Felsic Orthogneisses Research Articles

Tracing the origin of deposited sediments: A study applying proximal sensing in a drainage subbasin

Erosion is a major cause of soil degradation. Understanding the sources of sediment and the processes that exacerbate this problem is crucial for soil conservation. Remote and proximal sensing technologies have contributed to advance our understanding of soil erosion processes and support soil conservation research. The objective of this study is to use two proximal sensors, a portable X-ray fluorescence spectrometer (pXRF) and a magnetic susceptibilimeter, to identify the origin of sediments deposited in the lower portion of a drainage subbasin by using a fingerprinting approach. Five sediment tracing elements were selected: Fe, Si, Al, Ti, and Zr. The sediment source area is composed of gabbro, granite, and felsic orthogneiss, upon which two soil classes were formed: Rhodic Ferralsol (Dystric) (Latossolo Vermelho Distroférrico típico, LV) and Haplic Acrisol (Argissolo Vermelho-Amarelo Distrófico típico, PVA). In the source area, 35 samples were collected from the topsoil layer (0–2 cm of depth) using a regular grid of 50 × 50 m2. In the deposition area, 24 samples were collected in a regular grid of 7 × 7 m2. The source of the sediments was determined using cluster analysis with data provided by the proximal sensors and topographic attributes. Results indicated that the Haplic Acrisol, which is more susceptible to erosion, contributed most to sediment deposition. Analysis of pXRF data showed its potential to identify and differentiate sediments and their sources. Cluster models based on the abundance of Fe, Si and Al, as well as the Ti/Zr ratio, demonstrated the best results with a global accuracy of 88% and a Kappa coefficient of 0.74. The use of the susceptibilimeter data alone did not yield satisfactory results. Results indicate the usefulness of pXRF in identifying the source of sediments in drainage basins, making it a valuable tool in soil conservation and erosion assessments.

Neoproterozoic reworking of a Mesoproterozoic magmatic arc from the north-eastern part of the Central Indian Tectonic Zone: Implication for the growth and disintegration of the Indian shield in the Proterozoic supercontinental cycles

Thermotectonic evolution of the rocks of the Central Indian Tectonic Zone (CITZ) is crucial to understand the growth of the Indian shield and its response to the supercontinental cycles of the Earth. In this study we present field, petrology, geochemistry, in-situ U-Pb zircon and Th-U-total Pb monazite dates from a suite of felsic orthogneisses from the least studied Makrohar Granulite Belt (MKGB) of the CITZ. Petrology and geochemical data identify two rock groups: charnockitic orthogneiss (COG) and garnet orthogneiss (GOG). Geochemical fingerprints suggest that the magmatic protoliths of the gneisses are S-type granitoids, formed in a continental arc setting. In situ U-Pb zircon dates yield the time of arc magmatism at ca. 1400–1350 Ma. This is the first report of mid-Mesoproterozoic arc magmatism in the CITZ. These arc granites were subsequently metamorphosed at granulite facies condition that culminated at ∼800–850 °C, 7–7.5 kbar. A prominent gneissic (migmatitic) fabric was formed with garnet ± orthopyroxene + plagioclase + K-feldspar + quartz as peak metamorphic assemblages. After reaching the recorded P-T maxima, the studied rocks followed a decompression and cooling path to 4 kbar and 500 °C. The inferred clockwise P-T path is consistent with reworking of the Mesoproterozoic arc granites in a continent–continent collisional setting. In-situ Th-U-total Pb monazite and in-situ U-Pb ages of zircon overgrowth constrain the age of deformation and metamorphism in the span of ca. 974–913 Ma. In combination with the published information, the results of this study are consistent with the views that (a) the Indian shield participated as a coherent block in the Columbia supercontinent since ca. 1450 Ma (or before), (b) the ca. 1400–1350 Ma arc magmatism is presumably linked to collapse or closure of a basin (either north or south of the MKGB) in the CITZ in response to the prolonged accretionary phase of the Columbia supercontinent and (c) during the assembly of the Rodinia supercontinent, localized dismembered fragments of the Columbia got re-amalgamated during late Mesoproterozoic to early Neoproterozoic. Archean cratons got fused along the Proterozoic mobile belts (such as Central Indian Tectonic Zone, Eastern Ghats Mobile Belt). Different units within these mobile belts also experienced local compression during this time frame of the Grenvillian orogeny in the Indian subcontinent.

Latest Mesoproterozoic (ca. 1.2–1.1 Ga) amphibolite-facies metamorphism from the Dete-Kamativi Inlier, NW Zimbabwe: Implications for a Rodinia-related intracratonic orogen in Southern Africa

