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。     

Kinematics of large syn-orogenic intrusions: example of the Lower Proterozoic Saraya batholith (Eastern Senegal)

The large Saraya batholith intruded the Lower Proterozoic sequences of Eastern Senegal. Structural analysis indicates that this large granitic batholith corresponds to a composite body consisting of several coalescent plutons and interfering diapirs. Interpretation of the structural fabric patterns in these plutons and in the host-rocks constitutes the basis of a hypothetical model of initiation, ascent and emplacement as follows: (1.) development of a deep N-S granitic ridge controled by a major crustal fracture generated by an earlier extensional phase re-activated by a strike-slip fault which, probably, would facilitate segregation, collection and ascent of magmas; (2.) intrusion and splitting up of the ridge into several diapirs which ascended simultaneously and separately in a setting of a N-S sinistral transcurrent deformation inducing a transverse NW-SE shortening and forceful emplacement of plutons and (3.) final emplacement characterized by interference between regional deformation and pluton ‘ballooning’, the latter leading to the coalescence of intrusions and to the formation of a larger, apparently homogeneous body. This model could be extended to the other large Birimian batholiths of the Lower Proterozoic in Eastern Senegal.     

Inferring a deep-crustal source terrane from a high-level granitic pluton: the Strathbogie Batholith,Australia

The Strathbogie Igneous Complex is comprised of the ignimbritic rocks of the Violet Town Volcanics and the granitic rocks of the Strathbogie batholith. It is Late Devonian in age and postorogenic-extensional in tectonic setting. The batholith was constructed from peraluminous, metasediment-derived magmas emplaced as several internally heterogeneous plutons. Chemical variation in the magmas was largely inherited from the protolith rather than having been produced by differentiation (crystal–liquid separation) or magma mixing. The Strathbogie magmas formed during a granulite-facies metamorphic event that caused partial melting of the rocks of the Proterozoic Selwyn Block, which forms the basement in this region. The chemistry of the Strathbogie batholith, the Violet Town Volcanics and various other felsic complexes of similar age, implies that the Selwyn Block here originally consisted of andesite, dacite, greywacke and pelite, probably deposited in a back-arc extensional setting. The sedimentary components of this protolith may have been deposited in a basin that was extending and deepening with time, so that the sediments contained progressively higher ratios of clay to volcanic materials. Much later, in the Late Devonian, extensional tectonics allowed the emplacement of mantle magmas into the deep and middle crust, causing the low-pressure granulite-facies metamorphic event that was responsible for the production of the crustal components in the granitic magmas of Central Victoria.     

新疆东准噶尔北部青格里河下游花岗岩类的时代及地质意义

选取新疆东准噶尔北部青格里河下游一带乌图布拉克、阿热勒托别和哈旦逊岩体等具一定规模的3个代表性岩体,通过全岩Rb-Sr等时线同位素定年,确定了研究区内玛音鄂博断裂南侧花岗岩类的形成时代分别为(334.1±9.5)M a、(300±9)M a和(280±12)M a,其初始87Sr/86Sr比值分别为0.704 33、0.704 35和0.703 84。结果表明研究区内花岗岩分别形成于华力西中晚期的不同阶段。乌图布拉克岩体形成于早石炭世,为碰撞造山作用晚期的钙碱性花岗岩,可能是早泥盆世富Nb玄武岩部分熔融的产物;结合前人厘定的板块俯冲时间(408~376 M a),推测西伯利亚板块与哈萨克斯坦—准噶尔板块的碰撞时限为376~334 M a。阿热勒托别岩体形成于后碰撞拉张环境,可能是形成于岛弧环境的中基性火山岩部分熔融的产物;哈旦逊岩体虽然也形成于后碰撞拉张环境,但拉张程度更高,属于与岩浆底侵作用有关的花岗岩类。     

