Granitic orthogneiss contributions to the generation of Himalayan leucogranites: insights from the eastern Himalayas

ABSTRACT

Multi-component crustal sources are universally acknowledged as the overriding factor in causing geochemical heterogeneities of the Cenozoic Himalayan leucogranites. In previous studies, metasedimentary rocks from the Greater Himalayan Crystalline Complex were always underlined to be the dominant origin for leucogranites after the Eocene and Oligocene transition (ca. <36 Ma). However, given that the petrological diversity of the Greater Himalayas, especially the widespread and high-grade metamorphosed granitic gneisses; this traditional standpoint has been increasingly questioned nowadays. To further demonstrate the role of granitic gneiss in leucogranite generation, a rounded compilation of geochronological and geochemical data for leucogranites, granitic gneisses, and other related rocks from the specified N – S striking Yardoi – Cuonadong – Tsona transect has been conducted. After making comprehensive comparison and discussion between leucogranites and granitic gneisses, we argue that Cenozoic Himalayan leucogranites may not be pure metasediments derived S-type granites as orthogneisses could be another important endmember for the provenance of them based on the following evidence: (1) Abundant relict zircons within the Himalayan leucogranites display two evident U – Pb age clusters at ca. 850–800 Ma and ca. 520–470 Ma, which are contemporaneous with the Neoproterozoic and early Paleozoic granitic magmatism, respectively. (2) Zircon Hf isotopes of Himalayan-aged rims (−11.21 to −4.82) could be perfectly constrained by two evolution lines derived from Neoproterozoic (−6.40 to 0.16) and early Paleozoic (−2.37 to 6.15) zircon groups. (3) In terms of whole-rock Sr – Nd isotopes (all corrected to 20 Ma), there is a notable overlap between leucogranites (0.7142 to 0.8429 for Sr; and −17.34 to −9.86 for Nd) and granitic gneisses (0.7703 to 0.8716 for Sr, and −16.27 to −9.80 for Nd). (4) Although the fertility of granitic gneisses should be poorest in the absence of a separate aqueous phase; however, the evolutionary P – T – XH₂O conditions triggered by compressional thrust activity of the Main Central Thrust and arc-parallel extension have remarkably modified original source structures (infiltration by LHS-derived fluids) and melting behaviours (more recognized fluid-present partial melting cases). Consequently, the role the granitic gneisses would be strengthened because of the greatly improved fertility via fluid-present melting; and the Sr – Nd isotopic signatures of Himalayan leucogranites would be spatially-temporally evolved.

KEYWORDS:

• Himalayan leucogranites
• granitic gneisses
• crustal sources
• anatexis

Acknowledgment

We always appreciate the help and support from Dr. Robert J. Stern. Numerous insightful questions and constructive comments from two anonymous reviewers are of significance for us to improve the quality of this paper. The detailed typesetting work by Subashini from TIGR production team is also worthy of our gratitude.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Data availability statement

Data will be made available on request.

Supplementary material

Supplemental data for this article can be accessed online at https://doi.org/10.1080/00206814.2024.2335498

Additional information

Funding

The author(s) reported there is no funding associated with the work featured in this article.