The role of extensional tectonics at different crustal levels on granite ascent and emplacement: an example from Tuscany (Italy)

The role of regional extension on the rise and emplacement of granites in the crust is still debated. Pluton ascent and emplacement widely occurred in Tuscany (Italy) since late Miocene during the post-orogenic collapse of the inner Apennines, and are presently occurring in the geothermal areas of Amiata and Larderello. Tuscany offers a preferred test site to study the role of regional extension on pluton ascent and emplacement at different crustal levels. Ductile extension enhanced the segregation and ascent of granitic melts in the lower crust, controlling pluton emplacement in correspondence with the brittle–ductile transition. In the brittle crust, magma ascent occurred through subvertical faults and fractures compatible with the regional extension direction; pluton emplacement mainly occurred by means of roof lifting. The case of Tuscany suggests that the extensional structures enhance melt segregation and ascent in the ductile crust, but are not efficient alone to provide a pathway for the ascent of granitic magmas in the brittle-extending crust. The estimated magmatic strain rates due to pluton emplacement in the geothermal areas are much larger than the regional tectonic strain rates. This suggests that regional tectonics did not control magma emplacement in the brittle crust and explains why nontectonic processes (roof lifting) accommodated the space required for pluton emplacement.     

Emplacement Mechanism of the Akebasitao Pluton: Implications for Regional Tectonic Evolution of West Junggar, NW China

Late Carboniferous to Early Permian A-type granites are extensively distributed throughout the West Junggar region, NW China, and the Akebasitao pluton is extremely distinguished among these plutons. In this paper, we reported new anisotropy of magnetic susceptible (AMS) data combine with detailed field study and audio magnetotelluric (AMT) sounding to assess the three-dimensional shape and magmatic emplacement mechanism of the Akebasitao pluton. The geological features and the AMT sounding indicate that the pluton had a slightly oblique movement of magma from northwest to southeast, which was most likely to correspond to an asymmetrical torch with a laccolith-shaped upper part, and a lower part formed by sub-vertical “root” that was located within its northwestern part, probably controlled by the NE-trending Anqi fault. The AMS fabrics of all the specimens reveal a low Pj value (mean of 1.02) and a low T value (mean of ?0.024), suggesting that the deformation of the AMS ellipsoid is relatively weak. The specimens exhibit both oblate and prolate shapes of the AMS ellipsoid. Magnetic lineations and foliations are randomly distributed throughout the pluton without any preferred orientation. These AMS patterns indicate that the pluton formed in a relatively stable structural environment with no regional extrusion. Therefore, we propose a complex emplacement process in which the magmas reached the shallower crust levels via deep-faults and subsequently occupied the room created by doming, accompanied by stoping near the pluton roof. Additionally, the regional tectonic setting was relatively stable during the emplacement of the Akebasitao pluton, indicating the termination of compressional orogeny during the late Late Carboniferous in the West Junggar region. This conclusion perfectly coincides with the regional tectonic paleogeography, magmatic system, and paleostress field.     

花岗岩构造与侵位机制研究进展

近年来对造山带花岗岩构造与侵位机制的研究表明，花岗岩不但可以侵位在区域伸展的构造背景，也可以侵位在区域挤压（缩短）的构造背景。花岗岩侵位受断裂的控制并不是像以前认为的那样明显，而是受多种侵位机制的共同作用，而构造样式和变形组构则是侵位机制研究的基础。提出了一些新的研究思路和方法。此外，对大别山中生代花岗岩构造、侵位机制作了简要讨论。     

Study of the Emplacement Mechanism of the Fenghuangshan Granite Pluton and Related Cu-Au Mineralization in Tongling, Anhui Province

The Fenghuangshan Pluton is located in the Tongling polymineral-cluster district in the middle-lower section of the Yangtze metallogenic belt. In tectonic terms, it is in the middle of the Guichi-Fanchang faulted fold bundle of the lower Yangtze Platform fold belt between the Dabie Orogenic Belt and the Jiangnan Massif. Analyses of the structural deformation characteristics of both the contact zone and the interior of the pluton are used to explain its emplacement mechanism. The foliation and lineation of the pluton, consisting of the oriented distribution of dark minerals and xenoliths, mainly concentrate along the margin of the pluton. Toward the center of the pluton, the foliation structure becomes weak, showing intense compression formation at the margin, and reflecting the conformable intrusion of the pluton. The relatively gentle lineation is evidence of a rotatory emplacement mechanism. Relatively steep marginal foliation reflects compression expanding. Affected by the thermal power of the pluton,     

