Impact of gneissic layering and localized incipient melting upon melt flow during experimental deformation of migmatites

In this study, we test experimentally the role of compositional layering as a key parameter for controlling melt flow in a natural migmatite during coaxial deformation. We performed in – situ pure-shear experiments on two natural gneisses. The first gneiss is weakly foliated with minerals homogenously distributed. The second gneiss shows a pronounced compositional layering of alternating quartz – feldspar – rich and biotite – muscovite – rich layers. Experimental conditions were selected to obtain homogeneous melt distribution in the homogeneous gneiss and heterogeneous melt distribution in the layered gneiss. Initial melt distribution is not modified by deformation in experiments on the homogeneous gneiss, implying that melting products did not migrate from their initiation sites. In contrast, melt flowed in shear zones or in inter-boudin positions during experimental deformation of the heterogeneous gneiss. These experiments attest to the strong influence of initial gneissic layering on melting pattern, melt segregation and flow during deformation of partially molten rocks.     

Crustal xenoliths from Tertiary volcanics of the Northern Hessian Depression

A suite of crustal xenoliths from Tertiary basaltic tuffs of the Northern Hessian Depression (NHD) volcanic field comprises abundant meta-igneous pyroxene granulites of mafic, noritic to anorthositic, IAT and tonalitic composition. Less abundant are granitic, tonalitic and leucogranitic gneisses and metasedimentary xenoliths. A total of 49 samples were analyzed for modal compositions, for major and trace elements (including Li, Rb, Sr, Ba, Cs, V, Sc, Cr, Co, Ni, Y, Zr, Nb, Ta, Hf, Th and REE) and oxygen isotopes. Two-pyroxene thermometry yields temperatures between 700 and 900° C for mafic and noritic granulites. Feldspar thermometry indicates temperatures of 660°–710° C for tonalitic granulites and 470°–520° C for granitic and tonalitic gneisses. One highly depleted sillimanite-rich metasediment contains cordierite and garnet which have equilibrated at temperatures of 780° C. The general lack of garnet in the mafic and noritic granulites and the presence of sillimanite in felsic xenoliths indicates that metamorphic pressures have not exceeded 10 kb. Major and trace element data and oxygen isotope compositions of the mafic granulites are compatible with an origin from spilitized enriched-type MORB rocks (enrichment in ¹⁸O to 11 and in Li to 34 ppm at average SiO₂ contents of 44.1 wt%). These low-T spilites were transformed into amphibolites and then pyroxene granulites during subsequent high temperature metamorphic events. Low Si, Al, K, and Rb concentrations along with An contents in plagioclase ranging from near 50 to 98 mole percent suggest that amphibolite facies protoliths have generated tonalitic melts during partial melting at temperatures above 700° C. The mafic granulite xenoliths are interpreted as restites whereas the tonalitic samples probably represent the extracted partial melts derived by 20 to 30 percent degree of melting. Metasedimentary xenoliths strongly depleted in granitic component could represent restites from which granitic S-type partial melts have been extracted. Tonalitic and leucogranitic gneisses including one trondhjemite xenolith have many chemical characteristics (e.g. REE distribution) in common with tonalite-trondhjemite-granodiorite suites of the North Atlantic region but cannot be accounted for a more specific origin. Estimated elastic properties of the main types of NHD xenoliths yield P-wave velocities of 6.0–6.4 km^-1 for granitic, tonalitic and trondhjemite gneisses including tonalitic granulites and 6.5–7.0 for the more mafic xenoliths. When compared with two seismic depths-V_p profiles these data are in accordance with a model where the mafic, andesitic, noritic and tonalitic granulites comprise abundant rock types at depths between 29 km (Moho) and 20 km which mainly consists of old oceanic crust including subduction related volcanic products. The more felsic xenoliths probably represent material from depths between 12 and 20 km.     

