Ultrahigh temperature deformation microstructures in felsic granulites of the Napier Complex, Antarctica

Detailed electron microscope and microstructural analysis of two ultrahigh temperature felsic granulites from Tonagh Island, Napier Complex, Antarctica show deformation microstructures produced at  1000 °C at 8–10 kbar. High temperature orthopyroxene (Al 7 wt.% and  11 wt.%), exhibits crystallographic preferred orientation (CPO) and frequent subgrain boundaries which point to dislocation creep as the dominating deformation mechanism within opx. Two different main slip systems are observed: in opx bands with exclusively opx grains containing subgrain boundaries with traces parallel to [010] and a strong coupling of low angle misorientations (2.5°–5°) with rotation axes parallel to [010] the dominating slip system is (100)[001]. Isolated opx grains and grain clusters of 2–5 grains embedded in a qtz–fsp matrix show an additional slip system of (010)[001]. The latter slip system is harder to activate. We suggest that differences in the activation of these slip systems is a result of higher differential stresses imposed onto the isolated opx grains and grain clusters. In contrast to opx, large qtz grains (up to 200 μm) show random crystallographic orientation. This together with their elongate and cuspate shape and the lack of systematic in the rotation axes associated with the subgrain boundaries is consistent with diffusion creep as the primary deformation mechanism in quartz.Our first time detailed microstructural observations of ultrahigh temperature and medium to high pressure granulites and their interpretation in terms of active deformation mechanisms give some insight into the type of rheology that can be expect at lower crustal conditions. If qtz is the mineral phase governing the rock rheology, Newtonian flow behaviour is expected and only low differential stress can be supported. However, if the stress supporting mineral phase is opx, the flow law resulting from dislocation creep will govern the rheology of the rock unit; hence, an exponential relationship between stress and strain rate is to be expected.     

东南极格罗夫山镁铁质麻粒岩的变质作用

东南极内陆－格罗夫山存在一套经历了麻粒岩相变质作用的镁铁质麻粒岩和斜长角闪岩。变质反应结构显示该区多为单一的区域性麻粒岩相变质作用。但是，对含石榴石的镁铁质麻粒岩的详细工作则显示了包括3个阶段的近等温降压（ITD）的顺时针PT演化轨迹，M1：0．93GPa＞800℃：M2:0.65GPa,733-850℃;M3:0.46-0.61GPa，并有着与拉斯曼丘陵相似的演化历史。根据矿物组合和成分、变质反应结构及温压计算结果，格罗夫山镁铁质麻粒岩可能为高压麻粒岩。     

Beryllium and Other Trace Elements in Paragneisses and Anatectic Veins of the Ultrahigh-Temperature Napier Complex, Enderby Land, East Antarctica: the Role of Sapphirine

Anatectic veins containing the Be minerals khmaralite and berylliansapphirine as primary phases (or surinamite derived therefrom)are associated with Mg–Al-rich paragneisses at three localitiesin the ultrahigh-temperature Napier complex, Antarctica, a uniqueBe mineralization in the granulite facies. Likely precursorsof the paragneisses are volcaniclastic deposits that were hydrothermallyaltered by heated seawater prior to metamorphism. Regular distributionof Be among minerals in the paragneisses suggests an approachto equilibrium with Be greatly concentrated in sapphirine (25–3430ppm Be) or cordierite (560–930 ppm Be) relative to plagioclaseAn53–66 (14–43 ppm Be) > cores of coarse-grainedorthopyroxene (0·7–29 ppm Be) > coronitic orthopyroxene(0·4–14 ppm Be) sillimanite (0·1–26ppm Be) plagioclase An18–33 (0·6–15 ppmBe) > biotite (0·06–8 ppm Be) > K-feldspar,quartz, garnet (0·05–0·7 ppm Be). Sapphirine-bearingparagneisses have average Be concentrations, 4·9 ±2·4 ppm (13 samples), about twice that of typical pelites,whereas paragneisses lacking sapphirine and primary cordieritehave only 2·9 ± 2·1 ppm Be (12 samples),implying some loss of Be during metamorphism. The likely sourcerocks for the Be-rich melts were biotitic rocks lacking theBe sinks sapphirine and cordierite. These gneisses were probablyless competent than the sapphirine-bearing gneisses, so themelts were drawn to the latter and collected in spaces openedduring deformation and boudinage of the more competent paragneisses.Fractionation of the melts concentrated Be to the extent thatBe minerals could crystallize. The final result was Be-mineralizedanatectic veins hosted by relatively Be-rich sapphirine-bearingparagneisses. KEY WORDS: Antarctica; beryllium; granulite facies; microprobe; sapphirine     

