Mafic Source Research Articles

Neoproterozoic Collision Granitoids in the Southwestern Margin of the Siberian Craton: Chemical Composition, U−Pb Age, and Formation Conditions of the Gusyanka Massif

The paper provides evidence that collisional magmatism related to the Neoproterozoic (880−860 Ma) orogenic event occurred in the southwest of the Siberian Craton. Newly obtained data are presented on the major-component and trace-element composition, U−Pb (SHRIMP II) zircon age, and Sm−Nd isotope composition for rocks of the Gusyanka granitoid massif in the Yenisei fault zone of the Yenisei Ridge. The concordant U−Pb zircon age of the Gusyanka massif is 871 ± 11 Ma indicates that its rocks were formed in the mid-Early Neoproterozoic, simultaneously with the rocks of the Kalama and Eruda massifs in the Tatarka−Ishimba fault system, during the same stage of the collisional events at approximately 880–860 Ma. The calc-alkaline granites, granodiorites, and leucogranites of the Gusyanka massif are classified, on the basis of their high alumina content and trace element composition, as S-type and were derived from a metapelitic source. Many trace-element parameters of rocks of the Kalama and Eruda massifs correspond to those of low-potassium I-type granites, which were most likely derived from mafic rocks and tonalites. The granitoids of the Gusyanka massif, on the one hand, and the Kalama and Middle Tyrada massifs, on the other, differ contrastingly in Nd isotope composition. The source of the former was either metapelites of the Tungusik Group or metasedimentary rocks of the Sukhoi Pit Group, with the involvement of juvenile material. The melts of granites of the Kalama and Middle Tyrada massifs might have been derived from a source with the involvement of an older, possibly Paleoproterozoic, crustal material and a juvenile mafic source. Thus, the orogenic events at 880−860 Ma led to the generation of melts at different levels of the Paleo- to Mesoproterozoic crust of the trans-Angara region of the Yenisei Ridge. The geodynamic history of the region is correlated with the synchronous successions and similar style of tectono-thermal events along the peripheries of the large Precambrian cratons of Laurentia and Baltica, and this is consistent with paleocontinental reconstructions of the close spatiotemporal relations between these cratons, Siberia, and their incorporation into Rodinia.

The Fundamental Role of H2O in the Generation of Coeval Sodic and Potassic Granitoids at Continental Arcs: An Example from the Yangtze Block, South China

AbstractThe bulk rock composition of granitoids reflects the composition of their source and the conditions of partial melting, which are functions of the geodynamic setting in which they formed. Granitoids in active continental margins (continental arcs) are dominated by calc-alkaline rocks with subordinate alkaline compositions, although how these different magma compositions formed is not well understood. Neoproterozoic magmatic rocks are widely distributed along the western margin of the Yangtze Block in South China to form the >1000-km long Panxi continental arc system, which is dominated by granitoids with minor mafic–ultramafic and intermediate plutons. The granitoids are subdivided into sodic and potassic variants that occur as belts along the western and eastern sides of the continental arc, respectively. Sodic granitoids from the western part consist of tonalite, granodiorite, and monzogranite with crystallisation ages ranging from 870 Ma to 740 Ma. They have low K2O/Na2O ratios (0.1–1.0) and high Na2O contents (3.5–6.7 wt%), high but variable SiO2 (61–75 wt%) concentrations, and negative to positive whole-rock εNd(t) values (−1.7 to +2.9). Zircon grains from the sodic granitoids have εHf(t) values ranging from +0.3 to +9.6 and δ18O from 3.90‰ to 7.71‰. The potassic granitoids from the eastern side consist of monzogranite and syenogranite with crystallisation ages from 820 Ma to 790 Ma. They have high K2O/Na2O ratios (0.6–2.2), K2O (2.6–5.9 wt%) and SiO2 contents (69–78 wt%), but whole-rock εNd(t) (−0.9 to +2.9) and zircon εHf(t) (+1.8 to +12.9), and δ18O values (2.98‰ to 6.41‰) similar to those of the sodic granitoids. The isotopic compositions of both the sodic and potassic granitoids are similar to those of spatially- and temporally-related mantle-derived (mafic to ultramafic) rocks, and are considered to have been derived from juvenile mafic continental crust. Phase equilibrium modelling shows that the H2O content of the granitoid source rocks played a key role in their petrogenesis, both in lowering solidus temperatures and in controlling the compositions of the derived partial melts. Our results indicate that calc-alkaline sodic granitoids can be formed by water-fluxed melting of juvenile mafic crust at 750–900°C and 9–12 kbar in which the required H2O was derived from the dewatering of underplating mafic arc magmas. By contrast, the potassic granitoids were generated by fluid-absent (H2O-undersaturated) partial melting of a similar juvenile mafic source at 725–900°C and 6–9 kbar. We conclude that the sodic granitoids were derived from partial melting of the newly-formed mafic lower crust in the continental arc, whereas the potassic granitoids were likely generated in the back-arc setting induced by upwelling of asthenospheric mantle.