The western border of the Archaean province of the Baltic shield: evidence from northern Sweden

Restricted areas of acid to intermediate gneisses in northernmost Sweden are known to be Archaean and several other gneissic granodioritic to tonalitic massifs have been suggested to be of this age. To delimit the western border of the Archaean province of the Baltic Shield, and to characterize the Swedish Archaean, we have studied the 2.83 Ga old Soppero gneiss and 8 of the proposed Archaean massifs in northern Sweden by Sm-Nd isotopic analyses, U-Pb zircon dating (in two cases), and geochemistry.Of the Archaean gneisses proposed thus far, only those from the Soppero and Kukkola areas have Archaean ages (2.83 and 2.67 Ga, respectively). These gneisses have geochemical characteristics such as LREE-enrichment, low U-contents, and low K/Na-ratios, which are typical of Archaean TTG (tonalite-trondhjemite-granodiorite) rocks worldwide. The REE results indicate that the Soppero and the Kukkola gneisses were generated by partial melting of basic rocks, presumably amphibolites. According to the Sm-Nd isotopic data for the Soppero gneiss, this process occurred only a short time after initial segregation from the mantle ( _Nd values between +0.9 and +3.5). The Kukkola gneiss, however, has longer crustal residence time as indicated by negative _Nd values (–1.0 and –2.5) at the U-Pb zircon age of 2.67 Ga. Evidence in support of an important event at that time is given by secondary overgrowth on zircon cores in the 2.83 Ga old Soppero gneiss.The Sm-Nd isotope results show that the Proterozoic granitoids in northern Sweden have a decreasing involvement of Archaean source material with increasing distance from the Archaean areas. Before the Proterozoic reworking, the rather small Archaean areas in northern Sweden presumably belonged to a large continuous Archaean craton. The western palaeoboundary of this craton in Sweden probably follows approximately a line extrapolated from the western limit of the Archaean of central Finland, passing west of Kukkola, and then continues to the Lofoten area in northern Norway.     

A 3500 Ma plutonic and volcanic calc-alkaline province in the Archaean East Pilbara Block

Variably foliated, predominantly granodioritic plutonic rocks from the northern part of the Shaw Batholith in the east Pilbara Archaean craton are dated at 3,499±22 Ma (2σ errors) by a whole-rock Pb-Pb isochron. These rocks intrude the surrounding greenstone sequence, and their age is indistinguishable from that sequence. High strain grey gneisses which occupy much of the western and southern Shaw Batholith are chemically and isotopically similar to the North Shaw suite and are inferred to have been derived from this suite by tectonic processes. Felsic volcanics within the greenstones together with a major portion of the granitic batholiths apparently formed in a calc-alkaline volcanic and plutonic province at ～3,500 Ma. This volcanic and plutonic suite is similar to modern calc-alkaline suites on the basis of major element, rare earh element and most other trace element contents. The Archaean suite contrasts with modern equivalents only in having lower concentrations of HREE and higher concentrations of Ni and Cr. The average composition of the North Shaw suite is similar to that of Archaean gneiss belts for most elements and is consistent with the previously formulated hypothesis that the Shaw Batholith is transitional to the upper crustal level of a high-grade gneiss belt. Enrichment of the gneissic crust in the Shaw Batholith in alkali and heat-producing elements is inferred to have taken place by both igneous and hydrothermal processes over a protracted time interval. Late- and post-tectonic adamellite and granite melts intrude the gneissic rocks and there is isotopic evidence consistent with the gneisses being substantially enriched in Rb by pegmatite injection at ～3,000 Ma.     

