Two subduction events in a polycyclic basement: Alpine and pre-Alpine high-pressure metamorphism in the Suretta nappe, Swiss Eastern Alps

This study is essentially based on coupling macrostructures, microstructures and metamorphic petrology in polymetamorphic mafic rocks from the Swiss Eastern Alps (Suretta nappe, Penninic domain). Petrographic criteria are used in conjunction with structural analysis and microprobe work to define crystallization/deformation relationships and to establish a relative but precise sequence of tectono-metamorphic events. A first eclogite facies overprint and related exhumation occurred before emplacement of late Palaeozoic intrusives. During the Alpine cycle, the Suretta nappe was part of the thinned European continental margin. The Tertiary burial due to subduction and collision is responsible for D1 ductile thrusting and blueschist facies metamorphism. Late deformation phases, related to exhumation, are responsible for the development of extensional structures under greenschist facies conditions. Quantitative metamorphic petrology based on Gibbs free energy minimization (DOMINO by de Capitani) gives a constraint on the P–T  conditions during the polymetamorphic and polycyclic evolution. The first high- P metamorphic event related to pre-Alpine structures occurred at c . 700  °C and at least 2.0  GPa. These conditions are compatible with pre-Alpine high- P re-equilibration already described in several Alpine units. The Alpine high- P metamorphism occurred under blueschist facies conditions at c . 400–450  °C and 1.0  GPa. Similar high- P , low- T  conditions have already been described in the Mesozoic and Permian rock types. The two high- P events are clearly related to two different geothermal regimes and geodynamic environments.     

造山带双层结构的厘定及意义

当前,增生型造山带和碰撞型造山带的研究均取得了丰富的成果和创新性认识。二者过渡期间常常发生陆壳俯冲。然而,该俯冲具有什么样的构造变形特点,并如何影响造山带演化过程,长期未受到足够的关注。基于此,文中选择曾发生了陆壳俯冲的两个新生代时期的造山带（中国台湾造山带和雅鲁藏布江造山带）和一个中生代时期的造山带（羌塘造山带）开展研究,以期阐明陆壳俯冲的独特构造变形特征以及和造山过程的交互作用。研究发现,陆壳俯冲常常在造山带形成双层结构,上部为一套由史密斯地层组成的逆冲叠瓦扇构造体系,下部为一套具“岩块–基质”结构特征的俯冲杂岩。双层结构的上下部分物质组成相似,均以斜坡相–海底扇相沉积为主,也有陆棚相沉积。因此,由于构造变形时间相近,双层结构应是由同一套被动陆缘物质俯冲形成的深浅不同的构造体系。研究认为,在陆壳俯冲过程中,早期的斜坡–海底扇俯冲是形成双层结构的主要因素。后续的陆棚俯冲则对碰撞作用的发生起到了主导作用,从而使应变逐渐向克拉通内部扩展,形成前陆褶皱–冲断带。随着碰撞作用的持续,双层结构常常遭到构造破坏,深部的俯冲杂岩因此得以剥露至浅表。因此,文章的研究强调了陆壳俯冲和深俯冲物质的折返在造山带演化中的重要意义。      

Very-low-temperature record of the subduction process: A review of worldwide lawsonite eclogites

Lawsonite eclogites preserve a record of very-low-temperature conditions in subduction zones. All occur at active margin settings, typically characterized by accretionary complexes lithologies and as tectonic blocks within serpentinite-matrix mélange. Peak lawsonite-eclogite facies mineral assemblages (garnet + omphacite + lawsonite + rutile) typically occur in prograde-zoned garnet porphyroblasts. Their matrix is commonly overprinted by higher-temperature epidote-bearing assemblages; greenschist- or amphibolite-facies conditions erase former lawsonite-eclogite relics. Various pseudomorphs after lawsonite occur, particularly in some blueschist/eclogite transitional facies rocks. Coesite-bearing lawsonite-eclogite xenoliths in kimberlitic pipes and lawsonite pseudomorphs in some relatively low-temperature ultrahigh-pressure eclogites are known. Using inclusion assemblages in garnet, lawsonite eclogites can be classified into two types: L-type, such as those from Guatemala and British Columbia, contain garnet porphyroblasts that grew only within the lawsonite stability field and E-type, such as from the Dominican Republic, record maximum temperature in the epidote-stability field.