The Magondi Belt has been regarded as a Paleoproterozoic orogen formed by the collision of the Zimbabwe Craton and an unknown continental block. The Dete-Kamativi Inlier (DKI) located approximately 200 km west-southwest of the main Magondi Belt has been regarded as an extension of the belt. Here, we report new geochronological data for pelitic schists and a felsic orthogneiss from the DKI using monazites (CHIME method) and zircons (LA-ICP-MS analysis), and discuss the tectonic evolution of the region. Zircons from a felsic orthogneiss in the Kamativi area yielded magmatic and metamorphic ages of 2279 ± 25 Ma and 2020 ± 28 Ma, respectively. Similar Paleoproterozoic ages of ca. 2.1–1.8 Ga were also obtained from subhedral and rounded monazite grains in the pelitic schists. In contrast, irregular-shaped monazite intergrown with biotite in a different pelitic schist gave three latest Mesoproterozoic isochron ages of 1196 ± 37 Ma, 1143 ± 32 Ma, and 1070 ± 25 Ma, suggesting a long-lived (>120 million years) thermal event with several monazite-growing stages. Consistent isochron ages of 1062 ± 41 Ma and 1061 ± 26 Ma were obtained from monazites in the felsic orthogneiss and metapelite samples from an adjacent region. The application of phase equilibrium modeling for the peak mineral assemblage in the garnet-andalusite-biotite-cordierite-bearing pelitic schist with 1196–1070 Ma metamorphic ages indicated a peak P–T condition of 520–600 °C and 1.5–2.5 kbar, suggesting low-pressure amphibolite-facies metamorphism. The condition is lower than that obtained from the hornblende-plagioclase geothermometry of amphibolites (>700 °C) from the southwest part of the DKI, which probably corresponds to an earlier (ca. 2.0 Ga) high-grade metamorphic condition. The youngest thermal event, 994–982 Ma, from monazite rim in a mylonitic orthogneiss might correspond to the timing of later deformation. The latest Mesoproterozoic (1.2–1.1 Ga) amphibolite-facies metamorphism was likely associated with an intracratonic orogeny related to the activity of broadly coeval orogenic events, such as the Namaqua-Natal orogenic belt related to the amalgamation of the Rodinia supercontinent. Regional magmatic activity of the Umkondo large igneous province at 1112–1108 Ma could have also been an additional heat source.

Paleo-Mesoarchean crustal growth and reworking in the western Dharwar Craton, southwestern India: Evidence from trondhjemitic gneiss and granitic gneiss

Paleo-Mesoarchean felsic orthogneisses are widely exposed in the southern part of the western Dharwar Craton (WDC), southern India shield. The felsic orthogneisses studied in this paper from the WDC can be divided into two groups based on their geological, geochronological, and geochemical data: the 3.4–3.3 Ga trondhjemitic gneiss (including trondhjemitic veins) and the ∼3.1–3.0 Ga granitic gneiss. The 3.4–3.3 Ga trondhjemitic gneiss is characterized by high contents of SiO2 (70.4–73.1 wt%), Na2O (5.49–5.91 wt%) and Sr (647–740 ppm), high ratios of Na2O/K2O (4.7–7.7) and Sr/Y (53–106), but relatively lower Mg# values (32–39), and Cr (9.5–28.7 ppm) and Ni (1.34–5.33 ppm) concentrations, as well as insignificant Eu anomalies, fractionated REE patterns and mostly positive zircon εHf(t) values (+0.8-+5.9). These data indicate that the trondhjemitic gneiss was derived from partial melting of juvenile mafic crust in a relatively moderate-shallow depth, with residual mineral assemblage of garnet, amphibole and clinopyroxene, implying crustal growth event at 3.4–3.3 Ga. By contrast, the ∼3.1–3.0 Ga granitic gneiss is characterized by relatively lower contents of Na2O (3.82–4.63 wt%) and Sr (96.9–291 ppm), low ratios of Na2O/K2O (1.4–1.7) and Sr/Y (3.02–8.65), as well as significantly negative Eu anomalies and negative to positive zircon εHf(t) values (-5.6-+4.5). Therefore, the granitic gneiss was probably derived from partial melting of pre-existing crustal rocks, suggesting a prominent crustal reworking event at ∼3.1–3.0 Ga. This study emphasizes that the western Dharwar Craton is dominated by Paleoarchean (3.4–3.2 Ga) crustal growth event and a strong crustal reworking event in early Mesoarchean (∼3.1–3.0 Ga), though the Mesoarchean crustal growth event is discernible, evidenced by regional ∼3.0 Ga TTG rocks. The early Mesoarchean (∼3.1–3.0 Ga) crustal reworking event, represented by series of potassium-rich granites, is also reported in other Archean cratons, and thus it may have global significances.

Recovering P–T–t paths from ultra-high temperature (UHT) felsic orthogneiss: An example from the Southern Brasília Orogen, Brazil

Mineral assemblages that are diagnostic of ultrahigh-temperature (>900 °C) metamorphism are generally restricted to Mg- and Al-rich rocks. However, in many metamorphic terrains such compositions do not occur, and the rocks are dominated by orthogneisses that typically contain high-variance mineral assemblages that are not diagnostic of particular pressure–temperature (P–T) conditions. Here we use whole-rock and mineral chemistry, thermodynamic phase equilibrium modelling, and zircon U–Pb–Hf isotopic data to constrain the P–T–time history of granulites from the Socorro-Guaxupé Nappe of the Southern Brasília Orogen. These rocks occur as part of a stack of Neoproterozoic nappes that were assembled during the formation of West Gondwana. Based on phase equilibrium modelling, the peak assemblage in a garnet-bearing felsic granulite (ortho- and clinopyroxene, garnet, K-feldspar, plagioclase, quartz, ilmenite, and silicate melt) records pressures of around 11.5 kbar and temperatures of ~1000 °C, consistent with the high-Al content of orthopyroxene. The retrograde path records a small decrease in pressure to 9 kbar marked by orthopyroxene replacing garnet and the exsolution of magnetite from ilmenite followed by near-isobaric cooling that led to the development of coronas of hornblende around pyroxene due to crystallization of the last vestiges of melt. Zircon cores yielding U–Pb ages of ca. 625 Ma are interpreted as dating close-to-peak conditions during high-temperature metamorphism. In one sample, zircon rims and ‘soccer ball’ zircon neoblasts both have high U contents and yield ages of ca. 615 Ma, interpreted to mark the onset of melt crystallization. In two other samples, low-U rims and ‘soccer ball’ zircons grains yield ages of ca. 600–595 Ma, interpreted to date the end of melt crystallization. These data reveal a protracted regional metamorphic event in which the lower crust remained at granulite-facies conditions for at least 30 Myr during collision and assembly of West Gondwana.

