accommodated a large percentage of India-Asia convergence. In the eastern Himalaya in Bhutan, the MCT has been mapped where aluminosilicate-bearing, migmatitic Greater Himalayan (GH) gneisses have been emplaced over garnet-bearing schist and quartzite of the Jaishidanda Formation (JF). Shearing on the MCT throughout Bhutan has been bracketed between ~23 and ~15 Ma. In south-central Bhutan, recent work has proposed that the MCT is best defined as a 900 m-thick zone, including 700 m of metamorphosed JF and the lowest 200 m of GH rocks; temperature estimates from these rocks define an inverted metamorphic gradient. To quantify the timing of this metamorphism, we determined in-situ Th-Pb monazite dates and Lu-Hf garnet-whole rock isochron dates from five samples within and above the MCT zone. Garnets from GH paragneiss samples collected 0.8 km and 0.2 km above the GH-JF contact yielded ages of ~31 Ma and ~25 Ma, respectively. Trace element electron microprobe mapping revealed zoning within the monazites of the structurally-higher sample that correlates with different 208Pb/232Th dates: Y-poor cores grew between ~21 and ~18 Ma, and Y-enriched rims grew from ~18 to ~15 Ma, while the lower sample did not show clear monazite zoning and yielded 208Pb/232Th dates between ~29 and ~22 Ma. Garnets from three JF samples collected 0.2, 0.4, and 0.4 km below the GH-JF contact yielded ages of ~31 Ma, 29 Ma, and ~27 Ma, respectively. Monazite grains from the structurally-highest JF sample showed zoning: Y-poor cores grew between ~23 and ~21 Ma, and Y-enriched rims grew from ~22 to ~19 Ma. The structurally-lowest two JF samples contained monazite that showed no zoning and yielded dates between ~27 and ~18 Ma. Garnet ages from within and above the MCT zone range from ~31 to ~25 Ma, indicating that both the GH and JF rocks underwent early burial to garnet stable depths, which is also supported by older prograde monazite dates. Garnet and monazite ages that predate movement on the MCT rule out conductive heating, or the "hot-iron effect", as a mechanism for producing the inverted metamorphism observed within the MCT zone.
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Details
- Title
- Timing of metamorphism in the main central thrust zone in south-central Bhutan
- Creators
- Laura Storey Pianowski
- Contributors
- Jeffrey D. Vervoort (Degree Supervisor)
- Awarding Institution
- Washington State University
- Academic Unit
- Environment, School of the (CAHNRS)
- Theses and Dissertations
- Master of Science (MS), Washington State University
- Publisher
- Washington State University; [Pullman, Washington] :
- Identifiers
- 99900525016501842
- Language
- English
- Resource Type
- Thesis