Petrology and geochemistry of granulite xenoliths from Central Hoggar (Algeria) — Implications for the lower crust

Granulitic xenoliths, enclosed in Neogene alkali basalts from the Atakor and Adrar N'Addjer volcanic massifs in Central Hoggar, southern Algeria, include mafic metaigneous and felsic metasedimentary rocks. The main mineral phases of metaigneous xenoliths are plagioclase and pyroxenes whereas quartz, feldspars, sillimanite, garnet and sapphirine occur in metasedimentary granulites. The metaigneous xenoliths represent a magmatic sequence of cumulitic and non-cumulitic rocks which resembles gabbroic bodies associated with anorthosites and has calc-alkaline affinities. Similar types and distributions of granulitic xenoliths are widespread and are probably characteristic of the lower crust in general, suggesting its homogeneity on a large scale but heterogencity on a local scale.     

Granulite and pyroxenite xenoliths from the Deccan Trap: insight into the nature and composition of the lower lithosphere beneath cratonic India

Granulite and pyroxenite xenoliths in lamprophyre dykes intruded during the waning stage of Deccan Trap volcanism are derived from the lower crust beneath the Dharwar craton of Western India. The xenolith suite consists of plagioclase-poor mafic granulites (55% of the total volume of xenoliths), plagioclase-rich felsic granulites (25%), and ultramafic pyroxenites and websterites (20%) with subordinate wehrlites. Rare spinel peridotite xenoliths are also present, representing mantle lithosphere. The high Mg #, low SiO₂/Al₂O₃ and low Nb/La (<1) ratios suggest that the protoliths of the mafic granulites broadly represent cumulates of sub-alkaline magmas. All of the granulites are peraluminous and light rare-earth element-enriched. The felsic granulites may have resulted from anatexis of the mafic lower crustal rocks; thus, the mafic granulites are enriched in Sr whereas the felsic ones are depleted. Composite xenoliths consisting of mafic granulites traversed by veins of pyroxenite indicate intrusion of the granulitic lower crust by younger pyroxenites. Petrography and geochemistry of the latter (e.g. presence of phlogopite) indicate the metasomatised nature of the deep crust in this region.Thermobarometric estimates from phase equilibria indicate equilibration conditions between 650 and 1200 °C, 0.7-1.2 GPa suggestive of lower crustal environments. These estimates provide a spatial context for the sampled lithologies thereby placing constraints on the interpretation of geophysical data. Integration of xenolith-derived P-T results with Deep Seismic Soundings (DSS) data suggests that the pyroxenites and websterites are transitional between the lower crust and the upper mantle. A three-layer model for the crust in western India, derived from the xenoliths, is consistent with DSS data. The mafic nature of this hybrid lower crust contrasts with the felsic lower crustal composition of the south Indian granulite terrain.     

Granulite—facies Middle—Lower Crustal Xenoliths from Nueshan Alkali Basalt in Northeastern Anhui Province, China

Metamorphic xenoliths within the Nushan alkali basalt of northeastern Anhui (NEA),China ,are from the middle-lower crust.They could be divided into two end-members:basic and acid.Interme-diate xenoliths are scarcely found.Basic two-pyroxene granulites(pyriclasites) were formed at 720-810℃ and 7-8kb.Petrological and geochemical studies indicate that the primary magma of the protoliths of basic granulites was derived from the metasomatized upper mantle, while the pa-rental magma of the acid end-member was probably produced by partial melting of the basic rocks. The protoliths of charnockites and grey gneisses represent respectively the early and late crystallization products of the granitic magma.The Nushan granulites are much different in many aspects from the granulites exposed in the northern part of North China ,which implies the inhomogeneity regarding to the early evolution of the North China terranc.     

