Hornblende Biotite Gneiss Research Articles

Zr, Hf, U, Th and REE‐Fertile Lower Proterozoic Potassic Granite from Parts of Andhra Pradesh, South India

Abstract The medium‐ to coarse‐grained and porphyritic granitoid of Dharmawaram, Karimnagar district, Andhra Pradesh, south India is a biotite‐hornblende granite with notable contents of rare metal (Zr, Hf, Th) and rare earth (including Y) minerals like zircon, thorite, allanite, monazite and xenotime. Chemically, it is metaluminous (average A/C+N+K = 0.95)‐type, potassic (av. 5% K2O) granite, with dominantly sub‐alkaline characters. It shows up to 8 times enrichment of rare metals (Zr, Hf, U, Th) and rare earths (including Y, Sc), with reference to their abundances in normal unevolved granite, and hence, fertile for some of these elements. Field, petrological, geochemical and isotopic data of potassic granite (PG) indicate involvement of silica‐rich metasedimentary‐basic crustal rocks (amphibole‐quartzite, amphibolite, hornblende‐biotite gneiss, etc.) in its genesis, at a depth range of 30 km. Further, chondrite‐normalized REE patterns demonstrate that low‐degree partial melting of source rocks is the major controlling factor in the genesis of PG. Mild negative Eu‐anomaly (av. Eu/Eu* = 0.48), plots of Ba‐Rb‐Sr in the field of anomalous granite and K/Rb ratios (av. 239) in the range that is shown by normal unevolved granite together indicate less fractionated nature of the PG. Limited fractionation of metalumination‐type, involving hornblende, led to occasional weak alumina saturation. Interestingly, geochemical and petrogenetic features of the studied PG broadly match with those potassic granites which are already known to host anomalously high enrichment of rare metals and rare earths in other parts of Andhra Pradesh and adjoining Karnataka.

Mechanism of arrested charnockite formation at Nemmara, Palghat region, southern India

The formation of arrested charnockite is an excellent example of structurally controlled channellised fluid flow along specific sites accompanied by selective elemental mobility and mineralogical changes. The present paper recognises and focuses study on three types of arrested charnockite formation from Palghat region, namely, shear-controlled, foliation parallel and boudin-neck types, and address their spatio-temporal relations to regional-scale charnoenderbite. The shear-controlled and foliation parallel types post-date deformation and migmatisation. The boudin-neck type, on the contrary, is coeval with partial melting and followed the path of cooling and decreasing water activity in the gneiss. K-feldspar veining around plagioclase and quartz, symplectitic intergrowth of biotite+quartz after orthopyroxene and K-feldspar, and fluid inclusion data suggests the presence of alkalic supercritical brine and low-density CO 2-rich fluid during charnockite formation. Charnockite domains developed following the breakdown of hornblende, biotite and quartz are characterised by a more or less pronounced depletion of Fe, Ca, Mg and Ti and trace elements Y and Zr, compared to their counterpart gneiss. REE spectra indicate a subtle depletion in the HREE near the centre of the charnockite domain. Although close-pair samples of gneiss–charnockite are isochemical, on a scale of a few millimetres, bi-directional element movement, related to the formation of new mineral was noted. It is postulated that arrested charnockite formation developed in situ on local scale within the granitic domains of the hornblende-biotite gneiss, in the presence of CO 2-rich fluids and alkalic supercritical saline brine. This process post-dated the time of regional granulite (charnoenderbite) and large regional scale retrogression and migmatisation.

Multiple Tectonothermal Events in the Granulite Blocks of Southern India Revealed from EPMA Dating: Implications on the History of Supercontinents

