Series Of Gneisses Research Articles

Petrogenesis and Tectonic Implications of the Neoproterozoic Peraluminous Granitic Rocks from the Tianshui Area, Western Margin of the North Qinling Terrane, China: Evidence from Whole-Rock Geochemistry and Zircon U–Pb–Hf–O Isotopes

The source and petrogenesis of peraluminous granitic rocks in orogenic belts can provide insights into the evolution, architecture, and composition of continental crust. Neoproterozoic peraluminous granitic rocks are sporadically exposed in the Tianshui area of the western margin of the North Qinling Terrane (NQT), China. However, the source, petrogenesis, and tectonic setting of these rocks still remain unclear, which limits our understanding of the Precambrian tectonic and crustal evolution of the Qinling Orogenic Belt (QOB). Here, we determined the whole-rock geochemical compositions and in situ zircon U–Pb ages, trace-element contents, and Hf–O isotopic compositions of a series of peraluminous granitic mylonites and granitic gneisses in the Tianshui area at the west end of North Qinling. Zircon U–Pb dating revealed that the protoliths of the studied granitic mylonites and granitic gneisses crystallized at 936–921 Ma. The granitic rocks displayed high A/CNK values (1.12–1.34) and were enriched in large-ion lithophile elements (e.g., Rb, Ba, Th, U, and K) and light rare earth elements, and they were depleted of high-field-strength elements (e.g., Nb, Ta, and Ti). These rocks showed variable zircon εHf(t) (−12.2 / 9.7) and δ18O (3.56‰ / 11.07‰) values, suggesting that they were derived from heterogeneous crustal sources comprising predominantly supracrustal sedimentary rocks and subordinate igneous rocks. In addition, the U–Pb–Hf isotopic compositions from the core domains of inherited zircons were similar to those of detrital zircons from the Qinling Group, suggesting that the Qinling Group was an important crustal source for the granitic rocks. The lithological and geochemical features of these granitic rocks indicate that they were generated by biotite dehydration melting of heterogeneous sources at lower crustal depths. Combining our results with those of previous studies, we suggest that the NQT underwent a tectonic transition from syn-collision to post-collision at 936–874 Ma in response to the assembly and breakup of the Rodinia supercontinent.

Geophysical Analysis of Thermo-physical Properties of Rocks in Ikogosi field for Geothermal Energy Prospect.

The thermal and physical properties of rock units from Ikogosi Warm Spring (IKGWS) in the Southwestern part of Nigeria were examined in order to characterize and explore its geothermal prospect and to provide an insight into the different thermal properties of rocks of the study area. A total of 40 rock samples made up of granite, quartzite and gneiss series were collected from the outcrops of the study area and analyzed for Thermal conductivity (TC), Radiogenic heat production (RHP), Heat flow (HF), Porosity, Density and surface spring temperature measurements. The RHP values for all samples varied from 1.8 to 3.5μWm-3 with an average value of 2.5μWm-3 and standard deviation (SD) of 0.4 while the heat flow values varied from 14 to 27mWm-2 with an average of 19mWm-2 and SD of 3.4. The TC values varied from 2.95 to 4.11 with a mean of 3.49mWK-1 with a SD of 0.4 while the porosity values varied from 0.21-1.15 % with a mean of 0.62% with SD of 0.39. The density values varied from 2.68 to 2.85gcm-3 with a mean of 2.76gcm-3 and SD of 0.067. The surface temperature of the spring varied from 32 to 45°C with a mean of 38.9°C. From these results the average RHP and HF values estimated from all samples was below the 4μWm-3 and 100mWm-2 recommended value of heat to be considered for economic importance. Thus, the IKGWS geothermal field cannot be explored for power generation but for other geothermal activities and may be classified as a low enthalpy geothermal system (

Uncovering the Leichhardt Superbasin and Kalkadoon-Leichhardt Complex in the southern Mount Isa Terrane, Australia