SHRIMP U–Pb zircon geochronology of the Strzelin gneiss,SW Poland: evidence for a Neoproterozoic thermal event in the Fore-Sudetic Block,Central European Variscides

Zircon ages recorded in gneissic rocks have recently been used as criteria to define and correlate various tectonic units and crustal blocks in the central European Variscides. A SHRIMP U–Pb zircon geochronological study of the Strzelin gneiss in the Fore-Sudetic Block (SW Poland) indicates the presence of: (1) inherited zircon cores of Palaeo- to Mesoproterozoic ²⁰⁶Pb-²³⁸U ages (between ca. 2,000 and 1,240 Ma), and (2) zoned rims of Neoproterozoic age with two distinct means of 600±7 and 568±7 Ma. The Proterozoic age range of the cores suggests that different Precambrian crustal elements were the source for the protolith of the gneiss. A likely scenario is the erosion of various Proterozoic granites and gneisses, sedimentation (after 1,240 Ma), and partial resistance of the original components to subsequent metamorphic dissolution and/or anatectic resorption (in Neoproterozoic times). The zoned zircon rims of both of the younger Neoproterozoic ages are indistinguishable in the cathodoluminescence images. The data are interpreted in terms of two different thermal events inducing zoned zircon overgrowth at ca. 600 and 568 Ma. In general, the new results confirm earlier assumptions of the Proterozoic age of the gneiss protoliths, and indicate their similarity to orthogneisses in the East Sudetes tectonic domain (e.g. the Velké Vrbno and Desná gneisses). The Neoproterozoic dates are different from the age of 504±3 reported earlier for the Gocicice gneiss from a neighbouring locality in the Strzelin Massif. The new data strongly indicate a Moravo-Silesian (Bruno-Vistulian) affinity for the Strzelin gneiss and support the hypothesis that the Strzelin Massif lies within the tectonic boundary zone between the West- and East Sudetes domains, which represents the northern continuation of the Moldanubian Thrust.     

海南岛昌江灰色片麻岩包体及其地球化学特征

灰色片麻岩包体出露于海南岛昌江中元古代花岗质岩石中,主要由花岗闪长质-英云间长质片麻岩及其糜棱岩组成,遭受过强烈的剪切作用和变质作用,野外产出与岩石矿物学特点及其地球化学特征均表明灰色片麻岩是正片麻岩,是以片麻岩形式存在的高Al型英云间长岩-花岗间长岩深成岩体。具有轻稀土富集型和轻稀土亏损型两种分配模式,岩石成因复杂,其Nd模式年龄为2735～2791Ma,说明海南岛存在太古宙古陆壳。||关键词##4灰色片麻岩包体;;英云间长岩-花岗间长岩;;地球化学特征;;海南昌江     

The Isotopic Composition of Strontium in the Skaergaard Intrusion, East Greenland

The isotopic composition of strontium in the different partsof the differentiated Skaergaard intrusion has been determined.The average Sr⁸⁷/Sr⁸⁶ ratio for the basic rocks was found tobe 0?7065?0?002. Higher values, between 0?7101 and 0?7303, wererestricted to the late-stage acid granophyres. The Sr⁸⁷/Sr⁸⁶ratios for the basic Skaergaard rocks are similar to those foundby previous workers. The enrichment in Sr⁸⁷ expressed in theSr⁸⁷/Sr⁸⁶ ratio is taken to indicate contamination of the acidgranophyres by a source enriched in Sr⁸⁷. From considerationsbased upon circumstantial evidence the average country rock,composed of old Precambrian acid to intermediate gneiss, isnot sufficiently enriched in.Sr⁸⁷ to account for the Sr⁸⁷ enrichmentobserved in the acid granophyres by a simple assimilation process.At the present stage of the investigation the enrichment ofthe acid granophyres in Sr⁸⁷ is unexplained.     

Geochemistry and origin of granitic rocks,Scourian Complex,NW Scotland

Concordant granite sheets from the granulite facies Scourian Complex, N.W. Scotland exhibit the following features:

1. a common planar fabric with their host pyroxene granulites;
2. the presence of an exsolved ternary feldspar phase;
3. a low-pressure, water-saturated minimum composition;
4. K/Rb ratios (450–1,350) distinctly higher than most upper crustal granites but similar to the surrounding granulites;
5. low absolute concentrations of the rare earth elements (REEs), light REE enrichment, and large positive Eu anomalies.

