Pillow structures in serpentinite,Tagadur, Nuggehalli schist belt,Karnataka

Pillow structures developed in serpentinite is described from the Nuggehalli schist belt, Karnataka. Chemistry of the rock reveals that the serpentinite is a direct representation of mantle material.     

Gold-bearing rocks of Mangalur greenstone belt,Gulbarga District,Karnataka

The Mangalur greenstone belt of Dharwar Craton, South India, is an Archaean schist belt dominated by metavolcanic rocks. The gold mineralization occurs within the metavolcanics and the fabric, mineralogy and geochemistry of these host rocks indicate that they were tholeiitic basalts regionally metamorphosed under medium to low-grade greenschist facies. The basic metavolcanic rocks occur as tholeiitic metabasalts and amphibolites. The rocks have undergone some fractionation and appear to be derived from melts generated by 10 to 25% melting of the mantle at depths 30 to 35 km around temperature 1200°C and pressure 12 kb. The source of gold is mainly in the basalts and not in the surrounding granites.     

Secondary geochemical dispersion in the Precambrian auriferous Hutti-Maski schist belt, Raichur district, Karnataka, India. Part I: anomalies of As, Sb, Hg and Bi in soil and groundwater

The nature of secondary geochemical dispersion of As, Sb, Hg and Bi in soil and ground water of the semi-arid, tropical, Archaean, auriferous, Hutti-Maski greenstone belt has been investigated for identification of appropriate geochemical techniques for Au exploration in similar terrains.Results indicate that the <180 μm size-fraction of C-horizon soil is an appropriate sampling medium for delineating pedogeochemical anomalies of As, Sb, Hg and Bi related to gold mineralisation. These pedogeochemical anomalies along with anomalous values of alkalinity, chloride, sulphate, As and Sb in groundwater are controlled significantly by primary mineralisation located along shear zones in the greenstone belt. Arsenic anomalies in soil are broad, whereas, those of Sb and Bi are restricted to narrow zones directly over mineralised areas. In contrast, Hg anomalies around known mineralised areas are irregular and do not clearly demarcate the mineralised areas. The study indicates that anomalies of As, Sb and Hg in soil are principally hydromorphic, whereas those of Bi are clastic.The study recommends use of groundwater sampling at 2–3 km spacing with routine analysis of chloride, sulphate and alkalinity along with As and Sb in the first phase. This may be followed up with sampling of C-horizon of soils on a 1 km square grid for As-anomalies. Arsenic-anomalous areas may be sampled for As, Sb, Hg and Bi on a 500 m square grid for detailed exploration.     

Chemical remnants of the Archaean protocrust in the Sargur schist belt of Karnataka craton,India

The high-grade Sargur schist belt in the southern part of Karnataka craton is characterised by two contrasting magmatic suites: an older suite of metavolcanics and basic granulites of tholeiitic chemistry and a differentiated metabasic-serpentinised ultramafic sequence of calc-alkaline chemistry. The metavolcanics have a distinctive chemistry of their own and are not comparable to modern mid-oceanic ridge basalts and island-arc tholeiites.The chemistry of the metasediments, which mostly comprise pelites, semipelites, minor Ca-rich marbles and quartzites, indicates a mixed source consisting mostly of basic rocks and minor granitic material.     

Auriferous lode of Hira-Buddini gold mine,Hutti-Maski schist belt,Dharwar craton: Mineralogy,alteration, types and mechanism of vein emplacement

The auriferous lode in the Hira-Buddini deposit is confined to the sheared contact of amphibolite and felsic metavolcanic rock. Gold mineralization in the deposit is associated with sub-horizontal, sub–vertical, irregular and with few conjugate veins. These veins were emplaced during deformation in a ductile-brittle regime as inferred from the megascopic features and microstructures of the vein minerals. Fluid pressure was higher than the sum of the minimum principal stress and lithostatic load as well as the tensile strength of the shear zone. Crack-seal process appears to be the mechanism of vein formation. The microstructures of the vein minerals indicate a temperature of ~500ºC during the vein emplacement. In the auriferous lode, amphibolite and felsic metavolcanic rock have been subjected to intense alteration by the ore fluid with development of biotite-chlorite-tourmaline-calcite and muscovite (sericite)-chlorite-calcite-feldspar-biotite assemblages, respectively. Both the altered rocks contain significant amount of pyrite and chalcopyrite with native grains of gold and silver. Post-dating the fluid activity associated with gold mineralization, there is another stage of fluid activity manifested by the calcite veins and micro-veinlets.     

