P–T–t–D record of crustal‐scale horizontal flow and magma‐assisted doming in the SW Mongolian Altai

The Chandman massif, a typical structure of the Mongolian Altai, consists of a migmatite–magmatite core rimmed by a lower grade metamorphic envelope of andalusite and cordierite‐bearing schists. The oldest structure in the migmatite–magmatite core is a subhorizontal migmatitic foliation S1 parallel to rare granitoid sills. This fabric is folded by upright folds F2 and transposed into a vertical migmatitic foliation S2 that is syn‐tectonic, with up to several tens of metres thick granitoid sills. Sillimanite–ilmenite–magnetite S1 inclusion trails in garnet constrain the depth of equilibration during the S1 fabric to 6–7 kbar at 710–780 °C. Reorientation of sillimanite into the S2 fabric indicates that the S1–S2 fabric transition occurred in the sillimanite stability field. The presence of cordierite, and garnet rim chemistry point to decompression to 3–4 kbar and 680–750 °C during development of the S2 steep fabric, and post‐tectonic andalusite indicates further decompression to 2–3 kbar and 600–650 °C. Widespread crystallization of post‐tectonic muscovite is explained by the release of H₂O from crystallizing partial melt. In the metamorphic envelope the subhorizontal metamorphic schistosity S1 is heterogeneously affected by upright F2 folds and axial planar subvertical cleavage S2. In the north, the inclusion trails in garnet are parallel to the S1 foliation, and the garnet zoning indicates nearly isobaric heating from 2.5 to 3 kbar and 500–530 °C. Cordierite contains crenulated S1 inclusion trails and has pressure shadows related to the formation of the S2 fabric. The switch from the S1 to the S2 foliation occurred near 2.5–3 kbar and 530–570 °C; replacement of cordierite by fine‐grained muscovite and chlorite indicates further retrogression and cooling. In the south, andalusite containing crenulated inclusion trails of ilmenite and magnetite indicates heating during the D2 deformation at 3–4 kbar and 540–620 °C. Monazite from a migmatite analysed by LASS yielded elevated HREE concentrations. The grain with the best‐developed oscillatory zoning is 356 ± 1.0 [±7] Ma (²⁰⁷Pb‐corrected ²³⁸U/²⁰⁶Pb), considered to date the crystallization from melt in the cordierite stability ~680 °C and 3.5 kbar, whereas the patchy BSE‐dark domains give a date of 347 ± 4.2 [±7] Ma interpreted as recrystallization at subsolidus conditions. The earliest sub‐horizontal fabric is associated with the onset of magmatism and peak of P–T conditions in the deep crust, indicating important heat input associated with lower crustal horizontal flow. The paroxysmal metamorphic conditions are connected with collapse of the metamorphic structure, an extrusion of the hot lower crustal rocks associated with vertical magma transfer and a juxtaposition of the hot magmatite–migmatite core with supracrustal rocks. This study provides information about tectono‐thermal history and time‐scales of horizontal flow and vertical mass and heat transfer in the Altai orogen. It is shown that, similar to collisional orogens, doming of partially molten rocks assisted by syn‐orogenic magmatism can be responsible for the exhumation of orogenic lower crust in accretionary orogenic systems.     

Reversals in the polarity of structural facing across an early ductile thrust; the Central Donegal Slide,northwest Ireland

The Dalradian succession of central Donegal represents a tectono-stratigraphic sequence everywhere separated into two distinct successions by the Central Donegal Slide. Each stratigraphic package consistently youngs away from this major D₁ structure which originated during lower greenschist facies metamorphism and corresponds with the development of locally preserved penetrative S₁ fabrics. The tectonic slide and associated reversal in the direction of structural facing are commonly marked by mylonites and tectonic schists, which have been subsequently folded and reactivated by kilometric-scale sheath folds and the Ballybofey Nappe. Structural correlations with northwest Donegal suggest that the Central Donegal Slide relates to the Knockateen Slide, and represents an early thrust with top to the northwest sense of shear. The Central Donegal Slide cuts up-section through the Middle Dalradian sub-basin centred on southern Donegal, emplacing younger, anoxic basinal sediments and turbidites over an older, inverted, shelf succession. The location of the Central Donegal Slide adjacent to the margin of this sub-basin suggests that the siting and form of ductile thrusts may be strongly influenced by the geometry of the pre-existing sedimentary basin.     