铜陵凤凰山岩体侵位构造变形特征

通过对凤凰山岩体的野外研究,显微构造,包体测量及磁组构分析,详尽解析凤凰山岩体的侵位构造变形特征,并厘定了岩体变形过程中的应变状。结果表明表明凤凰山岩体ＮＮＥ向水平左旋剪切应力及重力的联合用用下呈气囊膨胀式同构造侵位的机制。     

Multi-stage emplacement of the G?temar Pluton, SE Sweden: new evidence inferred from field observations and microfabric analysis, including cathodoluminescence microscopy

The emplacement of the Mesoproterozoic G?temar Pluton into Paleoproterozoic granitoid host rocks of the Transscandinavian Igneous Belt is re-examined by microfabric analysis, including cathodoluminescence microscopy. Field data on the pluton-host rock system are used to strengthen the model. The G?temar Pluton, situated on the Baltic Shield of SE Sweden, is a horizontally zoned tabular structure that was constructed by the intrusion of successive pulses of magma with different crystal/melt ratios, at an estimated crustal depth of 4–8?km. Initial pluton formation involved magma ascent along a vertical dike, which was arrested at a mechanical discontinuity within the granitoid host rocks; this led to the formation of an initial sill. Subsequent sill stacking and their constant inflation resulted in deformation and reheating of existing magma bodies, which also raised the pluton roof. This multi-stage emplacement scenario is indicated by complex dike relationships and the occurrence of several generations of quartz (Si-metasomatism). The sills were charged by different domains of a heterogeneous magma chamber with varying crystal/melt ratios. Ascent or emplacement of magma with a high crystal/melt ratio is indicated by syn-magmatic deformation of phenocrysts. Complex crystallization fabrics (e.g. oscillatory growth zoning caused by high crystal defect density, overgrowth and replacement features, resorbed and corroded crystal cores, rapakivi structure) are mostly related to processes within the main chamber, that is repeated magma mixing or water influx.     

Space accommodation by roof lifting during pluton emplacement at Amiata (Italy)

The Amiata area is characterized by an uplift attributed to pluton emplacement during the Pliocene epoch. The shape of the buried pluton is constrained by gravity data. Fieldwork was carried out to identify possible deformations within the overburden, in order to evaluate the mechanism of emplacement. The attitudes of Lower Pliocene marine sediments, deposited immediately before the uplift, were measured also. The overall pattern of the deposits defines a dome, approximately 2.5 km high and 35 × 50 km wide. Consistency between these field data and the existing geophysical data suggests that the dome results from the emplacement of a laccolith-like intrusion which was responsible for lifting and flexuring the overburden. The Amiata case shows that roof lifting is a viable process by which space in the upper crust, to a depth of 7 km, can be accommodated during pluton emplacement.     

西准噶尔阿克巴斯陶岩体三维形态及其地质意义

西准噶尔地区广泛发育晚石炭世-二叠纪不同规模、形态各异的花岗岩体,阿克巴斯陶岩体是其中最具代表性岩体之一,但对于该岩体三维形态和侵位过程的研究尤显薄弱.基于详细的野外路线地质调查,通过对阿克巴斯陶岩体NE、NW、SE和SW侧接触边界产状、接触热变质带宽度、岩脉方位和发育程度、顶垂体和围岩捕虏体发育特征的研究,揭示出岩体NE、SE和SW侧与围岩呈低角度外倾接触,而岩体NW侧与围岩呈高角度接触.在此基础上,结合岩体出露区音频大地电磁反演结果,揭示出阿克巴斯陶岩体三维形态总体为不对称蘑菇状,岩体侵位时岩浆主要由NW向朝SE向斜向侵位,并建立了岩体的三维形态模型.阿克巴斯陶岩体三维形态的确定,揭示了西准噶尔地区晚石炭世晚期-早二叠世为后造山伸展环境.      

