Oxygen isotope geothermometers for metamorphic rocks

The Chicago mineral-carbonate oxygen isotope fractionation curves have been combined with mineral-water fractionation data for jadeite, zoisite and rutile and new data for grossular-water to provide a set of self-consistent mineral-pair calibrations. The A coefficients in the equation 1000 In α= A × 10⁶T^-2 of the new mineral-pair fractionations are
Jadeite Zoisite Grossular Rutile
Quartz 1.69 2.00 3.03 5.02
Jadeite 0.31 1.34 3.33
Zoisite 1.03 3.02
Grossular 1.99
The isotopic fractionation properties of natural pyralspite garnet [(Ca, Fe, Mg, Mn)₃Al₂Si₃O₁₂] can be approximated by those of the grossular end-member. Appropriate substitutions also yield coefficients for the solid-solution minerals: sodic pyroxene and epidote, e.g.
A _{quartz-sodic pyroxene}= 2.75 - 1.06X_jd,
A _{quartz-epidote}= 2.00 + 0.75X_ps
where X _Jd and X _Ps are the mole fractions of the jadeite and pistacite components, respectively.
The new data set is particularly suitable for the geothermometry of metamorphic rocks. δ¹⁸O data from minerals of the high-pressure metamorphic rocks of the Sesia Zone of Italy and Cyclades Complex of Greece yield well-constrained mean temperatures of 572 and 478 C, respectively. Type III blueschist metabasalts of the Franciscan Formation of California give mean quartz-garnet temperatures of 354 C.     

大别山双河地区超高压变质岩矿物超微构造的HRTEM研究

报道了大别山双河地区超高压榴辉岩和硬玉石英岩矿物超微构造及缺陷结构的高分辨透射电镜研究结果。在天然变形绿辉石晶内的自由位错、位错倾斜壁、位错环、位错网、亚晶界、堆垛层错等亚构造中 ,发现了纳米级水分子团 ,这种球形状水分子团包体的存在是导致绿辉石晶体水解弱化和塑性变形的重要因素。在榴辉岩矿物中广泛发育的层错、(10 0 )变形双晶、晶畴结构、界面与晶面的交叉滑移、晶格畸变等变形构造及缺陷结构 ,指示超高压岩石经历了快速折返。在硬玉单晶大约 5 0 0nm的微晶畴内 ,发现了C2 /c和P2 1/n两种结构 ,C2 /c结构的晶体学参数对应于硬玉 ,而P2 1/n结构的晶体学参数对应于绿辉石 ,纳米级P2 1/n出溶结构的存在 ,表明在退变质过程中 ,硬玉在纳米尺度上部分转变为绿辉石 ,并且未能达到平衡。也说明在主体岩石的抬升过程中 ,硬玉晶体伴随有复杂的非平衡退变质作用。对于大别山超高压变质岩的p T轨迹描述及其构造解释具有重要意义。     

大别山石马地区榴辉岩P-T-t轨迹及其构造意义

在大别山石马地区榴辉岩石榴石中包裹有与柯石英同期的绿辉石,据此,将这里的榴辉岩相变质作用划分为早期的柯石英榴辉岩阶段和晚期重结晶榴辉岩阶段。它们的P-T-t轨迹表明:产于片麻岩中的榴辉岩至少经历了两阶段的抬升过程:(1)在221Ma(印支期)时,由于陆壳俯冲所产生的大规模逆冲构造使其快速抬升;(2)在134Ma(燕山期)陆壳俯冲后的拉张抬升环境下,榴辉岩随山体一起缓慢上升。     