It is proposed that the granite sheets have originated by anatexis of gneisses undergoing granulite facies metamorphism — gneisses that were already essentially dry and depleted in incompatible elements. Their unusual trace element chemistry may be explained by either disequilibrium melting and/or sub-solidus reequilibration of the granite sheets with the surrounding gneisses. Isotopic and trace element data suggest that cross-cutting, potash-rich pegmatites represent reworking of the granite sheets during a later amphibolitization.     

Lead ages,reset rubidium-strontium ages and implications for the Archaean crustal evolution of the Diemals area,Central Yilgarn Block,Western Australia

The Pb-Pb whole-rock geochronology of Archaean granitic and gneissic rocks from the Diemals area in the Central Yilgarn granite-greenstone terrain provides important constraints on crustal evolution. The regionally extensive banded gneisses, previously considered as candidates for basement to the greenstones give a Pb-Pb whole-rock age of 2700 ± 97 Ma (2σ errors). This is within error of previously published Rb-Sr and Sm-Nd gneiss ages and also within error of the Sm-Nd ages on the greenstones in the Eastern Goldfields Province. Two synkinematic plutons give Pb-Pb whole-rock ages (2737 ± 62 Ma and 2700 ± 100 Ma) and Pb isotopic compositions consistent with the hypothesis, based on field and geochemical relations, that these plutons were derived by partial melting of the precursors to the banded gneisses. Assuming this, the combined data date the melting event at 2723 ± 25 Ma with a model source μ value of 8.18 ± 0.02. This source μ value is close to the range postulated for mantle values and restricts the crustal history of the precursors to less than ~200 Ma. A post-kinematic pluton with a whole-rock Pb-Pb age of 2685 ± 26 Ma and μ value of 8.26 ± 0.02 puts a younger limit on this relatively short lived crustal accretion-differentiation event.Comparison of Pb-Pb and Rb-Sr whole-rock dates for the plutons suggests that the latter became closed systems up to 200 Ma after the Pb-Pb ages, and that the plutons gained or lost Rb or Sr at this time.     

U–Pb ID-TIMS dating of igneous and metaigneous rocks from the El-Sibai area: time constraints on the tectonic evolution of the Central Eastern Desert,Egypt

This paper presents new ID-TIMS U–Pb zircon and titanite ages from the El-Sibai gneiss complex in the Eastern Desert of Egypt. The zircon data support previous studies, indicating that the protoliths of the gneissic (oldest) units in the area were emplaced during the East African orogeny, and do not represent an older pre-Neoproterozoic, reworked cratonic basement. The crystallization ages of three compositionally distinct orthogneiss protoliths are c. 685, 682 and 679 Ma, respectively. A U–Pb titanite age from one orthogneiss overlaps with the protolith age, indicating that the gneisses did not undergo post-magmatic high-temperature metamorphism. The gneissic textures of the rocks are therefore interpreted to reflect syn-emplacement deformation. This, and evidence for static amphibolite facies metamorphism in country-rock metavolcanics, lead us to conclude that the gneisses of El-Sibai do not represent an exhumed middle crustal gneiss dome, but are part of the island arc affined allochthon into which they were emplaced synchronously with NW-ward nappe translation. We also report ages from rocks cross-cutting the gneisses and the surrounding island arc affined assemblages that yield the hitherto youngest robust pre-Cretaceous intrusive ages in the Eastern Desert. The dated rocks are an anorthosite and a cross-cutting syenogranite giving ages of c. 541 and 540 Ma, respectively. We consider this late magmatic pulse to be anorogenic, most likely reflecting a separate extensional event involving asthenospheric upwelling and decompression melting of the mantle.     