陕西勉县地区关帝坪闪长岩体的LA—ICP—MS锆石U—Pb年龄及其地质意义

秦岭造山带印支期花岗岩的成因研究是当前秦岭造山带研究的热点问题。通过对出露于陕西勉县地区勉略缝合带中光头山岩体西端的关帝坪黑云母闪长岩锆石LA—ICPMSU-Pb年代学研究,探讨其地质意义。岩石主要由斜长石、角闪石和黑云母组成,蚀变较轻。LA—ICP—MS锆石U—Pb测年得到的加权平均年龄为220．5士3．1Ma（MSWD=0．66,2σ）,代表该闪长岩体的结晶年龄。该年龄与附近的光头山黑云母斜长花岗岩的年龄216±2Ma很接近,由此推断两者应产出于相同的构造背景。结合前人研究结果,推断该黑云母闪长岩体可能为勉略洋闭合后的碰撞后期产物。     

Experiments simulating surface deformation induced by pluton emplacement

We present centrifuge experiments to study the surface deformation induced by shallow pluton emplacement in a rheologically stratified crust. Sand simulates the topmost brittle crust; plastilina and denser silicone represent more and less competent crustal portions, respectively; lighter silicone simulates a buoyant intrusion. In the models, density differences affect the rate of intrusion but not their evolution or shape, whereas viscosity and strength stratifications control both the shape and rate of the intrusions. With a higher viscosity contrast (10^2–4) between the intrusion and the embedding media, the rise of the lighter silicone induces a laccolith-like intrusion, responsible for doming and thinning of the overburden; an apical depression may form, inducing silicone extrusion. Conversely, with a lower (10¹) viscosity contrast, the overburden and the intrusion exhibit a lens-shaped form, with a broad central depression bordered by an upward flexure towards the periphery. A sag in the floor of the intrusion is commonly observed; no silicone extrusion occurred. The intrusion is a hybrid between a laccolith and a lopolith. The comparison with nature (1) confirms roof uplift as an important means of accommodating space during pluton emplacement and (2) suggests that, where roof uplift plays a major role, pluton emplacement can induce a well-correlated sequence of events at surface: doming, the development of a depression and extrusion.     

Translation and the resolution of the pluton space problem

The Late Cretaceous Mono Creek granite has a pronounced NW–SE elongate shape, 60 km long by 10 km wide, characteristic of plutons from the eastern Sierra Nevada batholith. An 8 km-wide bulge exists on the NE side of this pluton, which exhibits evidence of forceful emplacement (or in-situ ballooning), such as deflection of metamorphic wallrock and igneous foliation, and the orientation of fracture patterns. Three-dimensional strain analysis indicates that wallrock strains do not provide enough volume to accommodate the emplacement of the bulge, a recurring problem in studies of plutonic terranes.We suggest that emplacement of the Mono Creek bulge was accommodated by all components of the three-dimensional displacement field—including translation, rotation, and pure strain (shape change)—of the surrounding units. Classical strain analysis only addresses the rotation and pure strain components, and is incapable of quantifying the translation component. However, our analysis suggests that translation plays the dominant role in the emplacement process. A shell model of translation of the surrounding igneous and metamorphic units is proposed for the Mono Creek bulge, which suggests that the translation component decreases dramatically away from the intrusion, consistent with the observed geology and finite strain analysis. We propose that translation is the solution to the recurring pluton `space' problem, either through tectonically controlled (passive) or magmatically controlled (active) movement of the wallrocks. Translation is generally the neglected component of the displacement field, but it may often be evaluated through judicious use of finite strain analysis and tectonic reconstruction.     

Multi-batch, incremental assembly of a dynamic magma chamber: the case of the Peninsula pluton granite (Cape Granite Suite, South Africa)

The S-type Peninsula Pluton (South Africa) exhibits substantial compositional variability and hosts a large variety of mafic and felsic magmatic enclaves with contrasting textures and compositions. Moreover, the pluton is characterized by mechanical concentrations of K-feldspar megacrysts, cordierite and biotite, generating a complex array of magmatic structures including schlieren, pipes, and spectacular sheeted structures. Chemical evidence indicates that the pluton is constructed incrementally by rapid emplacement of numerous magma pulses. Field, and textural data suggest that magmatic structures form by local flow at the emplacement level of highly viscous crystal-rich magmas (i.e. crystallinity up to 50?vol.%) through magma mushes assembled from older batches. At the time of arrival of relatively late magma batches, some areas within the pluton had achieved crystal fractions that allowed the material to act as a solid, whilst maintaining enough melt to prevent formation of sharp intrusional contacts. Magmatic structures represent “snapshots” of processes that operate in multiphase crystal-rich mushes and their genesis is due to mechanical and thermal instabilities in the crystal-rich magma chamber that are triggered by the emplacement of pulses of new magma derived from the melting of a compositionally variable metasedimentary source.     

