Amphibole Biotite Gneiss Research Articles

The Age and Origin of the Rocks of the West Azov Group (Lozuvatka Antiform, the Ukrainian Shield)

Geochronological studies of amphibole-biotite gneisses of the Verkhnii Tokmak Unit and amphibolites and Banded Iron Formation (BIF) of the Kaiinkulak Unit of the West-Azov Group in the Lypovets area of the Lozuvate antiform in the West Azov block of the Ukrainian Shield were carried out. A multi-stage history of the formation of amphibole-biotite gneisses has been established. In terms of chemical composition they correspond to quartz diorites. Rare earth elements are differentiated — (La/YbN = 7.54), with a significant negative europium anomaly — Eu/Eu* = 0.66. The REE distribution indicates that the initial melt for quartz diorites was produced in a crustal magmatic source. In zircon crystals from amphibole-biotite gneisses, relics (cores) of older zircon and mantles that surround them were distinguished. Relict zircon is coarse-zoned, cracked, and its mantles have a thin-zoned structure. A small amount of transparent non-zonal zircon crystals was also found. Based on the results of zircon dating by the LA-ICP-MS method, it was determined that the age of the relict zircon is 3.16 Ga. The thin-zonal mantles have an age of 2.9-2.8 Ga, which probably corresponds to the time of crystallization of quartz diorites. Non-zonal transparent zircon with an age of 2.1 Ga corresponds to the time of dynamometamorphism (collision). BIF of the Lypovets deposit in the West Azov iron ore province, which belongs to the Kaiinkulak Unit of the West Azov Group, has a high Ni/Fe×10–4 = 0.5-2.1 ratio, typical for the Archean BIFs of the Algoma type. This deposit is confined to an Archean greenstone belt in a synclinal structure composed of metamorphosed basaltic komatiites and sedimentary rocks. Thus, the Verkhnii Tokmak Unit comprises Mesoarchean quartz diorites of crustal genesis, while the Kaiinkulak Unit probably represents heavily metamorphosed rocks of an Archean greenstone belt.

Assessment of resources of the Varvara thermo-mineral water deposit, Pazardzhik District

In order to determine the resources of the Varvara thermo-mineral water deposit, a hydrogeological model has been created, determined by the Finite Difference Method (FDM). The model was developed in the mode of stabilized filtration. The Varvara deposit occupies an area of 2.4 km2 in the Pazardzhik District, located in Southern Bulgaria. The collector for those thermo-mineral waters are biotite and amphibole-biotite gneisses. The total water exchange in the system of the Varvara mineral water deposit is 26 l/s (2 260 m3/d). The regional resources of the deposit are 25 l/s (2 171 m3/d). The geothermal resources of the deposit have been estimated to be 4.98 MJ/s.

The Paleoarchean and Mesoarchean TTGs of the western Azov area, the Ukrainian Shield

A large anticline structure occurs in the western part of the Azov Domain of the Ukrainian Shield. It is composed of rocks of the Mesoarchean (3.2-3.0 Ga) granite-greenstone association and relics of an older basement. The anticline is divided into two parts by the Bilotserkivka structure of sub-latitudinal strike. The northern part includes the Huliaipole and Remivka blocks, and the southern part comprises the Saltycha anticline. The U-Pb age of plagiogneisses of the Lantsevo anticline of the Bilotserkivka structure is 3299 ± 11 Ma. In terms of geochemical characteristics, they correspond to TTGs. In the western part of the Bilotserkivka structure, we previously identified quartz diorites having an age of 3297 ± 22 Ma. These data show that the Bilotserkivka structure represents an ancient basement. Dislocated trondhjemites were studied in the Ivanivka area at the eastern part of the Saltycha anticline. They contain numerous relics of heavily altered amphibolites. The U-Pb age of zircons from trondhjemite is 3013 ± 15 Ma. These rocks are of the same age as TTGs of the Shevchenko Complex cutting through the sedimentary- volcanogenic rocks of the greenstone structures of the Azov Domain. They share age and geochemical characteristics with biotite and amphibole-biotite gneisses of the “Kainkulak beds” in the Zrazkove village located at the Mokra Konka river (3.1-3.0 Ga) and with biotite gneisses in the lower reaches of the Kainkulak river (2.92 Ga). Thus, gneisses of the “Kainkulak beds” actually represent the Mesoarchean TTGs of the Shevchenko Complex, transformed in the Paleoproterozoic time due to the dislocation metamorphism. The late Paleoarchean (3.3 Ga) tonalites are known in the West Azov and KMA domains; they probably also occur in the basement of the Middle Dnieper domains, where detrital zircons of this age have been reported. These data allow us to assume the existence of a large Late Paleoarchean (3.3 Ga) protocraton, in which the Mesoarchean (3.2-3.0 Ga) greenstone belts and TTGs of the eastern part of the Ukrainian Shield and the KMA Domain were formed.

