Mafic Source Research Articles

Rodingitization of Neoproterozoic Al-Barramiya ophiolite metagabbro in the Eastern Desert of Egypt, Arabian-Nubian Shield

ABSTRACT The present work studies the rodingite that is recorded for the first time as being associated to Al-Barramiya Neoproterozoic ophiolite at Eastern Desert of Egypt, Arabian-Nubian Shield (ANS). Al-Barramiya ophiolite is one of the most important ophiolitic sequences exposed in the ANS. It is affected by different styles of alterations included carbonatization, listvenitization, chloritization and rodingitization. The Neoproterozoic ophiolitic rocks of Al-Barramiya district consists of serpentinized peridotite and metagabbros. Serpentinized peridotites are highly altered to assemblages of talc-carbonate rocks, listvenite and magnesite, while metagabbro is partly converted into rodingite. The field studies indicate that serpentinite and rodingite in Al-Barramiya area display high-strain cataclastic deformation. Rodingite consists of garnet, diopside, vesuvianite and chlorite with minor epidote, prehnite, actinolite and opaque minerals. The garnets and vesuvianite are mostly concentrated near the peripheries of the studied rodingite masses, while clinopyroxene, chlorite, epidote and prehnite gradually increase inwards due to progressive decreasing water/rock ratio as fluids originally buffered by serpentinite move towards equilibrium with gabbroic bulk chemistry. Some rodingite samples preserve a few relics of igneous textures of ophiolitic metagabbros. Garnets are distinguished into andradite, hydroandradite and grossular. Pyroxene is represented mainly by secondary clinopyroxene beside primary fresh relics of diopside; the latter has high Mg# (0.92–0.97) similar to ophiolitic clinopyroxene in the ANS. Rodingite is enriched in CaO, Fe2O3 and MgO but depleted in SiO2, Al2O3 and Na2O compared to associated metagabbro. The association of rodingite with serpentinites and metagabbros suggests that Al-Barramiya rodingite is genetically related to serpentinization of ultramafic rocks. The rodingitization occurred during ocean floor alteration contemporaneous with the serpentinization process. The superposition of cataclastic deformation on the altered rocks indicates that alteration preceded obduction and occurred on the seafloor of the suprasubduction zone where Al-Barramiya ophiolite was originally emplaced. During serpentinization the olivine in the peridotite broke down and released Mg2+ in the metasomatic fluids and clinopyroxene broke down producing Ca-rich fluids. These fluids react with the mafic protoliths and enrich them in MgO and CaO. Also, serpentinization fluids react with the plagioclase of the metagabbro, releasing Ca from the mafic source and concentrated it in the rodingitized samples. The positive Eu anomalies beside high Sr contents of the rodingite can be inherited from a plagioclase-rich protolith such as ophiolitic metagabbro. The enrichment of rodingite in Th and U indicates two distinct solutions affected the gabbroic protolith because the alteration of depleted peridotite to serpentinite cannot be an efficient source to enrich these elements. The present study indicates that the rodingitization process underwent lower greenschist facies conditions associating metamorphism that caused serpentinization and released Ca-rich metasomatic fluids.

Mineralization, geochemical signatures, and provenance of stream sediments on the Jos Plateau, Northcentral Nigeria

This study aimed at determination of mineralization, geochemical signatures and provenance of stream sediments on the Jos Plateau, northcentral Nigeria. Ten (10) stream sediments samples collected were air-dried and subjected to X-Ray Fluorescence analysis. The SiO2 value of 49.79 to 77.03 wt%, Fe2O3 value of 1.30 to 28.84 wt%, Al2O3 value of 5.27 to 18.25 wt%, CaO value of 0.22 to 1.88 wt%, MgO value of 0.12 to 0.92 wt%, K2O value of 0.44 to 2.00 wt% and Na2O value of 0.02 to 0.65 wt%. The low values of CaO (0.22-1.88 wt %) and Na2O (0.02-0.65 wt %) could be associated with the destruction of plagioclase feldspar while low K2O composition (0.44-2.00 wt%) could be associated with the destruction of K-feldspar during weathering. The concentration of MnO (0.06-0.12 wt%) indicates manganite enrichment, the high TiO2 value (0.63-4.25 wt%) shows rutile enrichment while ZrO2 value (0.48-18.48 wt%) indicates zircon mineralization from the underlying lithologies. The high values of pathfinder elements (Y, Zn and Nb) could be attributed to the presence of Older and Younger Granites at the upstream of the study area. Samples 4, 5, 8 and 9 have Thorium (Th) concentrations >15 ppm threshold which point to areas favorable for Th mineralization. Enrichment Factor (EF) ranges from 3.03 to 42.42 indicating moderate, significant, very high enrichment and extremely high enrichment. The Contamination Factor (CF) of 1.27 to 201.03 indicates moderate and very high contamination possibly from geogenic sources. The provenance of stream sediments is associated with felsic and mafic sources

