Transhimalayan Batholiths Research Articles

Time-temperature pathways of himalayan and trans-himalayan crystalline rocks: A comparison of fission-track ages

Fission-track (FT) dates of apatate and zircon recording, respectively, paleo-temperatures of 100–120°C and ∼230°C provide a useful tool for understanding the thermotectonic history of the Himalaya, which is the world's loftiest and most dynamic orogenic belt, brought about by the continent-to-continent collision of the Indian and Asian plates. Systematic FT ages are now available for the crystalline rocks of the Higher Himalaya and Trans-Himalaya in Pakistan and N.W. India, and some FT ages have been reported from southern Tibet, the Karakorum and the Kunlum. These FT data are synthesized and discussed here in order to assess the contribution of FT thermochronology to Himalayan geology. FT zircon and apatite ages from the Higher Himalayan Crystalline rocks are confined to the Neogene. For example, in the Zanskar region of N.W. India and the Babusar-Kaghan region of Pakistan, zircon ages are 13–17 Ma and apatate ages range from 4–10 Ma (mostly ∼6 Ma) indicating an exhumation of at least 8 km for these rocks since the Middle Miocene and average unroofing rates of ∼ 6 mm yr −1. In the northwestern Himalaya, overall the FT ages become younger towards the Nanga Parbat Massif, which is the promontory of the Indian plate and has experienced a rapid Quaternary uplift and exhumation. Other areas of young uplift in the Himalayan include antiformal domes in Zanskar, such as the Kishtwar Window. The bulk of the Cretaceous-Paleocene Trans-Himalayan crystalline rocks, which are situated to the north of the Indus-Tsangpo Suture Zone and were generated from the consumption of the Tethyan ocean beneath Asia, seem to have cooled rapidly through paleo-temperatures of 200–500°C in the Eocene as a result of uplift and erosion that affected the Trans-Himalayan Batholith after the India-Asia collision. Apatite ages from these rocks in western Kohistan, southern Ladakh, and Lhasa are Early-Middle Miocene. In the Karakorum (western Tibet) and the Kunlun (northern Tibet), Miocene apatite ages demonstrate young tectonics of the ranges beyond the Himalaya related to the India-Asia convergence.

Oxygen isotopes in mantle related and geothermally altered magmatites of the Transhimalayan (Gangdese) ranges

In closed magma systems SiO2 approximately measures differentiation progress and oxygen isotopes can seem to obey Rayleigh fractionation only as a consequence of the behaviour of SiO2. The main role of δ18O is as a sensitive indicator of contamination, either at the start of differentiation (δ18Oinit) or as a proportion of fractionation in AFC. Plots of δ18O vs SiO2-allow to determine initial δ18O values for different sequences for source comparison. For NBS-28=9.60, the δ18O at 48% SiO2-varies between a high 6.4‰ for Kiglapait (Kalamarides 1984), 5.9‰ for Transhimalaya, 5.8‰ for Hachijo-Jima (Matsuhisa 1979), 5.6‰ for Koloula (Chivas et al. 1982) and a low 5.3‰ for the Darran Complex, New Zealand. The Transhimalayan batholiths (Gangdese belt) were emplaced in the ‘Ladakh-Lhasa terrane’, between the present-day Banggong-Nujiang, and Indus-Yarlung Tsangbo suture zones, after its accretion to Eurasia. The gradient of the least contaminated continuous (δ18O vs SiO2-igneous trend line is similar to that of Koloula, and AFC calculations suggest a low secondary assimilation rate of less than 0.05 times the rate of crystallisation. Outliers enriched in 18O are frequent in the Lhasa, and apparently rare in the Ladakh transsect. Low-δ18O (5.0‰−0‰) granitoids and andesites on the Lhasa-Yangbajain axis are the result of present day or recent near-surface geothermal activity; their quartzes still trace the granitoids to the Transhimalaya δ18O trend line, but the distribution of low total rock or feldspar δ18O values could be a guide to more recent heat flow and thermally marked tectonic lineaments. Two ignimbrites from Maqiang show hardly any 18O-contamination by crustal material.

Tectonic framework of the Himalaya, Karakoram and Tibet, and problems of their evolution

The Himalaya, the Karakoram and Tibet were assembled by the successive accretion to Asia of continental and arc terranes during the Mesozoic and early Tertiary. The Jinsha and Banggong Sutures in Tibet join continental terranes separated from Gondwana. Ophiolites were obducted onto the shelf of southern Tibet in the Jurassic before the formation of the Banggong Suture. The Kohistan—Ladakh Terrane contains an island arc that was accreted in the late Cretaceous on the Shyok Suture and consequently evolved into an Andean-type batholith. Further east this TransHimalayan batholith developed on the southern active margin of Tibet without the prior development of an island arc. Ophiolites were obducted onto the shelf of India in the late Cretaceous to Lower Palaeocene before the closing of Tethys and the formation of the Indus—Yarlung Zangbo Suture at about 50 Ma. Post-collisional northward indentation of India at ca.5 cm a-1 since the Eocene has redeformed this accreted terrane collage; palaeomagnetic evidence suggests this indentation has given rise to some 2000 km of intracontinental shortening. Expressions of this shortening are the uplift of mid-crustal gneisses in the Karakoram on a late-Tertiary breakback thrust, folding of Palaeogene redbeds in Tibet, south-directed thrust imbrication of the foreland and shelf of the Indian Plate, north-directed back-thrusts along the Indus Suture Zone, post-Miocene spreading and uplift of thickened Tibet, giving rise to N—S extensional faults, and strike-slip faults, which allowed eastward escape of Tibetan fault blocks.

Contrasting 18O 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 most 18O-enriched granites known (11.5–12.4‰ δ 18O 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 δ 18O vs. SiO 2 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 the 87Sr 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 the 18O levels and the concordance between minerals rule out significant cumulative water/rock ratios in syn- or post-magmatic interaction with subsurface waters.