Petrogenesis and Tectonic Implications of the Cryogenian I-Type Granodiorites from Gabgaba Terrane (NE Sudan)

The widely distributed granitic intrusions in the Nubian Shield can provide comprehensive data for understanding its crustal evolution. We present new bulk-rock geochemistry and isotopic (zircon U-Pb and Lu-Hf) data from the Haweit granodiorites in the Gabgaba Terrane (NE Sudan). The dated zircons presented a 206Pb/238U Concordia age of 718.5 ± 2.2 Ma, indicating that they crystallized during the Cryogenian. The granodiorites contain both biotite and amphibole as the main mafic constituents. The samples exhibit metaluminous (A/CNK = 0.84–0.94) and calc-alkaline signatures. Their mineralogical composition and remarkable low P2O5, Zr, Ce, and Nb concentrations confirm that they belong to I-type granites. They exhibit subduction-related magma geochemical characters such as enrichment in LILEs and LREEs and depletion in HFSEs and HREEs, with a low (La/Yb)N ratio (3.0–5.9) and apparent negative Nb anomaly. The positive Hf(t) values (+7.34 to +11.21) and young crustal model age (TDMC = 734–985 Ma) indicates a juvenile composition of the granodiorites. The data suggest that the Haweit granodiorites may have formed from partially melting a juvenile low-K mafic source. During subduction, the ascending asthenosphere melts might heat and partially melt the pre-existing lower crust mafic materials to generate the Haweit granodiorites in the middle segment of the Nubian Shield.

Hydroclimatic conditions and sediment provenance in the northeastern Arabian Sea since the late Miocene: insights from geochemical and environmental magnetic records at IODP Site U1457 of the Laxmi Basin

AbstractPalaeo-monsoon and palaeoclimate conditions over Southeast Asia are a matter of debate despite notable studies on the continental and oceanic sedimentary record. The present study investigates the environmental magnetic and geochemical records preserved in the deep marine sediments of the northeastern (NE) Arabian Sea to elucidate the erosion history of the western Himalayas and its link with the prevailing hydroclimatic conditions since the late Miocene. For this, the sediment core retrieved during International Ocean Discovery Program (IODP) Expedition 355 at Site U1457 in the NE Arabian Sea has been explored. The results reveal that the hydroclimatic conditions were predominantly arid during the late Miocene, except for humid intervals from 6.1 Ma to 5.6 Ma. Humid climate conditions in the Indus River Basin returned during the mid-Pliocene and continued to the Pleistocene with an intense chemical weathering regime from 1.9 Ma to 1.2 Ma. The dominant sediment source to the NE Arabian Sea at Site U1457 during the late Miocene and the Pliocene was the Indus River, while during the Pleistocene, mixed sediments brought by the Indus River and the Peninsular Indian rivers were observed. The sediment contribution from a chemically less altered mafic source (the Deccan basalts) increased between 1.2 Ma and 0.2 Ma, possibly linked to a weak Indian Summer Monsoon. The summer monsoon wind strength and associated shift in the Inter-Tropical Convergence Zone (ITCZ) influenced the dominant sediment provenance at Site U1457 of the Laxmi Basin.

Clay mineralogy and provenance modeling of the Paleoproterozoic Kaladgi shales, Dharwar Craton, Southern India: Implications on paleoweathering and source rock compositions

Proterozoic clastic sediments reveal vast information regarding provenance, depositional conditions, and environmental evolutions. Peninsular India comprising Archean cratons, also have numerous intracratonic Proterozoic sedimentary basins along their margins. The Archean Dharwar craton in southern India has many Proterozoic successions, namely Cuddapah, Kurnool towards the East, and Kaladgi, Badami, and Bhima towards the northern margin. The Paleoproterozoic Kaladgi Basin (∼1.85 Ga) consists of siliciclastic sedimentary rocks with stromatolitic carbonate formations. XRD analysis of shale layers of the Lower Lokapur and Upper Simikere Subgroups have been carried out to understand the primary clay mineral assemblages, weathering history, and provenance. The Lower and Upper shale layers of Lokapur and Simikere Subgroups show a dominance of montmorillonite and kaolinite, respectively. The geochemical affinities and the clay mineral assemblages indicate a more mafic source to the lower shales (Manoli and Hebbal Formations) and increased felsic contribution to the upper shales (Govindakoppa and Daddanhatti Formations). Illite is ubiquitous in all the shales of the Kaladgi Supergroup possibly representing the diagenetic transformation of montmorillonite and kaolinite to illite. Geochemical modeling of provenance has been carried out using (Eu/Eu*)N, (La/Yb)N, (Gd/Yb)N, and (La/Sm)N of all the plausible source rocks and the average compositions of lower and upper shales. The modeling results suggest that the lower shales are derived from a source of mafic rocks – 45 %, K-rich granite – 35 %, and TTG – 20 %. While the upper shales are derived from source characterized by K-rich granites – 61 % and intermediate volcanic rocks – 39 %. These results signify the classical unroofing of TTG-greenstone belts exposing K-rich granites with the progression of sedimentation. Further, a good correlation between K-enrichment (a measure of % difference between CIA and pre-metasomatic CIA) and ƩLREE is attributed to the abundance of kaolinites that fractionate more LREE.