Evolution of Hutti-Maski greenstone belt of the Eastern Dharwar Craton: Evidence for metamorphic and hydrothermal phases from the Hira-Buddini deposit,Raichur district,Karnataka, India

The Hira-Buddini gold deposit is located along the steeply dipping ENE trending sheared contact of felsic and mafic rocks of strike length of about 600 m with mylonitic foliation parallel to the S1 schistosity in amphibolites. Second-generation open folds with axial planes (S2) marked by fractures that are often filled by later calcite veins are observed in surface and underground exposures. Garnetiferous amphibolites occur in patches on the footwall side of the shear in the western part of the deposit. This rock shows garnet porphyroblasts, coarse second-generation hornblende and large grains of biotite that grow over an early S1 fabric which is made up of early hornblende, plagioclase, ilmenite and retrograde first-generation chlorite. Second-generation hornblende and biotite grains make high angles to S1 schistosity and are sub-parallel to S2. Late hydrothermal alteration is marked by an albite-epidote-chlorite-zoisite assemblage. Geothermometric estimates based on garnet-biotite, and garnet-hornblende pairs, as well as Ti in biotite, show that temperatures during D2 deformation that led to the growth of the porphyroblasts were \(530{\pm }20^{\circ }\hbox {C}\). The fabric and mineralogy of the rock indicate that porphyroblastic growth of garnet, hornblende and biotite was preceded and succeeded by stages of hydrothermal alteration. Primary gold mineralization is inferred to be associated with the early stage of hydrothermal ingress.     

Geochemistry and petrogenetic evolution of metasedimentary rocks in Bunu Area,part of Kabba-Lokoja-Igarra schist belt,SW Nigeria

Field and geochemical studies of the Bunu area, SW Nigeria, were carried out on gneiss-metasedimentary rocks that consist of migmatised gneiss, quartz-mica schist, and foliated and massive quartzites. These gneiss-metasedimentary rocks are interbedded with meta-igneous rock (amphibolite), all of which are intruded by granitoid and cut by basic and felsic dykes. Geochemical data on major, trace, and rare Earth elements of these rocks from the area were used to speculate on the petrogenetic and geodynamic evolution of the rocks in the area. Compositionally, the metasedimentary rocks in the area have a restricted range of major oxides such as SiO₂ and Al₂O₃ with low average values of Fe₂O₃, MnO, CaO, and P₂O₅. Concentrations of average values of HSFE such as Zr, Nb, and Y are moderately high in migmatitic gneiss, quartz-mica schist, and low in both massive and foliated quartzite. ∑LREE average values are 124.25, 132.41, and 18.64 ppm respectively for migmatite gneiss, quartz-mica schist, and low in both massive and foliated quartzite. These rocks are also generally enriched in Ba, Cs, Pb, U, Cr, Rb, and Zr and depleted in Be, Sr, Ti, Mo, Th, and W. This enrichment–depletion in major oxides and trace elements of the rocks in this area is probably a consequence of the movement of metamorphic remobilized fluids within the rocks in the area during the Pan-African or earlier events as noted in the adjacent Egbe-Isanlu Schist belt. A further geochemical characterisation of the rocks in the area using Log (Na₂O/K₂O) vs Log (SiO₂/Al₂O₃ indicates that while the protolith of migmatised gneiss and quartz-mica schist are greywackes to litharenite in compositions that of quartzites are sublitharenite to quartz arenite. On the ternary plot of Al₂O₃-(CaO + Na₂O)-K₂O for metasedimentary rocks in the area, most migmatitic gneiss and quartz-mica schist rocks plot close to average shale while quartzites mostly tend to illite compositions and almost toward the Al₂O₃-apex of the diagram attesting to depletion of CaO and NaO with the removal of K₂O. CIA vs PIA plot of the rocks in the area shows that while migmatitic-gneiss and quartz-mica schist are moderately weathered, both massive and foliated quartzites are high to extremely weathered. They are all derived mostly from predominantly felsic igneous to quartzose sedimentary provenance and mostly emplaced in passive continental margins. These nature of the protoliths suggests gradual subsidence of the basin during its genesis, and/or tectonic stable or inactive environment from which the sediments were derived.

