The PTst Evolution of Metamorphism and Development Mechanism of the Thermal - Structural - Gneiss Domes in the Chinese Altaides1

Abstract A series of thermal-structural-gneiss domes (briefly TSG domes) are developed in the Chinese Altaides. Sericite-chlorite zone, biotite zone, garnet zone, staurolite zone, kyanite (andalusite) zone, sillimanite - cordierite (sillimanite - garnet) zone, migmatite zone and migmatic granite - gneiss field are developed from the low-grade metamorphic area to the centre of the TSG domes. The succession of the formation and evolution of the progressive metamorphic zone, migmatite zone and migmatic granite-gneiss corresponds to the spatial sequence from the outer part to the centre of the TSG domes. The peak temperature of the metamorphism and granitization increases progressively from 400 °C to 800 °C while the pressure decreases progressively from 1.05 GPa to 0.10 GPa from the biotite zone to the migmatic granite-gneiss field. The metamorphism of the orogenic belt may be described by the pressure-temperature-space-time model (PTst). In the main episode of orogeny, the deep heat flow and structural flow upsurged along a series of the centres of the regional thermodynamic anomalies, giving rise to the progressive metamorphism, granitization, and the differential uplift and the formation of TSG domes.     

The first data on U–Pb (SHRIMP) dating of zircon from metamorphic rocks of the crystalline base of the Taigonos-Paren Uplift (southern margins of the Omolon Massif)

U–Pb SHRIMP dating of zircons from metamorphic rocks of the crystalline basement of the Omolon-Taigonos area (northeastern margins of Asia) has been used to determine the periods of early endogenic activity of the crust. These periods correlate to the time of formation of the magmatic rock protolith (3.2–3.3 Ga), the stage of regional metamorphism (2.6–2.8 Ga), and the superimposed granitization. The resulting data can be used to reconstruct the history of development of the crust in the Precambrian.     

青海都兰东北部哈莉哈德山花岗片麻岩LA-ICP-MS锆石U-Pb年龄及构造意义

青海都兰东北部哈莉哈德山花岗片麻岩原岩为具有S-型花岗岩特征的酸性侵入岩。来自该片麻岩的20粒锆石的LA-ICP-MS U-Pb年龄测定表明,该片麻岩的原岩花岗岩岩浆于约950Ma侵位结晶,岩浆物源源区存在中—新元古代的物质成分。正确甄别哈莉哈德山花岗片麻岩的变质围岩是否隶属于全吉地块的达肯大坂岩群,是理解全吉地块与柴达木地块在新元古代时期构造演化关系的关键。     

青海都兰东北部哈莉哈德山花岗片麻岩LA-ICP-MS 锆石U-Pb年龄及构造意义

青海都兰东北部哈莉哈德山花岗片麻岩原岩为具有S-型花岗岩特征的酸性侵入岩。来自该片麻岩的20粒锆石的LA-ICP-MS U-Pb年龄测定表明,该片麻岩的原岩花岗岩岩浆于约950Ma侵位结晶,岩浆物源源区存在中—新元古代的物质成分。正确甄别哈莉哈德山花岗片麻岩的变质围岩是否隶属于全吉地块的达肯大坂岩群,是理解全吉地块与柴达木地块在新元古代时期构造演化关系的关键。     

辽东古元古代造山带花岗片麻岩穹窿:热造山带典型构造样式

中下地壳的底辟上升是地壳中物质运移和热传递的一种重要机制,由这种机制产生的一系列穹窿构造不仅为揭示区域构造环境和构造演化提供了重要的线索,而且还提供了一个了解地壳深部物质流动的窗口。辽东半岛的古元古代造山带内就发育有这样一套花岗片麻岩穹窿构造,为了深入理解这套花岗片麻岩穹窿的成因以及对造山带演化的影响,本文对其不同构造层次进行了详细的构造特点和变形演化研究。结果显示,典型的花岗片麻岩穹窿可分为三层结构:混合岩化的花岗岩内核、发育顺层韧性剪切带的幔部以及含大型构造透镜体的外壳。其中核部花岗岩塑性流动变形发育,并具有明显的交代现象。顺层韧性剪切带的变形环境由靠近岩体的角闪岩相到远离岩体的低绿片岩相,并且没有明显的退化变质特点,拉伸线理具有统一的NW-SE方向。根据年代学数据与区域构造分析,花岗片麻岩穹窿构造是在区域收缩体制下花岗岩底辟形成的产物,其出现标志着辽东古元古代造山带变为一个由垂向和横向对流作用为主导的热造山带。     