THE PALEOARCHEAN (3.3 Ga) AND MESOARCHEAN (3.0 Ga) TTGs OF THE WESTERN AZOV AREA, THE UKRAINIAN SHIELD

A large anticline structure that includes the West Azov and Remivka blocks occurs in the western part of the Azov Domain of the Ukrainian Shield. These blocks are composed of rocks of the Mesoarchean (3.2-3.0 Ga) granite-greenstone association and relics of an older basement. The anticline is divided into two parts by the Bilotserkivka structure of sub-latitudinal strike; the northern part includes the Huliaipole and Remivka blocks, and the southern part is comprised of the Saltycha anticline. The Archean plagiogranitoids of the West Azov underwent intense dislocation metamorphism during the Paleoproterozoic. In many areas they were transformed into plagioclase gneisses that were attributed to the Paleoarchean “Kainkulak thickness” of the Azov Series. Detailed geological-structural and geochronological studies are required to define the age of these gneisses.We have chosen two areas for our studies: the Lantsevo anticline within the Bilotserkivka structure, and the Ivanivka area in the eastern part of the Saltycha anticline. The Bilotserkivka structure is composed of rocks of the Central Azov Series and highly deformed Archean formations. We have dated plagiogneisses of the Lantsevo anticline. These rocks contain large relics of metamorphic rocks of unknown age, including two-pyroxene and pyroxene crystalline schists, and pyroxenemagnetite quartzites (BIF). In terms of chemical composition, two-pyroxene crystalline schists correspond to tholeiitic basalts and basaltic komatiites. Ferruginous-siliceous rocks belong to the Algoma type typical for the Archean greenstone belts. Biotite gneisses are similar to the medium-pressure tonalite-trondhjemite-granodiorite rocks (TTGs). The U-Pb age of zircon crystallization from biotite gneisses is 3299 ± 11 Ma. At 30 km in the western part of the Bilotserkivka structure, we have previously identified quartz diorites having an age of 3297 ± 22 Ma. In terms of geochemical characteristics, they correspond to low-pressure TTGs. These data show that the Bilotserkivka structure is a block representing an ancient basement. In the Ivanivka area in the eastern part of the Saltycha anticline, the strike of the Archean rocks was reorientated from northwestern to latitudinal. The studied dislocated trondhjemites of the Ivanivka area correspond to TTGs in terms of the geochemical characteristics. They contain numerous relics of highly altered amphibolites. The U-Pb age of zircon crystallization from trondhjemite is 3013 ± 15 Ma. These rocks are of the same age as TTGs of the Shevchenko Complex cutting through the sedimentary-volcanogenic rocks of the greenstone structures of the Azov Domain. They share age and geochemical characteristics with biotite and amphibole-biotite gneisses of the “Kainkulak thickness” in Zrazkove village located at the Mokra Konka river (3.1-3.0 Ga) and with biotite gneisses in the lower reaches of the Kainkulak river (2.92 Ga). Thus, gneisses of the “Kainkulak thickness” in fact represent the Mesoarchean TTGs of the Shevchenko Complex, which were transformed in the Paleoproterozoic time due to the dislocation metamorphism. Late Paleoarchean (3.3 Ga) tonalites are known in the West Azov and the KMA domains; they probably also occur in the basement of the Middle Dnieper domains, where detrital zircons of this age have been reported. These data allow us to conclude the existence of a large Late Paleoarchean (3.3 Ga) protocraton, in which the Mesoarchean (3.2-3.0 Ga) greenstone belts and TTGs of the eastern part of the Ukrainian Shield and the KMA Domain were formed.

Eoarchean to Neoproterozoic Detrital Zircons from the South of Meiganga Gold-Bearing Sediments (Adamawa, Cameroon): Their Closeness with Rocks of the Pan-African Cameroon Mobile Belt and Congo Craton

