DM Ages Research Articles

The Congo deep-sea fan: Mineralogical, REE, and Nd-isotope variability in quartzose passive-margin sand

ABSTRACT The Congo deep-sea fan, the largest on Earth fed entirely with anorogenic detritus, is characterized by quartzose to pure quartzose sand, reflecting multiple recycling coupled with extreme chemical weathering in cratonic equatorial Africa. The very youthful lower course of the Congo River connects directly to a steep canyon, where detritus including quartz grains up to a few millimeters in diameter is funneled towards Atlantic Ocean floors and deposited at abyssal depths more than a thousand kilometers away from shore. This article illustrates for the first time in detail the mineralogical and geochemical signatures of Congo Fan sands and discusses the factors controlling their intersample and intrasample variability as a key to understand how sediment is generated, recycled, and finally transferred to the deep sea. Compositional variability is largely grain-size-dependent. Combined petrographic and Raman spectroscopy analyses demonstrate that quartz increases in coarser samples and size classes, whereas feldspars are concentrated in finer sizes, plagioclase relative to K-feldspar and orthoclase relative to microcline, defining an order of mechanical and chemical durability among detrital tectosilicates. Because of overwhelming quartz abundance and very low heavy-mineral concentration, quartz contributes significantly to the REE budget and up to 40–50% of Nd in coarser samples, characterized by εNd values as low as –21. The strong grain-size-dependent variability of εNd suggests that quartz carries a markedly more negative εNd signature than monazite and other detrital components. This is chiefly ascribed to the durability of quartz grains, able to survive repeated cycles of weathering and diagenesis through Proterozoic and Phanerozoic time better than all other minerals. Neodymium model ages are influenced less by grain size and quartz abundance but more by the Sm/Nd ratio of different detrital components, and samples hydrodynamically enriched in LREE-rich minerals display TNd,CHUR and TNd,DM ages 1.2–1.4 Ga younger than samples enriched in HREE minerals. Not all detritus in the Congo Fan is supplied transversally by the Congo River. Forward-mixing calculations based on mineralogical data indicate that sand entrained northward by longshore currents mixes progressively with Congo River sand along the northernmost Angola coast, penetrates in the Soyo estuary, and is eventually captured in the canyon and transferred to the deep-sea fan, where it is estimated to represents 7 ± 2% of turbidite deposits.

Petrogenesis of the 130 Ma Taolin granitic intrusion: Implications for the tectonic setting and diversity of Early Cretaceous felsic rocks in the Sulu orogenic belt, eastern China

• The parent magma of the ~130 Ma Taolin intrusion was derived from melting of the Yangtze crust. • The Early Cretaceous igneous event in the Sulu orogenic belt was related to an extensional setting. • Variable protoliths and complex magmatic processes led to the diversity of felsic rocks. Early Cretaceous felsic intrusions are widely exposed in the Sulu orogenic belt (SOB), eastern China. The Taolin granitic intrusion is an example of these intrusions and mainly comprises monzogranite, syenogranite, and granite. Here we present new geochronological and geochemical data for the intrusion. The ~130 Ma Taolin intrusion belongs to normal to fractioned I-type granite and is divided into two groups based on geochemical differences. Rocks in the Daxiezhuang and Malingzhan units belong to Group 1 and have relatively low SiO 2 (61.35–69.44 wt%), high Sr/Y ratios (26.04–60.18), low εHf(t) values (−27.2 to −21.4), and old T DM2 ages (2541–2903 Ma). They also display weak to no negative Eu anomalies (Eu/Eu*=0.65–1.29). Rocks from the Shibu and Tiqiushan units belong to Group 2 and have relatively high SiO 2 (76.38–77.68 wt%), low Sr/Y ratios (0.61–12.38), high εHf(t) values (−24.3 to −17.2) and young T DM2 ages (2271–2722 Ma). They have obviously negative Eu anomalies (Eu/Eu*=0.10–0.42). The partial melting of the Yangtze continental crust at different depths led to variable elemental and isotopic compositions of magma in Group 1 and Group 2. The SOB was in an extensional environment during the Early Cretaceous, which allowed the formation of felsic magmatic rocks with low Sr, Sr/Y, and Sr/CaO values. Early Cretaceous felsic magmatic rocks in the SOB were mainly sourced from the continental crust by crustal reworking. There are diverse rock types with variable geochemical features in the SOB, which is ascribed to the heterogeneity of the continental crust and the complex evolution of felsic magma.