Post-collisional granite magmatism in the central Damaran (Pan-African) Orogenic Belt,western Namibia

The central Damaran (Pan-African) Orogenic Belt in western Namibia is dominated by elongated granite-gneiss domes, surrounded by Neoproterozoic Damaran cover as periclinal synclinoria, mainly composed of amphibolite-facies metasedimentary rocks. The domes consist of remnants of pre-Damaran gneissic basement and/or Damaran granitoids. There is a high strain zone along the margin between the granite-gneiss domes and the Damaran cover, interpreted in terms of a late extensional mid-crustal detachment, following oblique north-south collision of the Kalahari and Congo Cratons (ca 550 Ma). The present paper focusses on two granite-gneiss domes (Khan and Ida) in western-central Namibia, where a previously unrecognised generation of early post-collisional Damaran granitoids (ca 530 Ma old), have invaded the pre-Damaran basement and cross-cut the detachment zone. These granitoids are classified as syeno-monzogranites with peraluminous high-K calc-alkaline compositions and I/S-type signatures. Geochemical data suggest that the syeno-monzogranites were derived from a lower crustal source, tentatively ascribed to the pre-Damaran gneissic basement. The granitoids also show evidence for magma contamination through partial assimilation with dark, amphibolitic cover rocks. As a result of this hybridisation of the syeno-monzogranites, granitoids with more dioritic compositions were developed. The syeno-monzogranites were then cross-cut by Damaran pegmatitic leucogranites (ca 510 Ma). Further evidence of episodic post-collisional magmatic activity is provided by late-stage mineralisation, and by a ca 465 Ma (⁴⁰Ar-³⁹Ar) thermal overprint in the cover rocks. It is proposed that the post-collisional magmatic activity emanated from the endogenic part of a mid-crustal Damaran granitic batholith, whose cupola-like emplacement in dome structures coincided with regional uplift, and subsequent cooling before ca 465 Ma.     

Oxygen isotope studies of early Precambrian granitic rocks from the Giants Range batholith,northeastern Minnesota,U.S.A.

Oxygen isotope studies of granitic rocks from the 2.7 b.y.-old composite Giants Range batholith show that: (1) δ(O¹⁸)_quartz values of 9 to 10 permil characterize relatively uncontaminated Lower Precambrian, magmatic granodiorites and granites; (2) granitic rocks thought to have formed by static granitization have δ(O¹⁸)_quartz values that are 1 to 2 permil higher than magmatic granitic rocks; (3) satellite leucogranite bodies have values nearly identical to those of the main intrusive phases even where they transect O¹⁸-rich metasedimentary wall rocks; (4) oxygen isotopic interaction between the granitic melts and their O¹⁸-rich wall rocks was minimal; and (5) O¹⁸/O¹⁸ ratios of quartz grains in a metasomatic granite are largely inherited from the precursor rock, but during the progression — sedimentary parent → partially granitized parent → metasomatic granite → there is gradual decrease in δ(O¹⁸)_quartz by 1 to 2 permil.     

济阳坳陷埕东凸起基底岩石组合、原岩恢复及地质意义

济阳坳陷埕东凸起太古界基底分布广泛,岩石类型丰富。岩心及显微薄片观察表明,岩石组合以花岗片麻岩、二长片麻岩和黑云母斜长片麻岩等各类片麻岩和石英闪长岩、花岗闪长岩等古老岩浆岩为主,少量石英岩、变粒岩等变质表壳岩以及云煌细晶岩、钾长花岗岩等年轻侵入岩脉。岩石地球化学分析及区域对比揭示,各类片麻岩的原岩为中酸性侵入岩,属于TTG质片麻岩,而石英岩和变粒岩属于变质表壳岩范畴,原岩为沉积岩。基底岩石构造样式为片麻岩或侵入岩形成构造穹窿,规模较小的变质表壳岩零星分布在周边。盆地基底岩石组合、变质岩原岩恢复与构造样式分析表明,渤海湾盆地覆盖区基底性质与露头区相似,是华北克拉通变质基底重要组成部分。这些认识对弄清盆地基底潜山油藏储层特征与展布具有重要指导作用,为深入研究变质岩潜山内幕油藏储层演化规律提供了依据。     