Coupled role of climate and tectonics in the deposition of the late Quaternary sedimentary sequence in the southern margin of the central Ganga Plain, India

ABSTRACT The Ganga Plain’s sedimentation is primarily controlled by Himalayan tectonics, variations in monsoon intensity, and glacier size. However, the significance of intrabasinal tectonics, which is an interplay of the Himalaya tectonics and basement tectonic configuration of the Ganga Plain, in comprehending the late Quaternary Ganga Plain sedimentation remains unclear. In this study, the sediment provenance and extent of weathering experienced by the sediments are studied using the mineralogical and geochemical (major, trace, and rare earth elements) composition of a sedimentary cliff sequence exposed at the Ramnagar locality near the cratonic peripheral bulge in the southern Ganga Plain. In the Ramnagar cliff sediment section, two sediment packages, designated SP-A and SP-B, are identified based on mineralogy, texture, and geochemistry. These packages of sediments show mild chemical weathering and distinct geochemical compositions. The lower part (SP-A) of the Ramnagar cliff section is characterized by higher contributions from mafic sources (pyroxene, feldspar, and mica as dominant minerals), while the upper part (SP-B) is predominately derived from felsic sources (mica and feldspar as dominant minerals), which are supplied via the peninsular and the Himalayan rivers, respectively. The southern part of the central Ganga Plain shows evidence of a shift in the sediment provenance from mafic to felsic source rock at 40 ka. This study demonstrates how weathering and sedimentation are controlled by both regional tectonics and climate in the southern margin of the central Ganga Plain.

Zircon U-Pb-Hf isotopes constraints on the petrogenesis of Neoarchean granitic rocks from the Northeastern part of the Eastern Dharwar Craton, Southern India

In the present study, we integrate bulk rock chemistry and in-situ zircon U-Pb-Hf isotopes of high-K Neoarchean granitoids from the northeastern part of the Eastern Dharwar Craton (EDC) to investigate their source, petrogenesis, and plausible tectonic setting. Based on the mineral assemblage, in coherence with their geochemical characteristics, these granitoids are classified as Hornblende-biotite granite (HBG) with Microgranular enclaves (ME), Biotite granite (BTG) and Monzogranite (MG). Field observations and zircon U-Pb ages reveal coeval emplacement of these granitoids between 2.53 and 2.50 Ga. The HBG, including ME, has low silica, metaluminous affinity, and high ferromagnesian element content, consistent with their derivation from a mafic source. Some of the HBG and ME samples are strongly enriched in incompatible elements, similar to the sanukitoids. The geochemical attributes and strongly evolved zircon Hf isotopic compositions (ɛHf(t) = −4.3 to +1.8 and −7.3 to +0.2) of the HBG and ME suggest their derivation from a mantle source metasomatized by subducted sediments. The BTG and MG are compositionally similar, with high silica, low ferromagnesian element content, and moderate peraluminous affinity, suggesting the involvement of felsic crustal sources. The sub-chondritic to chondritic zircon Hf isotopic compositions (ɛHf(t) = −2.7 to +1.4 and −6.9 to +1.9) of these granitoids support the involvement of heterogeneous ancient crustal sources. While all granitoids exhibit similar zircon Hf isotopic compositions, their geochemical attributes suggest distinct sources. The HBG reflects juvenile crustal additions, whereas BTG and MG are products of the crustal reworking. We propose that these granitoids formed during the continent-continent collision between Western and Eastern Dharwar cratons, that took place after the break-off of the eastward subducting slab. The asthenospheric upwelling induced by slab break-off and/or lithospheric delamination of thickened crust led to the genesis of metasomatized mantle derived magma, which drove the crustal reworking. The observed zircon Hf isotopic composition of the EDC granitoids are similar to the present day Phanerozoic Alpine-Himalayan type orogenies supporting subduction followed by continental collision model for the stabilisation of Archean cratons. Evidence for the existence of Paleo- Mesoarchean felsic crust from this part of the EDC warrants further studies to test the three-terrane model for the Dharwar Craton.