We report age data on zircon, monazite, uraninite and huttonite from a suite of 29 samples covering four major granulite blocks in southern India using an electron microprobe technique. The rocks analysed in this study cover all of the major lithounits in these terrains and include garnet-bearing and garnet-free charnockites, garnet-biotite gneisses, khondalites, calc-silicate rocks, and a suite of orthogneisses (biotite gneiss, biotite-hornblende gneiss). Two pink metagranites representing the magmatic phase were also analysed. Zircons from the Madras Block yield well-defined isochrons at 2.5-2.6 Ga. Core to rim analyses of single zircon grains show age zoning with 2.6-2.9 Ga igneous cores mantled by 2.4-2.5 Ga rims. Detrital zircons show age up to ca. 3.2 Ga. Monazites in this block have cores and rims with 2.5-2.3 Ga. A suite of 19 samples from the Madurai Block brings out the multiple tectonothermal events in this terrain. Zircons from an orthogneiss yield well-defined isochrons at 1.7±0.1 Ga, 0.82±0.05 Ga and 0.58±0.04 Ga from core, inner rim and rim portions, respectively. Zircon grains in other rocks preserve either core or secondary growth ages at 0.8-1.0 Ga. Zircons in a pink metagranite from this block show sharply defined isochrons of 0.68±0.03 Ga for the core and 0.57±0.01 Ga for the secondary portion. A late Pan-African overprint is observed throughout this block with zircon rims, monazite, uraninite and huttonite yielding age values in the range of 0.45-0.60 Ga. Zircons from both the Trivandrum and Nagercoil blocks show a major tectonothermal event at 0.55 Ga with faint indications of previous tectonothermal events during 0.8-1.0 and 1.7-2.0 Ga. Monazite data from both the Trivandrum and Nagercoil blocks are essentially similar to those from the Madurai Block except for presence of relic monazite in the former. Our study confirms the notion that the Palghat-Cauvery Shear Zone marks the major terrain boundary between an Archean craton in the north and Proterozoic terrains in the south. It also strengthens the view of Paleoproterozoic accretion and Pan-African reworking of blocks south of this shear zone. The ages of production, accretion, and reworking in the terrains of southern India yield important information for the histories of Columbia (∼1.8-1.5 Ga), Rodinia (∼1.1-1.0 Ga), and Gondwana (∼0.6-0.5 Ga) supercontinents. The southern Indian terrains formed part of a worldwide network of orogenic belts that is centered around 1.8 Ga and outlines the configuration of Columbia. Accretion of terrains in the Madurai block to Archean rocks north of the Palghat-Cauvery Shear Zone at this time is consistent with the ages suggested to be the oldest metamorphic event in the Eastern Ghats orogen of eastern India and the Rayner belt of coastal East Antarctica. Our data confirm earlier evidence that all of southernmost India underwent resetting of isotopic systems during the final accretion of Gondwana at the time of the Pan-African orogeny (∼0.5 Ga). The possibility that the Trivandrum and Nagercoil terrains accreted to the Madurai block after ∼0.7 Ga suggests that this Pan-African zone may also have involved accretion and closure of ocean basins. If that happened, then this zone may be the long-sought suture between East and West Gondwana.

Origin of the Wadi Haimur–Abu Swayel gneiss belt, south Eastern Desert, Egypt: petrological and geochronological constraints

The Wadi Haimur–Abu Swayel gneiss belt is composed of biotite-gneiss, biotite–hornblende-gneiss and garnet-gneiss. Amphibole and muscovite compositions indicate that these gneisses belong to low- to intermediate-pressure metamorphic facies. Temperature values calculated using the hornblende–plagioclase geothermometer are in the range of 495–550 °C. The retrograde part of the metamorphic evolution for these rocks is documented by the presence of epidote and chloritization of biotite. The chlorite geothermometer indicates that chloritization occurred around 260–300 °C. The sedimentary origin of the gneisses is indicated by the high values of normative quartz and corundum. It is suggested that the sedimentary protoliths of the Wadi Haimur–Abu Swayel gneisses evolved in a back-arc basin. Compression of this basin led to intense deformation, metamorphism and granite magmatism. K/Ar ages have been determined on hornblende and biotite. The hornblende ages ≈600 Ma and the ≈585 Ma of coexisting biotite from the gneisses are interpreted as cooling ages following the peak of metamorphism. These ages are within the range of granitic emplacement in the study region (570–675 Ma), which indicates that the peak of metamorphism could have been contemporaneous with the granitic magmatism in the Wadi Haimur–Abu Swayel gneisses.

