独居石LA-ICP-MS U-Pb法测定北大别木子店片麻杂岩变质年龄

北大别构造带木子店片麻杂岩是目前华南陆块范围内出露的最古老岩石,但有关其变质时代和构造性质等问题仍缺乏研究。对这些岩石的变质时代开展研究,对于了解华南陆块早期地壳演化过程具有重要意义。本文利用LA-ICP-MS方法对木子店片麻杂岩中两件片麻岩样品进行了独居石的微区原位U-Pb定年和微量元素分析,采用独居石标准物质44069对年龄进行校正,独居石标准物质Trebilcock作为监控,测试结果与其推荐值在误差范围内相一致。结果显示,大多数独居石具有均一、无环带特点,且存在重稀土元素明显亏损以及负Eu异常,暗示其可能形成于变质成因。两件片麻岩样品的独居石U-Pb年龄分别为129±1Ma和130±1Ma,代表了木子店片麻杂岩发生变质作用的时间,这一年龄与木子店地区早白垩世花岗岩的侵位年龄基本一致,也与北大别造山带内大量出露的混合岩时代相吻合。研究认为大别造山带早白垩世加厚地壳的伸展垮塌和造山山根拆沉,引起了变质作用和同时代的岩浆活动,将北大别构造带从挤压向伸展的构造转换时间限定在~130Ma。

TWO EPISODES OF MONAZITE CRYSTALLIZATION DURING METAMORPHISM AND CRUSTAL MELTING IN THE EVEREST REGION OF THE NEPALESE HIMALAYA

New monazite U\|Pb geochronological data from the Everest region suggest that 20～25Ma elapsed between the initial India—Asia collision and kyanite\|sillimanite grade metamorphism. Our results indicate a two\|phase metamorphic history, with peak Barrovian metamorphism at (32 2±0 4)Ma and a later high\|temperature, low\|pressure event (620℃, 400MPa) at (22 7±0 2)Ma.. Emplacement and crystallization of the Everest granite subsequently occurred at 20 5～21 3Ma. The monazite crystallization ages that differ by 10Ma are recorded in two structurally adjacent rocks of different lithology, which have the same post collisional p—T history.. Scanning electron microscopy reveals that the younger monazite is elaborately shaped and grew in close association with apatite at grain boundaries and triple junctions, suggesting that growth was stimulated by a change in the fluid regime. The older monazite is euhedral, is not associated with apatite, and is commonly armoured within silicate minerals. During the low\|pressure metamorphic event, the armouring protected the older monazites, and a lack of excess apatite in this sample prevented new growth. Textural relationships suggest that apatite is one of the necessary monazite\|producing reactants, and spots within monazite that are rich in Ca, Fe, Al and Si suggest that allanite acted as a preexisting rare earth element host. We propose a simplified reaction for monazite crystallization based on this evidence.

High- and Ultrahigh-pressure Metamorphism and Retrogressive Textures of Gneiss in the Donghai Area——Evidence from gneisses in drillhole ZK2304

In the gneisses from the drillhole ZK2304 of the Donghai area, there have been preserved high- and ultrahigh-pressure metamorphic mineral assemblages, a series of complicated retrogressive textures and relevant metamorphic reactions. In addition to garnet, jadeititic-clinopyroxene and rutile, other peak stage (M2) minerals in some gneisses include phengite, aragonite and coesite or quartz pseudomorphs after coesite. The typical peak-stage mineral assemblages in gneisses are characterized by garnet + jadeitic-clinopyroxene + rutile + coesite, garnet + jadeitic-clinopyroxene + phengite + rutile ± coesite and garnet + jadeitic-clinopyroxene + aragonite + rutile ± coesite. The grossular content (Gro) in garnet is high and may reach 50. 1 mol%. The SiO2 content of phengite ranges from 54.37% to 54.84% with 3.54-3.57 p.f.u. Quartz pseudomorphs after coesite occur as inclusions in garnet.The gneisses of the Donghai area have been subjected to multistage recrystallization and exhibit a closewise P-T evolutional path characterized by the near-isothermal decompression. The inclusion assemblage (Hb+Ep+Bi+Pl+Qz) within garnet and other minerals has recorded a pre-peak stage (Mi) epidote amphibole fades metamorphic event. High- and ultrahigh-pressure peak metamorphism (M2) took place at T=750-860℃ and P>2.7 GPa. The symplectitic assemblages after garnet, jadeitic-clinopyroxene and rutile imply a near-isothermal decompression metamorphism (M3, M4) during the rapid exhumation. Several lines of evidence of petrography and metamorphic reactions indicate that both gneisses and eclogites have experienced ultrahigh-pressure metamorphism in the Donghai area. This research may be of great significance for an in-depth study of the metamorphism and tectonic evolution in the Su-Lu ultrahigh-pressure metamorphic belt.

