Basement Gneisses Research Articles

Major and trace elements in German bottled water, their regional distribution, and accordance with national and international standards

The market for mineral water has been growing steadily over the last few years. Germany is the country with the highest number of bottled mineral water brands (908 bottled water samples from 502 wells/brands were analyzed). The per capita consumption of mineral water in Germany in 2003 was 129 L. A wide range of values of one to seven orders of magnitude was determined for 71 elements in the bottled water samples analyzed by ICP-QMS, ICP-AES, IC, titration, photometric, conductometric and potentiometric methods. A comparison of the element concentrations and the legal limits for both bottled and tap water (EU, Germany, US EPA, WHO) shows that only 70% of the 908 mineral water samples fulfill the German and EU drinking water (i.e., tap water) regulations for all parameters (not including pH) for which action levels are defined. Nearly 5% of the bottled water samples not fulfill the German and EU regulations for mineral and table water. Comparison of our results with the current German and European action levels for mineral and table water shows that only 42 of the bottled water samples exceed the limits for one or more of the following elements: arsenic, nitrate, nitrite, manganese, nickel and barium concentrations. Ten of the bottled water samples contain uranium concentrations above the 10 μg/L recommended limit. A selection of existing water quality classification systems was used in order to characterize the different mineral water types. For example, the relationships among selected variables were examined using Durov, Piper and van Wirdum diagrams. Univariate and multivariate (component analysis) statistical methods were used to analyze the data to characterize the bottled water from the different well regions of Germany. Maps of the element distributions are presented. Most of the elements show a clear regional dependency. The bottled water shows conspicuous geochemical influences by the limestone aquifers of the Muschelkalk (Middle Triassic), the sandstone aquifers of the Bunter and Keuper (Lower and Upper Triassic), the gypsum of the Keuper, and the aquifers in gneiss basement rocks.

Significance of “gneissic” rocks in the Liscomb Complex, Nova Scotia

The Liscomb Complex comprises Late Devonian intrusive rocks (principally peraluminous granite) and medium- to high-grade metamorphic rocks (“gneisses”) that collectively are hosted by low-grade (greenschist facies) metasediments of the Cambro-Ordovician Meguma Group. The conventional view that these “gneisses” contain high-grade mineral assemblages and represent basement rocks has recently been challenged, and indeed, some of the rocks previously mapped as gneisses, particularly metapelites, have isotopic compositions resembling the Meguma Group. Amphibole-bearing enclaves in the Liscomb plutons, however, are isotopically distinct and in this regard resemble xenoliths of basement gneisses in the Popes Harbour lamprophyre dyke, south of the Liscomb area. Metasedimentary enclaves with Meguma isotopic signatures can contain garnets with unzoned cores (implying high temperatures) that host high-grade minerals (prismatic sillimanite, spinel, and (or) corundum) and are enclosed by retrograde-zoned rims. These features are interpreted here as having formed during and following the attainment of peak temperatures related to Liscomb magmatism. The amphibole-bearing meta-igneous rocks described here contain cummingtonite or hornblendic amphibole and occur as enclaves in granodioritic to tonalitic plutons. They are mineralogically, texturally, and isotopically distinct from Meguma metasediments and at least some of the plutonic rocks that enclose them, so remain the most likely candidate for basement rocks in the Liscomb Complex.

Tectonic implications for a Cordilleran orogenic base in the Frenchman Cap dome, southeastern Canadian Cordillera

The Frenchman Cap gneiss dome of the Monashee Complex in the Canadian Cordillera sits in basement rocks of the orogen. It records a stepwise downward disappearance of penetrative deformation indicative of a frozen downward-migrating base of easterly verging Cordilleran shearing. This tectonic setting is incompatible with the commonly held views that gneiss domes of the Canadian Cordillera are extensional core complexes and that the presence of gneiss domes in orogens implies vertical flow. An important structural-time marker in our study is a widely distributed suite of ∼1850 Ma granite dykes that allow Cordilleran-aged structures to be distinguished from the older structures. The dykes show that only the uppermost ∼1.5 km structurally thick carapace of basement gneiss records penetrative Cordilleran strain, whereas the lowermost ∼5 km thick basement section does not and instead preserves a Paleoproterozoic migmatitic gneissosity. Cordilleran high strain in the upper basement carapace is characterized by penetrative easterly verging shear strain on both the westerly dipping and easterly dipping flanks of the dome, whereas Cordilleran deformation in the lower basement is limited to a meter-scale, top-to-the-east shear zone and NNE-trending, upright folds. New and previously published U–Pb data from accessory minerals indicates that the Cordilleran structures formed between 53 and 49 Ma, immediately prior to regional cooling and extension. The dome is interpreted as an incipient upright drag fold developed during top-to-the-east shearing.