Multiple,isotopically heterogeneous plagioclase populations in the Bushveld Complex suggest mush intrusion

The Bushveld Complex and other layered intrusions show significant initial isotopic heterogeneity, both between and within co-existing cumulate minerals. Various processes have been proposed to account for this, including (i) intrusion of variably contaminated crystal mushes from deeper staging chambers, (ii) blending of semi-consolidated crystal mushes as a result of subsidence during cooling, (iii) variable infiltration of contaminants into a partially solidified crystal mush, (iv) density-driven mixing of minerals from isotopically distinct magma pulses, (v) contamination of crystals at the roof of the intrusion and mechanical incorporation of such contaminated crystals into the lower crystallisation front as a result of gravitational instability at the upper crystallisation front, and (vi) late-stage metasomatic processes. In order to assess the likely process(es) responsible for initial isotopic heterogeneities within the Bushveld Complex, we analysed core and rim domains of 12 plagioclase crystals from the Main and Upper zones of the Bushveld Complex for their Sr-isotopic compositions. The data show the presence of multiple, isotopically heterogeneous populations of plagioclase occurring within the same rocks. The data presented here are best explained through the intrusion of variably contaminated crystal mushes derived from a sub-compartmentalized, sub-Bushveld staging chamber that underwent different degrees of contamination with crustal rocks of the Kaapvaal craton.     

An interface-condition substitution strategy for theoretical study of dissolution-timescale reactive infiltration instability in fluid-saturated porous rocks

A theoretical study of reactive infiltration instability is conducted on the dissolution timescale. In the present theoretical study, the transient behavior of a dissolution-timescale reactive infiltration system needs to be considered, so that the upstream region of the chemical dissolution front should be finite. In addition, the chemical dissolution front of finite thickness should be considered on the dissolution timescale. Owing to these different considerations, it is very difficult, even in some special cases, to derive the first-order perturbation solutions of the reactive infiltration system on the dissolution timescale. To overcome this difficulty, an interface-condition substitution strategy is proposed in this paper. The basic idea behind the proposed strategy is that although the first-order perturbation equations in the downstream region cannot be directly solved in a purely mathematical manner, they should hold at the planar reference front, which is the interface between the upstream region and the downstream region. This can lead to two new equations at the interface. The main advantage of using the proposed interface-condition substitution strategy is that through using the original interface conditions as a bridge, the perturbation solutions for the dimensionless acid concentration, dimensionless Darcy velocity, and their derivatives involved in the two new equations at the interface can be evaluated just by using the obtained analytical solutions in the upstream region. The proposed strategy has been successfully used to derive the dimensionless growth rate, which is the key issue associated with the theoretical study of dissolution-timescale reactive infiltration instability in fluid-saturated porous rocks.     

Granulites of the Kolpakovskaya Series in the sredinny range,Kamchatka: A myth or reality?

Geological, mineralogical, and geothermobarometric data testify that the regional metamorphism of the terrigenous protolith of the Kolpakovskaya Series, which composes the stratigraphic basement of the Kamchatka Median Crystalline Massif, corresponded to the kyanite mineral subfacies of the amphibolite facies at temperatures of 560–660°C and pressures of 5.9–6.9 kbar. This metamorphism predetermined wide kyanite development in high-Al garnet-biotite plagiogneisses. The younger granitization and migmatization of the plagiogneisses took place at a decrease in the pressure (depth), as follows from the textures of kyanite reaction replacement by andalusite in both the metamorphic rocks and the vein synmetamorphic granitoids and pegmatites. The temperature of the granitization and migmatization processes in the plagiogneisses was estimated at 620–650°C, and the pressure was evaluated at 1.9–3.0 kbar. Acid leaching that accompanied the granitization and migmatization processes resulted in the intense replacement of biotite by sillimanite (fibrolite) and, to a lesser degree, muscovite in the metamorphic and vein magmatic rocks. The highest temperature orthopyroxene-cordierite-biotite-orthoclase-plagioclase-quartz mineral assemblages were determined to have been formed in the Kolpakovskaya Series at a temperature of 830–840°C not by the regional metamorphism but in contact aureoles around gabbro-granitoid intrusions of the Lavkinskii intrusive complex of Oligocene-Miocene age in garnet-biotite and kyanite-garnet-biotite plagiogneisses of the amphibolite facies and cannot thus be regarded as evidence of an early granulite stage in the metamorphism of these rocks.     

