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Dave Waters - Metamorphic Petrology Research at Oxford
Metamorphic rocks retain a wealth of information about their history in their microstructures and compositional patterns. Making use of this record involves two strands of study:
- Learning about the fundamental processes of metamorphic change, and in particular how the observed patterns reflect the thermodynamics and kinetics of mineral reactions.
- Using this knowledge to determine pressure-temperature-time paths for deep-seated metamorphic rocks, with the aim of solving problems in continental tectonics.
Microscopic observations, therefore, are a key to understanding crustal-scale processes. My research forms an integral part of collaborations with Dr M. P. Searle and external co-workers, and involves several graduate students. Our principal tools are the polarising microscope, scanning electron microscope, and the electron probe microanalyser.
Fundamental processes: microstructure and kinetics
Large thermal aureoles, such as that beneath the
mafic rocks of the Bushveld Complex, South Africa,
provide an environment of known thermal history for
testing the predicted relationships between the rates
of heating, nucleation and growth, which are
ultimately expressed in crystal size distributions.
We are developing a quantitative model which can
account for the decrease in observed porphyroblast
size with increasing grade and constrain the
overstepping required to drive metamorphic reactions.
Subtle detail in the microstructure (see
illustration) indicates sequences of mineral growth
that cannot be matched by equilibrium thermodynamic
models, suggesting that barriers to nucleation are
sufficient to allow metastable reaction sequences to
occur, at least in the static environment of contact
metamorphism.
Applications to continental tectonics
1. The metamorphic core of the Himalaya and Karakoram
The
metamorphic rocks in the axial zones of mountain
belts preserve a record of the behaviour of
continental crust during collision. In the NW Zanskar
Himalaya, for example, crustal thickening is revealed
both in kilometre-scale structures (pictured here is
the closure of the Donara nappe) and at the
microscopic scale of mineral growth and fabric
development. We extract pressure-temperature paths
from compositionally-zoned garnets that grew during
compressive deformation, using the thermodynamic data
sets and calculation programs THERMOCALC
and
GIBBS. Here, we find a steady increase in both P
and T, rather than the P increase followed by thermal
relaxation commonly proposed for idealised
collisional orogeny. We are also working to constrain
the amount, timing and rate of exhumation associated
with major fault systems (such as the South Tibetan
detachment) in the Everest area, in Zanskar, and at
Nanga Parbat in the NW Himalaya. To circumvent shaky
assumptions about thermal gradients during cooling
through mineral blocking temperatures, this is being
done by direct pressure determinations on peak and
retrograde mineral associations, and by numerical
modelling of the temperature-depth evolution.
2. The formation and exhumation of eclogites in collision zones
We are
studying eclogite-facies rocks, formed from
continental material of shallow origin, in Norway,
Oman, and parts of the Himalayan chain. Eclogites
form at depths in excess of 50 km, deeper than the
base of normal continental crust, and special
mechanisms must operate to drive buoyant crust down
to mantle depths, and to return some of it to the
surface. In western Norway, parts of the Western
Gneiss Region have experienced pressures > 30 kbar
(100 km depth) and contain relics of coesite, the
high-density silica polymorph. We have shown that
these ultra-high pressure rocks form a distinct unit
that can be differentiated from lower-pressure (ca.
20 kbar) eclogites on petrographic and mineral
composition criteria as well as on evidence for the
former presence of coesite.
Mechanisms for the exhumation of Norwegian eclogites can be illuminated by studying decompression textures. The fine-grained mineral intergrowths that replace eclogite-facies phases (see photomicrograph) show systematic trends in composition and lamellar spacing. These appear to record a significant amount of cooling during exhumation from ca. 50 to 25 km depth, favouring a mechanism controlled by extensional shear zones rather than homogeneous thinning or erosion.
3. The cratonisation process: granulites, migmatites and charnockites in Precambrian terrains
The consolidation of new or reworked
continental crust involves granulite-facies
metamorphism, partial melting, and the emplacement of
water-undersaturated magmas. Even in rocks subjected
to such high temperatures, a record of the processes
remains, either in outcrop-scale textures of
migmatitic rocks, or in the small-scale compositional
patterns within refractory minerals. The rare-earth
phosphate monazite (illustrated) holds particular
promise, as the growth zones can in principle be
dated as well as correlated with the reaction history
of the rock. For example in Namaqualand (South
Africa), field relationships, reaction
microstructures, calculated mineral equilibria, in
situ monazite dating (provisional) and
geochemical mass balance tell us that granulites
formed in the middle crust (15 - 20 km depth) at ca.
1060 - 1030 Ma on a path of increasing pressure
during heating to 850°C. They lost a small amount
of melt, sufficient to remove the water from the
breakdown of hydrous minerals, and underwent only
limited back-reaction with residual melt at ca. 1020
- 1010 Ma.