The meta-igneous granulite xenoliths from Kerguelen Archipelago: evidence of a continent nucleation in an oceanic setting

Xenoliths of meta-igneous origin occur as one of the two main types of ultramafic and mafic xenoliths entrained by alkaline lavas of the Kerguelen islands. These are designated type II xenoliths and are subdivided into three mineralogical groups. Subtype IIa and IIc xenoliths are interpreted as crystallisation products of basaltic melts that were emplaced near the crust-mantle boundary during the early tholeiitic–transitional magmatic activity of the Kerguelen islands. Younger magmatism became more alkaline and subtype IIb xenoliths were formed as high-pressure alkaline cumulates related to the last alkaline volcanic stage. Subsequently, the plagioclase-bearing type II rocks have been re-equilibrated under granulite facies conditions. This addition of mafic material around the crust-mantle boundary is consistent with seismic evidence for crustal thickening to 14–20 km. Calculated compressional seismic velocities (Vp) for the basic granulites are consistent with the range of observed Vp in the low-velocity region underlying the oceanic crust. Such growth in the thickness of the oceanic crust may be caused by intrusion of basalts at different levels in the lithosphere and may provide the heat responsible for granulitic metamorphism in the oceanic setting. This study suggests that basic granulites can account for the observed seismic characteristics of oceanic plateaux and can be important components of Kerguelen oceanic lithosphere where there has been large-scale magma production. Moreover we speculate that the Kerguelen islands and perhaps the surrounding plateau represent a continental nucleation process. Received: 30 September 1997 / Accepted: 17 June 1998     

Petrology of high-grade crustal xenoliths in the Chalcatzingo Miocene subvolcanic field,southern Mexico: buried basement of the Guerrero-Morelos platform and tectonostratigraphic implications

The Miocene Chalcatzingo trondhjemitic volcanic field, sited along the southern margin of the Trans-Mexican Volcanic Belt, is a newly discovered locality with deep-seated crustal xenoliths that provide fundamental petrologic information on the nature of the unexposed metamorphic basement. The volcanic field lies along the eastern edge of the Cretaceous Guerrero-Morelos platform, which juxtaposes the Guerrero and Mixteco terranes of southern Mexico. Xenoliths consist of high temperature to ultra-high temperature metapelites as well as mafic and quartzofeldspathic gneisses, all of which show evidence of multiple granulite to amphibolite facies metamorphism and ductile deformation. A detailed petrologic study of representative xenoliths indicates a metamorphic evolution that apparently followed a clockwise pressure–temperature path leading from biotite-sillimanite₁/kyanite(?)-quartz assemblages (M₁) to the assemblage plagioclase-garnet-sillimanite₂-rutile/ilmenite (M₂) with a peak at ～9–11 kbar and >870°C. These conditions were followed by rapid uplift to <6 kbar and >800°C, which produced the decompression assemblage spinel-cordierite-sillimanite₃-corundum ± orthopyroxene ± quartz (M₃) before shallow emplacement of the xenolith-bearing trondhjemitic magma. Three possible sources for the xenoliths are considered: (1) early Mesozoic metasediments buried in the middle crust; (2) Precambrian lower crust; and (3) subducted Cenozoic sediments trapped in the mantle wedge. Based on the deep-seated, polymetamorphic nature of the xenoliths, the Nd depleted mantle model age of an orthogneissic xenolith, and on regional tectonostratigraphic considerations, we suggest that the xenolith source was Proterozoic continental crust. Although old zircon inheritance in the host trondhjemite is minimal, it may be explained by a lack of interaction of the magma with the traversed lithosphere. Studies of Palaeogene shallow intrusions exposed 140 km west of Chalcatzingo in the Guerrero terrane (Pepechuca plug) and 80 km southeast of that place in the Mixteco terrane (Puente Negro dikes) reveal the presence of similar very high-grade aluminous xenoliths. However, these magmas were probably generated by partial melting of Triassic–Jurassic metasediments of the Guerrero terrane underplated by basaltic magmas in Jurassic–earliest Cretaceous times or from Precambrian crust assimilated by underplated mafic magmas of Oligocene age, respectively.     

The western part of the Shuqra volcanic field,South Yemen

The western part of the Shuqra volcanic field consists of a wide area of geologically recent alkali basalt flows and numerous cinder cones, overlying a faulted monoclinal sequence of Jurassic limetones dipping towards the Gulf of Aden and themselves overlying Precambrian basement. The volcanic activity is substantially younger than the series of central vent volcanoes forming the Aden line somewhat to the west along the coast, and the volcanic products are more uniformly basaltic and more silica-undersaturated than those of the Aden volcanoes. Preliminary chemical data reveal an unusual whole-rock compositional trend within the Shuqra lavas which is not explicable in terms of the fractionation of the sparse phenocrysts they contain. A widespread suite of megacrysts not in equilibrium with their host liquids includes hornblende, pleonaste spinel, olivine, plagioclase and clinopyroxene. These may have a close genetic connection with the lavas and possibly represent fragments of refractory source material.     

