Contrasting18O enrichment and origins of High Himalayan and Transhimalayan intrusives

Twelve analysed leucogranites of the High Himalaya in Bhutan (Chung La, Mönlakarchung) and Garhwal (Badrinath) are among the most¹⁸O-enriched granites known (11.5–12.4‰ δ¹⁸O_SMOW with two exceptions) and separate minerals show good isotopic concordance. The data strongly support an origin of the granites by anatexis of continental basement such as the Indian crystalline basement sheet or slab, undercut by the Main Central Thrust, of which five samples were analysed. In contrast, the pre-collision Transhimalayan (Gangdese) batholiths to the north of the Indus-Tsangpo suture, as exemplified by the Ladakh intrusives, show an initially oceanic trend of δ¹⁸O vs. SiO₂ that becomes gradually somewhat enriched with respect to Hachijo-Jima. While not completely outside the range of enrichment that seems possible by fractional crystallisation, this could tie in with the⁸⁷Sr inhomogeneities reported by Honegger et al. 9], which may be due to assimilation of variably radiogenic Eurasian continental basement. For both the leucogranites and Ladakh intrusives the¹⁸O levels and the concordance between minerals rule out significant cumulative water/rock ratios in syn- or post-magmatic interaction with subsurface waters.