Very high-density carbonic fluid inclusions in sapphirine-bearing granulites from Tonagh Island in the Archean Napier Complex, East Antarctica: implications for CO2 infiltration during ultrahigh-temperature (T>1,100 °C) metamorphism

The ultrahigh-temperature (UHT) metamorphism of the Napier Complex is characterized by the presence of dry mineral assemblages, the stability of which requires anhydrous conditions. Typically, the presence of the index mineral orthopyroxene in more than one lithology indicates that H₂O activities were substantially low. In this study, we investigate a suite of UHT rocks comprising quartzo-feldspathic garnet gneiss, sapphirine granulite, garnet-orthopyroxene gneiss, and magnetite-quartz gneiss from Tonagh Island. High Al contents in orthopyroxene from sapphirine granulite, the presence of an equilibrium sapphirine-quartz assemblage, mesoperthite in quartzo-feldspathic garnet gneiss, and an inverted pigeonite-augite assemblage in magnetite-quartz gneiss indicate that the peak temperature conditions were higher than 1,000 °C. Petrology, mineral phase equilibria, and pressure-temperature computations presented in this study indicate that the Tonagh Island granulites experienced maximum P-T conditions of up to 9 kbar and 1,100 °C, which are comparable with previous P-T estimates for Tonagh and East Tonagh Islands. The textures and mineral reactions preserved by these UHT rocks are consistent with an isobaric cooling (IBC) history probably following an counterclockwise P-T path. We document the occurrence of very high-density CO₂-rich fluid inclusions in the UHT rocks from Tonagh Island and characterize their nature, composition, and density from systematic petrographic and microthermometric studies. Our study shows the common presence of carbonic fluid inclusions entrapped within sapphirine, quartz, garnet and orthopyroxene. Analysed fluid inclusions in sapphirine, and some in garnet and quartz, were trapped during mineral growth at UHT conditions as 'primary' inclusions. The melting temperatures of fluids in most cases lie in the range of -56.3 to -57.2 °C, close to the triple point for pure CO₂ (-56.6 °C). The only exceptions are fluid inclusions in magnetite-quartz gneiss, which show slight depression in their melting temperatures (-56.7 to -57.8 °C) suggesting traces of additional fluid species such as N₂ in the dominantly CO₂-rich fluid. Homogenization of pure CO₂ inclusions in the quartzo-feldspathic garnet gneiss, sapphirine granulite, and garnet-orthopyroxene gneiss occurs into the liquid phase at temperatures in the range of -34.9 to +4.2 °C. This translates into very high CO₂ densities in the range of 0.95-1.07 g/cm³. In the garnet-orthopyroxene gneiss, the composition and density of inclusions in the different minerals show systematic variation, with highest homogenization temperatures (lowest density) yielded by inclusions in garnet, as against inclusions with lowest homogenization (high density) in quartz. This could be a reflection of continued recrystallization of quartz with entrapment of late fluids along the IBC path. Very high-density CO₂ inclusions in sapphirine associated with quartz in the Tonagh Island rocks provide potential evidence for the involvement of CO₂-rich fluids during extreme crustal temperatures associated with UHT metamorphism. The estimated CO₂ isochores for sapphirine granulite intersect the counterclockwise P-T trajectory of Tonagh Island rocks at around 6-9 kbar at 1,100 °C, which corresponds to the peak metamorphic conditions of this terrane derived from mineral phase equilibria, and the stability field of sapphirine + quartz. Therefore, we infer that CO₂ was the dominant fluid species present during the peak metamorphism in Tonagh Island, and interpret that the fluid inclusions preserve traces of the synmetamorphic fluid from the UHT event. The stability of anhydrous minerals, such as orthopyroxene, in the study area might have been achieved by the lowering of H₂O activity through the influx of CO₂ at peak metamorphic conditions (>1,100 °C). Our microthermometric data support a counterclockwise P-T path for the Napier Complex.     

东南极内陆格罗夫山地区冰厚及冰下地形特征

中国第30次南极科学考察队格罗夫山分队(CHINARE30, 2013-2014年)利用雪地车载深层探冰雷达在东南极格罗夫山地区开展了测线总长度超过200 km大范围、高分辨率的冰厚及冰下地形调查, 获得了哈丁山北部和萨哈罗夫岭与阵风悬崖之间详细的冰厚及冰下地形特征. 通过对雷达数据分析表明, 哈丁山北部区域平均冰厚为580 m, 最大冰厚超过1 000 m, 出现在该区域的东北方向, 而东南方向冰厚相对较小; 萨哈罗夫岭与阵风悬崖之间区域的平均冰厚为610 m, 最大冰厚超过1 100 m, 该区域槽谷发育十分成熟, 槽谷形态近似呈U型. 通过对雷达剖面影像的筛选和分析, 推测在格罗夫山地区可能存在2个液态冰下湖泊.     