Provenance and sedimentary processes on Pleistocene storm deposits in Harhoura (Northern Coastal Atlantic, Morocco): New constraints from a source to sink perspective

The present work studies the Harhoura coastal deposits located immediately southwest of Rabat, Morocco. The base of this sequence has been previously dated by 14C as 9900 ± 150 BP. These deposits exhibit a thick record potentially related to the footprint of a high-energy storm. They are primarily characterized by coarse-grained dark shelly/gravelly units bearing highly fragmented shells inherited from shallow water platform. A high amount of organic matter was also observed.The provenance signature using modal analysis of detrital grains, and major and trace elements indicates two main phases. The first sedimentation is a marine within the shallow platform, followed by a coastal phase belonging to the littoral zone.The first source to sink pathway indicate that platform was supplied by sediments from felsic magmatic source, low-grade and medium metamorphic rocks belonging to the fold and thrust belt of the Sehoul Block, with a minor contribution from mafic source rocks. The second pathway indicates their subsequent remobilization and deposition on the calcarenite coastal plain by mean of high-energy storm(s). The provenance relationship of the latter phase was corroborated by the geochemical signature using major, trace and rare earth elements, assigning their latest source to a coastal-marine origin.

Europium anomalies in detrital zircons record major transitions in Earth geodynamics at 2.5 Ga and 0.9 Ga

Abstract Trace elements in zircon are a promising proxy with which to quantitatively study Earth’s long-term lithospheric processes and its geodynamic regimes. The zircon Eu anomaly reflects the crystallization environment of its felsic or intermediate parental magma. In particular, it provides insight into the water content, magmatic redox conditions, and the extent of plagioclase fractionation in the source rock or its occurrence as a cogenetic crystallizing phase from the magma. We performed a statistical analysis of Eu anomalies from a compilation of detrital zircons over geologic time and found a major decrease in Eu anomaly ca. 2.5 Ga and an important increase ca. 0.9 Ga. Coupled with thermodynamic modeling, we suggest that these variations could be due to long-term change in the chemical system of the mafic source from which the intermediate to felsic melt and derived zircons were produced. The 2.5 Ga drop was likely associated with an enrichment in incompatible elements in the mafic source, which extended the pressure-temperature field of plagioclase stability as a cogenetic melt phase. We interpret the 0.9 Ga rise to record increasing hydration of magmagenetic sites due to the general development of cold subduction systems, which would delay and/or suppress the saturation of plagioclase in hydrous magmagenetic sites.

Europium anomalies in detrital zircons record major transitions in Earth geodynamics at 2.5 Ga and 0.9 Ga

Trace elements in zircon are a promising proxy with which to quantitatively study Earth’s long-term lithospheric processes and its geodynamic regimes. The zircon Eu anomaly reflects the crystallization environment of its felsic or intermediate parental magma. In particular, it provides insight into the water content, magmatic redox conditions, and the extent of plagioclase fractionation in the source rock or its occurrence as a cogenetic crystallizing phase from the magma. We performed a statistical analysis of Eu anomalies from a compilation of detrital zircons over geologic time and found a major decrease in Eu anomaly ca. 2.5 Ga and an important increase ca. 0.9 Ga. Coupled with thermodynamic modeling, we suggest that these variations could be due to long-term change in the chemical system of the mafic source from which the intermediate to felsic melt and derived zircons were produced. The 2.5 Ga drop was likely associated with an enrichment in incompatible elements in the mafic source, which extended the pressure-temperature field of plagioclase stability as a cogenetic melt phase. We interpret the 0.9 Ga rise to record increasing hydration of magmagenetic sites due to the general development of cold subduction systems, which would delay and/or suppress the saturation of plagioclase in hydrous magmagenetic sites.

Ediacaran to Cambrian tectonomagmatic events in the Southern Dom Feliciano Belt, Uruguay: From a plate margin to an intraplate setting and the assembly of SW Gondwana

