The role of partial melting and extensional strain rates in the development of metamorphic core complexes

Orogenic collapse involves extension and thinning of thick and hot (partially molten) crust, leading to the formation of metamorphic core complexes (MCC) that are commonly cored by migmatite domes. Two-dimensional thermo-mechanical Ellipsis models evaluate the parameters that likely control the formation and evolution of MCC: the nature and geometry of the heterogeneity that localizes MCC, the presence/absence of a partially molten layer in the lower crust, and the rate of extension. When the localizing heterogeneity is a normal fault in the upper crust, the migmatite core remains in the footwall of the fault, resulting in an asymmetric MCC; if the localizing heterogeneity is point like region within the upper crust, the MCC remains symmetric throughout its development. Therefore, asymmetrically located migmatite domes likely reflect the dip of the original normal fault system that generated the MCC. Modeling of a severe viscosity drop owing to the presence of a partially molten layer, compared to a crust with no melt, demonstrates that the presence of melt slightly enhances upward advection of material and heat. Our experiments show that, when associated with boundary-driven extension, far-field horizontal extension provides space for the domes. Therefore, the buoyancy of migmatite cores contributes little to the outer envelope of metamorphic core complexes, although it may play a significant role in the internal dynamics of the partially molten layer. The presence of melt also favors heterogeneous bulk pure shear of the dome as opposed to the bulk simple shear, which dominates in melt-absent experiments. Melt presence affects the shape of P-T-t paths only slightly for material located near the top of the low-viscosity layer but leads to more complex flow paths for material inside the layer. The effect of extension rate is significant: at high extension rate (cm yr^− 1 in the core complex region), partially molten crust crystallizes and cools along a high geothermal gradient (35 to 65 °C km^− 1); material remains partially molten in the dome during ascent. At low strain rate (mm yr^− 1 in the core complex region), the partially molten crust crystallizes at high pressure; this material is subsequently deformed in the solid-state along a cooler geothermal gradient (20 to 35 °C km^− 1) during ascent. Therefore, the models predict distinct crystallization versus exhumation histories of migmatite cores as a function of extensional strain rates. The Shuswap metamorphic core complex (British Columbia, Canada) exemplifies a metamorphic core complex in which an asymmetric, detachment-controlled migmatite dome records rapid exhumation and cooling likely related to faster rates of extension. In contrast the Ruby Mountain-East Humboldt Ranges (Nevada, U.S.A.) exhibits characteristics associated with slower metamorphic core complexes.     

The Ordovician Sierras Pampeanas–Puna basin connection: Basement thinning and basin formation in the Proto-Andean back-arc

The Ordovician Sierras Pampeanas, located in a continental back-arc position at the Proto-Andean margin of southwest Gondwana, experienced substantial mantle heat transfer during the Ordovician Famatina orogeny, converting Neoproterozoic and Early Cambrian metasediments to migmatites and granites. The high-grade metamorphic basement underwent intense extensional shearing during the Early and Middle Ordovician. Contemporaneously, up to 7000 m marine sediments were deposited in extensional back-arc basins covering the pre-Ordovician basement. Extensional Ordovician tectonics were more effective in mid- and lower crustal migmatites than in higher levels of the crust. At a depth of about 13 km the separating boundary between low-strain solid upper and high-strain lower migmatitic crust evolved to an intra-crustal detachment. The detachment zone varies in thickness but does not exceed about 500 m. The formation of anatectic melt at the metamorphic peak, and the resulting drop in shear strength, initiated extensional tectonics which continued along localized ductile shear zones until the migmatitic crust cooled to amphibolite facies P–T conditions. P–T–d–t data in combination with field evidence suggest significant (ca. 52%) crustal thinning below the detachment corresponding to a thinning factor of 2.1. Ductile thinning of the upper crust is estimated to be less than that of the lower crust and might range between 25% and 44%, constituting total crustal thinning factors of 1.7–2.0. While the migmatites experienced retrograde decompression during the Ordovician, rocks along and above the detachment show isobaric cooling. This suggests that the magnitude of upper crustal extension controls the amount of space created for sediments deposited at the surface. Upper crustal extension and thinning is compensated by newly deposited sediments, maintaining constant pressure at detachment level. Thinning of the migmatitic lower crust is compensated by elevation of the crust–mantle boundary. The degree of mechanical coupling between migmatitic lower and solid upper crust across the detachment zone is the main factor controlling upper crustal extension, basin formation, and sediment thickness in the back-arc basin. The initiation of crustal extension in the back-arc, however, crucially depends on the presence of anatectic melt in the middle and lower crust. Consumption of melt and cooling of the lower crust correlate with decreasing deposition rates in the sedimentary basins and decreasing rates of crustal extension.     