Metastable growth of corundum adjacent to quartz in a spinel-bearing quartzite from the Archaean Napier Complex, Antarctica

In a granulite-facies spinel-bearing quartzite, corundum, orthopyroxene and sapphirine (and rarely cordierite and sillimanite) form partial rims separating spinel from quartz. Textures indicate the reactions:
spinel + quartz = orthopyroxene + corundum, and
spinel + quartz = orthopyroxene + sapphirine.
Thus, corundum and sapphirine are produced by reactions involving quartz. The low Al-content of the orthopyroxene (0.5–2.8 wt %) and low values for Mg–Fe distribution coefficient for spinel–sapphirine and spinel–orthopyroxene reflect low-temperature conditions during formation of the reaction products. Absence of zoning in spinel and a constant Mg–Fe distribution coefficient for spinel–sapphirine and spinel–orthopyroxene, over a compositional range, indicate Mg–Fe equilibration. It is suggested that stable reactions such as spinel + quartz = cordierite or spinel + quartz = garnet + sillimanite were over-stepped and that metastable reactions give rise to the anomalous juxtaposition of corundum + quartz.     

东南极格罗夫山泛非期麻粒岩相变质作用

东南极格罗夫山主要由麻粒岩相高级变质岩和花岗岩类组成，其中变质岩以浅色和暗色含斜方辉石长英质片麻岩占主导地位，夹有少量镁铁质麻粒岩、变沉积岩和含方柱石钙硅酸盐岩。这些岩石一般都展示了平衡的矿物共生结构，但在镁铁质麻粒岩的单斜辉石中普遍发育斜方辉石(易变辉石)的出溶片晶。根据出溶辉石的重组分析获得麻粒岩相变质作用的峰期温度约为850℃，而浅色片麻岩中的石榴子石—斜方辉石—斜长石—石英组合给出的变质压力为0．61～0．67GPa。镁铁质麻粒岩中火成亚钙质普通辉石斑晶的保存表明格罗夫山地区可能只发育单一的泛非期高温麻粒岩相变质事件，岩石在高温变质之后经历了缓慢冷却过程，这主要归因于花岗质岩浆的板底垫托作用。     

Evolution of the Archean crust in Finland

An Archean age for Finnish rocks in the range 2500–3000 Ma has been determined north of the NW-striking Ladoga—Raahe shear belt. The Archean may be divided into two main units: the granitoid association and the greenstone-belt association. The complex is characterized by stockwork tectonics. The granitoid association forms the basement infrastructure and the greenston-belt association forms the suprastructure which is present in synforms between granitoid diapirs. The infrastructure has been subjected to ultrametamorphism, and the second and third generation palingenetic magmas so formed have intruded the suprastructure. The granitoid association contains widespread migmatized relicts of the greenstone-belt association, indicating that the latter originally covered much larger areas, but the granitoids are also thought to be partly transformed primitive ensialic crust on which rocks of the greenstone-belt association were deposited. The Archean rocks have been deformed in at least four subsequent phases, of which part developed in Proterozoic time. The youngest deformation is the overthrust of the granulite belt of Lapland towards SSW. NWSE striking transcurrent faults played a major role in Proterozoic time and affected cratonized Archean crust. On the whole the greenstone belts in eastern and northern Finland form a NNW-trending zone 750 km long. On a geochemical basis the volcanic rocks of the greenstone belts can be divided into two groups: tholeiites with a low potassium content and extremely low aluminium content and a calc-alkalic group with some alkalic affinities.     

东南极格罗夫山变质基性岩地球化学特征

东南极格罗夫山变质基性岩的地球化学研究表明，该区存在两类玄武岩，即洋岛型玄武岩（OIB）和洋中脊型玄武岩（MORB）。OIB型具有大体类似的地球化学性质，它们均富集Ti（TiO2＝2．68％）、REE＝（202μg/g）、LREE[（La/Yb)N＝4．8]、Ti/Y（＝343）、Zr／Y(＝3．1），具洋岛玄武岩的特征，推测岩浆来源于富休地幔源区（EM）。而MORB型以低Ti（TiO2＝1．1％－1．31％），明显低于OIB的P的含量（P2O5＝0．1％－0．2％），低REE（47－93μg/g）。LREE／HREE（2．27－2．54）、（La/Yb)N（＝1．30－1．62）为特征，具洋中脊玄武岩的特征。MORB和OIB组合的出现说明在泛非期该区可能存在过洋分。     