We present new zircon U-Pb geochronological and Hf-O isotope data as well as whole rock geochemical and Sr-Nd-Pb isotope geochemical data of Ediacaran magmatic rocks from the Cuchilla Dionisio (CDT), Nico Pérez (NPT) and Tandilia (TT) terranes in Uruguay. Three different rock suites are recognized in the NPT. The Group 1 rocks are high Ba-Sr granodiorite, monzonite and syenite that formed from a subduction-related mafic source between 610 ± 3 and 593 ± 5 Ma, most likely in an extensional setting. The Group 2 plutons, syenite, formed from a mafic source between 573 ± 8 and 563 ± 7 Ma and were emplaced in an intraplate setting at the extended eastern Rio de la Plata cratonic margin. The Group 3 rocks as well as the La Paz Granite in the TT are intraplate granites that formed by crustal melting and/or by the incorporation of crustal melts at c. 557 ± 4 Ma. The Early Ediacaran Valdivia Pluton in the CDT is a subduction-related granite that formed from a mafic melt at an active continental margin. The 571 ± 3 and 566 ± 6 Ma subduction-related CDT José Ignacio and Florencia granites were emplaced at or close to an active margin. They, like the Valdivia Granite provide only little evidence of incorporating crustal rocks and/or melts. The coeval Maldonado and Rocha granites in the CDT are subduction-related but were emplaced in a distal position to the arc. The Early Cambrian Santa Teresa Granite is a collision-related pluton that formed during the final assembly of SW Gondwana at 532 ± 6 Ma. It is evident that the Ediacaran to Early Cambrian igneous rocks in the southern Dom Feliciano Belt in Uruguay originated from dissimilar source regions and were emplaced in different tectonic settings at different times. The data indicate a transition from Early Ediacaran subduction-related, arc-type magmatism to intraplate and, finally to Late Ediacaran/Early Cambrian collision-related igneous activity in the Uruguayan sector of the southern Dom Feliciano Belt.

Formation of Paleo- to Meso-Archean continental crust in the western Dharwar Craton, India: Constraints from U[sbnd]Pb zircon ages and Hf-Pb-Sr isotopes of granitoids and sedimentary rocks

The combination of UPb zircon ages with Hf-Sr-Pb isotopes of different intrusive and extrusive felsic and sedimentary rocks provides constraints on the petrogenetic evolution of the continental crust in the western Dharwar Craton, India. The oldest detrital zircon preserved formed at ∼3.6 Ga and represents a relic of the oldest felsic crustal material in the region. The dominant granitoid units of the western Dharwar Craton contain zircon grains with magmatic ages between 3.4 Ga and 3.0 Ga that indicate the formation of major felsic continental crust during this interval.Trace element abundances of the granitoids indicate that the oldest members of the intermediate to felsic suite derived by partial melting of mafic material at ∼3.6–3.4 Ga. The initial bulk rock Hf isotope compositions of these granitoids are consistent with their formation by melting of even older mafic material that was slightly enriched relative to the depleted mantle composition. This mafic and slightly enriched material formed by mantle melting at ∼ < 3.8 Ga. The Hf isotope compositions of individual zircon grains, obtained by two different analytical techniques (in-situ and complete dissolution followed by chromatographic separation) give evidence for the presence of such older mafic material (<3.8 Ga) that formed the immediate precursor of their granitoid host rocks. Such a mafic source for the granitoids is consistent with PbSr isotope systematics of these that shows no indication of Eoarchean enriched/evolved material in the western Dharwar Craton. The mafic source material of the granitoids thus represents an intermediate stage of crust formation that started after 3.8 Ga with the formation of mafic crust by mantle melting. The combined geochronological and isotopic constraints suggest that the Mesoarchean felsic crust of the Dharwar Craton formed by differentiation of melts derived from an amphibolite/eclogite source rock and included increasing contributions of reprocessed crustal material with time from ∼3.6 to 3.0 Ga. The major interval of growth of felsic continental crust was from 3.4 to 3.0 Ga. The younger generation of granitoids formed mostly by reworking of older intermediate to felsic crust. These different felsic magmatic bodies with distinct petrogeneses and sources, that include the depleted mantle, older mafic crust and the evolved continental crust, became essential elements of the stable continental crust of the western Dharwar Craton, the majority of which was generated from 3.4 to 3.0 Ga.

Evolution of the sources of TTG and associated rocks during the Archean from in-situ 87Sr/86Sr isotope analysis of apatite by LA-MC-ICPMS

Radiogenic isotopes provide an important means towards elucidating Archean crustal evolution. The global Hf and Nd isotope record of Archean crustal fragments has been instrumental to unveiling the history of ancient crustal growth and differentiation. The Rb-Sr system could provide valuable complementary constraints in this regard, as this system is particularly sensitive to magmatic fractionation processes, and the chemical and isotopic evolution of magma sources. Application of this system has so far been complicated, however, by its susceptibility to isotope re-equilibration or alteration of the Rb/Sr parent-daughter ratio. In-situ Sr isotope analysis of primary igneous minerals with very low Rb/Sr, such as apatite, provides a new means to determine the initial 87 Sr/ 86 Sr ( 87 Sr/ 86 Sr i ) values for igneous rocks directly. In this study, we apply in-situ Sr isotope analysis of apatite by LA-MC-ICPMS to tonalite-trondhjemite-granodiorite (TTG) rocks and end-member sanukitoids from Archean cratons worldwide. The 87 Sr/ 86 Sr i values of sanukitoids are relatively radiogenic, supporting the model in which such rocks are formed by flux melting of a mantle strongly enriched by metasomatism, possibly by slab-derived fluids. The 87 Sr/ 86 Sr i values for TTGs formed between 3.72 and 3.45 Ga are generally radiogenic, indicating aged amphibolite sources. The 87 Sr/ 86 Sr i values of younger TTGs are systematically lower and were derived from mafic sources that had an average age of ≤0.2 Gyr. This evolution matches with observations from Hf isotopes for TTGs of similar age and indicates a systematic change in the nature or efficiency of TTG crust formation during the Paleoarchean. In-situ Sr isotope analysis of apatite provides a useful method to uncover the Sr record of the early continental crust, and enables constraints on local source evolution and the general two-step evolutionary process of Archean crust formation. • New Sr isotope record of Archean TTGs from LA-MC-ICPMS analysis of apatite • TTG Sr isotope compositions change from radiogenic to unradiogenic at c. 3.5 Ga. • Neoarchean sanukitoids uniformly provide unradiogenic Sr isotope signatures. • The change represents a systematic shift in the average age of mafic TTG sources. • This shift occurred within a small time window (3.6–3.4 Ga) in different cratons.