The Cambrian collision of the Yangtze Block with Gondwana: Evidence from provenance analyses

The Neoproterozoic to Paleozoic Proto-Tethys Ocean has preserved critical records of Earth’s evolutionary history. The Lower Cambrian detrital strata along the northern margin of the Yangtze Block provide valuable information for gaining insight into the evolution of the Proto-Tethys Ocean, which is important in understanding the collision between the Yangtze Block and the Gondwana assembly. Here, we present comprehensive U-Pb ages and trace elements of zircons from Lower Cambrian detrital strata in the Yangtze Block. Our results revealed prominent age peaks at 600−500 Ma and 1100−800 Ma for detrital zircons in the Cambrian strata. The trace element characteristics of the detrital zircon indicate that the source rocks were mainly intermediate-felsic magmatic suites with minor input from mafic sources. These results suggest that the detrital materials were not only supplied from the interior of the Yangtze Block but also from magmatic sources in the internal orogenic belt of Gondwana. Furthermore, the kernel density estimate plots of detrital zircon U-Pb ages indicate a stable source-sink system in the Yangtze Block during the Early Cambrian. However, our data indicate that the crustal thickness of the Yangtze Block increased significantly since ca. 526 Ma, and the cumulative characteristics of the detrital zircons indicate that the rocks were deposited in a compressional tectonic environment after ca. 526 Ma, in contrast to those during the late Neoproterozoic, which were generally deposited in an extensional tectonic environment. This transition indicates that the initial collision between the Yangtze Block and the Gondwana supercontinent might have occurred at ca. 526 Ma, marking the rapid aggregation of the northern domain of the Gondwana assembly and potentially serving as an important indicator of the closure of the Yangtze-associated Proto-Tethys Ocean. Our findings provide key insights into late Neoproterozoic−early Paleozoic ocean-land evolution.

Control of paleoclimate and paleoweathering on chromium contents in a non-ultramafic aquifer hosting high chromium groundwater.

Cr(VI) is a carcinogen with proven mutagenic and genotoxic effects. The effects of the depositional environment (e.g., paleoweathering, paleoclimate, and paleoredox condition) on Crenrichment in non-ultramafic aquifer solids are unclear. In this study, we presented the sedimentary characteristics of a borehole from a typical non-ultramafic aquifer with high Cr groundwater in Jingbian, central Ordos Basin, China. Chromium was enriched in the K1h sandstone aquifer, especially at depths of 400-500m, with the highest value of mass transport coefficient (τAl,Cr) up to 92.13% and τAl,Fe up to 33.5%. The provenance of aquifer Cr was predominantly intermediate and felsic igneous rocks with a mafic rock mixture. This mafic source was inferred from Cr-rich granodiorite and mafic/ultramafic rocks in the Yinshan (Daqingshan-Wulashan) Block, northern Ordos Basin. The Cr-rich aquifer in K1h was developed due to a moderate chemical index of alteration (CIA) (mean, 56.7) under relatively warm and humid paleoclimate, as evidenced by high CIA-temperature (CIA-Temp) (mean, 6.79°C) and paleoclimatic index values (mean, 0.40). Fe-Mn redox cycling in the oxic to suboxic environments contributed to aquifer Cr accumulation. Using path analysis, we identified that paleoclimate created favorable weathering conditions and enrichment of Fe contributed to the formation of high-Cr aquifers. The study reveals the formation of positive Cr anomalies in non-ultramafic aquifers, which is the potential source of groundwater Cr, and highlights the effects of depositional factors on Cr accumulation during aquifer deposition or early diagenesis. It can provide new insights into the natural processes of high-Cr sediments occurring in non-ultramafic aquifers.

METAMORPHISM AND DEFORMATION OF GOLD-BEARING NEOPROTEROZOIC WONAKA SCHIST BELT, NORTHWEST-NIGERIA.

The role of metamorphism and deformation is indispensable in the occurrences of gold mineralization worldwide. In this work, deformation and metamorphic conditions for gold-bearing Neoproterozoic Wonaka Schist Belt; located around Kutcheri town of Tsafe Local Government of Zamfara State, was investigated. This is achieved using metamorphic litho-minerals obtained from ternary plots via X-Ray fluorescence (XRF) geochemical data, and directly using minerals phases from X-Ray Diffraction (XRD) technique. Index minerals identified from petrographic analysis previously suggest low to medium-grade metamorphism (M1). XRD analysis indicates quartz, albite, oligoclase, microcline, chlorite, and biotite, suggesting greenschist to lower amphibolite facies (M2). Sillimanite, andalusite, kyanite, staurolite, chlorite, biotite, and garnet were identified from the ternary plots using XRF major oxides, indicating upper amphibolite to granulite facies metamorphism (M3). This is typical of prograde metamorphism, granulite facie metamorphic grade is indicated. Na2O/Al2O3 versus K2O/Al2O3 for petrogenetic character suggests shale provenance, while the trace elements spider diagram indicates Wonaka litho-units as co-genetic compositionally, as high concentrations of V and Cr linked the petrogenetic affinity to mafic sources. Three circles of deformations are indicated; ductile deformation (D1) of the paleosome Schist producing foliations and lineation, brittle type (D2) in mid Pan-African and was accompanied by several fractures and felsic intrusions. Late Pan-African (D3) involves the folding of banded orthogneisses, the development of boudinage as well as intense shearing (ductile fault). Geospatial analysis of the fractures suggests that they represent regional Pan-African sutures cross-cutting Nigeria into the Atlantic and up to South American plate. The research therefore concludes that Au-fluid emanating through this regional event, utilizes D2 as channel ways and loci. D3 with M3 engulfed the entire structures repositioning the geometry to its present disposition.

