Hot orogens and supercontinent amalgamation: A Gondwanan example from southern India

The Southern Granulite Terrane in southern India preserves evidence for regional-scale high to ultrahigh temperature metamorphism related to the amalgamation of the supercontinent Gondwana. Here we present accessory mineral (zircon and monazite) geochronological and geochemical datasets linked to the petrological evolution of the rocks as determined by phase equilibria modelling. The results constrain the duration of high to ultrahigh temperature (> 900 °C) metamorphism in the Madurai Block to be c. 40 Ma with peak conditions achieved c. 60 Ma after the formation of an orogenic plateau related to the collision of the microcontinent Azania with East Africa at c. 610 Ma. A 1D numerical model demonstrates that the attainment of temperatures > 900 °C requires that the crust be moderately enriched in heat producing elements and that the duration of the orogenic event is sufficiently long to allow conductive heating through radioactive decay. Both of these conditions are met by the available data for the Madurai Block. Our results constrain the length of time it takes for the crust to evolve from collision to peak P–T (i.e. the prograde heating phase) then back to the solidus during retrogression. This evolution illustrates that not all metamorphic ages date sutures.     

增生型造山带的基本特征

增生型造山带是20世纪90年代新识别出来的一种造山带类型.这类造山带在中国有广泛的分布.增生型造山带的基本特征是①具有很宽的增生楔,增生楔中的复理石基质向着海沟后退方向时代逐渐变新;②增生楔中有多条蛇绿岩带,是海沟后退到适宜的构造位置时沿滑脱断层就位形成的;③增生型造山带中有多条钙碱性火山岩和花岗岩带,其生成时代也向着海沟后退方向变新;④增生地体内含有海山、大洋岛和大洋台地的构造碎块,使增生型造山带复杂化;⑤增生型造山带中具有多条韧性剪切带,可能是蛇绿岩构造就位的滑脱带;⑥增生型造山带含有大型、超大型铜、金和多金属矿床.增生型造山带尚有许多有待解决的基本问题,中国的增生型造山带分布广泛、规模巨大,是研究和解决这些问题的最佳地区.     

Ocean, continents and orogens

In this paper, the processes related to subduction and mountain building are discussed, and some new models and notions are proposed.At all known epochs, the Earth's surface comprised essentially migrating plates and large belts (of the order of 10,000 × 2000 km) where the lithosphere is mobilised, so that subductions and crustal resorption occur in complex structural patterns. Through time, these orogens start as island arcs and evolve into folded ranges.The formation and location of island-arc belts is shown to be related to the obliquity between rifts and continental margins i.e. to lateral relative movements of plates: torsion couples are developed bringing arcs to bulge through the inducted arc mechanism. This accounts for tensions prevailing in the back-arc basin while compressive stresses accumulate as vertical deformations in the arc-trench or uplift—subsidence couple. When the rupture limit is reached, a tangential tectonic phase occurs.It is suggested that the energy output (heat flow) varies with the pressure exerted by the top (elastic) lithosphere upon the underlying mantle. A compensation tends to be established between the inner flow and tectonic stress, as the latter brings about pressure variations through uplift-subsidence couples. These may therefore be related to the generation of paired metamorphic belts.The evolution from island arcs to completed folded ranges is briefly described, with special attention being paid to ophiolitic and crystalline basement nappes, as well as to strike-slip faults, which are the final expression of lateral relative movements.     

P-T-t paths of collisional orogens

Pressure-temperature-time (P-T-t) paths summarize the changes in pressure and temperature imposed on a metamorphic rock during orogenesis. They provide a convenient framework for the interpretation of complex metamorphic histories and also offer insight into the thermal and tectonic factors controlling metamorphism in collisional orogens. P-T-t data are acquired through a combination of textural observations, thermobarometry, and thermochronometry, and assembled into a P-T-t path using geological constraints. One-dimensional P-T-t models, assuming instantaneous deformation and thermal relaxation by conduction, are flexible and useful for testing tectonic models, particularly where geochronological constraints are available. Two-dimensional models allow more sophisticated deformation geometries and allow the effects of advection to be incorporated. Analysis of collisional orogens in terms of critical wedge theory can yield P-T-t paths that reflect coupling between thickening, uplift, exhumation, erosion, and convergence. Where rates of erosion approach rates of tectonic uplift, as is currently happening in the Southern Alps of New Zealand, high-grade metamorphic rocks can be exhumed rapidly from considerable depth. Alternatively, rapid exhumation may reflect gravity-driven extension in an over-steepened or thermally weakened orogen.