Neoproterozoic crustal growth in southern Malawi: New insights from petrology, geochemistry, and U–Pb zircon geochronology, and implications for the Kalahari Craton–Congo Craton amalgamation

The southern part of the Malawi Basement Complex is regarded as being part of the Mozambique Belt, which was formed by a series of collisions between Archean cratons and/or Proterozoic magmatic arcs during Neoproterozoic to Cambrian time. The region is particularly important because of its location near the junction between the East-African (ca. 600–550 Ma) and Kuunga (ca. 560–530 Ma) orogens, which means that complex magmatic or metamorphic episodes may have been recorded in high-grade metamorphic rocks in the region. Here we report on new petrological, geochemical, and zircon U–Pb age data on felsic to mafic orthogneisses as well as on a syn-tectonic syenite from the Lilongwe–Zomba–Blantyre area in southern Malawi, with a view to evaluating the timing and pressure–temperature (P–T) conditions of the collision events as well as the protolith formation. The geochemical data on the felsic orthogneisses suggested magmatic-arc signatures, whereas the mafic orthogneisses had both magmatic-arc and non-arc signatures. Metamorphic P–T estimates for the orthogneisses based on conventional geothermometry and phase-equilibria modeling indicated high-pressure granulite-facies conditions of 9.1–11.6 kbar and 840 °C–930 °C. Zircons obtained from charnockite, biotite gneiss, and garnet-bearing mafic granulite yielded magmatic ages of 1011–808 Ma, 1013 Ma, and 1076 Ma, respectively, suggesting the presence of Stenian–Tonian crustal growth related to arc magmatism. Ediacaran (573 Ma) intrusion of granitic rock was also inferred from magmatic zircons found within a granitoid gneiss. The timing of the high-grade metamorphism was constrained by the ages of the zircon rims and found to be 569–557 Ma, thus coinciding with the intrusion of syn-tectonic syenite at 562 Ma. The results of this study suggest that the southern part of Malawi is composed of remnants of the Stenian–Tonian magmatic arc, which were metamorphosed together with surrounding metasediments around 560 Ma. The Ediacaran high-grade metamorphism is consistent with the age of the Zambezi Belt located immediately southwest of the Malawi Basement Complex. Both the Lilongwe–Zomba–Blantyre area and the Zambezi Belt may correspond to a single suture zone formed by the latest Neoproterozoic collision between the Kalahari and Congo Cratons.

Further evidence of Rhyacian arc magmatism in the basement of the Brasília Belt, western São Francisco pericraton

A recently mapped area in central Brazil revealed a slice of Paleoproterozoic basement within Neoproterozoic metasedimentary rocks of the Brasília Belt. The area, named here the Artulândia area for its proximity to the homonymous village, contains metatonalites and metagranodiorites in tectonic contact with a meta-volcanosedimentary sequence that encompasses felsic, intermediate and mafic metavolcanic rocks interlayered with clastic and chemical metasedimentary rocks. A sample of felsic orthogneiss from the metavolcano-sedimentary sequence, yielded a U–Pb zircon age of 2142 ± 4.4 Ma, which is interpreted as the crystallization age of the volcanic protolith. Samples from the metagranite unit returned U–Pb zircon ages of 2130 ± 7.4 Ma for a biotite metatonalite, 2157 ± 9 for a biotite metagranite, and 2147 ± 5 Ma for a sulfide-bearing metagranodiorite, indicating roughly contemporaneous plutonic activity. Whole-rock geochemistry shows calc-alkaline signatures for both units, and trace element results are compatible with an extensional setting, such as back-arc or intra-arc, for the metavolcano-sedimentary sequence, and a magmatic arc system for the metagranite unit. These rocks were generated in an episode of juvenile crust formation, as suggested by εNd(2.14Ga) values between 0 and + 3.18. Based on lithological association, age and chemical signature, we suggest that the studied rocks represent a lateral continuation of the Campinorte Arc beyond the Rio Maranhão Thrust. Thus, the occurrence of Campinorte Arc rocks on both sides of the thrust argues against the thrust being a Neoproterozoic collisional suture. The geology of the Artulândia area in the context of the Campinorte Arc attests to the complexity of the Rhyacian amalgamation that generated the São Francisco Paleocontinent during a Rhyacian Orogeny.

Grenville-age metamorphism in the Larsemann Hills: P-T evolution of the felsic orthogneiss in the Broknes Peninsula, East Antarctica

ABSTRACT The gneisses outcropping in the Larsemann Hills of East Antarctica are ideal for studying the composition and evolution of the Prydz tectonic belt. In this study, detailed petrological analyses, phase equilibrium modelling, and zircon analyses were performed on a felsic orthogneiss from the Broknes Peninsula in the Larsemann Hills. The study results show that the protolith of the felsic gneiss is granite, which is further identified to have been an S-type granitoid. The felsic orthogneiss intruded into the basement complex at ~976 Ma. The magmatic zircons in the felsic orthogneiss have lower LREE and higher HREE contents than those of the metamorphic zircons in the same rock. The felsic gneiss consists of the mineral assemblage garnet +ilmenite+ plagioclase +K-feldspar + sillimanite + quartz and underwent granulite facies metamorphism in the Grenvillian period (～899 Ma), with peak conditions of ～870°C and ～9.5 kbar. The Larsemann Hills should be categorized as the orogenic process from arc-continent collision to continent-continent collision during the Grenvillian period, crust thickening(~899 Ma) and subsequent collapse(550–500 Ma).