Provenance of sediments from the Niger Delta, Gulf of Guinea: Evidence from geochemistry

The Gulf of Guinea is surrounded by numerous waterways, where several rivers flow into the ocean, and the Niger River brings large amounts of land-derived debris into the Niger Delta every year. However, geochemical characteristics and sources of sediments in this region are still poorly constrained. Here, we collected two cores (GC09 and GC10) on the shelf and one core (GC05) on the shelf break of the Niger Delta, Gulf of Guinea to assess their provenance using total carbon (TC), total organic carbon (TOC), major elements, and trace elements. The results indicate that sediments on the Niger Delta are classified as iron-rich silty clay and silty clay based on their geochemical compositions. Surface sediments are enriched in quartz and feldspar, evidenced by relatively high levels of SiO 2 , Al 2 O 3 , NaO 2 , K 2 O and CaO. Trace element ratios and TOC demonstrate organic matters were mainly derived from terrestrial inputs and were deposited under aerobic conditions. Geochemical index data (TOC, REEs, δCe, δEu) reflect that the redox conditions of deposition are more inclined to aerobic environment; the sea level gradually decreased and induced more oxic depositional environment over time. According to the location of the three sampling sites, the proportion of felsic in the provenance of sediments increased towards the mainland while the proportion of mafic decreased. With the increasing of water depth, the proportion of felsic source decreases from ~67% at GC10 site (Water depth is 1418 m) to ~50% at GC05 site (Water depth is 3245 m). In contrast, the proportion of mafic source increase from ~25% at GC05 site to ~50% at GC09 site (Water depth is 1377 m). Using chemical index of alteration (CIA), it is manifested that the sediments show high maturity and have experienced strong chemical weathering. Statistical analyses indicate terrigenous inputs contribute more than half materials for the Niger Delta sediments. The relatively stable depositional environment in this region favors the formation and development of gas hydrates. This study could be used to infer the sediment sources and paleoenvironment changes during sedimentation and also provide reliable information for a better understanding of sedimentary processes in global gas hydrate-related deltas. • Provenance of marine sediments in Niger Delta, Gulf of Guinea by geochemistry methods. • The evolution of sedimentary environment in the Niger delta region. • Inorganic geochemical signatures of marine sediments from where gas hydrates can form.

Petrogenesis of low-pressure intra-oceanic arc granitoids: Insights from the late Neoproterozoic Avalonian–Cadomian orogen, Bohemian Massif

The processes leading to the formation of siliceous (intermediate to felsic) magmatic rocks from juvenile sources within intra-oceanic island arcs govern the formation of continental crust and tempos of crustal growth. To unravel the predominant process responsible for the formation of the intermediate to felsic intra-oceanic arc crust, we performed inverse geochemical modelling using major elements and fluid-immobile trace elements. Subvolcanic trondhjemite from a volcanic arc – Davle volcanic complex (DVC) situated in the Teplá–Barrandian unit, Bohemian Massif – that formed during the Neoproterozoic Avalonian–Cadomian orogeny was used as a proxy for the most felsic endmember of the tonalite–trondhjemite suite. The DVC trondhjemite that follows the calc-alkaline trend is enriched in several fluid-mobile elements such as Cs, Ba and U, but depleted in high field strength elements (Nb, Ti) and lithophile elements (e.g., K, Rb, Sr and Th), and exhibits flat rare earth element (REE) primitive mantle-normalized distributions with slightly negative Eu anomalies. Together with the largely radiogenic Hf-Nd isotopic signatures (ε Nd +6.0 to +8.7 and ε Hf +10.2 to +11.7), these data indicate formation in an intra-oceanic arc setting with no significant assimilation of older crustal material. In general, the composition of the studied trondhjemite markedly resembles that of other intra-oceanic arc-derived tonalites and trondhjemites worldwide, formed at low pressures below the garnet stability field. These are herein referred to as “low-pressure intra-oceanic arc granitoids” or “LP IOAG”. The performed inverse geochemical modelling revealed that dehydration partial melting or fractional crystallization of a mafic source can produce a primitive tonalitic melt. Consequently, fractional crystallization of that melt accounts for the compositional heterogeneity observed within the tonalite–trondhjemite suite. We conclude that LP IOAG, commonly forming large portions of intra-oceanic arc middle crust, can form in a nearly closed system with a negligible contribution of subduction-related melts or assimilation of older siliceous crust. Therefore, our findings suggest that the formation of LP IOAG within oceanic crust may represent the predominant process of post-Archean crustal growth. • The DVC trondhjemite has flat REE patterns resembling typical LP IOAG. • The LP IOAG play a key role in the post-Archean crustal growth. • The source composition is established by an inverse geochemical modelling. • The trondhjemite formed by fractional crystallization of a tonalitic source.