Garnet-Orthopyroxene Bearing Granulites from Enderby Land, Antarctica: Metamorphic Pressure Temperature-Time Evolution of the Archaean Napier Complex

Garnet-orthopyroxene bearing granulite assemblages from theArchaean Napier Complex, Enderby Land, Antarctica, display avariety of exsolution, recrystallization and corona textureswhich result both from near-isobaric cooling from the peak ofmetamorphism and from later overprinting. Compositional dataon distinct generations of phases and on zoning patterns incoexisting minerals, have been used to estimate (a) peak metamorphicconditions attained between the first and second major deformationphases (Dl and D2); (b) cooling paths from this peak, and (c)ambient metamorphic conditions at the time of a later deformation(D3). Experimentally calibrated geothermobarometers indicateinitial metamorphism at 900–950?C and 7–10 kb duringand subsequent to Dl and D2, at 3100–3000 Ma. The presentlyexposed granulites indicate a regional increase in the pressuresof this metamorphism south-west to the Scott Mountains-CaseyBay region, where minimum crustal thicknesses of 10 kb wereattained at c. 3000 Ma. Subsequently, the Napier Complex granulitesevolved through a prolonged period of near-isobaric coolingprior to further metamorphism at 600–750 and 4–8kb during D3 at c. 2500 Ma. The near-isobaric pressure-temperature-timepath (P-T-t) suggests that the Napier Complex acted as an essentiallystable craton as early as 3000 Ma, and that the major magmaticand tectonic crustal thickening events associated with Dl precededthe thermal peak represented by the earliest recognized metamorphism.     

Chemical discontinuities in Archean metavolcanic terrains and the development of Archean crust

Most large Archean greenstone belts ( 2.7 Ga), comprise thick (12–15 km) mafic to felsic metavolcanics sequences which exhibit consistent but discontinuous geochemical patterns resulting from mantle-crust processes. In a typical Archean metavolcanic sequence, thick (5–8 km) uniform tholeiitic basalt is followed by geochemically evolved rock units (4–7 km thick) containing intermediate and felsic calc-alkaline rocks. This major geochemical discontinuity is marked by a change from LIL-element depleted basalts which show unfractionated REE abundance patterns, to overlying andesites with higher LIL-element contents, fractionated REE patterns and relatively depleted HREE. A less well marked discontinuity separates andesitic rocks from still later more felsic dacite-rhyolite extrusive assemblages and their intrusive equivalents, and is identified by a further increase in LIL element content and REE fractionation. The major geochemical discontinuity apparently separates rocks derived by partial melting of mantle (either directly or through shallow fractionation processes) from those which originated either by partial melting of mantle material modified by crustal interactions or by partial melting of crustal material.We suggest that accumulation of a great thickness of mantle derived volcanic rocks can lead to sagging and interaction of the lower parts of the volcanic piles with upper mantle material. The resulting modified mantle acts as a source for some of the geochemically evolved rocks observed in volcanic successions. Subsequent direct melting of the volcanic pile produces the felsic magmas observed in the upper parts of Archean volcanic successions. This process, termed sag-subduction, is the inferred tectonic process operating in the comparatively thin, hot Archean crustal regime. By this process, large masses of ultimately mantle-derived material were added to the crust.     

