Calc-silicate Gneisses Research Articles

Relict Mesoarchean (2.99–2.94 Ga) metamorphism overprinted by late Neoarchean tectonothermal event(s) from the Sukma Group supracrustal rocks, Bastar craton, India: Evidence from new Lu-Hf and Sm-Nd garnet isochron and Th-U-total Pb monazite ages

Mesoarchean metasupracrustal successions can preserve evidence of tectonothermal reworking involving Paleoarchean felsic crust. Paleoarchean tonalite and trondhjemite of the Bastar craton, central India contain remnants of amphibolite-facies meta-supracrustal rocks of the Sukma Group, which likely represents the oldest Archean terrestrial sedimentary rocks. In order to constrain the time of metamorphic events combined garnet Lu-Hf and Sm-Nd geochronology and Th-U-total Pb dating of texturally preserved monazite from garnet-bearing metapsammite samples, representative of cover rocks overlying orthopyroxene-bearing tonalite basement, were conducted. Relics of an earlier garnet porphyroblast-forming metamorphic stage 1 (M1) developed at upper amphibolite-to granulite-facies conditions, are texturally distinct from the widespread metamorphic stage 2 (M2) marked by the garnet-breakdown to cordierite-anthophyllite symplectite. The M2-stage conditions, at ∼ 650–700 °C and ∼ 5.7 kbar, in the metasupracrustal remnants, are similar to those in garnet-amphibole-bearing quartzite and also as inferred from associated calc-silicate gneiss. Samarium-Nd chronology on whole-rock and porphyroblastic garnet fractions from three garnet-bearing quartzite samples yielded a weighted mean age of 2990 ± 10 Ma whereas Lu-Hf isotopic systematics on the same garnets from two of these quartzite samples yielded a weighted mean age of 2940 ± 31 Ma. Superposed Late Neoarchean M2 stage tectonothermal events include mylonitization of a ∼ 3.5 Ga basement tonaliteat 2525 ± 20 Ma and garnet formation within a Fe-rich quartzite at 2413 ± 21 Ma, as constrained from Sm-Nd isochron ages and similar to the in-situ weighted mean chemical age of 2367 ± 19 Ma of monazite included within M2-stage symplectitic cordierite around the older M1-stage garnet. The relict 2.99–2.94 Ga mid-crustal amphibolite-to granulite-facies M1 metamorphic assemblages stabilized within metasupracrustal rocks, formed in response to crustal thickening during Mesoarchean convergent tectonics, and were likely related to amalgamation of Paleoarchean crustal fragments.

Strengths and limitations of in situ U–Pb titanite petrochronology in polymetamorphic rocks: An example from western Maine, USA

AbstractTitanite is a potentially powerful U–Pb petrochronometer that may record metamorphism, metasomatism, and deformation. Titanite may also incorporate significant inherited Pb, which may lead to inaccurate and geologically ambiguous U–Pb dates if a proper correction is not or cannot be applied. Here, we present laser ablation inductively coupled plasma mass spectrometry (LA‐ICP‐MS)‐derived titanite U–Pb dates and trace element concentrations for two banded calcsilicate gneisses from south‐central Maine, USA (SSP18‐1A and SSP18‐1B). Single spot common Pb‐corrected dates range from 400 to 280 Ma with ±12–20 Ma propagated 2SE. Titanite grains in sample SSP18‐1B exhibit regular core‐to‐rim variations in texture, composition, and date. We identify four titanite populations: (1) 397 ± 5 Ma (95% CL) low Y + HREE cores and mottled grains, (2) 370 ± 7 Ma high Y + REE mantles and cores, (3) 342 ± 6 Ma cores with high Y + REE and no Eu anomaly, and (4) 295 ± 6 Ma LREE‐depleted rims. We interpret the increase in titanite Y + HREE between ca. 397 and ca. 370 Ma to constrain the timing of diopside fracturing and recrystallization and amphibole breakdown. Apparent Zr‐in‐titanite temperatures (803 ± 36°C at 0.5 ± 0.2 GPa) and increased XDi suggest a thermal maximum at ca. 370 Ma. Population 3 domains dated to ca. 342 Ma exhibit no Eu anomaly and are observed only in compositional bands dominated by diopside (>80 vol%), suggesting limited equilibrium between titanite and plagioclase. Finally, low LREE and high U/Th in Population 4 titanite dates the formation of hydrous phases, such as allanite, during high XH2O fluid infiltration at ca. 295 Ma. In contrast to the well‐defined date–composition–texture relationships observed for titanite from SSP18‐1B, titanite grains from sample SSP18‐1A exhibit complex zoning patterns and little correlation between texture, composition, and date. We hypothesize that the incorporation of variable amounts of radiogenic Pb from dissolved titanite into recrystallized domains resulted in mixed dates spanning 380–330 Ma. Although titanite may reliably record multiple phases of metamorphism, these data highlight the importance of considering U–Pb data along with chemical and textural data to screen for inherited radiogenic Pb.