Formation and preservation of lawsonite eclogites requires cold subduction to mantle depths and rapid exhumation. The earliest occurrences of lawsonite-eclogite facies mineral assemblages are Early Paleozoic in Spitsbergen and the New England fold belt of Australia; this suggests that since the Phanerozoic, secular cooling of Earth and subduction-zone thermal structures evolved the necessary high pressure/temperature conditions. Buoyancy of serpentinite and oblique convergence with a major strike-slip component may facilitate the exhumation of lawsonite eclogites from mantle depths.     

Geology and geodynamic evolution of the high-P/low-T Maksyutov Complex, southern Urals, Russia

The high-pressure/low-temperature Maksyutov Complex is situated in the southern Urals between the Silurian/Devonian Magnitogorsk island arc and the East European Platform. The elongated N-S-trending complex is made up of two contrasting tectono-metamorphic units. Unit 1 consists of a thick pile of Proterozoic clastic sediments suggested to represent the passive margin of the East European Platform. The overlying unit 2, composed of Paleozoic sediments, volcanic rocks, and a serpentinite mélange with rodingites, is interpreted as a remnant of the Uralian Paleo-ocean. Devonian eastward subduction of oceanic crust beneath the Magnitogorsk island arc resulted in an incipient blueschist-facies metamorphism of unit 2 indicated by lawsonite pseudomorphs in the rodingites. While unit 2 was accreted to the upper plate, subduction of the continental passive margin caused the high-pressure metamorphism of unit 1. Buoyancy-driven exhumation of unit 1 into the forearc region led to its juxtaposition with unit 2 along a retrograde top-to-the-ENE shear zone. Further exhumation of the Maksyutov Complex into its present tectonic position was accomplished by later shear zones that were active as normal faults and are exposed along the margins of the complex. At the western margin a top-to-the-west shear zone juxtaposed a low-grade remnant of a Paleozoic accretionary prism (Suvanyak Complex) above the Maksyutov Complex. Along the eastern margin a top-to-the-east shear zone and the brittle Main Uralian Normal Fault emplaced the Maksyutov Complex against the Magnitogorsk island arc in the hanging wall.     

High-pressure blueschists and eclogites in Hong'an: a framework for addressing the evolution of high- and ultrahigh-pressure rocks in central China

The Hong'an region in the Qinling–Dabie collisional zone in eastern China hosts a series of metamorphic rocks exposing a south-to-north distribution from blueschist/blueschist–greenschist, amphibolite, eclogite (kyanite free) and kyanite–eclogite to coesite–eclogite facies rocks that represent progressively deeper levels of the Mesozoic subduction–collision complex. The Hong'an area is interesting for three reasons: (1) it escaped the thermal and structural overprint imparted on much of the Dabie Mountains during Early Cretaceous intrusion of voluminous granites and granodiorites; (2) the high-pressure (HP) Hong'an eclogites are widely distributed, often preserve prograde crystallization histories and can be directly linked in time and space to the blueschist/blueschist–greenschist rocks exposed to the south; (3) the blueschist/blueschist–greenschist facies rocks are generally better exposed than their equivalents in the southeastern Dabie Mountains and offer some opportunity for simultaneous structural and metamorphic analysis. The Hong'an area HP rocks offer perhaps the closest approximation to a preserved snapshot of Mesozoic pressure–temperature (P–T) conditions attending early subduction–exhumation in the region, and are thus essential to generating a coherent picture of the dynamics attending both metamorphism and exposure of the coeval ultrahigh-pressure (UHP) rocks. The purpose of this contribution is twofold: (1) to document previously unpublished metamorphic and structural data characterizing these HP sequences and their relative continuity in Hong'an; (2) to incorporate these data with recent geochronologic, structural and paleomagnetic information in the context of protracted, late Paleozoic through Mesozoic subduction, collision and exhumation. Metamorphism and exhumation of some of the southern Hong'an HP sequences appear to have occurred concomitant with oceanic subduction immediately to the west, and thus may have preceded widespread continental subduction/collision. Moreover, all of the HP–UHP sequences in the region were exhumed before the end of collision between the Sino-Korean and Yangtze cratons at ca. 160 Ma. Exhumation of HP–UHP rocks both before and during continental plate collision is neither novel for central China nor for other HP–UHP zones, but is important to take into account when reconstructing the evolution of such orogens.     