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Zusammenfassung Druck-Temperatur-Zeit (P-T-t) Pfade fassen die Änderungen in Druck und Temperatur zusammen, die den metamorphen Gesteinen während der Orogenese aufgeprägt wurden. Sie liefern einen passenden Rahmen für die thermischen und tektonischen Faktoren die die Metamorphose in Kollisionsorogenen steuern. P-T-t Daten werden erworben durch eine Kombination von textureilen Beobachtungen, Thermobarometrie, Thermochronologie, und werden in einem P-T-t Pfad unter Berücksichtigung von geologischen Grenzen zusammengefaßt. 1-dimensionale P-T-t Modelle die gleichzeitige Deformation voraussetzen sind flexibel und hilfreich um tektonische Modelle zu testen, insbesondere dort, wo geochronologische Grenzen zur Verfügung stehen. 2-dimensionale Modelle gestatten raffiniertere Deformationsgeometrien, und erlauben es die Advektionsprozesse zu integrieren. Analysen von Kollisionsorogenen im Sinne der kritischen Grenzwinkeltheorie kann P-T-t Pfade liefern, die eine Kopplung zwischen Verdickung, Heraushebung, Freilegung, Erosion und Konvergenz wiederspiegeln. Wo sich die Erosionsraten den Heraushebungsraten annähern, wie es zur Zeit in den südlichen Alpen Neuseelands passiert, können hochgradig metamorphe Gesteine aus erheblichen Tiefen freigelegt werden. Andererseits kann schnelle Freilegung auch gravitationsgesteuerte Ausdehnung in einem übersteilten oder thermisch instabilen Orogen widerspiegeln.

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Résumé Les trajets pression-température-temps (P-T-t) représentent les changements de pression et de température subis par les roches métamorphiques au cours d'une orogenèse. Ils fournissent un cadre adéquat à l'interprétation d'histoires métamorphiques complexes en même temps qu'ils éclairent les facteurs thermiques et tectoniques qui régissent le métamorphisme dans les orogènes de collision. Les données (P, T, t), obtenues par combinaison des observations structurales, de la thermo-barométrie et de la thermochronométrie, sont assemblées dans un trajet P-T-t qui tient compte des contraintes géologiques. Des modèles P-T-t, à une dimension, qui supposent une déformation instantanée et une relaxation thermique par conduction, présentent une certaine souplesse et sont utiles lorsqu'il s'agit de tester des modèles tectoniques, en particulier si on dispose de contraintes géochronologiques. Les modèles à 2 dimensions autorisent des geometries déformatives plus sophistiquées et permettent d'incorporer les effets d'advection. L'analyse d'orogènes de collision avec mise en æuvre de la théorie du coin critique, peut fournir des trajets P-T-t qui reflètent les connexions existant entre l'épaississement crustal, le soulèvement, l'exhumation, l'érosion et la convergence. Lorsque la vitesse d'érosion avoisine celle du soulèvement tectonique, comme c'est couramment le cas dans les Alpes du Sud de Nouvelle Zélande, des roches de degré métamorphique élevé peuvent être exhumées rapidement à partir de profondeurs considérables. Alternativement, une exhumation rapide peut traduire une extension à contrôle gravifique dans un orogène très redressé ou thermiquement instable.

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Granitoid magmatism of Early Paleozoic orogens

Global manifestations of Early Paleozoic granite formation in the Central Asian Mobile Belt and some other orogenic areas worldwide are considered. The work is based on the author’s studies of Early Paleozoic granitoids from the Altai–Sayan and West Transbaikalian orogenic areas as well as abundant literature data on other world provinces. Special attention is paid to the scales of granitoid magmatism in the Early Paleozoic, its geodynamic settings, periods, and stages, compositional evolution over time, lateral variability in structures of different types, relationship with LIPs, and, correspondingly, connection with mantle plumes and superplumes..     

Recognizing soft-sediment structures in deformed rocks of orogens

Soft-sediment deformation structures are common on passive continental margins, in trenches at subduction zones, and in strike-slip environments. Rocks from all these tectonic environments are incorporated into orogens, where soft-sediment deformation structures should be common. However, recognizing soft-sediment structures is difficult where superimposed tectonic structures are present. In seeking characteristic features of soft-sediment deformation, it is important to separate questions that relate to physical state (lithified or unlithified) from those that address the overall kinematic style (rooted or gravity driven). One recognizable physical state is liquefaction, which produces sand that has much lower strength than interbedded mud. Hence structures which indicate that mud was stronger than adjacent sand at the time of deformation can be used as indicators of soft-sediment deformation. These include angular fragments of mud surrounded by sand, dykes of sand cutting mud, and most usefully, folded sandstone layers displaying class 3 geometry interbedded with mud layers that show class 1 geometry. All these geometries have the potential to survive overprinting by later superimposed tectonic deformation; when preserved in deformed sedimentary rocks at low metamorphic grade they are indicators of liquefaction of unlithified sediment during deformation.     