Diagenesis and Metamorphism of Triassic Flysch Along Profile Zoige-Lushan,Northwest Sichuan,China

We used illite and chlorite crystaHinities, index minerals, mineral assemblages, polytype and domain size of white mica, electron microprobe analysis （EMPA）, b0 geobarometer and chlorite geothermometer to quantify the diagenetic and metamorphic overprint on the Triassic flysch formations in the Songpan-Garze orogen along profile Zoige-Lushan, northwest Sichuan. Two anchizones, two epizones, one diagenetic zone and a transition belt in-between them are defined on the basis of the obtained data. lllite crystallinity correlates strongly with the best mean domain size of mica and moderately with chlorite crystallinity. 2M1 white mica polytype are observed within the epizone whilst 2M1 and 1M polytypes occur in the anchizone and diagenetic zone. Epizonal metamorphism reached maximum temperatures of 370℃±21℃ at low-to intermediate pressure conditions. Clay minerals underwent Ostwald ripening during metamorphism. Rocks from both sides of the Longmenshan fault reveal contrasting degrees of metamorphic overprint： on the northwest side of the Longmenshan fault, epimetamorphic conditions contrast with diagenetic rocks on the southeast side.

Paleozoic post-collisional magmatism and high-temperature granulite-facies metamorphism coupling with lithospheric delamination of the East Kunlun Orogenic Belt,NW China

Lithosphere extension and upwelling of asthenosphere at post-collisional stage of an orogenic cycle generally induce diverse magmatism and/or associated high-temperature metamorphism. Nevertheless, the intimate coexistence of post-collisional magmatic activity and high-temperature metamorphism is rare.In this contribution, a lithological assemblage composing of diverse magmatic rocks deriving from distinct magma sources and coeval high-temperature metamorphism was identified in eastern Kunlun.Petrography, ages, mineral chemistry and whole-rock geochemistry demonstrated that those intimately coexistent diverse rocks were genetically related to post-collisional extension. The garnet-bearing mafic granulites in Jinshuikou area interior of the East Kunlun Orogenic Belt are mainly composed of garnet,orthopyroxene, and plagioclase, with peak metamorphic P–T conditions of ~ 701–756 ℃and 5.6–7.0 kbar,representing a granulite-facies metamorphism at 409.7 ± 1.7 Ma. The diverse contemporaneous magmatic rocks including hornblendites, gabbros and granites yield zircon U–Pb ages of 408.6 ± 2.5 Ma,413.4 ± 4.6 Ma, and 387–407 Ma, respectively. The hornblendites show N-MORB-like REE patterns with(La/Sm)Nvalues of 0.85–0.94. They have positive zircon εHf(t) values of 0.1–4.9 and whole-rock εNd(t) values of 3.9–4.7 but relatively high(^(87)Sr/^(86)Sr)_(i)values of 0.7081 to 0.7088. These features demonstrate that the hornblendites derived from a depleted asthenospheric mantle source with minor continental crustal materials in source. As for the gabbros, they exhibit arc-like elemental signatures, low zircon εHf(t) values(-4.3 to 2.5) and variable whole-rock εNd(t) values(-4.9 to 1.2) as well as high(^(87)Sr/86 Sr)ivalues(0.7068 to 0.7126), arguing for that they were originated from partial melting of heterogeneous lithospheric mantle anteriorly metasomatized by subducted-sediment released melts. Geochemistry of the granites defines their strongly peraluminous S-type signatures. Zircons from the granites yield a large range of εHf(t) values ranging from -30.8 to -5.1, while the whole-rock samples yield consistent(^(87)Sr/86 Sr)ivalues(0.7301 to 0.7342) and negative εNd(t) values(-10.1 to -12.4). These features indicate that the S-type granites could be generated by reworking of an ancient crust. Taken together, the penecontemporaneous magmatism and metamorphic event, demonstrated the early-middle Devonian transition from crustal thickening to extensional collapse. The post-collisional mantle-derived magmas serve as an essential driving force for the high-temperature granulite-facies metamorphism and anataxis of the crust associated with formation of S-type granite. This study not only constructs a more detail Proto-Tethys evolution process of the eastern Kunlun, but also sheds new light on better understanding the intimate relationship between magmatism and metamorphism during post-collisional extensional collapse.