Implications for catchment weathering, provenance, and climatic records from a late Pleistocene to present sedimentary sequence in Gujarat, India

AbstractTo ascertain weathering, provenance, and paleoclimate of the last ca. 29 ka in mainland Gujarat, western India, a sedimentary profile of ~7.5 m was measured, described, and sampled at Pratappura and a multiproxy analysis was conducted. To determine weathering, silty-sand and sandy-silt facies were analyzed, and log Na2O/K2O versus log SiO2/Al2O3 was plotted, which shows clustering in the quartz arenite and sub-litharenite categories, indicating low to moderate weathering. The chemical index of alteration (CIA) is 55–74, and was plotted versus the index of chemical variability (ICV) of 1.50, with samples clustered mainly between subalkali basalt and picrite, indicating the dominance of a mafic component. While depleted chondrite normalized light rare earth element (REE) (La/Yb &lt;1) levels suggest the prevalence of a mafic source in the catchment, identical chondrite normalized REE patterns indicate that sediments were well homogenized. Using multiple proxies, the measured profile was subdivided into five paleoclimatic zones. Zone-I (29–18 ka) exhibits decreasing moisture, Corg, χlf, Al2O3, TiO2, and Fe2O3 trends, while higher values of CO3−2 and δ13C indicate a change from a warm-humid to semiarid climate. Zone-II (18–11 ka) shows signs of the beginning of aridity ca. 18 ka during the Last Glacial Maximum (LGM). Several proxies in zone-III show wetter climatic conditions from the early Holocene (ca. 11–4 ka) due to the onset of the SW monsoon, with the trend continuing in zone-IV (4–2 ka). In zone-V, the climate appears to have been similar to the modern conditions in the area from 2 ka–present.

Provenance, paleoweathering, depositional setting and paleoclimatic constraints of cretaceous and neogene deposits of The Mamfe Basin, southwest Cameroon

The Cretaceous and Neogene deposits from the Mamfe Basin consisting of sandstone, shale and claystone were studied using petrography, and major, traces and REEs analyses to address sediment source, environment setting, prevailing paleoclimate as well as tectonic regime of the basin. The angular to subangular shape of detrital grains reflects the mineralogical and textural immaturity of sediments and the proximity of the sediment supply source. Sedimentary rocks are composed of a significant number of lithic debris, organic matter, ostracods as well as subrounded heavy minerals referring to notable igneous and metamorphic rocks bordering the Mamfe Basin. The plots of major element ratios including iron oxide/potasium oxide (Fe2O/K2O) against silicium oxide/aluminium oxide (SiO2/Al2O3) combined with that of sodium oxide/potassium oxide (Na2O/K2O) compared to silicium oxide/aluminium oxide (SiO2/Al2O3) are characteristic of greywacke and shale with few arkoses. The pronounced Eu negative anomaly of chondrite normalized REEs along with the plot of La/Th vs Hf and Co/Th vs La/Sc suggest that sediments are in general from felsic and intermediate source rock provenance, only subordinated contribution of mafic source. The negative anomaly of Yb suggests igneous fractionation under highly reducing conditions. The chemical index of alteration values of 47–70 combined with chemical index of weathering values of 0.6–84 suggest low to moderate weathering process of the sediment in the basin. This result is further confirmed by an index of chemical variability values of 0.6–100 and Zr/Sc ratio of 0.06–2.96. The REEs distribution displays a substantial content in LREE, low content in HREE and noticible proportion of (La/Yb)N ratio (mean >9), poor (Gd/Yb)N ratio in the Cross River Formation (mean <2) and slightly moderate (Gd/Yb)N ratio in the other formations (mean >2). This result implies that sediments from the Ngeme, Nfaitok and Baso formations derived from post-Archean rocks. Geochemical paleoenvironmental proxies including Sr/Cu, Sr/Ba, Ga/Rb vs Sr/Cu and SiO2 vs K2O + NaO2+Al2O3 are in favor of arid to semi-arid conditions during the deposition. Trace Elemental ratios such as Sr/Cu, Sr/Ba, V/Ni, U/Th, Ni/Co, V/Sc, and V/Cr values indicate a predominance of oxic conditions during deposition. In contrast, some authigenic pyrite, hematite, siderite and vivianite which are iron-rich minerals suggests episodic reducing conditions in the basin. The study provides a valuable information in evaluating sediments source, depositional environment, tectonic regime as well as the paleoclimatic conditions prevailing in the basin during the depositional period. The geochemistry of rocks of the Ngeme and Baso formations suggest passive continental margin setting and Ngeme, Nfaitok and Cross River formations suggest oceanic island Arc tectonic setting.