碰撞带热结构与碰撞成矿系统

造山带热结构对大陆碰撞带的形态大小、构造式样、岩浆活动和变质作用具有重要控制作用。然而,热结构对碰撞成矿作用的控制还不清楚。本文概述比利牛斯、阿尔卑斯、加里东、扎格罗斯、青藏高原和华力西等全球主要碰撞带的热结构与成矿系统发育特征,对比各个造山带内不同矿床类型成矿温度变化,探讨热结构对碰撞成矿的控制作用。研究表明,碰撞带主要发育盆地流体有关的密西西比河谷型铅锌矿床、变质流体有关的造山型金矿床和岩浆热液有关矿床(斑岩铜矿床、云英岩型钨锡矿床和岩浆热液有关的铌钽锂铍矿床等)。其中,前两者在大多数碰撞带内均有发育,代表了大陆碰撞成矿作用的基本类型。这些矿床的成矿温度在热碰撞带比较高而在冷碰撞带则偏低。岩浆热液有关矿床一般只出现在比较热的碰撞带内,这些热碰撞带的温度压力条件有很大区域在湿固相线以内,热扰动能够造就地壳发生部分熔融形成含矿岩浆。     

Early Paleozoic accretionary orogens along the Western Gondwana margin

Early Paleozoic accretionary orogens dominated the Western Gondwana margin and were characterized by nearly continuous subduction associated with crustal extension and back-arc basin development.The southwestern margin is represented by Famatinian and Pampean basement realms exposed in South America,both related to the protracted Paleozoic evolution of the Terra Australis Orogen,whereas the northwestern margin is mainly recorded in Cadomian domains of Europe and adjacent regions.However,no clear relationships between these regions were so far established.Based on a compilation and reevaluation of geological,paleomagnetic,petrological,geochronological and isotopic evidence,this contribution focuses on crustal-scale tectonic and geodynamic processes occurring in Western Gondwana accretionary orogens,aiming at disentangling their common Early Paleozoic evolution.Data show that accretionary orogens were dominated by high-temperature/lowpressure metamorphism and relatively high geothermal gradients,resulting from the development of extended/hyperextended margins and bulk transtensional deformation.In this sense,retreating-mode accretionary orogens characterized the Early Paleozoic Gondwana margin,though short-lived pulses of compression/transpression also occurred.The existence of retreating subduction zones favoured mantle-derived magmatism and mixing with relatively young(meta)sedimentary sources in a thin continental crust.Crustal reworking of previous forearc sequences due to trenchward arc migration thus took place through assimilation and anatexis in the arc/back-arc regions.Therefore,retreating-mode accretionary orogens were the locus of Early Paleozoic crustal growth in Western Gondwana,intimately associated with major flare-up events,such as those related to the Cadomian and Famatian arcs.Slab roll back,probably resulting from decreasing convergence rates and plate velocities after Gondwana assembly,was a key factor for orogen-scale geodynamic processes.Coupled with synchronous oblique subduction and crustal-scale dextral deformation,slab roll back might trigger toroidal mantle flow,thus accounting for bulk dextral transtension,back-arc extension/transtension and a large-scale anticlockwise rotation of Gondwana mainland.     

碰撞造山带的碰撞事件时限的确定

在造山带研究中,引发变形作用和山脉隆升的造山作用的时代是一个重要问题。现在,Ｓｔｔｉｌｅ的造山幕术语已经被人们摈弃了,但是,迄今还有不少地质学家沿用诸如印支运动、燕山造山作用和喜马拉雅造山幕之类的术语。碰撞造山作用是一个在碰撞事件之后的均变过程。碰撞事件没有遗留下任何可以观察到的地质记录,因此,我们必须运用碰撞事件发生前和发生后产生的地质记录来限定碰撞事件的时代范围。在本文中,运用大洋岩石圈消减过程中的沉积作用、岩浆作用和变质作用来限定碰撞下限,运用碰撞后同造山时期的岩浆作用和变质作用以及磨拉石沉积作用来作为碰撞事件的时代上限。所有这些时代标志都是依据地质观察提出的,更多的精确的碰撞事件时代标志还有待于将来进一步的研究     