广东麒麟新生代玄武质角砾岩筒中岩石包体的研究

本文以广东麒麟玄武质角砾岩筒中的各种岩石包体为研究对象,通过详细的岩石学研究,按岩石类型将包体划分为四类：即橄榄岩、辉石岩、辉长岩质麻粒岩、花岗岩和火山－沉积岩,并依据包体的岩石地球化学特征,讨论了它们的成因;其中辉长岩质麻粒岩是来自本区下地壳的样品,是由上地幔部分熔融产生的熔体“底侵”于地壳底部结晶,又经变质作用的产物。同时,本文结合实测的和计算的地球物理参数,讨论了该区的岩石圈分层。     

Pyroxenite and granulite xenoliths from beneath the Scottish Northern Highlands Terrane: evidence for lower-crust/upper-mantle relationships

Xenolith suites from Permian host rocks in Orkney and the extreme NE of the Scottish mainland (Duncansby Ness) are described and compared to those from elsewhere in the Northern Highlands Terrane. Those from the Tingwall dyke, Orkney, comprise roughly equal proportions of ultramafic rocks (wehrlites, clinopyroxenites, websterites, hornblendites) and mafic to felsic rocks (gabbroic, noritic and dioritic granulites, with subordinate tonalites and trondhjemites). Those from Duncansby (45 km to the south) are dominantly olivine-poor ultramafic rocks (clinopyroxenites, pargasite pyroxenites, biotite-pyroxenites), together with granulites grading from gabbroic through to tonalites and trondhjemites. Most of the granulites are meta-igneous, comprising plagioclase and one- or two-pyroxene species with equilibration temperatures of 810-710 °C, and are regarded as samples of the lower crust. Absence of garnet and olivine, together with the association of relatively sodic plagioclase and aluminous pyroxenes, is consistent with derivation from depths corresponding to 5-10 kbar. Positive Eu anomalies in the granulites imply that most originated as plagioclase-rich cumulates from basaltic magmas. Scarce peraluminous quartzo-feldspathic xenoliths, such as a garnet-sillimanite-bearing sample from Duncansby, are regarded as metasedimentary in origin. Pyroxenes (and biotites) in the ultramafic xenoliths tend to have higher mg numbers than those of the granulites, reflecting higher temperatures of formation. Whereas the pyroxene-rich ultramafic rocks may be partly interleaved with the granulites in the lower crust, it is concluded that they also constitute a zone of substantial thickness at or around Moho level, separating the granulites from underlying peridotites, and that they originated as cumulates cognate to the granulites. They have, however, been variably metasomatised with formation of amphibole. This zone may constitute a density trap at which melt fractions, rich in K, Fe, Ti and OH and ascending from the asthenosphere, interact with the ultramafic cumulates, modifying them texturally and modally to produce a complex veined assemblage of clinopyroxene- and pargasite-rich rocks. The metasomatism involved an increase in LREE, HFSE and LILE contents. Some modal and cryptic metasomatism may also have affected the granulites, accounting for the presence of amphibole and relatively high LREE/HREE values (La/Lu 38-206). Since closely comparable xenolith assemblages also occur in Mull at the southwestern extremity of the Northern Highland Terrane, such metasomatised olivine- and orthopyroxene-deficient ultramafic rocks may characterise the shallowest part of the mantle beneath the entire terrane. The strongly bimodal character of the xenolith populations (either ultramafic or mafic grading to felsic) is taken to reflect the sharpness of the petrological Moho in this region.     