Adakite generation as a result of fluid-fluxed melting at normal lower crustal pressures

Adakites with continental origin have K-rich compositions distinctive from the sodic, subduction-related adakites and are generally considered as partial melting products of thickened mafic lower crust at high pressure conditions (above 1.5 GPa; >50 km depth). In order to compare the geochemical differences and to constrain their source compositions and partial melting conditions, we compiled published data for the sodic adakites related to subduction processes in the Central Asian Orogenic Belt (CAOB) and the potassic adakite-like rocks related to continental collision in the Lhasa Terranes (LT) of the Tibetan Plateau. Based on the spatial similarity of Sr-Nd isotopes and nature of inherited zircons, the origin of the CAOB sodic adakites is best explained by fluid-fluxed melting of low-K arc basalts, while the LT potassic adakites are best explained as generated by fluid-fluxed melting of rocks similar to the voluminous high-K Linzizong mafic volcanic successions (LVS). The phase equilibria and trace-element modeling reveal that these low Mg# mafic sources would expand garnet stability to pressures as low as 7-8 kbar and fluid-fluxed melting (X(H2O) = 2-3 wt%) would significantly decrease the modal content of plagioclase, resulting in melts with “high pressure” signature (e.g., high Sr/Y and La/Yb). This paper provides an alternative model for the origin of sodic or potassic continental adakites arguing that they originate from fluid-fluxed melting at garnet amphibolite facies at normal lower crustal pressures (10-11 kbar). Our findings highlight the importance of the source composition and fluid content in generating the “high pressure” signatures of adakites, which may also explain the origin of Archean tonalite-trondhjemite-granodiorites (TTGs).

Evolution of the Paleo-Tethys Ocean in Eastern Kunlun, North Tibetan Plateau: From continental rift-drift to final closure

The dynamic evolution of the Paleo-Tethys Ocean from continental rift-drift and seafloor spreading to initial subduction and final closure has been hotly debated. This paper tries to bring new constraints to this longstanding debate based on the compiled the latest data and investigations into Carboniferous-Triassic magmatism in the westernmost segment of the Eastern Kunlun orogenic belt, North Tibetan Plateau. New results indicate that the ca. 335 Ma appinite stocks were derived from the asthenosphere mantle wedge slightly modified by slab-derived materials. The ca. 244 Ma dioritic appinite stocks were sourced from the partial melting of hydrothermally altered upper oceanic crust following the closure of the Paleo-Tethys Ocean. The ca. 231 Ma adakitic granite stock originated from the partial melting of the thickened eclogitic lower crust in the syn -collisional setting, and the ca. 219–215 Ma granite batholith had a mafic source in the post-collisional setting. By integrating records available, new scenarios of the evolution of the Paleo-Tethys Ocean in Eastern Kunlun were established. The ca. 390–380 Ma continental rifting leading to the opening of the Paleo-Tethys Ocean is attributed to the post-collisional gravitational instability subsequent to the closure of the Proto-Tethys Ocean and the far-field extensional stress induced by the retreating subduction of the Paleo-Asian Oceanic lithosphere along the northern margin of Tarim-Qaidam. The Paleo-Tethys Oceanic lithosphere initially northward subducted at ca. 366–335 Ma. The closure of the Paleo-Tethys Ocean leading to the “soft” collision between the Eastern Kunlun and Bayanhar terranes occurred at ca. 235–225 Ma followed by the ca. 225–200 Ma post-collisional extension. • The ca. 390–380 Ma opening of the PTO due to gravitational instability and retreating subduction. • The initial northward subduction of PTO occurred at ca. 366–335 Ma. • The closure occurred at 235–225 Ma followed by 225–200 Ma post-collisional extension.

Generation of continental crust by remelting of enriched oceanic crust in accretionary orogen: Geochemical evidence of granitoids in the Tongbai Orogen, Central China