Linking titanite U\u2013Pb dates to coupled deformation and dissolution\u2013reprecipitation

Titanite U–Pb geochronology is a promising tool to date high-temperature tectonic processes, but the extent to and mechanisms by which recrystallization resets titanite U–Pb dates are poorly understood. This study combines titanite U–Pb dates, trace elements, zoning, and microstructures to directly date deformation and fluid-driven recrystallization along the Coast shear zone (BC, Canada). Twenty titanite grains from a deformed calc-silicate gneiss yield U–Pb dates that range from ~ 75 to 50 Ma. Dates between ~ 75 and 60 Ma represent metamorphic crystallization or inherited detrital cores, whereas ~ 60 and 50 Ma dates reflect localized, grain-scale processes that variably recrystallized the titanite. All the analyzed titanite grains show evidence of fluid-mediated dissolution–reprecipitation, particularly at grain rims, but lack evidence of thermally mediated volume diffusion at a metamorphic temperature of > 700 °C. The younger U–Pb dates are predominantly found in bent portions of grains or fluid-recrystallized rims. These features likely formed during ductile slip and associated fluid flow along the Coast shear zone, although it is unclear whether the dates represent 10 Myr of continuous recrystallization or incomplete resetting of the titanite U–Pb system during a punctuated metamorphic event. Correlations between dates and trace-element concentrations vary, indicating that the effects of dissolution–reprecipitation decoupled U–Pb dates from trace-element concentrations in some grains. These results demonstrate that U–Pb dates from bent titanite lattices and titanite subgrains may directly date crystal-plastic deformation, suggesting that deformation microstructures enhance fluid-mediated recrystallization, and emphasize the complexity of fluid and deformation processes within and among individual grains.

Geological setting and mineralization characteristics of the Tick Hill Gold Deposit, Mount Isa Inlier, Queensland, Australia

The high grade (~22.5 g/t), gold-only Tick Hill deposit presents a unique mineralization style in the Mt Isa Inlier. The deposit was mined in the early 1990s, and is hosted in biotite schist, calc-silicate gneiss, quartzite and quartz-feldspar mylonite. These rocks were affected by D1 shearing and D2 upright folding at high-grade metamorphic conditions, followed by D3 events including normal faulting with widespread quartz-feldspar alteration and D4 strike-slip faulting. The Tick Hill ore body was linear in shape and paralleled the D1 mineral elongation lineation, and the intersection lineation between sets of D3 faults. Gold is dominantly hosted within intensely silicified units affected by D3 fracturing and associated alteration. However, some gold grains are contained within syn-D1, peak-metamorphic minerals indicating that mineralization involved two separate events; an early event predating or concomitant with D1 when gold was first introduced and concentrated, and a second event during D3 when gold was remobilized and/or further enriched to be deposited in its final configuration. Mineralization at Tick Hill is characterized by coarse grained gold associated with Bi-selenides, minor sulfides, and a general paucity of other metals except for a slight enrichment of Cu in the footwall to the ore zone. Alteration is characterized by early silicification and magnetite alteration that formed during the initial D1-2 events and may have coincided with the first stage of gold enrichment. During syn-D3 (re-)mobilization of gold, alteration involved the destruction of magnetite, the emplacement of abundant laminar quartz veins, and the deposition of proximal albite, hematite, chlorite, amphibole, epidote overprinted by later K-feldspar, sericite, clay minerals and minor calcite. The ore zone is bounded by a strongly silicified zone and surrounded by a chlorite-epidote shell. The low pressure D3 gold (re-)mobilization event involved an oxidized, S-under-saturated, acidic, saline aqueous fluid. In terms of alteration characteristics, the Tick Hill deposit shares similarities with IOCG deposits in the area. However, its high-grade gold-only nature, lack of Cu, and the presence of Bi-selenides makes it a unique deposit in the Mt Isa Inlier.

Geology and LA-ICP-MS U–Pb geochronology of the Nova Brasilândia belt, SW Amazonian Craton: New ages, re-evaluation of existing geochronological data, and implications for the evolution of the Sunsás orogen

New geological and LA-ICP-MS U–Pb geochronological data, combined with literature data, are used to characterize more precisely the timing of tectonic events in the Nova Brasilândia belt in the southwestern Amazonian Craton, Rondônia, Brazil. The U–Pb ages of zircon crystals were obtained in the rocks of the Rio Branco domain in the southern part of the belt. They reveal a period of sedimentation between 1241 ± 7.5 and 1137 ± 9 Ma and high-grade metamorphism between 1137 ± 9 and 1127.6 ± 2.3 Ma. Mafic-felsic magmatism of the Rio Branco Suite was dated between 1119.7 ± 2.7 and 1106.2 ± 2.8 Ma. U–Pb ages from specific groups of recrystallized zircon (cores and rims) extracted from high-grade calc-silicate gneiss (1019 ± 15 Ma), metagabbro (1016.7 ± 3.9 Ma), and anatectic granite (1011.3 ± 9.6 Ma) may represent episodes of metamorphic recrystallization and the youngest migmatization. The LA-ICP-MS U–Pb zircon ages establish the timing of two orogenic phases: an accretionary phase between approximately 1137 and 1106 Ma and overprinted metamorphism during the final collisional phase between approximately 1096 and 1011 Ma. The Nova Brasilândia orogeny recorded on the southwest margin of the Amazonian Craton can be described as an accretionary-collisional type that developed between approximately 1137 and 1010 Ma.