From back-arc rifting to arc accretion: the Late Jurassic–Early Cretaceous evolution of the Guerrero terrane recorded by a major provenance change in sandstones from the Sierra de los Cuarzos area,central Mexico

The Guerrero terrane is composed of Middle Jurassic–Lower Cretaceous arc assemblages that were rifted from the North American continental mainland during Late Jurassic–Early Cretaceous back-arc spreading within the Arperos Basin, and subsequently accreted back to the continental margin in the late Aptian. The Sierra de los Cuarzos area is located just 50 km east of the Guerrero terrane suture belt and, therefore, its stratigraphic record should be highly sensitive to first-order tectonic changes. Two Upper Jurassic–Lower Cretaceous clastic units were recognized in the Sierra de los Cuarzos area. The Sierra de los Cuarzos Formation is the lowermost exposed stratigraphic unit. Petrographic data and U-Pb zircon ages suggest that the Sierra de los Cuarzos Formation was derived from quartz-rich sedimentary and igneous sources within the North American continental mainland. The Sierra de los Cuarzos Formation is overlain by the Pelones Formation, which is composed of volcanoclastic sandstones derived from a mix of sources that include the mafic arc assemblages of the Guerrero terrane and quartz-rich sedimentary and volcanic rocks exposed in the continental mainland. The provenance change documented in the Sierra de los Cuarzos area suggests that the Pelones Formation was deposited when the Arperos Basin was closed and the Guerrero terrane was colliding with the North American continental mainland. Based on these data, we interpret the Pelones Formation as the syn-tectonic stratigraphic record associated with the accretion of the Guerrero terrane.     

The lithosphere beneath the Sangilen Plateau, Siberia: evidence from peridotite, pyroxenite and gabbro xenoliths from alkaline basalts

Summary Ultramafic and mafic xenoliths in Ordovician Agardag alkaline basalt dikes from the Sangilen Plateau, southeastern Siberia, provide samples from the upper mantle and crust beneath central Asia. Three major groups were distinguished among the xenoliths: Group I xenoliths are spinel lherzolites, Group II xenoliths are spinel-garnet clinopyroxenites, and Group III comprises gabbroic xenoliths with two subgroups: Group IIIa comprises garnet bearing gabbroids and Group IIIb is represented by garnet-free gabbroids. The spinel lherzolite xenoliths represent the uppermost lithospheric mantle beneath the Sangilen Plateau and have geochemical characteristics similar to those of primitive mantle. Spinel-garnet clinopyroxenite and gabbroic xenoliths are of igneous origin and represent fragments of intrusive bodies crystallized at depths close to the mantle-crust boundary, as well as in the lower and the upper crust. The gabbroic xenoliths are evidently the crystallization products of melts similar in major and trace element composition to parental magma of the Bashkymugur gabbronorite-monzodiorite intrusion. Gabbroic xenoliths from the Ordovician Agardag alkaline basalt dikes demonstrate the presence of intermediate magmatic chambers within the crust beneath the Sangilen Plateau during the Early Palaeozoic. The relatively high equilibration temperatures of the mantle and lower crust xenoliths in the Agardag alkaline basalt dikes are largely attributable to a plume occurring beneath the Sangilen Plateau during the Ordovician.     

Fluid inclusions as petrogenetic indicators in granulite xenoliths,Pali-Aike volcanic field,Chile

Granulite xenoliths within alkali olivine basalts of the Pali-Aike volcanic field, southern Chile, contain the mineral assemblage orthopyroxene + clinopyroxene + plagioclase + olivine + green spinel. These granulites are thought to be accidental inclusions of the lower crust incorporated in the mantle-derived basalt during its rise to the surface. Symplectic intergrowths of pyroxene and spinel developed between olivine and plagioclase imply that the reaction olivine+plagioclase = Al-orthopyroxene + Al-clinopyroxene + spinel (1) occurred during subsolidus cooling and recrystallization of a gabbroic protolith of the granulites.Examination of fluid inclusions in the granulites indicates the ubiquitous presence of an essentially pure CO₂ fluid phase. Inclusions of three different parageneses have been recognized: Type I inclusions occur along exsolution lamellae in clinopyroxene and are thought to represent precipitation of structurally-bound C or CO₂ during cooling of the gabbro. These are considered the most primary inclusions present. Type II inclusions occur as evenly distributed clusters not associated with any fractures. These inclusions probably represent entrapment of a free fluid phase during recrystallization of the host grains. IIa inclusions are found in granoblastic grains and have densities of 0.68–0.88 g/cm³. Higher density (=0.90–1.02 g/cm³) IIb inclusions occur only in symplectite phases. Secondary Type III CO₂+glass inclusions with =0.47–0.78 g/cm³ occur along healed fractures where basalt has penetrated the xenoliths. Type III inclusions appear related to exsolution of CO₂ from the host basalt during its ascent to the surface. These data suggest that CO₂ is an important constituent of the lower crust under conditions of granulite facies metamorphism, indicated by Type I and II fluid inclusions, and of the mantle, as indicated by Type III inclusions.Correlation of fluid inclusion densities with P-T conditions calculated from both two-pyroxene geothermometry and reation (1) indicate emplacement of a gabbroic pluton at 1,200–1,300° C, 4–6 kb; cooling was accompanied by a slight increase in pressure due to crustal thickening, and symplectite formation occurred at 850±35° C, 5–7 kb. Capture of the xenoliths by the basalt resulted in heating of the granulites, and CO₂ from the basalt was continuously entrapped by the xenoliths over the range 1,000–1,200° C, 4–6 kb. Examination of fluid inclusions of different generations can thus be used in conjunction with other petrologic data to place tight constraints on the specific P-T path followed by the granulite suite, in addition to indicating the nature of the fluid phase present at depth.     