The parts of the Mount Isa Terrane that crop out have been instrumental in deciphering the configuration of the supercontinent Nuna. In addition, these rocks host prolific base and precious metals. Further south, the Mount Isa Terrane is buried by >800 m of Phanerozoic cover that, to date, has primarily been investigated via geophysical surveys. Here, we present zircon U-Pb and Hf isotopic data of granitoids, metabasalts and metasedimentary rocks from seven >1 km deep drill holes that provide a window into the basement geology of the southern parts of the Mount Isa Terrane. The oldest rocks, intersected in two of the drill holes, are a series of granitic gneisses dated at ca. 1865 Ma, corresponding to the Kalkadoon-Leichhardt Complex. The majority of the encountered lithology in the drill holes are metasedimentary rocks with associated volcanic rocks and intruding granitoids, which bracket the remaining stratigraphy to ca. 1800–1760 Ma, corresponding to the lower and middle parts of the Leichhardt Superbasin. The ca. 1865 Ma Kalkadoon-Leichhardt Complex and ca. 1800 Ma granitoids have unradiogenic Hf isotopic signatures that necessitate an Archean lithosphere underneath the Mount Isa Terrane. Detrital zircon from the Leichhardt Superbasin may have been derived locally but also share age and Hf isotopic signatures with the eastern and northern Gawler Craton, implying possible long-distance transport. Metamorphic events recorded in discrete zircon grains and rims reveal punctuated tectonic activity at ca. 1760–1750 Ma and ca. 1720–1710 Ma in the southern Mount Isa Terrane, related to the pre-Wonga Event and the second stage of the Wonga Orogeny. The stratigraphy and events in the southern Mount Isa Terrane can be correlated to the northern and eastern Gawler Craton in South Australia, which may only have been ∼500–1000 km from the Mount Isa Terrane at the time of deposition. Ultimately, the results from these new drill holes reveal the extent of the Mount Isa Terrane under deep sedimentary cover and strengthen the links between the North and South Australian Cratons during the protracted assembly of the supercontinent Nuna.

Structural elements and evolution of the Yangmuchuan dome: Constraints on the tectonics of the Paleoproterozoic Jiao-Liao-Ji belt in the Eastern Block of the North China Craton

The nature of assembly of the Columbia supercontinent is well-characterized by the orogenic belts that were formed during amalgamation of continental blocks. The Jiao-Liao-Ji Belt (JLJB) in the eastern North China Craton possesses complex deformation structures that exemplify the Paleoproterozoic orogeny. A series of granitic gneiss domes in the southern JLJB characterizes the structural deformation in the Paleoproterozoic orogenic belts.Located in the middle segment of the JLJB, the Yangmuchuan dome is composed of three major elements, i.e., the Liaoji granites in the core, and sheared metamorphic rocks in the mantle and outer shell. Bedding-parallel ductile shear zones and weakly deformed metamorphic rocks between them are well-preserved in the latter two elements. Structural analysis reveals that the Yangmuchuan dome mainly experienced two deformation stages (D1 and D2). The D1 deformation is characterized by pure shear-dominated deformation structures with occurrence of penetrative foliations, symmetrical boudinages, mineral lineations and rootless folds in all the rocks from the mantle and outer shell. Dominated by simple shearing, the D2 deformation is the key stage contributing to the formation of the dome structure in the area. The D2 deformation is manifested as occurrence of A-type folds, S-C fabrics, mylonitic foliation S2, mineral stretching lineation L2 in granites, and in layered rocks in the mantle and outer shell. They were superposed on and transformed, along high strain zones, D1 deformation structures. Magnetic fabrics of the Liaoji granites from the core show a generally consistent orientation with the foliations and stretching lineations in the sheared granites. LA-ICP-MS zircon U-Pb dating on syn-tectonic leucogranitic dykes indicates that the peak deformation (D2) of the Yangmuchuan dome occurred from >1887 to 1808 Ma. Geometrical and kinematic analysis from the structural, microstructural and magnetic fabric data reveal that formation of the Yangmuchuang gneiss dome resulted from dominant subhorizontal flow and subsidiary vertical flow in the middle and lower crust as the consequence of orogenic activity along the JLJB during the assembly of the Columbia supercontinent.

Geoelectrical Resistivity and Geological Characterization of Hydrostructures for Groundwater Resource Appraisal in the Obudu Plateau, Southeastern Nigeria

Electrical resistivity tomography (ERT) and vertical electrical sounding (VES) techniques were employed to assess the hydrogeological structures and groundwater potential of the Obudu Basement Complex. The Wenner electrode configuration was used to acquire the ERT data along 4 profiles, while the Schlumberger configuration was employed in the VES investigation, where 30 locations were occupied during the data acquisition campaign. Results show that rock distribution is predominantly the granite gneiss series that are considerably weathered in several locations especially within the western portion of the study area. Three potential hydrogeological scenarios were identified from three of the ERT profiles. These include the regolith-, doleritic- and kaolin-induced groundwater conduits. Within the central segment of the study area, transtensional stress regimes traceable to the neo-Proterozoic syn-orogenic event were observed to have resulted in the establishment of some regional fault systems. Over time, several episodes of minor tectonic events have resulted in cataclastic deformation of the gneissic rocks, creating pathways for groundwater circulation. Additionally, it was observed that dolerite intrusion in some locations cataclastically deformed the host rock resulting in syn-emplacement fractures that can create pathways for groundwater to flow. Saprolite units and locations within major fault lines were inferred to have moderate to good groundwater potential due to their large width and the presence of multiple interconnected fractures as inferred from the pseudo-sections and lineament map. More so, streams and rivers within these areas can infiltrate through the pathways created by the faults, hence increasing groundwater circulation within the conduits.

