Petrology, Economic, and Tectonic Research & Analysis (PETRA)

The Variscan granites of SW England are Permian (295-275 Ma) S-type granites containing tourmaline, lithium micas, rare topaz and are pervasively mineralised with tin (cassiterite) tungsten (wolframite), copper (chalcopyrite, bornite), arsenic (arsenopyrite) and zinc (sphalerite) mineralised lodes. We are working on all the main Cornubian granite plutons, and their metamorphic aureoles, especially on the Land’s End, Cligga Head, Tregonning, Carnmenellis, and St. Austell plutons, combining geology and geophysics (gravity, seismic, bathymetry, topography) to determine the shape, structure, and emplacement mechanism of the granite batholith.

The Devonian Lizard ophiolite complex in Cornwall is the largest exposed ophiolite in the Variscan orogenic belt and contains an almost complete section from mantle peridotites (lherzolites, harzburgites), Moho transition zone, crustal gabbros, up to the sheeted dykes. It has an extensive metamorphic sole around the base consisting of amphibolites, and greenschist facies assemblages, and an enigmatic sequence of granitoids around the base (Kennack gneiss). Our project involves field structural mapping, thermobarometry, geochemistry, and U-Pb zircon dating in order to unravel the origin and emplacement history of the Lizard complex.

Key References

Lundy Island in the Bristol Channel is an anomalous peraluminous granite intrusion of similar mineralogy (quartz + plagioclase + K-feldspar + biotite ± garnet ± topaz) and mineralisation (tin, tungsten) as the Permian Cornubian granites, but has a U-Pb zircon age of 59.8 ± 0.4 Ma making it related to the British Cenozoic Igneous Province, and not to the Permian granites of Devon and Cornwall. The similarity of mineralogy and composition is the result of melting similar source rocks at different times.

Key Reference

The collision of India with Asia and closing of the intervening NeoTethys ocean at ca 50 Ma resulted in crustal shortening and thickening along the Indian plate margin. Obduction of the Spontang ophiolite onto the north Indian plate margin preceded continental collision. Deformation propagated south (or SW) from the Tethyan Himalaya to the Greater Himalaya with time, culminating in kyanite grade regional metamorphism between ~45-35 Ma, and sillimanite grade regional metamorphism from ~35-12 Ma. At the highest temperatures, partial melting of pelitic gneisses resulted in migmatisation and formation of the classic Himalayan leucogranites with widespread magmatic tourmaline, garnet, biotite, muscovite, and rare andalusite and cordierite-bearing leucogranites. The Zanskar Himalaya has the greatest extent of kyanite grade metamorphism and field mapping along the Suru Valley shows large-scale recumbent folding and deep thrusting.

Key Recent References

The Nepal Himalaya shows spectacular exposures of kyanite and sillimanite gneisses, migmatites and large leucogranites, emplaced as giant sill complexes along the top of the Greater Himalayan Slab, bounded above by a low-angle normal fault. the South Tibetan Detachment. Our project involves detailed regional mapping in the Annapurna – Manaslu Himalaya, the Langtang valley, around the Everest – Makalu region in both Nepal and south Tibet, combined with thermobarometry, metamorphic modelling, and U-Pb dating of zircons and monazites.

Key References

Searle, M.P., Cottle, J.M., Streule, M.J. and Waters, D.J. 2010. Crustal melt granitoids and migmatites along the Himalaya: melt source, segregation, transport and granite emplacement mechanisms. Transactions of the Royal Society of Edinburgh, v. 100, 219-233, doi:10.1017/SI75569100901617X

Streule, M.J., Searle, M.P., Waters, D.J. & Horstwood, M. 2011. Metamorphism, melting and channel flow in the Greater Himalayan Sequence and Makalu leucogranite: constraints from thermobarometry, metamorphic modeling and U-Pb geochronology. Tectonics, 29, TC5011, doi: 10.1029/2009TC002533

Law, R.D., Jessup, M.J., Searle, M.P., Francsis, M.K., Waters, D.J. & Cottle, J.M. 2011. Telescoping of isotherms beneath the South Tibetan Detachment System, Mount Everest Massif. Journal of Structural Geology, 33, 1569-1594, doi: 10.1016/jsg.2011.09.004