Geochemistry and petrology of mafic Proterozoic and Permian dykes on Bornholm, Denmark: Four Episodes of magmatism on the margin of the Baltic Shield

More than 250 dykes cut the mid Proterozoic basement gneisses and granites of Bornholm. Most trend between NNW and NNE, whereas a few trend NE and NW. Field, geochemical and petrological evidence suggest that the dyke intrusions occurred as four distinct events at around 1326 Ma (Kelseaa dyke), 1220 Ma (narrow dykes), 950 Ma (Kaas and Listed dykes), and 300 Ma (NW-trending dykes), respectively. The largest dyke at Kelseaa (60 m wide) and some related dykes are primitive olivine tholeiites, one of which has N-type MORB geochemical features; all are crustally contaminated. The Kelseaa type magmas were derived at shallow depth from a fluid-enriched, relatively depleted, mantle source,but some have a component derived from mantle with residual garnet. They are suggested to have formed in a back-arc environment. The more than 200 narrow dykes are olivine tholeiites (some picritic), alkali basalts, trachybasalts, basanites and a few phonotephrites. The magmas evolved by olivine and olivine + clinopyroxene fractionation. They have trace element characteristics which can be described mainly by mixing of two components: one is a typical OIB-magma (La/Nb < 1, Zr/Nb = 4, Sr/Nd = 16) and rather shallowly derived from spinel peridotite; the other is enriched in Sr and has La/Nb = 1.0 - 1.5, Zr/Nb = 9, Sr/Nd = 30 and was derived at greater depth, probably from a pyroxenitic source. Both sources were probably recycled material in a mantle plume. A few of these dykes are much more enriched in incompatible elements and were derived from garnet peridotite by a small degree of partial melting. The Kaas and Listed dykes (20-40 m) and related dykes are evolved trachybasalts to basaltic trachyandesites. They are most likely related to the Blekinge Dalarne Dolerite Group. The few NW-trending dykes are quartz tholeiites, which were generated by large degrees of rather shallow melting of an enriched mantle source more enriched than the source of the older Bornholm dykes. The source of the NW-trending dykes was probably a very hot mantle plume.

Pb isotopes during crustal melting and magma mingling — A cautionary tale from the Miki Fjord macrodike, central east Greenland

Pb isotopic data are presented for hybrid rocks formed by mingling between mantle-derived tholeiitic magma of the Eocene Miki Fjord macrodike (East Greenland) and melt derived from the adjacent Precambrian basement. Bulk mixing and AFC processes between end-members readily identified in the field fail to model the Pb isotope systematics. Selective contamination during diffusional exchange, which can explain the complex Sr and Nd isotope compositions of the hybrid rocks (Blichert-Toft et al., 1992), cannot fully account for the variability of the Pb isotopic data using the identified crustal end-members. The crustal anatectic end-member, although similar in Sr and Nd isotope composition, has a markedly different Pb isotopic composition than its source gneiss. The differences are consistent with preferential incorporation of radiogenic Pb from accessory phases such as metamict zircon or loosely-bound Pb from grain boundaries during disequilibrium melting of the host rock by the mafic magma. The crustal melt involved in magma interactions was therefore heterogeneous with respect to Pb isotopes on a metre-scale. These results illustrate the difficulties inherent in interpreting isotopic variations in contaminated mafic magmas even when the end-members are well constrained by field relations. We show that the Pb isotopic composition of the crustal contaminants and contamination trajectories for the Miki Fjord hybrid magmatic lithologies are markedly different from regional basement gneisses and contaminated lavas of the associated flood basalt province that were contaminated by characteristically unradiogenic Pb and Sr lithologies resembling Lewisian basement not exposed locally.