Multiple fluid migration events and REE+Th mineralisation during Alpine metamorphism in the Sopron micaschist from the Eastern-Alps (Sopron area, Western Hungary)

Four fluid migration events were recorded during the Alpine metamorphism in the Sopron micaschist from the Grob gneiss series of the Lower Austroalpine Unit of the Eastern Alps near Sopron, using mineral chemistry data, geothermo-barometry and fluid inclusion studies.1. Tourmaline mineralisation in quartz veins and to some extent in the host rock. Similar mineral compositions in the quartz-tourmaline veins and in the host rock show equilibrium between fluid and the host rock. Geothermo-barometry gives 560-610oC temperature and 950-1230 MPa pressure for the formation of quartz-tourmaline veins which is the same as the determined P-T peak (T=560 and 600°C p= 840-1230 MPa).2. Fluids causing Mg-metasomatism in the shear zones. The result of this fluid invasion was the formation of leucophyllite in the shear zones and Mg-enrichment of some minerals (chlorite, muscovite, garnet) in the close vicinity of the shear zone. The effect of this fluid was confined to the shear zones and the neighbouring host rock.3. The rock was infiltrated along the shear zones and quartz veins with CO2-bearing hypersaline fluids during retrograde metamorphism. The presence of this fluid is evidenced by secondary CO2 inclusions and hypersaline aqueous fluid inclusions ± CO2. The aqueous fluid had high concentrations of Na, Ca, Fe, Al, Cl and contained moderate amounts of Mg, Zn, Ti, K, Mn, S and P. This fluid was the carrier of the REE and Th and locally precipitated florencite, monazite, allanite, apatite, thorite and thorianite in the shear zone. Traces of this mineralisation are found in quartz-tourmaline veins, postdating the tourmaline mineralisation.4. Late retrograde metamorphic fluid represented by two phase (liquid+vapor) aqueous inclusions of the NaCl-CaCl2-H2O system with total salinity between 25 and 28.5% and homogenisation temperatures between 229.6 and 322oC

Two-Stage Late Jurassic to Early Cretaceous Hydrothermal Activity in the Sakar Unit of Southeastern Bulgaria

Southeastern Bulgaria is composed of a variety of rocks from pre-Variscan (ca. 0.3 Ga) to pre-Alpine sensu lato (ca. 0.15 Ga) time. The Sakar Unit in this region comprises a series of granitoids and gneisses formed or metamorphosed during these events. It is cut by a series of post-Variscan hydrothermal veins, yet lacks pervasive Alpine deformation. It thus represents a key unit for detecting potential tectonism associated with the enigmatic Cimmerian Orogenic episode, but limited geochronology has been undertaken on this unit. Here we report age constraints on hydrothermal activity in the Sakar Pluton. The investigated veins contain mainly albite–actinolite–chlorite–apatite–titanite–quartz–tourmaline–epidote and accessory minerals. The most common accessory minerals are rutile and molybdenite. Apatite and titanite from the same vein were dated by U–Pb LA–ICP-MS geochronology. These dates are interpreted as crystallization ages and are 149 ± 7 Ma on apatite and 114 ± 1 Ma on titanite, respectively. These crystallization ages are the first to document two stages of hydrothermal activity during the late Jurassic to early Cretaceous, using U–Pb geochronology, and its association with the Cimmerian orogenesis. The Cimmerian tectono-thermal episode is well-documented further to the east in the Eastern Strandja Massif granitoids. However, these are the first documented ages from the western parts of the Strandja Massif, in the Sakar Unit. These ages also temporally overlap with previously published Ar–Ar and K–Ar cooling ages, and firmly establish that the Cimmerian orogeny in the studied area included both tectonic and hydrothermal activity. Such hydrothermal activity likely accounted for the intense albitization found in the Sakar Unit.

Geochemistry, geochronology and evolution of Paleoproterozoic granitoid gneisses in the Khondalite Belt, North China Craton