Using Sm-Nd systematics to constrain sediment sources of the Ikorongo Group, north-eastern Tanzania.

Shales and siltstone samples from the Ikorongo Group of north-eastern Tanzania yield T DM ages of between 2049 Ma and 2597 Ma and ɛ Nd (0) ranging from ca. Ȓ28.3 to Ȓ17.3. These ages are interpreted as representing the mean mantle extraction ages of the protolith of the Ikorongo rocks. The data are consistent with most of the detritus being derived from lithologies of the Archaean Musoma-Mara Greenstone Belt. The T DM data also suggest participation of the Mozambique Belt rocks, although less in proportion when compared to the cratonic protolith. Mixing calculations indicate that the samples’ protolith can be modelled as mixtures of detritus from cratonic granitoids (42.2%) and andesites (40.4%) of the MMGB with those from Proterozoic granulites (17.4%) of the Mozambique Belt. The fact that the basin received material from the Pan-African Mozambique Belt, which is known to have cooled below ~300°C at about 540 Ma, points to a Cambrian or younger age for deposition of the Ikorongo strata.

ISOTOPE GEOCHEMISTRY AND GEOCHRONOLOGY OF SYN-DEPOSITIONAL VOLCANISM IN PORONGOS METAMORPHIC COMPLEX, SANTANA DA BOA VISTA ANTIFORM, DOM FELICIANO BELT, BRAZIL: ONSET OF AN 800 MA CONTINENTAL ARC

The Porongos Metamorphic Complex (PMC) is composed of supracrustal rocks, predominantly pelitic micaschist, quartzite and felsic metavolcanic rocks. It represents the remnants of a Neoproterozoic sedimentary basin developed in the western Gondwana, now located in the central section of Dom Feliciano Belt in southern Brazil. Isotope signature comparison and U-Pb ages shows that there are at least three main source-areas for the Porongos sediments: (i) one that can be related to Paleoproterozoic terrains; (ii) a younger component with Mesoproterozoic (1.5 - 1.2 Ga) ages; and (iii), a Neoproterozoic source with magmatic zircons (0.8 Ga), with relative age distribution mostly between 1.7 and 1.0 Ga, T DM ages between 2.58 and 1.71 Ga, and younger ages of ca. 1.21 Ga, which can be related to 1.4 - 1.0 African signature sources. Ten concordant analysis obtained in zircon grains from a metarenite rock yield 206 Pb/ 238 U median age of 802 +26/-33 Ma, which is related to metavolcanic samples from PMC and correlates with the Neoproterozoic Cerro Bori Continental Arc, indicating a syn-depositional volcanism in the sedimentary record. The depositional setting for these sequences can be understood as an evolution between a passive margin developed after 1.6 Ga and a terrain-collisional setting (ca. 570 Ma). doi: 10.12957/jse.2016.22722

Chamomile tea improves glycemic indices and antioxidants status in patients with type 2 diabetes mellitus

ObjectivesOxidative stress is a major factor in the pathogenesis of diabetes complications. The objectives were to investigate the effects of chamomile tea consumption on glycemic control and antioxidant status in subjects with type 2 diabetes mellitus (T2 DM). MethodsThis single-blind randomized controlled clinical trial was conducted on 64 subjects with T2 DM (males and females) ages 30 to 60 y. The intervention group (n = 32) consumed chamomile tea (3 g/150 mL hot water) 3 times per day immediately after meals for 8 wk. The control group (n = 32) followed a water regimen for same intervention period. Fasting blood samples, anthropometric measurements, and 3-d, 24-h dietary recalls were collected at the baseline and at the end of the trial. Data were analyzed by independent t test, paired t test, and analysis of covariance. ResultsChamomile tea significantly decreased concentration of glycosylated hemoglobin, serum insulin levels, homeostatic model assessment for insulin resistance, and serum malondialdehyde, compared with control group (all P < 0.05). Total antioxidant capacity, superoxide dismutase, glutathione peroxidase, and catalase activities were significantly increased by 6.81%, 26.16 %, 36.71 % and 45.06% respectively in chamomile group compared with these variables in control group at the end of the intervention (all P < 0.05). ConclusionsShort term intake of chamomile tea has beneficial effects on glycemic control and antioxidant status in patients with T2 DM. A larger sample population and a longer intervention period may be required to show significant clinical improvements.

