东昆仑石榴石闪长玢岩的形成机制及其对埃达克岩成因的制约     

袁超  孙敏  肖文交  刘小汉  李继亮  龙晓平  叶凯 《地质科学》2010,45(1):122-145

本文报道在东昆仑地区发现的一种较为稀少的含石榴石英云闪长玢岩,这也是此类岩石在中国的首次发现。该次火山岩形成于晚三叠纪,主要由富钙(CaO5 wt%)、贫锰(MnO3 wt%)的石榴石,富铝的角闪石(15.9 wt%),中性斜长石和石英等斑晶以及基质物质组成。岩石含有中等的SiO_2(61.1～62.2wt%),低的MgO(2.0 wt%),K_2O(1.3 wt%)以及较高的Al_2O_3(17 wt%)含量,呈现出次铝质至轻微过铝质的特征(ACNK=0.89～1.05)。在微量元素方面,该岩石富集大离子亲石元素和轻稀土元素,同时亏损Nb-Ta-Ti,显示出典型的消减带特征。而异常低的重稀土(Yb0.8×10~(-6))和相对高的Sr/Y比值(约38)表明石榴石是一个残留相,而较高的Al_2O_3含量,大多为正的铕异常反映了斜长石因结晶受到抑制而在岩浆演化晚期的聚集,同时表明岩浆具有较高的水含量。Nd-Sr同位素组成(ε_(Nd)(t)=-2.33～-1.38;~(87)Sr/~(86)Sr=0.706 5～0.706 7)和斜长石的反向分带显示,壳幔间岩浆混合作用在岩体的形成过程中扮演了重要角色。石榴石斑晶和其中的钛铁矿包体均含有较低的MgO,且包裹石榴石的角闪石形成于较高的压力(8～10 kb)条件下,显示这些矿物结晶自一个长英质岩浆中,且很可能形成于壳幔过渡带附近。尽管该岩石在富铝、低重稀土、高Sr/Y等很多方面均类似于埃达克岩石,但其中等含量的Sr(260×10~(-6))和La/Yb比值(16～21)却明显低于典型的埃达克岩和太古代的TTG。结合石榴石斑晶中广泛存在的磷灰石包体以及其较高的Sr和轻稀土分配系数,本文提出在岩浆演化早期结晶的磷灰石有效地降低了残余熔体中的Sr和轻稀土。这进一步表明,即使在高水逸度的条件下,磷灰石在岩浆早期的大量结晶可以有效地阻止一些弧岩浆演化成为埃达克质岩石。  相似文献   

Polymetamorphic evolution of pelitic schists and evidence for Permian low-pressure metamorphism in the Vepor Unit, West Carpathians     

P. JE&#;ÁBEK  M. JANÁK  S. W. FARYAD  F. FINGER  P. KONE&#;NÝ 《Journal of Metamorphic Geology》2008,26(4):465-485

Phase equilibrium modelling and monazite microprobe dating were used to characterize the polymetamorphic evolution of metapelites from the northern part of the Vepor Unit, West Carpathians. Three generations of garnet and associated metamorphic assemblages found in these rocks correspond to three distinct metamorphic events related to the Variscan orogeny, a Permian phase of crustal extension and the Alpine orogeny. Variscan staurolite‐bearing and Alpine chloritoid‐bearing assemblages record medium‐temperature and medium‐pressure regional metamorphisms reaching 540–570 °C/5–7.5 kbar and 530–550 °C/5–6.5 kbar respectively. The Permian metamorphic assemblage involves garnet, andalusite, sillimanite, biotite, muscovite, plagioclase and corundum and locally forms silica‐undersaturated andalusite‐biotite‐spinel coronas around older staurolite. The transition from andalusite to sillimanite indicates a prograde low‐pressure and medium‐temperature metamorphism characterized by temperature increase from 500 to 650 °C at ～3 kbar. As accessory monazite is abundant in the rocks, an attempt was made to derive its age of formation by means of electron microprobe‐based Th‐U‐Pb chemical dating. Despite the polymetamorphic nature of the metapelites, the monazite yielded uniform Permian ages. Microstructures confirm that monazite was formed in relation to the low‐pressure and medium‐temperature paragenesis, and the weighted average ages obtained for two different samples are 278 ± 5 and 275 ± 12 Ma respectively. The virtual lack of Variscan and Alpine monazite populations points to interesting aspects concerning the growth systematics of monazite in metamorphic rocks.  相似文献   

