Petrology and metamorphism of khondalites from the Jining complex,North China craton

The basement of the North China craton (NCC) can be divided into eastern and western blocks separating the Trans-North China orogen on the basis of petrologic associations, structures, metamorphic processes, and isotopic ages. Aluminous gneiss khondalites occur in the western block, and record a clockwise metamorphic P–T history characterized by nearly isothermal decompression following peak metamorphism at ca. 1.3 GPa and 825°C. Four metamorphic stages are recognized based on mineral assemblages. The early prograde metamorphic assemblage contains Ky+Bt+Ms+Grt+Pl+Qtz. The peak metamorphic mineral assemblage is characterized by Grt+Sil+Bt+Kfs+Pl+Qtz and the formation of cordierite after garnet, leading to a retrograde assemblage of Grt+Sil+Crd+Pl+Kfs+Qtz. Garnet retrogrades to biotite and the formation of pervasive matrix muscovite define a final metamorphic stage, inferred at ca. < 0.6 GPa and 700°C. Quantified metamorphic stages and a related clockwise P–T path derived from pseudosection analysis in the KMASH system suggest collision of the north Yinshan block with the South Ordos block at 1.92 Ga, before final suturing of the entire NCC basement.     

High-temperature 'clockwise'P-T paths and melting in the development of regional migmatites: an example from southern Brittany, France

The Southern Brittany Migmatite Belt (SBMB), which evolved through the metamorphic peak between c. 400 Ma and c. . 370 Ma ago, consists of a heterogeneous suite of high-grade gneisses and anatectic migmatites, both metatexites and diatexites. Rare garnet-cordierite gneiss layers record evidence of an early prograde P-T path. In these rocks, growth-zoned garnet cores and a sequence of included mineral assemblages in garnet, from core to rim, of Qtz + Ilm + Ky, Pl + Ky + St + Rt + Bt and Pl + Sil + St + Rt + Bt constrain a prograde evolution during which the reactions Ilm + Ky + Qtz→ Aim + Rt, Ms + Chl→ St + Bt + Qtz + V and St + Qtz→ Grt + Sil + V were crossed. Parts of this prograde evolution are preserved as inclusion assemblages in garnet in all other rock types. In all rock types, garnet has reverse zoned rims, and garnet replacement by cordierite and/or biotite and plagioclase suggests the following reactions have occurred: Grt + Sil + Qtz→ Crd → Hc ± Ilm, Bt + Sil + Qtz → Crd ± Hc → Ilm → Kfs + V and (Na + Ca + K + Ti) + Grt → Bt + Pl + Qtz. Microstructural analysis of reaction textures in conjunction with a petrogenetic grid has enabled the construction of a tightly constrained 'clockwise' P–T path for the SBMB. The high-temperature part of the path has a steep dT/dP slope characteristic of near isothermal decompression. It is proposed that the P-T path followed by the SBMB is the result of the inversion, by overthrusting, of a back-arc basin and that such a tectonic setting may be applicable to other high-temperature migmatite terranes. The near isothermal decompression is at least partly driven by the upward (diapiric) movement of the diatexite/anatectic granite core of the SBMB.     

青藏高原东南缘哀牢山构造带泥质高压麻粒岩的发现及其构造意义

哀牢山构造带泥质高压麻粒岩主要由石榴石、夕线石、钾长石和斜长石变斑晶及尖晶石、铁假蓝宝石、蓝晶石、石英、金红石和钛铁矿包裹体组成,为确定印支地块和华南地块的边界提供了关键性标志。石榴石-黑云母-斜长石-石英地质温压计(GBPQ)计算结果及标志性高温矿物组合(Spl+Qz)表明泥质高压麻粒岩的形成和演化经历了高压/高温进变质到中温/低压退变质的顺时针P-T演化过程。其中:1)高压/高温进变质阶段的矿物组合为Ky+Sil+Grt1+Kf1+Pl1+Spr+Ter(Kf+Pl)+Bt1+Spl+Qtz+Ilm1+Rut1,形成于850～919℃,≥10.4kbar;2)中温/低压退变质阶段的矿物组合为Grt2+Bt2+Pl2+Ms+Qtz+Ilm2+Rut2,早期和晚期的温压条件分别为664～754℃,4.9～6.5kbar和572～576℃,3.5～3.9kbar。反映陆壳物质在碰撞过程中俯冲到地下深处(≥30km)经高压高温变质后快速折返到中上地壳的动力学演变轨迹。     

The Meatiq dome (Eastern Desert, Egypt) a Precambrian metamorphic core complex: petrological and geological evidence