Pan-African Alkali Granitoids from the Sør Rondane Mountains, East Antarctica

Alkali granitoids (500-550 Ma) representing a prominent Pan-African magmatic event are widely distributed in the Sør Rondane Mountains, Dronning Maud Land, East Antarctica. Geochemically, they are granitic to syenitic in composition and show an alkaline affinity of A-type granites. They are characterized by high K₂O+Na₂O (7-13 wt%) and K₂O/Na₂O (1-2), low to intermediate Mg#, wide ranges of SiO₂ (45-78 wt%), Sr (20-6500 ppm) and Ba (40-13000 ppm) and have Nb and Ti depletion in the primitive mantle normalized diagram. The granitoids are subdivided into Group I granites, Group II granites, Lunckeryggen Syenitic Complex and Mefjell Plutonic Complex. The Group I granites have higher Mg#, Sr/Ba, Sr/Y, (La/Yb)_N and LREE/HREE, lower A/CNK, SREE and initial ⁸⁷Sr/⁸⁷Sr ratios and lack Eu anomalies compared to those with negative Eu anomalies in the Group II granites. The syenitic rocks from the Mefjell Plutonic Complex are higher in alkali, Ga, Zr, Ba, and have lower Mg#, Rb, Sr, Nb, Y, F and LREE/HREE with positive Eu anomaly, whereas the granites from the Mefjell Plutonic Complex have high LREE/HREE ratios with negative Eu anomaly. The Lunckeryggen syenitic rocks have intermediate Mg#, higher K₂O, P₂O₅, TiO₂, Fe₂O₃/FeO, Ba, Sr/Y and LREE/HREE ratios with lack of Eu anomalies and are lower in Al₂O₃, Ga, Y, Nb and Rb/Sr ratios. Based on chemical characteristics combined with isotopic data, we suggest that the Lunckeryggen syenitic body and Group I granitic bodies may be derived from the mantle-derived hot basic magma by fractional crystallization with minor assimilation. We also suggest that the Group II granites may be derived from assimilation with crustal rocks to varing degrees and then fractional crystallization in higher crustal levels (ACF model). The Mefjell Plutonic Complex seems to be derived from a heterogenetic magma source compared with other granitoids from the Sør Rondane Mountains. The syenitic rocks in the Mefjell Plutonic complex have a unique source (iron-enriched) and have a chemical affinity with the charnockites in Gjelsvikjella and western Mühlig-Hofmannfjella, but not like the Yamato syenites in adjacent areas.     

Aureoles of Pb(II)-enriched feldspar around monazite in paragneiss and anatectic pods of the Napier Complex,Enderby Land,East Antarctica: the roles of dissolution-reprecipitation and diffusion

Extraordinarily high Pb content in K-feldspar and plagioclase has been found contiguous to monazite in two occurrences in the ultrahigh-temperature Napier Complex of Antarctica. Monazite shows a variety of textures and compositions. In a garnet-sillimanite-orthopyroxene paragneiss at Mount Pardoe (Amundsen Bay), grains range 80–150 μm across and are anhedral; two grains are Th- and Si-dominant. In pods that crystallized from anatectic melts at 2500 Ma at Zircon Point, Casey Bay, monazite grains range 0.05 mm–1 cm in length and are highly variable in texture. The coarsest grains (>0.7 cm) are skeletal and euhedral, whereas the smallest grains are anhedral and associated with fine- to medium-grained quartz, K-feldspar, plagioclase, garnet, sillimanite and rutile in aggregates that form interstitial veinlets interpreted to be a second generation of anatexis during an event at 1100 Ma. The huttonite component (ThSiO₄) reaches 30 mole% in the cores of the coarsest skeletal grains, whereas other grains, particularly smaller ones, show complex and irregular zoning in Th and U. The latter zoning is attributed to dissolution-reprecipitation, which also resulted in complete Pb loss during the 1100 Ma event. In the paragneiss at Mount Pardoe, K-feldspar and myrmekitic plagioclase (An16) are found in a 70–80 μm band between monazite and orthopyroxene and contain up to 12.7 wt.% and 2.7 wt.% PbO, respectively, corresponding to 18.5% and 3.4% PbAl₂Si₂O₈ component, respectively. Cathodoluminescence of both feldspars increases with distance from a nearby monazite grain and is not correlated with Pb content. Incorporation of Pb in K-feldspar and plagioclase could be a result of diffusion, even though the monazite adjacent to feldspar apparently lost little Pb, i.e., Pb could have been transported by fluid from the Th-rich grains, which did lose Pb. In contrast to the paragneiss, cathodoluminescence correlates with Pb content of K-feldspar in aureoles surrounding skeletal monazite grains 0.7–1 cm across in anatectic pods at Zircon Point. Pb content of K-feldspar decreases monotonically to near detection limits within several millimetres of monazite grains; the greatest PbO concentration is attained in K-feldspar inliers and embayments in monazite, 8.8 wt.%, corresponding to 11.7% PbAl₂Si₂O₈ component. Fine-grained quartz in the K-feldspar suggests that the mechanism for Pb incorporation involved breakdown of feldspar: Pb²⁺ + 2(K,Na)AlSi₃O₈ → PbAl₂Si₂O₈ + 4SiO₂ + 2(K,Na)⁺ . The smooth decrease of Pb in the aureoles is not characteristic of dissolution-reprecipitation, which is characterized by abrupt changes of composition, and it seems more likely that Pb was incorporated in K-feldspar by diffusion at 1100 Ma. We suggest a model whereby fluid introduced during the 1100 Ma event flowed along grain boundaries and penetrated mineral grains. Temperatures were sufficiently high, i.e., 700°C, assuming burial in the mid-crust, for the fluid to induce localized melting of quartzofeldspathic matrix of the anatectic pods. Loss of radiogenic Pb was complete. Some penetration of K-feldspar by aqueous fluid is suggested by the presence of scattered galena specks and by rays of turbidity emanating from monazite. Aqueous fluid or water-rich granitic melt may have mediated the diffusion of Pb in feldspar, but it did not cause dissolution-reprecipitation. Although Pb was mobilized by aqueous fluid or water-rich granitic melt, it was not entirely flushed from the immediate vicinity of the monazite, but nearly half was incorporated in adjacent feldspar. Fluid activity that could cause Pb loss in monazite does not always leave an obvious trace, i.e., hydrous minerals, such as sericite, are very sparse, and biotite is absent in the anatectic pods at Zircon Point. Nonetheless, electron microprobe dating of monazite from the pods could not detect the 2500 Ma age of original crystallization determined by isotopic dating.     