藏南冈底斯带中段始新世岩浆作用的厘定及其大地构造意义

本文对藏南冈底斯带中段的花岗岩类和角闪辉长岩进行了锆石U-Pb年代学和全岩地球化学分析,据此阐明了岩体的形成机制与演化过程,并探讨了成岩时的大地构造背景。分析结果显示,研究区内花岗岩类和角闪辉长岩体的LA-ICPMS锆石U-Pb定年结果为41~55Ma,为始新世早-中期岩浆活动的产物,代表了区内岩体的成岩年龄。在地球化学组成上,花岗岩类属于钙碱性到高钾钙碱性系列,均富集轻稀土(LREE)和大离子亲石元素(LILE)(Rb、Ba和K),强烈亏损Nb、Ta、P等高场强元素(HFSE),具有弧型岩浆岩的地球化学组成。此外,花岗岩类的铝饱和指数(A/CNK)小于1.1,属于准铝质到弱过铝质的I型花岗岩。角闪辉长岩为石榴橄榄岩部分熔融的产物,并在后期侵位的过程中遭受到了壳源物质的混染。综合分析表明,研究区内的岩体形成于初始碰撞向主碰撞的转化阶段。始新世早期(～50Ma)新特提斯洋板片的断离引起软流圈物质上涌,导致岩石圈地幔发生部分熔融形成基性岩浆,随后基性岩浆底侵至下地壳并诱发下地壳发生部分熔融形成花岗岩质岩浆,最后经过岩浆混合作用形成始新世早-中期冈底斯地区的花岗岩类。     

辽南变质核杂岩主拆离断层的波瓦状构造(corrugation)及其成因

基于对辽南变质核杂岩主拆离断层及下伏韧性剪切带的几何学分析以及拆离断层带构造岩的显微构造研究,结合变质核杂岩构造—岩浆活动及其形成过程和力学机制,重点讨论了主拆离断层的波瓦状构造的特点及其形成条件。结果表明:波瓦状构造的发育与下盘晚期阶段花岗质岩体的侵位具有同时性。伸展构造发育初期就位的深成岩体加热和软化了上部地壳,此时岩石力学属性控制的钝角锯齿状断裂构成了波瓦状构造的雏形,晚期阶段花岗质深成侵入岩的不均匀侵位制约了波瓦状构造的发育。     

华北东部晚中生代区域伸展背景下同构造花岗岩体的起源与就位

花岗质岩浆的起源、迁移及就位是研究大陆岩石圈流变学特性的重要方面。然而,板内伸展背景下同构造花岗岩体的岩浆来源、就位机制和岩浆流动与区域应力场的关系等问题缺乏系统性的总结。晚中生代期间华北板块东部逐渐变为区域伸展体制,同时中浅部地壳形成一系列的韧性剪切带、变质核杂岩和拆离断层,这些伸展构造往往伴有同剪切变形的花岗岩体。因此,华北东部是系统研究板内伸展背景下同构造花岗岩体的最佳区域。本文选取多个典型的同构造花岗岩体,进行综合分析。通过归纳总结这些同构造岩体的岩石地球化学和年代学资料,发现多数同构造岩体具有多个岩浆源区,且较早就位的中性岩席(单元)往往来自壳幔混合岩浆或新生下地壳的部分熔融,而较晚的酸性岩席(单元)则主要来源于古老下地壳的部分熔融。这一特点反映了同伸展岩体岩浆源区由深至浅的演化规律,也揭示了区域伸展背景下源自地幔的流体和热量是触发地壳部分熔融的重要因素。通过分析岩浆就位过程中围岩和岩体中形成的定向及变形组构,发现华北东部同伸展岩体的就位模式可分为三大类:以扁平岩床或岩基形式就位于中部地壳的水平韧性剪切带内;岩浆以近直立运移的方式形成长轴平行拆离断层的岩基,就位于变质核杂岩核部或拆离断层下盘;岩浆就位于再活化的先存断裂,通过膨胀作用、挤压围岩获得就位空间并使围岩变形,形成类似底辟作用的就位方式。剪切应力和浮力是影响岩浆运移方向的重要力学参数。岩浆自源区上升的过程中浮力起着主要控制作用,就位于韧性剪切带时剪切应力起着控制作用,就位于浅部地壳的脆-韧性过渡带时浮力的作用再次凸显。     

Geology and petrology of the Simav Magmatic Complex (NW Anatolia) and its comparison with the Oligo-Miocene granitoids in NW Anatolia: implications on Tertiary tectonic evolution of the region