Grenvillean evolution of the Beishan Orogen, NW China: Implications for development of an active Rodinian margin

Abstract New geochemical and geochronological data are used to characterize the geodynamic setting of metasediments, felsic orthogneisses, and eclogite and amphibolite lenses forming the Beishan complex, NW China, at the southern part of the Central Asian Orogenic Belt. The metasediments correspond compositionally to immature greywackes receiving detritus from a heterogeneous source involving a magmatic arc and a Precambrian continental crust. Metagranitoids, represented by felsic orthogneisses, show both composition of greywacke-derived granitic melt with incompatible trace element patterns similar to the host metasediments. The eclogite lenses are characterized by high Nb contents (5.34–27.3 ppm), high (Nb/La)N (>1), and low Zr/Nb ratios (<4.5), which together with variable and negative whole-rock εNd(t) (–4.3 to –10.3) and zircon εHf(t) (–5.0 to + 2.3) values indicate an origin of enriched mantle source as commonly manifested by back-arc basalts at stretched continental margins. Combined with monazite rare earth element analysis, the in situ monazite U-Pb dating of metagraywacke (880.7 ± 7.9) suggests garnet growth during a high-temperature (HT) metamorphic event. Together with U-Pb dating of zircon metamorphic rims in amphibolite (910.9 ± 3.0 Ma), this indicates that the whole crustal edifice underwent a Grenvillian-age metamorphic event. The protolith ages of the eclogite (889.3 ± 4.8 Ma) and orthogneiss (867.5 ± 1.9 Ma) suggest that basalt underplating and sediment melting were nearly coeval with this HT metamorphism. Altogether, the new data allow placing the Beishan Orogen into a Grenvillean geodynamic scenario where: (1) The late Mesoproterozoic to early Neoproterozoic was marked by deposition of the greywacke sequence coeval with formation of an early arc. (2) Subsequently, an asthenospheric upwelling generated basaltic magma underneath the thinned subcontinental mantle lithosphere that was responsible for HT metamorphism, melting of the back-arc basin greywackes and intrusion of granitic magmas. These events correspond to a Peri-Rodinian supra-subduction system that differs substantially from the Neoproterozoic ophiolite sequences described in the Mongolian part of the Central Asian Orogenic Belt, thus indicating important lateral variability of supra-subduction processes along the Rodinian margin.

Tectonic implications of P-T paths derived for garnet-bearing felsic gneisses from the Dabie and Sulu ultrahigh pressure terranes, east-central China

Felsic gneisses usually represent the vast majority of rocks in high-pressure (HP) and ultrahigh-pressure (UHP) terranes, but the derivation of metamorphic pressure-temperature (P-T) paths for these rocks have been rarely undertaken. We studied a felsic paragneiss with the mineral assemblage of plagioclase, quartz, phengite, biotite, garnet, epidote, K-feldspar, and rutile from the Chinese Continental Scientific Drilling main hole (depth: 775 m), located in the Sulu UHP terrane, eastern China, by analyzing minerals with the electron microprobe and computing P-T pseudosections with PERPLE_X. Garnet includes phengite, feldspar, quartz, and rutile and retains a Ca-rich core (XCa = 0.42). Towards the rim Ca contents decrease (XCa = 0.36) and Fe and Mg contents increase. This zoning points to a prograde P-T path at pressures lower than 1.3 GPa. In combination with Zr-in-rutile thermometry and garnet volume isopleths in the P-T pseudosection, peak P-T conditions of ca. 1.2 GPa and 610 °C were constrained which are below the wet granite solidus. K-feldspar formation at the expense of phengite indicates a nearly isothermal decompression accompanied by late fluid infiltration. A similar P-T path was additionally derived for a published felsic orthogneiss from the Dabie UHP terrane using P-T pseudosections. The prograde P-T paths obtained in this study imply that not all felsic gneisses from the Dabie and Sulu UHP terranes underwent UHP metamorphism. This is explained by tectonic juxtaposition of various felsic gneisses from different depths in an exhumation channel.

Early Palaeoproterozoic structural reconstitution of a suite of rocks from the Mahadevi Layered Complex, Tamil Nadu, India