English

This paper investigates and reports the lithological and compositional features of Igue marble deposit in southwestern Nigeria. The deposit occurs as low-lying heterogeneous units under a humus-laden dark brown tropical soil within Igarra Schist Belt on south eastern flank of basement complex of southwestern Nigeria. The marble deposit occurs as lenses sandwiched between quartz-biotite schist, calc-silicate gneiss and polygenetic metaconglomerate intercalated with mica schist and quartzite all resting on ancient migmatite gneiss. The three colour variants of the marble which are white, grey, and foliated have average SiO2 contents of 2.92, 1.72 and 4.04% respectively. In the same order, average CaO contents are 60.76, 59.35 and 51.7%; MgO contents are 1.62, 1.42 and 2.57% indicating the marble is calcitic in nature. The oxides P2O5, TiO2 and MnO altogether constitute 0.09, 0.11 and 0.41% of the bulk chemistry of the white, grey, and foliated marble, respectively. Average loss on ignition (LOI) in white marble (33.46%), grey marble (36.12%) and foliated marble (33.49%) are relatively high suggesting high volatile contents. Chemical features of the marble deposit are comparable to similar rock types in Obajana and Ososo areas in Southwestern Nigeria. The marble differs significantly from Igbeti marble deposit which is dolomitic in nature. Marble deposits from different parts of Nigeria basement exhibits variable geochemical features that are influenced by their protoliths, mode of formation and associated lithologies and forms the basis for various economic uses. Key words: Igue marble, lithological and compositional features, Igarra Schist Belt, Nigeria, calcitic.

Calculating pressure with elastic geobarometry: A comparison of different elastic solutions with application to a calc-silicate gneiss from the Rhodope Metamorphic Province

Raman elastic geobarometry has increasingly been used complementary to metamorphic phase equilibria to estimate the conditions of recrystallization in metamorphic rocks. The procedure of applying Raman elastic barometry to host-inclusion mineral systems requires several steps that involve various assumptions. One of the most essential assumptions is that the mineral host-inclusion system behaves in an elastic and reversible manner. We discuss the discrepant results obtained by different authors employing different analytical solutions for elasticity and explore the assumptions lying behind each method. Furthermore, we evaluate numerically linear and non-linear elastic solutions and show their discrepancies. Both formulations are tested against recently published experiments on quartz inclusions in garnet (QuiG) at pressures up to 3 GPa, and we find a very good agreement between calculated and experimental pressure values (within 10% relative error). We subsequently apply our new elastic geobarometer to a calc-silicate gneiss from the Rhodope Metamorphic Province (N. Greece). The results of Raman elastic barometry combined with garnet-clinopyroxene geothermometry yield eclogite-facies conditions (~720 ± 40 °C, ~1.5 ± 0.2 GPa). These results are comparable to a high-temperature metamorphic overprint deduced from phase equilibria modeling in surrounding lithologies (730 ± 40 °C, ~1.2 ± 0.1 GPa). Our findings indicate that the estimated pressure from Raman elastic barometry is consistent with a significant viscous relaxation at high temperatures. We conclude that although Raman elastic barometry is a powerful tool for pressure estimation in metamorphic rocks, its pressure estimates do not necessarily correspond to entrapment conditions. Our results are consequential for the estimates of reaction overstepping in high-grade metamorphic rocks.

Petrography and geochemistry of Neoproterozoic charnockite–granite association and metasedimentary rocks around Okpella, southwestern Nigeria

The Neoproterozoic charnockite–granite association of Okpella intrudes metasedimentary and migmatite-gneiss complex rocks in the eastern Igarra Schist Belt, southwestern Nigeria. In order to unravel the complex processes involved in the formation and tectonic evolution of the Igarra Schist Belt during the Neoproterozoic, detailed field, petrographic and whole-rock geochemical study of the charnockite–granite association and metasedimentary rocks around Okpella was conducted. Published data on the metasedimentary rocks and Pan-African granitoids in different sectors of the belt were also compiled for detailed interpretations. The charnockites and granites of Okpella show primary magmatic mineralogy and geochemical characteristics. They are silicic (> 63 wt% SiO2), metaluminous to peraluminous, high-K calc-alkaline, ferroan, post-collisional granitoids. The garnet-biotite schist, calc-silicate gneiss and quartzite in the area are low–medium grade metasedimentary rocks. The mineralogy and geochemistry of the charnockites suggest that the charnockitic melt was derived from mafic lower continental crust through partial melting and assimilation-fractionation processes. The granites probably originated from mixed melts derived from lower- to mid-crustal tonalites-trondhjemites-granodiorites and/or subducted metagreywackes and mantle-derived magmas (probably the charnockitic melt and/or its progenitor). The granites and charnockites are coeval and were presumably emplaced during the post-collisional stages of the Pan-African Orogeny. The garnet-biotite schist, calc-silicate gneiss and quartzite represent metamorphosed immature to slightly mature sedimentary rocks, probably greywacke, marl and subarkose, respectively, that were sourced from intermediate–acid rocks which underwent low–moderate chemical weathering with minor contribution from recycled sediment sources and deposited in active continental environments. The Neoproterozoic evolution of the Igarra Schist Belt, therefore, involved deposition and infolding of sediments in active continental margin during the early Pan-African followed by upwelling of basaltic magma from the mantle which underplated and crystallized in the lower continental crust and was subsequently partially melted to generate the charnockitic and granitic melts through mantle-crust interaction during the late Pan-African.