Sedimentary provenance, age and possible correlation of the Iona Group SW Scotland

Synopsis Most clast lithologies within the basal conglomerate of the Iona Group, exposed on Iona, Inner Hebrides, SW Scotland, resemble the adjacent Archaean–Palaeoproterozoic Lewisian Gneiss Complex, supporting the hypothesis that the Iona Group unconformably overlies Lewisian gneisses on Iona. Moreover, a whole-rock Sm–Nd t DM age of 2512 Ma from one of these clasts supports its lithological correlation with granitic pegmatites in the underlying gneisses, while a second quartz-rich paragneiss clast with a Sm–Nd t DM age of 2745 Ma, is similar to Lewisian paragneisses on Coll. However, whole-rock t DM ages of 1755–2060 Ma and pronounced c . 1780–1800 Ma peaks in U–Pb detrital zircon and titanite data for Iona Group metasediments suggest additional distal sources of Laurentian affinity including the Rhinns Complex as well as Labradorian, Pinwarian and Archaean detritus. The Iona Group was deposited and deformed after c . 1490 Ma and before c . 422 Ma, constrained by the youngest detrital zircon and titanite and the intrusion of the Ross of Mull Granite, respectively. The detrital data permit correlation of the Iona Group with the Colonsay Group and the Glenshirra Subgroup of the Dalradian Grampian Group, although correlation with parts of the Torridonian cannot be excluded. Supplementary material: Full analytical results areavailable at http://www.geolsoc.org.uk/SUP18768 .

Petrogenesis of the early Cretaceous intermediate and felsic intrusions at the southern margin of the North China Craton: Implications for crust–mantle interaction

Abstract New major and trace element, whole rock Sr and Nd isotopes and zircon U–Pb ages and Hf isotope data are presented for rocks from the early Cretaceous Tianqiaogou dioritic and Taishanmiao granitic plutons at the southern margin of the North China Craton (NCC), in order to investigate their petrogenesis and geological evolution. LA-ICP-MS U–Pb analyses for zircons from these two plutons yield similar 206 Pb/ 238 U ages of 122 Ma and 115–125 Ma, respectively. Monzodiorites from the Tianqiaogou pluton have whole rock e Nd ( t ) values ranging from −6.2 to − 1.3 and zircon e Hf ( t ) values from + 2.9 to + 6.2. They are variably enriched in Ra, Ba, and Sr, and depleted in Nb, Ta, Zr, Hf and Ti, indicating that they were derived from a depleted mantle and underwent subsequent magma differentiation and crustal contamination. The Taishanmiao pluton is composed of metaluminous to peraluminous highly fractionated I-type granites that have high SiO 2 , Na 2 O, K 2 O, Rb, Th, and U, and low P, Ba, Sr, Ti and Eu contents. The granites have strong negative whole rock e Nd ( t ) values (− 16.1 to − 7.5) and zircon e Hf ( t ) values (− 20.9 to − 6.1). Their Nd T DM ages (1.19 to 2.01 Ga) and zircon Hf T C DM ages (1565 to 2490 Ma) are much younger than the basement rocks beneath the southern margin of the NCC, suggesting derivation from an ancient crustal source with minor involvement of mantle-derived components. Therefore, rocks from the Tianqiaogou dioritic pluton were partial melts of the mantle source. Underplating of the mafic magmas initiated partial melting of the ancient continental crust, resulting in the formation of the Taishanmiao granitic pluton. Their complex petrogenesis reflects a strong crust–mantle interaction process related to lithospheric thinning beneath the southern margin of the NCC in early Cretaceous.