Recent Publications
Warren, C.J. and Waters, D.J. (2006). Oxidized eclogites and garnet-blueschists from Oman: P–T path modelling in the NCFMASHO system. Journal of Metamorphic Geology, 24 (9), 783-802.
Parrish, R.R., Gough, S.J., Searle, M.P. and Waters, D.J. (2006). Plate velocity exhumation of ultrahigh-pressure eclogites in the Pakistan Himalaya. Geology, 34 (11), 989–992.
Warren, C.J., Parrish, R.R., Waters, D.J. and Searle, M.P. (2005). Dating the geologic history of Oman’s Semail ophiolite: insights from U-Pb geochronology. Contributions to Mineralogy and Petrology, 150, 403-422.
Searle, M.P., Warren, C.J., Waters, D.J. and Parrish, R.R. (2004). Structural evolution, metamorphism and restoration of the Arabian continental margin, Saih Hatat region, Oman Mountains. Journal of Structural Geology, 26, 451–473.
Grew, E.S., Rao, A.T., Raju, K.K.V.S., Hejny, C., Moore, J.M., Waters, D.J., Yates, M.G. and Shearer, C.K. (2003). Prismatine and ferrohogbomite-2N2S in granulite-facies Fe-oxide lenses in the Eastern Ghats Belt at Venugopalapuram, Vizianagaram District, Andhra Pradesh, India; do such lenses have a tourmaline-enriched lateritic precursor? Mineralogical Magazine, 67 (5), 1081-1098.
Warren, C.J., Parrish, R.R., Searle, M.P. and Waters, D.J. (2003). Dating the subduction of the Arabian continental margin beneath the Semail Ophiolite, Oman. Geology, 31 (10), 889-892.
Searle, M.P., Simpson, R.L., Law, R.D., Parrish, R.R. and Waters, D.J. (2003). The structural geometry, metamorphic and magmatic evolution of the Everest massif, High Himalaya of Nepal–South Tibet. Journal of the Geological Society, London, 160, 345-366.
Waters, D.J. and Charnley, N.R. (2002). Local equilibrium in polymetamorphic gneiss and the titanium substitution in biotite. American Mineralogist (Holdaway issue), 87, 383-396.
Waters, D.J. and Lovegrove, D.P. (2002). Assessing the extent of disequilibrium and overstepping of prograde metamorphic reactions in metapelites from the Bushveld aureole. Journal of Metamorphic Geology, 20, 135-149.
Walker, C.B., Searle, M.P. and Waters, D.J. (2001). An integrated tectono-thermal model for the evolution of the High Himalaya in western Zanskar with constraints from thermobarometry and metamorphic modeling. Tectonics, 20, 810-833.
Wain, A.L., Waters, D.J. and Austrheim, H. (2001). Metastability of granulites and processes of eclogitisation in the UHP region of Western Norway. Journal of Metamorphic Geology, 19, 609-625.
Waters, D.J. (2001). The significance of prograde and retrograde quartz-bearing intergrowth microstructures in partially-melted granulite-facies rocks. In: Kriegsman, L. (ed.) Prograde and retrograde processes in crustal melting, Lithos, 56, 97-110.
Stephenson, B.J., Waters, D.J. and Searle, M.P. (2000). Inverted metamorphism and the Main Central Thrust: field relations and thermobarometric constraints from the Kishtwar Window, NW Indian Himalaya. Journal of Metamorphic Geology, 18, 571-590.
Simpson, R.L., Parrish, R.R., Searle, M.P., and Waters, D.J., (2000). Two episodes of monazite crystallisation during prograde metamorphism in the Everest region, Nepalese Himalaya. Geology, 28, 403-406.
Wain, A.L., Waters, D.J., Jephcoat, A. and Olijynk, H. (2000). The high-pressure to ultrahigh-pressure eclogite transition in the Western Gneiss Region, Norway. European Journal of Mineralogy, 12 (3), 667-687.
Robb, L.J., Armstrong, R.A. and Waters, D.J. (1999). The history of granulite-facies metamorphism and crustal growth from single zircon U-Pb geochronology: Namaqualand, South Africa. Journal of Petrology, 40, 1747-1770.
Searle, M.P., Waters, D.J., Dransfield, M.W., Stephenson, B.J., Walker, C.B., Walker, J.D. and Rex, D.C. (1999). Thermal and mechanical models for the structural and metamorphic evolution of the Zanskar High Himalaya. In: Mac Niocaill, C. and Ryan, P.D. (eds.) Continental Tectonics. Geological Society of London, Special Publications, 164, 139-156.