Ultrahigh temperature （UHT） metamorphism is the most thermally extreme form of regional crustal metamorphism, with temperatures exceeding 900 ℃. UHT crustal metamorphism is recognised in more than 50 localities glo...Ultrahigh temperature （UHT） metamorphism is the most thermally extreme form of regional crustal metamorphism, with temperatures exceeding 900 ℃. UHT crustal metamorphism is recognised in more than 50 localities globally in the metamorphic rock record and is accepted as ＇normal＇ in the spectrum of regional crustal processes. UHT metamorphism is Wpically identified on the basis of diagnostic mineral assemblages such as sapphirine ＋ quartz, orthopyroxene ＋ sillimanite ＋ quartz and osumilite in Mg-Al- rich rock compositions, now usually coupled with pseudosection-based thermobarometry using internally-consistent thermodynamic data sets and/or Al-in-Orthopyroxene and ternary feldspar thermobarometry. Significant progress in the understanding of regional UHT metamorphism in recent years includes： （1） development of a ferric iron activity-composition thermodynamic model for sapphirine, allowing phase diagram calculations for oxidised rock compositions; （2） quantification of UHT conditions via trace element thermometry, with Zr-in-rutile more commonly recording higher temperatures than Ti-in-zircon. Rutile is likely to be stable at peak UHT conditions whereas zircon may only grow as UHT rocks are cooling. In addition, the extent to which Zr diffuses out of rutile is controlled by chemical communication with zircon; （3） more fully recognising and utilising temperature-dependent thermal properties of the crust, and the possible range of heat sources causing metamorphism in geodynamic modelling studies; （4） recognising that crust partially melted either in a previous event or earlier in a long-duration event has greater capacity than fertile, unmelted crust to achieve UHT conditions due to the heat energy consumed by partial melting reactions; （5） more strongly linking U-Pb geochronological data from zircon and monazite to P-T points or path segments through using Y ＋ REE partitioning between accessory and major phases, as well as phase diagrams incorporating Zr and REE; and （6） improved insight into the settings and factors responsible for UHT metamorphism via geodynamic forward models. These models suggest that regional UHT metamorphism is, principally, geodynamically related to subduction, coupled with elevated crustal radiogenic heat generation rates.展开更多
Metamorphic provinces such as the^1 Ga Grenvillian,~400 Ma Caledonide and Triassic Qinling Provinces often contain rocks with high-pressure assemblages such as eclogites,which formed at mantle depths in subduction zon...Metamorphic provinces such as the^1 Ga Grenvillian,~400 Ma Caledonide and Triassic Qinling Provinces often contain rocks with high-pressure assemblages such as eclogites,which formed at mantle depths in subduction zones.These are evidence of the accretion of terranes by subduction of oceans and collision to form large tectonostratigraphic provinces.The Mesoproterozoic Namaqua-Natal Province comprises a number of terranes thought to have been assembled by plate-tectonic processes,but they have generally yielded metamorphic pressures below 5 kbar,corresponding to<20 km,crustal depths,lacking evidence for subduction processes.The Kaaien Terrane in the Namaqua Front contains two large garbenschiefer units with the unusual paragenesis garnet-hornblende-epidote-white mica-plagioclase-ilmenite-quartz.Their protoliths are graywackes influenced by andesitic volcanism during their deposition at^1870 Ma,in a passive margin of the Rehoboth Province or Kaapvaal Craton.Prograde garnet growth dated at 11655 Ma culminated in peak metamorphic conditions of 64530C and 10.40.7 kbar,corresponding to 40 km depth.This is attributed to subduction of these rocks before collision between the overriding arc-related Areachap Terrane,the Kaaien Terrane and the Kaapvaal-Rehoboth cratonic block during the Namaqua orogeny.Exhumation of the garbenschiefer slabs was followed by rapid cooling,as the 11435 Ma argon dates of hornblende and white mica,with closure temperatures^540C and^440C respectively,are the same within error.This was probably due to tectonic juxtaposition of the garbenschiefer slab with much cooler rock units.The exhumation was accommodated along the Trooilapspan-Brakbosch Shear Zone due to ongoing transpression.Other components of the Namaqua Front have distinctly different P-T-t paths,exemplified by greenschist metamorphism in the 1300 Ma Wilgenhoutsdrift Group,and medium-pressure metamorphism in the Areachap Terrane.They were juxtaposed by late-tectonic uplift and transpressional movements.The^40 km depth of garbenschiefer peak metamorphism is the deepest yet found in the Namaqua-Natal Province and strengthens the plate tectonic model of accretion by collision of terranes at the end of a Wilson cycle.The high pressure paragenesis of the garbenschiefer was preserved due to its location in the Namaqua Front,whereas most other parts of the Namaqua-Natal Province were overprinted by 1100–1020 Ma thermal events after the collision events.展开更多