Geochronology and geochemistry of late Paleozoic volcanic rocks on the western margin of the Songnen–Zhangguangcai Range Massif, NE China: Implications for the amalgamation history of the Xing'an and Songnen–Zhangguangcai Range massifs

Abstract We here elucidate the tectonic evolution of the Xing'an and Songnen–Zhangguangcai Range massifs during the early Carboniferous–early Permian, based on zircon U–Pb dating and whole-rock geochemical analyses of volcanic rocks of the Songnen–Zhangguangcai Range Massif in the Sunwu area, Heilongjiang Province, NE China. Euhedral–subeuhedral zircons from three rhyolites and one dacite from the study area display fine-scale oscillatory growth zoning, indicating a magmatic origin. Zircon U–Pb dating by LA–ICP–MS indicates that these acidic volcanic rocks formed in the early Carboniferous–early Permian; i.e., early Carboniferous (~ 351 Ma), early late Carboniferous (~ 319 Ma), and early Permian (295–293 Ma). The early Carboniferous rhyolites exhibit chemical affinities to A-type rhyolites, implying an extensional environment. Their positive eHf(t) values (+ 8.67 to + 13.4 except for one spot of + 1.63) and Hf two-stage model ages (T DM2 = 562–988 Ma) indicate that the primary magma was possibly derived from partial melting of newly accreted continental crust. The early late Carboniferous rhyolites and dacites (~ 319 Ma) exhibit calc-alkaline peraluminous signature [molar Al 2 O 3 /(CaO + K 2 O + Na 2 O) ratio, or A/CNK = 1.04–1.22]. The eHf(t) values and T DM2 ages of zircons from the 319 Ma dacites are in the range of + 5.33 to + 9.32 and 907–1268 Ma, respectively, suggesting that the primary magma was derived from partial melting of newly accreted crust. The early Permian rhyolites (295–293 Ma) show chemical affinities to A-type rhyolites, implying an extensional tectonic environment; their positive eHf(t) values (+ 8.82 to + 13.8) and Hf two-stage model ages (484–743 Ma) indicate that the primary magma was derived from partial melting of newly accreted crust. Combined with the geochemical features of coeval igneous rocks from the eastern margin of the Xing'an Massif, these data reveal the late Paleozoic tectonic history and relationships of the Xing'an and Songnen–Zhangguangcai Range massifs, i.e., early Carboniferous westward subduction of the Paleo-Asian oceanic plate beneath the Xing'an Massif, followed by early late Carboniferous collision and amalgamation of microcontinental blocks, and early Permian post-collisional extension.

Late Paleozoic intrusive rocks from the southeastern Lhasa terrane, Tibetan Plateau, and their Late Mesozoic metamorphism and tectonic implications

Abstract The Lhasa terrane in southern Tibet experienced Late Paleozoic and Mesozoic–Cenozoic composite orogenesis. This work reports a study on the petrology, geochemistry, zircon U–Pb chronology and Hf isotopes of Late Paleozoic and Late Mesozoic intrusive rocks from the southeastern Lhasa terrane. The Late Paleozoic intrusive rocks crystallized in the Late Devonian–Early Carboniferous of 371 to 355 Ma, representing a bimodal igneous association formed in the back-arc extensional setting. The mafic end-member originated from the enriched mantle and experienced contamination of crustal materials, characterized by a slight enrichment of LREE, positive anomalies of U, K and Pb and negative anomalies of Th, Nb, Ta and Ti. The felsic end-member was derived from the partial melting of the ancient continental crust, characterized by metaluminous, positive anomalies of Th, Zr and Hf, negative anomalies of Ba, Sr, Nb, Ta and Ti and negative e Hf (t) values of zircon with T DM2 ages from 1.90 to 1.40 Ga. The Late Cretaceous (ca. 107 Ma) mafic intrusions, along with the Late Paleozoic intrusive rocks, underwent nearly syn-intrusion amphibolite-facies metamorphism under P–T conditions of 0.56 to 0.69 GPa and 692 to 735 °C during the Andean-type orogeny correlated with the subduction of the Neo-Tethyan oceanic slab beneath the Lhasa terrane. This study provides a new insight into the pre-Cenozoic tectonic evolution of the Lhasa terrane.

