Garnet Biotite Gneiss Research Articles

Vein Mineralogy and Hydrothermal Gold Mineralization at the Kyaw Soe Thu Prospect in the Wabo Deposit, Mandalay Region, Central Myanmar

The Kyaw Soe Thu Prospect is a part of the Wabo deposit which is situated along Mogok Metamorphic Belt (MMB) in Central Myanmar. Gold bearing veins are hosted in the banded garnet-biotite gneiss, calc-silicate, marble and granitoid of the Wabo area. The granitic rocks intruded into banded garnet-biotite gneiss. The MMB was intensely deformed and it is a highly fractured metamorphic terrain in which metamorphic facies ranges from amphibolite to granulite facies. Gold mineralization at the Kyaw Soe Thu Prospect in the Wabo deposit occurred in the forms of gold-quartz veins. The quartz veins are a few centimeters in width. The color of the quartz vein at the Kyaw Soe Thu Prospect is commonly smoky and milky. The quartz vein exhibits chalcedonic, saccharoidal and granular textures. Three mineralizations were indicated. Stage I is the thin layered crustiform chalcedonic quartz. Stage II is chalcedonic quartz vein and stage III is Ankerite-quartz-sericite vein. Stage II and stage III are gold rich hydrothermal stages, and Ankerite-quartz-sericite veins were formed with a small amount of sericites in the Kyaw Soe Thu Prospect. The Ankerite-quartz-sericite veinlets range from a centimeter up to three centimeter in width within the stage II chalcedonic quartz vein. Textures of mineralized quartz vein samples were petrographically examined. The textures include a feathery texture that is most closely associated with electrum in both stage II chalcedonic quartz veins and stage III Ankerite-quartz-sericite veins. Ankerite varies in color from pink, white to buff. Among them, pink color is more common in the Kyaw Soe Thu Prospect. Ankerite in the Kyaw Soe Thu Prospect exhibits rhombohedral idiomorphic, which exhibits growth patterns. Ore minerals of the veins are electrum, pyrite, galena, and trace amount of very fine-grained chalcopyrite. On the basis of textures of the chalcedonic quartz veins, the Kyaw Soe Thu Prospect is ascribed to be a low-sulfidation epithermal deposit.

Geochemical Features and Geological Processes Timescale of the Achaean TTG Complexes of the Ingozero Massif and the Pechenga Frame (NE Baltic Shield)

This article provides a geological review and results of the structural, metamorphic, and geochronological studies of the Pechenga frame outcrops located in the NW part of the Central-Kola terrain and the Ingozero massif outcrops situated in the northeastern part of the Belomorian mobile belt of the Kola Region (NW Baltic Shield). As a result of the work, the deformation scales and ages of the geological processes at the Neo-Archaean–Paleoproterozoic stage of the area’s development were compiled, and the reference rocks were dated. The petrochemical and geochemical characteristics of the Ingozero rocks are similar to those of tonalite–trondhjemite–granodiorite (TTG) complexes established on other Archaean shields. The isotope U–Pb dating of individual zircon grains from the biotite gneisses provided the oldest age for magmatic protolith of the Ingozero gneisses, which is 3149 ± 46 Ma. Sm–Nd model ages showed that the gneisses protolite initial melt formed at 3.1–2.8 Ga. Ages of metamorphic processes were determined by using isotope U–Pb dating ID TIMS (isotope dilution thermal ionization mass spectrometry): Biotite gneisses—2697 ± 9 Ma; amphibole–biotite gneisses—2725 ± 2 Ma and 2667 ± 7 Ma; and biotite–amphibole gneisses 2727 ± 5 Ma. Ages of granitoids, which cut the deformed gneisses, are 2615 ± 8 Ma and 2549 ± 31 Ma for plagiogranites and pegmatoid veins in gneisses, respectively. The following age sequence of geological processes was established by using U–Pb zircon dating: 2.8 Ga—The time of the garnet–biotite gneiss metamorphism; 2722 ± 9 Ma—The granodiorite crystallization time; 2636 ± 41 Ma—The aplite emplacement age and 2620 ± 16 Ma—The age of pegmatites origin, which marked final stages of the Archaean evolution; 2587 ± 5 Ma—The age of gabbros emplacement and 2507 ± 7 Ma—The age of gabbros metamorphism; 2522–2503 Ma—The origin time of the iron quartzite interpreted as the age of gabbros and biotite gneiss metamorphism.

Recognition and significance of c. 800 Ma upper amphibolite to granulite facies metamorphism in metasedimentary rocks from the NW margin of the Yangtze Block