Cottle, J.M., Searle, M.P., Jessup, M., Crowley, J.L. and Law, R.D. 2015. Rongbuk Revisited: Geochronology of leucogranites in the footwall of the South Tibetan Detachment system, Everest region, Southern Tibet. Lithos, v. 227, p. 94-106, doi:http://dx.doi.org/10.1016/j.lithos.2015.03.019

Parsons, A.J., Phillips, R.J., Lloyd, G.E., Law, R.D., Searle, M.P. and Walshaw, R.D. 2016a. Mid-crustal deformation of the Annapurna-Dhaulagiri Himalaya, Central Nepal: an atypical example of channel flow during the Himalayan orogeny. Geosphere, v.12, no. 3, doi:10.1130/GESO1246.1

Parsons, A.J., Law, R.D., Lloyd, G.E., Phillips, R.J. and Searle, M.P. 2016b. Thermo-kinematic evolution of the Annapurna-Dhaulagiri Himalaya, central Nepal: The composite orogenic system. Geochemistry, Geophysics, Geosystems, v. 17 (4), p. 1511-1539, doi:10.1002/2015GC006184

The 25th April 2015 Gorkha earthquake (magnitude 7.8) in Nepal ruptured the Main Himalayan Thrust (MHT) for ~140 km from the epicenter north of Gorkha, eastwards towards the Mount Everest region. The earthquake nucleated at a depth of ~15-18 km, but did not rupture to the surface. Our project used geodetic measurements of surface displacement to determine about 1 meter of uplift of the mountains north of the Kathmandu valley, and 0.6 meter of subsidence in the Everest region, showing a northward tilt of the Himalaya during the earthquake. Our data suggest the earthquake ruptured above a ramp in the MHT. We link geological studies of the Cenozoic evolution of the Nepal Himalaya with the active tectonics and earthquake record.

Key References

The western Himalayan syntaxis is centered on the mountain of Nanga Parbat (8126 meters) in the Pakistan Himalaya and exposes high-grade sillimanite and cordierite-bearing gneisses and migmatites intruded by tourmaline, muscovite and garnet-bearing leucogranites. Our project combines monazite petrochronology with thermal modelling to evaluate the roles of crustal melting, surface denudation, and tectonics in processes of ultra-fast exhumation of deeply buried rocks. We determined a pulse of ultra-fast exhumation (9-13 mm/year) that began ~1 million years ago, and was preceded by a slower, but still rapid exhumation (2-5 mm/year). These rocks are the youngest crustal melt granites anywhere in the World and record the highest exhumation rates.

Key References

Crowley, J.L., Bowring, S.A., Waters, D.J., Searle, M.P., Hodges, K.V. & Ramezani, J. 2009. Timescale of accessory mineral growth in Pleistocene leucogranites and exhumation rates at Nanga Parbat, Pakistan Himalaya, Earth and Planetary Science Letters, 288, 408-420. Doi: 10.1016/j.epsl.2009.09.044

Boyce, J.W., Hodges, K.V., King, D., Crowley, J.L., Jercinovic, M., Chatterjee, N., Bowring, S.A. & Searle, M.P. 2009. Improved confidence in (U-Th)/He thermochronology using the laser microprobe: an example from a Pleistocene leucogranite, Nanga Parbat, Pakistan. Geochemistry, Geophysics, Geosystems 10, doi:10.1029/2009GC002497.

Strike-slip faults in Tibet and SE Asia are key to the interpretation of the continental extrusion of Tibet hypothesis. We are studying the geology of the Lhasa Block, the southern margin of the Asian plate, and the major strike-slips involved in eastward extrusion, particularly the Karakoram fault, SW Tibet and Ladakh, the Red River fault, Yunnan and north Vietnam, the Xianshui-he fault in Eastern Tibet, and the Sagaing fault in Myanmar. These projects all involve field mapping, structural mapping, combined with U-Th-Pb geochronology, with the aim of determining the amount of geological offsets and timing of slip along these faults.

Key Recent References

Searle, M.P., Elliott, J.R., Phillips, R.J. & Chung, S-L. 2011. Crustal-Lithospheric structure, geological evolution and continental extrusion of Tibet. Journal of the Geological Society, London, 167, 633-672, doi: 10.1144/0016-76492010-139.