Numerical modelling of the late stage of subduction zone transference after an accretion event

The resumption of subduction of an oceanic plate at a contact with a docked island (continental) margin is modelled numerically in 2D. The mechanical properties of the crustal and mantle rock masses are treated with visco‐plastic rheologies where the viscosities are described by a dynamic power law phenomenologically accounting for the transient texture evolution of the deforming rock masses. The study is limited to the extreme case of a fluid weakened lithosphere characterized by a uniform effective yield stress. At a constant push of 8 cm a^?1, the shear zones split the oceanic slab into blocks at the contact with the island. At a moderate yield stress (σ_Y < 80 MPa) and normal relaxation rate of viscosity, subduction resumes in less than ca 1 Ma. High σ_Y = 100 MPa leads to the island edge bending. A detached block of the island is subducted after the plunge of the oceanic plate reverses.     

Mineral isograds and metamorphic zones of the Anglesey blueschist belt, UK: implications for the metamorphic development of a Neoproterozoic subduction–accretion complex

The 560–550 Ma blueschists and associated rocks in Anglesey, UK were derived from a subduction–accretion complex. The blueschist unit is divided into three mineral zones by two newly mapped metamorphic isograds; zone I sub-greenschist facies, (crossite isograd), zone II blueschist facies, (barroisite isograd), zone III epidote-amphibolite facies. The zones and isograds dip gently to the east, and decrease in metamorphic grade from the central high-pressure zone III to lower grade zones II and I to the west and east. The P – T conditions estimated from zoned amphibole indicate an anticlockwise P – T path following adjustment to a cold geotherm. This path is well preserved in the compositional zoning of Na–Ca amphibole that have a core of barroisite surrounded by a rim of crossite, although this is only locally developed. The sense of subduction was to the east and exhumation to the west, as indicated by the metamorphic isograds. The symmetrical arrangement of the metamorphic zones with the deepest high-pressure rocks in the middle suggests an isoclinal antiformal structure that formed by wedge extrusion during exhumation in the subduction zone.     

Late Miocene to Early Pliocene blueschist from Taiwan and its exhumation via forearc extraction

Blueschist facies rocks in the Yuli Belt of Taiwan's Central Range record ongoing subduction of the Eurasian plate. We present a prograde Lu–Hf garnet–whole‐rock age of 5.1 ± 1.7 Ma from a retrogressed blueschist in the Yuli Belt. This age is considerably younger than the previously assumed age of 14–8 Ma for high‐pressure metamorphism in the Yuli Belt and represents the youngest Lu–Hf garnet age ever recorded for blueschist facies metamorphism. The age sheds new light on the palaeogeographic origin and exhumation scenario of the Yuli Belt. We propose that the Yuli Belt originated from the ocean–continent boundary of the Chinese passive margin. It was subducted eastward during collision with the Luzon island arc and rapidly exhumed when the forearc lithosphere was removed from above the continental slab by discrete subduction (extraction). This process reduces the pressure above the continental slab and may prompt the ascent of subducted crust into the opening gap. Thus, it can control the exhumation of high‐pressure rocks.     