Fission-track analysis unravels the denudation history of the Bonar Range in the footwall of the Alpine Fault, South Island, New Zealand

AbstractWe apply fission-track thermochronology to shed new light on the tectonic history of Zealandia during Late Cretaceous continental extension and the onset of Late Tertiary mountain building in the Southern Alps of New Zealand. The Southern Alps are one of the fastest erosionally exhuming mountain belts on Earth. Exhumation of the Bonar Range in Westland just to the northwest of the Alpine Fault is orders of magnitude slower. We report apatite and zircon fission-track ages from samples that were collected along an ENE–WSW profile across the central Bonar Range, parallel to the tectonic transport direction of a prominent ductile fabric in the basement gneiss. Zircon fission-track (ZFT) ages show a large spread from 121.9 ± 12.1 Ma to 74.9 ± 7.2 Ma (1σ errors). The youngest ZFT ages of 78 to 75 Ma occur at low elevations on either side of the Bonar Range and become older towards the top of the range, thereby showing a symmetric pattern parallel to the ENE-trending profile across the range. Age–elevation relationships suggest an exhumation rate of 50–100 m Ma−1. We relate the ZFT ages to slow erosion of a tectonically inactive spot in the Late Cretaceous magmatic arc of Zealandia. Therefore, the first main significance of the paper is that it demonstrates that not all of 110–90 Ma Zealandia was necessarily participating in extreme core complex-related extension but that there were enclaves of lithosphere that underwent slow erosion. The apatite fission-track (AFT) ages range from 11.1 ± 1.9 Ma to 5.3 ± 1.0 Ma and age–elevation relationships suggest an exhumation rate of c. 200 m Ma−1. We relate the AFT ages to the inception of transpressive motion across the Alpine Fault and modest exhumation in its footwall in Late Miocene times. If so, the second significant point of this paper is that transpressive motion across the Alpine Fault was already under way by c. 11 Ma.

Inherited drainage - paleochannels and preferential groundwater flow

It is suggested that in a localized remnant of Kalahari sand at Dufuya, central Zimbabwe, groundwater flows in an integrated pattern inherited from the paleochannel network of the underlying gneiss. Contact springs occur at discrete localities along the Kalahari sand/gneiss boundary and are associated with spring sapping and land surface subsidence. Subsidence is presumed to be due to preferential solute removal by leaching and dissolution as a result of concentration of groundwater flow within the buried paleochannel network and the location of the springs is presumed to occur where the paleochannel network intersects the Kalahari sand/gneiss boundary. Over time the surficial Kalahari sand is preferentially removed along these buried drainage lines by spring sapping and headwards erosion, exposing the gneiss. Multi-electrode direct current resistivity profiling and radar have been used to map the sub-surface, revealing the topography of the basement and nature of the Kalahari cover. Coincidence of gneiss basement depressions with the spring sites, leached sands and subsidence zones suggests inheritance of the gneiss fluvial paleochannel network pattern by the present day groundwater flow. Washed sand and gravel intersected in shallow boreholes in these areas provides further evidentiary support for the concept of inherited drainage.

The Moine Supergroup of NW Scotland: insights into the analysis of polyorogenic supracrustal sequences

Abstract The Moine Supergroup of NW Scotland is a thick sequence of early Neoproterozoic sedimentary rocks, with minor igneous intrusions, that display evidence for multiple phases of regional deformation and metamorphism. The descriptions and interpretations of the ‘Moine Schists’ provided by the 1907 memoir ( Peach et al. 1907 ) have been proved to be essentially correct and have laid the groundwork for a century of distinguished and influential research that has reached far beyond the confines of NW Scotland. The Survey workers recognized the sedimentary protoliths of these rocks, realized that they had been deposited unconformably on inliers of reworked basement gneisses that now occupy the cores of major folds, and understood the likely complexity of folding and the kinematic significance of mineral lineations. Further advances in understanding of the Moine rocks were mainly achieved through two techniques that were not available to the Survey workers of 100 years ago – geochronology and palaeomagnetism. Isotopic studies have confirmed the view of the Survey workers that the Moine rocks are of Precambrian age, and furthermore have demonstrated a complex, polyorogenic history.