Spatial and temporal radiogenic isotopic trends of magmatism in Cordilleran orogens

An intrinsic feature of Cordillera-style orogenic systems is a spatial trend in the radiogenic isotopic composition of subduction-related magmatism. Magmatism is most isotopically juvenile near the trench and becomes increasingly evolved landward. A compilation of radiogenic isotopic data from the central Andes, U.S. Cordillera, and Tibet (the most well-studied examples of modern and ancient Cordilleran systems) demonstrate such spatial trends are long-lived and persist throughout the life of these continental subduction margins. The consistency of the isotopic trend through time in magmatic products is surprising considering the plethora of orogenic processes that might be expected to alter them. In addition to longevity, spatial isotopic trends encompass a broad spectrum of geochemical compositions that represent diverse petrogenetic and geodynamic processes. The two end-members of the spatial isotopic trends are represented by melts sourced within isotopically juvenile asthenospheric mantle and melts sourced from isotopically evolved continental lithospheric mantle and/or lower crust. Mantle lithosphere generally thins toward the magmatic arc and trench in Cordilleran orogens because sub-lithospheric processes such as delamination, subduction erosion, and subduction ablation, operate to thin or remove the continental mantle lithosphere. With time, magmatic additions may impart the isotopic composition of the mantle source on the lower crust, giving rise to an isotopically homogenous deep lithosphere. The results of this analysis have significant implications for interpreting temporal and spatial shifts in isotopic composition within Cordilleran orogens and suggest that the continental mantle lithosphere may be a significant source of magmatism in orogenic interiors.     

Effect of metamorphic reactions on thermal evolution in collisional orogens

The effects of metamorphic reactions on the thermal structure of a collisional overthrust setting are examined via forward numerical modelling. The 2D model is used to explore feedbacks between the thermal structure and exhumation history of a collisional terrane and the metamorphic reaction progress. The results for average values of crustal and mantle heat production in a model with metapelitic crust composition predict a 25–40 °C decrease in metamorphic peak temperatures due to dehydration reactions; the maximum difference between the P–T–t paths of reacting and non‐reacting rocks is 35–45 °C. The timing of the thermal peak is delayed by 2–4 Myr, whereas pressure at peak temperature conditions is decreased by more than 0.2 GPa. The changes in temperature and pressure caused by reaction may lead to considerable differences in prograde reaction pathways; the consumption of heat during dehydration may produce greenschist facies mineral assemblages in rocks that would have otherwise attained amphibolite facies conditions in the absence of reaction enthalpy. The above effects, although significant, are produced by relatively limited metamorphic reaction which liberates only half of the water available for dehydration over the lifetime of the prograde metamorphism. The limited reaction is due to the lack of heat in a model with the average thermal structure and relatively fast erosion, a common outcome in the numerical modelling of Barrovian metamorphism. This problem is typically resolved by invoking additional heat sources, such as high radiogenic heat production, elevated mantle heating or magmatism. Several models are tested that incorporate additional radiogenic heat sources; the elevated heating rates lead to stronger reaction and correspondingly larger thermal effects of metamorphism. The drop in peak temperatures may exceed 45 °C, the maximum temperature differences between the reacting and non‐reacting P–T–t paths may reach 60 °C, and pressure at peak temperature conditions is decreased by more than 0.2 GPa. Field observations suggest that devolatilization of metacarbonate rocks can also exert controls on metamorphic temperatures. Enthalpies were calculated for the reaction progress recorded by metacarbonate rocks in Vermont, and were used in models that include a layer of mixed metapelite–metacarbonate composition. A model with the average thermal structure and erosion rate of 1 mm year^?1 can provide only half of the heat required to drive decarbonation reactions in a 10 km thick mid‐crustal layer containing 50 wt% of metacarbonate rock. Models with elevated heating rates, on the other hand, facilitated intensive devolatilization of the metacarbonate‐bearing layer. The reactions resulted in considerable changes in the model P–T–t paths and ～60 °C drop in metamorphic peak temperatures. Our results suggest that metamorphic reactions can play an important role in the thermal evolution of collisional settings and are likely to noticeably affect metamorphic P–T–t paths, peak metamorphic conditions and crustal geotherms. Decarbonation reactions in metacarbonate rocks may lead to even larger effects than those observed for metapelitic rocks. Endothermic effects of prograde reactions may be especially important in collisional settings containing additional heat sources and thus may pose further challenges for the ‘missing heat’ problem of Barrovian metamorphism.     