The Meatiq basement, which is exposed beneath late Proterozoic nappes of supracrustal rocks in the Central Eastern Desert of Egypt, was affected by three metamorphic events. The ophiolite cover nappes show only the last metamorphic overprint. The M1 metamorphic event (T ≥750 °C) is restricted to migmatized amphibolite xenoliths within the Um Ba′anib orthogneiss in the structurally lowest parts of the basement. Typical upper amphibolite facies M2 mineral assemblages include Grt–Zn-rich Spl–Qtz±Bt, Grt–Zn-rich Spl–Ms–Kfs–Bt–Sil–Qtz and locally kyanite in metasedimentary rocks. The mineral assemblages Ms–Qtz–Kfs–Sil in the matrix and Sil–Grt in garnet cores indicate that peak M2 P–T conditions exceeded muscovite and staurolite stabilities. Diffusional equilibration at M2 peak temperature conditions caused homogeneous chemical profiles across M2 garnets. Abundant staurolite in garnet rims and the matrix indicates a thorough equilibration during M2 at decreasing temperature conditions. M2 P–T conditions ranged from 610 to 690 °C at 6–8 kbar for the metamorphic peak and 530–600 °C at about 5.8 kbar for the retrograde stage. However, relic kyanite indicates pressures above 8 kbar, preceeding the temperature peak. A clockwise P–T path is indicated by abundant M2 sillimanite after relic kyanite and by andalusite after sillimanite. M2 fluid inclusions, trapped in quartz within garnet and in the quartz matrix show an array of isochores. Steepest isochores (water-rich H₂O-CO₂±CH₄/N₂ inclusions) pass through peak M2 P–T conditions and flatter isochores (CO₂-rich H₂O-CO₂±CH₄/N₂ inclusions) are interpreted to represent retrograde fluids which is consistent with a clockwise P–T path for M2. The M3 assemblage Grt–Chl in the uppermost metasedimentary sequence of the basement limits temperature to 460 to 550 °C. M3 temperature conditions within the ophiolite cover nappes are limited by the assemblage Atg–Trem–Tlc to<540 °C and the absence of crysotile to >350 °C. The polymetamorphic evolution in the basement contrasts with the monometamorphic ophiolite nappes. The M1 metamorphic event in the basement occurred prior to the intrusion of the Um Ba′anib granitoid at about 780 Ma. The prograde phase of the M2 metamorphic event took place during the collision of an island arc with a continent. The break-off of the subducting slab increased the temperature and resulted in the peak M2 mineral assemblages. During the rise of the basement domain retrograde M2 mineral assemblages were formed. The final M3 metamorphic event is associated with the updoming of the basement domain at about 580 Ma along low-angle normal faults.     

Deformation-induced reconstitution and local resetting of mineral equilibria in polymetamorphic gneisses: tectonic and metamorphic implications

A sequence of at least three Al₂SiO₅-bearing mineral assemblages are preserved in successively overprinted ductile shear zones in the Willimantic window, Connecticut. The ductile deformation, localized at and near the boundary between the Putnam-Nashoba terrane and underlying Avalon terrane is characterized by a network of anastomozing shear zones that outline metre-scale tectonic blocks of migmatitic Kfs + Sil + Gt + Bi + Pg + Qtz + Ilm + Ru gneiss. These assemblages record Acadian or older metamorphic conditions of 6 kbar, 700d? C. Mylonitic gneisses in shear zones that define block margins were formed by reconstitution and recrystallization of the migmatitic gneiss. The reconstituted rocks exhibit relict Ky + St + Grt (+Pl + Bt + Qtz + Rt + Ilm) assemblages and require a minimum pressure for the Ky-Str grade metamorphism of 8.5 kbar. Kyanite in block margins is widely replaced by sillimanite, and locally by andalusite, during a period of post-Alleghanian ductile deformation. The interiors of blocks do not record this sequence of polymorphs. The pattern of reconstitution is accounted for by localization of strain along block margins within a regionally extensive terrane-bounding fault zone. Strain provided the activation energy for recrystallization and retrograde mineral reactions. The P-T conditions of post-Alleghanian ductile deformation evolved from 600d? C and 6 kbar to 550d? C and 3 kbar. The occurrence of Ky + Str-bearing assemblages, overprinting Acadian Kfs + Sil-bearing assemblages and subsequently overprinted by Alleghanian sillimanite- and andalusite-bearing assemblages, along with reset hornblende ⁴⁰Ar/³⁹ Ar mineral ages from Mississippian to Permian, requires a prograde Alleghanian metamorphism of rocks previously metamorphosed during the Acadian. Thus, mineral assemblages from gneisses in the Willimantic fault zone retain evidence of a protracted tectonothermal evolution that included high-grade Acadian orogenesis, tectonic loading resulting from Alleghanian collision of Avalon with North America, and tectonic exhumation in Permo-Triassic time. The c.3-kbar pressure decrease between prograde and retrograde Alleghanian metamorphic conditions corresponds to 10 km of crust that must have been tectonically excised from the base of the Putnam-Nashoba terrane cover sequence following Alleghanian orogenesis in south-eastern New England.     