Structure and kinematics of the Jungfrau syncline, Faflertal (Valais, Alps), and its regional significance

The formation and structural evolution of the Jungfrau syncline is described, based on excellent outcrops occurring in the Lötschental, in the Central Alps of Switzerland. The quality of the outcrops allows us to demonstrate that the External Massifs of the Swiss Alps have developed due to internal folding. The Jungfrau syncline, which separates the autochtonous Gastern dome from the Aar massif basement gneiss folds, is composed of slivers of basement rocks with their Mesozoic sedimentary cover. In the Inner Faflertal, a side valley of the Lötschental, the 200 m thick syncline comprises four units, the Gastern massif with a reduced Mesozoic sedimentary cover in a normal stratigraphic succession, two units of overturned basement rocks with their Mesozoic sedimentary cover, and the overturned lower limb of the Tschingelhorn gneiss fold of the Aar massif with lenses of its sedimentary cover. Stratigraphy shows that the lower units, related to the Gastern massif, are condensed and that the upper units, deposited farther away from a Gastern paleo-high, form a more complete sequence, linked to the Doldenhorn Meso-Cenozoic basin fill. The integration of these local observations with published regional data leads to the following model. On the northern margin of the Doldenhorn basin, at the northern fringe of the Alpine Tethys, the pre-Triassic crystalline basement and its Mesozoic sedimentary cover were folded by ductile deformation at temperatures above 300 °C and in the presence of high fluid pressures, as the Helvetic and Penninic nappes were overthrusted towards the northwest during the main Alpine deformation phase. The viscosity contrast between the basement gneisses and the sediments caused the formation of large basement anticlines and tight sedimentary synclines (mullion-type structures). The edges of basement blocks bounded by pre-cursor SE-dipping normal faults at the northwestern border of the Doldenhorn basin were deformed by simple shear, creating overturned slices of crystalline rocks with their sedimentary cover in what now forms the Jungfrau syncline. The localisation of ductile deformation in the vicinity of pre-existing SE-dipping faults is thought to have been helped by the circulation of fluids along the faults; these fluids would have been released from the Mesozoic sediments by metamorphic dehydration reactions accompanied by creep and dynamic recrystallisation of quartz at temperatures above 300 °C. Quantification of the deformation suggests a strain ellipsoid with a ratio (1+ e1 / 1+ e3) of approximately 1000. The Jungfrau syncline was deformed by more brittle NW-directed shear creating well-developed shear band cleavages at a late stage, after cooling by uplift and erosion. It is suggested that the external massifs of the Alps are basement gneiss folds created at temperatures of 300 °C by detachment through ductile deformation of the upper crust of the European plate as it was underthrusted below the Adriatic plate.

High spatial resolution 40 Ar/ 39 Ar dating of pseudotachylites: geochronological evidence for multiple phases of faulting within basement gneisses of the Outer Hebrides (UK)

Abstract: Pseudotachylites within the Neoarchaean to Palaeoproterozoic basement of the Lewisian Gneiss Complex have been analysed using the high spatial resolution 40 Ar/ 39 Ar UV laserprobe technique. It has been possible to measure 40 Ar/ 39 Ar age and ‘apparent' age variations that would be beyond resolution by lower spatial resolution or bulk 40 Ar/ 39 Ar dating methods. We report a range of complexities including significantly younger narrow (<1 mm) margins on much older pseudotachylite veins, and heterogeneous excess argon contamination. We also report pseudotachylite veins of varying magnitudes that have been subject to later thermal reheating, whereby there is a relationship between vein thickness and extent of argon loss. In spite of these it is still possible to determine geologically meaningful 40 Ar/ 39 Ar ages for pseudotachylite-forming events. In Lewis the c . 1900 Ma event may relate to early stages of Laxfordian reworking. In Barra and South Uist a 1300–1200 Ma event is recorded, whereas in Barra, South and North Uist a second event is recorded at c . 700 Ma, which is likely to be associated with NW- and west-directed thrusting, and could represent far-field effects of tectonic events recorded at this time east of the Moine Thrust in NW Scotland. Supplementary material: 40 Ar/ 39 Ar data are available at http://www.geolsoc.org.uk/SUP18372 .