The core of detrital zircons from the southern Meiganga gold-bearing placers were analyzed by Laser Ablation Split Stream analytical techniques to determine their trace element abundances and U-Pb ages. The obtained data were used to characterize each grain, determine its formation condition, and try to trace the provenance. The Hf (5980 to 12,010 ppm), Y (27–1650 ppm), U (25–954 ppm), Th (8–674 ppm), Ti (2–256 ppm), Ta, Nb, and Sr (mainly <5 ppm), Th/U (0.06–2.35), Ti zircon temperature (617–1180 °C), ∑REE (total rare earth element) (98–1030 ppm), and Eu/Eu* (0.03 to <1.35) are predominant values for igneous crustal-derived zircons, with very few from mantle sources and of metamorphic origin. Crustal igneous zircons are mainly inherited grains crystallized in granitic magmas (with some charnockitic and tonalitic affinities) and a few from syenitic melts. Mantle zircons were crystallized in trace element depleted mantle source magmatic intrusion during crustal opening. Metamorphic zircons grown in sub-solidus solution in equilibrium with garnet “syn-metamorphic zircons” and in equilibrium with anatectic melts “anatectic zircons” during crustal tectono-metamorphic events. The U-Pb (3671 ± 23–612 ± 11 Ma) ages distinguish: Eoarchean to Neoproterozoic igneous zircons; Neoarchean to Mid Paleoproterozoic anatectic zircons; and Late Neoproterozoic syn-metamorphic grains. The Mesoarchean to Middle Paleoproterozoic igneous zircons are probably inherited from pyroxene-amphibole-bearing gneiss (TTGs composition) and amphibole-biotite gneiss, whose features are similar to those of the granites, granodiorites, TTG, and charnockites found in the Congo Craton, south Cameroon. The youngest igneous zircons could be grains eroded from Pan-African intrusion(s) found locally. Anatectic and syn-metamorphic zircons could have originated from amphibole-biotite gneiss underlying the zircon-gold bearing placers and from locally found migmatized rocks that are from the Cameroon mobile belt, which could be used as proxies for tracking gold.

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.

Ages and nature of the protolith of the Tulovchikha metamorphic complex in the Bureya Massif, Central Asian Orogenic Belt, Russia: Evidence from U–Th–Pb, Lu–Hf, Sm–Nd, and 40Ar/39Ar data

The Bureya Massif is located in the eastern segment of the Central Asian Orogenic Belt (CAOB) and its tectonic affinity and Precambrian evolution is highly controversial. Here we present mineralogical, whole-rock geochemical (major and trace elements and Sm–Nd isotopes) and zircon Hf isotopic data, together with zircon geochronological and mica 40Ar/39Ar dating of the biotite and amphibole gneisses from the Tulovchikha metamorphic complex (previously considered as Archean or Paleoproterozoic basement) in the Bureya Massif, in order to investigate their protolith, metamorphic and deformation ages, petrogenesis, and tectonic implications. LA-ICP-MS U–Pb dating of magmatic zircons from the biotite gneiss, the amphibole–biotite gneiss, and the amphibolite yields single zircon age populations cluster at ~950 Ma, ~920 Ma and ~940 Ma, respectively. The narrow zircon age ranges, together with the morphological features of the zircon crystals and the geochemical data, indicate that the studied metamorphic rocks have a magmatic origin and their protoliths were formed in the early Neoproterozoic (i.e., 950–920 Ma). The consistent formation ages further suggest that the Neoproterozoic gabbro–granitic rocks within the Nyatygran Complex in the Bureya Massif probably represent the protolith of the studied metamorphic rocks. The biotite and amphibole-biotite gneisses of the Tulovchikha metamorphic complex yield whole-rock Nd model ages (tNd(DM)) of 2.4–1.5 Ga and Hf zircon crustal model ages (tHf(C)) of 1.7–1.2 Ga, indicating that the precursors of the Tulovchikha Complex gneisses formed from the reworking of Meso–Paleoproterozoic rocks or mixed Neoproterozoic juvenile and Meso–Paleoproterozoic crustal sources. Considering that the Bureya and Jiamusi massifs experienced similar Neoproterozoic magmatic events, we suggest that these massifs form a contiguous crustal unit and share a common geological history. The rocks of Tulovchikha metamorphic complex in the studied area experienced epidote-amphibolite to amphibolite facies metamorphism probably in the early Paleozoic, as evidenced by the 471 ± 19 Ma age of a metamorphic zircon with a low Th/U ratio (<0.1). According to 40Ar/39Ar biotite dating of the biotite gneiss, the Tulovchikha Complex experienced a youngest thermal event at 227 ± 3 Ma. This event may be related to the final closure of the Paleo-Asian Ocean, or may have been caused by the deformation of the blocks of East Asia in connection with the initial stages of the closure of the Mongol–Okhotsk Ocean.