Metamorphism of the East Sector of the Southern Qinling Orogenic Belt and Its Geological Significance

The east sector of the southern Qinling belt is, lithologically, composed mainly of metapelites, ***qüartzites, marbles and small amount of metabasites and gneisses, whose protoliths are the Silurian, Devonian and less commonly the Sinian and Upper Palaeozoic. They have been subjected at least to two epochs of metamorphism. The early epoch belongs to progressive metamorphism which is centered on high amphibolite-granulite fades in the Fuping area and changed outwards into low amphibolite facies (staurolite-kyanite zone), epidote amphibolite facies (garnet zone) and greenschist facies (chlorite and biotite zones), the metamorphic age of which is about 220–260 Ma. This early-epoch metamorphism belongs to different pressure types: the rocks from greenschist to low amphibolite facies belong to the typical medium-pressure type which shows geothermal gradients of about 17–20 ***C/km and was probably produced by a crustal thickening process related to continental collision, and the high amphibolite-granulite facies belongs to the low-pressure type which shows geothermal gradients of about 25–38 ***C/km and was probably affected by some magmatic heats. Based on the basic characteristics of the P-T paths of the different facies calculated from the garnet zonations, it can be deduced that the metamorphism of medium-pressure facies series took place during an imbricated thickening process, rather than during the uplifting process after thickening. The late-epoch metamorphism belongs to dynamic metamorphism of greenschist facies which is overprinted on the early-epoch metamorphic rocks and is Yanshanian or Himalayan in age, probably related to intracontinental orogeny.

Timing of Metamorphism and Provenance of the Metamorphic Basement of the Xiangshan Uranium Orefield, Jiangxi Province, China

The origin, age and evolution of the Precambrian metamorphic basement of southern China provide useful insights into early crustal development. Here, we present new laser ablation-inductively coupled plasma-mass spectrometry （LA-ICP-MS） U-Pb age data for detrital zircons from five samples of the Precambrian metamorphic basement of the Xiangshan uranium orefield. Two of these samples, from the northern Xiangshan volcanic basin, yielded a total of 140 U-Pb ages that cluster within the Neoproterozoic （773-963 Ma; 79.3% of data points）, with the rest being scattered through the Paleoproterozoic and Mesoproterozoic, along with a single Archean age. These ages indicate that this basement material is associated with the Cathaysia Block. In comparison, the 172 concordant ages from the other three samples from the southern part of the Xiangshan volcanic basin cluster within the Neoproterozoic （767-944 Ma; 59.8%） as well as the Proterozoic （37.8%） and the Archean （2502-2712 Ma; 14.5%）. These samples are also free of zircons with Grenvillian ages, indicating that these units are associated with the southeastern Yangtze Block. Combining these data with the geochemistry of these units, which suggests that the metamorphosed sedimentary rocks within the northern and southern parts of the Xiangshan basin have a common component from a magmatic island arc that formed during the early Neoproterozoic, we infer that the basin was located along the boundary between the Cathaysian and Yangtze blocks. In addition, the zircons within the samples from the southern and northern parts of the Xiangshan basin show different pre-Neoproterozoic （963 Ma） age populations but similar post- Neoproterozoic zircon populations, indicating that the amalgamation of the Cathaysian and Yangtze blocks occurred after the Neoproterozoic （960 Ma）, with magmatism peaking at 830 Ma and rifting starting at -770 Ma, leading to the subsequent deposition （from bottom to top） of the Shenshan, Kuli, and Shangshi formations.