Lower proterozoic metapelites in the northern Yenisei Range: Nature and age of the protolith and the behavior of material during collisional metamorphism

The variable P-T metamorphic conditions studied in the Fe-Al metapelites of the Karpinskii Range Formation are regarded as typical of collision-related metamorphism in the trans-Angara part of the Yenisei Range. Recently obtained geochronologic (SHRIMP-II U-Pb zircon dating) and geochemical data on the distribution of major and trace elements are used to reproduce the composition of the protolith, the facies conditions under which it was formed, the tectonic setting, and the age of the eroded rocks. The metapelites are determined to be redeposited and metamorphosed material of Precambrian kaolinite-type weathering crusts of predominantly kaolinite-illite-montmorillonite-quartz composition. The protolith of the rocks was formed via the erosion of Lower Proterozoic granite-gneiss complexes of the Siberian craton (dated mainly within the range of 1962–2043 Ma) and the subsequent accumulation of this material in a continent-marginal shallow-water basin in a humid climate and tectonically calm environment. These results are consistent with data of lithologic-facies analysis and geodynamic reconstructions of the Precambrian evolution of geological complexes in the Yenisei Range. Mass-transfer analysis with the use of the evaluated rock compositions and calculated chemical reactions indicates that the differences in the REE patterns of metapelites from distinct zones can be explained mostly by the chemical heterogeneity of the protolithic material and, to a lesser extent, by metamorphic reactions at a pressure increase.     

煤层气试井设计方法

注入压降试井可以获取煤层气储层的重要参数。注入的压力、排量以及注入和压降段的时间是关系到试井成功与否的关键因素。用实例说明了如何进行煤层气井的注入压降试井设计。      

Possible relation of resistance of rocks to granitization and to their equilibrium and thermodynamic anisotropism

Resistance or susceptibility of different rocks to granitization and ultra metamorphism may be related to the degree of their equilibrium and to their energetic (thermodynamic) anisotropism. All rocks may be divided accordingly into active and passive ones, depending on their behavior in granitizing environments. The active group (sedimentary, elastic, clayey, mixed, noncrystalline effusives) consists of energetically anisotropic ones as well as of rocks that failed to attain their chemical equilibrium by the time of their stabilization. Such rocks are able to release a part of their energy in a certain stage of their granitization. The passive group includes sedimentary rocks (carbonate, ferruginous), fully crystalline magmatic rocks, energetically isotropic homogeneous sandstones, and quartzites). Granitization of such rocks, is, among other things, an endothermal phenomenon. Basic and ultrabasic magmatic rocks are susceptible to granitization, provided they are interbedded with water-bearing sediments (including the passive ones); the resistance of such combinations to granitization is lower than the resistance of their constituents separately. Exothermal phenomena in granitization, such as release of energy from thermodynamically anisotropic bodies or of the accumulated solar energy (by crystalline substance) in reactions involving an uptake of liquid and gaseous phases, insufficient as they may be to assure a continuity of the granitization, may have a decisive effect upon the granitization-resistance of the rocks. — V.P. Sokoloff     

Heat flow in salt-dome basins of Eurasia: A comparative study

The geothermal fields in the Pericaspian, Pripyat, and North German basins are considered. These basins are characterized by widespread Upper Paleozoic evaporite sequences, which underwent halokinesis with the formation of salt domes and plugs owing to tectonic and gravity instability. Heat flow refraction occurs at the boundaries of the domes with country rocks due to the contrast in thermal conductivity of evaporites and terrigenous rocks between the domal zones. This is the main cause of heat flow variation in the lateral and vertical directions in the salt-dome basins. Close correlation between zones of elevated temperature in the sedimentary rocks and petroleum occurrences is confirmed by the results of 2D and 3D modeling of the geothermal field. The previously noted relations of oil and gas fields to the deep faults in the studied basins create prerequisites for consideration of the geothermal field as a genetic factor controlling the tectonic features and petroleum resources of the salt-dome basins.     