The studied granitoid gneisses from the Daqingshan complex in the Khondalite Belt of the North China Craton consist of tonalitic, trondhjemitic, and granodioritic (TTG) gneisses, as well as monzogranitic and granitic (MG) gneisses. The respective melts forming these granitoid gneisses are geochemically similar. The granitoid gneisses are generally enriched in large-ion lithophile elements (LILEs) such as Rb, Ba, Nd, La, Ce and depleted in high field strength elements (HFSEs) such as Nb, Ta, and Ti. Chondrite-normalized rare earth elements (REE) patterns indicate that both TTGs and MG gneisses illustrate various fractionation with (La/Yb)N ratios of 7.43–58.29 and 3.59–87.82, respectively. However, most TTG gneisses show positive Eu anomalies, and relatively negative Th and positive Sr anomalies, while MG gneisses have opposite characteristics. These geochemical characteristics suggest that TTG gneisses were derived from a magma source at higher pressures than that to have formed MG gneisses. Some TTG gneisses yield high Sr/Y (36–139) and (La/Yb)N ratios, indicating a subduction-related affinity. The whole series of granitoid gneisses from the Daqingshan complex belong to fractionated calc-alkaline I-type granitoids, suggesting their igneous protoliths akin to island arcs and continental magmatic arcs. LA-ICP-MS U–Pb dating of zircons from four representative granitoid samples indicates that tonalite and granodiorite exhibit weighted mean ages of 2416 ± 12 Ma and 2348 ± 10 Ma, respectively, whereas trondhjemite provides two groups of zircon ages yielding weighted mean ages of 2437 ± 15 Ma and 1876 ± 3 Ma, respectively. The studied granitic gneiss reveals an upper intercept of 1947 ± 45 Ma. Integration of cathodoluminescence images, Th/U ratio, and REE patterns of zircons indicates that all zircons of granitoid gneisses have a magmatic origin. Therefore, three major episodes of granitic magmatism at ca. 2.35–2.42, 1.95, and 1.88 Ga can be inferred. The ages of 1.95 and 1.88 Ga are coeval with widespread metamorphic events recorded by mafic and pelitic granulites in the Khondalite Belt. The Khondalite Belt could have undergone long-term arc-continent accretion along the southern margin of the Yinshan Block during early to late Paleoproterozoic time, and subsequent continental collision between the Yinshan and Ordos Blocks occurred at ca 1.95–1.88 Ga.

Alteration mapping on drill cores using a HySpex SWIR-320m hyperspectral camera: Application to the exploration of an unconformity-related uranium deposit (Saskatchewan, Canada)

In mineral exploration, the search for geophysical, geochemical, and mineralogical pathfinders is of critical importance, and many techniques have been used to detect the presence of mineralization and related host-rock alteration. In the case of unconformity-type uranium deposits that are spatially-linked to unconformities between sedimentary basin and underlying basement rocks, hydrothermal fluid-rock interactions produced extended alteration envelopes which are used to target mineralization and are therefore important guides for uranium exploration.The purpose of this study is to evaluate the utility of hyperspectral mapping of alteration minerals in drill core samples using the HySpex SWIR-320m hyperspectral camera that covers the 1300–2500nm (SWIR) spectral range. A series of representative samples, including pegmatite, pelitic gneiss, and sandstone, from the area around the Cigar Lake uranium deposit (Athabasca Basin, Saskatchewan, Canada) were analyzed by SWIR reflectance spectroscopy and optical microscopy. An algorithm was developed and implemented in the IDL language to determine both the positions and depth of diagnostic absorption bands of various minerals, and to calculate semi-quantitative estimates of mineral content. Subsequent petrographic analysis using optical microscopy validated the hyperspectral mapping methodology.Selected alteration minerals present in the core samples, such as clay minerals and carbonates, were mapped. The “pseudo-modal” mineralogical composition for each sample was determined. The mineral maps highlight the mineralogy and the main petrographic textures such as foliation, bedding, veins, and the geometry of pervasive alteration.This study offers new perspectives for the use of this method in mineralogical analysis of drill core samples and the characterization of hydrothermal alteration. The proposed mineralogical semi-quantification methodology may be used in other geological contexts and should improve the understanding of the mineralogical alteration distribution around hydrothermal metal deposits.

Zircon U–Pb–Hf isotopes and geochemistry of two contrasting Neoarchean charnockitic rock series in Eastern Hebei, North China Craton: Implications for petrogenesis and tectonic setting