Mobility of components in metasomatic transformation and partial melting of gneisses: an example from Sri Lanka

Reaction textures, fluid inclusions, and metasomatic zoning coupled with thermodynamic calculations have allowed us to estimate the conditions under which a biotite–hornblende gneiss from the Kurunegala district, Sri Lanka [hornblende (NMg=38–42) + biotite (NMg=42–44) + plagioclase + quartz + K-feldspar + ilmenite + magnetite] was transformed into patches of charnockite along shear zones and foliation planes. Primary fluid inclusion data suggest that two immiscible fluids, an alkalic supercritical brine and almost pure CO2, coexisted during the charnockitisation event and subsequent post-peak metamorphic evolution of the charnockite. These metasomatic fluids migrated through the amphibolite gneiss along shear zones and into the wallrock under peak metamorphic conditions of 700–750 °C, 5–6 kbar, and aflH2O=0.52–0.59. This resulted in the formation of charnockite patches containing the assemblage orthopyroxene (NMg=45–48) + K-feldspar (Or70–80) + quartz + plagioclase (An28) in addition to K-feldspar microveins along quartz and plagioclase grain boundaries. Remnants of the CO2-rich fluid were trapped as separate fluid inclusions. The charnockite patches show the following metasomatic zonation patterns:

Palghat Gap — A Dextral Shear Zone from the South Indian Granulite Terrain

The origin of Palghat Gap, the most prominent discontinuity in the Western Ghats, is controversial. The rocks of the Gap — hornblende-biotite gneiss, mylonitic augen gneiss, charnockite and amphibolite are characterised by three groups of folds formed during two deformations. Shear indicators like augens, pull-aparts, rootless folds, sheath folds, minor shears and quartz fabric strongly suggest ductile dextral shearing. Kinematic analysis of structural elements supports shearing in a N-S compressional field with XY being subvertical. Features on both mesoscopic and microscopic scales strongly favour Palghat Gap being treated as a ductile shear zone. The present Gap topography is the product of shearing and erosion.

Petrogenesis of dolomitic marbles from Rongcheng in the Su‐Lu ultrahigh‐pressure metamorphic terrane, eastern China

Marbles are interbedded with biotite–hornblende gneiss in the Rongcheng area, Su‐Lu ultrahigh‐pressure (UHP) metamorphic terrane, eastern China. Both marble and gneiss include UHP eclogite layers and boudins. Seven dolomitic marbles were selected for petrologic investigation. Dolomitic marbles have assemblages of major constituent minerals: (i) Mg–Cal + Dol + Ol; (ii) Mg–Cal + Dol + Di + Ol; (iii) Mg–Cal + Dol + Di; (iv) Mg–Cal + Dol + Ti–Chu; (v) Mg–Cal + Ti–Chu + Di; and (vi) Mg–Cal + Dol + Ti–Chu + Di + Ol. Titanium–clinohumites of (iv) and (v) contain 3∼4 wt% in TiO2, but those in (vi) are < 3 wt% and are regarded as later‐stage replacement products from olivine. Assemblages indicate uneven distribution of XCO2 within an individual sample. Schreinemakers' analysis of assemblage Arg (Cal) + Dol + Chu + Di + Fo in the system CaO–MgO–SiO2–CO2–H2O with thermodynamic calculations indicates that Fo‐ and Chu‐bearing assemblages are stable at very low XCO2 conditions, whereas Arg (Cal) + Dol + Di is stable at relatively higher XCO2 conditions. When XCO2 conditions are extremely low, Fo‐bearing and Chu‐bearing assemblages are stable at T > 800 °C and P > 2.2 GPa (XCO2 = 0.01), and T > 730 °C and P > 2 GPa (XCO2 = 0.005). If such extremely low XCO2 is possible, the assemblages (i–v) could be regarded as products at UHP conditions excepting Mg–calcite transformed from aragonite + dolomite. Calcite–dolomite intergrowths are common in these assemblages. Mg–calcite inclusions were found in Ti–clinohumite of assemblage (iv) with well developed radial fractures. The maximum XMgCO3 value of 0.111 yields a solvus temperature of 698 °C. If this Mg–calcite formed from aragonite + dolomite, solvus thermometry gives a minimum P–T point along the equilibrium curve of the reaction aragonite + dolomite = Mg–calcite during the decompressional stage; T = 680 °C and P = 1.9 GPa. Along both the prograde and retrograde P–T paths for UHP dolomitic marbles, changes in model proportion among carbonate phases including aragonite, dolomite and Mg–calcite were proposed on the basis of the phase relations in CaCO3–CaMg(CO3)2. Aragonite + dolomite is a stable assemblage during the prograde stage. Appearance of the eutectoid point between aragonite and dolomite in the phase diagram plays an important role for phase changes in this system. After peak‐P, the following phase changes are predicted for the bulk compositions < XMgCO3 at the eutectoid point during the decompressional stage: (i) Arg + Dol; (ii) Arg + Dol → Mg–Cal (eutectoid composition); (iii) Arg + Mg–Cal (XMgCO3 < eutectoid composition); (iv) Mg–Cal; (v) Mg–Cal (low‐XMgCO3) + exsolved Dol.

