Friday, June 4, 2010

Mineralogy And Deep Subduction Of Indian Continental Crust

Science Daily describes some work on eclogites found in the Ladakh region of the Himalayas and its implications on the subduction of the Indian continental crust underneath the Asian plate.

The paper appeared in last month's Geology and uses the mineral majorite - a high pressure variety of garnet - to estimate the depth of mineral formation. The pattern in which atoms are arranged in a lattice structure and the ratio of certain elements in a mineral are often sensitive to changes in temperature and pressure and can be used to estimate and develop a history of change of these parameters as plate subduction drives crust deeper and mineral growth takes place at higher and higher temperatures and pressures . The results indicate a depth of around 200 km for the formation of the mineral majorite.

So the study throws light not on just on how deep the Indian crust sank during continental collision with Asia but also how quickly it has been exhumed during the formation of the Himalayan mountains. The mineral and its chemical ratios which were stable at 200 km depth have been essentially frozen in that state as the enclosing rock was brought to the surface geologically quickly along thrust faults and through rapid erosion and removal of overlying crust preventing majorite from completely altering to a lower temperature and pressure analog.

Rocks at depths of 200 km formed tens of millions of years ago are today on the surface at great heights of about fourteen thousand odd feet. These ecologites are exposed in the Lake Tso Moriri area of Ladakh in the high Himalayas. It is difficult terrain. Field seasons are restricted to a couple of months in summer. The altitude takes some getting used to.

Lake Tso Moriri / Source: Wolken Berge Wasser Wiese   Author: Jochen Westermann

A faculty friend has a graduate student working on this very same eclogite complex. I hope this publication by a different group does not mean a setback to her work. Finding that you have been scooped after you are in your third year of PhD can be demoralizing but my friend is sure that there are parallel stories of mineral genesis to be wrung out of these ecologites.

Last month's Accretionary Wedge threw open the diverse interests of the geology community... from the microscopic to the continental scale. Discoveries like the one on the Tso Morari ecologites makes you reflect on the fact that these geological features and processes that encompass such vast differences in scale don't occur as independent events not affected by or having no relation to or having no effect on processes larger or smaller.

The secrets of plate tectonics are sometimes hidden in one tiny mineral. I find it fascinating that the chemical ratios found in one mineral reveal a story of continents colliding, the subduction and burial of crust to great depths and its later ascent to icy heights, part of the geological saga of the great Himalayan mountains.


  1. Nice write-up. Ultra-high pressure metamorphism of continental crust is a fascinating topic. I agree with you that it's cool how the presence or composition of a single mineral or an assemblage of a few minerals can tell us so much about the geodynamics of tectonics. One of my favorite papers is Christian Chopin's (1984, CMP 86:2) documentation of coesite (high-pressure silica) in rocks from the Alps. It was, I think, the first proof that continental crust could subduct. Since then, diamond and other minerals, like majorite, keep extending the depth of burial and the necessary speed of excavation.

  2. CM - thanks for the link.. I've been to Tso Moriri as a tourist.. so this paper on ecologites makes me want to visit it again as a geologist..