Eocene migmatite formation and diachronous burial revealed by petrochronology in NW Himalaya, Zanskar
Abstract
In this contribution, we highlight the importance ofin-situmonazite geochronology linked toP-Tmodelling for identification of timescales of metamorphic processes. Barrovian-type micaschists, migmatites and augengneiss from the Gumburanjun dome in the southeastern extremity of the Gianbul dome, NW Himalaya, have been studied in order to correlate the early stages of Himalayan metamorphism at different crustal levels and infer the timing of anatexis.P-T-tpaths are constrained through combined pseudosection modelling andin-situand in-mount monazite and xenotime laser ablation-split-stream inductively coupled plasma-mass spectrometry.
Petrography and garnet zoning combined with pseudosection modelling show that garnet-staurolite schists record burial from similar to 530 to 560 degrees C and 5.5 kbar to similar to 630 to 660 degrees C and 7 kbar; staurolite-kyanite schists from similar to 530 to 560 degrees C and 5 kbar to similar to 670 to 680 degrees C and 7-9 kbar; and garnet-kyanite migmatites from 540-570 degrees C and 5 kbar to similar to 680 to 750 degrees C and 7-10 kbar, probably also to >750 degrees C and >9 kbar above the muscovite stability field. The decompression paths of garnet-staurolite schists indicate cooling on decompression, while garnet rim chemistry and local sillimanite growth point to a stage of re-equilibration at similar to 600 to 670 degrees C and 4-6 kbar in some of the staurolite-kyanite schists, and at similar to 670 to 700 degrees C and 6 kbar in garnet-kyanite migmatites.
Some of the staurolite-kyanite schists and garnet-kyanite migmatites also contain andalusite or andalusite-cordierite. Monazite and xenotime were analysed in thin sections in garnet, staurolite and kyanite, and in the matrix; and in mounts.
BSE images and compositional maps of monazite (xenotime was too small) show variable internal structures from homogeneous through patchy zoning with embayed to sharp boundaries. Two groups of samples can be identified on the basis of the presence or absence ofc. 44 - 37 Ma ages.
The first group of samples-two garnet-staurolite schists-recorded onlyc. 31 - 27 Ma ages in porphyroblasts and noc. 40 Ma ages. The second group (samples of staurolite-kyanite schist, garnet-kyanite migmatites, augengneiss) have both the older,c. 44 - 37 Ma monazite ages in porphyroblasts and younger ages down toc. 22 Ma.
These significantly different ranges of ages from porphyroblasts of 44-37 Ma, and 31-27 Ma, are interpreted as the duration of progradeP-Tpaths in Eocene and Oligocene, and indicate diachronous two-stage burial of rocks. Early migmatization occurred at 38 Ma.
Thec. 29 Ma is interpreted as the time when rocks from the lower and middle crustal levels were partially exhumed and came in to contact with rocks that were downgoing at this time. Localized monazite recrystallization is as young as 26-24 Ma.
The youngest ages of 23-22 Ma are related to leucogranite emplacement.