Anatexis of former arc magmatic rocks during oceanic subduction: A case study from the North Wulan gneiss complex

Migmatites are widespread in the North Wulan gneiss complex from the South Qilian–North Qaidam orogenic belt, but their petrogenesis and ages are poorly constrained. Here, an integrated study of petrography, whole-rock geochemistry, geochronology, zircon trace element and Hf isotope analysis deciphers the nature and timing of partial melting in migmatitic amphibole-biotite gneiss. Zircon U–Pb geochronology reveals that the protoliths crystallized at 506–494 Ma followed by metamorphism and anatexis at ca. 465 to 450 Ma. Hafnium isotope compositions of inherited cores and anatectic rims are very similar, suggesting that partial melting occurred in a relatively closed isotopic system and new zircon rims grew via dissolution–reprecipitation of pre-existing zircon cores. Anatexis occurred by water-fluxed melting of mafic-intermediate rocks through the breakdown of biotite and growth of peritectic amphibole. The protolith of the migmatites records Cambrian arc magmatism in an active continental margin, which was induced by northward subduction of the South Qilian ocean slab. Contemporary arc-like magmatism and high-temperature/low-pressure metamorphism in the region suggest that anatexis in the North Wulan gneiss complex likely took place in a continental arc setting, which reflects the reworking of former arc magmatic rocks in a late stage of oceanic subduction.

Zircon as a metamorphic timekeeper: A case study from the Caledonides of central Norway

During metamorphism, inherited or detrital zircon grains commonly serve as a template onto which new generations of metamorphic zircon precipitates, charting successive episodes in the thermal history of rocks. In addition, chemical modification of pre-metamorphic zircon can leave an imprint in the U–Pb systematics. We present a case study from a sample of migmatitic amphibole–biotite gneiss from central Norway that contains zircon grains that chart much of the history of an entire orogenic episode, not only through generation of new zircon, but also resetting of the U–Pb system via radiogenic-Pb loss. The investigated gneiss, from the Helgeland Nappe Complex, within the Uppermost Allochthon of the Scandinavian Caledonides records a prolonged metamorphic evolution. Phase equilibria modelling implies peak metamorphic conditions of 7.5–8.0 kbar and 650–675 °C, consistent with low-degree partial melting and the presence of small volumes of leucosome. Laser ablation split stream analysis that simultaneously measures U–Pb, REE and Lu–Hf in the same volume of zircon, yields a range of analyses that spread along concordia. A zircon population between 485 and 443 Ma is characterized by faded and blurred oscillatory-zoned domains that have gained nonformula elements. These analyses are interpreted to indicate the presence of reactive solutions during this time period that led to leaching and mobility of Pb. Another texturally distinct zircon component yields an age of 478 ± 4 Ma, has typical igneous REE zircon profiles, and records low apparent alpha doses, interpreted to reflect new zircon growth during migmitization. A further population of zircon rims and discrete grains yields an age of 453 ± 2 Ma, interpreted as a later episode of zircon growth during high-grade metamorphism. A final set of zircon overgrowths yield an age of 439 ± 3 Ma, also interpreted to reflect new metamorphic growth. Many zircon growth phases in this rock have similar REE patterns, implying broadly isochemical metamorphism, consistent with the evidence for only limited partial melting. Furthermore, the Hf isotopic signature in the analyzed zircon grains suggests that there was no release of radiogenic Hf during metamorphism. The metamorphic zircon in the studied rock records several episodes of growth and/or alteration over a period of >40 Ma that was related to protracted destruction of the Iapetus Ocean. In the context of regional data, the new results suggest that the Helgeland Nappe Complex, which is generally considered to be exotic to Baltica, underwent geographically widespread, long-lived Ordovician high-temperature metamorphism. This record of metamorphism is distinct from that recorded by tectonostratigraphically lower units, which are conventionally regarded to have been derived from Baltica. The results shed light on the assembly of the Scandinavian Caledonides and show that zircon from individual samples may be used to unravel complex orogenic histories.

Orebody geometry, fluid and metal sources of the Omitiomire Cu deposit in the Ekuja Dome of the Damara Belt in Namibia