Deformation localization in orogens: Spatiotemporal expression and thermodynamic constraint

Orogens are spatiotemporal expressions of instabilities in materials under load, constrained by thermodynamics, and preserved in the cold outer shell of the planet. Their pressure–temperature–time histories are consistent with the predictions of differential grade-2 (DG-2) materials in pure shear. We place the statistically invariant shear localization mechanism of these materials in a coherent thermodynamic context using an analysis of strained elastic materials. This prototype system exhibits non-classical thermodynamic symmetry-breaking, where the potentials are all functions of a single variable and the distinction between heat and work fades from view. Consequently, internal energy must be described by a monotonically decreasing function of the entropy in order for heat capacity and absolute temperature to be positive. The entropy itself exhibits an inverse dependence on length. These constraints are satisfied by the overall shape and slope of the distributed deformation threshold ψ^D for DG-2 materials, and its noted 1/length correlation with naturally observed folds as a function of thermomechanical competence κ/χ. We predict that temperature in this non-linear elastic material will vary in proportion to the slope of ψ^D, being high at low competence, and low at high competence. Similar constraints apply to a self-gravitating body, where the energy function varies inversely with radius. Assigning zero pressure at the surface of the body, we also predict that pressure, the tensor trace of its stress–energy density, will vary inversely with radius. Thus, the body force of gravity will be expressed in this elastic self-gravitating system through the interplay of elastic and thermal lengths. Deformation localization in DG-2 materials arises due to the dynamic rescaling of lengths in response to a spike in the intrinsic energy ψ^I at κ/χ = ½. While the intrinsic ψ^I and localization ψ^L thresholds are monotonically decreasing for κ/χ > ½, they exhibit positive slopes at lower competence, signaling a return to classical thermodynamics and Joule heating in this transitional domain. Numerous structural and tectonic observations can be correlated using this remarkably simple model, beginning with the thickness and mechanical character of the brittle crust and oceanic lithosphere. In effect, this model projects the kinematic theory of plate tectonics into four-dimensional spacetime.     

Some remarks on high-temperature—low-pressure metamorphism in convergent orogens

Many high-temperature–low-pressure (high- T –low- P ) metamorphic terranes show evidence for peak mineral growth during crustal thickening strain increments at pressures near the maximum attained during the heating–cooling cycle. Such terranes are not readily explained as the conductive response to crustal thickening since the resulting Moho temperatures would greatly exceed the crustal liquidus and because heating due to conductive equilibration on length scales appropriate to lithospheric-scale strains must greatly outlast the deformation. Consequently, high- T –low- P metamorphism may be generated during crustal thickening only when significant heat is advected within the crust, as for example may occur during the segregation of granitic melts. We show that without the addition of asthenospheric melts and at strain rates appropriate to continental deformation the conditions required for significant lower crustal melting during deformation are only likely to be attained if heat flow into the lower crust during crustal thickening is increased substantially, for example, by removing the mantle part of the lithosphere. A simple parameterization of lithospheric deformation involving the vertical strain on the scale of the crust, _c, and the lithosphere, ₁ respectively, allows the potential energy of the evolving orogen to be readily evaluated. Using this parameterization we show that an important isostatic consequence of the deformation geometries capable of generating such high- T –low- P metamorphism during crustal thickening (with _c1) is an imposed upper limit to crustal thicknesses which is much lower than for homogeneous deformations (f_c= f₁) for the same initial lithospheric configuration.     