Metamorphic structure and P–T conditions of the metapelitic constituent of the ZEV

Bounded by mylonite zones and imbrications the Zone of Erbendorf-Vohenstrauss is built up by medium- and low-pressure amphibolite-facies metapelites associated with amphibolites and biotite–hornblende gneisses, and intruded by the Leuchtenberg granite. Pelitic index assemblages define the staurolite– kyanite-, the garnet–kyanite-, the garnet–sillimanite- and the garnet–kyanite–sillimanite zone of medium pressure metamorphism, which is estimated at P=5–9.6 kbar and T=580–730 °C, and the cordierite zone of subsequent low-pressure metamorphism, which is estimated at P<4.5 kbar and T≤700 °C. Both facies series probably are tectonically juxtaposed. Staurolite–kyanite- and garnet zones are related by the reaction: staurolite+muscovite+quartz=garnet+ aluminium-silicate+biotite+H2O. This relation defines the only observable trace of a field gradient in the ZEV. An early sillimanite generation (sillimanite I) is found in all mineral zones as mineral inclusions in garnet rims. Post-P–Tmax evolution is indicated by the inversion kyanite⇒sillimanite (sillimanite II), the reaction: garnet+muscovite⇒sillimanite (sillimanite III)+biotite+quartz and by XFe decreasing in garnet rims. Garnet is resorbed under the conditions of stable kyanite+sillimanite+muscovite+quartz. Some staurolite–kyanite zone garnets show two stages separated by resorption which may be ascribed to published Ordovician and Late Devonian radiometric ages. The second garnet generation formed near 9.7 kbar/ 620–660 °C. No evidence for high-pressure metamorphism was found. Comparison of the medium-pressure part of the ZEV with the Zone of Tepla-Domazlice (Czech Republic) and with garnet–kyanite rocks at the northern edge of the Black Forest seems possible. The low-pressure cordierite rocks are similar to metapelites framing the Winklarn eclogites towards southeast of the ZEV, but are in contrast to the Moldanubian-type cordierite gneisses.

Fluid inclusion evidence for the physicochemical conditions of sulfide deposition in the Olympias carbonate-hosted Pb-Zn(Au, Ag) sulfide ore deposit, E. Chalkidiki peninsula, N. Greece

The Olympias Pb-Zn(Au, Ag) sulfide ore deposit, E. Chalkidiki, N. Greece, is hosted by marbles of the polymetamorphic Kerdilia Formation of Paleozoic or older age. The geologic environment of the ore also comprises biotite-hornblende gneisses and amphibolites intruded by Tertiary pegmatite-aplite dikes, lamprophyre dikes, the 30-Ma Stratoni granodiorite, and porphyritic stocks. Only limited parts of the deposit display shear folding and brecciation; most of it is undeformed. Microthermometry of fluid inclusions in gangue syn-ore quartz indicates three types of primary and pseudosecondary inclusions: (1) H2O-rich, 1–18 wt.% NaCl equivalent, <3.6 mol% CO2; (2) H2O-CO2 inclusions, <4wt.% NaCl equivalent, with variable CO2 contents, coexisting in both undeformed and deformed ore; (3) aqueous, highsalinity (28–32 wt,% NaCl equivalent) inclusions found only in undeformed ore. Type 2 inclusions are differentiated into two sub-types: (2a) relatively constant CO2 content in the narrow range of 8–15 mol% and homogenization to the liquid phase; (2b) variable CO2 content between 18 and 50 mol% and homogenization to the vapor phase. Type 1 and 2b inclusions are consistent with trapping of two fluids by unmixing of a high-temperature, saline, aqueous, CO2-bearing fluid of possible magmatic origin, probably trapped in type 2a inclusions. Fluid unmixing and concomitant ore mineralization took place at temperatures of 350 ± 30 °C and fluctuating pressures of less than 500 bar, for both undeformed and deformed ores. The wide salinity range of type 1 inclusions probably represents a complex effect of salinity increase, due to fluid unmixing and volatile loss, and dilution, due to mixing with low-salinity meteoric waters. High solute enrichment of the residual liquid, due to extreme volatile loss during unmixing, may account for high salinity type 3 inclusions. The Olympias fluid inclusion salinity-temperature gradients bear similarities to analogous gradients related to Pb-Zn ores formed in “granite”-hosted, low-T distalskarn, skarn-free carbonate-replacement and epithermal environments.