Lithostratigraphy of the Copperton Formation, Areachap Group

Abstract The type area of the Copperton Formation is on the farms Vogelstruisbult 104, Somuspan 105 and Dooniespan 108 in Prieska District. Outcrop is poor and the type material is preserved in exploration borehole cores from the Prieska Copper Mines and the Annex Cu-Zn deposits. It is highly deformed and variably metamorphosed. Thus it is a lithodemic unit, but interpreted as a supracrustal sequence and described as a formation including lithologically distinct members. The Copperton Formation comprises a wide range of rock types including metabasic and intermediate gneisses with minor amounts of metapelitic and calc-silicate rocks. Metamorphic parageneses generally reflect amphibolite facies metamorphism, but granulite and retrograde greenschist facies zones also occur. The protoliths are interpreted as an arc-related volcano-sedimentary package and the Smouspan Member metadacite is dated at 1284 ± 9 Ma. Members are distinguished as follows: The Magazine Member is dominated by calc-silicate rocks which are dominant in outcrop but rarely found in borehole cores. The Smouspan Gneiss is a fairly homogeneous hornblende-biotite intermediate gneiss which is up to 400 meters thick in an isoclinal fold structure. It comprises the footwall to the Prieska Copper Mines Member, in which the massive sulphide orebody occurs, enclosed in an alteration assemblage comprising dark gedrite fels and strongly foliated, leucocratic quartz-perthite-sillimanite gneiss. The ore is interpreted as a volcanogenic massive sulphide deposit formed in a Mesoproterozoic island arc system. The Vogelstruisbult Member is the hanging wall unit, comprising mainly laminated amphibolites and metapelites, but also containing a variety of rock types including hornblende gneiss, biotite gneiss, chlorite schist, and calc-silicate gneiss. Away from the Prieska Mines orebody, a similar variety of rock types is found, and not subdivided but classified as Copperton Formation, a mappable unit. The same assemblage of rock types, including massive sulphide mineralization, was intersected in drill holes on the farms Kielder (portion of Doonies Pan 108), Eierdop Pan and Kantienpan to the north. The Copperton Formation is the southernmost unit of the Areachap Group which is exposed between Prieska Copper Mines and Areachap Mine north of Upington, where the Jannelsepan and Bethesda formations occur. The Copperton Formation is partly obscured in many places by Dwyka Group tillite cover which thickens southwards. A sequence of structural and metamorphic events affected the Copperton Formation and Areachap Group during the 1.2 to 1.0 Ga Namaqua-Natal orogeny. These involved collision of the Areachap Terrane with the Kaapvaal-Rehoboth Craton at about 1220 Ma, a thermal and deformational event coeval with the continental-scale Umkondo mantle event at about 1100 Ma, followed by uplift, erosion and the development of right-lateral shear zones of the Doornberg Lineament, with cooling below 300°C by 920 Ma. A Cambrian peneplain developed in the region which was first covered by Nama Group sandstones, then glaciated and covered by Permian Dwyka Group tillites which are presently being eroded to expose the Copperton Formation.

Age and Origin of Silicocarbonate Pegmatites of the Adirondack Region

Silicocarbonate pegmatites from the southern Grenville Province have provided exceptionally large crystal specimens for more than a century. Their mineral parageneses include euhedral calc–silicate minerals such as amphibole, clinopyroxene, and scapolite within a calcite matrix. Crystals can reach a meter or more in long dimension. Minor and locally abundant phases reflect local bedrock compositions and include albite, apatite, perthitic microcline, phlogopite, zircon, tourmaline, titanite, danburite, uraninite, sulfides, and many other minerals. Across the Adirondack Region, individual exposures are of limited aerial extent (<10,000 m2), crosscut metasedimentary rocks, especially calc–silicate gneisses and marbles, are undeformed and are spatially and temporally associated with granitic pegmatites. Zircon U–Pb results include both Shawinigan (circa 1165 Ma) and Ottawan (circa 1050 Ma) intrusion ages, separated by the Carthage-Colton shear zone. Those of Shawinigan age (Lowlands) correspond with the timing of voluminous A-type granitic magmatism, whereas Ottawan ages (Highlands) are temporally related to orogenic collapse, voluminous leucogranite and granitic pegmatite intrusion, iron and garnet ore development, and pervasive localized hydrothermal alteration. Inherited zircon, where present, reflects the broad range of igneous and detrital ages of surrounding rocks. Carbon and oxygen isotopic ratios from calcite plot within a restricted field away from igneous carbonatite values to those of typical sedimentary carbonates and local marbles. Collectively, these exposures represent a continuum between vein-dyke and skarn occurrences involving the anatexis of metasedimentary country rocks. Those of Ottawan age can be tied to movement and fluid flow along structures accommodating orogenic collapse, particularly the Carthage-Colton shear zone.