中国大陆显生宙俯冲型、碰撞型和复合型片麻岩穹窿(群)

片麻岩穹窿(gneiss dome)是中下地壳热动力学过程产生的、与岩浆作用(或混合岩化作用)密切相关的穹窿状构造。片麻岩穹窿大部分是地壳深层次变动的产物,在世界范围内几乎出露在所有的折返造山带中,反映了所在地区地壳的大幅度抬升。片麻岩穹窿核部主要是无或弱岩浆组构的花岗岩体和高级变质岩(例如混合岩),边部是具有岩浆组构的花岗片麻岩,幔部由来自地壳深部的高级片岩和片麻岩组成。片麻岩穹窿的形成经历从垂直上升的地壳流导致的岩浆上涌的挤压收缩机制到岩浆体侵位的顶部伸展机制的转化过程。根据片麻岩穹窿的岩石组合、组构特征、成因机制和大地构造背景以及片麻岩穹窿与地壳流关系的分析,结合中国大陆典型片麻岩穹窿的研究,提出中国大陆显生宙的片麻岩穹窿和片麻岩穹窿群可以划分为与大洋岩石圈板片俯冲增生与随后的折返造山相关的"俯冲型"片麻岩穹窿(群),如秦岭片麻岩穹窿;与陆陆碰撞折返造山有关的"碰撞型"片麻岩穹窿(群),如北喜马拉雅拉轨岗日片麻岩穹窿(群)和松潘甘孜雅江片麻岩穹窿(群);与俯冲和碰撞的叠合作用有关的"复合式"片麻岩穹窿(群),如帕米尔空喀山片麻岩穹窿和东冈底斯林芝片麻岩穹窿(群)。     

Ruby–sapphire–spinel mineralization in marble of the middle and southern Urals: Geology,mineralogy, and genesis

Ruby and spinel occurrences hosted in marble on the eastern slope of the Urals are considered. Ruby- and spinel-bearing marble is a specific rock in granite-gneiss complexes of the East Ural Megazone, which formed at the Late Paleozoic collision stage of the evolution of the Urals. Organogenic marine limestone is the protolith of the marble. No relict sedimentary bedding has been retained in the marble. The observed banding is a secondary phenomenon related to crystallization and is controlled by flow cleavage. Magnesian metasomatism of limestone with the formation of fine-grained dolomite enriched in Cr, V, Ti, Mn, Cu, Zn, Ga, and REE took place at the prograde stage of metamorphism. Dedolomitization of rocks with the formation of background calcite marble also developed at the prograde stage. Mg-calcite marble with spinel and ruby of the first type formed in the metamorphic fluid circulation zone. Magnesian metasomatism with the formation of bicarbonate marble with ruby, pink sapphire, and spinel of the second type developed at the early retrograde stage. The formation of mica-bearing mineralized zones with corundum and spinel of the third type controlled by cleavage fractures is related to the pneumatolytic–hydrothermal stage. The data on ruby-bearing marble in the Urals may be used for forecasting and prospecting of ruby and sapphire deposits hosted in marble worldwide.     