The evolutionary history of the NW Yangtze Block is important in interpreting its location and relationship with the Rodinia supercontinent. Although a Neoproterozoic arc-related tectonothermal event is recognized in the Micangshan area of the NW Yangtze Block, its timing and P–T conditions are poorly constrained. We address this issue by focusing on the garnet–biotite gneiss that represents the main lithology of the Huodiya Group on the NW margin of the Yangtze Block. This gneiss is predominantly composed of garnet, biotite, feldspar and quartz. The peak mineral assemblage in the matrix consists of garnet + biotite + quartz + plagioclase + K-feldspar. The retrograde stage is characterized by embayed rims of the garnet and its associated biotite in the matrix. The P–T conditions in the peak and retrograde stages are constrained to 7–8 kbar and c. 710°C and 5–6 kbar and 650–675°C, respectively, and suggest that 4–5 vol.% melt was produced during an upper amphibolite–granulite facies metamorphic event. The first report of monazite U–Pb dating in the Huodiya Group of the Micangshan area yielded a weighted mean 206 Pb/ 238 U age of 802 ± 5 Ma. The studied samples contained detrital igneous zircons with 206 Pb/ 238 U dates >800 Ma, whereas the metamorphic zircons yielded a weighted mean 206 Pb/ 238 U age of 797 ± 9 Ma. Based on the compressional tectonic environment and previous studies, we propose that the Yangtze Block was probably located on the periphery of the Rodinia supercontinent. Supplementary material: The analytical methods are summarized in Supplementary Text File 1. Representative mineral composition and the results of U–Pb isotopic analyses of monazites and zircons are presented in supplementary table 1–7 . The results of trace element concentrations in garnets are presented in Supplementary table 8 . https://doi.org/10.6084/m9.figshare.c.4698356

Timing of two separate granulite-facies metamorphic events in the Helanshan complex, North China Craton: Constraints from monazite and zircon U–Pb dating of pelitic granulites

This study presents the results of monazite and zircon U–Pb dating of garnet–biotite gneiss and recently discovered spinel-bearing ultrahigh-temperature (UHT) pelitic granulites from the Paleoproterozoic Helanshan complex of the North China Craton, and considers implications for the formation of the complex and the tectonic history of this region. SHRIMP monazite U–Pb dating of the garnet–biotite gneiss yielded a weighted mean 207Pb/206Pb age of 1944.4 ± 4.2 Ma, which is the same within error as a weighted mean laser ablation–inductively coupled plasma–mass spectrometry (LA–ICP–MS) 207Pb/206Pb age of 1959 ± 18 Ma obtained from metamorphic zircons within the same unit. The age of 1959 ± 18 Ma is similar to the published metamorphic ages of ca. 1.96–1.95 Ga from high-temperature (Tmax < 900 °C) pelitic granulites in the Helanshan, which represent the timing of continental collision between the Yinshan and Ordos terranes as suggested by previous published works. Metamorphic monazites from the spinel-bearing UHT pelitic granulite unit in this area yielded weighted mean 207Pb/206Pb ages of 1930.8 ± 2.6 and 1933.6 ± 3.1 Ma, which are consistent with the ages of metamorphic zircons. These ages record the timing of UHT metamorphism within the Helanshan complex and are similar to 1.93–1.92 Ga ages previously reported from typical UHT pelitic granulites within the Jining and Daqingshan complexes of the Khondalite Belt, where ca. 1.92 Ga metamorphic zircons are thought to record the cooling of the UHT rocks to the solidus. The new data presented here indicate that the ca. 1.93 Ga UHT metamorphic event occurred across a wider area than previously thought and extended into the Helanshan complex. Combining these new data with the results of previous research suggests that the entire Khondalite Belt was influenced by a ca. 1.93 Ga UHT metamorphic event that was preceded by metamorphism associated with continental collision between the Yinshan and Ordos terranes at ca. 1.95 Ga. This suggests that the Khondalite Belt underwent a tectonic transition from compression to extension between 1.95 and 1.93 Ga, a process likely controlled by a late-stage shallow slab-breakoff event.

Timing of late Neoarchean to late Paleoproterozoic events in the North China Craton: SHRIMP U–Pb dating and LA-ICP-MS Hf isotope analysis of zircons from magmatic and metamorphic rocks in the Santunying area, eastern Hebei

Santunying is an important area for revealing nature of the late Neoarchean tectono-magmato-thermal events in the eastern Hebei part of the North China Craton. It is mainly composed of meta-intrusive rocks. Supracrustal rocks sporadically occur in the meta-intrusive rocks. The meta-intrusive rocks are subdivided into the Santunying tonalitic gneiss, Qiuhuayu tonalitic-trondhjemitic gneiss, Xiaoguanzhuang dioritic gneiss and Qingyangshu meta-gabbro. Respectively, SHRIMP U–Pb zircon dating on fourteen samples yielded weighted mean 207Pb/206Pb ages of 2525–2537, 2532–2546, 2530–2544 and ∼2531 Ma for magmatic zircons from them. Dioritic gneiss of the Xiaoguanzhuang gneiss contain abundant 2544–3487 Ma xenocrystic zircons. SHRIMP U–Pb dating on a garnet-biotite gneiss sample yielded a weighted mean 207Pb/206Pb age of 2537 Ma for detrital zircons. All rocks underwent strong metamorphism, deformation and anatexis, resulting in formation of leucosomes and residues, with some leucosomes concentrating to form large veins. They record a strong late Neoarchean event by metamorphic zircon ages of 2489–2519 Ma. Some rocks also record metamorphic zircon ages of 1772–1843 Ma. Magmatic zircons from the magmatic rocks show large variations in εHf(t) values ranging from −1.7 to +8.7. Combined with early studies, conclusions are: 1) Intrusive rocks with the involvement of mantle-derived materials have a narrow range of magmatic zircon ages from 2525 to 2546 Ma, and supracrustal rocks were formed during the same period. 2) Ancient crustal remnants (>2600 Ma) are present, consistent with the late Neoarchean arc magmatism involving older continental crust, similar to Phanerozoic Andean margins. 3) The Archean basement underwent a strong tectonothermal event at the end of the Neoarchean, with the metamorphic zircon ages being 10–30 million years younger than the timing of magmatism, a common feature of the North China Craton. 4) A late Paleoproterozoic tectonothermal event widely occurred in the western part of eastern Hebei, which is linked with regional ductile deformation.