The Omitiomire Cu deposit (resource of 137 Mt at 0.54% Cu) in the Ekuja Dome of the Damara Belt in Namibia is hosted by an anastomosing, low-angle Pan-African (ca. 520 Ma) shear zone system developed around an older (ca. 1100–1060 Ma), late Mesoproterozoic intrusive breccia between a suite of mafic rocks (originally lava flows) and later tonalitic gneisses. High-grade ore shoots preferentially formed along contacts between tectonically interleaved biotite-epidote-quartz-chalcocite schists and felsic gneisses, and are directly related to an increase in the number and cumulative thickness of thin, contact-parallel mineralized shear zones. Alteration and mineralization are associated with elevated concentrations of K2O, Cr, Rb, S, and Cu and a loss of Na2O, CaO, and MgO. Oxygen isotope fractionation for quartz-biotite, quartz-feldspar, and quartz-amphibole mineral pairs support equilibrium temperatures of between 500 and 650 °C during the fluid/rock interaction. Mineral separates from amphibole-biotite gneisses and mineralized schists have similar ranges in δ18O values of about 1.2 to 2 ‰ relative to VSMOW. Coexisting minerals are arranged in an order of increasing δ18O values from biotite, to epidote, amphibole, and quartz, suggesting that the Omitiomire Shear Zone was a rock-dominated system. Similarly, H-isotope results for mineral separates from biotite-epidote schists and amphibole gneisses do not show any reversals for D/H fractionations, with δD values of between −48 and −82 ‰, typical of metamorphic-magmatic rocks. The homogeneous and low δ34S values (−6.1 to −4.7 ‰ CDT) are compatible with a local redistribution of sulfur from magmatic rocks and interaction with sulfur derived from metamorphic fluids of metasedimentary origin. The relatively low fluid/rock ratios and elevated Cu values (>1500 ppm) from unaltered amphibolite point to a local redistribution of an earlier (late Mesoproterozoic) Keweenaw-type Cu mineralization into later Pan-African shear zones during the exhumation of the Ekuja Dome. The timing, polyphase evolution, and tectonic setting of the Omitiomire deposit show remarkable similarities with the large Cu deposits of the Domes Region in the adjoining Lufilian Arc of northern Zambia. This suggests the presence of a much larger, regionally significant Cu province extending from central Namibia, through northern Botswana, and into Zambia.

Petrology and U–Pb geochronology of zircon in a suite of charnockitic gneisses from parts of the Chotanagpur Granite Gneiss Complex (CGGC): evidence for the reworking of a Mesoproterozoic basement during the formation of the Rodinia supercontinent

Abstract Understanding the evolution of the Chotanagpur Granite Gneiss Complex (CGGC) of the East Indian Shield is crucial to decipher the role of the Indian Shield in the formation of the Rodinia supercontinent. The area around Deoghar–Dumka exposes a suite of granulite-facies orthogneisses (variably retrogressed to amphibole–biotite gneiss) that enclose remnants of Palaeoproterozoic metasedimentary and meta-igneous rocks. Results from mineral chemistry, laser ablation inductively coupled plasma mass spectrometry (LA ICP-MS) U–Pb dating of zircon and limited bulk-rock compositions of the studied rocks suggest that the magmatic protoliths of the felsic orthogneisses had A-type chemistry, and that these were emplaced at approximately 1450 Ma presumably in a continental rift setting. Intense deformation and metamorphism of the felsic rock culminated at approximately 9 kbar and 850°C along an apparent geothermal gradient of 26°C km −1 . These peak metamorphic conditions were successively followed by initially a steeply decompressive and then a weakly decompressive retrograde pressure–temperature path. The shape of the retrograde pressure–temperature path and the estimated geothermal gradient at the metamorphic peak are interpreted to be the products of continent–continent collision; U–Pb dates of metamorphic zircon overgrowths suggest an age of approximately 943 Ma for the collisional event. This study demonstrates that ‘Grenville-age’ orogenesis thoroughly reworked the approximately 1450 myr-old basement of the CGGC during the formation of the Rodinia supercontinent.

Paleoproterozoic migmatitic gneisses from the Tandilia belt (Argentina), Río de la Plata craton, record cooling at deep crustal levels

We studied high-grade metamorphic rocks of the El Cristo hill area of the Tandilia belt. Mineral analyses and thermodynamic calculations were carried out for two adjacent rock samples: an amphibole–biotite gneiss and a garnet–biotite-bearing migmatite. Peritectic garnets in the migmatite show core compositions of pyr4.5(gro + andr)10spes6alm79.5 changing to pyr3.5(gro + andr)17spes6alm73.5 at their thin rims. Garnet compositions in the gneiss are pyr6.5(gro + andr)26spes12alm55.5 and pyr4.5(gro + andr)34spes12alm49.5 for core and rim, respectively. A P–T path was constructed by calculating pseudosections in the 11-component system Si–Ti–Al–Fe–Mn–Mg–Ca–Na–K–O–H and contouring them by isopleths for garnet components using the PERPLE_X software package. Supra-solidus crystallization of garnet cores in the migmatite began at 5.8 kbar and 660 °C. Garnet rims equilibrated at 7.0 kbar and 640 °C compatible with garnet cores in the amphibole–biotite gneiss (7.6 kbar and 660 °C). The further chemical development of garnet in this rock points to P–T conditions of 11.6 kbar and 620 °C and 12.2 kbar and 595 °C (outermost garnet rim). At this high-pressure stage Ca-amphibole was not stable. Most biotite formed during exhumation whereas the high-pressure accessory minerals, titanite and epidote, persisted. According to the obtained anti-clockwise P–T path the originally partly melted material was tectonically transported from ∼22 km (middle crust) to ∼40 km (lower crust) depths reaching a geothermal gradient as low as 15 °C km−1. This transport probably occurred along a major suture zone, which was active during the Paleoproterozoic (2.25–2.10 Ga), before a terminating collision of terranes near the SW boundary of the Rio de la Plata craton.