Late Cretaceous and Cenozoic tectonics of the Malay Peninsula constrained by thermochronology

New thermochronological analyses of granites from the Malay Peninsula record the region’s thermal history during the Late Mesozoic and Cenozoic. ⁴⁰Ar/³⁹Ar and (U–Th–Sm)/He analyses are combined with existing fission track data to provide a comprehensive set of temperature and time data. Fully and partially reset K-feldspar and biotite mica ⁴⁰Ar/³⁹Ar analyses indicate a significant period of thermal perturbation between ∼100 and ∼90 Ma, and a second lesser perturbation between ∼51 and ∼43 Ma. Zircon (U–Th–Sm)/He analyses and existing fission track data indicate exhumation of the Malay Peninsula in the Cretaceous, and renewed, localised exhumation in the early Paleogene. Apatite (U–Th–Sm)/He and fission track data indicate rapid exhumation of the region in the Late Eocene and Oligocene. Late Cretaceous tectonism is linked to the reversal of a regional dynamic topographic low following the cessation of subduction along the Sundaland margin in the Late Cretaceous, causing regional uplift and exhumation and the addition of significant heat into the crust via mantle upwelling. Early Paleogene exhumation may reflect the continuation of Cretaceous tectonism or a discrete phase of Paleocene exhumation linked to localised transpressional tectonics. Eocene tectonism is coincident with major subsidence offshore of the Malay Peninsula, interpreted to reflect regional block faulting in response to north–south compression driven by the resumption of subduction along the southern margin of Sundaland in the Eocene.     

Origin and evolution of the Escambray Massif (Central Cuba): an example of HP/LT rocks exhumed during intraoceanic subduction

High‐pressure (HP) metabasites from the Sancti Spiritus dome (Escambray massif, Central Cuba) have been studied in order to better understand the origin and evolution of the Northern Caribbean boundary plate during the Cretaceous, in a global subduction context. Geochemical and petrological studies of these eclogites reveal two groups with contrasting origins and pre‐subduction metamorphic histories. Eclogites collected from exotic blocks within serpentinite (mélange zone) originated from a N‐MORB type protolith, do not record pre‐eclogitic metamorphic history. Conversely eclogites intercalated in Jurassic metasedimentary rocks (non‐mélange zone) have a calc‐alkaline arc‐like origin and yield evidence for a pre‐subduction metamorphic event in the amphibolite facies. However, all the studied Escambray eclogites underwent the same eclogitic peak (around 600 °C at 16 kbar), and followed a cold thermal gradient during their exhumation (estimated at around 13.5 °C km^?1), which can suggest that this exhumation was coeval with subduction. Concordant geochronological data (Rb/Sr and Ar/Ar) support that the main exhumation of HP/LT rocks from the Sancti Spiritus dome occurred at 70 Ma by top to SW thrusting. The retrograde trajectory of these rocks suggests that the north‐east subduction of the Farallon plate continued after 70 Ma. The set‐off to the exhumation can be correlated with the beginning of the collision between the Bahamas platform and the Cretaceous island arc that induced a change of the subduction kinematics. The contrasting origin and ante‐subduction history of the analysed samples imply that the Escambray massif consists of different geological units that evolved in different environments before their amalgamation during exhumation to form the present unit III of the massif.     

Middle-Late Alpine thermotectonic evolution of the southern Rhodope Massif,Greece

The transition from the Alpine tectonic assembly to the exhumation of the units in the Rhodope metamorphic province in northernmost Greece has been refined by ⁴⁰Ar/³⁹Ar laserprobe mica analyses. Preservation of pre-Alpine (∼ 280 Ma and 145 Ma) muscovite cooling ages at the western margin of the Rhodope indicate that subsequent events failed to reset the argon system thermally in white mica in the outcropping basement of this region. The central and eastern Rhodope are characterized by white mica cooling ages of 40–35 Ma with ages gradually decreasing to ca. 15 Ma near the eastern margin of the Strymon Valley. The Eo-Oligocene ages reflect the regional exhumation of the metamorphosed units to shallow crustal levels, with corresponding temperatures below ca. 350 °C, by 40–35 Ma. The younger cooling ages are attributed to the initiation and subsequent operation of the Strymon-Thasos detachment system since ca. 30 Ma. This study provides a crucial contribution to future regional tectonic models for the Rhodope region as it recognizes an early stage of development of the Strymon-Thasos detachment system, and has constrained the regional exhumation of the Rhodope metamorphic province since 40 Ma indicating that the regionally observed amphibolite facies metamorphism had terminated by this time.     