Timing of the deformation in the Gondwanide orogeny: A structural and geochemical shift from syn- to post-tectonic magmatism recorded in the Permian Mamil Choique granitoids, Patagonia

The Sierra de Mamil Choique calc-alkaline granite batholith, covering an area of 320 km2, serves as the key reference for Late Paleozoic magmatism in central-west Patagonia. It comprises I-type weakly to mildly peraluminous tonalites to granites (59.6–75.7%SiO2) displaying various deformational features. This paper presents a micro- and mesostructural study alongside temperature-pressure (T-P) constraints coupled with U-Pb zircon crystallization ages, as well as Ar-Ar and K-Ar mica ages, and mineral and whole-rock geochemistry. The older units (288 ± 1 Ma Cerro Mojón Monzogranite and 281 ± 2 Ma Huenchuquil Granodiorite), exhibit syn-kinematic banding and strong NW-SE foliation showing a transition from magmatic-sub magmatic to high-T subsolidus deformational features, such as parallel alignment of magmatic minerals, sub magmatic fractures, melt pockets, and chessboard subgrains in quartz. Quartz recrystallization by grain boundary migration is also observed. These microstructures developed concurrently with the regional D3 event that affected the Devonian metamorphic host. Their crystallization started at 8 Kbar and ∼ 790 °C within a thickened crust (LaN/YbN = 13–15, average). In contrast, the younger units (278 ± 2 Ma; Nahuelfil and Antinao Monzogranites) exhibit mainly magmatic deformation and display a NE-SW parallel alignment of mostly subhedral K-feldspar. A D4 deformation younger than 278 Ma, with a sigma 1 NW-SE (in plain view- horizontal) would have controlled the emplacement of Nahuelfil and Antinao Monzogranites. Younger zircon ages (ca 265 Ma) in the 278 Ma monzogranites would result from resetting due to the magmatic-hydrothermal alteration associated with the later magmatic pulse of leucogranites and pegmatites of the 267 ± 8 Ma (Rb-Sr WR isochron) La Pintada Leucogranites. These leucogranites were emplaced in an already thinner crust (LaN/YbN = 2.6 average). The latest magmatic activity is represented by two groups of pegmatitic bodies one from 265 ± 6 to 257 ± 3 Ma and a younger one of ca. 252–251 Ma (Ar-Ar and K-Ar muscovite cooling ages). All units share a common mafic source, but the younger units crystallized from melts at lower pressure and temperature (748–725 °C). The magmatism, involving crustal recycling, occurred at an active margin during a stage of thickened crust from 290 to 280 Ma, followed by gradual thinning after 280 Ma. This change in crustal thickness fits models proposing a continuous Permian subduction with a variable dip angle of the subducted slab along the southwestern margin of Gondwana.

Insights on Source Lithology and Pressure‐Temperature Conditions of Basalt Generation Using Machine Learning

AbstractIdentifying the origin and conditions of basalt generation is a crucial yet formidable task. To tackle this challenge, we introduce an innovative approach leveraging machine learning. Our methodology relies on a comprehensive database of approximately one thousand major element concentrations derived from glass samples generated through experiments encompassing a wide range of source lithologies, pressure (from 0.28 to 20 GPa) and temperature (850–2100°C). We first applied the XGBoost classification models to assess the compositional characteristics of melts from three principal mantle source categories: peridotitic, transitional, and mafic sources. We obtained an accuracy of approximately 96% on the test data set. Furthermore, we also employ an XGBoost regression model to predict the pressure and temperature conditions of generation of basalts from diverse lithologic sources. Our predictions of temperature and pressure exhibit remarkable precisions, of about 49°C and 0.37 GPa, respectively. To enhance accessibility of our model, we have implemented a user‐friendly web browser application, available at (https://huggingface.co/spaces/lilucheng/sourcedetection). The web application allows users to swiftly recover the source lithology as well as pressure and temperature conditions governing basalt generation for a broad array of samples within a matter of seconds.

