In their last stages of evolution (ie at the
magmatic-hydrothermal transition), peraluminous
granitic melts exsolve a large amounts of fluids
which can modify the chemical composition of
whole-rock granitic samples and lead to the
deposition of economicaly significant mineralization
(Sn, W)
Nb and Ta are lithophile elements considered to
be “geochemical twins” because they have the same
charge and a similar ionic radius However, Nb/Ta
ratios are highly variable in granites Some authors
have demonstrated that the Nb/Ta ratios decrease in
granites during fractional crystallization [1] Other
studies have suggested that Nb and Ta could be
fractionated in evolved peraluminous granites during
the interaction with late magmatic fluids [2]
In this study [3], we demonstrate, using a
compilation of whole-rock geochemical data
available in the litterature that fractional
crystallization alone is not sufficient to explain the
distribution of Nb-Ta in most peraluminous granites
However, we notice that most of the granitic samples
displaying evidence of interactions with fluids have
Nb/Ta < ∼5 We propose that the decrease of the
Nb/Ta ratio in evolved melts is the consequence of
both fractional crystallization and sub-solidus
hydrothermal alteration We suggest that the Nb/Ta
value of ~5 fingerprints the magmatic-hydrothermal
transition in peraluminous granites Furthermore, a
Nb/Ta ratio of ~5 appears to be a good marker to
discriminate mineralized from barren peraluminous
granites

Nb-Ta fractionation in peraluminous granites: a marker of the magmatic-hydrothermal transition

Timing and mechanisms of Tibetan Plateau uplift

Genesis of the Cuonadong tin polymetallic deposit in the Tethyan Himalaya: Evidence from geology, geochronology, fluid inclusions and multiple isotopes

Integrated pseudosection modeling and monazite petrochronology of paragneiss from the Kanchenjunga region of northeastern Nepal reveal the presence of cryptic tectonometamorphic discontinuities within the Himalayan metamorphic core. These new data outline a series of thrust-sense structures that juxtapose rocks that generally record a protracted history of early Eocene to latest Oligocene−early Miocene (ca. 41−23 Ma) prograde metamorphism and lateral extrusion against others that typically record short prograde (between 2 and 7 m.y.) and retrograde (between 3 and 6 m.y.) histories variably spanning the middle Oligocene to the middle Miocene (ca. 31−12 Ma). Retrograde metamorphism in the hanging wall of the thrust faults is typically coeval with prograde metamorphism in the footwall, indicating that overthrusting/underthrusting accommodated crustal shortening and drove metamorphic processes. The structures and juxtaposed panels were cut by early Miocene (ca. 20−18 Ma) out-of-sequence thrusting coincident with the previously mapped High Himal thrust. The resulting kinematic model for the evolution of the Himalayan metamorphic core in the Kanchenjunga area demonstrates that the Himalayan metamorphic core was dominated by underplating from at least the Oligocene through to present, and that the internal structure of the exhumed metamorphic core is significantly more complex than has been documented previously.

Ductile extrusion, underplating, and out-of-sequence thrusting within the himalayan metamorphic core , kanchenjunga, nepal

Miocene Sn polymetallic mineralization in the Tethyan Himalaya, southeastern Tibet: A case study of the Cuonadong deposit

Highly fractionated Himalayan leucogranites and associated rare-metal mineralization

First evidence of eclogites overprinted by ultrahigh temperature metamorphism in Everest East, Himalaya: Implications for collisional tectonics on early Earth

In-sequence buoyancy extrusion of the Himalayan Metamorphic Core, central Nepal: Constraints from monazite petrochronology and thermobarometry

Eocene Metamorphism and Anatexis in the Kathmandu Klippe, Central Nepal: Implications for Early Crustal Thickening and Initial Rise of the Himalaya

This paper introduces how crustal thickening controls the growth of the Himalaya by summarizing the P‐T‐t evolution of the Himalayan metamorphic core. The Himalayan orogeny was divided into three stages. Stage 60–40 Ma: The Himalayan crust thickened to ∼40 km through Barrovian‐type metamorphism (15–25 °C/km), and the Himalaya rose from <0 to ∼1000 m. Stage 40–16 Ma: The crust gradually thickened to 60–70 km, resulting in abundant high‐grade metamorphism and anatexis (peak‐P, 15–25 °C/km; peak‐T, >30 °C/km). The three sub‐sheets in the Himalayan metamorphic core extruded southward sequentially through imbricate thrusts of the Eo‐Himalayan thrust, High Himalayan thrust, and Main Central thrust, and the Himalaya rose to ≥5,000 m. Stage 16–0 Ma: the mountain roots underwent localized delamination, causing asthenospheric upwelling and overprinting of the lower crust by ultra‐high‐temperature metamorphism (30–50 °C/km), and the Himalaya reached the present elevation of ∼6,000 m. Underplating and imbricate thrusting dominated the Himalaya' growth and topographic rise, conforming to the critical taper wedge model. Localized delamination of mountain roots facilitated further topographic rise. Future Himalayan metamorphic studies should focus on extreme metamorphism and major collisional events, contact metamorphism and rare metal mineralization, metamorphic decarbonation and the carbon cycle in collisional belts.

Gautam Prashad Khanal

Papers

Highly fractionated Himalayan leucogranites and associated rare-metal mineralization

First evidence of eclogites overprinted by ultrahigh temperature metamorphism in Everest East, Himalaya: Implications for collisional tectonics on early Earth

In-sequence buoyancy extrusion of the Himalayan Metamorphic Core, central Nepal: Constraints from monazite petrochronology and thermobarometry

Eocene Metamorphism and Anatexis in the Kathmandu Klippe, Central Nepal: Implications for Early Crustal Thickening and Initial Rise of the Himalaya

The Himalayan Collisional Orogeny: A Metamorphic Perspective