The Palghat‐Cauvery Shear Zone (PCSZ) of the Granulite Terrane of South India is perceived to be a late Neoproterozoic transcontinental shear zone that develops extensive mylonite and sheath folds. The Mahadevi Layered Complex (MLC), the area of present study, is situated in the eastern part of the PCSZ. The studied area exposes vast expanse of felsic orthogneisses with large enclaves of mafic granulite interlayered with meta‐BIF. Our study records two distinct deformational events (D1–2) that produced E‐W closing regional folds with gently curved fold axis. The curved fold axes results in the regional dome‐and‐basin structure. Kinematic markers suggest that a weak shear couple was accompanied with the D2 deformation. Preponderance of the granoblastic fabric in the rocks, restriction of mylonites only in local conjugate shear zones, and the structures produced during D1–2 deformations are at variance with the view that (a) existence of a sheath fold as previously reported from the area and (b) the PCSZ marks the zone of intense shear deformation. Preliminary petrological information from the studied area shows that D1–2 deformational events were accompanied by high‐pressure granulite‐facies metamorphism that was followed by a steeply decompressive retrograde P‐T path. The U–Pb zircon ages from the felsic orthogneiss fixes the date of emplacement of its magmatic protolith and the regional high‐pressure metamorphism at ≥2,530 and ca. 2,479 Ma, respectively. Integrating all the information it is suggested that the evolution of the MLC occurred in a continent‐continent/arc collisional setting at the dawn of the Proterozoic Eon. This study does not support the existence of a “mega sheath fold” in the MLC as has been published in the recent literature.

Bulk rock and zircon geochemistry of granitoids from the Chotanagpur Granite Gneissic Complex (CGGC): implications for the late Paleoproterozoic continental arc magmatism in the East Indian Shield

The north-eastern part of the Chotanagpur Granite Gneissic Complex (CGGC) exposes a vast expanse of felsic orthogneisses (~ 1450 Ma magmatic crystallization age) containing a variety of metamorphosed supracrustal rocks and augen gneiss. However, origin and timing of emplacement of the protolith of the augen gneiss are not well constrained. Owing to lack of detailed study and paucity of exposures, the antiquity of the basements of the CGGC also remains poorly understood. To address this issue, we present bulk rock geochemical data and the U–Pb and Lu–Hf isotope composition of zircon from augen gneisses from two adjoining areas within the CGGC. Our obtained data suggest that the augen gneiss have been derived from hybrid source involving older crustal rocks and metasomatised mantle. The bulk rock and zircon geochemistry also suggest that the protolith of the augen gneiss was emplaced in an early Paleoproterozoic basement in a continental arc setting. U–Pb ages of the magmatic zircon grains constrain the age of intrusion at ca. 1710 Ma and ca. 1635 Ma. The timing of this late Paleoproterozoic felsic magmatism overlaps with the timing of the regional high-to-ultrahigh-temperature granulite facies metamorphism in the CGGC.

Petrology and geochronology of a suite of pelitic granulites from parts of the Chotanagpur Granite Gneiss Complex, eastern India: Evidence for Stenian‐Tonian reworking of a late Paleoproterozoic crust

The north‐eastern part of the Chotanagpur Granite Gneiss Complex (CGGC) in the East Indian shield contains enclaves of migmatitic pelitic granulites (PG) within felsic orthogneiss (FOG). Field observations, petrology and geochronology (LA–MC–ICP–MS U–Pb dating of zircon and EPMA Th–U–total‐Pb dating of monazite) of the PG suggest two distinct metamorphic events. The earliest event M1, which is characterized by high‐temperature (>850°C) granulite facies metamorphism, occurred in the timespan of ~1680–1580 Ma. Extensive dehydration melting of biotite + sillimanite + quartz‐rich protoliths led to stabilization of the restitic assemblage (garnet + alkali‐feldspar + quartz + sillimanite + ferrian‐ilmenite) together with large volumes of felsic melts (leucosomes). Collisional tectonics followed by delamination and asthenospheric upwelling could have triggered the M1 event. Subsequently, at ~1470–1400 Ma, the igneous protolith of the host FOG intruded and hydrated the PG. Thereafter, a second metamorphic event, M2, accompanied by compressional structures, affected both the rock types. A clockwise P–T path that culminated at ≥10 kbar ~760–850°C and is followed by a steeply decompressive retrograde path characterizes this event. The P–T path and the inferred geothermal gradient (<27°C/km) are compatible with a continent–continent collisional setting. Geochronological findings suggest a protracted orogeny for the M2 event with its major pulse during ~970–950 Ma. When combined with the published information, this study supports the view that a large (if not the entire) portion of the Indian shield and the granulite terranes of east Antarctica share similar tectonothermal events that led to the formation of two supercontinents, Columbia and Rodinia.

Late Paleoproterozoic and Mesoproterozoic magmatism of the Nico Pérez Terrane (Uruguay): Tightening up correlations in southwestern Gondwana

New geochronological, isotopic and geochemical data were obtained from the late Paleoproterozoic and Mesoproterozoic magmatic units of the Nico Pérez Terrane (Uruguay). A U-Pb LA-ICP-MS zircon age of 1768 ± 11 Ma confirms the Statherian age for the Illescas rapakivi intrusion, being thus comparable with the age of the Campanero Unit felsic orthogneisses. Though both the Illescas and Campanero intrusions exhibit dominantly granitic, meta- to peraluminous compositions, the first shows a dominant shoshonitic and ferroan composition, whereas high-K calc-alkaline and magnesian to ferroan compositions characterize orthogneisses of the Campanero Unit. Additionally, Sm/Yb and LaN/YbN ratios indicate that both suites were emplaced in a thickened crust, whereas Lu-Hf zircon data of the Illescas granite together with available Sm-Nd data of the Campanero Unit point to significant recycling of an older, Archean and/or Paleoproterozoic crust of the Nico Pérez Terrane. Results suggest a common origin for both the Illescas and Campanero magmatism, probably related to a post-collisional/post-orogenic setting. On the other hand, metagabbros of the Zanja del Tigre Complex present U-Pb LA-ICP-MS zircon ages of 1479 ± 4 and 1482 ± 6 Ma and tholeiitic gabbroic compositions. Geochemical and Sm-Nd isotopic data show similarities with LIP-related intracontinental mafic magmatism, suggesting a mixed magma source derived from depleted asthenosphere and fertile subcontinental lithospheric mantle contributions. Emplacement of the metagabbros was related to lithospheric extension in a continental rifting setting and, therefore, fertilization of the lithospheric mantle probably took place during older Paleoproterozoic subduction-related events. Coeval late Paleoproterozoic and early Mesoproterozoic magmatism is also recorded in the northern Nico Pérez Terrane of southernmost Brazil, and in basement inliers of the Kaoko Belt and the Angolan Shield (southwestern Congo Craton), thus suggesting a common evolution of these blocks prior to the Neoproterozoic.