Eclogites with a short-lived granulite facies overprint in the Moldanubian Zone,Czech Republic: petrology,geochemistry and diffusion modelling of garnet zoning   总被引：6，自引：1，他引：6  

Patrick J. O'Brien  Stranislav Vrána 《International Journal of Earth Sciences》1995,84(3):473-488

The petrology and geochemistry of a newly discovered suite of high-pressure garnet + clinopyroxene-bearing rocks from the Monotonous Series of the Moldanubian Zone of the Bohemian Massif, southwest Czech Republic have been investigated. Three types [common eclogites, quartz ± kyanite ± (clino)zoisite eclogites and garnet-hornblende-clinopyroxenites] are distinguished by petrography and geochemistry. All underwent a significant degree of partial breakdown under granulite and amphibolite facies conditions during exhumation. Important features include the growth of orthopyroxene in breakdown domains after garnet and omphacite and anorthite + spinel ± corundum ± exceedingly peraluminous sapphirine replacing kyanite. Garnet zoning and inclusion patterns support a prograde evolution from low pressures for at least some of the samples. The post-eclogite stage granulite facies overprint indicates that high temperatures prevailed during exhumation, but preservation of zoning in some garnets and the results of diffusion modelling suggest that this overprint took place over a very short time-scale. The geochemical and petrological results allow characteristic differences to be recognized between these eclogites and metabasites found in other tectonic units of South Bohemia and consequently the assigning of all high-pressure rocks to a single, now disrupted, tectonic unit is a gross simplification that seriously misrepresents the tectono-metamorphic history of the region.  相似文献   

石榴子石的高温塑性：地幔转换带的流变特性   总被引：1，自引：1，他引：1  

王子潮 《地球物理学进展》1994,9(2):17-32

介绍了近年来国际地球物理学界在石榴子石相矿物高温塑性实验研究领域取得的进展和它的意义。这些实验研究进展表明地幔转换带内的富石榴子石岩层的流变强度要大大高于相邻深度的其它矿物相，从而导致地幔转换带流变非均匀性。这种由于矿物组分差异产生的地幔流变性非均匀性对地幔转换带地球动力学特征有着十分重大的影响。首先，石榴子石相矿物在很大程度上决定了那里的强度结构，并控制了地幔转换带的流变性。第二，中、深源地震的成因在很大程度上与石榴子石相矿物流变特性有关，第三，地幔转换带流变的非均匀性将影响到俯冲板块的下插深度和消减过程，最后，对这一课题的未来研究方向和问题作了简单的介绍。  相似文献   

云南个旧打磨山钙质夕卡岩及石榴石的稀土元素地球化学特征   总被引：4，自引：3，他引：4  

韩松  黄忠祥 《岩石学报》1993,9(2):192-198

  相似文献   

Crystal chemistry of ferric iron in (Mg,Fe)(Si,Al)O<Subscript>3</Subscript> majorite with implications for the transition zone     