The iron-rich suite from the Amîtsoq gneisses of southern West Greenland: early Archaean plutonic rocks of mixed crustal and mantle origin

A distinctive group of augen gneisses and ferrodiorites (termed the iron-rich suite) is a component of the early Archaean Amîtsoq gneisses of southern West Greenland. The iron-rich suite outcrops south of the mouth of Ameralik fjord in an area that underwent granulite facies metamorphism in the early Archaean. The iron-rich suite forms approximately 30% of the Amîtsoq gneiss of this area and occurs as sheets and lenses up to 500 m thick. The rest of the Amîtsoq gneisses are predominantly tonalitic-granodioritic, banded grey gneisses. Despite intense deformation and polymetamorphism, there is local field evidence that the iron-rich suite was intruded into the grey gneisses after they had been affected by tectonism and metamorphism. The banded grey gneisses are interpreted as 3,700 to 3,800 Ma old; U-Pb zircon ages from the iron-rich suite give concordia intercepts at circa 3,600 Ma.Coarse grained augen gneisses with microcline mega-crysts are the dominant lithology of the iron-rich suite. They are mostly granodioritic, grading locally into granite and diorite, and are generally rather massive, but locally have well-preserved layering or are markedly heterogeneous. Mafic components are commonly concentrated into clots rich in hornblende and biotite and containing apatite, ilmenite, sphene and zircon. Variation in the proportion of these clots is the main reason for the compositional variation of the augen gneisses. The ferrodiorites of the suite occur as lenses in the augen gneisses. Leucocratic granitoid sheets locally cut the iron-rich suite. The augen gneisses and ferrodiorites have geochemical characteristics in common, such as high Fe/Mg values and high contents of FeOt, TiO₂, P₂O₅, Zr, Y and total REE (rare earth elements).The iron-rich suite probably formed as follows:Heating of the lower crust adjacent to mantle-derived basic intrusions caused melting of the lower crust, giving rise to granodioritic magmas. Disruption of partially crystallised basic intrusions caused mixing of the crustal melts and the fractionated mantle melts to produce the augen gneisses with their high FeOt, TiO₂, P₂O₅, Zr, Y and total REE enrichment. Fragmented, crystallised parts of the basic intrusions gave rise to the ferrodiorite inclusions. These heterogeneous plutons rose to higher crustal levels where they crystallised as sheets and possibly were responsible for the local granulite facies metamorphism. The granitoid sheets that cut the iron-rich suite are interpreted as crustal melts of local origin.The iron-rich suite resembles Proterozoic rapakivi granite-ferrodiorite-norite (anorthosite) associations which form characteristic suites in late- to post-tectonic environments in recently thickened sial. The occurrence of this type of magmatism in the early Archaean is evidence of the complex, polygenetic nature of the oldest known continental crust.     

Chemical interrelationships in a low-pressure granulite terrain in Namaqualand. South Africa,and their bearing on granite genesis and the composition of the lower crust

The results of a chemical study of a suite of low-pressure granulite facies rocks in Namaqualand, South Africa, are reported. The area is underlain by augen gneisses and quartzites, which contain interlayered granular quartz-feldspar rocks (termed ‘granulites’) derived by extensive partial melting of the gneiss. The K/Rb ratio of the gneiss increases from 140 to 250 over a melting interval of 70%: the rate of increase being influenced by the presence of biotite. Simultaneously K/Ba and Rb/Sr decrease from 80 to 25 and from 4 to 0.3, respectively. The partial melts (granulites), which reflect, in part, a cumulate character, have similar K/Rb ratios to the parent gneiss (175) but larger K/Ba (238) and Rb/Sr (5) ratios, due to the retention of Ba and Sr in the residue.Three granites intrude the gneisses. One of these was produced by very advanced partial melting of the gneiss. Continuity of chemical composition suggests that the remaining two granites, although spatially separate, are comagmatic, and evolved by feldspar fractionation during ascent. Lower Sr⁸⁷/Sr⁸⁶ ratio coupled with enrichment of Ba, Sr and Rb in the parent magma of these granites relative to the country rocks precludes local derivation and indicates a lower crustal source rock of intermediate composition.The progressive increase in cafemic character of the gneisses, which is similar to that observed in world granulite terrains as a whole, coupled with intrusive granite which reflects reworking of the lower crust in the area studied, supports a partial melting model for the development of a lower crust of progressively more cafemic composition.     

