川西沙坝麻粒岩的Sm-Nd和Rb-Sr同位素年龄及其地质意义

首次测定扬子板块西缘古老的结晶基底岩石沙坝麻粒岩Sm Nd矿物等时线年龄为 114 0± 110Ma ,代表中元古代末期麻粒岩相变质作用时代。Rb -Sr矿物等时线年龄为 2 18± 11Ma ,可能为晚古生代至早中生代基性岩浆侵入的热扰动改造年龄。其亏损地幔模式年龄TDM 为115 8Ma ,表明沙坝麻粒岩的原岩在成岩之后不久即经受麻粒岩相变质作用。根据上述年龄以及紫苏辉石中Nd同位素封闭温度和40 Ar/ 3 9Ar年龄等资料的计算表明 ,以沙坝麻粒岩为代表的下地壳岩石抬升运动速率是缓慢的     

Kyanite-paragonite-bearing assemblages, northern Fiordland, New Zealand: rapid cooling of the lower crustal root to a Cretaceous magmatic arc

Fiordland, New Zealand exposes the lower crustal root of an Early Cretaceous magmatic arc that now forms one of Earth's most extensive high‐P granulite facies belts. The Arthur River Complex, a dioritic to gabbroic suite in northern Fiordland, is part of the root of the arc, and records an Early Cretaceous history of emplacement, tectonic burial, and high‐P granulite facies metamorphism that accompanied partial melting of the crust. Late random intergrowths of kyanite, quartz and plagioclase partially pseudomorph minerals in the earlier high‐T assemblages of the Arthur River Complex, indicating high‐P cooling of an over thickened crustal root by c. 200 °C. The kyanite intergrowths are themselves partially pseudomorphed by paragonite, commonly in the presence of phengitic white mica. Biotite–plagioclase intergrowths that partially pseudomorph phengitic white mica and diopside–plagioclase intergrowths that partially pseudomorph jadeitic diopside, combined with published thermochronology results, are consistent with later rapid decompression. A short duration anticlockwise P–T path may be explained by the high‐P juxtaposition of comparatively cool upper crustal rocks following their tectonic burial and under thrusting during the waning stages of Early Cretaceous orogenesis. This was then followed by the decompression giving the rapid exhumation within 20 Myr of peak metamorphism, as suggested by the isotopic data.     

库鲁克塔格基性岩墙群K—Ar等时年龄测定及其有关问题讨论

新疆库鲁克塔格基性岩墙群的四个样品的Ｋ－Ａｒ表观年龄为４５５－６７３Ｍａ,等时年龄为２８７±１３Ｍａ。这一年龄与近几年报道的沿天山构造带的早二叠世火山岩年龄一致。氩初始值为５０８,在Ｐ型岩浆范围内,结合稀土、微量元素特征,认为岩墙群与地幔热柱有成因联系。     

Variscan terrane of deep-crustal granulite facies in Yushugou area, southern Tianshan

The Yushugou terrane of deepcrustal granulite facies in southern Tianshan consists of two parts, granulite and metaperidotite. The whole terrane is a metamorphism of (high-pressure) grunulite facies, and typical mineral associations are: Gt-Cpx-P1-Tit-Ilm (±Qz) (silica-saturated and oversaturated mafic rocks), Gt-Ky (pseudomorph)-P1-Ru-Ilm± Qz (metapelitic rocks) and Spi-Opx-Cpx-01 (meta-ultramafic rocks). The peak-stage P-T conditions are 795— 964°, 0.97—1.42 GPa, which are obtained with mineral chemistry, assemblage analyses and P-T estimation. The Sm-Nd isochron age of peak-stage metamorphic minerals is (315 ± 3.62) Ma. All of these indicate that the terrane is a deep-crustal body, which subducts to the depth of 40—50 km in the middle late-Paleozoic, undergoing metamorphism of (high-pressure) granulite facies, and exhumed again to the surface by tectonic uplifting. Project supported hy the National Natural Science Foundation of China (Grant No. 49472135).     