C. A. McCammon  N. L. Ross 《Physics and Chemistry of Minerals》2003,30(4):206-216

Fifteen samples of (Mg,Fe)SiO₃ majorite with varying Fe/Mg composition and one sample of (Mg,Fe)(Si,Al)O₃ majorite were synthesized at high pressure and temperature under different conditions of oxygen fugacity using a multianvil press, and examined ex situ using X-ray diffraction and Mössbauer and optical absorption spectroscopy. The relative concentration of Fe³⁺ increases both with total iron content and increasing oxygen fugacity, but not with Al concentration. Optical absorption spectra indicate the presence of Fe²⁺–Fe³⁺ charge transfer, where band intensity increases with increasing Fe³⁺ concentration. Mössbauer data were used in conjunction with electron microprobe analyses to determine the site distribution of all cations. Both Al and Fe³⁺ substitute on the octahedral site, and charge balance occurs through the removal of Si. The degree of Mg/Si ordering on the octahedral sites in (Mg,Fe)SiO₃ majorite, which affects both the c/a ratio and the unit cell volume, is influenced by the thermal history of the sample. The Fe³⁺ concentration of (Mg,Fe)(Si,Al)O₃ majorite in the mantle will reflect prevailing redox conditions, which are believed to be relatively reducing in the transition zone. Exchange of material across the transition boundary to (Mg,Fe) (Si,Al)O₃ perovskite would then require a mechanism to oxidize sufficient iron to satisfy crystal-chemical requirements of the lower-mantle perovskite phase.  相似文献   

黄镇低温榴辉岩中石榴石成分分带的扩散动力学研究     

程昊  陈道公  周祖翼 《地球科学进展》2005,20(4):421-426

对南大别黄镇低温榴辉岩中不同粒径的石榴石颗粒所具有的不同化学成分分带进行研究，通过建立相应的扩散调整模型，运用扩散动力学的方法进行了模拟计算。结果表明该榴辉岩中石榴石经过峰期榴辉岩相变质之后，其成分环带经约23 Ma的扩散调整形成现在的组分特征。黄镇榴辉岩在峰期变质之后曾经历的是一个约6℃/Ma冷却过程，该过程持续了约23 Ma。黄镇榴辉岩不大可能与南大别超高压榴辉岩一样受到过超高压变质作用。  相似文献   

SIMS analyses of oxygen isotopes: Matrix effects in Fe-Mg-Ca garnets     

Daniel Vielzeuf  Michel Champenois  Fabrice Brunet 《Chemical Geology》2005,223(4):208-226

Large instrumental mass fractionation (IMF) may occur during measurements of oxygen isotope ratios by SIMS. Part of this fractionation depends on crystal structure and mineral composition. In order to improve the accuracy of SIMS measurements, we gathered 6 commonly used garnet standards and prepared 6 others to adequately cover the composition range Alm_0-73, Prp_0-99, Grs_0-20. Electron microprobe analyses were performed at UBP-Clermont to check the chemical homogeneity of these standards. Oxygen isotope compositions were determined by laser fluorination and mass spectrometry at UW-Madison. Ten SIMS sessions and 336 δ¹⁸O measurements at CRPG-Nancy, on a Cameca IMS1270 instrument, demonstrate that the standards are homogeneous with external reproducibility of 0.3‰ (1σ). In terms of δ¹⁸O, SIMS measurements indicate that, during a single session, IMF can vary up to 6.3‰ from one garnet standard to another. In most of the sessions, IMF can be correlated with the grossular content. However, for a satisfactory correction scheme, we suggest the combination of the 3 main components (Ca, Fe, Mg). This is done using a simple least square calculation routine. The correction coefficients determined for each session can be used to calculate the IMF and correct the measured isotopic ratio of a garnet of known chemical composition. This way, we were able to reproduce the δ¹⁸O values of most of the Fe-Mg-Ca garnet standards within ± 0.6‰. Interestingly, the use of only 3 end-member standards (AlmCMG, PrpMM, GrsSE) plus a standard of intermediate composition (e.g. UWG-2) is sufficient to reproduce δ¹⁸O within the same precision. Thus, linear interpolation among end-member standards is satisfactory in the case of the garnet solid-solutions. Two studies carried out on zoned garnets from the Alps and the Pyrenees indicate that matrix effects become significant when variations in grossular contents are important (> 10%). In order to obtain reliable isotope ratio measurements on Fe-Mg-Ca garnets using a SIMS, we suggest a correction scheme using at least 3 reliable end-member standards plus a standard of intermediate composition (a garnet standard closest to the average composition of the analysed garnet). This allows cross-checking and incorporates a correction based on the variations in composition of zoned crystals.  相似文献   