     

Gravity inferred subsurface structure of Gadwal schist belt, Andhra Pradesh

Detailed gravity data collected across the Gadwal schist belt in the state of Andhra Pradesh show an 8.4 mgal residual gravity anomaly associated with meta-sediments/volcanics of the linear NNW-SSE trending schist belt that shows metamorphism from green schist to amphibolite facies. This schist belt is flanked on either side by the peninsular gneissic complex. The elevation and slab Bouguer corrected residual gravity profile data were interpreted using 2-D prism models. The results indicate a synformal structure having a width of 1.8 km at the surface, tapering at a depth of about 2.6 km with a positive density contrast of 0.15 gm/cc with respect to the surrounding peninsular gneissic complex.     

Structural appraisal of the Gadag schist belt from gravity investigations

Semi-detailed gravity investigations were carried out over an area of approximately 2750 sq km with maximum N-S and E-W extents of 55 and 50 km respectively in the Gadag region in the Dharwar craton with a view to obtain a clearer perception of the structural configuration of the region. From qualitative analysis of the gravity data, several tectonic features are inferred: the high density Gadag schist belt is characterized by a gravity high and occurs in two discontinuous segments — the main N-S trending segment, and its thinner NW-SE trending extension, the two separated by a NE-SW trending deep seated fault. While the N-S trend of the Gadag schist belt is bounded on its east by the NW-SE trending Chitradurga thrust fault and on its west by another major NNWSSE trending fault, the NW-SE extension is likewise bounded by two other NW-SE major faults. Quantitative evaluation from forward modeling/inversion of five profiles in the region, assuming a density contrast of 0.29gm/cc of the anomalous schistose body with the gneissic host rocks indicated a synclinal structure plunging to the southeast along its axis for the Gadag schist belt. The maximum width and depth from surface of the schist belt are 22 km and 5.6 km respectively.     

Geology, geochemistry and genesis of BIF of Kushtagi schist belt, Archaean Dharwar Craton, India

The Banded Iron-Formation (BIF) of the Kushtagi schist belt, Dharwar Craton is interbedded with metavolcanics. The oxide fades cherty (Al₂O₃ < 2%) and shaley (Al₂O₃ > 2%) BIFs show large-scale variations in their major and trace elements abundance. Cherty Banded Iron-Formation (CBIF) is depleted in Al₂O₃, TiO₂, Zr, Hf and other trace elements like Cr, Ni, Co, Rb, Sr, V, Y and REE in comparison to Shaley Banded Iron-Formation (SBIF). Depleted REE, positive Eu anomalies and the flat to HREE-enriched pattern of CBIF indicate that Fe and SiO₂ for these BIFs were added to ambient ocean water by hydrothermal solutions at the AMOR vent sites. It is inferred that the higher amount of hydrothermal fluid flux with a higher exit temperature provided enormous quantities of iron and silica. Fine-grained sedimentation in the basin gave rise to the observed variability in the composition of BIF. During transgression a wave base was raised up, consequently deposition of CBIF became possible, whereas, during the regressive stage, these chemical sediments were buried by and/or mixed with the terrigenous sediments resulting in deposition of SBIF and interbedded shales. Volcaniclastic activity within the basin appears to have contributed significantly to the composition of some SBIF and shales. The hydrothermal exhalative hypothesis combined with the Archaean miniplate model explains most of the chemical features of the BIFs of greenstone belts.     

Ductility contrast induced by silicification in pelitic schist of the Ryoke metamorphic belt,Japan