Metamorphic P–T paths for the Archean Caozhuang supracrustal sequence, eastern Hebei Province, North China Craton: Implications for a sagduction regime

Archean supracrustal rocks of amphibolite-facies occur as enclaves within granitoids gneiss domes and belts between domes, representing collision or sagduction regimes. In order to distinguish between tectonic regimes using metamorphic patterns, systematic data on metamorphic evolution and zircon age dating are presented for the Caozhuang supracrustal sequence of eastern Hebei Province, China, which occurs as enclaves in gneisses. A garnet biotite gneiss records a P–T path involving pre-peak isobaric heating to peak conditions at 780–800 °C and 10–11 kbar (medium-P/T type), followed by decompression to 5–6 kbar. Two Mg/Al-rich schists from the same locality show low P/T conditions of 6–7 kbar/690–750 °C and 4–7 kbar/750–780 °C, similar to that of the decompression stage for the biotite gneiss. Zircon age dating suggests that the amphibolite-facies metamorphism occurred at ∼2.49 Ga, coeval with TTG magmatic activities. Combined with a summary of the metamorphism from Kaapvaal and Pilbara Cratons, we concluded that Archean amphibolite-facies supracrustal rocks occurring as enclaves and belts show different metamorphic patterns. The former exhibits the metamorphic patterns the same as those recovered for the Caozhuang rocks, characteristic of pre-peak isobaric heating to 600–800 °C at 10–12 kbar followed by isothermal decompression with the overprinting of medium- and/or low-P/T assemblages in different enclaves. The latter shows medium- or high-P/T peak conditions mostly lower than 600–650 °C followed by decompression, lacking the pre-peak heating. These metamorphic patterns could be good indicators of a sagduction regime.

Monazite as a monitor for melt‐rock interaction during cooling and exhumation

AbstractGranulite facies cordierite–garnet–biotite gneisses from the southeastern Reynolds Range, central Australia, contain both orthopyroxene‐bearing and orthopyroxene‐free quartzofeldspathic leucosomes. Mineral reaction microstructures at the interface of gneiss and leucosome observed in outcrop and petrographically, reflect melt‐rock interaction during crystallization. Accessory monazite, susceptible to fluid alteration, dissolution and recrystallization at high temperature, is tested for its applicability to constrain the chemical and P–T–time evolution of melt‐rock reactions during crystallization upon cooling. Bulk rock geochemistry and phase equilibria modelling constrain peak pressure and temperature conditions to 6.5–7.5 kbar and ~850°C, and U–Pb geochronology constrains the timing of monazite crystallization to 1.55 Ga, coeval with the Chewings Orogeny. Modelling predicts the presence of up to 15 vol.% melt at peak metamorphic conditions. Upon cooling below 800°C, melt extraction and in situ crystallization of melt decrease the melt volume to less than 7%, at which time it becomes entrapped and melt pockets induce replacement reactions in the adjacent host rock. Replacement reactions of garnet, orthopyroxene and K‐feldspar liberate Y, REE, Eu and U in addition to Mg, Fe, Al, Si and K.We demonstrate that distinguishing between monazite varieties solely on the basis of U–Pb ages cannot solve the chronological order of events in this study, nor does it tie monazite to the evolution of melt or stability of rock‐forming minerals. Rather, we argue that analyses of various internal monazite textures, their composition and overprinting relations allow us to identify the chronology of events following the metamorphic peak. We infer that retrograde reactions involving garnet, orthopyroxene and K‐feldspar can be attributed to melt‐rock interaction subsequent to partial melting, which is reflected in the development of compositionally distinct monazite textural domains. Internal monazite textures and their composition are consistent with dissolution and precipitation reactions induced by a high‐T melt. Monazite rims enriched in Y, HREE, Eu and U indicate an increased availability of these elements, consistent with the breakdown of orthopyroxene, garnet and K‐feldspar observed petrographically. Our study indicates that compositional and textural analysis of monazite in relation to major rock‐forming minerals can be used to infer the post‐peak chemical evolution of partial melts during high‐ to ultrahigh‐temperature metamorphism.