Tectonometamorphic Evolution of Jutulsessen,Gjelsvikfjella, cDML,East Antarctica

The Jutulsessen area, can provide a vital clue to the supercontinent assembly of Gondwana Land as it is situated within the Circum East Antarctic Mobile Belt just east of the Penksockett rift marking the divide between the central Dronning Maud Land from the Western Dronning Maud Land. This landmass is dominated by migmatitic quartzo-feldspathic rocks intruded by syn to post-tectonic granites. The work highlights the data from western part cDML area with a view to arrive at a more comprehensive model for the cDML and subsequently to the super continent assembly. Granitic and migmatitic gneisses comprising of amphibolitic and biotite rich enclaves. The gneisses show variations from quartzo-felspathic gneiss to amphibolitic gneiss. The area has witnessed complex geological history involving at different deformational episodes with concomitant metamorphism. The pervasive dominant foliation trends NW-SE with shallow to medium dips towards SW. In the Stabben area, a nonfoliated intrusive syenite-gabbro pluton limits the gneissic exposures. Compositionally, the orthogneisses plot in the monzogranitegranodiorite field where as the mafic dykes/enclaves plot in the basalt-andesite-rhyodacite field. The bulk geochemical characteristics suggest significant crustal contamination. Garnet-biotite Fe-Mg exchange thermometry gives peak metamorphic temperature of 483° C for the gneisses and 628° C for the dioritic enclave within gneisses. A peak metamorphic grade of upper amphibolite to granulite facies is deduced from the mineral assemblages. Widespread anatexis has led to extensive occurrence of migmatites in the area. Recent geochronological studies assign an age of 1170 Ma to 970 Ma for the migmatites/gneisses and an emplacement age of 501 Ma for the Stabben gabbro and syenite. The discriminant plots of the Jutulsessen rocks indicate diverse origin ranging from pre-plate collision to post-collision orogenic tectonic setting. The mafic enclaves/dykes show ocean island arc to MORB affinities. Voluminous addition of juvenile crust during the Pan-African orogeny strongly overprints earlier structures.     

阿尔金造山带青白口纪亚干布阳片麻岩年龄、地球化学特征及其地质意义

出露于阿中地块库木塔什萨依一带的亚干布阳片麻岩主要岩性为黑云斜长片麻岩、黑云二长片麻岩。利用LA-ICPMS方法进行锆石微区U-Pb同位素定年,得到206Pb/238U年龄加权平均值为900.2±2.9Ma,表明亚干布阳片麻岩原岩形成于新元古代早期青白口纪;地球化学结果显示,主量元素具有高SiO_2、Al_2O_3、K_2O+Na_2O含量,低Na_2O、MgO、CaO和TiO_2含量的特征,A/CNK值介于0.95～1.22之间,属于高钾钙碱性系列的过铝质花岗岩。岩石富集Rb、Th、K等大离子亲石元素,亏损Nb、Sr、P、Hf、Ti等高场强元素;岩石轻稀土元素分馏较强而重稀土元素分馏较弱,具有明显的负Eu异常,总体呈右倾的"V"字形稀土元素配分模式,显示典型的地壳重熔型花岗岩特征。亚干布阳片麻岩的源岩主要为地壳中沉积岩类的部分熔融,形成于俯冲-同碰撞构造环境。综上说明亚干布阳片麻岩是新元古代早期俯冲碰撞热事件的产物,反映阿中地块和柴达木地块青白口纪处于汇聚碰撞阶段,构造岩浆活动强烈,与Rodinia超大陆汇聚事件具有一致性。     