Regional-scale Grenvillian-age UHT metamorphism in the Mollendo–Camana block (basement of the Peruvian Andes)

The Mollendo–Camana Block (MCB) is a 50 × 150 km Precambrian inlier of the Andean belt that outcrops along the Pacific coast of southern Peru. It consists of stromatic migmatites of Paleoproterozoic heritage intensely metamorphosed during the Grenville event (c. 1 Ga; U‐Pb and U‐Th‐Pb ages on zircon and monazite). In the migmatites, aluminous mesosomes (FMAS) and quartzofeldspathic leucosomes (KFMASH), contain various amounts of K‐feldspar (Kfs), orthopyroxene (X_Mg Opx = 0.86), plagioclase (Pl), sillimanite (Sil; exceptionally kyanite, Ky) ilmenite (Ilm), magnetite (Mag), quartz (Qtz), and minor amounts of garnet (X_Mg Grt = 0.60), sapphirine (X_Mg Spr = 0.87), cordierite (X_Mg Crd = 0.92) and biotite (X_Mg Bt = 0.83). The ubiquitous peak mineral assemblage is Opx‐Sil‐Kfs‐Qtz‐(± Grt) in most of the MCB, which, together with the high Al content of orthopyroxene (10% Al₂O₃) and the local coexistence of sapphirine‐quartz, attest to regional UHT metamorphism (> 900 °C) at pressures in excess of 1.0 GPa. Fluid‐absent melting of biotite is responsible for the massive production of orthopyroxene that proceeded until exhaustion of biotite (and most of the garnet) in the southern part of the MCB (Mollendo‐Cocachacra areas). In this area, a first stage of decompression from 1.1–1.2 to 0.8–0.9 GPa at temperatures in excess of 950 °C, is marked by the breakdown of Sil‐Opx to Spr‐Opx‐Crd assemblages according to several bivariant FMAS reactions. High‐T decompression is also shown by Mg‐rich garnet being replaced by Crd‐Spr‐ and Crd‐Opx‐bearing symplectites, and reacting with quartz to produce low‐Al‐Opx‐Sil symplectites in quartz‐rich migmatites. Neither osumilite nor spinel‐quartz assemblages being formed, isobaric cooling at about 0.9 GPa probably followed the initial decompression and proceeded with massive precipitation of melts towards the (Os) invariant point, as demonstrated by Bt‐Qtz‐(± pl) symplectites in quartz‐rich migmatites (melt + Opx + Sil = Bt + Grt + Kfs + Qtz). Finally, Opx rims around secondary biotite attest to late fluid‐absent melting, compatible with a second stage of decompression below 900 °C. The two stages of decompression are interpreted as due to rapid tectonic denudation whereas the regional extent of UHT metamorphism in the area, probably results from large‐scale penetration of hot asthenospheric mantle at the base of an over‐thickened crust.     

山东半岛高压麻粒岩中锆石的U-Pb定年及其地质意义

在山东半岛早前寒武纪变质基底中,高压麻粗岩常常呈透镜体或不规则脉状体广泛分布于TTG片麻岩之中.锆石中矿物包体激光拉曼测试、阴极发光图像分析及原位U-Pb定年结果表明,山东丰岛高压麻粒岩中锆石成因十分复杂,可划分为3种类型:第一类锆石显示明显或弱的岩浆结晶环带,部分锆石保存磷灰石(Ap)等矿物包体,U-Pb定年结果显示...     

