Chlorite Phyllites Research Articles

Geochronological evolution of the Pitangui greenstone belt, southern São Francisco Craton, Brazil: Constraints from U-Pb zircon age, geochemistry and field relationships

The Pitangui greenstone belt (PGB) – located in the southern São Francisco Craton, (Brazil) – is a volcano-sedimentary assemblage that is tectonically imbricated over the orthogneiss and granitoid rocks collectively termed the Divinópolis Complex. One diorite gneiss sample from the northern Divinópolis Complex yields an igneous zircon age of 2878 ± 5.7 Ma, constraining the time of crystallization. The basal sequence of PGB predominantly comprise metabasalt with restricted bodies of metagabbro/metadiorite, metaultramafic volcanic rocks and BIF. The upper sequences of the PGB exhibit a maturation of the basin, grading to more felsic and sedimentary lithologies, such as clastic metasedimentary rocks, intermediate metavolcanic rocks, and metapyroclastic rocks. The first recognized deformation phase in the PGB (Dn) is associated with greenschist facies conditions and the development of the main tectonic foliation. The second deformation phase (Dn+1) is associated with amphibolite facies metamorphism, while a later (Dn+2) phase may be related to younger Proterozoic deformation (ca. 2.0 Ga). PGB metabasalts yield a crystallization age of 2740.2 ± 12 Ma and are characterized by a tholeiitic affinity and flat LREE patterns. Based on mixed MORB- and arc-affinities, a back-arc basin setting is suggested for the formation of PGB basalt. This mafic magmatism (2740 Ma) is slightly younger than the ages reported for the Rio das Velhas greenstone belt, with which the PGB is generally correlated. The metasedimentary rocks of the PGB have age intervals spanning from 2690 to 3250 Ma. One mica-quartz phyllite has a maximum depositional age of 2769.8 ± 8.9 Ma while a chlorite phyllite records and age of 2693.2 ± 7.9 Ma. The metasedimentary rocks are derived from a source area of mafic to felsic composition that is similar to TTGs, cratonic Archean shales and E-MORB. The predominant sources were likely the sialic Archean crust of the granite-gneiss complexes (e.g. the Divinópolis Complex and potassic granitoids of the Mamona Event) and the mafic sequences of the arc (e.g. PGB back-arc basalt). One late potassic pluton (Casquilho monzogranite) crosscutting the fabrics of the volcano-sedimentary assemblage and yields an intrusion age of 2694.7 ± 9 Ma, constraining the minimum sedimentation age assembly of the package. Collectively, a Neoarchean primitive island arc or back-arc basin is proposed to have generated the PGB, which is here interpreted as distinct from the Rio das Velhas greenstone belt. We suggest that PGB developed during an accretion-collision event in the southern São Francisco Craton before cratonic stabilization of the crust by the end of the potassic 2760-2680 Ma Mamona Event.

Iron isotopic variations of the Cryogenian banded iron formations: A new model

The Fe isotope composition of banded iron formation (BIF) is regarded as a powerful tracer in paleoceanography, and the Cryogenian banded iron formation associated with “Snowball Earth Events” provides a precious record of environmental change in the Neoproterozoic. However, Fe isotope studies on Cryogenian BIFs are rare and previous interpretations of Cryogenian Fe isotope data are problematic. Here we present a systematic investigation of the Fe isotope composition of the Cryogenian Xinyu BIF sections from four localities in the Yangtze region, South China. These BIF sections comprise banded magnetite quartzite, banded chlorite-magnetite quartzite, and magnetite-bearing chlorite phyllite, in stratigraphically ascending order. The δ56FeIRMM-014 values of the Xinyu BIF vary significantly and show an overall increase upsection, from ca. 0‰ to ca. 1.5‰. This stratigraphic trend in Fe isotope compositions is similar to those reported previously for Cryogenian BIFs in North America and Australia, and thus seems to be a common phenomenon. We interpret Fe isotope variation in Cryogenian BIF to be essentially controlled by varying degrees of Fe precipitation in seawater, rather than resulted from “a water column Fe isotope gradient” as proposed previously. The variation in the degree of Fe precipitation can be controlled by changes of Eh and/or pH conditions in seawater resulting from transgression or ocean acidification.