Origin and thermometry of graphites from Itapecerica supracrustal succession of the southern Sao Francisco Craton by C isotopes, X-ray diffraction, and Raman spectroscopy

ABSTRACTIn the mines of the Nacional de Grafite Company around Itapecerica (Minas Gerais), located in the southern Sao Francisco Craton, occurs a supracrustal succession of high-grade metamorphic rocks including quartzite, garnet-biotite gneiss, and graphite schist formed in the Palaeoproterozoic (2.0 Ga). During metamorphic processes, organic matter was progressively transformed into graphite. From four graphite samples of three different mines (two samples from high-grade metamorphic graphite schist and two generated by hydrothermal recrystallization of the graphite schist), the origin and formation temperature of this mineral was obtained by C isotopes, X-ray diffraction (XRD), and Raman spectroscopy. The values of δ13C range between −21.23 and −27.89 ‰, indicating that the source of the graphite was a primitive biogenic carbon material. High-grade metamorphic graphites show average temperatures around 729°C, while hydrothermal recrystallizated graphites (vein-graphites) show temperatures around 611°C by XRD, which correspond to granulite- to amphibolite facies conditions. The hydrothermal process with percolation of C-O-H fluids leads to a decrease in the crystal size along stacking direction (Lc(002)) when compared with the previously formed high-grade metamorphism graphites. An update of the current tectonic model about the collisional process during Rhyacian-Orosirian orogeny in the Sao Francisco Craton is proposed to insert the formation of the Itapecerica graphite-rich metasedimentary sequence.

Geochronological characterization of a transition zone between the Mozambique Belt and Unango-Marrupa Complex in SE Tanzania

We present geochronological and petrological data of orthogneisses, granitoids, and supracrustal rocks from SE Tanzania to characterize the transition zone between the Mesoproterozoic Unango-Marrupa Complex and the Neoproterozoic Mozambique Belt.Geochronological data (U-Pb zircon; LA-ICP-MS) indicate that the emplacement of orthogneisses (protolith) and granitoids in SE Tanzania occurred in Stenian, Cryogenian, and Ediacaran Periods. The magmatic emplacement ages are determined between 1056 ± 19 Ma and 554 ± 3.9 Ma. No older ages that that were determined.The Stenian orthogneisses (biotite granitic gneiss dates at 1056 ± 19 Ma and the associated migmatites, charnockites, amphibolite) represents Mesoproterozoic magmatic inliers in the Neoproterozoic rocks and the extension of the Unango-Marrupa Complex in SE Tanzania.Supracrustal rocks (marble and calc-silicate, garnet-biotite (hornblende) gneisses, quartzo-feldspathic gneisses, graphite schists, and quartzites) are interlayered with granitoids dated between 790 ± 8.7 Ma and 614 ± 8.1 Ma. The supracrustal rocks represent the upper part of the Mozambique Belt (Eastern Granulites/Cabo-Delgado nappes), and are possibly relics of continental shelf sedimentary deposits of the Mozambique Ocean.Upon assessing metamorphic ages (U-Pb zircon) in SE Tanzania we determined the Ediacaran metamorphic event dated between 606 ± 6.6 and 560 ± 37 Ma, which reworked the Stenian – Cryogenian crusts. The U-Pb zircon metamorphic ages are supported by a precise U-Th-Pb monazite metamorphic age at 589 ± 5 Ma obtained from a biotite garnet gneiss rock sample (amphibolite facies). The Ediacaran metamorphism in SE Tanzania was followed by the emplacement of biotite alkali granite at 554 ± 3.9 Ma, which probably marks crustal relaxation and extension after metamorphism as the result of collision or accretion.Magmatic rocks of the Mozambique Belt in SE Tanzania were emplaced during the East African Orogeny. Metamorphic reworking occurred during the final amalgamation of Gondwana between 600 and 500 Ma (Kuunga Orogenic Cycle) in line with tectono-magmatic history of the neighbouring Unango-Marrupa Complex, the Ubendian Belt, and the Irumide-Southern Irumide Belt.

On the so-called “leukogranulite formation” and “greenlevada suite” of Early Precambrian of the Ukrainian Shield

It has been shown using as an example the geological structure of the Precambrian basement of the Middle Bug area (the Ukrainian Shield) that the chronostratigraphic scheme of the Lower Archean needs to be modified, in particular, greenlevada suite of the Dniester-Bug series represented by supercrustal leucogranulite rock formation sho-uld be included. It has been found that leucogranulite formation of the Bug area are rocks (granitized biotite and garnet-biotite gneisses) which belong to formation-gene-tic type of quartz-feldspar metasomatites, particularly, to structural-formational association of metasomatites from regional fault zones. According to the data of geological, geochemical, tectonophysical, radioisotopic studies of the similar rocks in the exposures of the middle stream of the river South Bug, it has been proved that they were formed around 2.03 Ga ago as a result of diaftoresis of rocks of the Archean basement and their granulitic appearance as well as subvertical occurrence are related to dyna-mometamorphic including shear processes. Methodic problems of the studies of the age relations of the Catarchean rock complexes are under discussion. The well-known geological cross-section of Khoshchuvato-Zavalye site of the Middle Bug area has been considered and it has been shown that the stratigraphic method based on the principle of sequence of stratifying of supercrustal rocks is impossible to be effectively used in such complexes because of their strong tectonic and intrusive-metasomatic reprocessing, practical impossibility of their initial bedding to be found. The best way under these conditions is to be supported by more objective petrostructural, microstruc-tural, petrochemical, including radioisotopic and geophysical data.