Constraining the mid-crustal channel flow beneath the Tibetan Plateau: data from the Nielaxiongbo gneiss dome,SE Tibet

Gneiss domes involving the South Tibetan Detachment System provide evidence for crustal extension simultaneous with shortening. The Nielaxiongbo gneiss dome is composed of a metamorphic complex of granitic gneiss, amphibolite, and migmatite; a ductilely deformed middle crustal layer of staurolite- or garnet-bearing schist; and a cover sequence of weakly metamorphosed Triassic and Lower Cretaceous strata. The middle crust ductilely deformed layer is separated from both the basement complex and the cover sequence by lower and upper detachments, respectively, with a smaller detachment fault occurring within the ductilely deformed layer. Leucogranites crosscut the basement complex, the lower detachment, and the middle crustal layer, but do not intrude the upper detachment or the cover sequence. Three deformational fabrics are recognized: a N–S compressional fabric (D₁) in the cover sequence, a north- and south-directed extensional fabric (D₂) in the upper detachment and lower tectonic units, and a deformation (D₃) related to the leucogranite intrusion. SHRIMP zircon U–Pb dating yielded a metamorphic age of ～514 million years for the amphibolite and a crystallization age of ～20 million years for the leucogranite. Hornblende from the amphibolite has an ⁴⁰Ar/³⁹Ar age of 18 ± 0.3 million years, whereas muscovites from the schist and leucogranite yielded ⁴⁰Ar/³⁹Ar ages between 13.5 ± 0.2 and 13.0 ± 0.2 million years. These results suggest that the basement was consolidated at ～510 Ma and then exhumed during extension and silicic plutonism at ～20 Ma. Continuing exhumation led to cooling through the 500°C Ar closure temperature in hornblende at ～18 Ma to the 350°C Ar closure temperature in muscovite at ～13 Ma. The middle crustal ductilely deformed layer within gneiss domes of southern Tibet defines a southward-extruding ductile channel, marked by leucogranites emplaced into migmatites and amphibolites. We propose a model involving thinned upper crust for the initial extension of the Tibetan Plateau in the early Miocene.     

Extensional Tectonic Framework of Post High and Ultrahigh Pressure Metamorphism in Dabieshan, China

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华北燕山带:构造、埃达克质岩浆活动与地壳演化(英文)

埃达克质火成岩在中国东部,包括燕山带是很常见的,一般认为它们是下地壳不均匀的镁铁质岩石及/或富集的上地幔岩石在高压(≥1.5 GPa)下部分熔融的结果。在燕山带内埃达克岩浆的形成有一个很长的时间(约190～80 Ma),然而岩浆活动的峰期却与约170～130 Ma间有基底岩石卷入的陆壳收缩期相一致。尽管埃达克质岩浆活动的历史很长,但那种把岩浆活动与岩石圈的拆沉效应相联系的模式似乎是不适当的。在该带内,埃达克质与非埃达克质岩浆活动有一部分是同时的,而且在地理分布上也是相间的,这说明了在下地壳和上地幔岩石的部分熔融中成分是相当不均匀的。侏罗纪及白垩纪熔融作用的热源应当是与古太平洋板块俯冲相关的中生代板底垫托的玄武岩浆。除了局部例外,在燕山带,埃达克质岩浆活动的终结和碱性岩浆活动的开始约在130～120 Ma,在此时期收缩作用使东亚大达200万km~2以上的地区发生了NW—SE向的区域性伸展作用。强烈的地壳伸展仅局限于华北克拉通北缘分布的少数几个变质核杂岩中。陆壳的伸展减薄合理地解释了130～120 Ma间发生高压埃达克质熔融条件的终结,尽管还有局部年轻的埃达克火山活动(约120～80Ma)可以在伸展规模有限而厚的地壳依然存在的地区继续出现。燕山区早白垩世的碱性侵入体中的锆石不存在前寒武纪?     