Geochemistry and petrogenesis of late Miocene granitoids, Cyclades, southern Aegean: Nature of source components

Neogene igneous activity in the Aegean region comprised both extrusive volcanism and intrusive granitoid plutonism. Granitoid plutonism in the Cyclades was established in the mid-Miocene period and lasted until late Miocene (18 to 9 Ma). The timing of granitoid intrusion was associated with the initiation of extensional tectonics in the Aegean. These intrusions form a broad belt about 200 km long running from the west (Lavrium and Serifos) to the eastern Aegean. Both S-type and I-type granitoids are present, the former generally being emplaced earlier than the I-types (~ 15 to 8.3 Ma). Major and trace element variations reveal that three end-member components are involved in the granitoids, but the proportions of these vary in the different plutons. Initial isotopic compositions of all the granitoids are typical of crust-derived magmas from heterogeneous metasedimentary sources (I-type: 87Sr/86Sr = 0.7091–0.712, εNd = − 6.4 to − 10.4; S-type: 0.710–0715, εNd = − 7.5 to − 10.1). Three end-member sources have been identified: 1) One end-member appears to be a metasedimentary biotite-gneiss (greywacke-type) such as that forming the metamorphic core complexes (Naxos and Paros); this is a dominant (but not the only) component in the S-types. 2a) Major, trace element and Sr–Nd isotopic composition correlations of the S-type granitoids with basement gneiss require an extra source of Sr and Ca, having lower initial 87Sr/86Sr indicating a more depleted metasedimentary source at depth, due to possible interaction, of metamorphic fluids with marbles and amphibolites and infiltration through the gneissic units, at mid-crustal levels. 2b) A possible second end-member could be the marble component, as indicated by the buffered values of the Initial Sr isotopic ratios. Major element variation of the mafic microgranular (quartz diorites and tonalites) enclaves are compatible with dehydration melting of a mafic source similar to the amphibolites (island arc tholeiites) at medium pressure (~ 8 kb) conditions. 3) Amphibolite is another end-member which may contribute mostly to the source of the younger intrusions, along the western flank of the arc.

Formation of high‐pressure metabasites in the southern Appalachian Blue Ridge via Taconic continental subduction beneath the Laurentian margin

Tectonic models that account for the occurrence of eclogite, retrograde eclogite, peridotite, and migmatitic basement gneisses in the southern Blue Ridge invoke closure of a small ocean basin between the eastern Blue Ridge (EBR) and western Blue Ridge (WBR). In this “obducted ophiolite” model, Franciscan‐type, eastward subduction of oceanic crust beneath the Piedmont arc occurred prior to obduction of the arc onto the Laurentian margin. The discovery of kilometer‐scale eclogite bodies near the EBR‐WBR boundary are cited as evidence for obducted ophiolite/accreted island arc models. However, thermobarometric estimates based on mineral chemistry from HP metabasites of the western, central, and eastern Blue Ridge indicate that eclogites and granulites are medium‐temperature type (550–900°C), inconsistent with the obducted ophiolite model. Mid‐ocean ridge basalt subducted beneath continental margins and island arcs rarely reaches temperatures in excess of 550°C. Thermobarometric comparisons of the present study and a reexamination of deep seismic profiles from the Blue Ridge allow for a model of westward subduction of continental material beneath Laurentia during the Taconic orogeny that produced a megamélange now preserved in the central and eastern Blue Ridge tectonic provinces. P‐T paths determined from Blue Ridge HP metabasites are similar to P‐T paths determined previously from Caledonian and Variscan eclogites. Thermobarometric similarities imply a common tectonic history of continental collision, subduction of continental crust to depths of 50–65 km, followed by rapid eduction.