东喜马拉雅构造结南迦巴瓦岩群花岗质片麻岩的初步研究

野外地质填图和研究发现,东喜马拉雅构造结高喜马拉雅结晶岩系中有古老的花岗岩侵入,并在鲁霞地区圈定了9个花岗质侵入体。古老的花岗质岩石主要侵位于南迦巴瓦岩群直白岩组中,与南迦巴瓦岩群一起经历了麻粒岩相变质作用而形成花岗质片麻岩套。岩石类型有花岗闪长质片麻岩、黑云母花岗质片麻岩、闪长质片麻岩等。岩石化学研究表明这些花岗片麻岩套具“S”型特征,可能有深部幔源物质的加入。花岗岩形成深度在2～5km之间．侵位时代为552～525Ma,为新元古代晚期,属泛非期陆内演化阶段的产物。高喜马拉雅地区在元古宙末期形成了结晶基底。     

On the origin of early Archaean gneisses: A reply

The use of metamorphosed basic dykes as one of the most important single field criteria for subdividing gneisses in high grade areas into different chronological units is defended. The universal applicability of the lower greenstone-granite-upper greenstone model to all Archaean terrains is questioned using documented sequences of events in the North Atlantic craton. We prefer a chronology based on field criteria to one based on the application of theoretical crustal development models taken from one tectonic environment and used to explain the sequence of events in another.It is shown that the average K₂O and Rb content from the 3600 m.y. sialic rocks of the North Atlantic craton ranges between 2.26 (Amîtsoq grey gneisses, Greenland) to 2.66 (Uivak grey gneisses, Labrador). Average K/Rb ratios are respectively 200 and 177, Rb/Sr, 0.33 and 0.29 for the two areas. K and Rb values are thus markedly higher than those reported from most other Archaean gneiss suites. Secondary redistribution of K and Rb at about 3600 m.y. is demonstrated by the documentation of the massive addition of these elements to basic rocks included in the gneisses. Whole sale addition of alkalies during migmatisation to the level of crust now exposed is postulated as one explanation of the unusually high K and Rb contents. It is argued on statistical grounds that if Rb metasomatism occurred it is not possible to use low initial Sr ratios alone to preclude the possibility that part of the Archaean gneiss complexes consist of tonalitic gneiss which are much older than conventional Sr₀ interpretations allow.     

Geochemical and Sm–Nd isotopic study of titanite from granitoid rocks of the eastern Dharwar craton,southern India

Titanite occurs as an accessory phase in a variety of igneous rocks, and is known to concentrate geologically important elements such as U, Th, rare earth element (REE), Y and Nb. The differences in the abundances of the REEs contained in titanite from granitoid rocks could reflect its response to changes in petrogenetic variables such as temperature of crystallization, pressure, composition, etc. Widespread migmatization in the granodiorite gneisses occurring to the east of Kolar and Ramagiri schist belts of the eastern Dharwar craton resulted in the enrichment of the REEs in titanite relative to their respective host rocks. A compositional influence on the partitioning of REEs between titanite and the host rock/magma is also noticed. The relative enrichment of REEs in titanite from quartz monzodiorite is lower than that found in the granodioritic gneiss. Depletion of REE and HFSE (high field-strength elements) abundances in granitic magmas that have equilibrated with titanite during fractional crystallization or partial melting has been modelled. As little as 1% of titanite present in residual phases during partial melting or in residual melts during fractional crystallization can significantly lower the abundances of trace elements such as Nb, Y, Zr and REE which implies the significance of this accessory mineral as a controlling factor in trace element distribution in granitoid rocks. Sm–Nd isotope studies on titanite, hornblende and whole rock yield isochron ages comparable to the precise U–Pb titanite ages, invoking the usefulness of Sm–Nd isochron ages involving minerals like titanite.     