大别山高压-超高压变质期后伸展构造格局

大别山高压、超高压变质期后构造格架的最显著特征是以罗田片麻岩穹隆为核部的多层伸展拆离滑脱带的发育,并由它们将超高压变质单元、高压变质单元和蓝闪－绿片岩单元分隔成垂向叠置的席状岩片,类似于变质核杂岩的基本结构样式。这种伸展构造格架制约了高压、超高压岩石的展布,而在较大榴辉岩体中保存的缩短或挤压组构则是以高压、超高压变质作用为标志的陆－陆碰撞事件的记录。正确地区分挤压组构与伸展组构是识别大别山带内高压     

Composition and evolution of the continental crust: Retrospect and prospect

Until the middle of the 20th century, the continental crust was considered to be dominantly granitic. This hypothesis was revised after the Second World War when several new studies led to the realization that the continental crust is dominantly made of metamorphic rocks. Magmatic rocks were emplaced at peak metamorphic conditions in domains, which can be defined by geophysical discontinuities. Low to medium-grade metamorphic rocks constitute the upper crust, granitic migmatites and intrusive granites occur in the middle crust, and the lower crust, situated between the Conrad and Moho discontinuities, comprises charnockites and granulites. The continental crust acquired its final structure during metamorphic episodes associated with mantle upwelling, which mostly occurred in supercontinents prior to their disruption, during which the base of the crust experienced ultrahigh temperatures (>1000 °C, ultrahigh temperature granulite-facies metamorphism). Heat is provided by underplating of mantle-derived mafic magmas, as well as by a massive influx of low H₂O activity mantle fluids, i.e. high-density CO₂ and high-salinity brines. These fluids are initially stored in ultrahigh temperature domains, and subsequently infiltrate the lower crust, where they generate anhydrous granulite mineral assemblages. The brines can reach upper crustal levels, possibly even the surface, along major shear zones, where granitoids are generated through brine streaming in addition to those formed by dehydration melting in upper crustal levels.     

Major Constituents of the Dabie Collisional Orogenic Belt and Partial Melting in the Ultrahigh-Pressure Unit

The present constitution and architecture of the Dabieshan orogenic belt is the combined result of Triassic subduction collision, extensional tectonics postdating the HP and UHP metamorphism, and thermo-tectonic evolution in Mesozoic-Cenozoic time. In addition to Yanshanian and post-Yan-shanian magmatic intrusion, volcanic eruption, and basin deposition, lithotectonic constituents of the Dabie orogenic belt consist mainly of a core complex (CC) unit, an UHP unit, an HP unit, an epidote-blueschist (EBS) unit, and a sedimentary cover (SC) unit. Minor mafic-ultramafic plutons were intruded into or preserved within the CC, UHP, HP, or EBS units. Slices of UHP, HP, and EBS units are progressively sandwiched between the underlying core complex and the overlying sedimentary cover. The distribution of lithotectonic units is controlled by an extensional tectonic framework, which postdates the collisional event. The tectonic pattern of the Dabieshan orogenic belt as a whole is characterized by a general doming, with the development of multi-layered detachment zones.

The study of partial melting associated with decompressive retrogression in the UHP unit during exhumation of the eclogites provides us with a better understanding of the relationship between eclogites and the surrounding country rock (socalled UHP gneisses), and the foliated garnet-bearing granites (the non-HP country rocks). It supports the “in situ” interpretation. Anatexis occurred under conditions of amphibolite-facies metamorphism at lower to middle crustal levels. This partial melting associated with decompression is one of the most important physico-chemical processes that postdate the collisional event in the Dabieshan. It signaled the evolution of the deformation regime from compression to extension, and reflected thinning of the continental crust and rapid uplift of UHP metamorphic rocks to middle to lower crustal levels by regional-scale extension.     

变质核杂岩与岩浆作用成因关系综述

对岩浆与伸展作用的关系、伸展作用中岩浆的成因和需加强的工作进行了讨论,并重点论述了变质核杂岩形成机制与侵入作用的关系。在造山带重力势能差和深部作用等各种因素导致的拉伸应力场作用下,岩石圈地幔和地壳通过减压或深部热活动发生部分熔融而形成岩浆,岩浆的上涌强化了地壳伸展,对地壳的弱化作用触发伸展构造的发生。岩浆作用是变质核杂岩形成的主导因素之一,其主要包括对地壳的加热、弱化导致拆离断层的形成及由其浮力和密度产生不均一隆升而形成穹隆。     