The Oligo-Miocene granitic plutons and their related volcanic-subvolcanic successions form a NW–SE trending magmatic belt along the northern border of the Menderes Massif. This belt evolved within a nappe package consisting of the Menderes metamorphics, Sakarya Continent, Afyon Zone and Tav?anl? Zone and also intruded this nappe package. The Ezine, Evciler, Eybek, Kozak, Alaçam, Koyunoba, E?rigöz and the Baklan plutons emplaced along this belt and show similarities in their internal structures, emplacement mechanisms, and petrological characteristics. These different granitic plutons cut and stitch various combinations of the nappe package of the above-mentioned tectonic belts, and evolved during and following the Alpine collision. They all show characteristic map patterns of shallow-seated plutons and range from granite to monzogranite. The granitic plutons display calc-alkaline, I-type and post-collisional geochemical characteristics. The E?rigöz, Koyunoba plutons and their subvolcanic–volcanic phases (Simav Magmatic Complex) were studied in detail. The geochemical characteristics and field occurences of the Simav Magmatic Complex were compared to the other magmatic associations in western Anatolia and it was determined that it is of collisional origin and not related to an extensional tectonic regime as suggested in some recent studies.     

Hornblende thermobarometry of the Carboniferous granitoids from the Inner Mongolia Paleo-uplift: implications for the tectonic evolution of the northern margin of North China block

Summary Hornblende thermobarometry has been widely used to estimate the emplacement pressure (P) and temperature (T) of calc-alkaline igneous rocks. Application of hornblende thermobarometry to the newly discovered Carboniferous granitic plutons from the Inner Mongolia Paleo-uplift (IMPU) provides useful information on the exhumation and geotectonic evolution of the northern margin of the North China block (NCB) during the Late Paleozoic to Early Mesozoic. Emplacement depths estimated from aluminum-in-hornblende geobarometry indicate that the Longhua, Daguangding and Boluonuo plutons were emplaced at depths of 15.7–18.7 km. Temperatures of emplacement calculated with the hornblende-plagioclase thermometer range from 676 °C to 780 °C. Because most of these plutons are unconformably overlain by Jurassic-Cretaceous volcanic or sedimentary rocks and, regionally, the oldest strata overlying them are the Nandaling and Xiahuayuan Formations of Early Jurassic age, most of these plutons must have been exposed at the surface prior to the Early Jurassic. Therefore, the large-scale uplift and exhumation of the IMPU occurred from the Late Carboniferous to Early Jurassic; at least 15 km thick crustal rocks in the IMPU must have been eroded during this period. It is also inferred that the IMPU was not always an uplifted domain from the Neoproterozoic to Early Triassic. Some Meso-Neoproterozoic and, possibly Paleozoic sedimentary rocks or Early-Middle Triassic volcanic rocks were present in the IMPU until the Late Triassic, but were almost entirely eroded before the end of Triassic. The exhumation of the crystalline rocks and formation of the IMPU is a result of this strong erosion during the Late Paleozoic to Early Mesozoic. Supplementary material to this paper is available in electronic form at Tables 1-2 available as electronic supplementary material     

Age,origin, and thermal histories of some plutonic rocks from the Salinian block of California

U-Pb isotopic measurements on zircons from some granitic rocks of the Salinian block indicate emplacement and crystallization of these rocks about 104 m.y. ago (mid-Cretaceous). The relatively radiogenic nature of initial Sr and common Pb in these rocks, and the presence of an inherited component of zircon in some of them strongly suggest the involvement of continental crust in the genesis of the magmas. Possibly the magmas were generated in a zone of melting that overlapped the boundary between the mantle and the continental crust. U-Pb measurements on sphene, apatite, and feldspar from the plutons, along with previously published K-Ar and fission-track ages shed light on the post-emplacement thermal histories of the plutons. Most of the samples from the northern part of the Salinian block (Bodega Head, Point Reyes) show relatively simple thermal histories. Evidently these plutons were emplaced at moderate levels in the crust, crystallized, and cooled to moderate temperatures over an interval of about 10–15 m.y. Plutons from the central Salinian block (Santa Lucia Range) show more complex and prolonged thermal histories. These plutons evidently were emplaced at greater depths in the crust than were the plutons from the northern Salinian block. They remained at elevated temperatures for ca. 25 m.y., then cooled fairly rapidly, probably as a result of rapid uplift and erosion. One sample from the northern Salinian block shows an even longer span of time between emplacement and cooling. The thermal evolution of the Salinian plutonic rocks predates major offset along the San Andreas fault zone and thus reflects the thermal evolution of the undisrupted source terrane of the Salinian block. Isotopic measurements of the type reported here might therefore prove valuable in correlations across the San Andreas fault zone. Moreover, detailed study of thermal evolution within the Salinian block could shed light on major offsets within the block.     