Deciphering structures and deformation of the Obuom Gold Prospect, Central Ashanti Belt of Ghana: A lithostructural approach

The Obuom Gold Prospect is located in the central part of the Paleoproterozoic Birimian Ashanti belt of southwest Ghana. The prospect is approximately 45 km2 and is underlain by basalt, andesite and dacite volcanic beds, graphitic and chlorite phyllites of sedimentary origin and granites. Published works on the Obuom geological terrane, using gravity models, showed that, the structures are more complex than previously thought and may involve multiple thrust slices. This research identified two deformational episodes; the first has a remarkable sinistral sense probably synchronous with an Eoeburnean event with the second, an overprint of dextral shearing during the Eburnean event. The first episode, Do1 is characterized by NW-SE compression which resulted in subvertical NE-SW trending S1 axial plane metamorphic foliation in tight to isoclinal F1 folds with NW-SE trending L1 lineations. The second episode, Do2 indicates continuous shortening which produced nearly upright F2 folds with east-west striking axial planes, slight plunging hinges and associated subparallel crenulation cleavages. The first deformational event Do1 which represent NW-SE compression is observed affecting only the Sefwi Group. The Do2 resulting from N-S compression affected both the Sefwi and Kumasi Groups.

Geochronological and geochemical constraints on the origin of clastic meta-sedimentary rocks associated with the Yuanjiacun BIF from the Lüliang Complex, North China

Abstract The Luliang Complex is situated in the central part of the western margin of the Trans-North China Orogen (TNCO) in the North China Craton (NCC), and consists of metamorphic volcanic and sedimentary rocks and granitoid intrusions. The Yuanjiacun Formation metasediments occupy roughly the lowest part of the Luliang Group and are mainly represented by well-bedded meta-pelites (chlorite schists and sericite–chlorite phyllites), banded iron formations (BIFs) and meta-arenites (sericite schists), which have undergone greenschist-facies metamorphism. The youngest group of detrital zircons from the meta-arenite samples constrains their maximum depositional age at ~ 2350 Ma. In combination with previous geochronological studies on meta-volcanic rocks in the overlying Jinzhouyu Formation, the depositional age of the Yuanjiacun Formation can be constrained between 2350 and 2215 Ma. The metasediments have suffered varying degrees of source weathering, measured using widely employed weathering indices (e.g., CIA, CIW, PIA and Th/U ratios). Source rocks of the low-Al meta-pelites have undergone severe chemical weathering, whereas those of the meta-arenites and high-Al meta-pelites have suffered relatively moderate chemical weathering. Significant secondary K addition is recognized in the A–CN–K diagram for most of the studied samples. Diagnostic geochemical features like the Al 2 O 3 /TiO 2 values, trace element ratios (e.g., Th/Sc) and REE patterns, suggest that the meta-arenites and high-Al meta-pelites are predominantly derived from felsic igneous sources, whereas the low-Al meta-pelites are sourced mainly from mafic rocks. Coupled with Nd isotopic data, it is proposed that the meta-arenites and high-Al meta-pelites were sedimentary erosion products of the less differential felsic terrain (likely the old upper continental crust). The low-Al meta-pelites, however, have geochemical affinities with both pelite- and BIF-like components, suggesting that they were mixtures of these two sediments. Moreover, the U–Pb ages of detrital zircons from the Yuanjiacun metasediments yield three age populations of 2.8–2.65 Ga, 2.6–2.5 Ga and 2.5–2.35 Ga. The dominant Archean detrital zircons were most likely sourced from both the Archean igneous rocks in the Hengshan–Wutai–Fuping belt and old hidden Archean basement rocks in the Western Block, whereas the subordinate Paleoproterozoic detrital zircons were probably derived from the granitoid plutons within this age range in the Luliang Complex. Taking the geochemical and geochronological data together, the Yuanjiacun Formation is interpreted as having been deposited in a passive margin setting (likely the eastern margin of the Western Block), most probably on a stable continental shelf.