Contrasting ductility is recognized in the rocks of Cretaceous Ryoke metamorphic belt in Iwakuni area, southwest Japan. Pelitic schist is ubiquitous in the region and differences in mineral assemblages mark increase in metamorphic grade. The area has been graded as chlorite-biotite zone in the north progressing into biotite- and muscovite-cordierite zones in the south. Pelitic schist near the boundary between the biotite- and muscovite-cordierite zones has undergone partial silicification to form whitish silicified schist layers which contain two types of quartz veins: those parallel to foliation in the host rock are called schistosity-concordant veins, and those inclined to host rock foliation, schistosity-discordant veins. In this study we examined the quartz structure in the silicified schist and in both types of veins to understand the ductility contrast induced by the silicification process. Crystallographic orientations of quartz in the veins and silicified schist rocks were studied using the Scanning Electron Microscopy (SEM) based Electron Back Scatter Diffraction (EBSD) technique. Quartz c-axis orientations in the silicified schist are nearly random, demonstrating an absence of post-silicification ductile deformation. Quartz grains in the schistosity-concordant veins have preferred c-axis orientations perpendicular to the schistosity indicating ductile shortening. In contrast, schistosity-discordant veins display distinct quartz c-axis fabric than that found in the schistosity-concordant veins. This is because the two types of host rocks exhibit a difference in ductility during deformation. The presence of deformed quartz veins in the undeformed silicified schist indicates transformation of the ductile pelitic schist into the brittle silicified schist at mid-crustal levels where these rocks originate, hence forming contrasting rock layers. Schistosity-concordant veins in the biotite-rich pelitic schist deformed with its host rock in a ductile manner while the schistosity-discordant veins in the neighboring silicified schist were left intact. Silicification of the pelitic schist may have been caused by the silica-rich geofluids produced by subsurface processes. Geofluids responsible for the occurrence of such mechanically contrasting layers mark an increase in seismic reflectivity at mid-crustal depths and may be potential reflectors of seismic waves giving rise to the so-called “bright spots”.     

Secondary geochemical dispersion in the Precambrian auriferous Hutti–Maski schist belt, Raichur district, Karnataka, India. Part II. Application of factorial design in the analysis of secondary dispersion of As

The study uses the technique of factorial design for analysis of factors governing secondary dispersion of As in an auriferous Archean greenstone belt. Details of the sampling procedure and analytical techniques used for As in groundwater and soil of the Hutti–Maski schist belt are discussed in Part I of the paper in this issue. The main effects and interactions of topographic slope, drainage texture, proximity to lineament, depth of water table and soil types have been considered. The study shows that secondary dispersion of As is governed by proximity to lineament and factors that govern drainage and ground water movement. The factors retained were used to predict As values using a response surface model obtained by least square fitting using orthogonal polynomials. The goodness of fit of the model shows that the factors identified explain the secondary dispersion of As adequately.     

Gold mineralisation and alteration of Penakacherla schist belt, India, constraints on Archaean subduction and fluid processes

In the Dharwar Craton, southern India, gold deposits are found mostly along the six arcuate shear zones passing through late Archaean greenstone belts (2.7 Ga). One such shear zone complex extends for about 400 km within and along the Ramagiri–Hungund schist belt. The Penakacherla sector of this shear zone is excellently exposed, enabling a detailed investigation of synorogenic gold mineralisation and its relationship to associated hydrothermal alteration.Metamorphism and deformation under NE–SW compression associated with Archaean subduction processes converted mafic volcanic rocks into amphibolites and intermediate to felsic volcanic rocks into quartz mica schists. Continued compression generated a 50–100-m-wide shear zone complex consisting of mafic phyllonites. Advection of hydrothermal fluids through this shear zone and reaction between fluids and the mafic phyllonites resulted in a silicified, K-metasomatic assemblage mainly consisting of chlorite, amphibole, K-mica, plagioclase, ankerite, quartz, Fe-oxides, pyrite, chalcopyrite and arsenopyrite. Networks of quartz and carbonate veinlets, a few millimeters to a few centimeters thick, formed along the foliation planes giving rise to microscopic alteration envelope, in which individual veinlet systems are merged into one another to form a composite alteration system. Gold is found within these quartz veinlets, mafic phyllonites and at their mutual contacts.Hydrothermal fluids have modified the primary major, minor, trace and LREE compositions of host rocks such that their mutual behaviour became non-systematic. Some HFSE and HREE also show minor mobility but the overall REE pattern generally resembles that of the precursor mafic volcanic rocks. Mass and volume loss/gain by Si and Ca has made significant impact on Al, Ti and Zr abundances, which are generally immobile during hydrothermal alteration. However, element pairs such as Zr–Hf, V–Sc and Nb–Ta maintain primary inter-element ratios, although their absolute abundances are drastically diluted. Similarly, ΣREE in highly silicified and carbonatised samples are reduced, but patterns remain similar to those of relatively least altered mafic phyllonites with (La_N/Yb)_N between 1 and 3. In some samples, LREE enrichment is observed elevating in (La_N/Yb)_N from 3 to 11. Pathfinder elements and base metals such as As, Cd, Cu, Pb, Zn and Sb have been added along with the Au and Ag.δ¹³C of carbon varies from −16‰ to −21‰ suggesting a biogenic origin, whereas coexisting pyrite δ³⁴S ranges from 1‰ to 3‰, pointing towards the involvement of magmatic or average crustal sulphur. Overall concentrations of K, Rb, Sr, Ba, Nb, Ta, Ti, Cs, Cr, Co, V, Y and Sc and many of the ratios such as K/Rb, La/Sc, La/Yb indicate that metamorphism, devolatilisation and dehydration of an oceanic subducting slab might have partially contributed the mineralising fluids and generated the alteration assemblage observed in the host rocks. Fluid sources were mantle and greenstone belt dehydration and devolatilisation generating observed compositional and alteration diversity.     