Unlocking clues on the provenance of the Zigzag Formation in Mountain Province, Northern Luzon, Philippines using sedimentary geochemistry

Abstract Provenance analysis using whole-rock geochemical data was carried out on the Late Oligocene to Early Miocene Zigzag Formation in southwestern Mountain Province, North Luzon, Philippines. Major and trace element geochemistry of the siliciclastic samples from the Zigzag Formation revealed a first-cycle mafic source of the detrital materials. This is consistent with other discrimination diagrams such as those using La, Sc, and Th to determine the provenance. Tectonic discrimination diagrams suggest an oceanic island arc signature for the source rocks of the Zigzag Formation. The Cretaceous to Eocene Lepanto Metavolcanics is proposed to be the source of the detrital materials comprising this clastic sequence. The results present additional information on the current understanding of the evolution of the Central Cordillera.

Paleoproterozoic Dacite Dikes of the Vorontsovka Terrane, Volga–Don Orogen: Geochemistry, Age, and Petrogenesis

Metamorphosed dacitic porphyry dikes were first found in the western part of the Vorontsovka terrane, which is located in the Paleoproterozoic Volga–Don orogen at the margin of Archean Sarmatia and Volga–Ural cratons. The magmatic protolith age for the metadacites is ca. 2.07 Ga. These are ferrous, metaluminous calc-alkali I-type granitoids. The sodium specialization of the rocks and their low concentrations of Mg, Cr, Ni, and incompatible elements, with significant REE fractionation, the absence of Eu* anomalies, high Sr/Y ratio, remarkably high (Gd/Yb)n values (>10), and the radiogenic Nd isotopic composition indicate that the dacitic melts were derived from a juvenile mafic source. According to petrogenetic estimations, such conditions could be caused by the partial melting of depleted N-MORB basites in equilibrium with an eclogitic residue. The dacitic magmas were likely generated by the partial melting of mafic rocks at lower levels of the significantly thickened crust (>60 km) in relation to collision processes.

Paleozoic evolution of the Yukon-Tanana terrane of the North American Cordillera, NW British Columbia

Abstract The origins and primary relationships between tectono-stratigraphic units are fundamental to the terrane concept in accretionary orogens, but they are challenging to assess in metamorphic terranes. In NW British Columbia, three tectonically bounded metamorphic suites of the Yukon-Tanana terrane formed in distinct tectonic settings, based on high-spatial-resolution geochronology and immobile trace-element geochemistry. The Florence Range suite comprises late Neoproterozoic or younger to pre–latest Devonian metasedimentary rocks derived from continental crust, 360 ± 4 Ma calc-alkaline intermediate orthogneiss, and 357 ± 4 Ma amphibolite with oceanic-island basalt composition, consistent with rifting of a continental margin. The detrital signature is dominated by late Mesoproterozoic zircon, which indicates different sources than other parts of the Yukon-Tanana terrane. The Boundary Ranges suite comprises pre–Late Devonian metasedimentary rocks derived in part from a mafic source, amphibolite derived from subduction-zone metasomatized mantle, and 369 ± 4 Ma to 367 ± 7 Ma calc-alkaline felsic to intermediate orthogneiss. The Whitewater suite comprises meta-chert, graphite-rich metapelite, and amphibolite with back-arc basin basalt composition consistent with an anoxic basin near a volcanic source. Our data indicate that the Florence Range and Boundary Ranges suites were separate until at least the Early Mississippian and may have formed a composite terrane since the Permian, whereas the relationship with the Whitewater suite is uncertain. We compare the Paleozoic evolution of the Yukon-Tanana terrane in NW British Columbia with several modern analogues in the west and southwest Pacific Ocean.

Titanium isotope constraints on the mafic sources and geodynamic origins of Archean crust

Titanium isotope constraints on the mafic sources and geodynamic origins of Archean crust

Monogenetic volcanoes as windows into transcrustal mush: A case study of Slamet and Loyang volcanoes, Central Java