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.     

桂东南南渡—莲塘岩体的应变测量与应变特征

桂东南南渡—莲塘岩体及其围岩的应变测量结果表明：岩体总体以压扁变形为主，收缩变形为辅；热接触变质带内近岩体—侧以压扁变形为主，而远离岩体则转为以收缩变形为主。花岗岩浆沿博白—岑溪断裂带内由左旋走滑剪切所产生的构造位张空间侵位，故岩体和围岩的应变强度均软弱。     

哀牢山-红河剪切带左行走滑作用起始时间约束

位于哀牢山-红河剪切带NW延伸方向上的点苍山变质杂岩体遭受强烈的左行走滑剪切变形、变质作用改造,岩石中保存了典型的高温矿物组合以及由它们构成的宏观和微观高温变形构造特征,其中糜棱岩中具有极其发育的长石矿物拉伸线理而形成典型的L与LS型构造岩是其一个明显的特征。本文对点苍山地区高温糜棱岩主要矿物开展了显微构造与矿物变形、变形机制及组构分析,并对于遭受高温糜棱岩化改造的一个花岗质岩体开展了SHRIMP锆石U-Pb定年分析。结果表明岩石中长石、角闪石、石英等主要矿物具有典型的达角闪岩相条件下的高温晶质塑性变形和动态生长特征,它们也为走滑剪切变形活动提供了充分的微观构造证据。对于点苍山高温糜棱岩化改造的眼球状或似斑状二长花岗岩的显微构造分析结果表明,这套花岗质岩石从走滑剪切前期岩浆的侵位之后经历了早期强烈的岩浆期后交代作用—亚岩浆流动—高温固态塑性剪切变形的递进演化过程。由此可见,岩浆的上升与就位受左行走滑剪切作用的制约,岩体又遭受了强烈剪切变形改造。同时对这套构造前期就位花岗质岩石中的锆石进行定年分析,获得33.88±0.32Ma的岩浆结晶年龄,为此,我们有充分的理由认为,在点苍山地区哀牢山-红河剪切带左行走滑剪切作用的起始时间至少应该为早渐新世30.88±0.32Ma。     

兴凯湖花岗杂岩体的锆石U——Pb 年龄及其地质意义

佳木斯地块东南缘的兴凯湖花岗杂岩体主要由花岗闪长岩、二长花岗岩和正长花岗岩组成,其锆石的LA-ICP-MS U-Pb 测年结果显示,杨田寨南山岩体形成于257 ± 2 Ma,双子山岩体形成于215 ～212 Ma; 花岗岩的时空展布表明,两期花岗质岩浆的就位可能分别与古亚洲洋板块的俯冲作用和后造山伸展进程密切相关。结合本区和黑龙江东部其他花岗质岩石的年龄,揭示了佳木斯地块和兴凯地块在早古生代、二叠纪及三叠纪共同经历了三期大规模岩浆活动,暗示两者具有相同的构造属性,共同组成了同一陆块。     

Granitoid emplacement during syn-convergent transtension:An example from the Huamenlou pluton in North Qinling,central China