绿泥石片岩各向异性特性研究

为研究十堰地区片岩的各向异性特性,开展了武当群绿泥石片岩长方体试样的单轴压缩和圆盘试样的间接拉伸试验,探讨了试样各向异性的力学特性和在不同受力状态下的变形破裂特性,揭示了不同变形破裂的力学机制。研究表明,武当群绿泥石片岩具有明显的各向异性性质,平行片理方向强度高,垂直片理方向强度低。该片岩特殊的定向排列片束状构造和片理间的弱胶结作用,致使不同方向上的破坏特征具有明显差异,其力学机制也不相同。在压应力作用下,泊松效应容易引起平行片理面的张拉劈裂和压杆失稳,垂直片理方向容易发生片理面间的剪切破坏。在同一方向上强度具有一致性,即平行片理面抗压和抗拉强度均较垂直片理面强度大。由于片理面间的抗拉承载力极低,在小角度劈裂荷载下,容易发生张拉劈裂和拉剪破坏,因此,实际工程中应尽可能避免片理面间的受拉破坏和沿片理的拉剪破坏。研究结果可以为隧道、边坡支护加固和防水处理提供参考。     

Morphology and textures of komatiite flows of J.C. Pura schist belt,Dharwar craton

Study of komatiites for their structures and textures in cratonic blocks could provide more insights into the early Archaean volcanism, mantle processes and associated metallogeny. Jayachamarajapura (J.C.Pura) belt in Western Dharwar craton is a komatiitc milieu, where outcrop features display several flow characteristics and sub-volcanic emplacement features typical of well known komatiitic areas of the world. In spite of deformation, metamorphism and alterations the komatiites still preserve many of the primary cooling structures, which stand testimony for their extrusive volcanic nature. Distinct features like pillows, flow-top polyhedral joints, ocelli, vesicular, flow-top breccia and cumulate segregations and crude layering are observed. However, massive, undifferentiated nature of komatiitic flows is more predominant. Because of serpentinisation, carbonitization and chloritization, the original mineralogy and textures are obliterated and scantily preserved. Still, these observed features provide vital clues to imply the formation of komatiite sequences in a submarine to subaerial conditions when episodic pulses of komatiite lava piled up (about 3.35 Ga ago) to form the ultramafic milieu of J.C. Pura belt.     

Strain analysis and tectonite classification using polymictic metaconglomerates in the Igarra schist belt,southwestern Nigeria

Deformed conglomerates in the Igarra schist belt display contrasting strains between different lithological clast populations. We analysed three different clast populations (pegmatite, metasediment, quartz) across three sites in the Igarra metaconglomerates of Edo state, Nigeria. We calculated finite strain using the R_f/ø method and Flinn graph. Quartz clasts exhibited the least amount of strain, while the pegmatite and metasedimentary clasts had greater strains (pegmatite > metasediment > quartz). The variability in the finite strains gotten from the three sites is controlled by clast composition and probably grain size. The differences in finite strain and maximum elongation direction (λ ₁) in the three sites indicates that the Igarra metaconglomerates was subjected to a heterogenous simple shear deformation which is probably associated with the general transpressional deformation that affected the Pan-African mobile belt considering its (Igarra schist belt) spatial proximity with regional dextral shear zones. Three-dimensional strain analysis in site one indicates a constrictive deformation with the dominance of L tectonites. Spatial analysis of two-dimensional strain suggests a strain gradient where finite strain decreases from north to south.     