Mineral and fluid inclusions in zircon of UHP metamorphic rocks from the CCSD-main drill hole: A record of metamorphism and fluid activity

The Chinese Continental Scientific Drilling (CCSD) main drill hole (0–3000 m) in Donghai, southern Sulu orogen, consists of eclogite, paragneiss, orthogneiss, schist and garnet peridotite. Detailed investigations of Raman, cathodoluminescence, and microprobe analyses show that zircons from most eclogites, gneisses and schists have oscillatory zoned magmatic cores with low-pressure mineral inclusions of Qtz, Pl, Kf and Ap, and a metamorphic rim with relatively uniform luminescence and eclogite-facies mineral inclusions of Grt, Omp, Phn, Coe and Rt. The chemical compositions of the UHP metamorphic mineral inclusions in zircon are similar to those from the matrix of the host rocks. Similar UHP metamorphic P–T conditions of about 770 °C and 32 kbar were estimated from coexisting minerals in zircon and in the matrix. These observations suggest that all investigated lithologies experienced a joint in situ UHP metamorphism during continental deep subduction. In rare cases, magmatic cores of zircon contain coesite and omphacite inclusions and show patchy and irregular luminescence, implying that the cores have been largely altered possibly by fluid–mineral interaction during UHP metamorphism.

Abundant H₂O–CO₂, H₂O- or CO₂-dominated fluid inclusions with low to medium salinities occur isolated or clustered in the magmatic cores of some zircons, coexisting with low-P mineral inclusions. These fluid inclusions should have been trapped during magmatic crystallization and thus as primary. Only few H₂O- and/or CO₂-dominated fluid inclusions were found to occur together with UHP mineral inclusions in zircons of metamorphic origin, indicating that UHP metamorphism occurred under relatively dry conditions. The diversity in fluid inclusion populations in UHP rocks from different depths suggests a closed fluid system, without large-scale fluid migration during subduction and exhumation.     

辽东半岛古元古代北辽河群浪子山组巴罗式变质带分布、变质特征及其构造意义

巴罗式递增变质带能为地壳增厚及造山作用带来全新的认识.为了解辽东半岛北辽河群浪子山组内巴罗式变质带的野外分布和变质特征,对本溪地区连山关-祁家堡巴罗式变质带进行了详细的野外调查和室内研究工作.野外填图表明,浪子山组巴罗式变质带由南向北可以分为黑云母带、石榴石带、十字石带和蓝晶石带.岩相学研究表明,蓝晶石带的蓝晶十字石榴云母片岩保存了3阶段的矿物组合:（1）进变质阶段（M₁）矿物组合为Pl+Qz+Ms+Bt+Ctd+Chl±Grt;（2）峰期变质阶段（M₂）以Ky+St+Grt+Bt+Ms+Qz+Pl+Ilm为特征;（3）退变质阶段以毛发状的矽线石（M_3-1）和变斑晶边缘或裂隙处生长的绿泥石和绢云母为特征（M_3-2）.相平衡模拟表明,蓝晶十字石榴云母片岩所经历的进变质和峰期变质温压条件分别为~440℃/~3.7 kbar,~670℃/~7.9 kbar,具有典型的顺时针P-T轨迹.变质锆石U-Pb定年结果得到蓝晶十字石榴云母片岩经历了~1.96 Ga的峰期变质作用;碎屑锆石U-Pb年龄分布于2 631~2 020 Ma,浪子山组蓝晶十字石榴云母片岩碎屑物源可能来自于太古宙基底（~2.5 Ga）和2.2~2.1 Ga的岩浆岩.结合前人研究资料表明,浪子山组巴罗式变质带所记录的P-T-t轨迹以及大量的逆冲推覆构造揭示了胶-辽-吉带在~1.96 Ga经历了与造山作用相关的地壳增厚过程.      

P–T–t paths and tectonic history of an early Precambrian granulite facies terrane, Jining district, south-east Inner Mongolia, China

Abstract The widespread khondalite series of south-east Inner Mongolia consists largely of biotite–sillimanite–garnet gneiss and quartzo-feldspathic gneiss with some marble and mafic granulite layers. It has experienced two metamorphic events at c. 2500 and 1900–2000 Ma.
A pre-peak stage of the first metamorphism at T = 600–700°C and P > 6–7 kbar is recognized by the relict amphibolite facies assemblage Ky–Grt–Bt–Pl–Qtz and 'protected'inclusions of biotite, hornblende, sodic plagioclase and quartz in garnet or orthopyroxene. The peak stage, with T = c. 800 ± 50°C and P 8–10 kbar, is characterized by the widespread granulite facies assemblages Sil–Grt–Bt–Kfs–Pl–Qtz in gneiss and Opx–Cpx–Pl ± Hbl ± Grt in granulite. The P–T–t path suggests that the supracrustal sequence was buried in the lower crust by tectonic thickening during D1–D2.
The beginning of the second metamorphism is characterized by further temperature rise to 700°C or more at lower pressure. This stage is manifested by the appearance of cordierite after garnet, fibrolite (Sil2) after biotite in gneiss and transformation of Hbl1 into Opx2 and Cpx2 in granulite. Coronas of symplectitic Opx2 + Pl2 surrounding Grt1 and Cpx1 in mafic granulite are interpreted as products of near-isothermal decompression. The P–T–t path may be related tectonically to waning extension of the crust by the end of the early Proterozoic.     