A new occurrence of alkaline magmatism on the Kola Peninsula: An agpaitic dyke in the Kandalaksha region

1125 Alkaline magmatism has been widely abundant on the territory of the Kola Peninsula with the maximal activity in the Paleozoic, when most of the alkaline– ultrabasic complexes, alkaline intrusions of the agpaitic series, and rocks of the dyke series were formed [1–3]. Paleozoic dykes of alkaline rocks were registered (1) in the Khibiny and Lovozero agpaitic massifs and their frames; (2) in the massifs of alka� line–ultrabasic rocks with carbonatite (Kovdor, Turii Mys, Vuorijarvi, Afrikanda, Kandaguba, etc.); (3) in autonomous swarms, which are not spatially related to alkaline intrusions (dykes on the coast of the Kan� dalaksha Bay, swarms of pipes and dykes of kimberlite and melilitite of the Tersk coast, etc.) [3]. A new occurrence of alkaline dyke magmatism was discovered in 2010 during the geological works per� formed by the Murmansk Geological Prospecting Expedition in the “Mokhnatye Roga” area located 55 km to the southeast of the Kovdor massif ( 67°15′ N, 31°30′ E) (Fig. 1). The “Mokhnatye Roga” area is located in the Ensk segment of the northwestern part of the Belomorian mobile belt. We performed a complete petrological and geochemical description of the sections in Holes 19 and 24, which reveal one of the largest dykes in the area. The dyke, with a length of ≈4 km and a width of ≈160 m, has an eastern orientation with steep dipping (60°–90°) to the north. The thickness of Quaternary deposits ranges from 0.8 to 4.0 m; alkaline rocks occur in the range of 4.0–93.9 m being followed by host amphibole–biotite gneiss with interbeds of plagioam� phibolite (AR2 mt) along the section. The contacts with host rocks are sharp, secant, magmatic; there are no

Incipient charnockite: Characterization at the type localities

Fifty years after the discovery of incipient charnockite (orthopyroxene-bearing granitic rocks) partially replacing amphibole–biotite gneiss in southern India, this phenomenon has been found in transitional amphibolite facies to granulite facies terrains on every continent, and may be a characteristic feature of progressive deep-crustal metamorphism. The alteration is essentially a metasomatic transformation. Mass balance calculations for localities in southern India and Sri Lanka for which analytic data are most comprehensive show that the main reaction producing orthopyroxene in orthogneiss is breakdown of hornblende by reaction with biotite, with loss of FeO, MgO, TiO2, and CaO, as well as H2O from the host rock. There is concomitant gain of SiO2. No significant change of alkalis can be demonstrated as essential to the transformation, though advanced charnockitic alteration at the type locality of Kabbal, Karnataka may also show large increase of K2O, with removal of mafic constituents.Very similar orthopyroxene-bearing veins in garnet–biotite gneiss at the Ponmudi type locality in Kerala show definite, but more subtle metasomatism, with loss of FeO and MgO and increase of SiO2 and alkalis. Both the Kabbal and Ponmudi alterations occurred at mid-crustal levels: 5–6kbar and 700–750°C.It is postulated that both types of charnockitic alteration may have been produced as open-system processes by much the same kind of metasomatizing fluids – nearly coeval CO2-rich and ultrasaline fluids, based on fluid inclusion evidence. The paragneiss alteration is mainly biotite breakdown with some involvement of garnet, and leads to a distinct metamorphic facies type. The presence of graphite in some of the garnet–biotite gneisses at Ponmudi would have resulted in lower oxygen fugacity and, probably, lower H2O activity than in the similar Kabbal-type metamorphism. The fluid/rock ratios for both types were small (about 10 mass percent of the rocks), and the fluids, whatever their origin, were largely rock-governed.The source of the metasomatizing fluids has not been definitely identified. Carbon isotopes of fluid inclusions seem to show an igneous or primitive mantle source at both type localities, but ultrasaline fluids may be more plausibly explained by metasedimentary sources, and calcareous metasediments have been identified as a source of CO2 in some occurrences. Much remains to be learned about incipient charnockite and its significance in processes of crustal evolution.