Combined U–Pb geochronology and Lu–Hf isotope systematics by LAM–ICPMS of zircons from granites and metasedimentary rocks of Carrazeda de Ansiães and Sabugal areas, Portugal, to constrain granite sources

The Carrazeda de Ansiães and Sabugal areas in the Central Iberian Zone of the Iberian Variscan Belt contain abundant S-type granites formed during the post-thickening stage of the Variscan orogeny (syn- to post-D3, i.e. the last ductile deformation phase). Magmatic zircons and inherited zircon cores from two granites (G and G5), magmatic zircons from three other granites (G1, G4 and G6) and detrital zircons from a chlorite phyllite and a metagraywacke were dated by LAM–ICPMS. Hf isotope data were obtained by the same method to constrain the sources for granites. Some granites have zircons with negative εHft values (G: from −8 to −4; G1: from −8 to −1; G4: from −6 to −3), suggesting an origin from heterogeneous crustal anatectic melts or, from mantle-derived magmas contaminated in the crust. Magmatic zircons from granites G5 and G6 show a larger variation, yielding also some positive εHft values (G5: from −8 to +6; G6: from −4 to +4) indicating crystallization from mildly contaminated mantle-derived magmas or from a lower crustal source. U–Pb isotope data of inherited zircon cores from granites G and G5 show that Neoproterozoic ages dominate, but only the inherited zircon cores from granite G have similar initial 176Hf/177Hf ratios to those from a chlorite phyllite of the Formation of Ervedosa do Douro (Carrazeda de Ansiães area), suggesting that this metasedimentary unit could have been involved in the genesis of the granite G. On the other hand, the Lu–Hf data suggest that the metagraywacke from Sabugal area should not be considered a component involved in the partial melting process that formed the granite G5. The occurrence of Mesoproterozoic detrital zircons in both metasediments suggests that these rocks are likely to have evolved from a peri-Amazonian realm, close to West Avalonian terranes, during the Neoproterozoic/Early Palaeozoic. However, a West African Craton provenance could also be considered, if a long distance river transportation of Mesoproterozoic zircons, from the southwest Baltica or Arabian–Nubian Shield, is admitted. The U–Pb isotope data obtained in detrital zircons from both metasedimentary rocks suggest that their maximum depositional ages are ca. 578Ma for the chlorite phyllite and ca. 577Ma for the metagraywacke.

P– T modeling of kyanite and sillimanite paramorphs growth after andalusite in late Paleozoic Pyeongan Supergroup, South Korea: Implication for metamorphism during the Mesozoic tectonic evolution

Metasedimentary rocks of the Pyeongan Supergroup in the northeastern flank of the Taebaeksan Basin, South Korea, commonly contain two Al 2SiO 5 polymorphs (And + Ky and And + Sil), or more rarely, three coexisting Al 2SiO 5 polymorphs. Lower-grade rocks preserve chloritoid + andalusite ± kyanite in muscovite–chlorite phyllite, and higher-grade rocks contain andalusite + sillimanite (± kyanite) + staurolite ± garnet in mica schists. Kyanite and sillimanite occur as partial paramorphs after andalusite. Textural relations and relative metamorphic P– T conditions of the Al 2SiO 5-bearing mineral assemblages suggest a crystallization sequence of andalusite → kyanite → sillimanite, with the growth of staurolite and garnet in the kyanite and sillimanite stability fields. Metamorphic P– T conditions determined from geothermobarometry, compositional isopleths of chloritoid and garnet, and petrogenetic grids in the MnO–K 2O–FeO–MgO–Al 2O 3–SiO 2–H 2O (MnKFMASH), MnKFMASH–T(TiO 2)–O(Fe 2O 3), and Mn–N(Na 2O)–C(CaO)–KFMASH systems could be divided into two regimes temporally: a clockwise path at lower temperature (LT)/medium pressure (MP) conditions, and higher temperature (HT)/low pressure (LP) conditions. The peak metamorphic P– T conditions for regional LT/MP metamorphism are ca. 5.0–6.0 kbar at 550–580 °C, and those for HT/LP metamorphism are ca. 3.0–4.8 kbar and 590–610 °C. Kyanite paramorphs after andalusite may result from a LT/MP clockwise P– T path associated with a rapid contractional deformation during the early to middle Mesozoic, and then sillimanite paramorphs after andalusite may have been formed by magmatic heating due to the later intrusion of leucocratic granite.