Reworking of deep-seated gabbros and associated contact metamorphosed paragneisses in the southeastern part of the Seiland Igneous Province, northern Norway

The Seiland Igneous Province of the North Norwegian Caledonides consists of a suite of deep-seated rift-related magmatic rocks emplaced into paragneisses during late Precambrian to Ordovician time. In the south-eastern part of the province, contact metamorphism of the paragneisses and later reworking of intrusives and associated contact aureoles have resulted in the development of three successive metamorphic stages. The contact metamorphic assemblage (M1) Opx + Grt + Qtz + Pl + Kfs + Hc + Ilm ± Crd is preserved in xenolithic rafts of paragneiss within metagabbro. Geothermobarometric calculations yield 930-960d? C and 5-6.5 kbar for the contact metamorphism. M1 was followed by cooling, accompanied by strong shearing, formation of the gneiss foliation and recrystallization at intermediate-P granulite facies conditions (M2). Stable M2 phases are Cpx + Opx + Pl +Ilm ± Hbl in metagabbro and Grt ± Sil ± Opx + Kfs + Qtz + Pl ± Bt + Ilm in host paragneiss. The M2 conditions are estimated to 700-750d? C and 5-7 kbar. A subsequent pressure increase is recorded in the M3 episode, which is associated with recrystallization in narrow ductile shear zones and secondary growth on M2 minerals. M3 is defined by the assemblages Grt + Cpx ± Opx + Pl + Ru + Qtz in metagabbro, and Grt ± Ky + Qtz + Pl ± Kfs + Bt + Ru in host paragneiss. M3 conditions are estimated to 650-700d? C and 8-10 kbar. The substantial pressure increase related to the M2 → M3 transition is interpreted to be a result of (early?) Caledonian overthrusting. Chemical zoning in cordierite and biotite suggest rapid cooling following the M3 event. The proposed P-T-t evolution implies that the tectonic evolution of the Seiland Igneous Province was long (at least 330 Ma) and complex and involved initial rifting and extension followed by crustal thickening and compression.     

拉萨地块东南缘晚白垩世高温变质作用及其地质意义:来自石英出溶金红石的证据和锆石U-Pb定年

文中对拉萨地块东南缘林芝杂岩中的含榴斜长角闪岩进行了详细的岩相学研究和锆石U-Pb定年。岩相学观察表明,含榴斜长角闪岩经历了峰期麻粒岩相变质和角闪岩相退变质作用。峰期麻粒岩相矿物组合为石榴子石+高Ti角闪石+紫苏辉石+斜长石+石英+金红石,其中的石榴子石、石英和角闪石中含有大量金红石出溶体,说明这些矿物的初始成分具有高Ti含量。角闪岩相退变质矿物组合为低Ti角闪石+斜长石+斜黝帘石+石英+金红石。利用Ti在石英中的含量(TitaniQ)温度计计算得到峰期麻粒岩相变质温度为803～924℃,后期角闪岩相退变质温度为555～732℃。样品中的锆石具有明显的核边结构,核部为典型岩浆型锆石,具有高的Th/U值,强烈富集HREE,明显的正Ce异常和负Eu异常,206Pb/238U年龄为(89.3±0.6)Ma,代表含榴斜长角闪岩原岩结晶年龄。锆石边部呈无环带结构,同核部相比,具有低的Th/U值,低的M-HREE和弱的Eu负异常,为变质峰期生长的锆石,206Pb/238U年龄为(81.1±0.8)Ma,代表麻粒岩相峰期变质年龄。我们认为约81Ma高温麻粒岩相变质作用可能与洋中脊俯冲造成的板片窗导致的软流圈上涌有关。     

An example of ultrahigh-temperature metamorphism: orthopyroxene–sillimanite–garnet, sapphirine–quartz and spinel–quartz parageneses in Al–Mg granulites from In Hihaou, In Ouzzal, Hoggar