Geochemistry and geochronology of the Méiganga metadiorite: implications on the timing of D2 deformational phase in Adamawa Yadé Domain in Cameroon

The metadiorite of M&eacute;iganga forms an ovoid body intruding metasedimentary gneisses in the area of M&eacute;iganga city. It shows schistosity, banded structure and is made up of green hornblende, biotite, antiperthitic plagioclase, quartz, zircon, opaque minerals and chlorite. Imprints of D2, D3 and D4 deformation phases are identical in the metadiorite and the amphibole biotite gneiss, leaving no doubt as to the syn-D2 emplacement and crystallization of the protolith of the metadiorite. The metadiorite samples display high-K calc-alkaline to shoshonitic characteristics and possess features of I-type granitoids. They exhibit relative low enrichment of LREE with respect to HREE, (La/Yb)N = 9.5-11.3, and moderate negative Eu anomalies (Eu/Eu* =0.80-0.83). Optical features of the zircon grains of the metadiorite and its syn-D2 emplacement and crystallisation indicate that the zircon evaporation 207Pb/206Pb ages (614.1 &plusmn; 3.9 Ma and 619.8 &plusmn; 9.8 Ma) of this rock date the D2 deformation phase in the M&eacute;iganga area. Comparison with other sectors of the Adamawa Yad&eacute; Domain (AYD) shows the close synchronic character of the D2 deformational phase in central Cameroon (M&eacute;iganga, Tibati, Ngaound&eacute;r&eacute; areas) and the diachronic character of this phase in central and western Cameroon. We found that a similar chronological evolution has been demonstrated in the Macurur&eacute; Domain of the Sergipano Belt (NE Brazil).Keywords: syn-D2 granitoid; 207Pb/206Pb age; Central Pan-Africa Fold Belt; Sergipano Belt

Ultrahigh-pressure metamorphism in the archean-proterozoic mobile belt (Gridino complex, Karelia, Russia)

412 Ultrahighpressure metamorphism is considered as an indicator of the modern style of plate tectonics, generally, and of subduction, in particular (1). Accord� ing to some data, the formation of this style started on the Earth in the Neoproterozoic (1); according to other data, it began 2 b. y. before, at the Meso- Neoarchean boundary (2). The absence of ultrahigh� pressure complexes with an older age than the Neoproterozoic is an important argument in favor of the first hypothesis. In this paper we describe the results of petrological investigation of a metagabbro dyke in the Gridino eclogite complex (Karelia) dem� onstrating that ultrahighpressure events were realized in the subducting lithospheric slab already in the Pale� oproterozoic. The Gridino complex is part of the Belomorian mobile belt of Karelia (3-5). It represents a tectonic plate of northwestern strike occurring in the coastal area and on the islands of the White Sea up to 60 km from Sukhaya Bay in the northwest to the Suprotivnye Islands in the southeast at a width of 6-7 km. The main volume of the complex is represented by migma� tized gneiss containing rock boudines of various sizes and compositions (3, 4). Rocks of the basic composi� tion (metabasites), namely eclogite and amphibolites, strongly prevail among boudines in gneiss; amphiboli� tized metaultramafites, marbles, zoisite, and scapolite rocks are rarely observed. The complex is cut by dykes and small massifs of metagabbronorites metamor� phosed under the conditions of the eclogite and upper amphibolite facies, as well as intrusions and veins of plagiogranites (3). Dating of eclogites from this com� plex demonstrates a wide range of ages from the Neoarchean (2.74 Ga) to the Paleoproterozoic (1.9 Ga) (3-5). This paper contains the results of study of metagab� bro dyke cutting migmatized amphibole-biotite gneiss on Bezymyannyi Island. Metagabbro has massive structure, with fineto mediumgrained, heterogranoblastic texture. The main rockforming minerals are garnet, clinopyrox� ene, plagioclase, orthopyroxene, biotite, amphibole, and quartz, which are related to different stages of metamorphic evolution of the rock, according to the

New Constraints from Pb-Evaporation Zircon Ages of the M&amp;#233;iganga Amphibole-Biotite Gneiss, Central Cameroon, on Proterozoic Crustal Evolution

The amphibole biotite gneiss (ABGn) in the Meiganga area forms part of a meta volcano-sedimentary sequence of the Adamawa Yade domain (AYD), Central African Fold Belt (CAFB). This sequence shows affinity with immature sediments (greywackes, arkoses) with intercalation of mafic lavas or tuffs. New 207Pb/206Pb zircon evaporation ages for two ABGn samples range from 1887 - 2339 Ma and from 675 - 889 Ma, respectively. These ages and evidence from internal zircon structures indicate that igneous rocks of Archean to Paleoproterozoic and of early Neoproterozoic age contributed to the detritus of the sedimentary sequence. The deposition of detritus took place prior to 614 - 619 Ma which represent the syntectonic emplacement of the Meiganga metadiorite. Leucogranites north to the Meiganga area were generated by melting of crust identical to that which provided the source of the ABGn. The metasedimentary sequence investigated in this study is similar to that of the southern part of the AYD and in the Borborema Province, NE Brazil. The tectonic and geochronologic characteristics of the AYD in the Meiganga area support the idea that during the Proterozoic, Central Africa and NE Brazil were part of the same continental landmass.