Orogeny, migmatites and leucogranites: A review

The type ofP-T-t path and availability of fluid (H₂O-rich metamorphic volatile phase or melt) are important variables in metamorphism. Collisional orogens are characterized by clockwiseP-T evolution, which means that in the core, where temperatures exceed the wet solidus for common crustal rocks, melt may be present throughout a significant portion of the evolution. Field observations of eroded orogens show that lower crust is migmatitic, and geophysical observations have been interpreted to suggest the presence of melt in active orogens. A consequence of these results is that orogenic collapse in mature orogens may be controlled by a partially-molten layer that decouples weak crust from subducting lithosphere, and such a weak layer may enable exhumation of deeply buried crust. Migmatites provide a record of melt segregation in partially molten crustal materials and syn-anatectic deformation under natural conditions. Grain boundary flow and intra-and inter-grain fracture flow are the principal grain scale melt flow mechanisms. Field observations of migmatites in ancient orogens show that leucosomes occur oriented in the metamorphic fabrics or are located in dilational sites. These observations are interpreted to suggest that melt segregation and extraction are syntectonic processes, and that melt migration pathways commonly relate to rock fabrics and structures. Thus, leucosomes in depleted migmatites record the remnant permeability network, but evolution of permeability networks and amplification of anomalies are poorly understood. Deformation of partially molten rocks is accommodated by melt-enhanced granular flow, and volumetric strain is accommodated by melt loss. Melt segregation and extraction may be cyclic or continuous, depending on the level of applied differential stress and rate of melt pressure buildup. During clockwiseP-T evolution, H₂O is transferred from protolith to melt as rocks cross dehydration melting reactions, and H₂O may be evolved above the solidus at lowP by crossing supra-solidus decompression-dehydration reactions if micas are still present in the depleted protolith. H₂O dissolved in melt is transported through the crust to be exsolved on crystallization. This recycled H₂O may promote wet melting at supra-solidus conditions and retrogression at subsolidus conditions. The common growth of ‘late’ muscovite over sillimanite in migmatite may be the result of this process, and influx of exogenous H₂O may not be necessary. However, in general, metasomatism in the evolution of the crust remains a contentious issue. Processes in the lower-most crust may be inferred from studies of xenolith suites brought to the surface in lavas. Based on geochemical data, we can use statistical methods and modeling to evaluate whether migmatites are sources or feeder zones for granites, or simply segregated melt that was stagnant in residue, and to compare xenoliths of inferred lower crust with exposed deep crust. Upper-crustal granites are a necessary complement to melt-depleted granulites common in the lower crust, but the role of mafic magma in crustal melting remains uncertain. Plutons occur at various depths above and below the brittle-to-viscous transition in the crust and have a variety of 3-D shapes that may vary systematically with depth. The switch from ascent to emplacement may be caused by amplification of instabilities within (permeability, magma flow rate) or surrounding (strength or state of stress) the ascent column, or by the ascending magma intersecting some discontinuity in the crust that enables horizontal magma emplacement followed by thickening during pluton inflation. Feedback relations between rates of pluton filling, magma ascent and melt extraction maintain compatibility among these processes.     

Ollo de Sapo Cambro‐Ordovician volcanics from the Central Iberian basement—A multiphase evolution

The Cambro‐Ordovician rhyodacitic to dacitic volcanics from the Central Iberian basement, currently known as Ollo de Sapo (toads eye), have been reported as a specific group of felsic porphyritic rocks with blue quartz and large phenocrysts of K‐feldspar, in a partly vitreous or fine‐grained matrix. Interpreted to form Cambro‐Ordovician volcanic domes, they are accompanied by tuffs, ignimbrites and products of reworking in a near‐surface environment. The coarse‐ to fine‐grained rocks exhibit rather large K‐feldspar phenocrysts, plagioclase and rounded blue quartz, representing former corroded phenocrysts. Their colouration indicates unmixing of TiO₂ at around 900°C during cooling from relatively high crystallisation temperatures, indicating their origin at hot lower crustal conditions. We propose at least a two‐step evolution (1) starting around 495 Ma in the lower crust of a collapsing cordillera, generating a phenocryst‐rich mush and adiabatic melting of the lower crustal protolith to produce the spectacular Ollo de Sapo porphyrites, before (2) magma ascent and crustal extension leading to a different thermal regime around 483 Ma.     

Protolith and deformation age of the Gneiss-Plate of Kartali in the southern East Uralian Zone

The southern East Uralian Zone consists of granite-gneiss complexes that are embedded in geological units with typical oceanic characteristics. These gneisses have been interpreted as parts of a microcontinent that collided during the Uralian orogeny. The gneiss-plate of Kartali forms the south eastern part of the gneiss mantle surrounding the Dzhabyk pluton. Its post-collisional protolith age of 327±4 Ma is inconsistent with the microcontinent model. The deformation of the gneisses took place in 290±4 Ma at the time of the intrusion of the Dzhabyk magmas. Granites and gneisses cooled and were exhumed together. Therefore, we interpret the gneiss complexes of the East Uralian Zone as marginal parts of the granitic batholiths that were deformed during the ascent and emplacement of the pluton. From Nd and Sr isotope constraints we conclude that the magma source of the gneiss protolith was an island arc. Since no evidence for old continental crust has been discovered in the East Uralian Zone, the Uralian orogeny can no longer be interpreted as a continent-island arc-microcontinent collision. Instead, the geochemical data presented within this paper indicate that the stacking and thrusting of island arc complexes played an important role in the Uralian orogeny.