Savcılıebeyit (Kaman-Kırşehir) Gold Mineralization and Its Genetic Relationship With Post-Collisional Exhumation and Extensional Tectonics in Central Anatolia, Turkey

Gold exploration works in central Anatolia, Turkey, were intensified during the last decade, and potentially important gold mineralizations were discovered. In the region, important gold discoveries hosted by metamorphic and granitic rocks are, in the west, Savciliebeyit and Terziali (Kirsehir), in the middle part, Himmetdede and Mahmatlar (Kayseri), Akcatas (Nevsehir), and at the south-east , Gumusler (Nigde) mineralizations. The Savciliebeyit gold mineralization consists mainly of gold-quartz veins, and is the first discovered gold enrichment hosted by metamorphic rocks of west-central Anatolia. These veins are situated at the south-eastern margin of the Cefalikdag-Baranadag granite-migmatite dome, at 60 km west of the city of Kirsehir. In this study, preliminary results of geological and structural investigations of the gold-quartz veins are presented. The study area is part of the Palaeozoic-Mesozoic Kirsehir Massif that comprises mainly high-grade metamorphic and plutonic rocks covered by Cenozoic sediments. The gold-quartz veins are hosted by migmatite, migmatitic gneiss, gneiss-schist-marble intercalations, calc-silicate gneiss and marble. Quartz veins are discordant with respect to the foliation and major lithologic boundaries, and have sharp contacts with the enclosing metamorphic wall rocks. Veins occur as single or composite, relatively continuous veins that are up to about 2 km long and extend for at least 200 m down dip. Their thickness ranges from a few millimetres to two metres. The gold-quartz veins consist mainly of quartz, arsenopyrite, pyrite and secondary haematite and limonite. Other common minerals in the veins are calcite, muscovite, biotite, chlorite and epidote. The distribution of gold is erratic, and only weathered, haematite- and limonite-rich parts of the veins near the surface consistently display higher gold contents. Homogenisation temperatures obtained from fluid inclusions in the quartz vein have a wide range, from 160˚C to well above 400˚C. The fluid inclusions are generally rich in carbon dioxide and also have high salinities (20-33 % NaCl equivalent). The gold-quartz veins have many similarities to orogenic gold deposits in terms of metal associations, wall-rock alteration assemblages, mineralogy, formation conditions and structural control. The spatial association of the veins with migmatites, high-grade metamorphic rocks and granitoids suggests an origin related to metamorphism, uplift and/or migmatite doming and granitoid emplacement in central Anatolia.

The geologic record of the exhumed root of the Central African Orogenic Belt in the central Cameroon domain (Mbé – Sassa-Mbersi region)

We present structural, petrological, geochemical and geochronological data compiled in a new geological map on the high-grade metamorphic rocks exposed in the Mbé – Sassa-Mbersi region located along the Tcholliré-Banyo Shear Zone, at the northern edge of the Central Cameroon domain.The region exposes a complex assemblage of metabasic (hornblendite, metagabbro, amphibolite) and metasedimentary (garnet paragneiss, calc-silicate gneiss) rocks alternating with ubiquitous migmatitic intermediate to felsic gneisses. U-Pb ages on detrital zircon in paragneiss point to the contribution of Archean, Paleoproterozoic and Neoproterozoic sources, and suggest a maximum deposition age of 725 ± 12 Ma for their sedimentary protolith. The protolith of some amphibolites was emplaced during the Neoproterozoic as indicated by U–Pb ages on magmatic zircon at 600 ± 4 and 599 ± 6 Ma. Plutonic rocks with I-type chemical signatures were emplaced discontinuously over a period of ca. 80 Ma. The magmatic evolution started with the intrusion of magnesian medium-K tonalites and trondhjemites at 651 ± 3 Ma, followed by magnesian high-K hornblende-biotite granites and ferroan shoshonitic biotite granites until 599 ± 2 Ma. Finally, pegmatite dykes were emplaced along dextral shear zones at 573 ± 2 Ma. The metamorphic rocks show a penetrative shallow-dipping foliation partially to totally transposed in a NE–SW trending steep-dipping foliation bearing a shallow-dipping lineation. These ductile syn-migmatitic fabrics, developed under high-grade metamorphic conditions in the presence of melt, between the metamorphic peak at ca. 1.3 GPa–770 °C, and the following isothermal decompression down to ca. 0.8 GPa. In situ U–Pb dating of metamorphic zircon rims from a garnet migmatitic paragneiss yields an age of 582 ± 4 Ma. Localization of high-grade deformation is indicated by the development of the Tcholliré-Banyo Shear Zone, characterized by mutually crosscutting subvertical and shallow-dipping mylonitic to ultramylonitic shear zones associated with sinistral and top-to-the SW kinematic criteria, respectively. These data indicate that the Central Cameroon domain exposed in the Mbé – Sassa-Mbersi region, represents the exhumed mid-to lower part of the former orogenic root of the Pan-African Central Africa Orogenic Belt that has undergone partial melting, lateral flow and intrusion of mafic to felsic calc-alkaline magmas between 650 and 580 Ma. The waning stage of the Pan-African orogeny is marked by the emplacement of syntectonic pegmatitic granite at ca. 575 Ma in subvertical E-W trending retrogressive dextral shear zones.