The Zunhua–Qinglong microblock in Eastern Hebei, North China Craton, exposes two contrasting charnockitic rock series in the Yuhuzhai–Taipingzhai and Cuizhangzi areas: charnockitic plagioclase gneiss series with chemical compositions of dioritic and tonalitic rocks, and charnockite series showing tonalitic and granodioritic components. Petrographic observation reveals that two types of orthopyroxenes are preserved in these rocks. Medium-fine grained anhedral orthopyroxenes are preserved in both series, and coarse grained subhedral orthopyroxenes are preserved only in the charnockite series. In situ trace element analyses reveal that the coarse grained subhedral orthopyroxenes exhibit typical chemical features of magmatic orthopyroxene, whereas the medium-fine grained anhedral orthopyroxenes display lower medium to heavy rare earth elements (REE) and no obvious Eu anomalies, resulting in left-inclined REE patterns. Combined with petrographic features, these new data indicate that the medium-fine grained anhedral orthopyroxenes were formed by granulite–facies metamorphism caused by dehydration reactions, whereas the coarse grained subhedral orthopyroxenes were directly crystallized from their magmatic precursors. Laser ablation inductively coupled plasma mass spectrometry (LA–ICP–MS) zircon U–Pb isotopic dating reveals that the magmatic precursors of the charnockitic plagioclase gneiss series were emplaced between 2530±17 and 2523±12Ma, and those of the charnockite series were emplaced between 2527±28 and 2515±22Ma. We conclude that these magmatic precursors are contemporaneous emplacements of two Neoarchean granitoid magmatic events, which were followed by regional granulite–facies metamorphism at ∼2.45Ga.The charnockitic plagioclase gneiss series and charnockite series exhibit similar ɛHf(t) values of −2.4 to +5.3 and −3.2 to +5.5, indicating either depleted mantle or juvenile crust affinity, and involvement of ancient crustal components. The charnockitic plagioclase gneiss series exhibit relatively low SiO2 contents (54.1–65.2%), with various MgO contents (1.93–5.36%), Mg# (0.36–0.55), and (La/Yb)N values (3.49–56.00); the charnockite series exhibit higher SiO2 contents (60.1–69.2%), with similar (La/Yb)N values (14.61–42.69), MgO contents (2.10–3.99%), and Mg# (0.29–0.52). These geochemical and zircon Lu–Hf isotopic features, together with chemical modeling, suggest that the magmatic precursors of the charnockitic plagioclase gneiss series may have been primarily derived from the partial melting of a depleted mantle that was metasomatized by slab-derived melts/fluids, and experienced fractional crystallization processes with hornblende and plagioclase as primary fractionated phases. The aforementioned features further suggest that the charnockite series originated from a partial melting of the subducted slab, and it was strongly contaminated by mantle peridotites. These new data, combined with the previous studies of Neoarchean basement rocks in the Zunhua–Qinglong microblock in Eastern Hebei, suggest that the magmatic precursors of these two contrasting charnockitic rock series were formed in a back-arc-related tectonic environment at a convergent plate margin.

Geology of the Eoarchean, > 3.95 Ga, Nulliak supracrustal rocks in the Saglek Block, northern Labrador, Canada: The oldest geological evidence for plate tectonics

The Earth is a unique planet, which has been highly evolved, diversified and complicated through geologic time, and underwent many key events, including giant impact, magma ocean, core formation, large-scale mantle differentiation and late heavy bombardment, especially in its dawn. But, our knowledge of early Earth is limited due to the lack of the Hadean supracrustal rocks. The supracrustal rocks with the Eoarchean ages provide key evidence for the Earth's early evolution, but few supracrustal rocks have been comprehensively investigated. Therefore, we mapped in seven areas of the Saglek Block, northern Labrador, where ancient supracrustal sequences are interleaved with a diverse assemblage of orthogneisses. Early studies suggested that some of them have the Mesoarchean ages because of the lack of the Mesoarchean Saglek dyke, but we found the Saglek dykes in the areas to recognize the Eoarchean Nulliak supracrustal rocks and Uivak Gneiss in all the areas. Recent reassessment of U–Pb dating and cathodoluminescence observation of zircons from the oldest suites of the Uivak Gneiss showed that the Uivak Gneiss has the Eoarchean age, >3.95Ga, and forms the Iqaluk–Uivak Gneiss series. Because our geological survey clearly showed that the Iqaluk–Uivak Gneisses were intruded into the Nulliak supracrustal belts, the Nulliak supracrustal rocks are the oldest supracrustal rock in the world. The supracrustal belts consist of piles of fault-bounded blocks, which are composed of the ultramafic rocks, mafic rocks and sedimentary rocks in ascending order, similar to modern ocean plate stratigraphy (OPS). In addition, small-scale duplex structures are found over the areas. The presence of duplex structure and OPS indicates that the >3.95Ga Nulliak supracrustal belts originate from an accretionary complex. The presence of the accretionary complex, ophiolite and granitic continental crust provides the oldest evidence for the plate tectonics on the early Earth.