Evolução crustal do setor ocidental do Bloco Arqueano Gavião, Cráton do São Francisco, com base em evidências U-Pb, Sm-Nd e Rb-Sr

The western part of the Gavião Block (São Francisco Craton) is one of the oldest fragments of the South American Platform. It comprises the Riacho de Santana Gneissic Migmatitic complex, the Santa Isabel complex and the Riacho de Santana greenstone belt. T DM ages of country rocks of these units reveal that the main accretion periods were: 3.9 Ga, 3.2 - 3.0 Ga and 2.6 Ga. The Gneissic Migmatitic complex yields U-Pb age of 3648 ± 69 Ma, while the negative εNdt value is compatible with crustal reworking. The Rb-Sr isochron provided age of 3247 ± 120 Ma, tentatively interpreted as migmatization event. U-Pb data indicate that crystallization of a rock from the Santa Isabel Complex occurred at 2954 ± 100 Ma ago, with migmatization at 2748 ± 100 Ma ago (Rb-Sr isochron). This complex displays T DM ages of 3.3 - 3.1 Ga and variable εNdt parameters (-4.7 to +0.3), which suggest juvenile and reworking processes from Mesoarchean protholiths with short crustal residence time. Relative the metamafic (tholeiitic) rock from the Riacho de Santana greenstone belt, at least one of the samples indicates anomalous fractionation, yields a 2.6 Ga T DM age whereas the U-Pb crystallization age is 2218 ± 18 Ma. This suggests crustal contamination of the particular mantle source. Shortly after, the country rocks were intruded by the (U/Pb = 2054 ± 8 Ma) Guanambi batholith which was mainly derived from Archean protholiths (T DM ages of 2.8 - 2.7 Ga). Eventually, tectonic and thermal overprints took place in the study area during Meso- and Neoproterozoic periods, as indicated by K-Ar apparent ages and lower intercept age of a U-Pb discordia line from rocks of the Riacho de Santana Gneissic Migmatitic Complex. We believe that these younger episodes reflect the onset of the Northern Espinhaço intracratonic system and the Neoproterozoic dynamics, respectively, as similar as recorded over the Paramirim Corridor.

Marins Granite (MG/SP): petrography, geochemistry, geochronology, and geotectonic setting

The Marins Granite forms an ellipsoidal body that crops out along the border between Sao Paulo and Minas Gerais states. It intrudes meta-sedimentary and meta-igneous rocks of Neoproterozoic age, belonging to the Socorro Nappe and Embu Complex, in the interference zone between the Brasilia and Ribeira fold belts. Geochemical analyses indicate a high-K calc-alkaline signature, weakly peraluminous, type I character, poorly differentiated with restricted silica range and similar rare earth element patterns enriched in light rare earth elements. Two lithofacies were defined: (1) Marins facies, predominantly inside the body, isotropic, light gray with inequigranular porphyritic texture containing euhedral to subhedral microcline phenocrysts (< 1 cm), and (2) Mendanha facies, restricted to the border zone of the body, are pinkish with elongated microcline phenocrysts and aligned biotite grains, defining a deformational foliation. Neodymium isotopes point to a crustal signature, revealed by a eNd(0.6): -10.4. T DM ages indicate a paleoproterozoic source. U-Pb LA-ICPMS ages in zircon yielded concordia crystallization ages of 606.9 ± 1.9 Ma for the Mendanha facies and 603.7 ± 4.8 Ma for the Marins facies. These ages are consistent with the interpretation that the Marins Granite is a late to post-collisional body with respect to the collision of the southern Brasilia Belt, also reinforced by the location of the granite in the upper plate (Socorro Nappe), a few tens of kilometers away from the suture. The apparent lack of deformation related to collision in the Ribeira Belt (590 - 570 Ma), in the Marins facies, after crystallization of the body, is interpreted as the result of its rheological behavior, more resistant to deformation than the surrounding schists, similar to the behavior of a pre- or syntectonic porphyroblast.