Proterozoic orogens in southern Peninsular India: Contiguities and complexities

The Precambrian terranes of southern Peninsular India have been central to discussions on the history of formation and breakup of supercontinents. Of particular interest are the Proterozoic high grade metamorphic orogens at the southern and eastern margins of the Indian shield, skirting the 3.4 Ga Dharwar craton which not only preserve important records of lower crustal processes and lithospheric geodynamics, but also carry imprints of the tectonic framework related to the assembly of the major Neoproterozoic supercontinents – Rodinia and Gondwana. These Proterozoic orogens are described as Southern Granulite Terrane (SGT) in the southern tip and the Eastern Ghats Mobile Belt (EGMB) in the eastern domains of the peninsula. The contiguity of these orogens is broken for a distance of ∼400 km and disappears in the Bay of Bengal. These orogens expose windows of middle to lower crust with well-preserved rock records displaying multiple tectonothermal events and multiphase exhumation paths.Recent studies in these orogens have led to the recognition of discrete crustal blocks or terranes separated by major shear zone systems, some of which represent collisional sutures. The SGT and EGMB carry several important features such as fold-thrust tectonics, regional granulite facies metamorphism of up to ultrahigh-temperature conditions in some cases, multiple P–T paths, development of lithospheric shear zones, emplacement of ophiolites, presence of alkaline and anorthositic complexes, development of crustal-scale “flower structures”, transpressional strains, and reactivation tectonics. A heterogeneous distribution of different metamorphic and magmatic assemblages with distinct spatial and temporal strain variations in shaping the fabric elements in different blocks is identified. Both EGMB and SGT share a common transpressional deformation history during the latest Neoproterozoic characterized by the steepening of the initial low angle crustal scale structures leading to a subvertical grain conducive to reactivation tectonics. Our synthesis of the spatial distribution, geometry, kinematics and the transpressional strain of the shear zone systems provides insights into the tectono-metamorphic history of the Proterozoic orogens of southern India and their contiguity and complexities. Recent understanding of subduction, accretion and collisional history along these zones together with a long lived transpressional tectonic regime imply that these orogens witnessed identical tectonic regimes at different times in Earth history, although the major and common structural architecture was built during the final assembly of the Gondwana supercontinent.     

Problems of geodynamics, tectonics, and metallogeny of orogens

This is an overview of papers published in the present volume of Russian Geology and Geophysics (Geologiya i Geofizika), a special issue that covers presentations at the International Conference “Geodynamic Evolution, Tectonics, and Metallogeny of Orogens”, held on 28–30 June 2010 in Novosibirsk (http://altay2010.igm.nsc.ru). The workshop concerned the general evolution of the Central Asian orogenic system, with a special focus on continental growth, history of oceans and continental margins, and role of plumes in accretionary-collisional tectonics and metallogeny. The discussed papers are grouped in three sections: 1. General issues of geodynamics and geodynamic evolution; 2. Role of mantle plumes in tectonics, magmatism, and metallogeny; 3. Regional tectonic and geodynamic problems of Asia.The synthesis of data reported at the workshop demonstrates critical importance of mantle plumes for the evolution of the Paleoasian ocean and for orogenic processes in Central Asia.In addition to three large pulses of continental growth at about 2900–2700, 1900–1700, and 900–700 Ma, three orogenic stages have been distinguished in the geological history of Eurasia: Late Cambrian–Ordovician (510–470 Ma), Late Devonian–Early Carboniferous (380–320 Ma), and Permian–Triassic (285–230 Ma). In the evolution of the Central Asian orogen, these stages were associated with events of ultramafic-mafic and bimodal plume magmatism which promoted translithospheric strike-slip faulting. Plume magmatism was an active agent in ocean opening when the Paleotethys, Ural, Ob–Zaisan, and Turkestan basins appeared while the Late Cambrian–Ordovician orogen was forming in Central Asia (North Kazakhstan, Altai–Sayan, Tuva, and Baikal areas). Closure of the Ob–Zaisan ocean and collision of the Kazakhstan–Baikal continent with Siberia in the Late Devonian–Early Carboniferous was coeval with the maximum opening of the Turkestan ocean, possibly, as a consequence of plume activity. The Tarim (285–275 Ma) and Siberian (250–230 Ma) superplume events corresponded in time to closure of the Ural ocean and opening of the Meso- and Neotethys, as well as to major metallogenic events.     