Eclogites of the Dabie Region: Retrograde Metamorphismand Fluid Evolution

Based upon fluid effects, retrograde metamorphism of eclogites in the Dabieregion can be divided into the fluid-poor, fluid-bearing and fluid-rich stages. The fluid-poor stageis marked by polymorphic inversion, recrystallization and exsolution of solid solutions, and isthought to represent eclogite-facies retrograde environments. The fluid-bearing stage is likely tohave occurred at the late stage of ecologite-facies diaphthorosis and is represented by kyaniteporphyroblasts, rutile, and sodic pyroxene in association with high-pressure hydrous minerals suchas phengite and zoisite (clinozoisite) without significant amount of hydrous minerals such asamphibole, epidote and biotite. The fluid-rich stage might have commenced concomitantly with loweramphibolite-facies diaphthoresis and persisted all the way towards the near-surface environment. Theproduct of this stage is characterized by plentiful hydrous and volatile-bearing phases.The dissemination-type rutile mineralizations in eclogites might have formed by preferentialshearing-induced pressure solution of gangue minerals at the fluid-bearing stage. The accompanyingvein rutile was precipitated from fluids of this stage after local transport and concentration, andmay hence represent proximal mobilization of titanium from the eclogite. Therefore, rutile veins canbe used as an exploration indicator for dissemination-type rutile deposits.

Gold mineralisation and orogenic metamorphism in the Lena province of Siberia as assessed from Chertovo Koryto and Sukhoi Log deposits

The Chertovo Koryto gold deposit（80 t Au at 1.84 g/t） in the Lena world-class province,Siberia,is hosted in a metamorphosed sequence of the Paleoproterozoic Mikhailovsk Formation that comprises the oldest black shale strata of the Baikal-Patom region.The mineralisation is confined to the thrust zone complicated with a conjugate anticline fold,zones of shearing and dislocation.The struaural position of the mineralisation is similar to that at the giant Sukhoi Log deposit in the neighbouring Mama-Bodaibo zone.In the latter,the isotope age data suggest that Khomolkho black shales,hosts to Sukhoi Log mineralisation,are of Ediacaran age and underwent prograde metamorphism during early Paleozoic.The geochemical composition of the terrigenous rocks that host Sukhoi Log,Chertovo Koryto,and a number of other deposits at the various stratigraphic levels throughout the Proterozoic sequence have much in common.They do not show elevated metal contents above the common black shale abundances,except for Au and As,which is at variance with the accepted view on diagenetic enrichment of black shales in the Lena province.The occurrence of sagenitic rutile in quartz and chlorite pseudomorphs after biotite and other petrographic observations provide evidence on a retrograde nature of the metamorphic mineral assemblages in the Mikhailovsk rocks.The sulphides are pyrrhotite and arsenopyrite with very minor pyrite at Chertovo Koryto,whereas pyrite is the predominant sulphide in the Sukhoi Log ore.Fluid inclusion data on both deposits emphasise a high-temperature nature of the mineralisation albeit revealing great contrast in the fluid composition.Sukhoi Log mineralisation was formed at mixing between low-salinity aqueous solutions and dense gaseous carbonic fluids,which facilitated effective gold scavenging and precipitation,as demonstrated by thermodynamic simulation.The precursory devolatilisation of the Mikhailovsk sediments at the prograde stage results in the paucity of gaseous carbonic fluid during retrograde metamorphism and mineralisation.The similarity in the styles and chemical parameters of mineralisation,and the predominant structural control of ore localisation within the same Precambrian regional tectonic unit support an idea that orogenic gold mineralisation in the Lena province was produced during a single early Paleozoic event.