On the origin and fluid content of some rare crustal xenoliths and their bearing on the structure and evolution of the crust beneath the Bakony–Balaton Highland Volcanic Field (W-Hungary)

Rarely occurring clinopyroxene-plagioclase bearing, felsic granulite and skarn xenoliths were studied from the mantle and crustal xenolith-bearing alkaline basaltic and pyroclastic localities of the Bakony–Balaton Highland Volcanic Field (W-Hungary). Geobarometry and geothermometry of the xenoliths made it possible to categorise them in three groups according to their depth of formation. The first group formed in the lower crust together with mafic and metasedimentary granulites. The second group represents magmatic intrusions of the middle crust, and the third one comprises contact metamorphic rocks of relatively shallow origin. The calculated pressure difference from the core and rim compositions of plagioclase and clinopyroxene as well as garnet breakdown reactions in some xenoliths show evidence for pressure decrease due to crustal thinning both in lower crustal and middle crustal xenoliths during formation of the Pannonian Basin. Fluid inclusion studies reveal the dominance of the CO₂-rich fluids in the whole crustal section in contrast with fluids found in mafic garnet-bearing xenoliths. Crustal stratigraphy was constructed for the periods prior to the extension and after the extension on the basis of geobarometry and geophysical data. On the basis of mineral stabilities and geothermo-barometry, we estimated that the pre-extensional thickness of the lower and upper crust may have been 27–34 and 26–28?km, respectively. Comparison of pre-extensional and present-day thickness of the lower and upper crust indicate that thinning affected both the lower and the upper portion of the crust but on a different scale. The calculated thinning factors are between 2.25 and 3.4 for the lower crust and 1.3–1.56 for the upper crust.     

部分熔融与高级变质岩流变机制——以内蒙古大青山高级变质岩为例

部分熔融作用与高级变质岩变形作用是相互制约,变形作用能够提高岩石部分熔融程度,降低熔融温度。熔体存在影响和制约岩石强度和变形机制。大青山高级岩经历了下部地壳构造层次变质变形和深熔作用改造,形成了复杂构造要素组合。宏观与微观构造特点表明:高级变质岩变形机制主要为熔体增强颗粒边界扩散和颗粒流动,使岩石发生大规模的塑性流动。在宏观上形成了不对称流动组构、熔融线理、岩石和矿物条带、层内底辟褶皱和大型穹窿构造。但是,在微观上矿物颗粒变形不明显,晶内变形组构不发育,表现为三边平衡结构,与静态结晶变质岩结构相似,形成了地壳深部构造层次上变质构造岩-构造片麻岩。     

Convergence in a thermally softened thick crust: Variscan intracontinental tectonics in Iberian plate rocks

The subduction phase in the development of the Variscan Orogen in SW Europe was followed by an extended period of ‘intracontinental’ tectonics. The progressive temperature rise in the hinterland during plate convergence was accompanied by widespread partial melting in the lower crust and the nucleation of kilometric buckle folds and crustal‐scale shear zones in the stronger upper crust. Thermal mechanical weakening in the core of the orogen was contemporaneous with shortening and thickening in the foreland fold‐and‐thrust belt. We evaluate lithospheric strength profiles in the hinterland and foreland based on the metamorphic and structural record for three tectonic stages. We find that lower crustal strength varied in space as well as in time during orogenesis. Strength contrasts between the foreland and the hot hinterland during convergence may have led to the additional indentation of the foreland into the hinterland of the Ibero‐Armorican Arc.     

Petrological and geochronological constraints on regional metamorphism along the northern border of the Bitterroot batholith

Quantitative thermobarometry in pelites and garnet amphibolites from the Bitterroot metamorphic core complex, combined with U–Pb dating of metamorphic monazite and zircon from footwall rocks, provide new constraints on the P – T  – t evolution of footwall rocks. The thermobarometric and geochronological results, when correlated with observations from other regions bordering the Bitterroot batholith, define a regional metamorphic history for the northern margin of the Bitterroot batholith consisting of three distinct events beginning with early prograde metamorphism (M1) coincident with arc-related magmatism and crustal shortening at c .  100–80 Ma. Magmatism and crustal thickening led to regional upper-amphibolite facies metamorphism (M2) and anatectic melting between 64 and 56 Ma. Mineral textures related to high-temperature isothermal decompression (M3), coincident with late stages of magmatism in the Bitterroot complex footwall (56–48 Ma), are only preserved in areas adjacent to extensional structures. The close temporal relationship between peak metamorphism and the onset of footwall decompression indicates that thermal weakening was an important factor in the initiation of Early Eocene regional extension and tectonic denudation of the Bitterroot complex and possibly the Boehls Butte metamorphic terrane. The morphology of the decompressional P – T  – t path derived for Bitterroot footwall rocks is similar to other trajectories reported for Cordilleran core complexes and may represent a transition in the deformational style of core-bunding detachments responsible for exhumation.     