Geochemistry of calcic horizons in relation to hillslope processes, southern India

The study concerns chemical and mineralogical properties of calcic horizons in relation to their physical environment such as position in the soil profile and the terrain. The carbonates were calcitic in upslope positions but dolomite was abundant in valley sites. The content of fluorine increased downslope. The distribution of the composition in the terrain indicates a sequential precipitation of calcite, dolomite and fluorite from soil water and groundwater as the water flows downslope to discharge areas. Carbonates in Vertisols formed from the more basic charnockites were of the low Mg calcite type, while dolomite occurred only in Entisols and Alfisols formed from biotite-hornblende gneisses. Manganese in calcic horizons increased as the groundwater level was approached. Black intercalations in laminated petrocalcic horizons contained elevated concentrations of Mn, K, Al and Si. This may indicate periods of higher soil moisture and clay formation. The presence of low-Mg calcitic carbonates overlying dolomitic ones in valley sites is likely to be due to deposition of calcite-rich material from upslope. The amount of soil eroded from upslope positions was between 0.2 and 2 m in four investigated profiles.

ポロシリオフィオライト中にトラップされた日高変成帯起源の異地性岩塊

The boundary between the Hidaka metamorphic belt (HMB) and the Poroshiri ophiolite is a major thrust fault ; the Hidaka Main Thrust. The westernmost part of the HMB consists of an intense mylonite zone (western-margin mylonite zone) along the Hidaka Main Thrust. The ophiolite is cut by several brittle shear zones which strike NW-SE and dip NE. Exotic slices consisting of tonalitic mylonite (including hornblende-biotite gneiss and garnet bearing biotite gneiss) and brown hornblende amphibolite occur along the shear zones. The brown hornblende amphibolite is exposed as blocks in the tonalitic mylonite or intercalated with the mylonite. The bulk compositions of the hornblende-biotite gneiss and garnet bearing biotite gneiss are comparable with the lower or basal tonalite of the HMB. Major and minor element chemistries of the brown hornblende amphibolite show MORB features. K2O contents of these exotic rocks are different from those of amphibolites in the Poroshiri ophiolite, but are very similar to those of brown hornblende amphibolite of the HMB. Garnets in the tonalitic mylonite have the composition similar to those of basal tonalites of the HMB, and exhibit a reverse zoning at the margin. Hornblendes of the hornblende-biotite gneiss and the brown hornblende amphibolite have a brownish core enclosed by a greenish rim. Ti contents of the hornblende cores are similar with those of brown hornblende amphibolite in the HMB, and decrease in the green rims. Such compositional changes suggest that they underwent retrograde metamorphism. On the other hand, surrounding ophiolite rocks suffered from prograde metamorphism. The above facts indicate that these exotic slices are not originated in the ophiolite, but derived from the HMB. Studies of deformed structures in these slices and the surrounding rocks lead to the following conclusions: these rocks are ‘far-travelled horse’ accreted to the footwall (the Poroshiri ophiolite) derived from the hangingwall (the lower sequence of the HMB) of the HMT. The lower sequence of the HMB thrust to the west over the ophiolite and formed a nappe. Then, a part of the lower sequence of the HMB was tectonically intermingled with the ophiolite along the NW-SE trending shear zones.