Mineralogical Evidence for Regional Metamorphism Overprinted by Contact Metamorphism

The Soursat metamorphic complex （SMC） in northwestern Iran is part of the Sanandaj- Sirjan metamorphic belt. The complex is composed of different metamorphic and plutonic rocks, but is dominated by metapelites composed of garnet, staurolite, kyanite, fibrolite, cordierite, and andalusite. Porphyroblasts in schists have the same fabric, and three stages of schistosity are present. The internal schistosity （Sn） inclusion trails are also offset by conjugate sets of extensional schistosity （Sn＋l） and a second （Sn＋2） that crenulates （Sn＋l）. Polyphase metamorphisms are present in the complex. Garnet, staurolite, kyanite, and fibrolite assemblage preserves conditions during the M1 metamorphic event. This assemblage yields a P-t estimate of 645±11℃ and 6.5±0.5 kbar. Other samples of the central part of SMC contain cordierite and andalusite （M2） overgrowth that yields a P- t estimate of 532±33℃ and 2.1±1.1 kbar.

Study of oxidative desulphurization process of coal with different metamorphism degrees

The oxidative desulphurization process of coal with different metamorphism degrees treated by an air-steam mixture has been studied.It has been shown that the pyrite present in black coal and anthracite is oxidized with the sulphur dioxide formation,and the process chemical mechanism does not depend on the quality of organic matter.The medium-metamorphized coal,capable of turning into a plastic state and cake in the range of investigated temperatures(350～450 ℃),is desulphurized with the greatest difficulty.The chemical mechanism dealing with the transformations of pyritic sulphur present in brown coal differs from similar processes taking place in black coal and anthracite,because FeS2 is converted with hydrogen sulphide formation at desulphurization.

Extensional Tectonic Framework of Post High and Ultrahigh Pressure Metamorphism in Dabieshan,China

The most prominent feature of the extensional tectonic framework of post high pressure (HP) and ultrahigh pressure (UHP) metamorphism in Dabieshan is the development of the multi layered extension detachment zones surrounding the core of the Luotian dome, and the separation of the UHP, HP and epidote blueschist units by the detachment zones, which form the vertically stacking sheet like slices of the HP and UHP metamorphic rocks. From the core outwards, exist the HP and UHP rock barren Dabie complex, UHP unit, HP unit and epidote blueschist unit. The extension tectonics of post HP and UHP metamorphic event constrain the distribution and present configuration of the HP and UHP metamorphic rocks, and the extensional tectonic framework bears some similarities to the Cordillera metamorphic core complex. It is suggested that partial melting happened in the Dabie gneiss complex (DGC) and UHP unit contemporaneously with the extrusion of UHP metamorphic rocks into the lower-middle crust. The formation and emplacement of the migmatite and granites are the response to the change in thermal state, facilitating the transfer from the compressive regime to extensional regime in the crust. The large scale crustal extension and uplift and the accompanying anatexis in Dabieshan are probably related to the delamination and magmatic underplating in the mantle and the lower crust.