Intracontinental subduction: a possible mechanism for the Early Palaeozoic Orogen of SE China

AbstractThe Early Palaeozoic Orogen of SE China consists of three litho‐tectonic elements, from top to bottom: a sedimentary Upper Unit, a metamorphic Lower Unit and a gneissic basement. The boundaries between these units are flat lying, south directed, ductile decollements. The lower one is coeval with an amphibolite facies metamorphism (M1). The belt is reworked by migmatite–granite domes, high‐temperature metamorphism (M2) and granitic plutons related to post‐orogenic crustal melting. We date here the syn‐M1 ductile shearing at 453 ± 7 Ma by U‐Th/Pb method on monazite. Previous ages and our new 40Ar/39Ar ages of biotites and muscovites show that the metamorphic rocks experienced syn‐M2 exhumation from 440 to 400 Ma. The Early Palaeozoic Orogen of SE China is an intracontinental belt in which decollements accommodated the north‐directed subduction of the Cathaysian continent. This orogen is an example of intracontinental subduction that was not preceded by oceanic subduction.

Pan-African strike–slip tectonics in eastern Cameroon—Magnetic fabrics (AMS) and structure in the Lom basin and its gneissic basement

Field, microstructural, and anisotropy of magnetic susceptibility (AMS) or magnetic fabric studies were applied to identify the sequence and character of the Pan-African structures in the basement of Eastern Cameroon at both sides of the regional scale Bétaré-Oya Shear Zone (BOSZ). The NE-SW trending BOSZ separates older gneisses and migmatites towards SE (domain I) from the younger rocks of the Lom meta-volcano-sedimentary basin towards NW (domain II). In domain I, early, ductile compressional deformation occurred in two events, D1 and D2, under relatively high T conditions. During subsequent cooling, strain partitioned between the competent basement gneisses with only mild compression and the bordering shear zone (BOSZ) with intense simple shear-wrenching (D3). Strain in the less competent rocks of domain II is dominated by simple shear, strike-slip wrenching (D3), with an earlier stage of compressional deformation preserved only in some low strain pods. Magnetic fabrics (AMS) document a progressive change from oblate ellipsoids towards prolate ellipsoids in domain I, when proceeding from the south towards the BOSZ. Foliations are mostly steep but define a girdle with a pole plunging gently towards WSW. The magnetic lineations also plunge mostly towards WSW at shallow angles. These fabrics indicate a compression approximately normal to the BOSZ, which is also the SE margin of the Lom Basin. In the Lom metasediments (domain II), AMS ellipsoids are typically oblate. Foliations trend NE-SW with mostly steep dips. Magnetic lineations plunge gently NE or SW. This fabric with foliations mostly steep and subparallel with the major BOSZ, combined with generally subhorizontal lineations implies the BOSZ as a Pan-African strike–slip shear zone with a subordinate component of compression. At a larger scale, the area is part of a continent-scale shear zone, separating external Pan-African domains of compression along the northern margin of the Congo craton from internal domains dominated by high-angle strike–slip and transpressional deformation. Together with published data, the present study thus demonstrates that transpression is a regional phenomenon in the Pan-African orogen of central and eastern Cameroon.

Mixing hydrology and chemical equilibria in Bakreswar geothermal area, Eastern India

A group of alkaline thermal springs of Bakreswar, having varying temperatures (45–71 °C) and identical compositions emerges through a nearly N–S trending fault in the gneissic basement. The thermal springs are of Na–Cl–SO 4 type and are rich in SiO 2 and F, contrary to the local groundwaters which are mostly of Ca–HCO 3 type. The springs undergo mild to moderate dilution by local groundwater, which does not significantly affect their original compositions. Agnikund, the main spring, with the highest temperature, mild dilution, consistent composition and negligible tritium, discharges almost pure deep-seated thermal water. Original thermal water is at equilibrium with quartz in the deep reservoir and, attains surface temperature largely by conductive cooling with no significant loss of dissolved SiO 2. Certain ionic ratios coupled with very low tritium contents in the springs are indicative of prolonged subsurface circulation of the original water. XRD results of the spring site sediments and the bore hole rocks fairly corroborate the predicted saturation state of thermal water with respect to quartz, feldspar, calcite, fluorite etc., which is believed to control the major chemical compositions of the springs. A geothermal reservoir having probable temperature of about 100 ±5 °C and possibly occurring at a depth of 1 km is the source of thermal water for the springs. The present study is aimed at understanding the mixing trend between thermal springs and local groundwaters as well as the water–rock equilibria.