The precambrian of Sri Lanka: a historical review

Early studies of the geology of Sri Lanka lasted from the beginning of the century until 1935. The modern phase began in the forties, whereas intensive studies of the Precambrian rocks took place in the last ten years. The term “Vijayan Gneiss”, first used for the gneisses around Colombo, was changed in 1947 to “Vijayan Series” for all rocks outside the “Central Khondalite-Charnockite Belt”. The latter was named the “Highland Series” in 1961, and the “Vijayan Series” was subdivided into an “eastern” and a “western” component, and the name was altered to Vijayan Complex in 1978. In 1959 a “Southwestern Group” was recognized, and in 1935 the “Kadugannawa Gneiss” was named. At present there is a general agreement that the following nomenclature should be used by all workers on the Precambrian of Sri Lanka: Highland Complex (HC) (formerly “Highland Series” and “Southwestern Group”); an intimate association of interbanded ortho- and paragneisses including pelitic gneiss, metaquartzite, marble and charnockitic gneiss, which forms the broad belt of rocks extending from northeast to southwest through the central highlands of the island. Outliers of HC rocks (tectonic klippen) are found in the Vijayan Complex at Kataragama and Maligawila. Vijayan Complex (VC) (formerly “Eastern Vijayan Complex”), consisting of migmatites, granitic gneisses, granitoids and scattered metasediments, and occurring in the east and south of the HC. Wanni Complex (WC) (formerly “Western Vijayan Complex”), consisting of migmatites, granitic gneisses, charnockitic gneisses, minor metasediments and granitoids, lying west of the HC. Kadugannawa Complex (KC) (partly the former “Kadugannawa Gneiss”), for the hornblende and biotite-hornblende gneisses within the elongate synformal basins (“Arenas”) around Kandy. In the early sixties it was noted that the “Highland Series” rocks had been subjected to granulite facies metamorphism and the “Vijayan Series” rocks to amphibolite facies metamorphism. Charnockitic rocks in the “Highland Series” showed the characteristics of metamorphic rocks. Recently, near-peak metamorphic conditions in the HC were established at 770–900°C and 5 kbar in the west (WC) to 9 kbar in the east (HC), with a pronounced regional gradient in palaeopressure. Relict inclusions of staurolite and kyanite in garnet of HC pelitic gneisses have been shown to document a late stage of a prograde metamorphic P-T path. A retrograde P-T path, with coronitic assemblages of retrograde mineral phases like andalusite has also been recorded. Outliers of HC rocks in the VC are klippen thrust over the VC. Parts of KC have been interpreted as a metamorphosed layered basic complex. In the late seventies three phases of deformation were recognized in the HC; presently four phases are known to occur. An early view was that the “Highland Series” was younger than the “Vijayan Series”, the former sediments having been laid down on a Vijayan floor. The HC is now known to be older than the VC, the boundary between them being a thrust contact. The exact position and nature of the HC-WC boundary, however, is still to be resolved. Recent geochronological data suggest that: (1) HC sedimentation as well as igneous activity in the form of basic lava flows, basic sills and dykes took place ∼2.0 Ga ago; (2) intrusion of the earliest granitoids into the HC took place 1.8 to 1.94 Ga ago; (3) orthogneisses of the WC were intruded between 0.67 and 1.1 Ga ago; (4) orthogneisses of the VC were intruded 1030 to 1040 Ma ago; and (5) high-grade regional metamorphism of the HC, VC and WC took place between 455 and 610 Ma ago.

Carbon‐isotope constraints on fluid advection during contrasting examples of incipient charnockite formation

Abstract Incipient charnockite formation within amphibolite facies gneisses is observed in South India and Sri Lanka both as isolated sheets, associated with brittle fracture, and as patches forming interconnected networks. For each mode of formation, closely spaced drilled samples across charnockite/gneiss boundaries have been obtained and δ13C and CO2 abundances determined from fluid inclusions by stepped‐heating mass spectrometry.Isolated sheets of charnockite (c.50 mm wide) within biotite–garnet gneiss at Kalanjur (Kerala, South India) have developed on either side of a fracture zone. Phase equilibria indicate low‐pressure charnockite formation at pressures of 3.4 ± 1.0 kbar and temperatures of about 700°C (for XH2O= 0.2). Fluid inclusions from the charnockite are characterized by δ13C values of −8% and from the gneiss, 2 m from the charnockite, by values of −15%. The large CO2 abundances and relatively heavy carbon‐isotope signature of the charnockite can be traced into the gneiss over a distance of at least 280 mm from the centre of the charnockite, whereas the reaction front has moved only 30 mm. This suggests that fluid advection has driven the carbon‐isotope front through the rock more rapidly than the reaction front. The carbon‐front/reaction‐front separation at Kalanjur is significantly larger than the value determined from a graphite‐bearing incipient charnockite nearby, consistent with the predictions of one‐dimensional advection models.Incipient charnockites from Kurunegala (Sri Lanka) have developed as a patchy network within hornblende–biotite gneiss. CO2 abundances rise to a peak near one limb of the charnockite, and isotopic values vary from δ13C of c.−5.5% in the gneiss to −9.5% in the charnockite. The shift to lighter values in the charnockite can be ascribed to the formation of a CO2‐saturated partial melt in response to influx of an isotopically light carbonic fluid.Thus, incipient charnockites from the high‐grade terranes of South India and Sri Lanka reflect a range of mechanisms. At shallower structural levels non‐pervasive CO2 influxed along zones of brittle fracture, possibly associated with the intrusion of charnockitic dykes. At deeper levels, in situ melting occurred under conditions of ductile deformation, leading to the development of patchy charnockites.