An integrated microtextural and chemical approach to zircon geochronology: refining the Archaean history of the Napier Complex,east Antarctica

Integrated textural and chemical characterisation of zircon is used to refine the U–Pb geochronology of the Archaean, ultra-high temperature Napier Complex, east Antarctica. Scanning electron microscope characterisation of zircon and the rare earth element compositions of zircon, garnet and orthopyroxene are integrated to place zircon growth in an assemblage context, thereby providing tighter constraints on the timing of magmatic and metamorphic events. Data indicate that magmatism occurred in the central and northern Napier Complex at ca. 2,990 Ma. A regional, relatively low-pressure metamorphic event occurred at ca. 2,850–2,840 Ma. Mineral REE data from garnet-bearing orthogneiss indicate that ca. 2,490–2,485 Ma U–Pb zircon ages provide an absolute minimum age for the ultrahigh temperature (UHT) foliation preserved in this rock. Internal zircon zoning relationships and estimated zircon-garnet D_REE values from paragneiss suggest that an absolute minimum age of ultra-high temperature metamorphism is ca. 2,510 Ma, but that it is more likely to be older than ca. 2,545 Ma. We suggest that the high proportion of published zircon U–Pb data with ages between ca. 2,490–2,450 Ma reflects late, post-peak zircon growth and does not date the timing of peak UHT metamorphism.Electronic Supplementary Material Supplementary material is available for this article at     

关于东天山花岗岩与陆壳垂向增生的若干认识

东天山海西期主碰撞以后形成的花岗岩可分为三个阶段：挤压．伸展转折阶段（310～285Ma）、碰撞后伸展阶段（285-250Ma）和板内阶段（250-208Ma）。这三个阶段在岩石圈厚度、等温线形态和动力学状态等方面的差异，造成了不同阶段花岗岩岩体形态、岩石组合、岩相学和地球化学等方面的差异。除了底侵以外，幔源岩浆的内侵可以造成地壳不同层次岩石的部分熔融，也是花岗质岩浆生成的重要机制和地壳垂向增生的重要方式。东天山的片麻状花岗岩有一部分是变质交代成因的，这类花岗岩的形成与碰撞后幔源岩浆的底侵和内侵有关。变质峰期后韧性剪切带中构造细粒化的岩石是形成片麻状花岗岩的最有利部位。虽然这类花岗岩多数定位于地壳较深层次，但在内侵热量的影响下也可以定位于较浅层次。康古尔韧性剪切带的形成除了构造动力作用以外，还与地壳垂向增生，尤其是内侵有着密切联系，是构造动力、岩浆活动、变质作用和流体运移等复杂反馈的结果。博格达造山带碰撞前和碰撞后岩浆岩均具有正的εNd（t）值，表明该造山带地幔早在碰撞前就已亏损，而碰撞后的地幔则继承了碰撞前地幔的亏损特征。东天山在印支期有一次重要的地壳垂向增生事件，其岩浆活动和成矿作用与古特提期洋的俯冲和随后的碰撞密切有关，因此是东天山从中亚构造体制向特提斯体制转换的产物。     

Metamorphic history and tectonic evolution of the Qinling Complex, eastern Qinling Mountains, China