Oceanic arcs are known as the critical sites to generate new continental crust. However, additional processes are needed to transform mafic oceanic crust to mature continental crust. Ascertaining the petrogenesis of granitoids therein is vital to understand such a maturation process. In this study, we carried out a combined study of in-situ zircon U-Pb dating and Hf-O isotopic analyses, as well as whole-rock geochemical and Sr-Nd isotopic compositions for the Changwan granitoids in the Erlangping oceanic arc unit of the Tongbai orogen. This intrusion mainly consists of biotite granite with subordinate trondhjemite. LA-ICPMS and SIMS zircon U-Pb dating yielded identical U-Pb ages of ca. 460 Ma, representing the intrusion ages. Whole-rock analyses demonstrated that the Changwan granitoids have relatively high SiO 2 (72.71–77.14 wt%) and Al 2 O 3 (12.61–14.24 wt%), but low MgO (0.31–0.74 wt%) contents and Mg# (32.0–40.2). The relatively high K 2 O (2.81–3.63 wt%) contents and K 2 O/Na 2 O values (0.76–1.03) of the granitic samples suggest they belong to middle-K weakly peraluminous I-type granite, while the trondhjemitic samples with low K 2 O (1.06–1.96 wt%) and K 2 O/Na 2 O values (0.23–0.42) are tholeiitic to middle-K series. In both cases, the samples show arc-type trace element distribution patterns. They are characterized by depleted Sr-Nd-Hf isotopic compositions with ( 87 Sr/ 86 Sr) i of 0.7038–0.7050, ε Nd (t) of +0.52 ~ +2.85, zircon ε Hf (t) of +3.4 ~ +9.6, and relatively low zircon δ 18 O values of 4.71 ± 0.17–4.97 ± 0.18‰. These isotopic fingerprints suggest the Changwan granitoids were derived from a relatively depleted source that had been hydrothermally altered under high temperature conditions. The most likely candidate is the accreted Erlangping lower oceanic crust which might have been heterogeneously modified in a supra-subduction zone. This is further verified by the trace element modeling on potential mafic source rocks enriched in incompatible elements in the Erlangping unit. We suggest that the high K 2 O/Na 2 O granitic samples were derived from K-rich segment of the Erlangping lower oceanic crust whilst the low K 2 O/Na 2 O trondhjemites from K-poor part. Therefore, our results show that partial melting of enriched oceanic crust in supra-subduction zone plays an important role for crust maturation in accretionary orogens. • The Changwan granitoids were formed at ca. 460 Ma. • They were derived from an incompatible-element-enriched lower oceanic crust source. • Remelting of enriched oceanic crust holds a key for continental crustal maturation.

Source Characteristics of the Carboniferous Ortokarnash Manganese Deposit in the Western Kunlun Mountains

The specific source of ancient sedimentary manganese (Mn) deposits is commonly complex. Here we use systematic major and trace element data with strontium (Sr) and neodymium (Nd) isotopic analyses of the Ortokarnash Mn(II) carbonate ores and associated carbonate rocks from the Upper Carboniferous Kalaatehe Formation (ca. 320 Ma) in order to constrain the Mn source. This formation consists of three members: the first member is a volcanic breccia limestone, the second member is a sandy limestone, and the third member is a black marlstone with the Mn(II) carbonate interlayers. Petrographic observations in combination with low Al2O3 (&lt;3.0 wt%) and Hf (&lt;0.40 ppm) contents and the lack of correlations between the Al2O3 and 87Sr/86Sr ratios as well as εNd(t) values demonstrate a negligible influence of terrigenous detrital contamination on both Sr and Nd isotopic compositions of the Mn(II) carbonate ores. The Sr isotopes of Mn(II) carbonate ores are most likely affected by post-depositional alteration, while Nd isotopes remain unaltered. The initial 87Sr/86Sr ratios in the associated carbonate rocks are likely the result of a mixture of the chemical components (i.e., seawater) and the Al-rich components (e.g., volcanoclastic material), while the detrital effects on Nd isotopes are negligible. In addition, both Sr and Nd isotopes in these non-mineralized wall rocks remained unchanged during post-depositional processes. The relatively low Th/Sc ratios and positive εNd(t) values suggest that the aluminosilicate fraction in the calcarenite and sandy limestone was mainly derived from the weathering of a depleted mafic source, representing the riverine input into the seawater. Given that the Mn(II) carbonate ores are characterized by negative εNd(t) values, these suggest that seafloor-vented hydrothermal fluids derived from interaction with the underlying old continental crust mainly contribute to the source of the Mn(II) carbonates.

The Role of Heavy Minerals in Understanding the Provenance of Sandstone: An Example from the Upper Cretaceous Tanjero Formation, Surdash Region, Northeastern Iraq

The Tanjero Formation is the Upper Cretaceous (Campanian-Maastrichtian) crops out in northeastern Iraq at the High-Folded and Imbricate Zones. To study heavy minerals, twenty sandstone samples were obtained from selected sites of the Tanjero Formation exposed at the southwestern limb of the Surdash anticline in the northeast part of Sulaimanyia city. The analyzed data indicates that the most abundant heavy minerals are opaque, followed by pyroxene, epidote, amphibole, chlorite, rutile, and garnet. The significant abundance of unstable and metastable minerals in the heavy mineral suites shows their direct derivation from the adjacent main basic igneous and metamorphic source rocks, which are situated in the High-Folded and Imbricate Zones in the northern and northeastern parts of Iraq. The estimated zircon-tourmaline-rutile index reveals that the majority of the sandstone samples have less than 75% zircon-tourmaline-rutile index, indicating that the sediments are generally immature. Furthermore, the relationship between an ultra-stable and a meta-stable heavy mineral indicated the immaturity of the Tanjero Formation sandstone and its moderate stability, proving that these minerals cannot be transported over extremely long distances close to the source rocks area. The heavy mineral assemblage demonstrated that the studied area is distinguished by high tectonic activity. In addition, according to the MF- MT-GM ternary diagram, the analyzed sandstone samples lie within the active continental margins field, which is distinguished by a higher percentage of MF minerals than GM minerals generated from mafic magmatic source rocks.