Structural relations of charnockites of the Archaean Dharwar craton, southern India

Abstract Two varieties of charnockites are recognized in the Dharwar craton of southern India. The style and sequence of structures in one charnockite variety, and related intermediate to basic granulites, are similar to those in the supracrustal rocks of the Dharwar Supergroup and the adjacent Peninsular Gneiss. This style has isoclinal folds with long limbs and sharp hinges with an axial planar fabric in some instances. Additional evidence of flattening is provided by pinch‐and‐swell and boudinage structures, with basic granulites forming boudins in the more ductile charnockites/enderbites in the limbs of isoclinal folds. These folds are involved in near‐coaxial upright folding resulting in the bending of the axial planes of the isoclinal folds and the associated boudins. All these structures are overprinted by non‐coaxial upright folds with axial planes striking nearly N–S. The map pattern of charnockites suggests that this sequence of structures is present not only on a mesoscopic scale, but also on a macroscopic scale. Charnockites of this variety provide, in some instances, evidence of having been migmatized to give rise to hornblende–biotite gneiss and biotite gneiss, which form a part of the Peninsular Gneiss terrane.The second variety comprises charnockite sensu stricto with an entirely different structural style. This type occurs in the tensional domains of the hinge zones of the later buckle folds, in the necks of foliation boudinage, in shear zones and in release joints parallel to the axial planes of the later folds in the Peninsular Gneiss. Because the non‐coaxial later folds are associated with a strain pattern different from, and later than, that of the isoclinal folds of the first generation, it follows that charnockites of the Dharwar craton have evolved in at least two distinct phases, separate both in time and in process.

Rubidium-Strontium Whole-Rock Ages of Kataragama and Pottuvil Charnockites and East Vijayan Gneiss: Indication of a 2 Ga Metamorphism in the Highlands of Southeast Sri Lanka

Highland Group granulite-facies rocks of the Kataragama klippe in southeast Sri Lanka yield a Rb-Sr whole-rock apparent age of , MSWD = 39, and a intercept of , indicating a Highlandian metamorphism about 2.0 Ga ago. A charnockitic gneiss at Komari near Pottuvil, east Sri Lanka, gives a Rb-Sr whole-rock isochron age of , MSWD = 0.78, initial , suggesting a metamorphic resetting at about 0.8 Ga. The Rb-Sr whole-rock data of an East Vijayan biotite-hornblende gneiss fit a reference isochron of 800 Ma with a intercept of 0.705; the low intercept may be explained by a juvenile addition to the older crust. A review of available data from various isotopic dating methods suggests that the Highland Group supracrustals were deposited 2.5-2.0 Ga ago, metamorphosed in the granulite-facies about 2.0 Ga (Ml) ago, and disturbed by resetting events about 1.1 Ga (M2), 0.8 Ga (M3), and 0.55 Ga (M4) ago. The East Vijayan supracrustals were deposited 2.0-1.1 Ga ago, invaded by granites and metamorphosed in the amphibolite-facies about 1.1 Ga. (M2) ago, and disturbed by resetting events about 0.8 (M3) and 0.55 Ga (M4) ago. Overthrusting of the Kataragama granulites over the East Vijayan gneisses occurred post-M3.