Abstract The metamorphic history and tectonic evolution of the Qinling Complex is divided into formation and modification stages. During the Proterozoic formation stage, three deformational sequences are recognized. Andalusite–muscovite, sillimanite–muscovite and sillimanite–K-feldspar zones of amphibolite facies regional metamorphism are earlier than, or synchronous with the first or second phase of folding. Ductile shear zones were formed and Caledonian granites were emplaced during the modification stage. The granites superimposed contact aureoles (garnet–K-feldspar zone) on the regional metamorphic fabric.
Metamorphic reactions, P–T conditions of metamorphism and P–T–t paths were estimated by analysis of mineral textures and standard thermobarometric techniques. The P–T–t path of the Proterozoic tectonometamorphic cycle shows prominent clockwise decompression. The P–T–t path of the Caledonian tectonometamorphic cycle is characterized by an early rise of pressure and temperature, followed by isothermal decompression (rapid uplift) and finally with isobaric cooling.
The P–T–t paths of the two tectonometamorphic cycles reflect two major stages of collision and uplift in the evolution of the Qinling orogenic belt during the Proterozoic and Caledonian–Hercynian periods, respectively.     

Two-stage decompression in orthopyroxene–sillimanite granulites from Forefinger Point, Enderby Land, Antarctica: implications for the evolution of the Archaean Napier Complex

Magnesian metapelites of probable Archaean age from Forefinger Point, SW Enderby Land, East Antarctica, contain very-high-temperature granulite facies mineral assemblages, which include orthopyroxene (8–9.5 wt% Al₂O₃)–sillimanite ± garnet ± quartz ± K-feldspar, that formed at 10 ± 1.5 kbar and 950 ± 50°C. These assemblages are overprinted by symplectite and corona reaction textures involving sapphirine, orthopyroxene (6–7 wt% Al₂O₃), cordierite and sometimes spinel at the expense of porphyroblastic garnet or earlier orthopyroxene–sillimanite. These textures mainly pre-date the development of coarse biotite at the expense of initial mesoperthite, and the subsequent formation of orthopyroxene (4–6 wt% Al₂O₃)–cordierite–plagioclase rinds on late biotite.
The early reaction textures indicate a period of near-isothermal decompression at temperatures above 900°C. Decompression from 10 ± 1.5 kbar to 7–8 kbar was succeeded by biotite formation at significantly lower temperatures (800–850°C) and further decompression to 4.5 ± 1 kbar at 700–800°C.
The later parts of this P–T evolution can be ascribed to the overprinting and reworking of the Forefinger Point granulites by the Late-Proterozoic ( c . 1000 Ma) Rayner Complex metamorphism, but the age and timing of the early high-temperature decompression is not known. It is speculated that this initial decompression is of Archaean age and therefore records thinning of the crust of the Napier Complex following crustal thickening by tectonic or magmatic mechanisms and preceding the generally wellpreserved post-deformational near-isobaric cooling history of this terrain.     

Carbonation of Cl-rich scapolite boudins in Skallen, East Antarctica: evidence for changing fluid condition in the continental crust

Spectacular reaction textures in poikiloblastic scapolitite boudins, within marbles in the continental crust exposed in the Lützow–Holm Complex, East Antarctica, provide insights into the changing fluid composition and movement of fluid along grain boundaries and fractures. Petrographic and geochemical features indicate scapolite formation under contrasting fluid compositions. Core composition of scapolite poikiloblasts (Scp_I) are marialitic (Cl = 0.7 apfu) whereas rims in contact with biotite or clinopyroxene are meionite rich. Fine‐grained recrystallized equigranular scapolite (Scp_II) shows prominent chemical zoning, with a marialitic core and a meionitic rim (Cl = 0.36 apfu). Scapolite poikiloblasts are traversed by Scp_III reaction zones along fractures with compositional gradients. Pure CO₂ fluid inclusions are observed in healed fractures in scapolite poikiloblasts. These negative crystal‐shaped fluid inclusions are moderately dense, and are believed to be coeval with Scp_III formation at temperatures >600 °C and a minimum pressure of c. 3.8 kbar. Grain‐scale LA‐ICPMS studies on trace and rare earth elements on different textural types of scaplolites and a traverse through scapolite reaction zone with compositional gradient suggest a multistage fluid evolution history. Scp_I developed in the presence of an internally buffered, brine‐rich fluid derived probably from an evaporite source during prograde to peak metamorphism. Recrystallization and grain size reduction occurred in the presence of an externally sourced carbonate (CaCO₃)‐bearing fluid, resulting in the leaching of Cl, K, Rb and Ba from Scp_I along fractures and grain boundaries. Movement of fluids was enhanced by micro‐fracturing during the transformation of Scp_I to Scp_III. Fractures in fluorapatite are altered to chlorapatite proving evidence for the pathways of escaping Cl‐bearing fluids released from Scp_I. The present study thus provides evidence for the usefulness of scapolite in fingerprinting changing volatile composition and trace element contents of fluids that percolate within the continental crust.     

