Scientists have decoded the evolution of the Ladakh Magmatic Arc (LMA) in the northwestern Himalaya, uncovering a 130‑million‑year geological record that traces the subduction, maturation and eventual collision of the Indian and Eurasian plates. The findings shed new light on how the Himalaya began to take shape long before the mountains rose to their present heights.
Researchers from the Wadia Institute of Himalayan Geology, an autonomous body under the Department of Science and Technology, examined the chemistry and isotopic signatures of rocks from Ladakh to reconstruct the slow but powerful processes that once unfolded beneath the Neo‑Tethys Ocean. Millions of years ago, the region that is now Ladakh lay above this ancient ocean, where tectonic slabs plunged into the mantle in a process known as subduction. This movement fuelled the formation of the LMA, a belt of igneous rocks formed between the Jurassic and Eocene periods, from about 201 million to 34 million years ago.
To map the arc’s evolution, scientists compared geochemical and isotopic data from three key rock groups: the pre‑collisional Dras–Nidar Island Arc Complex, the Ladakh Batholith formed during the pre‑ to syn‑collisional stages and part of the larger Kohistan–Ladakh Batholith, and the post‑collisional mafic dykes that intruded after the Indian and Eurasian plates collided. Their analysis revealed that long‑term magmatic activity in the region unfolded through three major episodes—160–110 million years ago, 103–45 million years ago and after 45 million years ago—each shaped by shifts in the dynamics of the subducting Neo‑Tethyan plate and its interaction with the mantle and crust.
The earliest phase resembled a volcanic island arc rising from the Neo‑Tethys Ocean. Rocks from the Dras–Nidar complex preserve evidence that the magma then originated largely from the mantle with minimal influence from sediments dragged down by the subducting plate. As convergence intensified, the arc evolved and large granitic bodies known as the Ladakh Batholith formed at depth. These rocks display stronger continental signatures, indicating that sediments and crustal fragments were increasingly being recycled into the magma as the Indian plate pushed closer to Eurasia.
With the eventual collision of the two plates, the Neo‑Tethys Ocean closed and the Himalaya began to rise. Even after the main collision, molten material continued to intrude the crust, forming narrow mafic dykes that cut through older rocks. These later magmas originated from mantle sources already enriched by earlier subduction‑related processes.
The researchers pieced together this tectono‑magmatic history by analysing rare elements and isotopes such as strontium and neodymium, which help identify whether magma was derived from deep mantle sources, recycled oceanic sediments or continental crust. Their study concludes that sediment subduction played a far more significant role in forming the Kohistan–Ladakh Batholith than in the earlier Dras–Nidar island arc rocks, offering new insights into the complex tectonic evolution that ultimately gave rise to the Himalaya.