Geochemistry of magmatic rocks and time constraints on deformational phases and shear zone slip in the Méiganga area, central Cameroon

The Méiganga area is situated in the Adamawa–Yadé domain (AYD) of the Pan-African fold belt in Cameroon. The AYD is characterized by abundant plutonic rocks that intruded Palaeoproterozoic gneisses. It is cut by the transcurrent Central Cameroonian Shear Zone (CCSZ). The studied area is made up of metadiorite (MD), pyroxene-bearing granite (PGr) and biotite-muscovite granite (BMGr), hosted in a metasedimentary sequence of amphibole-biotite gneisses. The granitoids are metaluminous to slightly peraluminous, and mainly of I-type (ASI ≤ 1.1), representing a high-K calc-alkaline to shoshonitic suite. They were derived from crustal protoliths (BMGr), from rocks of mantle origin (MD, PGr), and/or from the remelting of metabasalt or metatonalite (MD, PGr). Four successive deformational phases (D1, D2, D3, and D4) are present in the Méiganga area. The S1 foliation is formed by tectonic transposition of the S0 primitive surface represented by contacts between the gneiss and intercalated amphibolites. The synmigmatitic D2 deformational phase is characterized by S2 banded schistosity, S2/C2 sinistral shear planes, and F2 folds with axes parallel to a L2 stretching lineation. Imprints of the D2 and subsequent deformational phases are similar in the metadiorite and host rocks, implying syn-D2 emplacement and crystallization of the metadiorite; therefore 614–619 Ma 207Pb/206Pb zircon evaporation ages obtained for this rock type date the syn-D2 magmagenesis. Similarly, the D3 phase of the PGr is 601 ± 1 Ma, dated by the 207Pb/206Pb evaporation method. D4 is a late-stage brittle deformational phase. Sinistral movement of the CCSZ is associated with D2, whereas its latest activity, characterized by dextral slip, cannot be older than emplacement of the 558 ± 2 Ma BMGr (207Pb/206Pb zircon evaporation age).

Petrology and deformation history of the metamorphic basement in the Mezősas-Furta crystalline high (SE Hungary)

The metamorphic basement of the Pannonian Basin consists of uplifted highs and deep sub-basins among them. One of the best-known highs is the so-called Szeghalom Dome, which is surrounded by less intensely explored ones. The eastern neighbour, the Mezosas-Furta Dome (MFD), is studied in this paper. Based on detailed petrologic investigation, six main lithologies are distinguished for the MFD, which can be well compared to those described previously for the Szeghalom Dome. All these rock types (orthogneiss, mafic-ultramafic xenolith, granite, sillimanite-biotite gneiss, garnet-bearing amphibolite, amphibole-biotite gneiss) are classified into three main units based on different metamorphic and deformation history. Understanding their relative spatial position permitted the elaboration of geologic map and sections of the MFD.

The 1160 Ma Hidderskog meta-charnockite: implications of this A-type pluton for the Sveconorwegian belt in Vest Agder (SW Norway)

The Hidderskog massif is a charnockitic intrusion located in the Rogaland-Vest Agder segment of the Sveconorwegian province of SW Norway. The U-Pb zircon age (1159 ± 5 Ma) and the Rb-Sr whole-rock isochron age (1153 ± 39 Ma) of this pluton are concordant. This age is interpreted as the magmatic emplacement age. The Hidderskog charnockitic intrusion is deformed and at a large scale concordant with amphibolite facies gneisses. It has been partially transformed to amphibole-biotite gneiss during the main Sveconorwegian orogeny (1040mdash;980 Ma in Rogaland). Major and trace element composition as well as isotopic systems have been only weakly disturbed during this event. The Hidderskog intrusion displays A-type geochemical features. Most samples could correspond to mixing of a granitic liquid with feldspar crystals. It may be correlated in age with the Gloppurdi and Botnavatnet intrusions of Rogaland and tentatively to the Hovdefjell and Gjeving charnockites of the Bamble sector. This group of charnockitic intrusions defines an intraplate anorogenic geodynamic environment prior to the main Sveconorwegian orogenic phase. These anorogenic charnockites are distinct from the charnockitic magmas spatially related to the Rogaland anorthosite complex, which intruded after the main Sveconorwegian orogeny within a short period of time, from around 950 to 930 Ma.