Metamorphic evolution of calcsilicate granulites near Battye Glacier, northern Prince Charles Mountains, East Antarctica

Calcsilicate granulites of probable Middle Proterozoic age ( c .1000–1100  Ma) in the vicinity of Battye Glacier, northern Prince Charles Mountains, East Antarctica, contain prograde metamorphic assemblages comprising various combinations of wollastonite, scapolite, clinopyroxene, An-rich plagioclase, calcite, quartz, titanite and, rarely, orthoclase, ilmenite, phlogopite and graphite. Comparison of the prograde assemblages with calculated and experimentally determined phase relations in the simple CaO–Al₂O₃–SiO₂–CO₂–H₂O system suggests peak metamorphism at ≥835 °C in the presence (in wollastonite-bearing assemblages at least) of a CO₂-bearing fluid ( X _CO≥0.3) at a probable pressure of 6–7  kbar.
Well-preserved retrograde reaction textures represent: (1) breakdown of scapolite to anorthite+calcite±quartz; (2) formation of grossular–andradite garnet and, locally, (3) epidote, both principally by reactions involving scapolite breakdown products and clinopyroxene; (4) local coupled replacement of clinopyroxene and ilmenite by hornblende and titanite, respectively; and finally (5) local sericitization of prograde and retrograde plagioclase. These retrograde reactions are interpreted to be the result of cooling and variable infiltration by H₂O-rich fluids, possibly derived from crystallizing pegmatitic intrusions and segregations that may be partial melts, which are common throughout the area.     

Relict sapphirine + kyanite and spinel + kyanite associations in pyropic garnet from the eastern Sør Rondane Mountains,East Antarctica

Sapphirine, spinel, orthoamphibole ± quartz and kyanite are included in porphyroblastic garnet in biotitic gneiss enclosed in a lens of metamorphosed ultramafic rocks in the Cambrian granulite-facies metamorphic complex of the eastern Sør Rondane Mountains, Queen Maud Land, East Antarctica. A bulk analysis of the biotitic gneiss reveals features characteristic both of ultramafic rocks, e.g., high contents of Cr and Ni, and of metasomatism associated with fluids having a crustal source, e.g., relatively elevated contents of Li, Rb, Mo, Cs, Ba, Tl, and Pb. This trace element signature is consistent with the biotitic gneiss being a slice of blackwall skarn that developed between harzburgite and the enclosing biotite–hornblende ± garnet ± orthopyroxene ± clinopyroxene gneiss and was subsequently infolded or inserted by faulting. The matrix assemblage of the biotitic gneiss is garnet + corundum + hercynite + biotite +plagioclase +allanite + zircon. The included associations (all with biotite and rutile) are (1) sapphirine + kyanite, (2) spinel + kyanite, (3) sapphirine +spinel, (4) kyanite, and (5) orthoamphibole + plagioclase ± quartz. The garnet porphyroblasts are compositionally zoned with broad pyropic cores (X_Mg(=Mg / (total Fe + Mg)) = 0.45–0.55) surrounded by Fe-richer rims (X_Mg ≈ 0.3 at the outermost part). The garnet cores preserve compositions homogenized under peak conditions of the granulite-facies metamorphism (760–800 °C and 7–8 kb), whereas the Fe-enriched rims are attributed to an amphibolite-facies overprint at 500–600 °C. Theoretical calculations of garnet + corundum + spinel ± sapphirine + kyanite equilibria in the FMAS system constrain possible P–T conditions for a sapphirine + spinel + kyanite + garnet (X_Mg ≈ 0.55) assemblage to form near 450 °C and 4 kb on the prograde path. In contrast, a modified calibration of the Das et al. (Das, K., Fujino, K., Tomioka, N., Miura, H., 2006. Experimental data on Fe and Mg partitioning between coexisting sapphirine and spinel: an empirical geothermometer and its application. Eur. J. Mineral., 18, 49–58). sapphirine–spinel thermometer gives 860–895 °C for the included associations; pressures would have to be at least 12 kb to stabilize kyanite at these temperatures. Neither estimate is satisfactory and the stability range of kyanite + spinel–hercynite ± sapphirine assemblages remains an unresolved question. The Sør Rondane Mountains constitute the third region for kyanite, sapphirine and spinel–hercynite inclusions in garnet in granulite-facies rocks of the Neoproterozoic–Cambrian orogen extending from the Sør Rondane Mountains to Lützow–Holm Bay and onward to Sri Lanka, southern India and southern Madagascar, and thus determining the stability range of kyanite + spinel–hercynite ± sapphirine is critical for deducing the tectonic evolution of this orogen.