海南洋壳型高温榴辉岩: 基本特征及待解问题

榴辉岩常产于汇聚板块的边界,是鉴别古板块缝合带的重要标志之一。最近,这种特征的变质岩石在海南岛北部被第四系严重覆盖的木栏头地区被发现,其基本特征总结如下：（1）榴辉岩孤立地出露于潮滩鼻潮间-潮下带,主体露岩区域分布的总面积约1.8km²,由片麻理构成的优势构造走向为北东至近东西;（2）榴辉岩经历了顺时针变质演化,从绿帘角闪岩相（620~680℃、0.87~1.11GPa）、榴辉岩相/榴辉岩-高压麻粒岩过渡相（820~860℃、1.70~1.82GPa）、角闪岩相（700~730℃、0.71~0.85GPa）到绿片岩相;（3）榴辉岩的主体（占分析样品总数的65%）具有正常洋中脊玄武岩（N-MORB）属性,少数具有富集洋中脊玄武岩（E-MORB）和火山弧玄武岩（VAB）属性,初始Sr-Nd同位素成分表明他们来自于亏损的软流圈地幔;（4）榴辉岩原岩形成于355Ma之前,进变质和峰期-退变质的时代分别约为340~330Ma和310~300Ma,冷却至金红石U-Pb体系封闭温度的时代为292±6Ma。所以,海南榴辉岩主要是大洋（少数岛弧）玄武岩在石炭纪经高温高压变质作用的产物。由这种特殊的洋壳型高温榴辉岩自身及其引申出的科学问题包括：（1）海南榴辉岩是单体榴辉岩还是榴辉岩集合体?如果是单体,那将是国内出露规模最大的榴辉岩体;（2）海南榴辉岩到底是榴辉岩相岩石还是榴辉岩-高压麻粒岩过渡相岩石?榴辉岩相峰期变质的压力到底有多高?遍布的深熔作用是发生在温压峰期还是在减压过程中的温度峰期?（3）原岩形成于洋盆还是弧后盆地?其与同时代的金沙江-哀牢山-马江洋和邦溪-晨星弧后盆地具有怎样的联系?（4）榴辉岩起因于大洋热俯冲/增生还是大陆俯冲/碰撞环境?以其为代表的古板块缝合带（可称木栏头或潮滩鼻缝合带）向哪里延伸?（5）东、西古特提斯构造域的早期演化有无相似之处?海南陆块或其北部或西部地体是否在石炭纪就已与华南陆块碰撞对接在一起?显然,海南榴辉岩对重构全球古特提斯构造带的早期演化具有重要的科学意义,值得进一步深入研究。

     

大别山朱家冲高压榴辉岩变质演化特征分析

研究表明大别山朱家冲榴辉岩属于典型的高压榴辉岩,其具有复杂的变质演化历史。根据岩相学、矿物成份化学和热力学分析表明,该类榴辉岩记录了6期变质过程,具有“顺时针”的P?T演化轨迹。由Ⅰ?阶段、Ⅱ?阶段至Ⅲ?阶段显示了近于等温增压进变质特征,并在 Ⅲ?阶段压力值最高达到P＝2.53 GPa; Ⅳ?阶段和Ⅴ?阶段则表现为增温降压退变过程,温度在 Ⅴ?阶段最高达到T＝627 ℃; 到Ⅵ?阶段则为降温降压过程。该过程暗示了高压板片在俯冲和折返过程中可能处于一个非匀速的状态。推测朱家冲榴辉岩后期增温退变的过程可能是源于受扰动的地温线恢复、超高压榴辉岩退变过程的“散热”以及该类榴辉岩对温度变化极为敏感所致。     