A Late Tournaisian synmetamorphic folding and thrusting event in the eastern Variscan foreland: 40Ar/39Ar evidence from the phyllites of the Wolsztyn–Leszno High, western Poland

A Variscan foreland in western Poland comprises two NW-trending basement highs, which are concealed under Carboniferous through Triassic strata of the Fore-Sudetic Monocline (FSM). Both highs consist of multiply deformed quartz-sericite ± albite ± chlorite phyllites of unknown protolith age. 40Ar/39Ar laser probe dating of white micas in up to 0.5-mm-thick mica layers, which form the S2 axial-plane foliation in phyllites of the Wolsztyn–Leszno High, yielded an age of 340.1±2.6 Ma for the lower greenschists facies metamorphism and probably also for the F2 folding. This deformation was associated and followed by thrusting, which brought about the basement highs. The latter delivered clasts to overlying late Visean–early Namurian flysch basin that was mainly sourced from the Saxothuringian Sudetes in which most of the deformation occurred between 345–335 Ma. The FSM basement may represent an independent terrane, referred to as the Wielkopolska terrane, belonging to the Armorican Terrane Assemblage.

Stratigraphy, structure, and geochronology of the Anyox Pendant, northwest British Columbia, and implications for mineral exploration

Anyox Pendant contains complexly deformed greenschist-facies volcanic strata overlain by a thick succession of turbidite sedimentary rocks. At the contact between the two are well-studied copper-rich volcanogenic massive sulphide deposits; however the age and correlation of the volcanic strata, including the ore horizon, are poorly known. New stratigraphic, structural, and geochronology studies elucidate the regional stratigraphic, tectonic, and metallogenic settings and significance of these strata. The thick turbidite succession is correlated with Bowser Lake Group and conformably overlies the volcanic succession. Volcanogenic units include pillowed basalt, volcaniclastic rocks, chlorite phyllite, tuffaceous siltstone, and abundant gabbroic sills and dykes. Strained leucocratic intrusions yielded U–Pb zircon crystallization ages of 176.9 ± 0.2 Ma and 185.6 ± 0.3 Ma. Volcaniclastic rocks yielded detrital zircons that constrain the maximum ages of the volcanic succession. The youngest zircon grains from four samples in different structural segments limit strata to younger than ca. 186 Ma, ca. 178 Ma, ca. 183 Ma, and ca. 186 Ma. The results, combined with the regional oldest age limit of Bowser Lake Group, constrain the volcanic succession to late Early Jurassic(?) and early Middle Jurassic age, and support correlation with Hazelton Group of Stikinia. Furthermore, they enable comparisons with the coeval Au-Ag-rich stratiform Eskay Creek deposit, expanding southward the zone of extension in western Stikinia, which tapped primitive mafic magmas and accompanied the last vestiges of Stikinian volcanism in the early Middle Jurassic.