http://www.sciencedirect.com/science/article/pii/S1674987112000618

The Moyar Shear Zone(MSZ) of the South Indian granulite terrain hosts a prominent syenite pluton (～560 Ma) and associated NW-SE to NE-SW trending mafic dyke swarm(～65 Ma and 95 Ma). Preliminary magnetic fabric studies in the mafic dykes,using Anisotropy of Magnetic Susceptibly(AMS) studies at low-field,indicate successive emplacement and variable magma flow direction.Magnetic lineation and foliation in these dykes are identical to the mesoscopic fabrics in MSZ mylonites,indicating shear zone guided emplacement.Spatial distribution of magnetic lineation in the dykes suggests a common conduit from which the source magma has been migrated.The magnetic foliation trajectories have a sigmoidal shape to the north of the pluton and curve into the MSZ suggesting dextral sense of shear.Identical fabric conditions for magnetic fabrics in the syenite pluton and measured field fabrics in mylonite indicate syntectonic emplacement along the Proterozoic crustal scale dextral shear zone with repeated reactivation history.     

Magmatic history and geophysical signature of a post-collisional intrusive center emplaced near a crustal-scale shear zone: the Plechý granite pluton (Moldanubian batholith,Bohemian Massif)

The Plechy pluton, southwestern Bohemian Massif, represents a late-Variscan, complexly zoned intrusive center emplaced near the crustal-scale Pfahl shear zone; the pluton thus provides an opportunity to examine the interplay among successive emplacement of large magma batches, magmatic fabric acquisition, and the late-Variscan stress field associated with strike-slip shearing. The magmatic history of the pluton started with the emplacement of the porphyritic Plechy and Haidmühler granites. Based on gravity and structural data, we interpret that the Plechy and Haidmühler granites were emplaced as a deeply rooted, ∼NE–SW elongated body; its gross shape and internal fabric (steep ∼NE–SW magmatic foliation) may have been controlled by the late-Variscan stress field. The steep magmatic foliation changes into flat-lying foliation (particularly recorded by AMS) presumably as a result of divergent flow. Magnetic lineations correspond to a sub-horizontal ∼NE–SW finite stretch associated with the divergent flow. Subsequently, the Třístoličník granite, characterized by steep margin-parallel magmatic foliation, was emplaced as a crescent-shaped body in the central part of the pluton. The otherwise inward-younging intrusive sequence was completed by the emplacement of the outermost and the most evolved garnet-bearing granite (the Marginal granite) along the southeastern margin of the pluton. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Incremental emplacement and syn-tectonic deformation of Late Triassic granites in the Qinling Orogen: Structural and geochronological constraints

Granitoids are important components of major orogenic belts, and provide important information about the regional geodynamic evolution. The emplacement mechanism of granite plutons and its relationship with regional tectonics has long been discussed, although it still remains debated. The Qinling Orogen within the Central China Orogen was marked by the emplacement of numerous Late Triassic granitic plutons. Although the petrology, geochemistry and geochronology of these intrusions have been addressed in various studies, their tectonic setting remains controversial, particularly since the structural aspects not been evaluated in detail. In this study, we attempt to reconstruct the emplacement process of the Late Triassic Dongjiangkou pluton in the South Qinling Belt. Field observations show extensive syn-plutonic deformations both in the pluton and its contact zones. Microstructural observations demonstrate that fabrics in the pluton were mainly acquired during submagmatic flow to high-T solid-state deformation. Zircon U–Pb ages reveal that the pluton is a composite intrusion which is composed of two juxtaposed small plutons with distinct ages (~210 Ma and ~200 Ma). Al-in-hornblende thermobarometer indicates that the pluton was formed at depths ranging from 4.7 km to 8.8 km, with an increasing depth trend from the inner unit to the outer unit. Distribution of the internal fabrics shows two concentric patterns which are concordant with pluton margins at the pluton scale and were probably induced by the regional sinistral transpression. Integrating these analyses, an incremental emplacement model is proposed for the syn-tectonic pluton. This model not only solves the ‘room problem’ but also accounts for the zoned petrological features of the pluton. Combined with previous studies, we suggest that the Late Triassic granite plutons in the Qinling Orogen were emplaced under a syn-collisional convergence setting, and that the granite magmatism was probably controlled by regional tectonics. Additionally, the incremental emplacement model may be a common mechanism for the Late Triassic granite plutons.     