A Comparison of Structures in the Andean Orogen of Northern Chile and Exhumed Midcrustal Structures in Southern California,USA: An Analogy in Tectonic Style?

The modern Andean Cordillera has proven to be a good modern analog for the Mesozoic and early Tertiary tectonic evolution of the US Cordillera, particularly for the transition between the Sevier and Laramide orogenies. A detailed version of this analogy, based on the tectonic evolution of the northern Chilean Andes, may explain the tectonic style of intra-arc exhumation and the southward migration of tectonism associated with arc extinction in southern California. Two regionally extensive episodes of deformation and exhumation are identified in southern California; the first occurred in an intra-arc setting in mid-Late Cretaceous time, and the second followed extinction of the magmatic arc and tectonic underplating by a blueschist/greenschist-grade metagraywacke terrane. We develop a model of Laramide oblique subduction of an aseismic oceanic ridge to explain these observations, based on modern subduction of the Juan Fernandez Ridge beneath the northern Chilean Andes. Laramide oblique ridge collision and consequent shallow subduction beneath southern California extinguished the magmatic arc and its intra-arc thrust belt and caused tectonic burial of the forearc beneath the extinct magmatic arc.     

Low temperature eclogite facies rocks discovered in the Eastern Himalayan Syntaxis: Poly-cyclic metamorphic evolution and implications

We report the first occurrence of poly-cyclic high-pressure low-temperature (HP-LT) rocks from the easternmost Indus-Yarlung suture zone, formed during subduction of Neo-Tethyan oceanic lithosphere. Petrology, mineral composition and P–T pseudosection modelling reveal two low-temperature eclogite facies metamorphic events with an initial high-pressure P–T condition of 16.4–18.7 kbar and 510–520°C, exhumation to 10.5–12.0 kbar and 580–590°C and a subsequent second high-pressure P–T condition of ~16 kbar and ~560°C and exhumation to ≤9 kbar and ≤600°C. This history implies a complex ‘yo-yo type’ P–T path. In situ monazite dating and textural relationships show that late-stage exhumation, cooling and garnet breakdown occurred at c. ~25–22 Ma. We interpret the first burial event to represent subduction of the Neo-Tethys Ocean at the eastern Indus-Yarlung suture zone. Initial exhumation, reburial and final exhumation represent material transport in a large-scale convective circulation system in the subduction channel. Convective overturn in the subduction channel evidently serves both as a mechanism to produce poly-cyclic metamorphism and to exhume LT eclogite facies rocks.     

Evolution of Alpine eclogites in the Eastern Alps: Implications for Alpine geodynamics

In the Eastern Alps Alpine eclogites are generally associated with rocks of continental lithosphere, while eclogites that are associated with oceanic assemblages are restricted to minor exposures. Such eclogites are exposed both in the Penninic unit of the Tauern Window and in the Austroalpine nappe complex. (1) In the central southern part of the Tauern Window (Eclogite Zone) eclogites and associated high pressure metasediments of a distal continental margin are intercalated between Penninic basement units. A mylonitic eclogitic foliation and stretching lineation are contemporaneous to the high pressure metamorphism and are related to the subduction of distal Penninic continental margin sequences. Continuous subduction of cool lithosphere resulted in blueschist facies overprint of the whole Penninic nappe pile. (2) Within the Middle-AustroAlpine Koralm/Saualm region most eclogites are eclogitic mylonites documenting plastic deformation of omphacite and garnet. The meso- and macroscale structures indicate an overall extensional regime possibly related to a large-scale SE-directed ductile low-angle normal shear zone. The eclogites are associated with migmatite-like structures and are intruded by pegmatites. This indicates decreasing pressure, but isothermal or even increasing temperature conditions during exhumation.These relationships argue for the subduction of Penninic continental lithosphere in the foot-wall of the Austroalpine unit at the time of exhumation of the Koralm/Saualm eclogites. Formation of the Austroalpine eclogites is explained by subduction of continental lithosphere, and subsequent, rapid exhumation in an upper plate tectonic position within an extensional regime.     