Evolução geológica das sequências do embasamento do Cinturão Araguaia na região de Paraíso do Tocantins (TO), Brasi

Estudos isotopicos baseados nas metodologias Pb-Pb em zircao e Sm/Nd (rocha total), permitiram avancos no entendimento do quadro geologico evolutivo e litoestratigrafico do embasamento do segmento sul do Cinturao Araguaia. Os processos geologicos identificados aconteceram a partir do Arqueano (2,6 Ga e TDM 2,78 – 3,25 Ga) e se estenderam ate o Neoproterozoico. Os ortognaisses do Complexo Rio_dos Mangues posicionam-se no Paleoproterozoico (2,05 – 2,08 Ga) e TDM 2,35 – 2,21 Ga. Um forte encurtamento crustal e fusao parcial de compartimentos isolados e espessados, gerou corpos igneos de 1,85 e 1,82 Ga e o Granito Serrote (1,86 Ga), que provem de fontes entre 2,50 e 2,43 Ga. No final do Mesoproterozoico a regiao foi marcada por processos tafrogeneticos, evidenciados por magmatismo mafico e alcalino (1,05 Ga) e bacias deposicionais, como a que acolheu os sedimentos que originaram as supracrustais do Cinturao Araguaia. No Neoproterozoico, atraves da inversao nas condicoes geodinâmicas, ocorreu novo processo de encurtamento/espessamento crustal com fusoes que originaram expressivas massas batoliticas (Granitos Matanca e Santa Luzia). O Cinturao Araguaia foi edificado a partir dessa movimentacao tectonica. O transporte de massas tectonicas no sentido do Craton Amazonico teria ocorrido, resultando na atual arquitetura em que se encontram as varias unidades lito-estratigraficas, organizadas sob a forma de lascas imbricadas.

Tectonic evolution of the Juruena magmatic arc between the Aripuanã and Juruena rivers: northwest Mato Grosso State, Brazil

The area of study is located in the southwest of the Amazon Craton, northwest of Mato Grosso State and it comprises a region between Aripuanã and Juruena rivers. This paper proposes a geotectonic evolutionary model of a single continental magmatic arc for this Paleoproterozoic segment; its orogenesis starts around 1,820 Ma with subduction, oceanic plate consumption, crust generation followed by continental blocks oblique collision processes after approximately 160 Ma, resulting in the formation of the Juruena Magmatic Arc. The initial phase litotypes are represented by oceanic crust remnants from Bacaerí-Mogno Complex and sub-volcanic to volcanic rocks from Paranaíta Suite and Colíder Group respectively, which configures a marginal ridge. Southwestern, this belt is in contact with the Juruena Complex, a portion of the arc deformed in a ductile regime. This segment is composed by Vitória Suite, São Pedro and São Romão Granites and Vespor Mafic Suite. Volcanic rocks scattered in the complex, apparently filling retro-arc basins, are included in the Roosevelt Group. Post-collisional Type A granitogenesis, related to extensional tectonic, is represented by Serra da Providência Suite. The U-Pb LA-ICPMS geochronology results from two Paranaíta Suite samples, one Vitória Suite and one Vespor Mafic samples are 1,797 ±14, 1,769 ±17, 1,783 ±14 and 1,773 ±15 Ma, respectively. Ten rock samples were also examined by Sm-Nd method, resulting in T DM ages between 2.25 and 1.88 Ga and εND(t) values between -1,9 and +1,92. The isotopic results are compatible with the expected for the Juruena Magmatic Arc.

Paleoproterozoic source contributions to the São Roque Group sedimentation: LA-MC-ICPMS U-Pb dating and Sm-Nd systematics of clasts from metaconglomerates of the Boturuna Formation

The São Roque Group is characterized by volcano-sedimentary sequences, in which deposition probably started in the late Paleoproterozoic. U-Pb dating by LA-MC-ICPMS of zircons extracted from predominantly equigranular monzogranites clasts from Morro Doce and Morro do Polvilho regions, yield paleoproterozoic ages of 2199 ± 8.5 Ma and 2247 ± 13 Ma, respectively. These represent the ages for the main source of granite for the metaconglomerates from the Boturuna Formation (basal unit of São Roque Group). Its polycyclic history is reinforced by the presence of inherited Archean zircons (2694 ± 29 Ma) found within the clasts. Moreover, these clasts have also been affected by the Neoproterozoic overprinting event as indicated by their lower intercept Concordia ages. Sm-Nd isotope data for the main clast varieties from the Morro Doce metaconglomerates yield T DM ages of 2.6 to 2.7 Ga, demonstrating that these granites are the recycling products of an Archean crustal component. The metaconglomerate arkosean framework yields slightly lower &#949;Nd(t) values than those for the clasts, indicating that a younger and/or more primitive source also contributed to the Boturuna Formation.