Eoarchean to Neoproterozoic crustal evolution of the Mantiqueira and the Juiz de Fora Complexes, SE Brazil: Petrology, geochemistry, zircon U-Pb geochronology and Lu-Hf isotopes

The Mantiqueira and the Juiz de Fora Complexes (MQC and JFC) in the Araçuaí-Ribeira orogenic system, southeastern Brazil, have been regarded as parts of the Neoproterozoic to Cambrian Brasiliano Orogeny formed by a series of collisions of the São Francisco and the Congo Cratons. Here we report new petrological, geochemical, and zircon U-Pb and Lu-Hf isotopic data on felsic to mafic orthogneisses from the complexes with a view to evaluate the timing and P-T conditions of the collisional event as well as protolith formation. Metamorphic P-T estimates for orthogneisses indicate amphibolite-facies conditions of 650–730 °C and 5.8–9.1 kbar from the MQC, whereas slightly higher (upper amphibolite- to lower granulite-facies) conditions of 680–770 °C and 6.0–7.7 kbar were obtained from the JFC. Geochemical, zircon U-Pb geochronological, and Lu-Hf isotopic data of felsic orthogneisses indicate that the MQC was formed by a series of Neoarchean (ca. 2.67 and 2.61 Ga) volcanic arc magmatisms with magma sourced from Eoarchean to Paleoarchean (ca. 3.82–3.42 Ga) crustal components. In contrast, slightly younger U-Pb ages of ca. 2380 Ma and ca. 2050–1960 Ma were obtained from the JFC marking the timing of volcanic arc magmatism as Early to Middle Paleoproterozoic, with magma sourced from Paleoarchean (∼3.45 Ga) crust with addition of juvenile crustal components. The recycled ancient crustal components were also supplied for late Neoproterozoic (ca. 600 Ma) continental arc magmatism of the JFC. The high-grade metamorphism is possibly related to the final collision of the Congo and São Francisco Cratons at 590–560 Ma, which broadly compares with the formation of the Kuunga Orogen.

Tectonothermal imprints in a suite of mafic dykes from the Chotanagpur Granite Gneissic complex (CGGC), Jharkhand, India: Evidence for late Tonian reworking of an early Tonian continental crust

Tectonothermal evolution of a set of mafic dykes in the felsic orthogneiss basement of the Chotanagpur Granite Gneissic Complex (CGGC) is presented. The gneissic basement rocks preserve high pressure granulite facies metamorphism (M2, ~800 °C and ~9 kbar) and develop pervasive planar fabric (S2) during a continent-continent collision event at 1000–950 Ma. Mafic dykes cut the S2 fabric of its basement it intrudes. Together with the S2, the mafic dykes have been deformed by three sets of folds viz. DkF1, DkF2 and DkF3. A conspicuous NS trending foliation defined by metamorphic amphibole is developed parallel to the axial plane of DkF1–2. The mafic dykes preserve relict igneous textures (e.g. intergranular texture) defined by clinopyroxene, orthopyroxene and plagioclase. Numerically computed phase diagram (pseudosection) with a representative bulk composition (with <10 vol% metamorphic minerals namely amphibole+ plagioclase rim + quartz) constrain the depth of emplacement of the mafic dykes (15–18 km). The geological features are consistent with rifting of the early Tonian basement of the CGGC and the then mantle beneath it. This lithospheric extension resulted in outpouring of basaltic magma that punctured the early Tonian basement rocks in the form of mafic dykes. Subsequent metamorphism that affected the mafic dykes and its basement culminated at ~700 °C and ~7.5 kbar (M3 metamorphism).The estimated physical conditions during the M3 metamorphic event suggest burial of the previously rifted basement possibly in a continent-continent collisional tectonic setting. The Th-U-total Pb ages of monazite grains from the host felsic orthogneiss date the M2 (ca. 964 Ma) and M3 (ca. 917 Ma) metamorphism. Based on commonality of geological and geochronological history among the CGGC, the Eastern Ghats Mobile Belt and Rayner complex (and its equivalent parts) in east Antarctica corroborates the view that Indo-Antarctic landmass were united and shared the similar geological histories during the entire Tonian period. During the Tonian period the Indo-Antarctic landmass witnessed two major phases of orogenesis (1000–950 Ma and < 920 Ma) that are punctuated by a phase of lithospheric extension manifested by the emplacement of mafic dykes.