Evidence for 2.35 to 2.30 Ga juvenile crustal growth in the northwest Borborema Province, NE Brazil

Abstract The c . 600 Ma Brasiliano Borborema Province of NE Brazil comprises a complex collage of Precambrian crustal blocks cut by a series of continental-scale shear zones. The predominant basement rocks in the province are 2.1–2.0 Ga Transamazonian gneisses of both juvenile and reworked nature. U–Pb zircon and Sm–Nd whole-rock studies of tonalite-trondhjemite-granodiorite basement gneisses in the NW Ceará or Médio Coreaú domain in the northwestern part of the Borborema Province indicate that this represents a continental fragment formed by 2.35–2.30 Ga juvenile crust. This block has no apparent genetic affinity with any other basement gneisses in the Borborema Province, and it does not represent the tectonized margin of the c . 2.1–2.0 Ga São Luis Craton to the NW. The petrological and geochemical characteristics, as well as the Nd-isotopic signatures of these gneisses, are consistent with their genesis in an island arc setting. This finding documents a period of crustal growth during a period of the Earth's history which is known for its tectonic quiescence and paucity of crust formation.

Precambrian mafic magmatism of Shillong plateau, Meghalaya and their evolutionary history

The Archaean gneissic basement of Shillong plateau has been traversed by number of mafic dyke swarms. At least two suites of dykes are identified in the region represented by Proterozoic Khasi greenstone related dolerites and younger Cretaceous dolerite dykes in addition to mafic alkaline dykes. The older Khasi greenstone dolerites are altered and have undergone low-grade metamorphism compared to fresh Cretaceous dykes, which are well exposed in the West Garo Hills region. All the Khasi greenstone dolerites are tholeiite in composition and range from basalt to basaltic andesite in composition and show olivine or quartz normative character. Most of the dykes show continental nature of emplacement with some overlapping oceanic tectonic setting of origin. Petrochemical study suggests that they were derived from picrites that subsequently undergone low-pressure fractionation. Palaeomagnetic study of the older Khasi greenstone related dolerites show a direction of magnetization of Dm=17, Im= +57 (α95= 23.34; K=31.5; N=24) with a palaeolatitude of 29.7°N to the Indian subcontinent that clearly support the Proterozoic dyke/dyke swarm emplacement in the region. The magnetic carrier as inferred from K-T studies is in multi domain (MD) size and cation deficient (CD) domain states.

Phanerozoic tectonics of south China block: New insights from the polyphase deformation in the Yunkai massif

The Yunkai massif is a key region to decipher the tectonic evolution of the south China block since the contained high‐grade metamorphic rocks experienced a polyphase deformation. The earliest event (D1) corresponds to a top‐to‐the‐northwest ductile shearing, coeval with amphibolite facies metamorphism, probably developed during a postorogenic synmigmatization extensional event occurred in early Paleozoic time. The main ductile deformation (D2) is top‐to‐the‐northeast shearing, occurred in early Mesozoic time, coeval with amphibolite to greenschist facies metamorphism and associated with the development of northeast verging recumbent folds within the Neoproterozoic‐Paleozoic sedimentary cover of the south China block. The tectonic significance of D2 is possibly linked with the subduction of Indochina beneath south China along the Song Ma belt of North Vietnam. Another compressional event (D3) of Late Jurassic to Cretaceous age is indicated by SE verging recumbent folds with NE–SW trending axes developed both in the sedimentary cover and gneissic basement. The northwestward subduction of the Pacific plate or the SE thrusting of the Songpan‐Ganzi fold belt upon the Yangtze craton can be considered as the geodynamic causes of this late Mesozoic event. Last, NE–SW trending brittle sinistral strike‐slip and normal faulting (D4) control the massif uplifting and the subsiding of continental half grabens. This structure, which is widespread in the eastern margin of the Eurasia continent, can be related either to the westward subduction of the Paleo‐Pacific plate or to lithosphere removal during the late Mesozoic.