The buried southern continuation of the Oaxaca-Juarez terrane boundary and Oaxaca Fault,southern Mexico: Magnetotelluric constraints

Thirty magnetotelluric soundings were made along two NW–SE profiles to the north and south of Oaxaca City in southern Mexico. The profiles crossed the N–S Oaxaca Fault and the Oaxaca-Juarez terrane boundary defined by the Juarez mylonitic complex. Dimensionality analysis of the MT data showed that the subsurface resistivity structure is 2D or 3D. The Oaxaca and correlative Guichicovi terranes consist of ca. 1–1.4 Ga granulitic continental crust overlain by Phanerozoic sedimentary rocks, characterized by high and low resistivities, respectively. The Juarez terrane consists of oceanic Mesozoic metavolcanic and metasedimentary rocks, characterized by a low to medium resistivity layer, that is approximately 10 km thick. The Oaxaca Fault is a Cenozoic aged, normal fault that reactivated the dextral and thrust Juarez mylonitic complex north of Oaxaca City: its location south of Oaxaca City is uncertain. In the southern profile, the MT data show a ca. 20–50 km wide, west-dipping, relatively low resistivity zone material that extends through the entire crust. This is inferred to be the Juarez terrane bounded on either side by the ca. 1–1.4 Ga granulites. The Oaxaca Fault is imaged only by a major electrical resistivity discontinuity (low to the west, high to the east) along both the western border of the Juarez mylonitic complex (northern profile) and the San Miguel de la Cal mountains (southern profile) suggesting continuity.     

Petrogenetic evolution of basaltic lavas from Balhaf–Bir Ali Plio-Quaternary volcanic field, Arabian Sea, Republic of Yemen

The Plio-Quaternary Balhaf–Bir Ali volcanic field (BBAVF) constitutes one of the largest volcanic fields in SE Yemen, covering some 500 km². It comprises cinder cones complexes associated with vesicular lava flows and scoria–spatter cones. In many places, ultramafic xenoliths are encountered within these volcanics. The explosive volcanism is mainly of alkaline character including alkali olivine basalt, hawaiite and mugearite together with subordinate tuffaceous trachytes. Major, trace and REEs data from the basaltic rocks of the BBAVF are interpreted in terms of a mantle-lithospheric origin in which crustal contribution during the initial stage of rift magmatism has occurred. Magma genesis may have resulted from plume-derived melt introducing into the base of the lithosphere. A mantle plume source is proposed for the Balhaf–Bir Ali basaltic lavas that are here interpreted as having been generated by partial melting of garnet lherzolite in the uppermost part of asthenosphere. The magmatic evolution of Balhaf–Bir Ali volcanic field may be accounted for by the recent models developed for plume structure.     