Fluid-driven resetting of titanite following ultrahigh-temperature metamorphism in southern Madagascar

LA-ICP-MS U-Pb dates and EPMA trace-element maps of titanite were collected from calc-silicate gneisses of the Ikalamavony (peak T: 750–800 °C) and Anosyen (peak T: 900–950 °C) domains in southern Madagascar to evaluate how titanite responds to high-temperature metamorphism, cooling, and retrogression. Fluid-mediated replacement of precursor titanite by titanite domains of different composition (interface-coupled dissolution-precipitation, ICDR) was the primary mechanism by which titanite was reset following high-grade metamorphism. Comparison of titanite U-Pb dates (530–490 Ma) with independent petrology and thermochronology indicates that the alteration occurred at temperatures as low as 300–500 °C. Apparent Zr temperatures (temperatures calculated assuming titanite–quartz–zircon equilibrium) in altered titanite are less than or equal to the metamorphic peak, but higher than the inferred alteration temperature, implying that Zr was removed, but that titanite–quartz–zircon equilibrium was not achieved during alteration. Although evidence for ICDR was observed in titanite samples separated by 10's to ~100 km, differences in U-Pb dates among samples, and even among titanite grains in the same thin section suggest that alteration at any given time was localized.

U-Pb zircon and titanite ages from granulites of the Koraput area – Evidence for Columbia, Rodinia and Gondwana from the Eastern Ghats Province, India

The Eastern Ghats Province (EGP) of India, characterized by ∼1 Ga granulite metamorphism (M1), collided with the Archaean Indian craton in either Grenvillian (∼1 Ga) or Pan-African (∼0.6–0.5 Ga) time. At ∼850 Ma, the Koraput Alkaline Complex (KAC) intruded into M1 EGP granulites, and was metamorphosed during a second granulite event (M2) in the EGP. U-Pb zircon dates from gneisses in Koraput yield magmatic ages ∼1500 Ma, with overgrowths at 1100–900 Ma and 800–700 Ma. Cratonic zircons from the EGP contact zone have Archaean 2.8–2.6 Ga cores with overgrowths at ∼700 Ma. Titanites from a calc-silicate gneiss within the EGP define a U-Pb isochron around 700 Ma, whereas titanites from nepheline syenites within the KAC yielded a range of dates from ∼622 to 520 Ma. The c. 1500 Ma age in the EGP either implies Columbia break-up or proximity to a Columbia margin. The ∼700 Ma age is interpreted as the age of M2 granulite metamorphism that followed intracontinental thrusting of the EGP over ∼2.8 Ga granulites of the Jeypore Province. The ∼520 Ma titanite ages are correlated with final juxtaposition of the EGP-Jeypore Province amalgam with the Indian craton during Gondwana formation.

Деформаційний аналіз групи порід басейну Пур, Індія

Precambrian rocks exposed around Pur are a part of Pur-Banera belt that is stratigraphically placed within Aravalli Quartzite, Calc Silicate Gneiss, Quartzite and Garnetiferous Mica Supergroup. The different lithological units exposed in the area are Banded Magnetite Schist. These rocks have been subjected to multi-deformational episodes. In accordance with the modern system of multi-level deformation analysis proposed by Fedor Yakovlev in 2012, the results of the The work belongs to the scientific direction of tectonophysics, which was called “strain analysis of linear folding structures”. The present study involves analysis of strain in the rocks around Pur, Bhilwara District, Rajasthan (India). The deformation analysis at the level of folds (level three of seven) are presented in this work. The outcrop pattern is an isoclinal fold of D2 generation, whose axis plunges steeply NE’ly with an axial plane trending NNE-SSW. The quartzite band in this Pur Synform was sampled for the purpose of strain analysis. This section parallel to the YZ plane of finite strain ellipsoid was prepared (perpendicular to both Schistosity and the lineation). The accepted methods used to estimate strain are a) Fry method and b) “Isogon Rosette” method. The axial ratio of the strain ellipse and the orientation of the long axis of the strain ellipse were computed for every location. It was seen that the strain ratio is nearly uniform in magnitude. The orientation of the long axis of the strain ellipse (Y direction of the finite strain ellipsoid) is also constant in space and parallel to the axial trace of D2 Synform. From the analysis it was suggested a strong flattening following the buckling. The amount of this flattening strain was computed using the”Isogon rosette” method. We calculated reflects the flattening strain recorded in the rock with the very high constancy of the strain ratio and the orientation of the long axis of the strain ellipse irrespective of the methodology adopted. The results of performed researches bring tectonophysics database up-to-date providing an excellent opportunity for provenance analysis of the Pur-basin using the different types of undulose extinction of the quartzite for development of prospective mining projects around the area including iron deposits.