Metamorphic response in orogens of different obliquity,scale and geometry

Investigation of material flow within transpressional orogens must involve integration of structural and metamorphic datasets. To illustrate the problems in documenting flow vectors we present integrated structural-metamorphic datasets from two transpressional systems; the Kaoko Belt in Namibia and the Kalinjala Shear Zone in South Australia. These orogens experienced widely differing metamorphic responses to transpressional deformation. Integration of kinematic and metamorphic datasets from the Kaoko Belt indicate shallow up-plunging extrusion trajectories in the orogen core, and show that the maximum stretching direction pattern matches the inferred flow vectors. High-grade domains (800–840 °C and 7.0–8.0 kb) in the orogen core developed low-angle upward-verging maximum stretching direction trajectories, whereas a low-grade domain (575–600 °C and 5.0–5.5 kb) in the orogen core has downward-verging lineation trajectories. The barometric differential between these high-grade and low-grade domains is entirely consistent with the angle of plunge of maximum stretching directions within the high-grade domains that were extruded obliquely, for the amount of lateral shear estimated for the orogen core. The Kalinjala Shear Zone in South Australia contrasts strongly with the Kaoko Belt. In this example, the high-grade and high-strain shear zone core of the orogen, experienced high-T/high-P metamorphism with low thermal gradients of 21–26 °C/km and steep decompressive P–T paths. The lower-grade external domains experienced lower-T/lower-P metamorphism with high thermal gradients of 35–37 °C/km. Sub-horizontal maximum stretching directions do not match the vertical extrusional flow in the high-grade core that is indicated by the metamorphic data. This comparison shows that in general and on a gross scale, maximum stretching directions do not necessarily correlate with the real flow vectors experienced during orogenesis. In some cases maximum stretching direction recorded by deformation structures is to some degree decoupled from the vertical component of material flow. Consequently, information pertaining to flow is often partitioned into information derived from deformation structures and information derived from the metamorphic record. These two datasets must be used in concert to obtain realistic constraints on first-order material flow trajectories at orogenic scales. The horizontal component of flow is typically best recorded by structural fabrics (maximum stretching direction and sense of shear), whereas the vertical component is typically best recorded by metamorphic information, such as P–T paths, temperature over depth ratio (G) and metamorphic field gradients (i.e. ΔT, ΔP and ΔG) across the orogen.     

Interaction of metamorphism, deformation and exhumation in large convergent orogens

Coupled thermal‐mechanical models are used to investigate interactions between metamorphism, deformation and exhumation in large convergent orogens, and the implications of coupling and feedback between these processes for observed structural and metamorphic styles. The models involve subduction of suborogenic mantle lithosphere, large amounts of convergence (≥ 450 km) at 1 cm yr^?1, and a slope‐dependent erosion rate. The model crust is layered with respect to thermal and rheological properties — the upper crust (0–20 km) follows a wet quartzite flow law, with heat production of 2.0 μW m^?3, and the lower crust (20–35 km) follows a modified dry diabase flow law, with heat production of 0.75 μW m^?3. After 45 Myr, the model orogens develop crustal thicknesses of the order of 60 km, with lower crustal temperatures in excess of 700 °C. In some models, an additional increment of weakening is introduced so that the effective viscosity decreases to 10¹⁹ Pa.s at 700 °C in the upper crust and 900 °C in the lower crust. In these models, a narrow zone of outward channel flow develops at the base of the weak upper crustal layer where T≥600 °C. The channel flow zone is characterised by a reversal in velocity direction on the pro‐side of the system, and is driven by a depth‐dependent pressure gradient that is facilitated by the development of a temperature‐dependent low viscosity horizon in the mid‐crust. Different exhumation styles produce contrasting effects on models with channel flow zones. Post‐convergent crustal extension leads to thinning in the orogenic core and a corresponding zone of shortening and thrust‐related exhumation on the flanks. Velocities in the pro‐side channel flow zone are enhanced but the channel itself is not exhumed. In contrast, exhumation resulting from erosion that is focused on the pro‐side flank of the plateau leads to ‘ductile extrusion’ of the channel flow zone. The exhumed channel displays apparent normal‐sense offset at its upper boundary, reverse‐sense offset at its lower boundary, and an ‘inverted’ metamorphic sequence across the zone. The different styles of exhumation produce contrasting peak grade profiles across the model surfaces. However, P–T–t paths in both cases are loops where P_max precedes T_max, typical of regional metamorphism; individual paths are not diagnostic of either the thickening or the exhumation mechanism. Possible natural examples of the channel flow zones produced in these models include the Main Central Thrust zone of the Himalayas and the Muskoka domain of the western Grenville orogen.     