Patterns of fluid flow in the contact aureole of the Late Miocene Monte Capanne pluton (Elba Island,Italy): the role of structures and rheology

Fluid–rock interaction was investigated in the inner aureole of the Late Miocene Monte Capanne pluton on Elba Island (Tuscany, central Italy) by integrating structural, petrological, fluid inclusion, and stable isotope analyses. In the north-western sector of the aureole (Procchio–Spartaia area), calc–silicates alternate with nearly pure carbonate layers at the metre scale. Close to the pluton, the prograde metamorphic sequence includes calc–silicates that transition within a few metres to overlying nearly pure calcite marbles. The calc–silicates are extensively metasomatised to form massive wollastonite-grossular-bearing exoskarn. The mineralogical assemblage found in the marbles and the unshifted carbon and oxygen isotopic ratios in calcite attest that the fluid phase was internally buffered. On the other hand, the calc–silicates constituted channels for infiltration of disequilibrium fluids of magmatic origin. Fluid infiltration was enhanced by hydrofracturing and structurally-controlled by existing planar anisotropies in calc–silicates (layering and lithological boundaries). At the metamorphic peak (∼600°C and 1.5–2 kbar), the marble–calc–silicate interface acted as a barrier to fluids exsolved from the crystallising intrusions, separating two different flow patterns in the inner aureole: a high fluid–flux region on its higher grade side (Wol-zone) and a low fluid–flux region on the lower-grade side (Cpx zone). Results of this study: (1) documented that fluid pathways in the aureole rocks at the top of the pluton were largely horizontal, controlled by the lithological layering and the pluton–host rock contact; and (2) elucidated the primary control exerted by the structural and rheological properties of the host rocks on the geometry of fluid flow during pluton emplacement.     

铜陵凤凰山岩体SHRIMP锆石U-Pb年龄与构造变形及其对岩体侵位动力学背景的制约

地球动力环境对岩浆来源和侵位机制具有重要的控制作用.铜陵矿集区中生代中酸性侵入岩和大规模成矿作用有密切关系.凤凰山岩体为凤凰山矿田的主要组成部分, 主要岩石类型为花岗闪长岩、石英二长闪长岩.根据精确锆石SHRIMP测年结果, 凤凰山岩体206Pb/238U年龄为(144.2±2.3) Ma, 是晚侏罗世形成的一个典型岩体, 和铜陵矿集区主要成矿岩体的形成时代一致.接触带构造变形特征显示岩体侵位受控于区域左旋剪切应力场作用.内部构造变形特征表明凤凰山岩体在深部主要为左旋螺旋式强力上升, 浅部为气球膨胀式主动侵位.岩体形成时代及构造变形特征表现出晚侏罗世铜陵矿集区和成矿有关的侵入岩系列的形成可能和古太平洋板块的向北斜向俯冲所导致的左旋剪切应力场有关.      

The emplacement of the Late Miocene Monte Capanne intrusion (Elba Island,Central Italy): constraints from magnetic fabric analyses

Anisotropy of magnetic susceptibility (AMS) analysis has been carried out in the thermometamorphic aureole surrounding the Late Miocene Monte Capanne pluton (Elba Island, Central Italy). The identification and separation of the main carriers of the magnetic susceptibility by low-temperature and high-field AMS measurements demonstrate that a correct knowledge of the magnetic fabric is needed in order to use AMS for tectonic interpretations. Magnetic fabric data, combined with structural data from the aureole, and their comparison with data from the pluton itself, were used to constraint the mode of pluton emplacement. Results document an intimate linkage between the magmatic flow pattern and the syn-metamorphic fabrics acquired during pluton emplacement in the host rocks. The magnetic/structural fabric in the aureole rocks is dominated by flattening deformation and no systematic relationship with any regional tectonic feature is observed. These results suggest that local processes induced by magma ascent in the upper crust might have played a primary role in space generation for pluton emplacement in the Tuscan Magmatic Province, suggesting a revaluation of the modes of pluton emplacement during the post-orogenic evolution of the northern Apennine system as a whole.     

