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Constant elevation of southern Tibet over the past 15 million years (2003)

Harris, Nigel B. W. ; Widdowson, Mike ; Herman, Alexei B. ; [et al.]

[s.l.] : Macmillian Magazines Ltd.

Nature 421 (2003), S. 622-624

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ISSN: 1476-4687

Source: Nature Archives 1869 - 2009

Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics

Notes: [Auszug] The uplift of the Tibetan plateau, an area that is 2,000 km wide, to an altitude of about 5,000 m has been shown to modify global climate and to influence monsoon intensity. Mechanical and thermal models for homogeneous thickening of the lithosphere make specific predictions about ...

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1038/nature01356

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2

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The significance of garnet and cordierite from the Sioux Lookout region of the English River gneiss belt, Northern Ontario (1976)

Harris, Nigel B. W.

Springer

Contributions to mineralogy and petrology 55 (1976), S. 91-104

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ISSN: 1432-0967

Source: Springer Online Journal Archives 1860-2000

Topics: Geosciences

Notes: Abstract Garnet and cordierite bearing Archean gneisses are a common rock-type in the English River gneiss belt of the Superior Province, northern Ontario. Bulk compositions of such gneisses are found to be depleted in potassium with respect to the garnet and/or cordierite-free biotite gneisses. The mineral assemblages of the garnet and cordierite gneisses indicate that they have equilibrated at higher metamorphic grades than the garnet and/or cordierite-free biotite gneisses. These, observations suggest that anatexis has occurred in the garnetiferous regions of the gneiss belt resulting in a granite melt which had migrated from the source rocks, thereby depleting the garnetiferous restite in potassium.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF00372757

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3

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Carbon dioxide in the deep crust (1989)

Harris, Nigel

[s.l.] : Nature Publishing Group

Nature 340 (1989), S. 347-348

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ISSN: 1476-4687

Source: Nature Archives 1869 - 2009

Topics: Biology , Chemistry and Pharmacology , Medicine , Natural Sciences in General , Physics

Notes: [Auszug] IT is not only in the atmosphere that the effects of carbon dioxide are a cause for both debate and concern. It has been known for some time that the mineral assemblages we see in metamorphic rocks often have stabilized when in equilibrium with an unseen fluid phase, but it has been much harder to ...

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1038/340347a0

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4

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Post-collision magmatism and tectonics in northwest Anatolia (1994)

Harris, Nigel B. W. ; Kelley, Simon ; Okay, Aral I.

Springer

Contributions to mineralogy and petrology 117 (1994), S. 241-252

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ISSN: 1432-0967

Source: Springer Online Journal Archives 1860-2000

Topics: Geosciences

Notes: Abstract A suite of biotite-hornblende granodiorite intrusions has been emplaced into blueschist-facies metasediments in northwest Anatolia, following collision between two continental margins, now represented by the Tavşanli and Sakarya zones. The 40Ar/39Ar ages of phengites and glaucophanes from the blueschists, metamorphosed under unusually high P-low T conditions (P=20±2 kbar, T=430±30° C), suggest that metamorphism apparently occurred over a period spanning at least 20 Ma from 108 to 88 Ma. Post-tectonic granodiorites were emplaced during the Eocene (53 to 48 Ma) resulting in a cordierite and andalusite-bearing thermal aureole, indicative of pressures of ∼3 kbar. Trace-element systematics of the granodiorites are consistent with a derivation either from mantle-derived magmas by fractional crystallisation in shallow magma chambers, or from anatexis of crustal lithologies of internediate composition at pressures 〈10 kbar. The preservation of high P-low T assemblages in the blueschists together with the range of ages determined for blueschist-facies metamorphism are indicative of rapid exhumation of delaminated fragments from a subducted continental margin. However decompression melting of the crust is unlikely to have been a significant cause of magmatism, both because exhumation of the blueschists from deep crustal levels predated magmatism by at least 25 Ma, and because of the small melt fraction (〈0.1) that may be generated in crustal lithologies by this process. Melting in the mantle wedge is required either to generate a primary melt for the derivation of magmas of intermediate composition or to provide an advective heat source for crustal melting. The cause of melt formation in the upper mantle may be related to the termination of subduction following collision during the Mid-Eocene.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF00310866

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5

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The application of spinel-bearing metapelites to P/T determinations: An example from South India (1981)