Early Paleoproterozoic (2.45–2.20 Ga) magmatic activity during the period of global magmatic shutdown: Implications for the crustal evolution of the southern North China Craton

A global database of zircon ages from both granitoids and detrital sediments shows an exceptional and robust age gap between 2.45 and 2.20Ga. The early Paleoproterozoic magmatism dramatically decreased on the Earth, which was proposed to relate to the global plate tectonic shutdown. However, the available data indicate that 2.45–2.20Ga magmatic rocks are widespread in the Taihua Complex during the quiet interval. The Taihua Complex is located in both the Lushan and Xiaoqinling areas along the southern segment of North China Craton (NCC). The Complex is composed mainly of a gneiss series and the Khondalite-dominated supracrustal rocks. There was an original unconformable relationship between the two rock associations. The latter occurs as a linear structural belt and was named the Khondalite Belt. The gneiss series in the Lushan area has an age of 2.85–2.72Ga, whereas the Xiaoqinling region contains widespread tonalitic–trondhjemitic–granodioritic gneisses with ages of 2.45–2.20Ga. Significant variations of Hf isotopes in zircons and whole-rock ɛNd(t) values suggest that these rocks were produced by variable mixing of a juvenile materials with older crust in an Andean-type continental margin arc or island arc setting. The depositional age of the sedimentary protoliths of the khondalite series can be roughly constrained in the period between 2300 and 1970Ma. It has been considered that they were deposited in a stable continental margin environment. The U–Pb dating of metamorphic zircons from the Taihua Complex suggest that the southern NCC underwent a polyphase tectonic evolution during the period 1.97–1.82Ga, and the peak metamorphism coeval with crustal thickening has occurred at ∼1.94Ga.

Ultrasonic velocity of the upper gneiss series rocks from the Outokumpu deep drill hole, Fennoscandian shield — Comparing uniaxial to triaxial loading

Abstract P- and S-wave propagation velocities in dry mica schist samples obtained from the Outokumpu deep drill hole in Eastern Finland were measured under uniaxial compression in a custom built measurement device. At 300 MPa pressure the obtained velocities (5520 ± 130) m/s and (3270 ± 60) m/s, for P- and S-waves, respectively, agreed with those measured under triaxial compression, (5530 ± 200) m/s and (3300 ± 90) m/s. The increase in Vp with uniaxial pressure was steeper, whereas this was not the case with Vs. These results indicate that ultrasound velocity measurements can be performed under uniaxial compression without the need to alter the cylindrical geometry of a borehole sample. Results obtained with the device can improve the interpretation of seismic crustal structure maps by providing seismic velocity data of the Outokumpu drill core as well as seismic impedances and reflection coefficients measured under crustal conditions.

Geochemistry and Genesis of the Sodium Charnockitic Gneisses, Eastern Hebei Province, China

Abstract High in sodium and low in potassium (Na2O/K2O > 1), the charnockitic gneiss series in the Santunying- Taipingzhai area, eastern Hebei province, consists of hypersthene- quartz- diorite, hypersthene-granodiorite and hypersthene-plagioclase-granite. Geological, petrological and large ion lithophile element (LILE), high field strength element (HFSE) and REE geochemical studies suggest that the medium-coarse-grained hypersthene-granodiorite is the product of crystallization of anatectic magmas of the same composition. Under granulite facies conditions, the equilibrium crystallization differentiation of the magmas yielded the early crystallization phase—high- SiO2, LILE-depleted, low-ΣREE, positive Eu anomaly and REE- saturated hypersthene- plagioclase- granite. The residual phase, coarse- grained to pegmatitic hypersthene-granodiorite, is marked by low SiO2, LILE-enrichment, high ΣREE and REE-undersaturation. These rocks and hypersthene-quartz-diorite enclaves constitute the sodium-charnockitic gneiss series in eastern Hebei province. Model calculation for trace elements in the granitoids was applied. On the basis of a systematic geological study, the equation for calculation was chosen, the source magma was determined and the partition coefficients were obtained. The resulting curves are entirely consistent with those observed in the patterns of actual rocks. The study indicates that whole-rock REE patterns can not be used directly in the comparison of the sources and genesis of granitoids.

Elastic wave velocities, chemistry and modal mineralogy of crustal rocks sampled by the Outokumpu scientific drill hole: Evidence from lab measurements and modeling

The Outokumpu scientific deep drill hole intersects a 2500 m deep Precambrian crustal section comprising a 1300 m thick biotite-gneiss series (mica schists) at top, followed by a 200 m thick meta-ophiolite sequence, underlain again by biotite gneisses (mica schists) (500 m thick) with intercalations of amphibolite and meta-pegmatoids (pegmatitic granite). From 2000 m downward the dominating rock types are meta-pegmatoids (pegmatitic granite). Average isotropic intrinsic P- and S-wave velocities and densities of rocks were calculated on the basis of the volume fraction of the constituent minerals and their single crystal properties for 29 core samples covering the depth range 198–2491 m. The modal composition of the rocks is obtained from bulk rock (XRF) and mineral chemistry (microprobe), using least squares fitting. Laboratory seismic measurements on 13 selected samples representing the main lithologies revealed strong anisotropy of P- and S-wave velocities and shear wave splitting. Seismic anisotropy is strongly related to foliation and is, in particular, an important property of the biotite gneisses, which dominate the upper and lower gneiss series. At in situ conditions, velocity anisotropy is largely caused by oriented microcracks, which are not completely closed at the pressures corresponding to the relatively shallow depth drilled by the borehole, in addition to crystallographic preferred orientation (CPO) of the phyllosilicates. The contribution of CPO to bulk anisotropy is confirmed by 3D velocity calculations based on neutron diffraction texture measurements. For vertical incidence of the wave train, the in situ velocities derived from the lab measurements are significantly lower than the measured and calculated intrinsic velocities. The experimental results give evidence that the strong reflective nature of the ophiolite-derived rock assemblages is largely affected by oriented microcracks and preferred crystallographic orientation of major minerals, in addition to the lithologic control.