High-grade regional metamorphism of ultramafic and mafic rocks from the Archaean Sargur terrane,Karnataka, South India

Deformed and metamorphic ultramafic to mafic rocks emplaced into the Archaean Sargur supracrustal series (>3.0 Ga) in Karnataka, southern India, represent layered igneous bodies. The terrane has been affected by several episodes of deformation and metamorphism in the time span from 3.4 to 2.5 Ga ago.During the regional metamorphism about 2.5 Ga ago the igneous bodies re-equilibrated partly or completely at conditions of the upper amphibolite to granulite facies. The development of sagvandites with enstatite + magnesite and anthophyllite + magnesite-bearing assemblages, and of mafic garnet—pyroxene charnockites indicates the presence of CO₂-rich intergranular fluids (X_CO2 ? 0.5) in these rocks during metamorphism.The physical conditions of metamorphism have been estimated by applying methods of geothermobarometry to the recrystallized ultramafic assemblages with olivine, pyroxenes and spinel and to the charnockitic assemblages with garnet, pyroxenes, plagioclase and quartz. A best temperature estimate of 700 ± 30°C was derived with the geothermometers of Evans and Frost (Ol—Spi), Fabriès (Ol—Spi), Wells (Opx—Cpx), Powell (Opx—Cpx), Ellis and Green (Gra—Cpx), Lal and Raith (Gra—Opx), and Danckwerth and Newton (Al₂O₃-content in opx). A mean pressure estimate of 8.6 ± 0.8 kbar has been obtained with the models of Perkins and Newton (Gar—Opx/Cpx—Plag—Qtz). The P—T data indicate a minimum crustal thickness of about 35 km at c. 2.5 Ga in this part of the South Indian Archaean craton.     

拉萨地块松多超高压变质带含石榴石云母石英片岩的变质演化相平衡模拟

拉萨地块松多超高压变质带含石榴石白云母石英片岩为榴辉岩的围岩,岩石的矿物组合为石榴石、白云母、钠长石、绿泥石、石英及少量金红石、榍石。石榴石具有明显的成分环带,从核部到幔部Xprp=[Mg/(Mg+Fe+Mn+Ca)]缓慢升高,Xsps=[Mn/(Mg+Fe+Mn+Ca)]逐渐降低,表明石榴石从核部到幔部的成分记录了温度逐渐升高的进变质过程;幔部到边部,Xprp=[Mg/(Mg+Fe+Mn+Ca)]略微降低,Xgrs=[Ca/(Mg+Fe+Mn+Ca)]明显升高,Xsps=[Mn/(Mg+Fe+Mn+Ca)]先升高后降低,表明石榴石边部成分受到了退变质作用改造,呈现扩散环带的特征。利用Thermocalc变质相平衡计算软件在Mn NCKFMASHO体系下计算出含石榴石云母石英片岩的P-T、P-M(H2O)视剖面图,结合石榴石镁铝榴石等值线、钙铝榴石等值线及饱和水含量等值线限定出含石榴石云母石英片岩的峰期变质条件为约27×105k Pa,523/580℃,对应的峰期矿物组合为(g-Jd-Cr-Law(+Phn+q/Coe+H2O)。石榴石核部到幔部成分记录了主要的进变质演化,结合饱和水等值线的变化,判断进变质阶段为升温升压的冷俯冲过程,岩石经历了蓝片岩相至榴辉岩相的变质演化;P-M(H2O)视剖面图及饱和水等值线反映了岩石在减压中的流体行为,通过其变化特征可以确定岩石在峰期之后先经历近等温降压的早期退变质过程,晚期降温降压的退变轨迹则由石榴石边部成分所确定,此过程中,岩石发生了角闪岩相至绿帘角闪岩相变质,并在后期经历了绿片岩相变质叠加。近等温降压的退变质过程反映了快速抬升的构造运动过程,早期硬玉转变为钠长石可能发生在这个阶段。对比含石榴石云母石英片岩与榴辉岩的P-T轨迹,峰期变质温压及变质演化特征,提出含石榴石云母石英片岩曾经历过高压变质,结合野外相互伴生的地质关系,认为该片岩与榴辉岩经历了相同或者相似的俯冲折返过程。     