P–T–t path of the Hercynian low-pressure rocks from the Mandatoriccio complex (Sila Massif, Calabria, Italy): new insights for crustal evolution

The tectono‐metamorphic evolution of the Hercynian intermediate–upper crust outcropping in eastern Sila (Calabria, Italy) has been reconstructed, integrating microstructural analysis, P–T pseudosections, mineral isopleths and geochronological data. The studied rocks belong to a nearly complete crustal section that comprises granulite facies metamorphic rocks at the base and granitoids in the intermediate levels. Clockwise P–T paths have been constrained for metapelites of the basal level of the intermediate–upper crust (Umbriatico area). These rocks show noticeable porphyroblastic textures documenting the progressive change from medium‐P metamorphic assemblages (garnet‐ and staurolite‐bearing assemblages) towards low‐P/high‐T metamorphic assemblages (fibrolite‐ and cordierite‐bearing assemblages). Peak‐metamorphic conditions of ～590 °C and 0.35 GPa are estimated by integrating microstructural observations with P–T pseudosections calculated for bulk‐rock and reaction‐domain compositions. The top level of the intermediate–upper crust (Campana area) recorded only the major heating phase at low‐P (～550 °C and 0.25 GPa), as documented by the static growth of biotite spots and of cordierite and andalusite porphyroblasts in metapelites. In situ U–Th–Pb dating of monazite from schists containing low‐P/high‐T metamorphic assemblages gave a weighted mean U–Pb concordia age of 299 ± 3 Ma, which has been interpreted as the timing of peak metamorphism. In the framework of the whole Hercynian crustal section the peak of low‐P/high‐T metamorphism in the intermediate‐to‐upper crust took place concurrently with granulite facies metamorphism in the lower crust and with emplacement of the granitoids in the intermediate levels. In addition, decompression is a distinctive trait of the P–T evolution both in the lower and upper crust. It is proposed that post–collisional extension, together with exhumation, is the most suitable tectonic setting in which magmatic and metamorphic processes can be active simultaneously in different levels of the continental crust.     

A first coincident normal-incidence and wide-angle approach to studying the extending Aegean crust

The Aegean Sea is a broad area of submerged continental crust undergoing active extension to varying degrees. A combined near-normal incidence and wide-angle seismic recording programme was conducted in the western Aegean Sea in 1993, with the principal objective of testing the popular hypothesis that lower crustal deformation (particularily extension) is expressed as a seismically “layered lower crust” (LLC). Across the southern margin of the Cretan trough (i.e. North Cretan offshore margin), a LLC was indicated by wide-angle arrivals that was not apparent on either the coincident near-normal-in-cidence profile or on older low-frequency refraction records. North of the northern margin of the Cretan Trough, beneath the Cyclades, a domain of strong reflectivity is recorded from the middle to lower crust. Here, the near-normal incidence sections also show this typical LLC reflectivity. On the wide-angle sections, a distinct interface is suggested in addition, at a larger depth than that previously assumed for the Moho discontinuity. The structural images and interpretations derived from the new seismic data so far do not clearly support either a pure-shear crustal stretching or an asymmetric simple-shear extension model for the Aegean Sea. Our results appear to be consistent with a tectonic model, where middle crust mobilised by flow coincides spatially with upper crust that has been thinned by active extension of an orogenically thickened crust and expressed near the surface as an exhumed metamorphic core complex.     