Monogenetic volcanic fields (MVFs) are commonly associated with long-lived polygenetic composite volcanoes in many tectonic settings. The association between polygenetic and monogenetic volcanoes has raised questions as to the source of melts feeding monogenetic volcanoes – specifically, whether they are derived from the transcrustal mush underlying a nearby polygenetic volcano, or from a batch of parental magma derived from a different mantle source. To address this question, we studied a well-constrained suite of scoria samples from Gunung Slamet in Central Java, Indonesia, and its most distal monogenetic scoria cone, Gunung Loyang. Slamet is composed of calc-alkaline basalts, and has an extensive MVF on its eastern flank, similar to other Indonesian volcanoes. We analyzed the textural and chemical features of the olivine cargo in both volcanoes, including olivine- and clinopyroxene-hosted melt inclusions. These melt inclusions are the first such analyzed for volatile element contents (H2O, CO2) in the Slamet-Loyang system. We applied hierarchical clustering and mass balance models to constrain the features of the olivine populations in both systems. The olivines in both volcanoes cluster in five distinct textural groups, defined by the presence or lack of zoning and resorption, and according to the type and degree of zoning they exhibit. Olivines show a range of moderate to evolved Fo%, and they also show significant variation in nickel contents (50–1600 ppm). By estimating saturation pressures of entrapped melt inclusions, we observe that Loyang olivines are drawn from a similar petrological system as Slamet, but from deeper mid-crustal levels (12 km), and more mafic source. Our work suggests that monogenetic volcanoes like Loyang can be used as “petrological windows” into the deeper crustal levels of mush reservoirs at composite stratovolcanoes and their associated MVFs.

Radioelement abundances and heat production in the paleoarchaean and early neoarchaean granitoids of the Singhbhum Craton, eastern India: thermal and geodynamic implications

Summary Distributions of long-lived radioelements (Th, U, K) in the upper crust play a vital role in lithospheric thermal modeling and understanding of underlying geodynamic processes. In the present study, we report Th, U, K and radiogenic heat production (A) in Paleoarchaean and early Neoarchaean granitoids from the Singhbhum Craton, located in the eastern part of the Indian shield, for the first time in a systematic way, using laboratory gamma-ray spectrometric set-up. We have studied 204 samples consisting of Paleoarchaean gneiss, three phases of Paleoarchaean Singhbhum Granite and early Neoarchaean granitoid. Th, U, K and A in the Paleoarchaean Singhbhum Granite (10.8 ppm, 1.4 ppm, 2.4 per cent, and 1.3 μWm−3, N = 115) are found to be marginally higher than the Paleoarchaean gneiss (9.6 ppm, 1.7 ppm, 1.8 per cent, and 1.3 μWm−3, N = 74). In comparison, such values are much higher in the early Neoarchaean granitoid (31.1 ppm, 4.2 ppm, 3.9 per cent, and 3.6 μWm−3, N = 15). The surface heat production (1.36 μWm−3) of this craton is comparatively low compared to most cratons worldwide. In addition, it has the lowest average crustal heat production (0.42 µWm−3), due to which it will contribute little to the observed surface heat flow. Moreover, Th is a dominant component in heat production, followed by U and K in these granitoids. Spatially, radioelement ratios Th/U and K/U show similar trends as the radioelements, indicating systematic depletion of Th and U, compared to K. The study also indicates that the spatially separated coeval granitoids are formed from different magma sources in the heterogeneous crust that prevailed during the Archaeans. The lower radioelement abundances in the Paleoarchaean granitoids mostly resulted from the partial melting of mafic sources, whereas higher radioelement abundances in the early Neoarchaean granitoids resulted from the partial melting of the felsic source.

The Singhbhum Craton (India) records a billion year of continental crust formation and modification

The petrogenesis of continental crust from its ultimate mantle source can be reconstructed from the element abundances and radiogenic isotope compositions of ideally pristine igneous rocks. The initial isotope compositions of igneous rocks provide geochemical constraints on the age, composition and evolution of their source(s). Determining initial isotope ratios for rock samples can be challenging, especially in rocks with a long and protracted thermal history. The Rb-Sr system is highly sensitive to parent-daughter element fractionation during magma differentiation. This makes the Rb-Sr isotope systematics ideal to trace the precursor composition of Archean felsic crust and constrain the time of element fractionation during the formation and subsequent modification of continental crust. Initial isotope compositions can be obtained directly from minerals that strongly prefer the daughter element and effectively exclude the parent element of the radio-isotope system of interest. Apatite, having a near zero Rb/Sr ratio, is ideal for preserving its initial 87Sr/86Sr and zircon records initial 176Hf/177Hf compositions. Combined modelling of Sr and Hf isotope data from granitoids of the Archean Singhbhum Craton, indicates that the older Paleoarchean granitoids, emplaced between 3.53 Ga and 3.44 Ga, were derived from a mafic precursor (∼52–54 wt% SiO2) sourced from a depleted mantle at ∼3.71 Ga. Initial 87Sr/86Sr isotope signatures of matrix apatite and apatite inclusions in zircon from the younger Paleoarchean granitoids (3.4–3.2 Ga) of the Singhbhum Craton indicate these younger granitoids were produced by mixing of magma generated from an older mafic source and partial melts derived from the older granitoids. The combined Sr-Hf isotope modelling links the timing of mantle extraction of the precursor material for Paleoarchean Singhbhum granitoids with a known mafic crust extraction event at ∼3.71 Ga. In combination, the new Sr isotope data from apatite combined with whole rock and zircon Hf isotope data from the literature reveal a ∼1 Ga protracted crustal growth and differentiation history of the nucleus of the Singhbhum Craton. By combining radio-isotope systems like 87Rb-87Sr and 176Lu-176Hf, the petrogenesis of Archean felsic crust from the extraction of mafic material from the mantle to reworking in an orogenic cycle to emplacement can be reconstructed. This approach can be applied to other greenstone-gneiss terranes to quantify the spatio-temporal and compositional evolution of voluminous felsic crust and the formation of cratons in the Archean.