Petrology, geochemistry, and zircon U-Pb geochronology of the Zambezi Belt in Zimbabwe: Implications for terrane assembly in southern Africa

The Zambezi Belt in southern Africa has been regarded as a part of the 570–530 Ma Kuunga Orogen formed by a series of collision of Archean cratons and Proterozoic orogenic belts. Here, we report new petrological, geochemical, and zircon U-Pb geochronological data of various metamorphic rocks (felsic to mafic orthogneiss, pelitic schist, and felsic paragneiss) from the Zambezi Belt in northeastern Zimbabwe, and evaluate the timing and P-T conditions of the collisional event as well as protolith formation. Geochemical data of felsic orthogneiss indicate within-plate granite signature, whereas those of mafic orthogneiss suggest MORB, ocean-island, or within-plate affinities. Metamorphic P-T estimates for orthogneisses indicate significant P-T variation within the study area (700–780 °C/6.7–7.2 kbar to 800–875 °C/10–11 kbar) suggesting that the Zambezi Belt might correspond to a suture zone with several discrete crustal blocks. Zircon cores from felsic orthogneisses yielded two magmatic ages: 2655 ± 21 Ma and 813 ± 5 Ma, which suggests Neoarchean and Early Neoproterozoic crustal growth related to within-plate magmatism. Detrital zircons from metasediments display various ages from Neoarchean to Neoproterozoic (ca. 2700–750 Ma). The Neoarchean (ca. 2700–2630 Ma) and Paleoproterozoic (ca. 2200–1700 Ma) zircons could have been derived from the adjacent Kalahari Craton and the Magondi Belt in Zimbabwe, respectively. The Choma-Kalomo Block and the Lufilian Belt in Zambia might be proximal sources of the Meso- to Neoproterozoic (ca. 1500–950 Ma) and early Neoproterozoic (ca. 900–750 Ma) detrital zircons, respectively. Such detrital zircons from adjacent terranes possibly deposited during late Neoproterozoic (744–670 Ma), and subsequently underwent high-grade metamorphism at 557–555 Ma possibly related to the collision of the Congo and Kalahari Cratons during the latest Neoproterozoic to Cambrian. In contrast, 670–627 Ma metamorphic ages obtained from metasediments are slightly older than previous reports, but consistent with ∼680–650 Ma metamorphic ages reported from different parts of the Kuunga Orogen, suggesting Cryogenian thermal events before the final collision.

Characterising the southern part of the Hearne Province: A forgotten part of Canada’s shield revisited

The Hearne craton comprises the southeastern part of the western Churchill craton. The central part of the craton has been well studied in Nunavut and northeastern Saskatchewan in comparison with the southern part of the craton, which is mainly exposed south of the Athabasca Basin in Saskatchewan. Mapping and analytical results indicate that the southern Hearne craton is dominated by a felsic orthogneiss complex derived mainly from tonalites with crystallisation ages in the range 2.81–2.78 Ga and ca. 2.73 Ga. Sm-Nd isotopic analysis suggests that these evolved plutons (mean ƐNd = −3.3) were derived from a Mesoarchean to Paleoarchean source. Other components of the orthogneisses include leucotonalite intrusions dated at ca. 2.664 Ga and regionally extensive K-feldspar phyric granites emplaced between 2.63 and 2.60 Ga. Both of these suites have Sm-Nd isotopic signatures similar to the older tonalites. A suite of granodiorite and quartz diorite, one of which yielded a crystallisation age of 2.654 Ga, has unique Sm-Nd isotopic results with ƐNd values near CHUR. The felsic orthogneiss complex is unconformably overlain by the Black Birch Lake assemblage, which contains dominantly orthquartzites, mafic volcanic rocks and pelites. Detrital zircon signatures in the orthoquartzites are dominated by Archean sources and indicate that the supracrustal rocks were deposited after 2.60 Ga.The central Hearne Province extends into northeastern corner of Saskatchewan. It contains oceanic mafic volcanic rocks of the Ennadai Group that were intruded by juvenile tonalites at 2.71–2.70 Ga. The age and isotopic character of these tonalites contrasts markedly with those in the felsic orthogneiss complex of the southern Hearne Province. A comparison of plutonic suites and supracrustal assemblages of the central and southern Hearne Provinces suggests that there is heterogeneity between the two entities. Analysis of geophysical data, coupled with existing geological maps, suggest that an important tectonic boundary separates them. The current age relationships in the central and southern parts of the Hearne province imply that the two were juxtaposed after 2.60 Ga; however, the time of suturing remains equivocal.

Late Mesoproterozoic to Early Paleozoic history of metamorphic basement from the southeastern Chiapas Massif Complex, Mexico, and implications for the evolution of NW Gondwana