A geologic investigation of the Halloran Hills,central Mojave Desert,California

The Halloran Hills consist of Precambrian (?) metamorphites, Mesozoic igneous intrusive rocks, Tertiary volcanic and sedimentary breccias, and Quaternary alluvia. The Precambrian Halloran Complex has been subdivided into the following formations: (1) Silver Lake Peak Formation, mostly quartzofeldspathic gneisses; (2) Cree Camp Formation, quartzites and metarhyolites; (3) Riggs Formation, metamorphosed carbonate rocks. This complex is intruded by dioritic rocks. Regional metamorphism produced parageneses of the almandine amphibolite facies. Metablastesis was a major phenomenon, partial fusion was a local one. The rocks are, therefore, metatexites. Regional metamorphism was followed by diaphthoresis, accentuated in a zone of dislocation. A mesozonal pluton (Wander Mine Pluton) invaded the Halloran Complex during the Laramide (?) orogeny. The plutonic core consists of quartz monzonite, whereas the peripheral parts are more basic in composition. Contact relationships with the country rock are both concordant and discordant. Locally, a tectonic breccia marks marginal upthrusts. Foliation exists to various degrees in the periphery of the pluton. The sequence of the emplacement phases of the pluton is as follows: (1) metasomatism, (2) flow, and (3) movement of the dead body. All three phases overlapped each other, but at the present level of erosion the rocks in any one restricted area depict only two phases. Younger granitic rocks and various dike-swarm intrusions followed the emplacement of the pluton. During the Tertiary, autoclastic friction breccias of andesitebasalt composition intruded along faults. These intrusions were followed by the deposition of arkosic breccias (Halloran Spring Formation) and coarse sedimentary breccias (rubble). At the end of the Tertiary (Pliocene?), basalt flows buried much of the ancient land surface near Halloran Spring. The configuration of the Halloran complex indicates a domelike structure, partially bounded in the north by a fault zone (Cree Camp Fault Zone). Outlines of the Wander Mine Pluton indicate a phacolithic geometry modified by regional faults. The most important fault in the area is the Cree Camp Fault Zone, which has a general east-west trend. It was probably formed before the emplacement of the pluton, and was revived three times after its initiation. Faulting ceased after the outpouring of the basalt flows. Within the regional framework, the Halloran Hills lie east of the quartz-diorite line. There seems to be a trend for association of tonalites with sillimanite-cordierite-bearing metamorphites west of the quartz-diorite line, and of quartz monzonites with potassium-rich metamorphites east of it. If the plutonic rocks are anatexitic, or palingenetic, their compositional differences on either side of the line find a suitable explanation. Results of recent experiments agree with this assumption. “Anormal” rock suites are explained as ectectic mobilizates. The age of orogenies in California generally decreases from west to east, and the potassium-sodium ratio increases in the same direction. It is submitted that time determines the site of the orogeny and that the site determines the possible range in composition of the granitic magma. The spreading sea-floor hypothesis serves as the unifying framework for the above considerations.     

Fault-controlled pluton emplacement in the Sevier fold-and-thrust belt of southwest Montana,USA

Problems associated with syncompressional pluton emplacement center on the need to make room for magma in environments where crustal shortening, not extension, occurs on a regional scale. New structural data from the Pioneer and Boulder batholiths of southwest Montana, USA, suggest emplacement at the top of frontal thrust ramps as composite tabular bodies at crustal depths between 1 and 10 km. Frontal thrust facilitated pluton emplacement was accommodated by: (1) a magma feeder zone created along the ramp interface; (2) providing ‘releasing steps’ at ramp tops that serve as initial points of emplacement and subsequent pluton growth; and (3) localizing antithetic back-thrusts that assist in pluton ascent. A model of magma emplacement is proposed that involves these elements. This model for syntectonic ramp-top emplacement of plutons helps explain how space is made for plutons within fold-and-thrust belts.