Mineralogy, geochemistry, and zircon U-Pb-Hf isotopes of the Paleoproterozoic granulite-facies metamorphic rocks from the Aketashitage area, southeastern Tarim Craton

The key to understanding the generation of mantle-derived magma and the formation of new continental crust is the recognition of the granulite-facies metamorphism, associated partial melting, and coeval magmatism that are documented in active continental margins. We firstly present a combined analysis of petrography, mineral chemistry, zircon U-Pb ages and hafnium (Hf) isotopes, in addition to whole-rock geochemistry on the garnet-biotite gneisses (Grt-Bi gneisses) and mafic granulites from the Aketashitage area, southeastern Tarim Craton. The implications of this analysis are discussed with respect to the thermal and tectonic evolution of the Tarim Craton. The Grt-Bi gneisses and mafic granulites are strongly peraluminous in composition and have flat REE patterns with negative Eu anomalies that are identical to those from the middle crust but higher than lower continental crust. Both the Grt-Bi gneisses and the mafic granulites have negative Nb-Ta anomalies, slightly positive Zr-Hf anomalies, and pronounced positive spikes in U and Pb. The characteristics in whole-rock geochemistry and mineral compositions indicate that the Grt-Bi gneisses and mafic granulites have the same source locality, which is most likely the garnet-bearing middle crust. The retrograde metamorphic P-T estimates for the Grt-Bi gneisses and mafic granulites are T = 574 °C; P = 7.0 kbar and T = 715 °C; P = 7.0 kbar, respectively. SIMS zircon U-Pb dating results reveal a large spread in metamorphic ages (2036–1962 Ma) for these granulite-facies rocks, interpreted to be the consequence of a large amount of magmas being added to the crust in an uninterrupted manner. Zircon Hf isotopes are characterized by variable but mostly negative εHf (t) values (−18.2 to −0.6), reasonably uniform Hf isotopic compositions, and highly variable TDMC model ages between 2628 and 3599 Ma, representing a long-lived reworking of the preexisting crust. Our data, along with available geological evidences, lead us to propose a model for the evolution of the Dunhuang Block in the Paleoproterozoic. That is, the southeastern margin of the Tarim Craton, the Dunhuang Block was possibly an active continental margin during the Paleoproterozoic. The steep subduction of oceanic slab resulted in the thickening of the continental crust, which in turn caused the crust anatexis. The increase in asthenosphere high heat flow and the intrusion of mantle-derived magma induced extensive heating, which preceded the granulite-facies metamorphism and coeval magmatism in the middle crust of the southern Tarim Craton.

Monazite U–Th–Pb geochronology of the Central Metasedimentary Belt Boundary Zone (CMBbz), Grenville Province, Ontario Canada

The Central Metasedimentary Belt boundary zone (CMBbz) is a crustal-scale shear zone that juxtaposes the Central Gneiss Belt and the Central Metasedimentary Belt of the Grenville Province. Geochronological work on the timing of deformation and metamorphism in the CMBbz is ambiguous, and the questions that motivate our study are: how many episodes of shear zone activity did the CMBbz experience, and what is the tectonic significance of each episode? We present electron microprobe data from monazite (the U–Th–Pb chemical method) to directly date deformation and metamorphism recorded in five garnet–biotite gneiss samples collected from three localities of the CMBbz of Ontario (West Guilford, Fishtail Lake, and Killaloe). All three localities yield youngest monazite dates ca. 1045 Ma; most of the monazite domains that yield these dates are high-Y rims. In comparison with this common late Ottawan history, the earlier history of the three CMBbz localities is less clearly shared. The West Guilford samples have monazite grain cores that show older high-Y domains and younger low-Y domains; these cores yield a prograde early Ottawan (1100–1075 Ma) history. The Killaloe samples yield a well-defined prograde, pre- to early Shawinigan history (i.e., 1220–1160 Ma) in addition to some evidence for a second early Ottawan event. In other words, the answers to our research questions are: three events; a Shawinigan event possibly associated with crustal thickening, an Ottawan event possibly associated with another round of crustal thickening, and a late Ottawan event that resists simple interpretation in terms of metamorphic history but that coincides chronologically with crustal thinning at the base of an orogenic lid.

Three-dimensional Earth’s crust density model of the central part of the Golovanevsk suture zone of the Ukrainian Shield

Three-dimensional Earth’s crust density model of the central part of the Golovanevsk suture zone of the Ukrainian Shield at the scale 1:50 000 has been given. Model calculations were carried out using automated complex GMT-Auto. The model takes into account the change in density both laterally in accordance with petrophysical data; and with depth in accordance with seismometry data. In this case; an abrupt change in the density in one direction or another; of the inversion zone and zone of constant density was taken into consideration. The density of rated bodies on the surface of the basement was defined based on the percentage ratio of the constituent rocks; in the depth — from the dependence of density on velocity. The block structure of the Earth’s crust upper part; which was previously revealed by seismometry in some areas of the zone; was confirmed. During the modeling it was found out that the structure of investigated territory in the density model from the surface to a depth of ³ 20 km is divided into the central; eastern and western parts. The central one; which corresponds to the most part of the Golovanevsk suture zone; is represented by rocks; whose density is much higher than in the western and especially in the eastern parts. The existence of essentially decompacted rocks in the eastern and northern parts of the zone to a depth of 10 km indicates the presence of granitization processes in the past. From the depth of 30 km and up to the Moho discontinuity two parts are distinguished: the western and the eastern. These parts are separated by a sharp submeridional stage in the Moho discontinuity from 46 to 55—65 km; which are traced along the center line of the Golovanevsk suture zone. The density of rocks is much higher in the western part both in area and with depth. The density distribution across the entire crustal section was obtained both in local structures with an abnormally high density on the surface of the basement (represented by gabbro amphibolites; ferruginous quartzites; crystal shales and gneiss garnet-biotite gneisses) and in the enclosing rocks. The performed calculations made it possible in the first approximation to determine the nature of the density variation with depth in the anomalous structures and to establish the depth of their propagation. According to preliminary data; these structures can be traced to a depth of 3—5 km.