Harris, Nigel

Springer

Contributions to mineralogy and petrology 76 (1981), S. 229-233

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ISSN: 1432-0967

Source: Springer Online Journal Archives 1860-2000

Topics: Geosciences

Notes: Abstract Metasedimentary migmatites from the Archean charnockitic terrain of South India contain the five phase equilibrium assemblage spinel-cordierite-garnet-corundum-sillimanite. The assemblages is a result of anatexis which has generated a silica-deficient anhydrous restite. Peak metamorphic conditions are defined by the intersection of two divariant reactions in the A12O3-SiO2-FeO-MgO system at which the five phases coexist. These reactions are univariant and their intersection invariant if the Fe/Mg ratio of at least one femic phase is fixed. The location of the invariant point in P/T space is derived from extracting standard stage thermodynamic data from published equilibria experiments in the system Al2O3-SiO2-FeO. Microprobe analyses of coexisting spinel, almandine and cordierite specify the Fe/Mg distributions between phases and allow the computation of the five phase invariant point for $$P_{{\text{H}}_{\text{2}} {\text{O}}} $$ =P total (770° C, 5.9 kb) and $$P_{{\text{H}}_{\text{2}} {\text{O}}} $$ =O (740° C, 4.8 kb). A low $$P_{{\text{H}}_{\text{2}} {\text{O}}} $$ , implied by evidence of extreme anatexis, indicates a P/T field of T=740±20° C and Ptotal=4.8±0.5 kb which is consistent with the field of equilibration of interlayered charnockites computed from garnet-hypersthene and garnet-plagioclase pairs.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/BF00371962

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6

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40Ar-39Ar and Rb-Sr geochronology of high-pressure metamorphism and exhumation history of the Tavsanli Zone, NW Turkey (1999)

Sherlock, Sarah ; Kelley, Simon ; Inger, Simon ; [et al.]

Springer

Contributions to mineralogy and petrology 137 (1999), S. 46-58

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ISSN: 1432-0967

Source: Springer Online Journal Archives 1860-2000

Topics: Geosciences

Notes: Abstract Geochronological investigations in high- and ultra-high-pressure metamorphic rocks are problematic since firstly the low temperatures lead to fine grain size and disequilibrium assemblages, and secondly the problem of “excess argon” affects 40Ar-39Ar systematics, the most commonly used isotopic system. The Tavsanli Zone is a belt of high-pressure low-temperature (HP-LT) rocks spanning NW Turkey and is one such region where previous geochronological studies have produced a range of estimates for the age of HP-LT metamorphism, raising the question of whether they are geologically significant. This study presents new data from the Tavsanli Zone; 40Ar-39Ar ages are in the range 60 Ma to 175 Ma, whilst Rb-Sr ages are restricted to 79.7 Ma to 82.8 Ma, confirming the presence of excess argon. Detailed ultra-violet laser ablation microprobe (UVLAMP) studies have revealed younger 40Ar-39Ar ages in the cores of coarser white micas, which in conjunction with 40Ar-39Ar ages from the finest grained lithologies and the Rb-Sr white mica crystallisation ages, constrain the post-HP-LT metamorphism exhumation rates of these rocks. Petrological and regional constraints suggest that syn-subduction exhumation and cooling took place initially by synchronous subduction and exhumation by underplating. This is followed by a phase of syn-continent-continent collision at a rate of approximately 1.5 mma−1 and exhumation to the surface via thrusting. The 40Ar-39Ar hornblende data from a granodiorite intruding the HP-LT rocks constrain the later parts of exhumation path. This study highlights the importance of a multi-system geochronological approach when attempting to determine the history of HP-LT rocks.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/PL00013777

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7

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The application of single zircon evaporation and model Nd ages to the interpretation of polymetamorphic terrains: an example from the Proterozoic mobile belt of south India (1998)

Bartlett, Jessica M. ; Dougherty-Page, Jon S. ; Harris, Nigel B. W. ; [et al.]