High-grade calcareous metasediments from the Sawtooth Metamorphic Complex, Idaho, USA: evidence for passive margin strata and polymetamorphism within the Idaho batholith

ABSTRACTThe Sawtooth Metamorphic Complex (SMC) of central Idaho contains exposures of metasupracrustal rocks, in a crucial location between the Archaean Wyoming craton to the east and the adjacent Mesozoic terranes to the west, that provide constraints on Precambrian crustal evolution in the northwestern United States. Mineral textures, whole-rock geochemistry, and thermobarometry of calc-silicate gneisses record multiple stages of crustal evolution, including protolith deposition, burial and multiple metamorphic and deformational overprints. Whole-rock signatures are consistent with derivation from post-Archaean, continental sources that have undergone sedimentary maturation and recycling typical of clastic sedimentation in passive-margin environments and with a metamorphosed sequence of calcareous sandstones and marls, containing varying proportions of clay and quartzo-feldspathic detritus. The sandstone-to-marl continuum may reflect a shallow-to-deep water transition in the depositional environment of the calc-silicate protoliths. Two metamorphic events and three deformational events are preserved. The assemblage clinopyroxene + quartz + plagioclase + K-feldspar + rutile represents peak metamorphic conditions (M1), estimated at 750–775°C with relatively oxidized metamorphic fluids, during development of D1a foliation. Deformation twinning in clinopyroxene records a widespread, high strain D1b event at high temperature. A second static thermal event (M2), associated with randomly oriented amphiboles overprinting M1 and D1b features, records conditions of ~550–725°C and H2O-rich fluids. Late-stage, brittle–ductile D2 deformation is characterized by mylonitic lenses of quartz, fractures within M1 clinopyroxenes that crosscut D1b deformation twins, and localized fracturing of M2 amphiboles. Geochemical and mineral chemical signatures of SMC calc-silicates preserve fingerprints of the original protolith through burial to mid-crustal conditions and high-grade metamorphism and suggest crustal thickening to about 20 km. The SMC may reflect rocks at depth beneath the Idaho batholith. These data, indicative of newly recognized post-Archaean terrain with passive-margin, continental sediments suggest that a continuous Cordilleran passive margin sequence may have extended along the western edge of Laurentia.

Characterization and assessment of Okpella metacarbonate deposit in Nigeria

The metacarbonate deposits in the southern Nigeria intercalated with calc-silicate gneiss, intrusive granite and migmatite gneiss rocks are of Proterozoic age. The study focused on examining the suitability of the metacarbonate via characterisation for industrial applications. This study is based on X-ray diffractometry (XRD), scanning electron microscopy (SEM), polarizing/reflected light microscopy, chemical and differential thermal analysis/thermal gravimetric analysis (DTA/TGA). The calcite is the dominant phase while silica is the subordinating component of the metacarbonate. The metacarbonate is characterized with limited water absorption, appreciable compressive strength, high loss of ignition and bulk specific gravity. Quicklime calcined at 900, 1100 °C for 60, 90 and 120 min of soaking time were unreactive due to incomplete calcination and densification of the pore surface of the produced quicklime. However, the quicklime calcined at 1000 °C exhibited high reactivity after complete thermal decomposition associated with its unique microstructure. Marble has affluent industrial applications attributed to its established microstructure, chemical composition and thermal decomposition behaviour.

Seismic anisotropy in mid to lower orogenic crust: Insights from laboratory measurements of Vp and Vs in drill core from central Scandinavian Caledonides

Recent drilling of the first Collisional Orogeny in the Scandinavian Caledonides scientific borehole (COSC-1) near Åre, Sweden permitted a laboratory investigation of seismic anisotropy on high metamorphic grade and highly deformed core samples. The 2.5km deep borehole crosscuts the amphibolite-grade Lower Seve Nappe and intersects a high-strain shear zone in the lowermost 800m. Measurements of ultrasonic compressional (Vp) and shear (Vs) velocities are conducted at room temperature and pressures ranging from room conditions up to 260MPa on six core sections that represent the most abundant lithologies in the borehole. The core sections consist of two amphibolites, a calc-silicate gneiss, a felsic gneiss, an amphibole-rich gneiss, and a garnet-bearing micaschist from the shear zone. Three mutually perpendicular samples were taken to characterize the anisotropy induced by the clear foliation and lineation. The intrinsic (crack-free velocities) Vp0 and Vs0 in the direction perpendicular to foliation ranges from 5.51 to 6.67km/s and 3.31 to 4.13km/s, respectively. In the direction parallel to foliation the Vp0 and Vs0 ranges from 6.31 to 7.25km/s and 3.53 to 4.35km/s, respectively. Vp anisotropy ranges from 3% in the calc-silicate gneiss to 19% in the micaschist. In the upper crustal seismic reflection survey around the COSC-1 borehole, reflection coefficient analysis suggests that reflectors occur due to impedance contrast between commonly occurring amphibolites and gneisses in the upper 1800m and the micaschists below. When extrapolated to mid-crustal levels the analysis indicates that both rock types can produce reflection coefficients between adjacent lithologies in excess of 0.1. Similarities in lithologies, shear zone thickness, and reflectivity pattern in the Central Scandinavian Caledonides compared to other orogens (e.g., the Himalaya) demonstrate the importance of these measurements as a proxy for in-situ strongly anisotropic shear zones in the middle crust.