As a window of insight into the lower crust, high pressure granulite has received much attention since last decade. Yushugou high pressure granulite-peridotite Complex was located in the northeast margin of Southern T...As a window of insight into the lower crust, high pressure granulite has received much attention since last decade. Yushugou high pressure granulite-peridotite Complex was located in the northeast margin of Southern Tianshan, NW China. Previous ideas agreed that the peridotite unit in Yushugou, combined with the ultramafic rocks in Tonghuashan and Liuhuangshan, represent an ophiolite belt. However, the metamorphic evolution and tectonic mechanism of the Yushugou high pressure （HP） granulite remain controversial. Petrological investigations and phase equilibrium modelling for two representative fclsic granulitc samples suggest two stages metamorphism of the rocks in Yushugou Complex. Granulite facies metamorphism （Stage I） with P-T conditions of 9.8-10.4 kbar at 895-920℃ was recorded by the porphyroblastic garnet core; HP granulite facies metamorphism （Stage II） shows P-T conditions of 13.2-13.5 kbar at 845-860℃, based on the increasing grossular and decreasing pyrope contents of garnet rims. The Yushugou HP felsic granulites have recorded an anti- clockwise P-T path, characterized by the temperature decreasing and pressure increasing simultaneously. The LA-ICP-MS isotopic investigations on zircons from the felsic granulite show that the protolith ages of the granlulites are -430 Ma, with two age groups of-390 Ma and 340-350 Ma from the metamorphic rims of zircon, indicating the Stage I and II metamorphic events, respectively. A tectonic model was proposed to interpret the processes. The investigated felsic granulite was derived from deep rooted hanging wall, with Stage I granulite facies metamorphism of -390 Ma, which may he related to the Devonian arc magmatic intrusion; Stage II HP granulite facies metamorphism （340-350 Ma） may due to the involvement of being captured into the subducting slab and experienced the high pressure metamorphism.展开更多
基金Prof.M.Santosh is thanked for his warm hospitality and the invitation to write an updated review of progress on UHT metamorphism while DEK was on a research visit to China University of Geosciences,Beijing,in September 2013 funded by Australia's Group of Eight(Go8)and in China by the China Science and Technology Exchange Center(CSTEC).Thorough and constructive reviews by M.Brown and F.Korhonen were warmly welcomed
文摘Ultrahigh temperature （UHT） metamorphism is the most thermally extreme form of regional crustal metamorphism, with temperatures exceeding 900 ℃. UHT crustal metamorphism is recognised in more than 50 localities globally in the metamorphic rock record and is accepted as ＇normal＇ in the spectrum of regional crustal processes. UHT metamorphism is Wpically identified on the basis of diagnostic mineral assemblages such as sapphirine ＋ quartz, orthopyroxene ＋ sillimanite ＋ quartz and osumilite in Mg-Al- rich rock compositions, now usually coupled with pseudosection-based thermobarometry using internally-consistent thermodynamic data sets and/or Al-in-Orthopyroxene and ternary feldspar thermobarometry. Significant progress in the understanding of regional UHT metamorphism in recent years includes： （1） development of a ferric iron activity-composition thermodynamic model for sapphirine, allowing phase diagram calculations for oxidised rock compositions; （2） quantification of UHT conditions via trace element thermometry, with Zr-in-rutile more commonly recording higher temperatures than Ti-in-zircon. Rutile is likely to be stable at peak UHT conditions whereas zircon may only grow as UHT rocks are cooling. In addition, the extent to which Zr diffuses out of rutile is controlled by chemical communication with zircon; （3） more fully recognising and utilising temperature-dependent thermal properties of the crust, and the possible range of heat sources causing metamorphism in geodynamic modelling studies; （4） recognising that crust partially melted either in a previous event or earlier in a long-duration event has greater capacity than fertile, unmelted crust to achieve UHT conditions due to the heat energy consumed by partial melting reactions; （5） more strongly linking U-Pb geochronological data from zircon and monazite to P-T points or path segments through using Y ＋ REE partitioning between accessory and major phases, as well as phase diagrams incorporating Zr and REE; and （6） improved insight into the settings and factors responsible for UHT metamorphism via geodynamic forward models. These models suggest that regional UHT metamorphism is, principally, geodynamically related to subduction, coupled with elevated crustal radiogenic heat generation rates.