Geochronological constraints on the timing of granitoid magmatism, metamorphism and post-metamorphic cooling in the Hercynian crustal cross-section of Calabria

Exposed cross‐sections of the continental crust are a unique geological situation for crustal evolution studies, providing the possibility of deciphering the time relationships between magmatic and metamorphic events at all levels of the crust. In the cross‐section of southern and northern Calabria, U–Pb, Rb–Sr and K–Ar mineral ages of granulite facies metapelitic migmatites, peraluminous granites and amphibolite facies upper crustal gneisses provide constraints on the late‐Hercynian peak metamorphism and granitoid magmatism as well as on the post‐metamorphic cooling. Monazite from upper crustal amphibolite facies paragneisses from southern Calabria yields similar U–Pb ages (295–293±4 Ma) to those of granulite facies metamorphism in the lower crust and of intrusions of calcalkaline and metaluminous granitoids in the middle crust (300±10 Ma). Monazite and xenotime from peraluminous granites in the middle to upper crust of the same crustal section provide slightly older intrusion ages of 303–302±0.6 Ma. Zircon from a mafic to intermediate sill in the lower crust yields a lower concordia intercept age of 290±2 Ma, which may be interpreted as the minimum age for metamorphism or intrusion. U–Pb monazite ages from granulite facies migmatites and peraluminous granites of the lower and middle crust from northern Calabria (Sila) also point to a near‐synchronism of peak metamorphism and intrusion at 304–300±0.4 Ma. At the end of the granulite facies metamorphism, the lower crustal rocks were uplifted into mid‐crustal levels (10–15 km) followed by nearly isobaric slow cooling (c. 3 °C Ma^?1) as indicated by muscovite and biotite K–Ar and Rb–Sr data between 210±4 and 123±1 Ma. The thermal history is therefore similar to that of the lower crust of southern Calabria. In combination with previous petrological studies addressing metamorphic textures and P–T conditions of rocks from all crustal levels, the new geochronological results are used to suggest that the thermal evolution and heat distribution in the Calabrian crust were mainly controlled by advective heat input through magmatic intrusions into all crustal levels during the late‐Hercynian orogeny.     

Two-dimensional numerical modeling of tectonic and metamorphic histories at active continental margins

The evolution of an active continental margin is simulated in two dimensions, using a finite difference thermomechanical code with half-staggered grid and marker-in-cell technique. The effect of mechanical properties, changing as a function of P and T, assigned to different crustal layers and mantle materials in the simple starting structure is discussed for a set of numerical models. For each model, representative P–T paths are displayed for selected markers. Both the intensity of subduction erosion and the size of the frontal accretionary wedge are strongly dependent on the rheology chosen for the overriding continental crust. Tectonically eroded upper and lower continental crust is carried down to form a broad orogenic wedge, intermingling with detached oceanic crust and sediments from the subducted plate and hydrated mantle material from the overriding plate. A small portion of the continental crust and trench sediments is carried further down into a narrow subduction channel, intermingling with oceanic crust and hydrated mantle material, and to some extent extruded to the rear of the orogenic wedge underplating the overriding continental crust. The exhumation rates for (ultra)high pressure rocks can exceed subduction and burial rates by a factor of 1.5–3, when forced return flow in the hanging wall portion of the self-organizing subduction channel is focused. The simulations suggest that a minimum rate of subduction is required for the formation of a subduction channel, because buoyancy forces may outweigh drag forces for slow subduction. For a weak upper continental crust, simulated by a high pore pressure coefficient in the brittle regime, the orogenic wedge and megascale melange reach a mid- to upper-crustal position within 10–20 Myr (after 400–600 km of subduction). For a strong upper crust, a continental lid persists over the entire time span covered by the simulation. The structural pattern is similar in all cases, with four zones from trench toward arc: (a) an accretionary complex of low-grade metamorphic sedimentary material; (b) a wedge of mainly continental crust, with medium-grade HP metamorphic overprint, wound up and stretched in a marble cake fashion to appear as nappes with alternating upper and lower crustal provenance, and minor oceanic or hydrated mantle interleaved material; (c) a megascale melange composed of high-pressure and ultrahigh-pressure metamorphic oceanic and continental crust, and hydrated mantle, all extruded from the subduction channel; (d) zone represents the upward tilted frontal part of the remaining upper plate lid in the case of a weak upper crust. The shape of the P–T paths and the time scales correspond to those typically recorded in orogenic belts. Comparison of the numerical results with the European Alps reveals some similarities in their gross structural and metamorphic pattern exposed after collision. A similar structure may be developed at depth beneath the forearc of the Andes, where the importance of subduction erosion is well documented, and where a strong upper crust forms a stable lid.     