Geochemical Characteristics of Archean Clastic Metasediments of Gadag Gold Field, Southern India: Implications for Provenance and Tectonic Setting

The Gadag Gold Field in the western Dharwar craton consists of a thick pile of sediments, apart from the volcanic rocks. These rocks have undergone metamorphism of greenschists to lower amphibolite facies. The clastic metasediments (metagreywacke, chlonte phyllite and quartz-sericite phyllite) constitute the prominent lithounits in the eastern half of the area. Metagreywackes consist of abundant lithic fragments and are designated as lithic greywackes. Their mineralogical maturity index is very low (0.4). Metagreywackes have higher SiO, and Ba contents, lower Al, O, FeO', MgO and CaO contents than chlorite phyllites. Compared to chlorite phyllites, the quartz-sericite phyllites have higher SO, YO, Ba, Zr and Sr contents, and lower FeO', MgO, CaO, Ni, Co, Cr and V contents. YO/Na, O ratios of metagreywackes and chlorite phyllites are typically less than 1, and are chemically immature. Further, these sediments are characterised by higher concentrations of SiO, and Zr, higher SiOJMgO and Zr/Y ratios, suggesting felsic component in the provenance. While higher abundance of Cr and Ni with K, O/MgO ratios less than 1 and low O/MgO ratios suggest a more basaltic nature of the source area. High L a m ratios (42), negative Eu anomaly and overall REE enrichment in metagreywackes indicate a component of granite-granodioritic rocks in the provenance. The polymictic metaconglomerate with pebbles of gneiss, granite, quartzite, andesite, rhyolite and the metagreywackes with lithic fragments like quartzites, chert, andesite and rhyolite suggest a sialic dominant, mixed felsic and mafic source in the provenance. The geological setting, associated rock types, their field relationships and primary sedimentary structures coupled with petrographic and geochemical characteristics plus their modeling through various tectonic discrimination diagrams indicate that metagreywackes have continental island arc + active continental margin, chlorite phyllites have continental island arc + oceanic island arc and quartz-sericite phyllites have active continental margin + passive margin tectonic settings. It is suggested that these clastic metasediments of Gadag Gold Field, with different tectonic settings, were probably brought together (juxtaposed ?) by accretion processes during convergence of plates. However, geochronolgic and rare earth element data on a large number of samples of volcano-sedimentary rocks of this area is required to test this accretionary model.

Primary gold mineralization in the Pre-Cambrian and lower Paleozoic rocks, Lungri Khola region, Rolpa district

Primary gold mineralization occurring in the Lungri Khola region is observed in the Pre­ Cambrian green schist facies and Lower Paleozoic micaceous marble and limestone. The Pre-Cambrian green schist facies comprising mainly of sericite and chloritic quartzite, chlorite schist, quartz-chlorite schist, chlorite phyllite and schistose pebble beds include a discontinuous auriferous zone which persists laterally for about 30 km extension from the Gajul khola in the west to the upper reaches of Gam khola in the east. The auriferous host rocks are found confined close to the contact with the overlying Lower Paleozoic carbonates. Eight distinct auriferous hosts are encountered in the Pre-Cambrian green schists facies. Occurrence of auriferous host in the Lower Paleozoic limestone and marble is rather irregular and insignificant So far minor quartz-carbonate veins have indicated the presence of gold. Finely disseminated gold with minor amount of pyrite, chalcopyrite and rarely arsenopyrite occurs in the above hosts. Visible gold ranging from 0.08 to 2 mm dimension has been recovered. Gold content of upto 6.7 gm/ton has been recorded but in general they show 0.1 gm/ton gold which is significantly 30 to 60 times higher in magnitude than the background value. Depending upon the type of the hosts, three possible models, namely (1) volcanogenic (2) hydrothermal and (3) syn-sedimentary, could be suggested for the origin of the primary gold mineralization which has later undergone remobilisation after the initial deposition.