Genesis of the Olympias carbonate-hosted Pb-Zn (Au, Ag) sulfide ore deposit, eastern Chalkidiki Peninsula, northern Greece

Genesis of the Olympias carbonate-hosted Pb-Zn (Au, Ag) sulfide ore deposit, eastern Chalkidiki Peninsula, northern Greece

Chemical mineralogy and geothermometry of the Middle Shire granulites, Malawi

The Middle Shire area of Malawi lies roughly midway between the southern tip of Lake Malawi and the Zambesi river. It includes the western side of the Shire Highlands and the central section of the Shire rift valley. It is occupied by gneisses and granulites of the Mozambique Belt, a polymetamorphic fold belt, which extends down the eastern side of Africa from Ethiopia to southern Mozambique. In the Middle Shire section the eastern part of the area is occupied by two-pyroxene granulites together with subordinate skarns and biotide-garnet granulites. The western part consists of biotite-gneisses and hornblende-biotite gneisses together with subordinate pelitic and calcareous metasediments. A well defined transition zone lies between them separating prograde amphibolite facies gneisses from the granulite facies to the east. Later retrograde effects are related to a subsequent episode of folding and produced secondary amphibole and biotite. Mineral compositions reflect a temperature range of 680–1000°C, during the prograde regional metamorphism and pressures of 6–8 kbars in the granulites. Transition zone temperatures were 550–680°C at 4–7 kbars. Amphibolite facies temperatures were 450–600°C at pressures of 4–5 kbars.

Plagioclases in metamorphosed rocks of Lokoja S.E., Nigeria

Composition of plagioclases in schists, migmatites, augen gneiss, granite gneiss and hornblende-biotite gneiss in Lokoja S.E., Nigeria ranges from An 24 to An 32. Granite and gabbro which intruded these rocks have plagioclases ranging in composition from An8 to An 18 and An 32 to An 58 respectively. Albite twins (80%) and pericline twins (15%) predominate in the plagioclases of all the rock units. The structural state of the plagioclases is low and the value of 2 vz ranges from 82° to 85°.

K-Ar Ages of Allochthonous Mafic and Ultramafic Complexes and Their Metamorphic Aureoles, Western Brooks Range, Alaska: ABSTRACT

End_Page 656------------------------------New K-Ar ages from allochthonous mafic and ultramafic complexes of the western Brooks Range (Brooks Range ophiolite) show that igneous rocks yield ages nearly identical to those of underlying metamorphic aureole rocks. Dated rocks of the Misheguk igneous sequence from Tumit Creek consist of (1) hornblende gabbro with minor greenschist and lower grade alteration, hornblende age 147.2 ± 4.4 Ma; and (2) hornblende-bearing diorite, also slightly altered, age 155.8 ± 4.7 Ma. Both samples come from presumed higher levels of the Misheguk sequence. Dated samples of metamorphic aureole rocks come from outcrops near Kismilot Creek and lie structurally beneath the Iyikrok Mountain peridotite body. The rocks consist of amphibolite and garnet-bearing biotite-hornblende gneiss considere to be metamorphosed Copter igneous sequence and related sedimentary rocks. Hornblende ages are 154.2 ± 4.6 Ma and 153.2 ± 4.6 Ma. Metamorphism is clearly related to the structurally overlying peridotite, as the degree of alteration decreases downward. We suggest that the K-Ar ages of these rocks represent the effects of thermal metamorphism post-dating igneous crystallization and are related to tectonic emplacement of the complex. Earlier K-Ar data on igneous rocks give similar ages and have been interpreted as reflecting tectonothermal events. The age of igneous crystallization of the mafic and ultramafic rocks of the Misheguk igneous sequence remains uncertain. End_of_Article - Last_Page 657------------

Geochemistry and petrogenetic evolution of the diatexites of Central Kerala, India

The hornblende-biotite gneisses of Central Kerala which cover approximately 490km2 exhibit schlieric and nebulitic structures, tending towards a homophanous nature and are classified here as diatexites. Mafic protoliths and restite biotite, each representing the refractory residuum of two independent partial melting episodes are widely present in the gneisses. The general mineral assemblage of the gneisses comprise quartz, K-feldspar, oligoclase, biotite and hornblende. Chemically, they are dominantly adamellitic and the behaviour of major and trace elements is consistent with a magmatic parentage. Based on petrochemical criteria, a two-stage evolution model is proposed here, which involves (i) partial melting of mafic granulites under high Archean geothermal gradients and generation of tonalite/trondhjemite through amphibole and plagioclase fractionation and (ii) partial melting and subsequent quartz-alkali feldspar fractionation of the tonalite/trondhjemite under amphibolite facies conditions with synchronous K-enrichment resulting in the diatectic adamellites.