Progressive deformation and the rotation of contemporary fold axes in the Ballybofey Nappe,north-west Ireland

Internal regions of orogenic belts may be characterized by an alignment of fold axes with mineral elongation lineations. This relationship is commonly interpreted as representing progressive tightening and rotation towards the shear direction of early buckle folds, the hinges of which were initiated orthogonal to this direction. Detailed structural analysis of lower amphibolite facies Dalradian metasediments of the Ballybofey (fold) Nappe, north-west Ireland, shows that an intense S₃ schistosity is developed axial planar to mesoscopic and minor F₃ folds. In areas of low D₃ strain, F₃ fold axes plunge gently towards the north-east, whereas in regions of greater strain plunges are towards the south-east subparallel to the constant mineral lineation. Minor folds which initiated at angles of 70–80° from the mineral lineation subsequently rotated towards the shear direction in a consistent clockwise sense. Progressive and variable non-coaxial deformation oblique to the original mean F₃ orientation has resulted in a unimodal distribution pattern of fold axes. Analysis of the angular rotation of fold axes enables estimates of the bulk shear strain to be evaluated and models of progressive deformation to be assessed.     

Evolution of the Bhandara-Balaghat granulite belt along the southern margin of the Sausar Mobile Belt of central India

The Bhandara-Balaghat granulite (BBG) belt occurs as a 190 km long, detached narrow, linear, NE-SW to ENE-WSW trending belt that is in tectonic contact on its northern margin with the Sausar Group of rocks and is bordered by the Sakoli fold belt in the south. The Bhandara part of the BBG belt is quite restricted, comprising a medium to coarse grained two-pyroxene granulite body that is of gabbroic composition and preserves relic igneous fabric. The main part of the belt in Arjuni-Balaghat section includes metasedimentary (quartzite, BIF, Al- and Mg-Al metapelites) and metaigneous (metaultramafic, amphibolite and two-pyroxene granulite) protoliths interbanded with charnockite and charnockitic gneiss. These rocks, occurring as small bands and enclaves within migmatitic and granitic gneisses, show polyphase deformation and metamorphism. Geochemically, basic compositions show tholeiitic trend without Fe-enrichment, non-komatitic nature, continental affinity and show evolved nature. Mineral parageneses and reaction textures in different rock compositions indicate early prograde, dehydration melt forming reactions followed by orthopyroxene stability with or without melt. Coronitic and symplectitic garnets have formed over earlier minerals indicating onset of retrograde IBC path. Evidences for high temperature ductile shearing are preserved at places. Retrogressive hydration events clearly post-date the above paths. The present study has shown that the BBG belt may form a part of the Bastar Craton and does not represent exhumed oceanic crust of the Bundelkhand Craton. It is further shown that rocks of the BBG belt have undergone an earlier high-grade granulite metamorphism at 2672 ± 54 Ma (Sm-Nd age) and a post-peak granulite metamorphism at 1416 ± 59Ma (Sm-Nd age, 1380 ± 28Ma Rb-Sr age). These events were followed by deposition of the Sausar supracrustals and Neoproterozoic Sausar orogeny between 973 ± 63Ma and 800 ± 16Ma (Rb-Sr ages).     

Structural,petrological and chemical analysis of syn‐kinematic migmatites: insights from the Western Gneiss Region,Norway