Detrital zircon and igneous protolith ages of high-grade metamorphic rocks in the Highland and Wanni Complexes, Sri Lanka: Their geochronological correlation with southern India and East Antarctica

The high-grade metamorphic rocks of Sri Lanka place valuable constraints on the assembly of central parts of the Gondwana supercontinent. They are subdivided into the Wanni Complex (WC), Highland Complex (HC) and Vijayan Complex (VC), but their correlation with neighbouring Gondwana terranes is hindered by a poor understanding of the contact between the HC and WC. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U–Pb dating of remnant zircon cores from 45 high-grade metamorphic rocks in Sri Lanka reveals two domains with different age characteristics that correlate with the HC and WC and which help constrain the location of the boundary between them. The HC is dominated by detrital zircon ages of ca. 3500–1500 Ma from garnet–biotite gneiss, garnet–cordierite–biotite gneiss, some samples of garnet–orthopyroxene–biotite gneiss and siliceous gneiss (interpreted as paragneisses) and igneous protolith ages of ca. 2000–1800 Ma from garnet–hornblende–biotite gneiss, other samples of garnet–orthopyroxene–biotite gneiss, garnet–two-pyroxene granulite, two-pyroxene granulite and charnockite (interpreted as orthogneisses). In contrast, the WC is dominated by detrital zircon ages of ca. 1100–700 Ma from paragneisses and igneous protolith ages of ca. 1100–800 Ma from orthogneisses. This clearly suggests the HC and WC have different origins, but some of our results and previous data indicate their spatial distribution does not correspond exactly to the unit boundary proposed in earlier studies using Nd model ages. Detrital zircon and igneous protolith ages in the HC suggest that sedimentary protoliths were eroded from local 2000–1800 Ma igneous rocks and an older Paleoproterozoic to Archean craton. In contrast, the WC sedimentary protoliths were mainly eroded from local late Mesoproterozoic to Neoproterozoic igneous rocks with very minor components from an older 2500–1500 Ma craton, and in the case of the WC precursor sediments there was possibly additional detritus derived from early to middle Neoproterozoic metamorphic rocks. The relic zircon core ages in the HC are comparable with those of the Trivandrum Block and Nagercoil Block of southern India. In contrast, those ages in the WC match the Achankovil Shear Zone and Southern Madurai Block of southern India. These comparisons are also supported by Th/U ratios of detrital zircon cores from paragneisses (Th/U ratios of >0.10 for the former and not only >0.10 but also ≤0.10 for the latter). Comparisons with the Lützow-Holm Complex of East Antarctica indicate that the geochronological characteristics of the HC and WC broadly match those of the Skallen Group, and the Ongul and Okuiwa Groups, respectively.

COMMENTS ON «THE OPEN LETTER TO THE MEMBERS OF THE BUREAU OF THE NATIONAL STRATIGRAPHIC COMMITTEE OF UKRAINE AND FELLOW PRECAMBRIAN GEOLOGISTS» V.P. KYRYLYUK

V.P. Kyrylyuk proposals as to stratigraphic subdivision of early Precambrian formations of Ukrainian Shield as a whole and rock association of Dniester-Bug megablock in particular are analysed. On an example of supercrustal formations of Dniester-Bug megablock it is shown, that among rocks of granulitic association, in addition to that stratogenic formations (primarily sedimentary, volcanogenic, chemogenic, and others) there are more late (intrusive-magmatic, metasomatic) rocks which formation is caused by the superimposed endogenic activity. The morphology of these units, their relations with other supercrustal rocks and occurrence modes are formed as a the result of the same superimposed endogenic activity. Simple assumption of these formations to be the treated as suites will lead only to increase in volumes of these stratigraphic units and the interpreation of contacts of their vein (intrusive magmatic and metasomatic) varieties as primary stratification can cause wrong interpretation of tectonic structure of the territory. Plutonic (metamorphosed dikes of diabases) and metasomatic (biotite, a garnet-biotite gneisses, granulites and eulysite) formations are also found in rhythmical-layered alumina-basic formations. If these rocks be excluded from geological section, remained petroparagenesis will include hyperstene, bi-pyroxene plagiogneisses and crystalloshists, that is typical of Tyvrivka strata (enderbite-gneiss formation). Leucocratic biotite and garnet-biotite gneisses that comprize about 80 % of high-alumina-quartzitic formation volume (Kosharo-Oleksandrivka suite) are also metasomatic in origin.