Oxygen isotope study of metamorphic and granitic rocks of the Yanai district in the Ryoke belt,Japan

¹⁸O/¹⁶O ratios have been obtained for 134 whole-rocks and minerals from metamorphic and granitic rocks of the Yanai district in the Ryoke belt, Southwest Japan. The ¹⁸O/¹⁶O ratios of pelitic rocks of the marginal metamorphic zone decrease progressively with increasing metamorphic grade. In the gneiss-granite complex (zone of migmatite [1]), the most characteristic feature of the rocks is that oxygen isotopic homogenization proceeds on both local and regional scales in parallel with “granitization” or chemical homogenization. Granitic rocks of various origin are fairly uniform in isotopic composition with δ ¹⁸O of quartz of 12 to 14‰ (SMOW) and δ ¹⁸O of biotite of 7 to 9‰ and are about 3 to 4‰ enriched in ¹⁸O compared to other Cretaceous granites of non-metamorphic terranes in Japan. The high ¹⁸O/¹⁶O ratios of granitic rocks of this district were discussed in relation to the ¹⁸O-depletion in metasediments. Oxygen isotopic fractionations among coexisting minerals from various rock-types of the gneiss-granite complex indicate that these minerals were formed under near isotopic equilibrium at a temperature of about 600 to 700° C. Some abnormal fractionations of quartz-biotite pairs also were obtained for rocks which had undergone a progressive ¹⁸O-depletion or ¹⁸O-enrichment. This is due to high resistivity of quartz and contrastive susceptibility of biotite to isotopic exchange during metamorphism and “granitization”.     

Petrogenesis and source characteristics of metatholeiites from the Archean Ramagiri schist belt, eastern part of Dharwar craton, India

The N–S trending, 2–4 km wide Ramagiri schist belt is made up of three blocks dominated by metavolcanic rocks, separated and surrounded by granitic rocks of distinct characteristics. The metavolcanic rocks are tholeiitic in composition and are very similar in their major element composition as well as in their abundances of some trace elements. However, the rare earth elements (REE) require distinct sources. The rocks of the amphibolite facies eastern block have LREE depleted REE patterns ([Ce/Yb] = 0.7–0.9), requiring derivation from depleted mantle-like sources. The greenschist facies metatholeiitic rocks of the central block have LREE enriched REE patterns ([Ce/Yb] = 3–6), reflecting the nature of their source(s). The Nd isotopic data require that the LREE enriched nature could not have been attained significantly prior to its melting. The fine-grained, upper greenschist facies metatholeiites of the western block have flat to slightly LREE depleted patterns ([Ce/Yb] = 0.8–0.95). Minor fractional crystallization of rock forming minerals may relate a few samples to each other among samples from each of the three blocks. Different extents of partial melting of distinct mantle sources have played a dominant role in the generation of the parent magmas to the central versus eastern and western block metatholeiites. The geochemical data suggest that the mantle sources were non-lherzolitic, and that these sources may have seen previous episodes of melt addition and extraction prior to melting that gave rise to the parent melts to the rocks ∼2750 Ma ago. The REE data indicate that while the sources of the eastern and western block rocks were similar to depleted mantle (ɛ_{Nd( i )} about +2), the source of the central block rocks (ɛ_{Nd( i )} about +3.5) were enriched in large ion lithophile element (LILE)-rich fluids/melts probably derived from subducting oceanic crust. This and other trace element signatures point to magma extraction in tectonic settings similar to modern island arcs. Subsequent to magma emplacement and crystallization, all the three suites of rocks were affected by interaction with low-temperature, crustal derived fluids (ɛ_{Nd 2750Ma} of about −8 to −12), probably during the accretion of the three blocks of the belt in the present form. The inferred source characteristics, tectonic setting of magma generation and the crustal fluid processes seem to suggest that Phanerozoic-style tectonic processes may have been important in the generation of Archean crust in the Dharwar craton. Received: 31 July 1995 / Accepted: 12 May 1997