Project

Discipline

petrology ; monazite; allanite; NW Himalaya; geochronology

Lead
Lay summary
This project proposes to investigate the chemical and textural behavior of monazite and allanite with increasing metamorphic grade in a well constrained prograde pelitic suite in order to determine the P-T conditions of reactions forming or affecting monazite and allanite. The High Himalayan Crystalline (HHC) of SE Zanskar (NW India) is selected for this study for two completely different reasons: (1) it offers the unique advantage to consist of a single stratigraphic unit of pelitic rocks, that preserve a gradual increase of the metamorphic conditions from the chlorite zone to the sillimanite-migmatite zone, all in response to one orogeny. A meticulous account of the chemical and textural evolution of monazite and allanite along this already well studied section will yield detailed information on the conditions of formation of allanite and monazite during metamorphism. These results on mineral stability and reaction relations should be of general relevance to petrochronology in any regional metamorphic (Barrovian) terrane; and (2) the studied section is located in the only region of the Himalayas that preserves structures and metamorphism associated with NE-directed early-stage tectonic movements that precede the predominant Himalayan folding and thrusting towards the SW. The uniqueness of the location of the geological segment studied will ensure that our results will bear directly on strongly debated issues regarding the timing and P-T-t paths of the prograde metamorphism in this part of the Himalayan range. Quantitative data associated with the early convergence (e.g. timing, heating rate, burial rate) will have important tectonic implications affecting the Himalayan chain as a whole.

Direct link to Lay Summary

Last update: 21.02.2013

Active participation

1.ABSTRACT OF REASEARCH PLANGeologists aim to extract the P-T-t (Pressure-Temperature-time) history from metamorphic rocks in order to quantify the evolution of orogenic belts. Advances in combining geochronology with the petrologically determined P-T history of suitable samples over the last decade have led to increasingly detailed and reliable reconstructions of P-T-t paths of metamorphic rocks. However, of crucial importance for tectonic reconstructions is the structural and thermal evolution during the early metamorphic stages, such as those associated with continental collision. Because most minerals used in geochronometry may experience (at least partial) resetting of their isotope system owing to further heating and deformation during subsequent orogenic phases, the crucial early history is particularly difficult to reconstruct.Several recent studies have shown that, in addition to zircon, accessory phases such as monazite and allanite are promising minerals to date metamorphic events because the U-Th-Pb isotope systems in both of these phases have been found to be essentially closed under most crustal conditions. Hence the U-Th-Pb system should not be affected by subsequent temperature increases, implying that monazite and allanite U-Th-Pb ages can reflect crystallization ages. Combined with the development of increasingly precise spot analytical techniques (e.g. SHRIMP, SIMS, LA-ICPMS, ID-µTIMS), some of which allow carrying out in situ micro-chronometric analyses, these accessory phases represent good candidates to date metamorphic stages over a wide range of metamorphic conditions. However, for monazite and allanite crystallization ages to be useful in constraining the P-T-t path, it is necessary to know which reaction(s) produced the phases used for dating, and to determine under which P-T conditions the mineral-forming reaction(s) occurred. This is fully predictable provided the stability of these phases is known for a defined chemical system.However, the metamorphic conditions under which monazite and allanites in pelitic schists crystallized are not completely known. However, thermodynamic properties are adequately known for only a small number of REE-phases, and only for pure endmember compositions, not the observed solid solutions. Regrettably, there are also few direct experimental constraints on the respective mineral reactions. These gaps of knowledge thus far limit the integration of precise chronological information provided by monazite or allanite in the tectono-thermal history of samples. In turn, the power of U-Th-Pb dating by means of metamorphic REE phases is not yet fully exploitable as long as the behavior of these phases in metamorphic terrains is not more completely predictable.This project proposes to investigate the chemical and textural behavior of monazite and allanite with increasing metamorphic grade in a well constrained prograde pelitic suite in order to determine the P-T conditions of reactions forming or affecting monazite and allanite. The High Himalayan Crystalline (HHC) of SE Zanskar (NW India) is selected for this study for two completely different reasons: (1) it offers the unique advantage to consist of a single stratigraphic unit of pelitic rocks, which preserve a gradual increase of the metamorphic conditions from the chlorite zone to the sillimanite-migmatite zone, all in response to one orogeny. A meticulous account of the chemical and textural evolution of monazite and allanite along this already well studied section will yield detailed information on the conditions of formation of allanite and monazite during metamorphism. These results on mineral stability and reaction relations should be of general relevance to petrochronology in any regional (Barrovian) terrane; and (2) the studied section is located in the only region of the Himalayas that preserves structures and metamorphism associated with NE-directed early-stage tectonic movements that precede the predominant Himalayan folding and thrusting towards the SW. The uniqueness of the location of the geological segment studied will ensure that our results will bear directly on strongly debated issues regarding the timing and P-T-t paths of the prograde metamorphism in this part of the Himalayan range. Quantitative data associated with the early convergence (e.g. timing, heating rate, burial rate) will have important tectonic implications affecting the Himalayan chain as a whole.