冈底斯朱诺地区中新世板内热隆伸展成矿

位于冈底斯构造成矿带中段西侧的朱诺地区具有良好的成矿前景,已发现有朱诺大型斑岩铜矿床和铁雅铁铜矿点。野外基础地质和矿床地质调查、构造分析和岩石化学的研究表明,朱诺斑岩铜矿床在构造上受近EW向和近SN向伸展断裂控制,时间上属于中新世青藏高原南部板内构造过程,含矿斑岩具有典型的埃达克质岩特征。朱诺及其所在的冈底斯构造成矿带在中新世处于板内构造环境,板内伸展构造—埃达克质岩—斑岩铜矿系统叠加在早期的俯冲—碰撞构造岩石组合之上。与埃达克质岩形成直接相关的下地壳流动导致冈底斯上地壳及下地壳显著加厚,发生部分熔融作用,下地壳物质可能源于地壳减薄的锡瓦利克盆地,流经喜马拉雅,穿过并改造了雅鲁藏布江缝合带中挤入地壳的洋壳地幔岩石,造成被混入洋壳地幔成分的冈底斯下地壳发生部分熔融,形成埃达克质岩浆,上升并顶托冈底斯上地壳,致使冈底斯上地壳先后发生近EW向和近SN向的伸展,在上地壳伸展扩容空间中含矿埃达克质岩浆沿伸展断裂上升、侵位,并富集成矿。     

Low-pressure metamorphism of Palaeozoic pelites in the Aspromonte, southern Calabria: constraints for the thermal evolution in the Calabrian crustal cross-section during the Hercynian orogeny

During Hercynian low-pressure/high-temperature metamorphism of Palaeozoic metasediments of the southern Aspromonte (Calabria), a sequence of metamorphic zones at chlorite, biotite, garnet, staurolite–andalusite and sillimanite–muscovite grade was developed. These metasediments represent the upper part of an exposed tilted cross-section through the Hercynian continental crust. P–T information on their metamorphism supplements that already known for the granulite facies lower crust of the section and allows reconstruction of the thermal conditions in the Calabrian crust during the late Hercynian orogenic event. Three foliations formed during deformation of the metasediments. The peak metamorphic assemblages grew mainly syntectonically (S2) during regional metamorphism, but mineral growth outlasted the deformation. This is in accordance with the textural relationships found in the lower part of the same crustal section exposed in the northern Serre. Pressure conditions recorded for the base of the upper crustal metasediments are c. 2.5 kbar and estimated temperatures range from <350 °C in the chlorite zone, increasing to 500 °C in the lower garnet zone, and reaching 620 °C in the sillimanite–muscovite zone. Geothermal gradients for the peak of metamorphism indicate a much higher value for the upper crust (c. 60 °C km^?1) than for the granulite facies lower crust (30–35 °C km^?1). The small temperature difference between the base of the upper crust (620 °C at c. 2.5 kbar) and the top of the lower crust (690 °C at 5.5 kbar) can be explained by intrusions of granitoids into the middle crust, which, in this crustal section, took place synchronously with the regional metamorphism at c. 310– 295 Ma. It is concluded that the thermal structure of the Calabrian crust during the Hercynian orogeny – as it is reflected by peak metamorphic assemblages – was mainly controlled by advective heat input through magmatic intrusions into all levels of the crust.     

Production and recycling of oceanic crust in the early Earth

Because of the strongly different conditions in the mantle of the early Earth regarding temperature and viscosity, present-day geodynamics cannot simply be extrapolated back to the early history of the Earth. We use numerical thermochemical convection models including partial melting and a simple mechanism for melt segregation and oceanic crust production to investigate an alternative suite of dynamics which may have been in operation in the early Earth. Our modelling results show three processes that may have played an important role in the production and recycling of oceanic crust: (1) Small-scale (x×100 km) convection involving the lower crust and shallow upper mantle. Partial melting and thus crustal production takes place in the upwelling limb and delamination of the eclogitic lower crust in the downwelling limb. (2) Large-scale resurfacing events in which (nearly) the complete crust sinks into the (eventually lower) mantle, thereby forming a stable reservoir enriched in incompatible elements in the deep mantle. New crust is simultaneously formed at the surface from segregating melt. (3) Intrusion of lower mantle diapirs with a high excess temperature (about 250 K) into the upper mantle, causing massive melting and crustal growth. This allows for plumes in the Archean upper mantle with a much higher excess temperature than previously expected from theoretical considerations.