A synthesis of the geologic evolution of Taiwan

The island arc of Taiwan is composed of Cenozoic geosynclinal sediments more than 10,000 m thick, lying on a pre-Tertiary metamorphic basement. Pleistocene to Miocene andesitic islands surround the main island and are related mostly to arc magmatism. The Penghu Island Group in the Taiwan Strait is covered with Pleistocene flood basalt. Neogene shallow marine clastic sediments are exposed mainly in the western foothills with Pleistocene andesitic extrusives at the northern tip and the northeastern offshore islands. A thick sequence of Paleogene to Miocene argillitic to slaty metaclastic rocks underlies the western Central Range and forms the immediate sedimentary cover on the pre-Tertiary metamorphic complex to the east, which represents an older Mesozoic arc-trench system. The Coastal Range in eastern Taiwan is a Neogene andesitic magmatic arc, including also a large variety of volcaniclastic and turbiditic sediments. Cenozoic Taiwan is the site of arc-continent collision where the Luzon arc on the Philippine Sea plate overrides the Chinese continental margin on the Eurasian plate. East and northeast of Taiwan, the polarity of subduction changes whereby the oceanic Philippine Sea plate is subducting beneath the Ryukyu arc system on the Eurasian plate. Continent-arc collision in Taiwan island is anomalous and may occur in a broad belt of deformation rather than along a well-defined plate boundary or subduction zone.     

Geochronological and petrological constraints on Palaeoproterozoic granulite facies metamorphism in southeastern margin of the North China Craton

In the southeastern margin of the North China Craton, high-pressure (HP) granulite facies meta-basic rocks exposed as bands or lenses in the Precambrian metamorphic basement (e.g. Bengbu) and as xenoliths in Mesozoic intrusions (e.g. Jiagou) are characterized by the assemblage garnet + clinopyroxene + plagioclase + quartz + rutile ± Ti-rich hornblende. Cathodoluminescence imaging and mineral inclusions reveal that most zircon from the three dated samples displays distinct core-mantle-rim structures. The cores show typical igneous zircon characteristics and give ages of 2.5–2.4 Ga, thus dating the protolith of the metabasites. The mantles formed at granulite facies conditions as evidenced by inclusions of the HP granulite mineral assemblage garnet + clinopyroxene + rutile + plagioclase + quartz ± hornblende and Ti-rich biotite and yield ages of 1839 ± 31, 1811 ± 19 and 1800 ± 15 Ma. An inclusion-free rim yields an age of 176 ± 2 Ma with the lower Th/U ratio of 0.02. The geochronological and preliminary petrological data of this study suggest that the lower crust beneath the southeastern margin of the North China Craton formed at 2.5–2.4 Ga and underwent HP granulite facies metamorphism at c. 1.8 Ga. This HT-HP metamorphic event may be ascribed to large-scale crustal heating and thickening related to mantle-derived magma underplating at the base of the lower crust, as evidenced by widespread extension, rifting and related mafic magma emplacement in the North China Craton during this period. The age of 176 ± 2 Ma most likely records the late amphibolite facies retrogression occurring during exhumation.     

Middle-Late Alpine thermotectonic evolution of the southern Rhodope Massif,Greece

Abstract

The transition from the Alpine tectonic assembly to the exhumation of the units in the Rhodope metamorphic province in northernmost Greece has been refined by ⁴⁰Ar/³⁹Ar laserprobe mica analyses. Preservation of pre-Alpine (~ 280 Ma and 145 Ma) muscovite cooling ages at the western margin of the Rhodope indicate that subsequent events failed to reset the argon system thermally in white mica in the outcropping basement of this region. The central and eastern Rhodope are characterized by white mica cooling ages of 40–35 Ma with ages gradually decreasing to ca. 15 Ma near the eastern margin of the Strymon Valley. The Eo-Oligocene ages reflect the regional exhumation of the metamorphosed units to shallow crustal levels, with corresponding temperatures below ca. 350 °C, by 40–35 Ma. The younger cooling ages are attributed to the initiation and subsequent operation of the Strymon-Thasos detachment system since ca. 30 Ma. This study provides a crucial contribution to future regional tectonic models for the Rhodope region as it recognizes an early stage of development of the Strymon-Thasos detachment system, and has constrained the regional exhumation of the Rhodope metamorphic province since 40 Ma indicating that the regionally observed amphibolite facies metamorphism had terminated by this time. © 2000 Editions scientifiques et médicales Elsevier SAS     