Quartz Al–Mg granulites exposed at In Hihaou, In Ouzzal (NW Hoggar), preserve an unusual high-grade mineral association stable at temperatures up to 1050°C, involving the parageneses orthopyroxene–sillimanite–garnet–quartz, sapphirine–quartz and spinel–quartz. The phase relationships within the FMAS system show that a continuum exists between the earlier prograde reaction textures and those of the later decompressive event. The following mineral reactions involving sillimanite are deduced: (1) Grt+Qtz→Opx+Sil, (2) Opx+Sil→Grt+Spr+Qtz, (3) Grt+Sil+Qtz→Crd, (4) Grt+Sil→Crd+Spr, (5) Grt+Sil+Spr→Crd+Spl, (6) Grt+Sil→Crd+Spl, (7) Grt+Crd+Sil→Spl+Qtz and (8) Grt+Sil→Spl+Qtz. Minerals in quartz Al–Mg granulites display compositional variations consistent with the observed reactions. The Mg/(Mg+Fe²⁺) range of the main minerals is as follows: cordierite (0.81–0.97), sapphirine (0.77–0.88), orthopyroxene (0.65–0.81), garnet (0.33–0.64) and spinel (0.23–0.56). The reaction textures and the evolution of the mineral assemblages in the quartz Al–Mg granulites indicate a clockwise P–T trajectory characterized by peak conditions of at least 10 kbar and 1050°C, followed by decompression from 10 to 6 kbar at a temperature of at least 900°C.     

柴北缘大陆深俯冲板片折返过程中的深熔作用研究

柴北缘锡铁山地区长英质（花岗）片麻岩普遍经历了不同程度的部分熔融作用,常见新生的花岗质浅色体呈层状、脉状或网络状分布于长英质片麻岩中,并显示出混合岩化的特征。岩相学观察结果显示长英质片麻岩保留了关键的深熔作用显微结构证据:（1）石榴石内部发育有钾长石、石英和斜长石组成的矿物包裹体;（2）长石颗粒边界出现由石英+钾长石±斜长石±白云母组成的楔形矿物集合体;（3）云母颗粒边界发育尖锐的、不规则的微斜长石,而且云母边界溶蚀明显,形成锯齿状不规则的边界;（4）石英、斜长石或钾长石颗粒边界发育圆珠状（stringofbeads）结构,而且颗粒边界或三联点中尖锐状微斜长石与周围矿物的形成较小的二面角。阴极发光图像和锆石U-Pb定年结果表明花岗质浅色体中的锆石具有明显的核、幔、边三层结构,而且具有明显不同的年龄结果。发光较强的继承性锆石岩浆核部的²⁰⁶Pb/²³⁸U年龄约为~910Ma,而且具有高的Th/U比值;弱发光的变质锆石幔的²⁰⁶Pb/²³⁸U年龄结果约为~450Ma。新生的锆石增生边中等程度发光,并发育震荡环带和较低的Th/U比值,与世界典型地区混合岩中深熔锆石的特征十分相似,其²⁰⁶Pb/²³⁸U年龄结果为432±3Ma。野外关系、显微结构特征和年代学的研究结果显示柴北缘锡铁山地区花岗质浅色体可能是其寄主岩石长英质片麻岩在折返到高压麻粒岩相条件下深熔作用的产物,而且白云母的脱水熔融是引发岩石发生深熔作用的主要机制。柴北缘地区已有的资料综合研究表明,大陆深俯冲板片在俯冲/碰撞和折返过程中可能经历了多重深熔作用。     

Models for orthopyroxene–plagioclase and other corona reactions in metanorites, Dahomeyide orogen, West Africa

Coronal reaction textures occur in metanorite and related intrusions in the Pan-African Dahomeyide orogen of West Africa; they apparently formed by retrograde subsolidus reactions during cooling of the intrusions from 900 to 700 °C at c. 9±1 kbar. The coronas that formed around orthopyroxene (Opx) consist of sequential layers of diopside (Dio), hornblende (Hbl), garnet (Grt) and plagioclase (Pl) with inclusions of kyanite (Ky) or sillimanite (Sil). Three well-organized mineral assemblage sequences have been identified and modelled using steady-state diffusion theory for closed and open systems. The mineral sequence Opx|Hbl+Qtz|Hbl|Pl+Ky, formed by diffusion-controlled reactions in a closed system as layer thicknesses are very sensitive to relative component mobilities defined by Onsager diffusion coefficient (L_ii). Models of this corona (type i) that satisfy modes require L_CaCa≥L_MgMg≥L_FeFe>L_AlAl～L_SiSi ; however, small open-system fluxes involving loss of Al and gain of Ca are required to obtain the best fit between model and observed mineral proportions. Under steady-state diffusion, the monomineralic hornblende layer grew by replacing plagioclase whereas the |Hbl+Qtz| grew by Opx replacement. The type ii corona, which consists of the sequence Opx|Dio|Grt+Dio|Pl+Sil, is also stable under steady-state diffusion in a closed system. Modelling results show that the diopside grew by replacing Opx whereas most of Grt+Dio grew by replacing plagioclase. Stable solutions to the closed-system diffusion model that approximate the mode are restricted to the L-ratio regions where Fe, Mg and Ca are more mobile than Si and Al but are unstable when L_AlAl>100 L_SiSi . However, type ii corona mineral proportions were only closely matched when open-system loss of Al and gain of small amounts of Fewere considered in the diffusion models and relative mobilities were L_FeFe≥L_CaCa≥L_MgMg>L_AlAl～L_SiSi . The modelling results indicate that Ca and Mg were the most mobile elements in the formation of type i corona whereas Fe and Ca were the most mobile components in the growth of type ii coronas. A third corona type, consisting of the mineral sequence Act|Hbl|Grt|Pl+Sil, is only stable in open systems and requires large external fluxes involving gain of Fe, Al, Mg and Na and loss of Ca to obtain a solution of the diffusion model that approximates the estimated mineral proportions. Extensive recrystallization of plagioclase to produce sillimanite or kyanite inclusions accompanying corona formation may explain the open-system behaviour indicated by the diffusion models.     