Petrogenesis and evolution of late Mesozoic granitic magmatism in the Hohhot metamorphic core complex, Daqing Shan, North China

Late Mesozoic granitoid plutons of four distinct ages intrude the lower plate of the Hohhot metamorphic core complex along the northern margin of the North China craton. The plutons belong to two main groups: (1) Group I, deformed granitoids (148 and 140 Ma subgroups) with high Sr, LREE, and Na2O, low Y and Yb contents, high Sr/Y and La/Yb ratios, weak or no Eu anomalies, low Rb/Ba ratios, similar initial 87Sr/86Sr values (0.7064–0.7071) and low Mg# (<37 mostly, 100 × molar MgO/MgO + FeO t ); (2) Group II, non-deformed granitoids (132 and 114 Ma subgroups) with low Sr, relatively low Na2O, high Y and Yb contents, pronounced negative Eu anomalies, high Rb/Ba ratios, and initial 87Sr/86Sr values (0.7098–0.7161). The two groups share geochemical similarities in ϵNd(t) (–11.3 to –15.4) and T DM2 ages (1.85–2.18 thousand million years) as well as Hf isotopic ratios in zircons. Geochemical modelling (using the MELTS code) suggests that similar sources but different depths of magma generation produced the early, high-pressure low-Mg adakitic granitoids and late, low-pressure granitoids with A-type characteristics. The early granitoids likely represent a partially melted, deep-seated, thickened lower continental crust that involved a minor contribution from young materials, whereas the later group partially melted at shallower depths. This granitic magmatic evolution coincided with the tectonic transition from crustal contraction to extension.

Evolução tectônica do arco magmático Juruena entre os rios Aripuanã e Juruena: noroeste de Mato Grosso, Brasil

The area of study is located in the southwest of the Amazon Craton, northwest of Mato Grosso State and it comprises a region between Aripuanã and Juruena rivers. This paper proposes a geotectonic evolutionary model of a single continental magmatic arc for this Paleoproterozoic segment; its orogenesis starts around 1,820 Ma with subduction, oceanic plate consumption, crust generation followed by continental blocks oblique collision processes after approximately 160 Ma, resulting in the formation of the Juruena Magmatic Arc. The initial phase litotypes are represented by oceanic crust remnants from Bacaerí-Mogno Complex and sub-volcanic to volcanic rocks from Paranaíta Suite and Colíder Group respectively, which configures a marginal ridge. Southwestern, this belt is in contact with the Juruena Complex, a portion of the arc deformed in a ductile regime. This segment is composed by Vitória Suite, São Pedro and São Romão Granites and Vespor Mafic Suite. Volcanic rocks scattered in the complex, apparently filling retro-arc basins, are included in the Roosevelt Group. Post-collisional Type A granitogenesis, related to extensional tectonic, is represented by Serra da Providência Suite. The U-Pb LA-ICPMS geochronology results from two Paranaíta Suite samples, one Vitória Suite and one Vespor Mafic samples are 1,797 ±14, 1,769 ±17, 1,783 ±14 and 1,773 ±15 Ma, respectively. Ten rock samples were also examined by Sm-Nd method, resulting in T DM ages between 2.25 and 1.88 Ga and εND(t) values between -1,9 and +1,92. The isotopic results are compatible with the expected for the Juruena Magmatic Arc.

Geochemical and Nd isotope constraints on petrogenesis of granitoids from NW part of the eastern Dharwar craton: Possible implications for late Archaean crustal accretion