GEOCHEMISTRY AND U-Pb GEOCHRONOLOGY OF THE NEOARCHEAN GNEISSES AND PALEOPROTEROZOIC GRANITES FROM JANUÁRIA HIGH: RECORDS OF JUVENILE AND YOUTHFUL EARTH IN THE SÃO FRANCISCO CRATON NUCLEUS (BRAZIL)

Within the proposal of secular evolution of the Earth system divided into seven phases: “Proto-Earth” (4.57–4.45 Ga); “Primordial Earth” (4.45–3.80 Ga); “Primitive Earth” (3.8–3.2 Ga); “Juvenile Earth” (3.2–2.5 Ga); “Youthful Earth” (2.5–1.8 Ga); “Middle Earth” (1.8–0.8 Ga); and “Contemporary Earth” (since 0.8 Ga), the 2.61-2.64 Ga High-K, meta- to peraluminous Januária High orthogneisses are inserted in the context of almost rigid behavior of the lithosphere, by the end of the Juvenile Earth. With protolith origin suggestive of intracrustal partial melting of local older TTG (tonalite-trondhjemite-granodiorite), these orthogneisses are the representatives in the São Francisco craton nucleus of the 2.8 Ga to 2.6 Ga potassic rocks found in archean cratons worldwide, and the occurrence of these rocks is a defining characteristic of the Juvenile Earth. Supported on evidence of the plate tectonic activity in Paleoproterozoic, the 2.14-2.19 Ga Januária High biotite granites are inserted in the context of Youthful Earth and they are the record of accretionary orogeny delineated by the edification of magmatic arc at that time. They are calc-alkaline, I-type rocks typical of subduction-related continental magmatic arcs. These biotite granites are probably derived from a High-K mafic source associated with tonalites.

Precise U-Pb age models refine Neoproterozoic western Laurentian rift initiation, correlation, and Earth system changes

The upper Tonian ChUMP (Chuar-Uinta Mountains-Pahrump) strata of the southwestern U.S.A. are hypothesized to be regional correlatives and to record a time of rift basin evolution commencing at ca. 770 Ma in western Laurentia (modern-day coordinates). We test this correlation using U-Pb chemical abrasion-isotope dilution-thermal ionization mass spectrometry (CA-ID-TIMS) on detrital zircon grains from basal units within these successions. ChUMP units yield CA-ID-TIMS maximum depositional ages (MDA) between 775 and 766 Ma: the Chuar Group of AZ has an MDA of 770.1 ± 0.5 Ma (n = 1) and an additional young zircon mode at 775.7 ± 0.3 Ma (n = 11); the Uinta Mountain Group of northern UT has an MDA of 766.3 ± 0.5 Ma (n = 5) and contains a second young mode at 775.1 ± 0.7 Ma (n = 3); and the basal Horse Thief Springs Formation of the middle Pahrump Group CA has an MDA of 775.4 ± 0.7 Ma (n = 3). The ca. 775 and 770 Ma grains are interpreted to be from zircon-bearing mafic sources related to the 770–778 Ma Gunbarrel Large Igneous Province of Yukon and NW U.S.A. The 766 Ma population was either derived from the Mt Rogers complex of eastern Laurentia or could have come from conjugate margins that were in the process of rifting away, such as Tasmania.The CA-ID-TIMS dates on the Chuar Group in Grand Canyon anchor a Bayesian age model for evaluating late Tonian Earth systems. Faster sediment accumulation rates (80 + 150/-44 m/My) in the lower Chuar Group are consistent with the inception of an extensional basin related to Rodinia breakup; slower rates in the upper Chuar Group (25 + 12/-5 m/My) record are associated with relatively deeper water sedimentation and concomitant organic carbon burial during marine transgression. The model also constrains the timing of several biological events recorded in the Chuar Group, including eukaryovorous predation (>767 Ma), the first appearance of vase-shaped microfossils (∼741 Ma), and the ranges of Cerebrosphaera globosa (=C. buickii; 800–743 Ma) and Lanulatisphaera laufeldii.(766–740 Ma), both proposed as possible marine index fossils for late Tonian time. Finally, the model can also be used to search for stratigraphic evidence of a purported glaciation at ca. 751 Ma.