Metamorphism, graphite crystallinity, and sulfide anatexis of the Rampura–Agucha massive sulfide deposit, northwestern India

Located adjacent to the Banded Gneissic Complex, Rampura–Agucha is the only sulfide ore deposit discovered to date within the Precambrian basement gneisses of Rajasthan. The massive Zn–(Pb) sulfide orebody occurs within graphite–biotite–sillimanite schist along with garnet–biotite–sillimanite gneiss, calc–silicate gneisses, amphibolites, and garnet-bearing leucosomes. Plagioclase–hornblende thermometry in amphibolites yielded a peak metamorphic temperature of 720–780°C, whereas temperatures obtained from Fe–Mg exchange between garnet and biotite (580–610°C) in the pelites correspond to postpeak resetting. Thermodynamic considerations of pertinent silicate equilibria, coupled with sphalerite geobarometry, furnished part of a clockwise P–T–t path with peak P–T of ∼6.2 kbar and 780°C, attained during granulite grade metamorphism of the major Zn-rich stratiform sedimentary exhalative deposits orebody and its host rocks. Arsenopyrite composition in the metamorphosed ore yielded a temperature [and log f(S 2)] range of 352°C (−8.2) to 490°C (−4.64), thus indicating its retrograde nature. Contrary to earlier research on the retrogressed nature of graphite, Raman spectroscopic studies on graphite in the metamorphosed ore reveal variable degree of preservation of prograde graphite crystals (490 ± 43°C with a maximum at 593°C). The main orebody is mineralogically simple (sphalerite, pyrite, pyrrhotite, arsenopyrite, galena), deformed and metamorphosed while the Pb–Ag-rich sulfosalt-bearing veins and pods that are irregularly distributed within the hanging wall calc–silicate gneisses show no evidence of deformation and metamorphism. The sulfosalt minerals identified include freibergite, boulangerite, pyrargyrite, stephanite, diaphorite, Mn–jamesonite, Cu-free meneghinite, and semseyite; the last three are reported from Agucha for the first time. Stability relations of Cu-free meneghinite and semseyite in the Pb–Ag-rich ores constrain temperatures at >550°C and <300°C, respectively. Features such as (1) low galena–sphalerite interfacial angles, (2) presence of multiphase sulfide–sulfosalt inclusions, (3) microcracks filled with galena (±pyrargyrite) without any hydrothermal alteration, and (4) high contents of Zn, Ag (and Sb) in galena, indicate partial melting in the PbS–Fe0.96S–ZnS–(1% Ag2S ± CuFeS2) system, which was critical for metamorphic remobilization of the Rampura–Agucha deposit.

Delineation of potential groundwater zones in the Kagna river basin of Gulburga district, Karnataka, India using remote sensing and GIS techniques

The potential groundwater zones in parts of Kagna river basin of Gulburga district, Karnataka, have been delineated on the basis of geomorphic units alongwith geological setting, lineaments indicating bore well locations using remote sensing and GIS. The study area covers an area of 1318 sq km and it is divided into Wadi, Chitapur, Sedam and Kurkunta sub-basins which range in area from 184 to 537 sq km. Lithologically, the area consists of basement gneisses with granites overlain by conglomerate, sandstone, shale and limestone with a thin cover of Deccan basalts. Hydrogeomorphologically, the entire area is classified into escarpment slope, pediments, pediment inselberg complex, shallow pediplains, moderate pediplains, slightly to highly dissected and undissected plateaus, valleys and shallow valley fills. The lineaments representing drainages and dykes are oriented in NNW, NW, EW and NE directions. Most of boreholes are located nearer to the lineaments indicating that the lineaments are acting as pathways for the movement of groundwater. Based on lineament and hydrogeomorphic analysis, it is suggested that the valley fills and moderate pediplains are very good to good, shallow pediplains and dissected plateau are good to moderate, pediments are moderate to poor and the pediment inselburg complex and escarpment slope are the poor to very poor potential groundwater zones.