Very Low-Grade Metamorphism of Clastic Rocks from the Meso-Neoproterozoic and the Paleozoic along the Profile Yueyang-Linxiang in Northeastern Hunan Province and Its Geological Implications

This study uses illite crystallinity,chlorite crystallinity,illite polytypes,the b0 cell-dimension of K-white mica,clay mineral assemblages and mineral geothermo-geobarometers to investigate the overprint of diagenesis and metamorphism on the Meso-Neoproterozoic and the Lower Paleozoic along the profile Yueyang-Linxiang in northeastern Hunan Province,China.Illite crystallinity Kbler index（KI） of the 〈2μm fractions ranges from 0.225 to 0.485°Δ2θ while chlorite crystallinity Arkai index（AI） ranges from 0.244 to 1.500°Δ2θ.This indicates that the Meso-Neoproterozoic and the Lower Paleozoic along the profile Yueyang-Linxiang were overprinted with diagenesis and anchi- to epimetamorphism.Peak metamorphic temperature is estimated with the IV site chlorite geothermometer roughly at 360℃.The b0 cell dimension values of illites（K-mica） range from 0.9002 to 0.9054 nm and,on average,at 0.9030 nm for the Meso-Neoproterozoic.Based on cumulative frequency curves of illite（K-mica） b0 cell dimension,the peak metamorphic pressure of the MesoNeoproterozoic along the profile Yueyang-Linxiang is derived of an intermediate pressure type.Most illites occur in the 2M1 polytype and some of them in a mixture of 2M1＋1M types especially those in the Paleozoic.This result partly agrees with the conclusion of the lower greenschist and greenschist facies of the Lengjiaxi and Banxi Groups.However,it is not agreed with the sedimentary cover from the Sinian to the Lower Paleozoic or from the Banxi Group to the Lower Paleozoic.Crustal thickening due to ＂collision＂ between the Yangtze and Cathaysia blocks led to an increase in the thickness of the Meso-Neoproterozoic to ca.14 km and resulted in a temperature increase in those rocks due to burial.The very low grade to low grade metamorphism overprinting the Meso-Neoproterozoic implies that the so called ＂Chiangnania or Jiangnan orogen＂ was no relative with the ＂Grenvillian orogeney;instead,it might be a continuous amalgamation product between the Yangtze and Cathaysia blocks.

Precambrian Metamorphism of Tiya－Olokit Metamorphic Belt in Northern Balkal Region and its Metallogeny

The paper deals with results of petrological observations of the Precambrian rocks in the Tiya-Olokit metamorphic belt of the Northern Balkal region. Evoluhon of mstamorphic processes from Archean through to Paleozoic is characterized. Interrelations of metamorphic and ore formation processes are shown .Various genetic types of metamorphism and their metallogeulc speianzation (pyrihc-polylnetallic, gold-sulphide and others) are established.A great number of previously published analyses of garnets, biohtes, amphiholes, pyroxenes and muscovitde from metamorphic rocks have been made use of. The evoluhon of their compositions under progressive zoned metamorphism and diaphthoresis is revealed.

Distinguishing and Correlating Multiple Phases of Metamorphism across Multiply Deformed Regions Using the Axes of Inclusion Trails in Porphyroblasts

Successions of FIAs（foliation inflection/intersection axes preserved within porphyroblasts） provide a relative time scale for deformation and metamorphism.In-situ dating of monazite grains preserved as inclusions within garnet and staurolite porphyroblasts within the foliations defining each FIA from such successions provides a rigorous approach to grouping ages that formed over extended periods of deformation and metamorphism.Matching age and FIA progressions confirms the suitability of this approach plus provides access to lengthy histories that cannot otherwise be determined.Tectonism in the Big Thompson region of the Colorado Rocky Mountains,USA began about a SW-NE trending axis defined by FIA set 1 at around 1761±13 Ma.Subsequent periods of tectonism occurred around 1712±25 and 1672±21 Ma about W-E and NW-SE trending FIAs 2 and 3, respectively.Tectonism in the Balcooma Metamorphic Group NW of Greenvale in NE Queensland, Australia began around 470 Ma with the growth of garnet porphyroblasts about a W-E trending axis defined by FIA set 1.No monazite grains were preserved as inclusions in this mineral phase but they were in subsequently grown staurolite.Tectonism then occurred around 443.2±3.8,424±4 and 408.9±8.9 Ma about NNW-SSE,SSW-NNE and W-E trending FIAs 2,3 and 4,respectively.This test of a FIA based approach to monazite dating in tectonic regimes in different parts of the world and with dramatically different ages suggests it can be used in all orogens containing porphyroblasts preserving inclusion trails.