Heterogeneity of water in garnets from UHP eclogites, eastern Dabieshan, China     

Qun-Ke Xia  Ying-Ming Sheng  Xiao-Zhi Yang  Hui-Ming Yu 《Chemical Geology》2005,224(4):237-246

Garnets in UHP eclogites from Bixiling in Dabieshan were investigated by Fourier transform infrared spectroscopy (FTIR). The results indicate that all garnets contain structural water that occurs as hydroxyl (OH⁻) and non-structural molecular water (H₂O) possibly in the form of sub-microscopic fluid inclusions. The structural hydroxyl contents range from 92 to 1735 ppm (H₂O wt.) and most are between 200 and 1000 ppm. Therefore, garnet in eclogite can recycle surface water into the mantle. Various water contents were observed among different samples of the same outcrop (∼150 m) and in different domains of the same sample (∼1 cm). This variability in structural H₂O contents suggests that the mobility of fluids during UHP metamorphism was very limited, and that both subduction and exhumation processes of UHP rocks occurred in a short time interval.  相似文献   

Trace element fractionation during high-grade metamorphism and crustal melting—constraints from ion microprobe data of metapelitic, migmatitic and igneous garnets and implications for Sm–Nd garnet chronology     

S. Jung  E. Hellebrand 《Lithos》2006,87(3-4):193-213

Rare earth element (REE) and other trace element (Y, Sr, Ti, Cr, V, Na) abundances in garnet from a garnet-bearing metapelite, a pelitic migmatite, a syn-tectonic granite and a post-tectonic leucogranite were measured by secondary ion mass spectrometry (SIMS) in order to identify the effective variables on the trace element distribution between garnet and the host rock. Garnet from the garnet-bearing metapelite, the pelitic migmatite and the syn-tectonic granite is zoned with respect to REE. The cores are enriched by a factor of 2–3 relative to the rims. For the garnets from the garnet-bearing metapelite equilibrium distribution following a simple Rayleigh fractionation is responsible for the decreasing concentrations in REE from core to rim. Garnet from the pelitic migmatite shows a more complex trace element pattern following distinct enrichment and depletion patterns for Ti, V, Cr and REE from core to rim. These features suggest disequilibrium between garnet and the associated melt in which the enrichment of trace elements probably correspond to a period of open-system behaviour in these rocks at a time when the garnet, originally nucleated in the metamorphic environment was incorporated into the melt. The garnet from the syn-tectonic granite shows stepwise decreasing concentrations in REE from core to rim: a REE-rich core can be distinguished from a broad REE-depleted rim. Notably, from core to rim an inflection of the Yb / Er and Yb / Dy ratios is visible. Whereas the decrease of HREE abundance in the core region of the garnet from the syn-tectonic granite may arise from equilibrium partitioning during garnet growth, the inflection can be interpreted as a result of partial melting. Garnet cores with high Yb / Er and Yb / Dy >  1 nucleated in the metamorphic environment without the presence of a melt whereas the rims with lower Yb / Er and Yb / Dy <  1 crystallized in the presence of a melt. Garnet from the leucogranite has lower REE abundances and is considered to be of igneous origin. In contrast to garnet from the other samples, its core has low trace element abundances, whereas its rim is significantly enriched in REE but depleted in Ti. These features suggest that only the outermost rim was in equilibrium with the melt. For this garnet, liquid diffusion controlled partitioning is more likely to explain the extreme trace element variation. An evaluation of Sm and Nd concentrations in garnet and a comparison of Sm–Nd and U–Pb garnet ages and U–Pb monazite ages form the terrane indicate that the observed LREE systematics in the different garnet species are a primary feature and are not homogenized by volume diffusion during high grade amphibolite facies conditions.  相似文献