基金supported by a Swedish VR grant 2006-2402 to DHC
文摘Metamorphic provinces such as the^1 Ga Grenvillian,~400 Ma Caledonide and Triassic Qinling Provinces often contain rocks with high-pressure assemblages such as eclogites,which formed at mantle depths in subduction zones.These are evidence of the accretion of terranes by subduction of oceans and collision to form large tectonostratigraphic provinces.The Mesoproterozoic Namaqua-Natal Province comprises a number of terranes thought to have been assembled by plate-tectonic processes,but they have generally yielded metamorphic pressures below 5 kbar,corresponding to<20 km,crustal depths,lacking evidence for subduction processes.The Kaaien Terrane in the Namaqua Front contains two large garbenschiefer units with the unusual paragenesis garnet-hornblende-epidote-white mica-plagioclase-ilmenite-quartz.Their protoliths are graywackes influenced by andesitic volcanism during their deposition at^1870 Ma,in a passive margin of the Rehoboth Province or Kaapvaal Craton.Prograde garnet growth dated at 11655 Ma culminated in peak metamorphic conditions of 64530C and 10.40.7 kbar,corresponding to 40 km depth.This is attributed to subduction of these rocks before collision between the overriding arc-related Areachap Terrane,the Kaaien Terrane and the Kaapvaal-Rehoboth cratonic block during the Namaqua orogeny.Exhumation of the garbenschiefer slabs was followed by rapid cooling,as the 11435 Ma argon dates of hornblende and white mica,with closure temperatures^540C and^440C respectively,are the same within error.This was probably due to tectonic juxtaposition of the garbenschiefer slab with much cooler rock units.The exhumation was accommodated along the Trooilapspan-Brakbosch Shear Zone due to ongoing transpression.Other components of the Namaqua Front have distinctly different P-T-t paths,exemplified by greenschist metamorphism in the 1300 Ma Wilgenhoutsdrift Group,and medium-pressure metamorphism in the Areachap Terrane.They were juxtaposed by late-tectonic uplift and transpressional movements.The^40 km depth of garbenschiefer peak metamorphism is the deepest yet found in the Namaqua-Natal Province and strengthens the plate tectonic model of accretion by collision of terranes at the end of a Wilson cycle.The high pressure paragenesis of the garbenschiefer was preserved due to its location in the Namaqua Front,whereas most other parts of the Namaqua-Natal Province were overprinted by 1100–1020 Ma thermal events after the collision events.
基金financially supported by the National Natural Science Foundation of China(Grants 41330210,41520104004)
文摘As a window of insight into the lower crust, high pressure granulite has received much attention since last decade. Yushugou high pressure granulite-peridotite Complex was located in the northeast margin of Southern Tianshan, NW China. Previous ideas agreed that the peridotite unit in Yushugou, combined with the ultramafic rocks in Tonghuashan and Liuhuangshan, represent an ophiolite belt. However, the metamorphic evolution and tectonic mechanism of the Yushugou high pressure （HP） granulite remain controversial. Petrological investigations and phase equilibrium modelling for two representative fclsic granulitc samples suggest two stages metamorphism of the rocks in Yushugou Complex. Granulite facies metamorphism （Stage I） with P-T conditions of 9.8-10.4 kbar at 895-920℃ was recorded by the porphyroblastic garnet core; HP granulite facies metamorphism （Stage II） shows P-T conditions of 13.2-13.5 kbar at 845-860℃, based on the increasing grossular and decreasing pyrope contents of garnet rims. The Yushugou HP felsic granulites have recorded an anti- clockwise P-T path, characterized by the temperature decreasing and pressure increasing simultaneously. The LA-ICP-MS isotopic investigations on zircons from the felsic granulite show that the protolith ages of the granlulites are -430 Ma, with two age groups of-390 Ma and 340-350 Ma from the metamorphic rims of zircon, indicating the Stage I and II metamorphic events, respectively. A tectonic model was proposed to interpret the processes. The investigated felsic granulite was derived from deep rooted hanging wall, with Stage I granulite facies metamorphism of -390 Ma, which may he related to the Devonian arc magmatic intrusion; Stage II HP granulite facies metamorphism （340-350 Ma） may due to the involvement of being captured into the subducting slab and experienced the high pressure metamorphism.