Stratigraphy of the greenstone belts, Crixás region, Goiás, central Brazil

Archean rocks in the Crixás region, central Brazil, comprise polydeformed low-grade metamorphic greenstone belt volcano-sedimentary sequences of 2.8 Ga arranged in three subparallel belts separated by domal granitoid gneiss blocks. From west to east the supracrustal belts are named Crixás, Guarinos and Pilar de Goiás. A revised stratigraphy is proposed for each belt. The Crixás Group is composed of: (a) the Córrego Alagadinho Formation, consisting of an approximately 500 meter thick sequence of komatiites interlayered with banded iron formations; (b) the Rio Vermelho Formation (350 m), locally pillowed metabasalts with intercalations of oxide and/or silicate facies banded iron formations; and (c) the Riberrão das Antas Formation (700 m), with a lower sub-unit of carbonaceous phyllite-manganiferous metachert assemblage, a middle carbonaceous phyllite-marble assemblage, and an upper metapelite sub-unit. The Guarinos Group is subdivided into: (a) the Serra do Cotovelo Formation (500 m), made up of komatiites with intercalations of carbonaceous phyllite; (b) the Serra Azul Formation (300 m), consisting of metabasalts with intercalations of carbonaceous phyllite; (c) the São Patricinho Formation (300 m), metagraywacke interlayered with chlorite phyllites and locally banded iron formations and orthoquartzite; (d) the Aimbé Formation (50 m), a stratigraphic marker made up to diverse banded iron formations; and (e) the Cabaçal Formation (400 m), consisting of carbonaceous phyllite. The Pilar de Goiás Group contains four sub-units: (a) the Córrego Fundo Formation (900 m), komatiites interlayered with thin banded iron formations; (b) the Cedrolina Formation (500 m), consisting of metabasalts with thin iron and/or manganese formation layers; (c) the Boqueirão Formation (200 m), made up of 50 meters of lower metachert grading up into calc-silicate rocks with dolomitic marble lenses close to the top; and (d) the Serra do Moinho Formation (400 m), composed of phyllite with variable proportions of carbonaceous matter. Each of the greenstone belts has a gross stratigraphic framework made up of a lower volcanic and an upper sedimentary sequence. Major stratigraphic differences occur among the upper sedimentary sections of the three belts; these are interpreted as resulting from different paleographic settings. Vertical facies change from deep to shallow water facies in each belt. Lateral facies change from an outer shelf euxinic environment in the Crixás greenstone belt, eastward into slope graywackes in the Guarinos greenstone belt, and deep water cherts and calc-silicate rocks in the Pilar de Goiás greenstone belt. A carbonaceous sequence on top of both the Guarinos and Pilar de Goiás Groups may represent either a blanket of euxinic shelf prograding onto deep water facies or allochtonous slices of outer shelf type metasediments transported during late Archean thrusting.

Proterozoic contact relationships between gneissic basement and metasedimentary cover in the Needle Mountains, Colorado, U.S.A.

In the Needle Mountains of southwestern Colorado, Proterozoic siliciclastic metasedimentary rocks of the Uncompahgre Group occupy a synclinorium bounded by 1690 + Ma gneissic and plutonic basement rocks. Stratigraphic facing of the Uncompahgre Group away from basement and local preservation of erosional features along the basement—cover interface imply that the contact is an unconformity. A ≤12 m thick zone of phyllite, which commonly contains textural evidence of derivation from basement lithologies, occurs everywhere along the contact. Tectonic fabrics within and near the contact zones locally record non-coaxial deformation that resulted from upward movement of the cover relative to basement on each side of the synclinorium. Comparison of bulk chemistries of basement and contact zone samples indicates that hydration and the loss of CaO, MgO, SiO 2 and Na 2O accompanied alteration of plagioclase + quartz + biotite-bearing basement rocks to muscovite + quartz + Fe/Ti-oxide ± choritoid ± andalusite ± chlorite phyllites. The micaceous zone along the contact is interpreted as a metamorphosed regolith that localized out-of-synform movement during folding of the basement and its parautochthonous cover. Localization of intense deformation within mechanically weak lithologic horizons, such as regoliths, along basement—cover contacts can occur in situations where there is little relative displacement of the adjacent rocks and the stratigraphic significance of the boundary is preserved. Detailed petrographic and kinematic analysis may help distinguish such contacts from major fault boundaries.