In this paper, U-Pb zircon geochronology, Lu-Hf and Sm-Nd isotope systematics, geochemistry and geothermobarometry of metaigneous basement rocks exposed in the southeastern Chiapas Massif Complex are presented. Geologic mapping of the newly defined “El Triunfo Complex” located at the southeastern edge of the Chiapas Massif reveals (1) partial melting of a metamorphic basement mainly constituted by mafic metaigneous rocks (Candelaria unit), (2) an Ediacaran metasedimentary sequence (Jocote unit), and (3) occurrence of massif-type anorthosite. All these units are intruded by undeformed Ordovician plutonic rocks of the Motozintla suite. Pressure and temperature estimates using Ca-amphiboles, plagioclase and phengite revealed prograde metamorphism that reached peak conditions at ~650°C and ~6kbar, sufficient for partial melting under water saturated conditions. Relict rutile in titanite and clinopyroxene in amphibolite further indicate a previous metamorphic event at higher P-T conditions. U-Pb zircon ages from felsic orthogneiss boudins hosted in deformed amphibolite and migmatite yield crystallization ages of ~1.0Ga, indicating that dry granitic protoliths represent remnants of Rodinia-type basement. Additionally, a mid-Tonian (~920Ma) metamorphic overprint is suggested by recrystallized zircon from a banded gneiss. Zircon from folded amphibolite samples yield mainly Ordovician ages ranging from ~457 to ~444Ma that are indistinguishable from the age of the undeformed Motozintla plutonic suite. Similar ages between igneous- and metamorphic- zircon suggest a coeval formation during a high-grade metamorphic event, in which textural discrepancies are explained in terms of differing zircon formation mechanisms such as sub-solidus recrystallization and precipitation from anatectic melts. In addition, some amphibolite samples contain inherited zircon yielding Stenian-Tonian ages around 1.0Ga. Lu-Hf and Sm-Nd isotopes and geochemical data indicate that the protoliths of the amphibolite have E-MORB characteristics and were derived from a depleted mantle source younger than the Rodinia-type basement. Inasmuch as similar amphibolites also occur in the Ediacaran metasedimentary rocks as dykes or lenses, Late Neoproterozoic magmatism in a rift setting is suggested. Hence, the geologic record of the El Triunfo Complex includes evidences for Rodinia assemblage, Tonian circum-Rodinia subduction, and breakup during the Late Neoproterozoic. Metamorphism, and partial melting are interpreted in terms of a convergent margin setting during the Ordovician. The results place the southern Chiapas Massif along with Oaxaquia and similar Northern Andes terranes on the NW margin of Gondwana interpreted as the extension of the Famatinian orogen that evolved during the closure of the Iapetus Ocean.

Paleoproterozoic (ca. 1.8 Ga) arc magmatism in the Lützow-Holm Complex, East Antarctica: Implications for crustal growth and terrane assembly in erstwhile Gondwana fragments

The Lützow-Holm Complex (LHC) of East Antarctica forms a part of the latest Neoproterozoic–Cambrian high-grade metamorphic segment of the East African–Antarctic Orogen. Here we present new petrological, geochemical, and zircon U–Pb geochronological data on meta-igneous rocks from four localities (Austhovde, Telen, Skallevikshalsen, and Skallen) in the LHC, and evaluate the regional Paleoproterozoic (ca. 1.8Ga) arc magmatism in this terrane for the first time. The geochemical features reveal a volcanic-arc affinity for most of the meta-igneous rocks from Austhovde and Telen, suggesting that the protoliths of these rocks were derived from felsic to mafic arc magmatic rocks. The protoliths of two mafic granulites from Austhovde are inferred as non-volcanic-arc basalt such as E-MORB, suggesting the accretion of remnant oceanic lithosphere together with the volcanic-arc components during the subduction–collision events. The weighted mean 206Pb/238U ages of the dominant population of magmatic zircons in felsic orthogneisses from Austhovde and Telen show 1819±19Ma and 1830±10Ma, respectively, corresponding to Paleoproterozoic magmatic event. The magmatic zircons in orthogneisses from other two localities yield upper intercept ages of 1837±54Ma (Skallevikshalsen), and 1856±37Ma and 1854±45Ma (Skallen), which also support Paleoproterozoic magmatism. The earlier thermal events during Neoarchean to Early Paleoproterozoic are also traced by 206Pb/238U ages of xenocrystic zircons in the felsic orthogneisses from Austhovde (2517±17Ma and 2495±15Ma) and Telen (2126±16Ma), suggesting partial reworking of the basement of a 2.5Ga microcontinent during ca. 1.8Ga continental-arc magmatism. The timing of peak metamorphism is inferred to be in the range of 645.6±10.4 to 521.4±12.0Ma based on 206Pb/238U weighted mean ages of metamorphic zircon grains. The results of this study, together with the available magmatic ages as well as geophysical and lithological data from the region, suggest that the LHC can be divided into three units: Neoarchean (ca. 2.5Ga) unit in the southern LHC (Shirase Orthogneiss or “Shirase microcontinent”), Neoproterozoic (ca. 1.0Ga) unit in the northern LHC, and supracrustal unit in the central LHC with fragments of Paleoproterozoic (ca. 1.8Ga) and minor Neoarchean (ca. 2.5Ga) and Neoproterozoic (ca. 1.0Ga) magmatic arcs. The 1.8Ga arc magmatism inferred in this study has also been reported from adjacent Gondwana fragments such as the Highland Complex in Sri Lanka, and the Trivandrum and Nagercoil Blocks in southern India. Although the ca. 1.8Ga arc-magmatic event is coeval in these regions, the Paleoproterozoic supracrustal unit in the central LHC may not be contiguous with those in the Highland Complex of Sri Lanka because recent studies have shown that the Vijayan Complex in Sri Lanka and the ca. 1.0Ga northern LHC possibly were part of a single crustal unit (northern Lützow-Holm–Vijayan Complex) within the Kalahari Block. The supracrustal unit possibly marks part of a discrete suture formed by the collision of the ca. 2.5Ga southern LHC (Shirase microcontinent) and the ca. 1.0Ga northern Lützow-Holm–Vijayan Complex during the latest Neoproterozoic–Cambrian Gondwana amalgamation, which might be coeval with the collision of the Vijayan and Wanni Complexes and the formation of the Highland Complex in Sri Lanka. Our study provides new insights on crustal growth and terrane assembly in the ancient continental blocks of Gondwana.