造山带中成对出现的高压麻粒岩与榴辉岩及其地球动力学意义

在一些典型碰撞造山带中,高压麻粒岩与榴辉岩在空间和时间上密切相关,它们之间的关系对揭示碰撞造山带的造山过程和造山机制具有重要意义.本文以中国西部的南阿尔金、柴北缘及中部的北秦岭造山带为例,详细陈述了这3个地区榴辉岩和相关的高压麻粒岩的野外关系、变质演化和形成时代,目的是要建立大陆碰撞造山带中榴辉岩和相关高压麻粒岩形成的地球动力学背景模式.南阿尔金榴辉岩呈近东西向分布在江尕勒萨依,玉石矿沟一带,与含夕线石副片麻岩、花岗质片麻岩和少量大理岩构成榴辉岩一片麻岩单元,榴辉岩中含有柯石英假象,其峰期变质条件为P=2.8～3.0GPa,T=730～850℃,并在抬升过程中经历了角闪岩-麻粒岩相的叠加;大量年代学研究显示其峰期变质时代为485～500Ma.南阿尔金高压麻粒岩分布在巴什瓦克地区,包括高压基性麻粒岩和高压长英质麻粒岩,它们与超基性岩构成了一个大约5km宽的构造岩石单元,与周围角闪岩相的片麻岩为韧性剪切带接触.长英质麻粒岩和基性麻粒岩的峰期组合均具有蓝晶石和三元长石(已变成条纹长石),形成的温压条件为T=930～1020℃,P=1.8～2.5GPa,并在退变质过程中经历了中压麻粒岩相变质作用叠加.锆石SHRIMP测定显示巴什瓦克高压麻粒岩的峰期变质时代为493～497Ma.都兰地区的榴辉岩分布柴北缘HP-UHP变质带的东端,在榴辉岩和围岩副片麻岩中均发现有柯石英保存,形成的峰期温压条件为T=670～730℃和P=2.7～3.25GPa,退变质阶段经过了角闪岩相的叠加;榴辉岩相变质时代为420～450Mao都兰地区的高压麻粒岩分布在阿尔茨托山西部,高压麻粒岩包括基性麻粒岩长英质麻粒岩,基性麻粒岩的峰期矿物组合为Grt+Cpx+Pl±Ky±Zo+Rt±Qtz,长英质麻粒岩的峰期矿物组合为：Grt+Kf+Ky+Pl+Qtz.峰期变质条件为T=800～925℃,P=1.4～1.85GPa,退变质阶段经历了角闪岩-绿片岩的改造,高压麻粒岩的变质时代为420～450Ma.北秦岭榴辉岩分布在官坡-双槐树一带,榴辉岩的峰期变质组合为Grt+Omp±Phe+Qtz+Rt,所计算的峰期温压条件为T=680～770℃和P=2.25～2.65GPa,年代学数据显示榴辉岩的变质时代为500Ma左右.北秦岭高压麻粒岩分布在含榴辉岩单元的南侧松树沟一带,包括高压基性麻粒岩和高压长英质麻粒岩,与超基性岩在空间上密切伴生,高压麻粒岩的峰期温压条件为T=850～925℃,P=1.45～1.80GPa,锆石U-Pb年代学研究显示其峰期变质时代为485～507Ma.以上三个实例显示,出现在同一造山带、在空间上伴生的高压麻粒岩和榴辉岩有各自不同的变质演化历史,但榴辉岩中的榴辉岩相变质时代和相邻的高压麻粒岩中的高压麻粒岩相变质作用时代相同或相近,这种成对出现的榴辉岩和高压麻粒岩代表了它们同时形成在造山带中不同的构造环境中,即榴辉岩的形成于大陆俯冲带中,而高压麻粒岩可能形成在俯冲带之上增厚的大陆地壳根部.     

碰撞造山带斑岩型矿床的深部约束机制

在印度-亚洲大陆碰撞过程中,俯冲板片断离触发了幔源岩浆底侵作用、下地壳部分熔融和冈底斯岩基带以及同岩基斑岩的产生.在此过程中,幔源岩浆分离结晶的产物、下地壳岩石部分熔融残余和地壳分异过程中下沉的镁铁质块体,构成了加厚下地壳.随着造山岩石圈的冷却和加厚下地壳重力不稳定性的增加,岩石圈拆沉作用触发了后碰撞斑岩型岩浆活动.与此相应,碰撞造山带斑岩型矿床可以形成于同碰撞和后碰撞两个不同的构造阶段.同碰撞成矿作用发生于岩基带形成时期,成矿物质主要来自于底侵幔源岩浆及更深部的含矿流体,其触发机制是俯冲板片的断离.后碰撞成矿作用发生于加厚下地壳冷却之后,成矿物质主要来自于新生矿源层和更深部的含矿流体,其触发机制为岩石圈拆沉作用.在同碰撞构造阶段,伴随着幔源岩浆的底侵作用,深部流体和幔源岩浆所含的成矿物质被注入到岩基岩浆中,与从岩基岩浆源区萃取的成矿物质汇聚在一起,一部分受岩基热的驱使上升成矿.由于流体中成矿元素的浓度强烈依赖于压力,另一部分成矿元素则滞留在难熔残余中形成新的矿源层.当发生岩石圈拆沉作用时,由此矿源层部分熔融形成的斑岩岩浆将相对富含成矿物质,导致碰撞造山带第二次成矿作用大爆发.