A paleoproterozoic intra-arc basin associated with a juvenile source in the Southern Brasilia Orogen: Application of U–Pb and Hf–Nd isotopic analyses to provenance studies of complex areas

Early Proterozoic sedimentary basins are an important record of crust generation processes and consequently a fundamental key to unraveling Earth's evolution through geological time. Sediments within the basins are typically deformed and metamorphosed by subsequent tectonothermal events, which can obliterate their links to source terranes. Nd-whole-rock and detrital zircon U–Pb and Lu–Hf isotopic analyses are among the most reliable tools to be used in provenance investigations, since zircon is a resilient mineral and the Sm–Nd system is not extensively modified during metamorphism. These methods have been applied to a study of the provenance and tectonic setting of the São Vicente Complex, preserved in a Neoproterozoic passive margin related allochthon within the Southern Brasilia Orogen. The complex consists of siliciclastic and calc-silicate gneisses with mafic and minor ultramafic rocks, which were deformed and metamorphosed during late Neoproterozoic collision between the Paranapanema Block and the São Francisco-Congo plate. Detrital zircons indicate derivation from a juvenile Paleoproterozoic source terrane (peaks of crystallization ages of ca. 2130Ma, 2140Ma and 2170Ma; ɛHft between +0.1 and +6.0; NdTDM=2.31–2.21Ga; ɛNdt=+1.6 to +2.8), with a minor contribution from older continental crust. Interlayered amphibolite rocks, with juvenile signatures (ɛHft=+5.8 to +8.2; NdTDM=2.14 and 2.30Ga; ɛNdt=+2.2 and +3.2), yielded similar ages of 2136±17 and 2143±14Ma, suggesting syn-sedimentary magmatism. Thus, the maximum age of deposition at around 2130Ma represents the best estimate of the depositional age of the complex. The dominance of detrital zircons ages close to the age of deposition, along with syn-sedimentary magmatism, imply a convergent margin basin tectonic environment for the São Vicente Complex, with similarities to fore arc basin and trench deposits. Amphibolite and meta-sedimentary rocks point to important juvenile magmatism around 2.14Ga. Juvenile Rhyacian (ca. 2.1Ga) granite-granodiorite-tonalite orthogneisses with arc-related geochemical signatures (Pouso Alegre Complex) that override the São Vicente Complex, are the probable main source of detritus within the complex. Both basin and source were part of the southern edge of the São Francisco plate during the assembly of West Gondwana, and served as sources for early Neoproterozoic passive margin related basins. The age of intrusive anorogenic A-type Taguar granite indicates that by 1.7Ga the São Vicente Complex was in a stable tectonic environment.

Evolução metamórfica e estrutural do Complexo Várzea do Capivarita, Cinturão Dom Feliciano, Encruzilhada do Sul, RS

O Complexo Várzea do Capivarita está representado por paragnaisses dispostos como megaxenólitos nos granitos da Suíte Encruzilhada do Sul e como fragmentos menores nos granitos Quitéria e Cordilheira. Essas unidades graníticas afloram no extremo norte do Batólito Pelotas, porção leste do Cinturão Dom Feliciano, próximo a Encruzilhada do Sul. O complexo é constituído por gnaisses pelíticos, com ocorrência subordinada de gnaisses calci-silicáticos, mármores, gnaisses quartzo-feldspáticos e raros quartzitos. Os gnaisses pelíticos possuem injeções de corpos tabulares de muscovita leucogranitos peraluminosos com espessuras centimétricas a métricas. Neste trabalho foram feitas análises petrográficas e microestruturais de seções delgadas representativas das litologias do complexo e a caracterização estrutural da área de estudo. A integração desses dados permitiu avaliar a migmatização dos paragnaisses e a geração das injeções leucograníticas. As condições metamórficas estabelecidas com base nas paragêneses identificadas indicam temperaturas da ordem de 850 a 1000°C e pressões entre 6 e 10 kbar, caracterizando um metamorfismo orogênico de fácies granulito, da série de pressão intermediária e de ultra-alta temperatura (UAT). A concordância estrutural entre o bandamento (S2) dos paragnaisses e a foliação ígnea (S0 ) do leucogranito indica que ambas foram formadas no mesmo evento, implicando que a migmatização dos paragnaisses do Complexo Várzea do Capivarita gerou os leucogranitos peraluminosos durante o ápice do metamorfismo orogênico colisional.