The Huamenlou pluton,is an elongated granite intrusion with high aspect ratio,emplaced within the southern margin of the North Qinling(central China).Here we investigate this pluton through multiple techniques including the fabric study,microstructural observation and zircon geochronology.Our zircon U-Pb data confirm that the granite crystallized at ca.462 Ma which is consistent with the ages of other linear plutons in North Qinling.Microstructural observations of the Huamenlou granites illustrate that the pluton has undergone superimposed deformation during its emplacement,from magmatic to hightemperature solid state conditions.The internal fabric obtained by anisotropy of magnetic susceptibility(AMS)and shape preferred orientation(SPO)show similar results.The fabrics are relatively concordant and generally vary from NE-SW to NEE-SWW which are roughly oblique to the trend of the pluton elongation and the regional structures.Meanwhile,scalar parameters reflect two completely different strain regimes for the pluton and its host rocks,i.e.,the fabrics within host rocks are mainly oblate while the central part of the intrusion displays mainly prolate fabrics.It is inferred that the structural pattern recorded in this pluton was caused by local dextral transtension in consequence of oblique convergence between the South and North China Blocks.We propose that the local transtension in convergence setting probably evolved from vertical extrusion tectonics that provided room for the magma emplacement and imparted prolate fabrics in the Huamenlou pluton.     

Zoned Calc-Alkaline Plutons in Northeastern Kamchatka,Russia: Implications for the crustal growth in magmatic arcs

Summary The Upper Cretaceous oceanic and island-arc terranes of Northeastern Kamchatka (Russia) are intruded by numerous ultramafic and mafic to intermediate plutons. Ultramafic plutons north of Epilchik Lake show a concentric structure: dunites in the core pass progressively outwards into wehrlites and pyroxenites and into gabbros along the rim which are occasionally phlogopite- and amphibole-bearing. Compositional layering and mineral orientations marked by spinel clusters in the dunites and by acicular amphiboles in the marginal gabbros are parallel to the contacts. The mafic to intermediate plutons studied in the Machevna area show similar concentric structure with a dioritic core and a gabbroic rim. Magmatic flow lines are near-vertical in the central part of the plutons. These internal structures and general geometry of the plutons suggest vertical magma intrusion into mega-tension gashes developed initially at a high angle to the subduction zone.Olivine, pyroxenes, and plagioclase have primitive compositions. Al-rich, highpressure amphiboles are followed by Al-poor, low-pressure hornblendes. A two stage model of pluton crystallization and emplacement is proposed: initial crystallization and seggregation at the Moho level below the intraoceanic island are (approximately 22–25 km depth) and final emplacement followed by partial recycling of primary ultramafic cumulates and solidification at upper crustal levels (approximately 8–10 km depth). Emplacement of the arc plutons was accompanied by epidote amphibolite-facies contact metamorphism of host volcanic and pyroclastic rocks.Major- and trace-element analyses of plutonic rocks indicate calc-alkaline affinities and emplacement in a primitive arc setting. The tectonic setting, age, and emplacement structures suggest that these plutons mark the change of geodynamic conditions from arc-related compression to local extension probably related to the initial stages of opening of the proto-Komandorsky basin. A two stage model for the crustal growth in magmatic arcs is proposed based on northeastern Kamchatka plutons. During stage I, primitive basaltic melts were emplaced along the crust-mantle boundary below the arc. Continuous fractionation and accumulation of olivine-rich cumulate rocks at the base of the growing island-arc crust resulted in production of evolved melt which was further emplaced in the upper arc crust. Crystal fractionation of a shallow, stage II magmatic reservoir below the active arc volcanoes resulted in formation of a significant amount of plutonic rocks and eruption of mafic to intermediate calc-alkaline lavas at the surface.

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Zonierte kalk-alkalische Plutone im nardöstlichen Kamchatka, Russland: Hinweise auf Krusten Wachstum inmagmatischen Bögen