大别山桐城地区雷庄低温高压榴辉岩的增温退变P-T轨迹及其构造含义

早期的研究显示大别山东段(安徽部分)出露的榴辉岩均为中温榴辉岩,并没有其他类型榴辉岩存在.本研究通过对桐城地区雷庄附近的榴辉岩野外地质调查、岩相学、矿物成分化学以及热力学分析表明,该地区榴辉岩为低温高压榴辉岩.榴辉岩的P-T演化特征研究区分了5个变质阶段,展示了一个"发卡式"增温退变轨迹.Ⅰ-阶段的变质温压为T=390～411℃,P=0.89～1.40GPa;Ⅱ-阶段为T=527～544℃,P=1.95～2.34GPa.该阶段为峰期变质,代表了板片俯冲最大深度;Ⅲ-阶段为T=501～519℃,P=1.74～2.14GPa;Ⅳ-阶段为T=630和P=1.22GPa;Ⅴ-阶段为T=350℃,P=0.20GPa.由Ⅲ-阶段至Ⅳ-阶段随着压力的降低,温度明显增加,反映了一个退变增温的演化趋势.这种增温可能是由于低温榴辉岩对温度变化的敏感,以及榴辉岩板片在折返至地壳某一深度停滞时间较长,北大别热穹隆和地温梯度的恢复是导致增温的热源.     

Relics of eclogite facies assemblages in the Ceará Central Domain, NW Borborema Province, NE Brazil: Implications for the assembly of West Gondwana

The Borborema Province, in the NE of Brazil, is a rather complex piece in the Brazil–Africa puzzle as it represents the junction of the Dahomeyide/Pharusian, Central African, Araçuai and Brasilia fold belts located between the West-African/São Luis, Congo/São Francisco and Amazonas craton. The correlation between the Dahomeyides from W-Africa (Ghana, Benin, Togo, and Mali) and the Borborema Province involves the Médio Coreaú and Central Ceará domains. The inferred continuation of the main oceanic suture zone exposed in the Dahomeyides of W Africa is buried beneath the Phanerozoic Parnaíba Basin in Brazil (northwest of the Médio Coreaú domain) where some high density gravity anomalies may represent hidden remnants of an oceanic suture. In addition to this major suture a narrow, nearly continuous strip composed of mainly mafic pods containing relics of eclogite-facies assemblages associated with partially migmatized granulite-facies metapelitic gneisses has been found further east in the NW Borborema Province. These high pressure mafic rocks, interpreted as retrograded eclogites, are located between the Transbrasiliano Lineament and the Santa Quitéria continental arc and comprise primitive to evolved arc-related rocks with either arc- or MORB-type imprints that can indicate either deep subduction of oceanic lithosphere or roots of continental and oceanic magmatic arcs. Average peak P–T conditions under eclogite-facies metamorphism (T = 770 °C and P = 17.3 kbar) were estimated using garnet–clinopyroxene thermometry and Jd content in clinopyroxene. Transition to granulite-facies conditions, as well as later widespread re-equilibration under amphibolite facies, were registered both in the basic and the metapelitic rocks and suggest a clockwise P–T path characterized by an increase in temperature followed by strong decompression. A phenomenon possibly related to the exhumation of a highly thickened crust associated with the suturing of the Médio Coreaú and Central Ceará domains, two distinct crustal blocks separated by the Transbrasiliano Lineament.     

大别山榴辉岩（牌楼）P－T－t轨迹

本文重点研究了大别山潜山，牌楼地区榴辉岩Ｐ－Ｔ－ｔ轨迹，探讨了大别山地区榴辉岩成因，认为榴辉岩原岩主要是碱性玄武岩，少部分洋脊拉斑玄武岩岩浆早期结晶分异堆晶岩及部分钙质沉积岩。它们在华北与杨子板块碰撞时（２６０±３０Ｍａ），地壳向下俯冲到４０－６０ｋｍ处，温度５５０°－８３０°Ｃ，压力１．１－２．１ＧＰａ的条件下形成的。在构造抬升过程中，榴辉岩产生了角闪岩相、绿片岩相退变质作用，角闪岩相温压条件：４６０°－５７０℃，０．４－０．７ＧＰａ。流体包裹体研究表明：流体包体成分主要为ＮａＣｌ－Ｈ＿２Ｏ，Ｈ＿２Ｏ，ＮａＣｌ－ＣＯ＿２－Ｈ＿２Ｏ体系，变质作用过程从早至晚由氧化向还原环境转化。熔融包体的发现表明：榴辉岩形成具有局部深熔作用。从榴辉岩产状、岩石化学、地球化学特征表明本地区榴辉岩不是一种成因，同时具有Ｂ类、Ｃ类榴辉岩特征。     