Thermal gradient and geochronology of a Paleozoic high-grade terrane in the northeastern Cathaysia block, South China

The Chencai terrane in the northeastern Cathaysia Block forms part of the northeast extension of the Paleozoic orogeny in South China. Recent field investigations have revealed the occurrence of high grade rocks in this terrane including granulite facies metapelites (khondalites) and orthopyroxene-bearing granulites. However, the thermal gradients and metamorphic history of this region have not been well constrained, thus impeding a better understanding of the geodynamic history of the Paleozoic orogeny in South China. Here we report results from a systematic study on the metamorphic PT conditions and geochronology of the orthopyroxene granulites and metasedimentary rocks (metapelites) from the Chencai area. The orthopyroxene granulites preserve granulite facies peak assemblage of Opx+Grt+Pl+Kfs+Qtz and a retrograde P-T trajectory. Peak PT conditions are 845–860°C and 0.83–0.86Gpa, corresponding to a thermal gradient of ~30°C/km which is much hotter than the normal thermal gradient of continental crust. Mineral compositions of the metasedimentary rocks re-equilibrated during retrograde metamorphism so that the peak PT conditions recorded by these rocks are much lower than those obtained from the orthopyroxene granulite. Peak PT conditions are 710–720°C, 0.57–0.58Gpa for garnet-biotite gneiss and 760–780°C, 0.60–0.62Gpa for biotite garnetite. Zircon U-Pb dating shows that this episode of granulite facies metamorphism occurred at 430–450Ma and the detrital population shows minor or no contribution from the basement rocks of the Cathaysia Block. The Neoproterozoic rocks occurring along the Jiangnan belt are considered to have provided the sedimentary source as indicated by the two main age peaks of 0.79Ga and 0.83Ga from the detrital zircon cores. Lu-Hf isotope data on the zircons indicates extensive alteration and little new zircon growth during the Paleozoic metamorphism. The εHf(t) values and model ages (TDM1) of zircons suggest that the source rocks involved juvenile crustal components with Hf model ages of 1.1–1.5Ga. Sedimentation of such a continental shelf sequence as well as the high thermal gradient of ~30°C/km confirm the previously proposed continental arc formation and maturation model in the Chencai region.

Subduction or sagduction? Ambiguity in constraining the origin of ultramafic–mafic bodies in the Archean crust of NW Scotland

The Lewisian Complex of NW Scotland is a fragment of the North Atlantic Craton. It comprises mostly Archean tonalite–trondhjemite–granodiorite (TTG) orthogneisses that were variably metamorphosed and reworked in the late Neoarchean to Paleoproterozoic. Within the granulite facies central region of the mainland Lewisian Complex, discontinuous belts composed of ultramafic–mafic rocks and structurally overlying garnet–biotite gneiss (brown gneiss) are spatially associated with steeply-inclined amphibolite facies shear zones that have been interpreted as terrane boundaries. Interpretation of the primary chemical composition of these rocks is complicated by partial melting and melt loss during granulite facies metamorphism, and contamination with melts derived from the adjacent migmatitic TTG host rocks. Notwithstanding, the composition of the layered ultramafic–mafic rocks is suggestive of a protolith formed by differentiation of tholeiitic magma, where the ultramafic portions of these bodies represent the metamorphosed cumulates and the mafic portions the metamorphosed fractionated liquids. Although the composition of the brown gneiss does not clearly discriminate the protolith, it most likely represents a metamorphosed sedimentary or volcano-sedimentary sequence. For Archean rocks, particularly those metamorphosed to granulite facies, the geochemical characteristics typically used for discrimination of paleotectonic environments are neither strictly appropriate nor clearly diagnostic. Many of the rocks in the Lewisian Complex have ‘arc-like’ trace element signatures. These signatures are interpreted to reflect derivation from hydrated enriched mantle and, in the case of the TTG gneisses, partial melting of amphibolite source rocks containing garnet and a Ti-rich phase, probably rutile. However, it is becoming increasingly recognised that in Archean rocks such signatures may not be unique to a subduction environment but may relate to processes such as delamination and dripping. Consequently, it is unclear whether the Lewisian ultramafic–mafic rocks and brown gneisses represent products of plate margin or intraplate magmatism. Although a subduction-related origin is possible, we propose that an intraplate origin is equally plausible. If the second alternative is correct, the ultramafic–mafic rocks and brown gneisses may represent the remnants of intracratonic greenstone belts that sank into the deep crust due to their density contrast with the underlying partially molten low viscosity TTG orthogneisses.