Zusammenfassung Zahlreiche ultramafische, Bowie mafische bis intermediäre Plutone intrudieren die oberkretazischen ozeanischen und Inselbogen-Terrains in Nordost-Kamchatka (Russland). Ultramafische Plutone nördlich des Epilchik Lake zeigen eine konzentrische Struktur: Dunite im Kern gehen graduell in Wehrlite und Pyroxenite, und schließlich am Rand in Gabbros über, die gelegentlich Phlogopit und Amphibol führen. Lagenbau und Orientierung von Mineralkörnern sind durch Spinell-Kluster in den Duniten und durch nadelige Amphibole in den randlichen Gabbros gekennzeichnet und liegen parallel zu den Kontakten. Die mafischen bis intermediären Plutone in dem Machevna-Gebiet zeigen ähnliche konzentrische Strukturen mit einem dioritischen Kern und einem gabbroischen Rand. Magmatische Flußlinien Bind fast vertikal in den zentralen Teilen der Plutone. Diese internen Strukturen und die allgemeine Geometrie der Plutone legen es nahe, daß es sich hier um vertikale Magmen-Intrusion in Mega-Strukturen handelt, die sich ursprünglich in einem hohen Winkel zur Subduktionszone gebildet haben.Olivine, Pyroxene und Plagioklase haben primitive Zusammensetzungen. Auf Alreiche Hochdruck-Amphibole folgen Al-arme Tiefdruck-Hornblenden. Ein Modell, das zwei Stadien der Pluton-Kristallisation und Platznahme vorsieht, wird vorgeschlagen: Ursprüngliche Kristallisation und Segregation im Moho-Niveau zwischen dem intraozeanischen Inselbogen (ungefähr 22–25 km Tiefe) und schließliche Platznahme gefolgt von teilweise Recycling primärer ultramafischer Kumulate und Verfestigung in oberen Krustenniveaus (ungefähr 8–10 km Tiefe). Die Platznahme der Bogen-Plutone war von Epidot-Amphibolit-Fazies Kontaktmetamorphose der intrudierten Vulkanite und Pyroklastite begleitet.Haupt- und Spurenelement-Analysen der plutonischen Gesteine weisen auf kalkalkalischen Charakter und eine Platznahme in einem primitiven Insenbogensetting hin. Die tektonische Situation, das Alter und die Strukturen der Platznahme lassen erkennen, daß these Plutone den Wechsel in geodynamischen Bedingungen von Inselbogenbezogener Kompression zu lokaler Extension repräsentieren, Vorgänge die mit den initialen Stadien der Öffnung des proto-Komandorsky Beckens zusammenhängen. Auf der Basis der Plutone des nordöstlichen Kamchatka wird ein Modell des Krustenwachstums in magmatischen Bögen vorgeschlagen, das auf zwei Stadien beruht. Während des ersten Stadiums werden primitive basaltische Schmelzen längs der Kruste-MantelGrenze unter dem Bogen intrudiert. Andauernde Fraktionicrung und Akkumulation Olivin-reicher Kumulatgesteine an der Basis der wachsenden Inselbogenkruste resultiert dann in der Produktion einer entwickelten Schmelze die spdter in der Kruste des oberen Bogens Platz genommen hat. Kristallfraktionierung eines seichten magmatischen Reservoirs des zweiten Stadiums unter den aktiven Vulkanen des Bogens führte dann zur Bildung signifikanter Mengen plutonischer Gesteine und zur Eruption von mafischen bis intermediären kalk-alkalischen Laven an der Oberfläche.

     

Petrology of the Albany and Torbay Adamellite Plutons,near Albany,Western Australia

The Albany and Torbay Adamellites are composite plutons emplaced in Pre‐cambrian gneisses of the Albany‐Esperance Block in the vicinity of Albany, Western Australia. The gneissic country rocks have been metamorphosed to the lower granu‐lite facies at Albany and the upper amphibolite facies at Torbay. Granitized aureoles about 1 km wide, metasomatically enriched in SiO₂, K₂O, and various trace elements commonly including Rb, Ba, La, Pb, and Th, are developed in the gneisses around both plutons. Field relations suggest late‐kinematic magmatic emplacement of the Adamellites in the catazone. Both show chemical variation trends comparable with the trends normally associated with fractional crystallization of calc‐alkali magmas, and their normative compositions correspond with the thermal trough in the system An‐Ab‐Or‐Q‐H₂O at 4–7 kb P_H2o, suggesting an origin involving crystal‐liquid equilibria at a water vapour pressure of about this value. The initial ⁸⁷Sr/⁸⁶Sr ratio of 0.7118 for the Albany Adamellite is consistent with derivation of the magma from crustal rocks. The late‐kinematic field characteristics of the plutons and the limited isotopic data available are compatible with emplacement and crystallization during the closing stages of orogeny and regional metamorphism. The magmas are believed to have been generated not at their present sites of emplacement, but in a deeper, higher‐temperature zone of the crust, with magma generated during an earlier phase of the orogeny.     