Springer

Contributions to mineralogy and petrology 131 (1998), S. 181-195

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ISSN: 1432-0967

Source: Springer Online Journal Archives 1860-2000

Topics: Geosciences

Notes: Abstract The technique of single zircon dating from the thermal evaporation of 207Pb/206Pb (Kober 1986, 1987) provides a means of dating successive periods of growth and nucleation of zircons in polymetamorphic assemblages. In contrast Nd model ages may provide a measure of the period of crustal residency for the sample or its protolith. These two techniques have been combined to elucidate the tectonic history of the Proterozoic mobile belt of southern India, exposed south of the Palghat-Cauvery Shear Zone that marks the southern boundary of the Archaean craton of Karnataka. The two main tectonic units of this mobile belt comprise the Madurai and Trivandrum Blocks, both of which are characterised by massive charnockite uplands and low-lying polymetamorphic metasedimentary belts that have undergone a complex tectonic history throughout the Proterozoic. Evidence for early Palaeoproterozoic magmatism is restricted to the Madurai Block where single zircon evaporation ages from a metagranite (2436 ± 4 Ma) are similar to model Nd ages from a range of lithologies suggesting crustal growth at that time. The Trivandrum Block, to the south of the Achankovil shear zone, is comprised of the Kerala Khondalite Belt, the Nagercoil charnockites and the Achankovil metasediments. Single zircon evaporation ages, together with conventional zircon and garnet chronometry, suggest that all three units underwent upper-amphibolite facies metamorphism at ∼1800 Ma, an event unrecorded in the metagranite from the Madurai Block. This implies that the Madurai and Trivandrum blocks represent distinct terrains throughout the Palaeoproterozoic. Model Nd ages from the Achankovil metasediments are much younger (1500–1200 Ma) than those from the adjacent Kerala Khondalite Belt and Madurai Blocks (3000–2100 Ma), but there is no evidence for zircon growth in these metasediments during the Mesoproterozoic. Hence the comparatively young model Nd ages of the metasediments are indicative of a mixed provenance rather than a discrete period of crustal growth. Zircon overgrowths from the Madurai Block (547 ± 17 Ma) and Achankovil metasediments (530 ± 21 Ma) suggest that all tectonic units of the Proterozoic mobile belt of South India shared the same metamorphic history from the early Palaeozoic. This event has been recognised in the basement lithologies of Sri Lanka and East Antarctica, confirming that the constituent terrains of East Gondwana had assembled by this time.

Type of Medium: Electronic Resource

URL: http://dx.doi.org/10.1007/s004100050387

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Contribution of crustal anatexis to the tectonic evolution of Indian crust beneath southern Tibet (2011)

King, Jess ; Harris, Nigel ; Argles, Tom ; Parrish, Randall ; Zhang, Hongfei

Geological Society of America (GSA)

In: GSA Bulletin 123: 218-239.

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Publication Date: 2011-01-01

Description: This geochemical, geochronological, and structural study of intrusive rocks in the Sakya dome of southern Tibet has identified two distinct suites of anatectic granites that carry contrasting implications for the tectonic evolution of the India-Asia collision zone. The northern margin of the dome core was intruded by anastomosing, equigranular two-mica garnet granites between 28.1 {+/-} 0.4 Ma and 22.6 {+/-} 0.4 Ma, coeval with top-to-the-south shear. Trace-element and isotopic (Sr-Nd) characteristics indicate an origin from partial melting of a biotite-bearing source in the Indian crust under conditions of high-fluid-phase activity. These granites thus provide evidence for the melt weakening required by some thermo-mechanical models that predict the southward extrusion of a low-viscosity channel during the Oligocene. Evidence for subsequent shear-sense reversal may document initiation of this process. However, a younger suite of porphyritic two-mica granite plutons, emplaced between 14.5 {+/-} 0.9 Ma and 8.81 {+/-} 0.22 Ma, is derived from anatexis of muscovite-bearing metasediments of the High Himalayan Series under fluid-absent conditions. Ar-Ar cooling ages of 14.4-8.0 Ma from the Sakya dome postdate crystallization of the Oligocene granite suite by [~]10 m.y. but are coincident with mid-Miocene granite emplacement, suggesting uplift to depths of

Print ISSN: 0016-7606

Electronic ISSN: 1943-2674

Topics: Geosciences

Published by Geological Society of America (GSA)

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Crustal Melting and the Flow of Mountains (2011)

Jamieson, Rebecca A. ; Unsworth, Martyn J. ; Harris, Nigel B. W. ; Rosenberg, Claudio L. ; Schulmann, Karel

Mineralogical Society of America

In: Elements 7: 253-260.

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Publication Date: 2011-08-01

Description: As the continental crust thickens during mountain building, it can become hot enough to start melting, leading to a profound reduction in its strength. Melt-weakened crust can flow outward or upward in response to the pressure gradients associated with mountain building, and may be transported hundreds of kilometres laterally as mid-crustal channels. In the Himalayan-Tibetan system, melting began about 30 million years ago, and widespread granite intrusion began at 20-23 Ma. Geophysical data indicate that melt is present beneath the Tibetan plateau today, and deeply eroded mountain belts preserve evidence for melt-enhanced ductile flow in the past. Flow of partially molten crust may limit the thickness and elevation of mountain belts and has influenced the deep structure of continents.

Print ISSN: 1811-5209

Electronic ISSN: 1811-5217

Topics: Geosciences