Evidence of melting is presented from the Western Gneiss Region (WGR) in the core of the Caledonian orogen, Western Norway and the dynamic significance of melting for the evolution of orogens is evaluated. Multiphase inclusions in garnet that comprise plagioclase, potassic feldspar and biotite are interpreted to be formed from melt trapped during garnet growth in the eclogite facies. The multiphase inclusions are associated with rocks that preserve macroscopic evidence of melting, such as segregations in mafic rocks, leucosomes and pegmatites hosted in mafic rocks and in gneisses. Based on field studies, these lithologies are found in three structural positions: (i) as zoned segregations found in high‐P (ultra)mafic bodies; (ii) as leucosomes along amphibolite facies foliation and in a variety of discordant structures in gneiss; and (iii) as undeformed pegmatites cutting the main Caledonian structures. Segregations post‐date the eclogite facies foliation and pre‐date the amphibolite facies deformation, whereas leucosomes are contemporaneous with the amphibolite facies deformation, and undeformed pegmatites are post‐kinematic and were formed at the end of the deformation history. The geochemistry of the segregations, leucosomes and pegmatites in the WGR defines two trends, which correlate with the mafic or felsic nature of the host rocks. The first trend with Ca‐poor compositions represents leucosome and pegmatite hosted in felsic gneiss, whereas the second group with K‐poor compositions corresponds to segregation hosted in (ultra)mafic rocks. These trends suggest partial melting of two separate sources: the felsic gneisses and also the included mafic eclogites. The REE patterns of the samples allow distinction between melt compositions, fractionated liquids and cumulates. Melting began at high pressure and affected most lithologies in the WGR before or during their retrogression in the amphibolite facies. During this stage, the presence of melt may have acted as a weakening mechanism that enabled decoupling of the exhuming crust around the peak pressure conditions triggering exhumation of the upward‐buoyant crust. Partial melting of both felsic and mafic sources at temperatures below 800 °C implies the presence of an H₂O‐rich fluid phase at great depth to facilitate H₂O‐present partial melting.     

辽宁小盘岭金矿床混合岩特征及与金成矿的关系

辽宁绥中小盘岭金矿床位于混合岩化中心带的混合花岗岩内，其中间带主要为混合片麻岩，外带为以条带状混合岩为主的混合杂岩。通过对各类混合岩中造岩矿物光学特征的对比，发现随着混合岩化程度的增强，矿物光学特征有规律地变化。根据岩石中矿物的交代关系，确定在混合岩化作用过程中至少有4期热液活动。通过将区域原变质岩、混合岩和与金矿成矿有关的花岗岩类岩石地球化学成分的对比，中心带的混合花岗岩与金成矿关系密切，其原岩——厚层状黑云母斜长片麻岩很可能是金成矿的重要矿源岩，其中发育的长英质岩脉是金成矿的一个重要标志。     

Geological context of crustal anatexis and granitic magmatism in the northeastern Glenelg River Complex,western Victoria

The Cambro‐Ordovician Glenelg River Complex in the Harrow district, western Victoria, consists of extensive granitic rocks associated with a migmatitic metasedimentary envelope. Metasedimentary rocks comprise amphibolite facies massive‐laminated quartzo‐feldspathic schists and layered gneisses with minor sillimanite‐bearing horizons. Intercalated are stromatic and nebulitic migmatites of granitic and tonalitic character; textural evidence suggests that both varieties developed by in situ partial melting. Ranging from adamellite to leucotonalite, granitic rocks contain abundant magmatic muscovite, commonly with garnet and sillimanite, and exhibit generally unrecrystallised igneous textures. Heterogeneous structurally concordant plutons transitional to migmatites and more uniform intrusive phases are delineated with both types hosting diverse metasedimentary enclaves, micaceous selvages and schlieren; a gneissic foliation of variable intensity is defined by the latter. These petrographic attributes are consistent with derivation of plutons by anatexis of a peraluminous metasedimentary protolith. The schlieric foliation is not tectonically imposed, but rather directly inherited from the migmatitic precursor, compositional variations within which are preserved by the layered Schofield Adamellite. The most mafic granitic body (Tuloona Granodiorite) also has igneous microgranular enclaves indicating a more complex petrogenesis. Metasedimentary rocks experienced five episodes of folding, the latest involving macroscopic open warps. This is analogous to the structural history elucidated elsewhere in the Glenelg River Complex, by inference a coherent tectonic entity whose present metamorphic and stratigraphic configuration might be governed by F₅ folding. Structures within migmatites intimate that partial melting proceeded throughout the deformational history and peaked syn‐D₄ to pre‐D₅, whilst temperatures had waned to sub‐biotite grade in the southwestern Glenelg River Complex. Granitic rocks were generated during this anatectic culmination and were therefore emplaced late in the orogenic history relative to other syntectonic phases of the Glenelg River Complex.     