火成岩中的麻粒岩相捕虏体研究新进展及其对认识地壳生长与演化的意义

火成岩中的麻粒岩相捕虏体主要分为基性熔体捕虏体，酸性变质火成岩捕虏体与变质沉积岩捕虏体。麻粒岩相捕虏体的锆石离子探针年龄与上地壳花岗岩的侵位年龄，造山运动时代基本一致。基性熔体的下垫作用是地壳生长的一种重要方式，并且代表了一种有效的幔向壳的物质与能量迁移。地下壳部分熔与麻粒岩相变质作用是壳内分异的重要途径。     

滇西腾冲地块新生代火山岩中高温麻粒岩包体的发现及成因

腾冲地块芒棒乡新生代新近纪芒棒组火山岩的研究,发现粗面安山岩中含有较多麻粒岩包体,主要类型为长英质麻粒岩和二辉麻粒岩,直径一般为3~5 cm,略具定向排列构造,粒状变晶结构特征.长英质麻粒岩包体平衡温度和压力为:869~969 ℃,0.94~1.00 GPa,二辉麻粒岩包体平衡温度为:841~972 ℃,均显示为高温麻粒岩相变质;寄主岩粗面安山岩中斑晶结晶估算的温度和压力为:1 008~1 059 ℃,1.26~1.33 GPa,表明它们形成和起源于下地壳到上地幔顶部之间.二辉麻粒岩包体电子背散射衍射(EBSD)显微组构分析表明,斜方辉石(紫苏辉石)和单斜辉石(普通辉石)均具有明显晶格优选方位(LPO),斜方辉石和单斜辉石主滑移系分别为:(010)[001]和(100)[001]、(010)[001],变形机制为位错蠕变,说明麻粒岩包体经历了高温塑性变形变质作用.新生代新近纪火山岩中高温麻粒岩的发现和研究表明,腾冲新生代岩浆活动可能与印度板块和腾冲地块俯冲-碰撞造山后的伸展垮塌-拆沉和板片断离,以及随后的上地幔物质上涌减压熔融产生玄武质岩浆底侵,导致的下地壳发生高温变质及火山岩浆作用有密切关系.      

Neogene-Quaternary Volcanic forms in the Carpathian-Pannonian Region: a review

Neogene to Quaternary volcanic/magmatic activity in the Carpathian-Pannonian Region (CPR) occurred between 21 and 0.1 Ma with a distinct migration in time from west to east. It shows a diverse compositional variation in response to a complex interplay of subduction with rollback, back-arc extension, collision, slab break-off, delamination, strike-slip tectonics and microplate rotations, as well as in response to further evolution of magmas in the crustal environment by processes of differentiation, crustal contamination, anatexis and magma mixing. Since most of the primary volcanic forms have been affected by erosion, especially in areas of post-volcanic uplift, based on the level of erosion we distinguish: (1) areas eroded to the basement level, where paleovolcanic reconstruction is not possible; (2) deeply eroded volcanic forms with secondary morphology and possible paleovolcanic reconstruction; (3) eroded volcanic forms with remnants of original morphology preserved; and (4) the least eroded volcanic forms with original morphology quite well preserved. The large variety of volcanic forms present in the area can be grouped in a) monogenetic volcanoes and b) polygenetic volcanoes and their subsurface/intrusive counterparts that belong to various rock series found in the CPR such as calc-alkaline magmatic rock-types (felsic, intermediate and mafic varieties) and alkalic types including K-alkalic, shoshonitic, ultrapotassic and Na-alkalic. The following volcanic/subvolcanic forms have been identified: (i) domes, shield volcanoes, effusive cones, pyroclastic cones, stratovolcanoes and calderas with associated intrusive bodies for intermediate and basic calclkaline volcanism; (ii) domes, calderas and ignimbrite/ash-flow fields for felsic calc-alkaline volcanism and (iii) dome flows, shield volcanoes, maars, tuffcone/tuff-rings, scoria-cones with or without related lava flow/field and their erosional or subsurface forms (necks/ plugs, dykes, shallow intrusions, diatreme, lava lake) for various types of K- and Na-alkalic and ultra-potassic magmatism. Finally, we provide a summary of the eruptive history and distribution of volcanic forms in the CPR using several sub-region schemes.     