桑干地区高压麻粒岩中石榴石和斜长石的连续激光探针~(40)Ar~-~(39)Ar等时年龄及其地质意义

采用微区激光探针４０Ａｒ－３９Ａｒ定年方法, 对华北桑干地区高压基性麻粒岩中变质石榴石和斜长石直接进行了原位微区年代测定。石榴石变斑晶是高压麻粒岩相变质作用形成的矿物,石榴石周围后成合晶反应边组合中的斜长石是石榴石减压分解的产物。石榴石斑晶的４０Ａｒ－３９Ａｒ等时线年龄为２５１０ Ｍａ, 证明高压变质作用发生在太古宙末。斜长石４０Ａｒ－３９Ａｒ等时线年龄为１９６８Ｍａ, 代表石榴石在中压麻粒岩相条件下分解的时代。它们之间年龄相差大于５００Ｍａ, 说明高压麻粒岩可能没有经历近等温减压的ＰＴ轨迹。后成合晶组合很可能代表中压麻粒岩相变质作用的叠加。这一结果对探讨华北克拉通桑干地区早期地壳的构造演化具有重要意义。     

中国太古宙地质体组成、阶段划分和演化

我国太古宇陆壳主要见于华北克拉通区内，由高级变质的麻粒岩相带、角闪宕相区和低、中级变质的绿岩－花岗岩区组成。按地质时序可划分为始、古、中、新太古４个阶段。麻粒岩相带的组成主要由英云闪长质片麻岩为主和少量表壳岩或主要由表壳岩含一些花岗质岩石。绿岩－花岗岩区时代主要属新太古代，绿岩属基性火山－沉积建造，基性岩的原岩物质来自相对亏损的上地幔，形成于大陆边缘的裂谷环境。太古宙末，板块体制已经形成，麻粒岩相带是板块体制的俯冲碰撞机制的产物。对克拉通古陆块能长期成为大陆的解释，应认为是下地壳的挤压环境，促使陆壳以板底垫托叠置、垂直增生所致。     

Empirical calibration of the silica–Ca-tschermak's–anorthite (SCAn) geobarometer

The pressure-sensitive equilibrium among anorthite, quartz and the Ca-tschermak component in clinopyroxene (CaAl₂SiO₆; CaTs), CaAl₂SiOCpx6+SiOQtz2=CaAl₂Si₂OPl8 (SCAn) ,can be used as a geobarometer in granulites with the proper assemblage, and has been calibrated using mineral composition data from partial melting experiments of natural assemblages and from phase equilibrium experiments on the end-member CMAS system. The experimental data cover the P – T  range 4–32  kbar and 900–1400  °C. Linear least-squares regression analysis of the experimental data resulted in the following empirical expressions for pressure in terms of composition and temperature: P = 5.066 [±0.760]+ 1300 [±800] T  −ln K 276 [±16] · T  [±2.5  kbar]or P = 6.330 [±0.116]−ln K 301 [±9]· T  [±1.0  kbar] ,where K = a PlAn a CpxCaTs  .The first equation incorporates an enthalpy term, but is less accurate than the second equation, in which the enthalpy of reaction is ignored. Application of these expressions to natural and experimental equilibrium mineral assemblages demonstrates that the empirical barometers are applicable over a wide range of pressures (≥4  kbar), temperatures (≥700  °C) and bulk compositions (Mg#≥32.5).     

False metamorphic events inferred from misinterpretation of microstructural evidence and P–T data

Geometrical relationships involving inclusions and partial inclusions in metamorphic microstructures can be inadequate for inferring an order of crystallization and hence a metamorphic reaction. Unique spatial and/or chemical relationships need to be defined for mineral inclusions, in the context of a reference paragenesis, commonly the matrix assemblage. Corona microstructures are reliable indicators of metamorphic reactions, but require considerable care when used to infer reactions or changes in P–T conditions, owing to kinetic problems, as well as to changes in the effective reaction volume during changes across relatively broad P–T stability fields of assemblages. Mineral equilibria models, most commonly implemented through P–T pseudosections, may allow the order in which different minerals become stable along a given P–T path to be inferred. However, the order in which two minerals become stable may be different from the order in which two grains of these minerals nucleate. Furthermore, such diagrams cannot make predictions about which minerals will form porphyroblasts and which minerals will form inclusions in porphyroblasts. An evaluation of three examples from the Australian Proterozoic shows that modelling, in combination with inclusion‐host relationships, is a powerful tool for understanding the metamorphic evolution of a rock, but involves considerable uncertainty.     