Provenance and geotectonic setting of the Paleoproterozoic Cerro Figurita formation (Río de la Plata Craton, Uruguay)

The Cerro Figurita Formation (CFFm) represents a clastic sedimentation approximately 3000 m thick which underwent low greenschist metamorphic conditions. The sedimentary sequence is dominated by polymictic conglomerates and sandstones deposited in an environment of alluvial fans that evolve to marine turbidites. The geochemistry of the CFFm reveals derivation from sources with a mix composition similar to unrecycled Upper Continental Crust; alteration increases towards the top of the sequence (CIA 45–92). The contribution of mafic (probably ophiolitic) source rocks is revealed by petrography and low values of Th/Sc (0.3–1.5), Zr/Sc (6–20), Th/U (3–6) and high values of Cr/V (1.1–12.2); slightly negative Eu/Eu* anomalies (0.7–0.9) would indicate the presence of plagioclase in the source rocks. TDM ages are between 2001 and 2269 Ma, and the ƒSm/Nd values along with the relationship between εNd(t) andTh/Sc ratio indicate a provenance from an andesitic arc component. Detrital zircon age patterns are dominated by 2160, 2163 and 2177 Ma peaks, with the youngest zircon showing an age of 2112 ± 21 Ma, confirming a provenance from the Florida central granitic gneiss belt (FB). It is noteworthy the presence of subordinate populations of detrital zircon grains with ages of 2772, 2935 and 3222 Ma, indicating the presence of an inherited felsic source of Archean age, not yet detected in the Piedra Alta Terrane (PAT), and probably located within the Nico Pérez Terrane. A multi-approach to provenance analyses integrating geochemistry, petrography, U–Pb zircon ages, and Nd isotopes indicate ongoing active tectonism during the Orosirian in the PAT, whereas a retroarc foreland basin geotectonic setting is suggested for the CFFm deposition.

Record of short-lived “orogen” on Eurasian continental margin by South China Sea obduction preserved in Taiwan collision

The Taiwan mountain belt is the result of an arc-continent collision following the total subduction of the South China Sea and subsequent closure of the Luzon forearc, a process important in the accretionary growth of continents. Due to the oblique convergence, the southern tip of Taiwan Island is experiencing incipient collision, which is key to observing the oceanic-continental subduction transition. Within the monotonous turbidite extensively exposed on the Hengchun Peninsula as an uplifted Manila Trench accretionary wedge, the Shihmen Conglomerate, as a few intercalated lenses of coarse mafic pebbles, represents a dramatic change in sediment provenance and the causal tectonic event. New zircon U-Pb and amphibole 40Ar/39Ar ages are obtained from sediments, including sands and mafic pebbles that are either gabbro or foliated amphibolite. The 22–24 Ma zircon crystallization ages confirm the South China Sea origin of the mafic clasts, while the much younger 13 ± 2 Ma amphibole 40Ar/39Ar isochron ages from foliated amphibolites suggest a later thermal-tectonic event other than seafloor metamorphism. The amphibole 40Ar/39Ar ages overlap with the biostratigraphic age (∼11–14 Ma), indicating that the mafic source rocks were exhumed and eroded in a high-relief topography immediately after metamorphism. Detrital zircon U-Pb ages from a sandy layer within the conglomerate are also mostly identical to those from the mafic pebbles. Since the paleocurrent of the Shihmen Conglomerate was similar to that of the neighboring turbidites, which were derived from major rivers draining the southeastern Chinese continent, the provenance of the mafic pebbles and sands was best explained as an isolated subaerial mountain on the Eurasian continental margin with a very limited temporal and spatial extent, as the detrital products are poorly distributed. The most likely cause of the ephemeral mountain was the obduction of the South China Sea onto the Eurasian continental margin when the latter first impinged on the Philippine Sea Plate at the Manila Trench, where the gabbroic oceanic crust was uplifted and exhumed, followed by dynamic metamorphism along the basal thrust.