Illite Crystallinity Mapping of Very Low Grade Metamorphism of Triassic Metapelites in the Zoigê Area,Western China

X-ray diffraction methods for estimating the metamorphic grade of diagenetic, anchizone and epizone in metapelites are reviewed and applied to samples from a 7000 m＋ borehole in western China and surface samples from the surrounding Zoige area. Kiibler＇s illite crystallinity （IC） measurements provide more consistent results than calculated values of percentage of illite in the I/S mixed layers and percentage of I/S mixed layers. Down-borehole IC values display a typical burial metamorphic relationship between stratigraphic level and IC. A method for preparing very low grade metamorphic maps is described, and isograds plotted on a regional geological map at selected values of IC, delineating a high temperature diagenetic zone, an anchizone, and an epizone. The map shows that IC values are controlled by stratigraphic level in the north of the study area （i.e. burial metamorphism）, and proximity to an igneous intrusive body in the south （i.e. contact metamorphism）.

Metamorphism and Microstructure of Seasonal Snow:Single Layer Tracking in Western Tianshan,China

【Title】【Author】Snowpack is a combination of several snow layers. Accordingly, snowpack natural metamorphism is composed of several stages. The aim of this study is to investigate the natural snow metamorphism at the snow layer unit. The field investigation was conducted at the Tianshan Station for Snow Cover and Avalanche Research, Chinese Academy of Sciences （43°16＇ N, 84°24＇ E, and 1,776 m a.s.l.）, during the winter of 2010-2011. A complete metamorphic procedure and the corresponding microstructure of a target snow layer were tracked. The results indicate that： the ideal and complete metamorphic process and the corresponding predominant snow grain shape have 5 stages： 1） unstable kinetic metamorphism near the surface; 2） unstable kinetic metamorphism under pressure; 3） stable kinetic metamorphism; 4） equilibrium metamorphism; 5） wet snow metamorphism. Snow grain size sharply decreased in the surface stage, and then changed to continuously increase. Rapid increase of grain size occurred in the stable kinetic metamorphism and wet snow metamorphism stage. The characteristic length was introduced to represent the real sizes of depth hoar crystals. The snow grain circularity ratio had a variation of “rapid increase – slow decrease – slow increase”, and the snow aggregations continuously increased with time. Snow density grew stepwise and remained steady from the stable kinetic to the equilibrium metamorphism stage. The differences in metamorphism extent and stages among snow layers, led to the characteristic layered structure of snowpack.

Timing of metamorphism and deformation in the Swat valley, northern Pakistan:Insight into garnet-monazite HREE partitioning

New metamorphic petrology and geochronology from the Loe Sar dome in the Swat region of northern Pakistan place refined constraints on the pressure, temperature and timing of metamorphism and deformation in that part of the Himalayan orogen. Thermodynamic modelling and monazite petrochronology indicate that metamorphism in the area followed a prograde evolution from ～525 ± 25 ℃and 6 士 0.5 kbar to ～610 ± 25 ℃ and 9 士 0.5 kbar, between ca. 39 Ma and 28 Ma. Partitioning of heavy rare earth elements between garnet rims and 30-28 Ma monazite are interpreted to indicate coeval crystallization at peak conditions. Microtextural relationships indicate that garnet rim growth post-dated the development of the main foliation in the area. The regional foliation is folded about large-scale N-S trending fold axes and overprinting E-W trending folds to form km-scale domal culminations. The textural relationships observed indicate that final dome development must be younger than the 30-28 Ma monazite that grew with garnet rims post-regional foliation development, but pre-doming-related deformation. This new timing constraint helps resolve discrepancy between previous interpretations,which have alternately suggested that N-S trending regional folds must be either pre-or post-early Oligocene. Finally, when combined with existing hornblende and white mica cooling ages, these new data indicate that the study area was exhumed rapidly following peak metamorphism.