Weller, O.M., St-Onge, M.R., Waters, D.J., Rayner, N., Searle, M.P., Chung, S-L., Palin, R.M., Lee, Y-H. and Xu, X. 2013. Quantifying Barrovian metamorphism in the Danba Structural Culmination of eastern Tibet. Journal of Metamorphic Geology, doi:10.1111/jmg/12050

Phillips, R.J., Searle, M.P. and Parrish, R.R. 2013. The geochemical and temporal evolution of the continental lithosphere and its relationship to continental-scale faulting: The Karakoram Fault, eastern Karakoram, NW Himalayas. Geochemistry, Geophysics, Geosystems, 14, 3, doi:10.1002/ggge.20061

Palin, R.M., Searle, M.P., Morley, C.K., Charusiri, P., Hortswood, M.S.A. and Roberts, N.M.W. 2013b. Timing of metamorphism of the Lansang gneiss and implications for left-lateral motion along the Mae Ping (Wang Chao) strike-slip fault, Thailand. Journal of Asian Earth Sciences, doi.org/10.1016/j.seaes.2013.01.0121

Palin, R.M., Searle, M.P., St-Onge, M.R., Waters, D.J., Roberts, N.M.W., Horstwood, M.S.A., Parrish, R.R. & Weller, O.M. 2015. Two-stage cooling history of pelitic and semi-pelitic mylonite (sensu lato) from the Donjiu-Milin shear zone, northwest flank of the eastern Himalayan syntaxis. Gondwana Research, doi: org/10.1016/j.gr.2014.07.009

The metamorphic core complexes of Naxos and Ios Islands are compressional in origin with high-grade kyanite- and sillimanite gneisses in the core and extensional faults around the margin. Older high-pressure blueschists and eclogites record an early subduction related metamorphism. ‘Extensional’ faults related to exhumation of blueschists, and exhumation of the regional Barrovian metamorphic core rocks occurred in a compressional setting. Only the last phase of low-angle normal faulting relates to regional Aegean Sea extension. Our project involves regional mapping, strain analysis, petrological modelling, thermobarometry, and U-Pb geochronology, with the aim of unravelling the tectonic evolution of the Aegean Orogeny.

Key References

The HP-LT rocks on Tinos Island include lawsonite- garnet- and glaucophane-bearing schists metamorphosed at 22-26 kbar and 490-520ºC. Eclogites containing aegirine omphacite and chromium-rich kosmochlore on Syros Island formed at pressure around 20-23 kbar. Exhumation of these HP rocks from the subduction zone occurred during the late stage of ophiolite emplacement, and assisted uplift of footwall rocks below a passive roof fault. Our project on Tinos and Syros Islands in the Aegean Sea involves regional mapping, detailed petrology, thermobarometry and metamorphic modelling, as well as U-Pb geochronology.

Key References

The Tsniknias Island ophiolite is the best preserved obducted ophiolite in the Aegean Islands, and shows an amphibolite metamorphic sole accreted to the base of the harzburgite – lherzolite mantle sequence. The sole rocks comprise upper amphibolites and lower greenschist – blueschist facies meta-sediments. A plagiogranite intruded in the crustal sequence has a U-Pb zircon date of 161.9 ± 2.8 Ma. Partial melting occurred during subduction at ~ 74 Ma and the ophiolite was obducted some 90 m.y. after formation. Subduction beneath resulted in regional blueschist terrain exposed around the island of Tinos and HP eclogites on Syros Island. 

Key References

On the islands of Tinos, Delos, Mykonos, and Naxos in the Aegean Sea, Greece both hornblende, biotite bearing I-type granites and garnet + muscovite S-type granites occur together. U-Pb zircon ages suggest they formed simultaneously at ~15 – 14 Ma. The I-type granites are not related to Hellenic subduction but formed from dehydration melting of Variscan basement, whereas the S-type granites formed from muscovite-dehydration melting of a sedimentary protolith. Granite composition is related to different source rocks at different depths, at the same time. In this project we use field structural mapping, combined with major and trace-element geochemistry, Sr-Nd isotopes and U-Pb geochronology to constrain granite origin, and the tectonic evolution of the Aegean region.

Key References