Petrology, geochemistry and the mechanisms determining the distribution of platinum-group element and base metal sulphide mineralisation in the Platreef at Overysel, northern Bushveld Complex, South Africa

Platinum-group element (PGE) mineralisation within the Platreef at Overysel is controlled by the presence of base metal sulphides (BMS). The floor rocks at Overysel are Archean basement gneisses, and unlike other localities along the strike of the Platreef where the floor is comprised of Transvaal Supergroup sediments, the intimate PGE–BMS relationship holds strong into the footwall rocks. Decoupling of PGE from BMS is rare and the BMS and platinum-group mineral assemblages in the Platreef and the footwall are almost identical. There is minimal overprinting by hydrothermal fluids; therefore, the mineralisation style present at Overysel may represent the most ‘primary’ style of Platreef mineralisation preserved anywhere along the strike. Chondrite-normalised PGE profiles reveal a progressive fractionation of the PGE with depth into the footwall, with Ir, Ru and Rh dramatically depleted with depth compared to Pt, Pd and Au. This feature is not observed at Sandsloot and Zwartfontein, to the south of Overysel, where the footwall rocks are carbonates. There is evidence from rare earth element abundances and the amount of interstitial quartz towards the base of the Platreef pyroxenites that contamination by a felsic melt derived from partial melting of the gneissic footwall has taken place. Textural evidence in the gneisses suggests that a sulphide liquid percolated down into the footwall through a permeable, inter-granular network that was produced by partial melting around grain boundaries in the gneisses that was induced by the intrusion of the Platreef magma. PGE were originally concentrated within a sulphide liquid in the Platreef magma, and the crystallisation of monosulphide solid solution from the sulphide liquid removed the majority of the IPGE and Rh from it whilst still within the mafic Platreef. Transport of PGE into the gneisses, via downward migration of the residual sulphide liquid, fractionated out the remaining IPGE and Rh in the upper parts of the gneisses leaving a ‘slick’ of disseminated sulphides in the gneiss, with the residual liquid becoming progressively more depleted in these elements relative to Pt, Pd and Au. Highly sulphide-rich zones with massive sulphides formed where ponding of the sulphide liquid occurred due to permeability contrasts in the footwall. This study highlights the fact that there is a fundamental floor rock control on the mechanism of distribution of PGE from the Platreef into the footwall rocks. Where the floor rocks are sediments, fluid activity related to metamorphism, assimilation and later serpentinisation has decoupled PGE from BMS in places, and transport of PGE into the footwall is via hydrothermal fluids. In contrast, where the floor is comprised of anhydrous gneiss, such as at Overysel, there is limited fluid activity and PGE behaviour is controlled by the behaviour of sulphide liquids, producing an intimate PGE–BMS association. Xenoliths and irregular bands of chromitite within the Platreef are described in detail for the first time. These are rich in the IPGE and Rh, and evidence from laurite inclusions indicates they must have crystallised from a PGE-saturated magma. The disturbed and xenolithic nature of the chromitites would suggest they are rip-up clasts, either disturbed by later pulses of Platreef magma in a multi-phase emplacement or transported into the Platreef from a pre-existing source in a deeper staging chamber or conduit.     

恒山地区部分熔融地质体填图方法的尝试

五台群地层呈东西向分布在恒山东段中部,其北为高角闪岩相的恒山片麻杂岩,两者岩性呈渐变过渡关系,由此萌发出片麻杂岩是五台群地层部分熔融而来的思路。经过填图实践,总结出地层与片麻岩体之间存在着仍属地层范畴的片麻岩层这一无序岩石地层单位,并建立起它的填图标志。与地层呈渐变关系的片麻岩体外貌复杂、边界不清,不同于具有一定侵入关系、边界明显、成分均一的片麻岩体,据此划分了原地片麻岩体与异地片麻岩体两个系列。