Petrology and metamorphism of glaucophane eclogites in Changning-Menglian suture zone, Bangbing area, southeast Tibetan Plateau: An evidence for Paleo-Tethyan subduction

High/ultrahigh-pressure(HP/UHP)metamorphic complexes,such as eclogite and blueschist,are generally regarded as significant signature of paleo-subduction zones and paleo-suture zones.Glaucophane eclogites have been recently identified within the Lancang Group characterized by accretionary mélange in the Changning-Menglian suture zone,at Bangbing in the Shuangjiang area of southeastern Tibetan Plateau.The authors report the result of petrological,mineralogical and metamorphism investigations of these rocks,and discuss their tectonic implications.The eclogites are located within the Suyi blueschist belt and occur as tectonic lenses in coarse-grained garnet muscovite schists.The major mineral assemblage of the eclogites includes garnet,omphacite,glaucophane,phengite,clinozoisite and rutile.Eclogitic garnet contains numerous inclusions,such as omphacite,glaucophane,rutile,and quartz with radial cracks around.Glaucophane and clinozoisite in the matrix have apparent optical and compositional zonation.Four stages of metamorphic evolution can be determined:The prograde blueschist facies(M_(1)),the peak eclogite facies(M_(2)),the decompression blueschist facies(M_(3))and retrograde greenschist facies(M_(4)).Using the Grt-Omp-Phn geothermobarometer,a peak eclogite facies metamorphic P-T condition of 3000–3270 MPa and 617–658℃ was determined,which is typical of low-temperature ultrahigh-pressure metamorphism.The comparison of the geological characteristics of the Bangbing glaucophane eclogites and the Mengku lawsonite-bearing retrograde eclogites indicates that two suites of eclogites may have formed from significantly different depths or localities to create the tectonic mélange in a subduction channel during subduction of the Triassic Changning-Menglian Ocean.The discovery of the Bangbing glaucophane eclogites may represent a new oceanic HP/UHP metamorphic belt in the Changning-Menglian suture zone.

LA-ICP-MS U-Pb Zircon Dating for Felsic Granulite, Huangtuling Area, North Dabieshan: Constraints on Timing of Its Protolith and Granulite-Facies Metamorphism, and Thermal Events in Its Provenance

Information about the protolith of the Huangtuling granulite in North Dabieshan has been unavailable. The complex evolution history of the rock and its host basement must be further discussed. LA-ICP-MS U-Pb dating was conducted on three textural domains in zircon from a high-temperature, high-pressure felsic granulite in the Huangtuling area, North Dabieshan, Central China. The metamorphic growth-derived detrital zircon domain yields a 207^ pb/206^Pb age in the range of （2 49±54 ） -- （2 500±180） Ma. The magmatic genesis-derived detrltal zircon domain gives a 207^pb/ 206^Pb age ranging from 2 628 Ma to 2 690 Ma, with an oldest 206^ pb/ 238^U age of （2 790 ± 150） Ma. The metamorphic overgrowth or metamorphic recrystallization zircon domain yields a diesordia with an upper intercept age of （2 044. 7 ± 29.3 ） Ma. Compositions of the mineral assemblage, major element geochemistry, and especially the complex interior texture of the zircon suggest that the prololith of the felsic granulite is of sedimentary origin. Results show that the protolith material of the granulite came from a provenance with a complex thermal history, i.e. -2.8 Ga magmatlsm and -2.5 Ga metamorphism, and was deposited in a basin not earlier than 2.5 Ga. The high-temperature and high-pressure granulite-facies metamorphic age was precisely constrained at （2.04±0.03） Ga, which indicates the granulite in Huangtuling area should be a relict of a Paleoproterozoic UHT （ultrahigh temperature） metamorphosed slab.