NEW EVIDENCES OF THE LOW-P/HIGH-T PRE-ALPINE METAMORPHISM AND MEDIUM-P ALPINE OVERPRINT OF THE PELAGONIAN ZONE DOCUMENTED IN METAPELITES AND ORTHOGNEISSES FROM THE VORAS MASSIF, MACEDONIA, NORTHERN GREECE

Pelitic rocks of the pre-Alpine "gneiss series" from the Voras Massif record a polymetamorphic history of three metamorphic events; a first HT event indicated by migmatitization phenomena, a subsequent LP-HT event (andalusite-sillimanite series) at 2.5-3 Kbar and 610-640°C, and a third medium-P event at ~11 Kbar and ~550°C. The LP-HT metamorphism is associated with intrusion of Variscan granitoid plutois in a magmatic arc setting. The medium-P metamorphism records an Alpine event (probably Early Cretaceous) indicated by kyanite, chlohtoid, garnet and staurolite formation replacing andalusite and cordierite. The Variscan granitoids and the overlying lithologies of the "schist series" are affected only by the medium-P metamorphism. Granitoids are transformed into phengite orthogneisses having the mineral assemblage phen (Si=3.35-3.47 a.p.f.u.)±Bt-Ab-Kfs- Qtz±Czo±Grt and high-alumina pelites into gamet-chloritoid schists with the mineral assemblage Grt-Ctd-Chl-Ms-Qtz-Rt±llm. The "schist series" which underlies the Late Triassic-Early Jurassic marbles and overlies lithologies of the "gneiss series" and metagranitoids represents protoliths of the Permo-Triassic volcanosedimentary series deposited at the eastern margin of the Pelagonian continental block

<b>Earth Sciences</b><br><br>The geochemistry, tectonic setting and origin of the massive melanocratic amphibolite in the Ilesha schist belt, Southwestern Nigeria.

A massive melanocratic amphibolite, (MMA) occurs in Ilesha schist belt within a series of muscovite schists and amphibolite gneiss. Though metamorphosed, MMA shows no obvious textural deformation. Actinolite, tremolite, hornblende and boitite constitute the major minerals in MMA. Minor minerals in MMA include calcite, plagioclase and pyroxene while monazite, zircon and apatite form the accessory minerals. Chemical studies revealed that MMA contains low K2O and Na2O. Its Mg1, Cr and Ni contents are considerably lower than those of similar basalts derived from purely primitive mantle. Enrichment of LREE, negative Eu/Eu* anomaly and occurrence of monazite in its mineralogy are all indications that the precursor magma of MMA contains a sedimentary input. The plots of immobile trace (Zr, Ti, Nb, Y) and rare earth elements during the greenschist-amphibolite metamorphic grade for MMA show that it was derived from a low-K-Tholeiitic magma in a volcanic geotectonic setting (back arc basin). Global Journal of Pure and Applied Sciences Volume , No 1 January (2001) pp. 85-90

Grain reduction and growth in faulting and new classification of faulted rocks

New classification scheme about faulted rocks is proposed, according to the extent of grain reduction and growth and their sequence, and faulted rocks are classified as follows: (1) faulted rocks formed by the reduction action mainly include breccia series, cataclasite series, tectonobutchite series, mylonite series; (2) faulted rocks formed by growth action is mainly tectono-schist (gneiss) series; (3) blastomylonite series formed by grain reduction first and then growth; (4) mylonitic schist (gneiss) series formed by crystal growth first and then grain reduction. All series can be further classified according to matrix contents.