Mineral inclusions in zircons of para- and orthogneiss from pre-pilot drillhole CCSD-PP1, Chinese Continental Scientific Drilling Project

The pre-pilot drillhole CCSD-PP1, Chinese Continental Scientific Drilling Project (CCSD), with depth of 432 m, is located in the Donghai area in the southwestern Sulu terrane. The core samples are mainly comprised of paragneiss, orthogneiss and ultramafic rock with minor intercalated layers of eclogite and phengite-bearing kyanite quartzite. All analyzed paragneiss and orthogneiss samples were overprinted on amphibolite facies retrograde metamorphism. Coesite and coesite-bearing ultrahigh-pressure (UHP) mineral assemblages were identified by Raman spectroscopy and electron microprobe analysis as inclusions in zircons separated from paragneiss, eclogite and phengite-bearing kyanite quartzite samples. In the paragneiss samples, UHP mineral inclusion assemblages mainly consist of Coe+Omp+Grt+Phe, Coe+Jd+Phe+Ap preserved in the mantles (M) and rims (R) of zircons. These UHP mineral inclusion assemblages yield temperatures of 814–852 °C and pressures of ≥28 kbar, presenting the P–T condition of UHP peak metamorphism of these country rocks. According to the mineral inclusions and cathodoluminescence images of zircons, the orthogneisses can be divided into two types: UHP (OG1) and non-UHP (OG2). In OG1 orthogneisses, low-pressure mineral inclusion assemblage, mainly consisting of Qtz+Phe+Ab+Ksp+Ap, were identified in zircon cores (C), while coesite or coesite-bearing UHP mineral inclusions were identified in the mantles (M) and rims (R) of the same zircons. These features suggest that the OG1 orthogneisses, together with the paragneisses, phengite-bearing kyanite quartzite and eclogite experienced widespread UHP metamorphism in the Sulu terrane. However, in the zircons of OG2 orthogneiss samples, no UHP mineral inclusions were found. Inclusions mainly comprised Qtz+Phe+Ap and were identified in cores (C), mantles (M) and rims (R) of OG2 zircons; the cathdoluminescence images of all analyzed zircons showed clear zonings from cores to rims. These features indicate that the OG2 orthogneisses in pre-pilot drillhole CCSD-PP1 did not experience UHP metamorphism. Therefore, we should not rule out the possibility that some orthogneisses in Sulu terrane might represent relatively low-pressure granitic intrusives emplaced after the UHP event.     

滇西鲁甸金沙江结合带新发现退变榴辉岩

在滇西鲁甸地区金沙江结合带新发现退变榴辉岩,其在野外呈透镜体状产于石榴子石白云母石英片岩中.利用电子探针及激光拉曼分析发现石榴子石和锆石中残留绿辉石包体.石榴子石及基质中的白云母为多硅白云母(Si(p.f.u)=3.27~3.53),指示岩石经历了高压变质作用过程.石榴子石发育进变质生长成分环带.岩相学及矿物化学特征显示,退变榴辉岩大致经历了进变质角闪岩相、峰期榴辉岩相、早期退变质以及晚期强退变这4个世代矿物组合,各阶段典型的矿物组合依次为Grt+AmpI+Qtz、Grt+Omp+Rt+Qtz+Phe、Pl+Di+AmpⅡ+Ilm+Spn+Qtz、AmpⅢ+Pl+Czo+Ilm+Qtz.该新发现对金沙江结合带复杂的变质演化P-T-t轨迹样式及年代格架、以金沙江洋为代表的整个西南三江地区古特提斯洋-陆俯冲-碰撞-造山的复杂构造演化历史以及微陆块的拼贴机制等关键科学问题的解决提供了极为重要的素材和更多的约束,具有重要的科学意义.      