Syn-deformational migmatites and magmatic-arc metamorphism in the Xolapa Complex, southern Mexico

The Xolapa Complex (XC) is the largest plutonic and metamorphic mid‐crustal basement unit in Mexico and represents an ancient continental magmatic‐arc. A complete range from metatexite to diatexite migmatitic structures has been produced during a single high‐grade metamorphic event. However, structural relics reveal the existence of early Cpx + Pl + Qtz ± Opx and Grt + Opx + Pl + Qtz ± Cpx pre‐migmatitic metamorphic assemblages. Field relationships and microstructural observations allow us to constrain five pre‐, syn‐ and post‐migmatitic deformational phases. It is argued that migmatitic structures and minor anatectic granites were developed during ductile recumbent folding and shear structures related to the D2–D3 phases. Late post‐migmatitic ductile‐brittle deformation is evidenced by the development of NNE trending transpressional thrusting (D4), and E–W left‐lateral mylonitic shear zones (D5). Biotite‐breakdown melting in felsic rocks and amphibole‐breakdown melting in mafic rocks, as well as geothermobarometric results, indicate that metamorphism took place at temperatures from 830 to 900 °C and pressures ranging from ≥6.3 to 9.5 kbar. Late migmatitic assemblages equilibrated in the highest temperature range along a clockwise P–T path. The relationships between the large diversity of migmatitic structures and the progressive production of melt suggest that feedback relations prevailed as a time‐marker during a contractional regime. Deformation, metamorphism, and plutonism of the XC show that this terrane evolved as a north‐east‐verging thrust system with synkinematic metamorphism and partial melting, during the Late Cretaceous – Palaeogene. The tectonothermal history of XC is analogous to a Cordilleran metamorphic magmatic‐arc formed in an accretionary tectonic framework. This new model provides constraints on the exhumation mechanism and thermal evolution of southern Mexico.     

赣中变质岩带的Sm-Nd、Rb-Sr同位素年代研究

赣中变质岩带主要由斜长(云母)变粒岩、十字石榴云母片岩、云母石英片岩夹斜长角闪岩组成。斜长角闪岩Sm-Nd全岩等时线年龄为1113±19Ma,相当于该变质岩带的原岩形成年龄。ε_(Nd)(t)值为2.4±0.1,说明岩浆起源于亏损程度较低的地幔源区:726.6±1.1Ma、403.1±6.4Ma的Rb-Sr等时线年龄表明赣中地区在新元古代末期、加里东期经历了一次强烈的构造变质热作用,其地质、地球化学特征及时代可与浙闽地区陈蔡群、建瓯群进行对比。因此赣中变质岩带并非长期公认的华南加里东褶皱带,应是华夏地块的一部分,这一事实对华南大地构造单元的划分及构造演化具有重要意义。     

大陆造山带深熔垮塌的岩石学、地球化学证据:以北大别深熔混合岩为例

北大别位于大别造山带的核部,分布着大量的造山带垮塌时期形成的混合岩,其于理解大别造山带的形成和演化有着重要的意义。北大别混合岩的原岩为TTG（D）岩石,因黑云母和角闪石的脱水熔融诱发深熔作用产生。顺层产出的为富斜长石浅色体,主要矿物组成为斜长石+石英+黑云母±钾长石±角闪石。伟晶岩脉或团块为富钾长石浅色体,主要矿物组成为钾长石+石英±斜长石±黑云母±角闪石。暗色体为变晶结构,主要矿物组成为角闪石+黑云母+斜长石+石英±单斜辉石;其中,暗色矿物角闪石和黑云母常常定向排列,具有明显的溶蚀结构;暗色体中浅色矿物颗粒较小,以斜长石和石英为主,指示部分熔融的残余产物。全岩地球化学特征表明,碱金属元素（Na、K等）、大离子亲石元素（Ba、K、La等）和LREE等优先进入酸性熔体,而相容元素和中-重稀土元素等残留在残余体中。浅色体与本区花岗岩相比,二者都有右倾的稀土配分模式,富集LREE,亏损HREE。但浅色体具有明显的Eu正异常,δEu值为2.48~6.55,而花岗岩则有弱的Eu负异常,并且浅色体中大颗粒斜长石相互构成框架结构,含量明显高于正常花岗岩熔体,表明浅色体更可能是熔体早期结晶的产物。