Granulites and related rocks in variscan median Europe: A dualistic interpretation

On the basis of differences in structural, petrological and radiometric features, two main clear-cut groups can be distinguished for the granulites of the Variscan median Europe.Group I comprises lenses of eclogite-granulite scattered in the internal zones of the Hercynian belt, whereas Group II includes granulitic slices related to deep-seated Alpine lineaments and xenoliths scavenged by recent volcanoes.Group I granulites belong to early thrust nappes of the Variscan orogen; they display high-pressure assemblages and are commonly associated with eclogites and garnet-peridotites. From radiometric data, this HP granulitic-eclogitic metamorphism happened around 450-400 Ma throughout median Europe. A subduction context is suggested by the low T/P gradient, the occurrence of eclogites and the contemporaneity with some blueschist-facies rocks.Group II granulites are characterized by massive occurrences, intermediate-pressure granulite-facies parageneses and the absence of eclogites. Granulitic paragneisses often display a pronounced depletion in granitophile elements and may appear as degranitized restites. Numerous basic-ultrabasic complexes occur and are interpreted as deep-seated, layered igneous intrusions emplaced during the granulitic metamorphism. Radiometric dating indicates a late Hercynian (ca. 300 Ma) age. The high T/P gradient, the occurrence of synchronous basic intrusions and the post-collision context suggest a granulitic event due to a major thermal anomaly.The Variscan cycle is thus characterized by two granulite-forming episodes. “Older granulites” of the HP type result from an early compressive stage in a crustal subduction context and “younger granulites”, from a major thermal event which originated in the upper mantle and annealed the deepest parts of the Variscan belt. A broadly similar duality in the genesis of granulitic rocks may be anticipated in other collisional belts.     

内蒙古卓资县玛珥式火山群的发现和意义

玛珥式火山易被掩埋和不易识别,它们与人们常见的高出地面的锥状体火山口不同,而以负地形、圆形或椭圆形状、面积小和高度密集成群出现为特征。位于卓资县北部的玛珥式火山群无论在规模上和保存完好程度上都是世界上第一位的。进一步研究它们将填补我国地学界对负地形玛珥式火山口的认识、研究第四纪火山地质、推动当地旅游方面都具有意义。     

TECTONIC EVOLUTION OF THE EOCENE DROSH-VOLCANO-SEDIMENTARY BASIN IN NW-KOHISTAN ISLAND ARC TERRANE, HINDUKUSH, N. PAKISTAN

In NW Himalayas, the suture zone between the collided Indian and the Karakoram plates is occupied by crust of the Cretaceous Kohistan Island\|Arc Terrane [1] . Late Cretaceous (about 90Ma) accretion with the southern margin of the Karakoram Plate at the site of the Shyok Suture Zone turned Kohistan to become an Andean\|type margin. The Neotethys was completely subducted at the southern margin of Kohistan by Early Tertiary, leading to collision between Kohistan and continental crust of the Indian plate at the site of the Main mantle thrust.More than 80% of the Kohistan terrane comprises plutonic rocks of (1) ultramafic to gabbroic composition forming the basal crust of the intra\|oceanic stage of the island arc, and (2) tonalite\|granodiorite\|granite composition belong to the Kohistan Batholith occupying much of the intermediate to shallow crust of the terrane mostly intruded in the Andean\|type margin stage [2] . Both these stages of subduction\|related magmatism were associated with volcanic and sedimentary rocks formed in Late Cretaceous and Early Tertiary basins. This study addresses tectonic configuration of Early Tertiary Drosh basin exposed in NW parts of the Kohistan terrane, immediately to the south of the Shyok Suture Zone.     

Catazonal xenoliths in French Neogene volcanic rocks: Constitution of the lower crust

87 samples of granulitic xenoliths scavenged by the Neogene volcanoes in French Massif Central, have been analysed for major elements and Li, Rb, Sr, Ba, V, Cr, Co, Ni, Cu and Zn. These data confirm the presence of rocks with both sedimentary and igneous derivation chemically similar to other granulitic rocks, especially those of the Frazer Range in Australia. Relatively high K and Rb concentration of the metasedimentary rocks conflicts with the hypothesis of systematic depletion of lithophile elements in the lower continental crust.The composition and the elemental variations of the igneous derived rocks suggest a parent magma and a differentiation trend characteristic of tholeiitic rocks. It is proposed that this differentiation was produced at low pressure and consequently, the elemental variation existed before the high pressure granulite facies metamorphism of this series.Although an andesite average of the lower crust (between 25–26 and 12–16 km), directly beneath the Bournac pipe (Velay) is calculated, confirming earlier suggestions of a relatively basic composition but it does not mean that this crust was derived from andesitic rocks.