Geochemical constraints of the eclogite and granulite facies metamorphism as recognized in the Raobazhai complex from North Dabie Shan,China

A combined study of major and trace elements, fluid inclusions and oxygen isotopes has been carried out on garnet pyroxenite from the Raobazhai complex in the North Dabie Terrane (NDT). Well‐preserved compositional zoning with Na decreasing and Ca and Mg increasing from the core to rim of pyroxene in the garnet pyroxenite indicates eclogite facies metamorphism at the peak metamorphic stage and subsequent granulite facies metamorphism during uplift. A P–T path with substantial heating (from c. 750 to 900 °C) after the maximum pressure reveals a different uplift history compared with most other eclogites in the South Dabie Terrane (SDT). Fluid inclusion data can be correlated with the metamorphic grade: the fluid regime during the peak metamorphism (eclogite facies) was dominated by N₂‐bearing NaCl‐rich solutions, whereas it changed into CO₂‐dominated fluids during the granulite facies retrograde metamorphism. At a late retrograde metamorphic stage, probably after amphibolite facies metamorphism, some external low‐salinity fluids were involved. In situ UV‐laser oxygen isotope analysis was undertaken on a 7 mm garnet, and impure pyroxene, amphibole and plagioclase. The nearly homogeneous oxygen isotopic composition (δ¹⁸O_VSMOW = c. 6.7‰) in the garnet porphyroblast indicates closed fluid system conditions during garnet growth. However, isotopic fractionations between retrograde phases (amphibole and plagioclase) and garnet show an oxygen isotopic disequilibrium, indicating retrograde fluid–rock interactions. Unusual MORB‐like rare earth element (REE) patterns for whole rock of the garnet pyroxenite contrast with most ultra‐high‐pressure (UHP) eclogites in the Dabie‐Sulu area. However, the age‐corrected initial ε_Nd(t) is ? 2.9, which indicates that the protolith of the garnet pyroxenite was derived from an enriched mantle rather than from a MORB source. Combined with the present data of oxygen isotopic compositions and the characteristic N₂ content in the fluid inclusions, we suggest that the protolith of the garnet pyroxenite from Raobazhai formed in an enriched mantle fragment, which has been exposed to the surface prior to the Triassic metamorphism.     

Whole-rock, Phosphate, and Silicate Compositional Trends across an Amphibolite- to Granulite-facies Transition, Tamil Nadu, India

Chemical trends from north (amphibolite facies) to south (granulitefacies) along a 95 km traverse in Tamil Nadu, Southern India,include: whole-rock depletion of Rb, Cs, Th and U, enrichmentin Ti and F, and depletion in Fe and Mn in biotite and amphibole;increases in Al and decreases in Mn in orthopyroxene; enrichmentof fluorapatite in F. K-feldspar blebs are widespread alongquartz–plagioclase grain boundaries, and could indicateeither partial melting or metasomatism. In the northernmostportion of the traverse the principal rare earth element (REE)-bearingminerals are allanite and titanite. South of a clinopyroxeneisograd, monazite grains independent of fluorapatite are themajor REE- and Th-bearing phase. Further south independent monaziteis rare but Th-free monazite inclusions are common in fluorapatite.During prograde metamorphism, independent monazite was replacedby REE-rich fluorapatite in which the monazite inclusions laterformed. The loss of independent monazite was accompanied bya loss of whole-rock Th and possibly a small depletion in lightREE. Most mineralogical features along the traverse can be accountedfor by progressive dehydration and oxidation reactions. Trace-elementdepletion is best explained by the action of an externally derivedlow H₂O activity brine migrating from a source at greater depth,possibly preceded or accompanied by partial melting. KEY WORDS: granulite facies; charnockite; metasomatism; Archean; Tamil Nadu, India; fluorapatite; monazite; allanite; titanite; biotite