Interdependence of deformation, fluid infiltration and reaction progress recorded in eclogitic metagranitoids (Sesia Zone, Western Alps)

The effects of high-strain deformation and fluid infiltration during Alpine eclogite facies metamorphism have been studied across ductile shear zones in relatively undeformed metagranitoids at Monte Mucrone (Sesia Zone, Western Alps, Italy). Microfabrics together with bulk rock and stable isotope data indicate that the mineralogical and chemical variations are related to the degree of deformation, rather than to changes in P-T conditions or tectonic position. Transformation of meta-quartz diorite to recrystallized eclogitic mylonites involved the breakdown of biotite and plagioclase and required the influx of H₂O. Bulk-rock geochemical data show that ductile deformation to form eclogitic mylonites involved an increase in volume with a weight percent gain in H₂O and Si and variable loss of K, Na, Ca and Al. δ¹⁸O changes systematically across ductile shear zones into the undeformed country rocks. Constant values in shear zone centres indicate advection parallel to the shear zone and within 10 cm of the mylonites. A dominant component of diffusive oxygen exchange perpendicular to the shear zones produced isotopic fronts, evident from a gradual increase in δ¹⁸O values to the reference values of the country rocks. The degree of isotopic shift within the shear zones reflects increasing deformation and degree of reaction progress. Multiple phases of Alpine deformation and mineral growth are recognized in the Monte Mucrone metagranitoids, and in some cases, eclogite facies shear zones were reactivated under greenschist facies conditions. The results of this study suggest that high-strain deformation provided pathways for both synkinematic and post-kinematic metamorphic fluids which were necessary for complete reactions. Relict igneous fabrics, as well as the presence of corona textures around biotite and pseudomorphs after primary igneous plagioclase in the least deformed rocks, indicate a paucity of hydrous fluids and support the conclusion that fluid movement was channelled rather than pervasive.     

Variation in peak P–T conditions across the upper contact of the UHP terrane,Dabieshan, China: gradational or abrupt?

The Southern Dabieshan Terrane (SDT) has previously been divided into high‐pressure (HP) and ultrahigh‐pressure (UHP) terranes, and its regional extent and the tectonic nature of its boundaries are hotly debated topics. In this study, an eclogite‐bearing area of 100 km² near Taihu is mapped in detail, and divided into Northern, Middle and Southern Zones on the basis of lithological characteristics. The Northern Zone consists of epidote‐biotite gneiss and eclogite blocks, the Middle Zone includes granitic gneiss, biotite gneiss, eclogites and amphibolite, and the Southern Zone is composed mainly of garnet‐bearing mica schist. The eclogites occur mainly as lens or blocks in the Northern and Middle Zones. The peak P–T conditions for 61 eclogite samples across the area are estimated using the Grt‐Cpx Fe²⁺‐Mg thermometers and the Grt‐Cpx‐Phe barometers. The results indicate three different P–T regions: 2.82–4.09 GPa/759–942 °C in the Northern Zone, and 2.00–3.54 GPa/641–839 °C in the granitic gneiss and 1.38–2.36 GPa/535–768 °C in the biotite gneiss from the Middle Zone. Combined with the spatial distribution of eclogites across the area, the P–T values for eclogites increase continuously from the south to the north, defining a reference ‘geotherm’ of 5 °C km^?1. However, some unreasonable apparent gradients can be established along two south–north profiles across the area, and display a P–T difference between the Northern and Middle zones. On the basis of the average P–T data for eclogites across the area, a gap of at least 0.3 GPa/20 °C exists between the Northern and Middle zones. By contrast, the P–T values of eclogites from the Middle zone show a coherent pattern with transitional characteristics from HP in the south to UHP in the north. We suggest that the SDT was a coherent slab during subduction, and was broken up by a major fault during exhumation, which was formed under UHP metamorphic conditions.