Mesozoic granitic magmatism in extensional tectonics near the Mongolian border in China and its implications for crustal growth

The Yagan area of the southernmost Sino–Mongolian border is characterized by an extensional structure where a large metamorphic core complex (Yagan–Onch Hayrhan) and voluminous granitoids are exposed. New isotopic age data indicate that the granitoids, which were previously regarded as Paleozoic in age, were emplaced in early and late Mesozoic times. The early Mesozoic granitoids have 228±7 Ma U–Pb zircon age, and consist of linear mylonitic quartz monzonites and biotite monzogranites. Their chemical compositions are similar to those of potassic granites and shoshonitic series, and show an intraplate and post-collisional environment in tectonic discrimination diagrams. Their fabrics reveal that they experienced syn-emplacement extensional deformation. All these characteristics suggest that the adjustment, thinning and extensional deformation at middle to lower crustal levels might have occurred in the early Mesozoic. The late Mesozoic granitoids have a U–Pb zircon age of 135±2 Ma, and are made up of large elliptical granitic plutons. They are high-K calc-alkaline, and were forcefully emplaced in the dome extensional setting. Both the early and late Mesozoic granitoids have Nd (t) values of −2.3 to +5, in strong contrast with the negative Nd (t) values (−11) of the Precambrian host rocks. This suggests that juvenile mantle-derived components were involved in the formation of the granitoids. The similar situation is omnipresent in Central Asia. This study demonstrates that tectonic extension, magmatism and crustal growth are closely related, and that post-collisional and intraplate magmatism was probably a significant process for continental growth in the Phanerozoic.     

Intrusive rocks and tectono-metamorphic evolution of the Mako Paleoproterozoic belt (Eastern Senegal,West Africa)

The Kedougou Kenieba Inlier (KKI) (Paleoproterozoic of Eastern Senegal) is a portion of the West African Craton (WAC) containing a granite-greenstone terrain that experienced three distinct periods of magmatic activity, peaking at 2200, 2160–2130 and 2100–2070 Ma. In the Inlier, Paleoproterozoic granitoids and large-scale transcurrent shear zones are spatially associated, suggesting a genetic link between magma bodies and shear zones. Granitic intrusions are associated with all the volcanic episodes and phases of deformation, and have been used to constrain the age of many of these events. Our structural data and deformation sequence indicate that the Mako Greenstone Belt and the adjacent granitoid complexes have undergone a multi-phase evolutionary history that is spread over a prolonged period. The available geochronological data and field studies allowed classification of the granitoids of the KKI into four generations GI, GII, GIII and GIV.The current data suggest that the oldest rocks in the KKI, the Badon granites (2198 ± 2 Ma) and the tonalitic gneisses from Tonkouto (2200–2198 Ma) (GI), could be correlated with an early Birimian magmatic event. The gneisses, crystallized at depth, record the earliest deformation and in contrast to other tonalites, do not appear to have intruded volcanic rocks. The second manifestation of magmatism was intrusion of mafic diorite – the Gabbro Sandikounda Layered Igneous Complex type (GII) and development of the Laminia Kaourou Plutonic Complex (LKPC) (2160–2130 Ma). These bodies pre-date or are sometimes synchronous with a major deformational episode, and may, therefore, have formed very early in convergent Birimian orogenesis.The third major peak of magmatic activity occurred after the above major episode with the development of the oval shaped Diombalou and Bouroumbourou plutons (GIII). The orientation of these plutons parallel to the regional strike of the schistosity indicates structural control on granite emplacement. Eburnean magmatism was terminated in the Mako Belt following compressional Eburnean deformation, with the emplacement of the Tinkoto, Mamakono plutons (GIV) in the east of the complex and continued in the Dialé–Daléma supergroup with the syntectonic emplacement of the Saraya batholith. Garnitiferous granites of crustal derivation were emplaced in the final period of extensional activity around 2080 Ma.Field observations suggest the early plutons of the complex granitic (Kakadian) batholith intruded during convergent deformation whereas later igneous activity accompanied regional orogen-parallel extension, followed by exhumation. In the Mako Belt, thickening of the crust was proposed to have caused heating and the ‘apparent diapiric rise’ of the Diombalou and Bouroumbourou plutons.