Journal of Asian Earth Sciences
Journal of Asian Earth Sciences
Volume XX, Issue XX Abstract
In press Data Repository
Pages XX-XX    Thermobarometric Data
   Geochronologic Data
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Records of the evolution of the Himalayan orogen from in situ Th-Pb ion microprobe dating of monazite: eastern Nepal and Garhwal

E.J. Catloscorrespondence information, send an email, 1, T. Mark Harrison1, Craig E. Manning1, Marty Grove1, Santa Man Rai2, M.S. Hubbard3, B.N. Upreti2

1.Department of Earth and Space Sciences and Institute of Geophysics and Planetary Physics, University of California, Los Angeles, California, 90095-1567, USA

2.Department of Geology, Tri-Chandra Campus, Tribhuvan University, Kathmandu, Nepal

3.Department of Geology, Kansas State University, Manhattan, Kansas, 66506, USA

Received Sept. 28, 2000; revised March 22, 2001; accepted March 27, 2001. Available online XX.


Abstract

In situ Th-Pb monazite ages from rocks collected along two transects (the Dudh Kosi-Everest, eastern Nepal and the Bhagirathi River, Garhwal Himalaya, India) perpendicular to the Main Central Thrust (MCT) suggest a striking continuity of tectonic events across the Himalaya. The youngest age reported in this study, 5.90.2 Ma (MSWD= 0.4), from matrix monazite grains collected beneath the MCT in the Garhwal region is consistent with several age data from rocks at similar structural levels in central Nepal, providing support for widespread Late Miocene MCT activity. The lateral parallelism of orogenic events is further manifested by the 20.70.1 Ma age of a High Himalayan leucogranite from an injection complex that outcrops along the Dudh Kosi-Everest transect, resembling ages of these bodies reported elsewhere. The youngest monazite grain analyzed along the Dudh Kosi-Everest transect is 10.30.8 Ma. The absence of 7-3 Ma monazite in eastern Nepal may reflect a different nappe structure, which obscures the reactivated ramp equivalent exposed in the Garhwal and central Nepal. Garnets from the MCT hanging wall (Greater Himalayan Crystallines) and footwall (Lesser Himalaya) display different major element zoning, and the patterns are useful for constraining the location of the thrust system that separates the two lithologies. Pressure-temperature paths for two upper Lesser Himalayan garnets that contain monazite inclusions indicate the utility of an in situ methodology to constrain the metamorphic evolution of the shear zone. Along the Dudh Kosi-Everest transect, upper Lesser Himalayan monazite grains from three rocks record a clear signal at 14.50.1 Ma (MSWD= 8.4), and the ~23 Ma age that characterizes the hanging wall is notably absent. Monazite collected within a large-scale Greater Himalayan Crystallines fold yield the ~14 Ma age, consistent with the structure forming due to MCT-related compression. Paleo-Mesoproterozoic (1407 35 Ma) matrix monazite grains are found within an augen gneiss unit located beneath the MCT, whereas Cambro-Ordovician (4368 Ma; 54817 Ma) inclusions are preserved within garnets of the Greater Himalayan Crystallines. The presence of 45.2 2.1 Ma grains from lower structural levels of the Greater Himalayan Crystallines indicates the unit realized conditions conducive for monazite growth during the Eocene.


Data Repository

All data tables and figures are given in PDF format and can be accessed if you download Adobe Acrobat Reader .  This  program is free of charge.

1. Thermobarometric data

Detailed electron microprobe analyses of Everest region samples:

ET19 (Greater Himalaya)
ET52 (upper Lesser Himalaya)
ET33 (upper Lesser Himalaya)
ET45 (lower Lesser Himalaya)

Mineral compositions used in P-T paths modeling

X-ray element maps of Everest region garnets:
Adjust the brightness/contrast of these maps yourself.  For the scale bars, see the pdf files above.  Just download the .tif or .jpg file and open them using Image J (PC) or NIH Image (Mac). These programs are free of charge.

Sample

Mn Ca Mg Fe

ET19 (Greater Himalaya)

10.2K 33.2K 24.4K 20.1K
ET52 (upper Lesser Himalaya) 27.8K 22.2K 23.3K 18.1K
ET33 (upper Lesser Himalaya) 10.2K 14.4K 4.70K 4.85K
ET45 (lower Lesser Himalaya) 10.5K 16.6K 20.3K 20.3K

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2. Geochronologic data

Detailed Th-Pb  ion microprobe analyses of Himalayan monazite grains

BSE images of dated eastern Nepal and Garhwal monazite grains
ET7 ET12 ET18b ET19
ET22 ET23b ET25 ET26
ET52 ET33 85H20g GM74
ET38
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3. Full manuscript text
Manuscript text
Figure Captions
Manuscript tables
Table 1. Compositional parameters used in the geothermobarometric calculations.
Table 2. Th-Pb ages of spots on eastern Nepal monazite grains.
Table 3. Summary of the ion microprobe monazite Th-Pb age results.
Manuscript figures
Fig 1. Generalized geologic map of eastern Nepal.
Fig 2. Generalized geologic map of the Garhwal Himalaya.
Fig 3. Geologic traverse and sample location map along the Dudh Kosi, eastern Nepal.
Fig 4. Geologic cross section along the Dudh Kosi in eastern Nepal.
Fig 5. Two samples ET39 (upper) and ET38 (lower) from the Phaplu augen gneiss unit.
Fig 6.  Compositional zoning profiles across ET45, ET52, ET33, and ET19 garnets.
Fig 7. Backscattered electron images of upper Lesser Himalaya garnets.
Fig 8.  P-T paths calculated for upper Lesser Himalaya garnets.
Fig 9. Backscattered electron image of Greater Himalayan Crystallines sample ET26.
Fig 10. Backscattered electron image of Greater Himalayan Crystallines sample ET22.
Fig 11. Backscattered and   secondary electron images of upper Lesser Himalaya sample 85H20g.
Fig 12. Backscattered electron images of Garhwal sample GM74.
Figure 13. Schematic illustration of the tectonic development of the Dudh Kosi-Everest Himalayan region.
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mail1.gif (129 bytes) Corresponding author. Tel.: (310) 206-2940; Fax: (310) 825-2779; Email: catlos@argon.ess.ucla.edu; Web page: http://oro.ess.ucla.edu/~catlos/


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Volume XX, Issue XX
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