苏鲁地体折返与郯庐断裂活动:莱阳盆地中生界碎屑锆石年代学的制约

胶莱盆地位于苏鲁造山带的北缘,其莱阳群沉积对反演郯庐断裂和苏鲁造山带中生代的折返过程具有重要的制约作用。本文利用LA-ICP-MS方法对莱阳盆地的莱阳群碎屑岩和青山群火山岩进行锆石U-Pb年龄测定,分析其年龄谱特征,探讨其沉积物源区,进而为苏鲁造山带的折返机制提供依据。(1)莱阳盆地发育莱阳群和青山群为代表的中生代沉积岩,锆石年龄测试得到莱阳群形成时代为125±0.6Ma;青山群形成时代为119±1Ma,表明两者都是早白垩世中-晚期的沉积产物;(2)超高压带北缘莱阳盆地与合肥盆地的碎屑锆石年龄谱对比表明,莱阳盆地的沉积物无论沉积时代还是物源区均明显不同与郯庐断裂西侧的合肥盆地,表明郯庐断裂应该形成于两个盆地形成之前,可能为三叠纪-早侏罗世之间;(3)莱阳盆地内发育大量的华北型碎屑物质,进一步表明在扬子大陆板块俯冲过程中华北板块曾经仰冲到扬子板块之上;(4)莱阳盆地发育少量的新元古代岩浆和印支期变质锆石的年龄,表明早白垩世苏鲁超高压变质岩已经折返到地表;(5)超高压变质岩与中生代岩浆岩同时作为莱阳盆地的物源,结合五莲拆离断层的同期活动和莱阳盆地的同时代沉积,说明苏鲁超高压带中生代的折返具有与变质核杂岩类似的大型伸展构造背景。     

浅谈前寒武纪条带状铁建造的沉积-变质成矿过程

条带状铁建造(BIF)是3.5～1.8Ga前陆架和洋盆的常见沉积物.前寒武纪条带状铁建造构成了世界上重要的铁矿资源.虽然它们成矿过程及其演化的许多方面的问题仍未解决,但人们普遍认为,它们沉积方式的长期变化与地球的环境和地球化学演化有关.条带状铁建造记录了前寒武纪古海洋、古环境、大气条件和细菌代谢条件以及铁的来源和沉积过...     

北祁连榴辉岩相变沉积岩的特征及其构造意义

在北祁连造山带中,出露典型的高压/低温变质岩石,前人对其中的低温榴辉岩已做过较多的研究,但对其中的变沉积岩研究涉及很少.本文展示了榴辉岩相变质沉积岩的岩石学、地球化学、锆石U-Pb年代学和Hf同位素方面的一些新的研究结果.变沉积岩含有榴辉岩相的矿物组合,峰期温压条件为t= 450～520℃,p=1.9～2.3 GPa,与相邻榴辉岩的温压条件一致.地球化学显示这些岩石的原岩为不成熟的沉积岩,可能形成于大陆边缘或大陆岛弧环境.变沉积岩中的碎屑锆石U-Pb年龄主要集中在1800 Ma左右和540～600 Ma之间,结合锆石Hf同位素特征,表明其原岩的碎屑来源既有周缘陆块的前寒武纪变质基底物质,又有新元古代-早古生代新生洋壳或增生物质.同时,这些数据也表明北祁连早古生代洋壳俯冲过程中发生了活动大陆边缘的构造剥蚀作用,即形成于上盘的沉积物(弧前盆地或增生楔)被构造作用运移到俯冲带中,并俯冲到60～70km深处,遭受榴辉岩相变质作用,然后折返到地表.