Mesoarchean convergent margin processes and crustal evolution: Petrologic, geochemical and zircon U–Pb and Lu–Hf data from the Mercara Suture Zone, southern India

The Mercara Shear Zone is sandwiched between the Western Dharwar Craton and the Coorg Block in the Southern Granulite Terrain of India, and is marked by steep gravity gradients interpreted to suggest the presence of underplated high-density material in the lower crust. Here we present geological, petrological and geochemical data, together with zircon U–Pb ages and Lu–Hf isotopes from a suite of metaigneous (TTG-related gneisses, charnockite, metagabbro, mafic granulite) and metasedimentary (quartz mica schist, khondalite, garnet biotite gneiss, kyanite–sillimanite bearing metapelite) rocks from this zone. Geochemical data on the magmatic suite suggests formation through subduction-related arc magmatism, whereas the metasediments represent volcano-sedimentary trench sequences. Phase equilibrium modeling of mafic granulites from the Mercara Shear Zone suggests P–T range of 10–12kbar at 700°C to 900°C. The zircon data yield weighted mean 207Pb/206Pb ages of 3229±80Ma for metagabbro, 3168±25Ma for the charnockite, and 3181±20Ma for the mafic granulite. Ages ranging from 3248±28Ma to 3506±26Ma were obtained from zircons in the kyanite/sillimanite bearing metapelite, 3335±44Ma from khondalite, 3135±14Ma from garnet biotite gneiss, 3145±17Ma to 3292±57Ma from quartz mica schist and 3153±15Ma to 3252±36 from TTG gneiss. The tightly defined ages of 3.1 to 3.2Ga from igneous zircons in the magmatic suite suggest prominent Mesoarchean convergent margin magmatism. The timing of high grade metamorphism as constrained from metamorphic overgrowths in zircons is ca. 3.0Ga which might mark the collisional event between the Western Dharwar Craton and the Coorg Block. Hf isotope features suggest magma derivation mostly from juvenile sources and the Lu–Hf model ages indicate that the crust building might have also involved partial recycling of basement rocks as old as ca. 3.8Ga. Our study defines the Mercara Shear Zone as a terrane boundary, and possible Mesoarchean suture along which the Coorg Block was accreted to the Western Dharwar Craton. The accretion of these continental fragments might have coincided with the birth of the oldest supercontinent “Ur”.

Tectonothermal evolution of the continental crust beneath the Yakutian diamondiferous province (Siberian craton): U–Pb and Hf isotopic evidence on zircons from crustal xenoliths of kimberlite pipes

U–Pb age and Hf-isotope data were collected on zircons from crustal xenoliths from the Late Devonian kimberlite pipes of Markha terrane. The xenoliths include mafic garnet granulites (Gt+Pl+Cpx±Opx±Amp±Bt±Scp) and garnet–biotite gneisses (Grt+Bt+Pl+Kfs+Qtz±Scp). The data from this study demonstrate that the crust of the Markha terrane experienced several tectonothermal events. Zircons with Archean Hf model ages (TDM=3.13–2.5Ga) are predominant in all xenoliths with one exception. These data imply that most of the lower and middle crust beneath the Markha terrane was produced in the Archean time. Later this Paleoarchean crust was significantly reworked in several tectonothermal events, including a Neoarchean stage (2.9–2.5Ga), and several Paleoproterozoic metamorphic stages (1.98, 1.9 and 1.8Ga). There is very little evidence for generation of juvenile crust associated with these metamorphic events. Importantly, crustal xenoliths from kimberlite pipes of the Markha terrane record all the same events that occurred in the adjacent Daldyn and Magan terranes.

P–T path and timing of crustal thickening during amalgamation of East and West Gondwana: A case study from the Hafafit Metamorphic Complex, Eastern Desert of Egypt

The southeastern sector of the Hafafit Metamorphic Complex, southern Eastern Desert of Egypt comprises infrastructural orthogneisses of tonalite and syenogranite parentage, amphibolites, and a volcano-sedimentary association. These are overthrust by an obducted suprastructural ophiolite nappes via the Nugrus thrust. The protolith of the biotite–hornblende-gneisses was formed during island-arc accretion, while that of the garnet–biotite gneisses were formed in a within-plate regime, consistent with a transition to a post-collisional setting. The volcano-sedimentary association comprises interbedded and intercalated highly foliated metapelitic schists, metabasites, and leucocratic gneisses, deposited in a back-arc basin. The metapelites and the leucocratic gneisses originated from immature Fe-shales and arkoses derived from intermediate-mafic and acidic igneous rocks, respectively, via weak chemical weathering in a tectonically active island arc terrane. The intercalated amphibolites were derived from tholeiitic basalts generated in a back-arc setting.The volcano-sedimentary association was metamorphosed under upper-amphibolite facies conditions with pressures of 9–13kbar and temperatures of 570–675°C, as derived from conventional geothermobarometry and pseudosection calculation. A steep, tight clockwise P–T path is constrained and a geothermal gradient around 20°C/km is estimated for the peak metamorphism. We assume that deformation and metamorphism are due to crustal thickening during the collision of East and West Gondwana, where peak metamorphism took place in the middle to lower crust at 33km average crustal depth. This was followed by a subsequent quasi-isothermal decompression due to rapid exhumation during wrench tectonics. Sinistral transcurrent shearing with extensional denudation resulted in vertical ductile thinning that was accompanied by heat input from magmatism, as indicated by a higher geothermal gradient during retrograde metamorphism and exhumation of the complex.U–Pb data from magmatic zircons yields protolith ages of 731±3Ma for the biotite–hornblende gneisses and 646±12Ma for the garnet–biotite gneisses. Conforming to field evidence, our geochronology data point to a depositional age of the volcano-sedimentary cover at around 650Ma. The age of metamorphism is constrained by a low Th/U ratio of a zircon grain crystallized at an age of 597±6Ma.