P-T Conditions and the Influence of Graphite on Pelitic Phase Relations in the Ballachulish Aureole, Scotland

Pressure-temperature conditions of pelites in the Ballachulishaureole, Scotland, have been determined from a calibrated petrogeneticgrid and from published geothermometers and geobarometers. Tocalibrate the mineral reactions in the grid, thermodynamic datafor appropriate end members of Ms, Chi, Qtz, And, Sil, Ky, Crn,Crd, Kfs, and Bt were derived from experimental data. This approachwas hampered by the unknown compositions of many of the mineralsused in the experiments, and by apparent inconsistency betweenthe experiments. A best compromise grid that satisfies mostof the data was obtained, which is applicable to the Ballachulishand other contact aureoles. In this grid, the first developmentof sillimanite is constrained to lie between the Richardsonet al. (1969) and Holdaway (1971) andalusite-sillimanite boundaries. A pressure estimate of 3.0 + 0.5 kb is obtained from the calibratedgrid, within 0.3 kb of estimates from geobarometry and fromtwo other independent petrological studies. Temperatures rangedfrom 560?20?C at the first development of cordierite in theassemblage Ms+Qtz+Chl+Crd+Bt to 750–800?C in Grt+Crd+Hyassemblages in pelitic screens within the igneous complex. In graphitic slates, in contrast to non-graphitic pelites, anentire andalusite-bearing subzone is developed, and initialcordierite development occurs further from the igneous contacts.The presence of graphite lowered a_H2o in the slaters, expandingthe stability field of the andalusite-bearing assemblage And+Qtz+Bt+Ms+Crdrelative to the assemblage Kfs+Qtz+Bt+Ms+Crd developed in non-graphiticunits. Initial development of cordierite in the assemblage Ms+Qtz+Chl+Crd+Btwas also promoted by reduced a_H2o in graphitic slates. The regular sequence and spacing of mineral zones in the aureolesuggests that gross equilibrium was attained during contactmetamorphism, even though the thermal metamorphic pulse is estimatedto have been less than 0.2 Ma (Buntebarth, in press). Thereis no evidence for reaction overstepping in cordierite-producingreactions.     

大青山地区古元古代壳源碳酸岩:锆石特征及SHRIMP定年

内蒙古大青山地区孔兹岩系被划分为榴云片麻岩岩组、透辉片麻岩岩组和大理岩岩组。透辉片麻岩岩组中的部分透辉大理岩实际上为壳源碳酸岩。壳源碳酸岩锆石包裹斜锆石、方解石、磷灰石、榍石、石英、长石、辉石等矿物。锆石具核－边结构。透射光和阴极发光下,核部锆石多具密集平行环带,为岩浆成因;边部锆石无明显结构变化,为变质成因。斜锆石在岩浆锆石和变质锆石中都存在。岩浆锆石和变质锆石的年龄分别为1.95-1.93 Ga和1.90-189Ga。结合地质和岩相学研究,认为大青山地区壳源碳酸岩为透辉片麻岩岩组中的透辉大理岩深熔作用产物,其形成时代为古元古晚期（1.95-1.93 Ga）,并在稍后（1.90-1.89 Ga）遭受变质作用叠加改造。     

苏鲁地体超高压和非超高压花岗质片麻岩的判别标志——来自锆石中矿物包裹体的证据

中国大陆科学钻探工程预先导孔CCSD－PP1中存在两类不同性质的花岗质片麻岩，它们的岩相学性质，地球化学特征，锆石中的包体矿物组合和分布规律以及阴极发光图像均存在明显的判别，其中第一类花岗质片麻岩中的锆石呈完好的自形晶，不含任何超高压矿物包体，自晶体中心至边缘均具有典型的岩浆结晶生长环带，表明该类花岗质片麻岩未经历超高压变质作用，第二类花岗质征麻岩中的锆石呈半自形晶一它形晶，晶体核部保存典型的岩浆结晶生长环带，且无超高压矿物包体，具有继承性锆石的特点，而在晶体的过渡带及其与边缘的交界部位，普遍存在以柯石英粉 包体矿物，表明第二类花岗质片麻岩曾经历了超高压变质作用，且锆石在此变质过程进一步结晶生长，形成新的生长环带，该项成果对于深入探讨苏鲁超高压变质带的俯冲－折返机制及其与岩浆作用的相互关系以及对中国大陆科学钻探工程的选址和实施有着重要的科学意义。