Redistribution of rare earth elements, thorium, and uranium over accessory minerals in the course of amphibolite to granulite facies metamorphism: The role of apatite and monazite in orthogneisses from southwestern Norway

The amphibolite to granulite facies transition has been studied in a high-K calc-alkaline, hornblende-biotite, K-feldspar megacrystic augen gneiss series from the Rogaland-Vest-Agder (Rog-VA) sector of the Sveconorwegian province (southwest Norway). Hornblende begins to break down mainly to clinopyroxene at the Cpx-in isograd and biotite mainly to orthopyroxene at the Opx-in isograd (abbreviations for minerals following Kretz, 1983). The magmatic accessory mineral association comprises titanite and allanite, which begin to break down before the Cpx-in isograd. Titanite is preserved as relict inclusions in other minerals at higher grade. Monazite and thorite formed in the breakdown of allanite. Monazite abundance reaches a maximum between the Cpx-in and Opx-in isograds. The middle and heavy rare earth element (M-HREEs, except Eu) content of apatite steeply increases with increasing metamorphic grade and is directly correlated to the decrease of the modal abundances of titanite, hornblende, and biotite. The U contents of apatite are low and do not increase with metamorphic grade. The light rare earth elements (LREEs) and Th content of apatite are not correlated to the breakdown of allanite around the Cpx-in isograd but increase around the Opx-in isograd. A simplified equation is proposed for monazite crystallisation in the vicinity of the Cpx-in isograd that accounts for the allanite, titanite, and hornblende breakdowns and the M-HREE substitution in apatite: 3 (M-HREE) 2 O 3 in hornblende and litanite + 3 ( LREE) 2 in allanite O 3 + 2 Ca 5 (PO 4 ) 3 apatite ( F,OH ) + 6 SiO 2 quartz ⇔ 6 ( LREE)P monazite O 4 + 2 Ca 2 (M-HREE) 3 (SiO 4 ) 3 ( F,OH) lessingite in apatite + 6 CaO in plagioclase . At the Opx-in isograd, the increase of LREEs and Th content of apatite results either from the breakdown of some monazite: 3 ( LREE)PO 4 monazite + 3 SiO 2 quartz + 4 CaO in plagioclase + (F 2 ,H 2 O) fluid ⇔ Ca 5 (PO 4 ) 3 (F,OH) + apatite Ca 2 (LREE) 3 (SiO 4 ) 3 (F,OH) less ingite in apatite . or the breakdown of the remaining allanite: 3 ( LREE) 2 O 3 in allanite + 6 SiO 2 quartz + 4 CaO in plagioclase + (F 2 ,H 2 O) fluid ⇔ 2 Ca 2 (LREE) 3 (SiO 4 ) 3 (F,OH) lessingite in apatite . The release of fluorine from the breakdown of biotite at the Opx-in isograd may increase apatite stability relatively to monazite in the granulite facies. when compared to amphibolite facies, the granulite facies augen gneisses do not show any U or Th depletion. This means that changes in the accessory mineral associations and the simultaneous breakdown of hydrous minerals at the amphibolite-granulite facies transition do not inevitably result in depleted granulite facies rocks but rather to an isochemical element redistribution. The coexistence of small amounts of metamorphic monazite and of relict inclusions of titanite in upper amphibolite facies augen gneisses suggests that (high-K) calc-alkaline orthogneisses are a suitable material to date (with the U-Pb method) prograde path of amphibolite facies regional metamorphism on monazite and the cooling path on titanite.

K-Rich Calc-Alkaline Augen Gneisses of Grenvillian Age in SW Norway: Mingling of Mantle-Derived and Crustal Components

Augen gneisses were emplaced as phenocryst-bearing granodiorites at to the west of the Sveconorwegian belt (Rogaland-Vest-Agder, SW Norway) just before the main Sveconorwegian (Grenvillian) metamorphic event. They define a typical K-rich calc-alkaline trend. Initial Sr and Nd isotopic ratios are close to Bulk Earth values at 1040 Ma: ( and average ). Average initial Pb isotopic values of 11 K-feldspar phenocrysts are and . The augen gneisses contain ultrapotassic gabbroic inclusions of calc-alkaline lamprophyric composition. The geochemical evolution of the augen gneiss series can be accounted for by fractional crystallization, without contamination by surrounding banded gneisses, of a magma resulting from the mixing between ultrapotassic, mantle-derived mafic magma (20-25%) with a granodioritic magma generated in the lower crust. Accessory minerals (apatite, zircon, titanite, allanite) control the behavior of most trace elements. In Rogaland-Vest-Agder, the Sveconorwegian granitoids (1040 to 930 Ma) show a systematic geochemical evolution with time. They display an increase of , of ratio (2.2 to 18) and Zr content (360 to 900 ppm). This evolution corresponds to an increase in the partial melting temperature together with an increase of the proportion of upper crustal component. The calc-alkaline affinity of the augen gneisses implies that a subduction-related geodynamic regime prevailed just before the main Sveconorwegian event. The Sveconorwegian cycle ends by the emplacement of A-type charnockitic granites.