Geochemical and Nd isotope data on granitoids of the NW part of the late Archaean eastern Dharwar craton are presented to elucidate their petrogenesis and role in crust formation. The granitoids are divided into three suites viz. trondhjemite–granodiorite gneisses, biotite monzogranites and porphyritic biotite granodiorites. The gneisses are pre- to syn-kinematic (with respect to deformation in the adjacent Hungund–Kushtagi schist belt), which show variable SiO 2 and Al 2O 3, enriched LREE and depleted HREE with slightly negative to no Eu anomalies. They display unusual chemistry in having higher FeO(T), K 2O, Ba, Cr and Ni compared to the typical Archaean tonalite–trondhjemite–granodiorite (TTG). The biotite monzogranites are mostly syn- to late-kinematic and exhibit evolved calc-alkaline compositions with high SiO 2, K 2O, LILE and LREE, depleted to undepleted HREE and strongly negative to no Eu anomalies. The porphyritic granodiorites show syn- to late-kinematic calc-alkaline, sanukitoid-like character with a wide range of SiO 2, higher TiO 2, P 2O 5, Sr, Ba, Cr and Ni, and lower Rb. They, however, uniquely display higher K 2O, ΣREE and Th than typical sanukitoids. The trondhjemite–granodiorite gneisses are interpreted as product of melting of a subducted basaltic slab followed by slight contamination from the overlying metasomatized mantle wedge. Subsequent melting of the extremely metasomatized mantle wedge resulted in formation of the parental magma of the porphyritic granodiorites. Intrusion of the latter triggered melting of the TTG crust accreted earlier and generated the evolved monzogranites. The monzogranites occurring east of the Hungund–Kushtagi schist belt show higher εNd but lower T DM ages than those occurring to the west, indicating that terranes with different histories were juxtaposed by lateral accretion.

Nd isotope mapping of Grenvillian crustal terranes in the vicinity of the Manicouagan Impact Structure

Over 70 new Nd isotope analyses are presented for the Manicouagan area of the Grenville Province to estimate the crustal age of target rocks involved in the 214 Ma Manicouagan Impact Structure, and to reconstruct the Precambrian geological evolution of this crustal segment. The rocks fall into two main groups: Samples from the Archean-aged Gagnon Terrane to the north and west of the impact give T DM ages averaging 2.70 Ga. Samples from the Manicouagan Imbricate Zone (MIZ) and other allochthonous lithotectonic domains to the south of the impact yield Paleoproterozoic T DM ages averaging 2.01 Ga for the MIZ and 1.86 Ga for the southern domains. These Paleoproterozoic terranes are correlated with Makkovik-age crust in Labrador that was heavily reworked by Labradorian magmatism that increased in intensity southwards. The target rocks involved in the impact event would have consisted almost entirely of the MIZ, which formed a layer several kilometres thick, overlying Archean crust at depth.

Tracing Nile sediment sources by Sr and Nd isotope signatures (Uganda, Ethiopia, Sudan)

Strontium and neodymium isotopes, measured on diverse mud and sand fractions of sediment in transit along all major Nile branches, identify detritus sourced from Precambrian basements, Mesozoic strata, and Tertiary volcanic rocks exposed along the shoulders of the East African rift and in Ethiopian highlands. Sr and Nd isotopic ratios reflect the weighted average of detrital components generated in different catchments, allowing us to discriminate provenance, calculate sediment budgets, and investigate grain-size and hydraulic-sorting effects. 87Sr/ 86Sr and 143Nd/ 144Nd range, respectively, from as high as 0.722 and as low as 0.5108 for sediment derived from Archean gneisses in northern Uganda, to 0.705 and 0.5127 for sediment derived from Neoproterozoic Ethiopian and Eritrean basements. 87Sr/ 86Sr and 143Nd/ 144Nd, ranging 0.705–0.709 and 0.5124–0.5130 for Blue Nile tributaries, are 0.704–0.705 and 0.5127–0.5128 for largely volcaniclastic sediments of River Tekeze–Atbara, and 0.705–0.706 and 0.5126–0.5127 for main Nile sediments upstream Lake Nasser. Model mantle derivation ages ( t DM), oldest in Uganda where sediment is principally derived from the Congo Craton (3.4–3.0 Ga for Victoria and Albert Nile), progressively decrease northward across the Saharan Metacraton, from 2.6 Ga (Bahr el Jebel in South Sudan), to 2.4–2.2 Ga (Bahr ez Zeraf across the Sudd), and finally 1.6–1.3 Ga (White Nile upstream Khartoum). Instead, t DM ages of Sobat mud increase from 0.9 to 1.5 Ga across the Machar marshes. T DM ages are younger for sediments shed by Ethiopian (1.2–0.7 Ga) and Eritrean basements (1.5–1.2 Ga), and youngest for sediments shed from Ethiopian flood basalts (0.3–0.2 Ga). Integrated geochemical, mineralogical, and settling-equivalence analyses suggest influence on the Nd isotopic signal by volcanic lithic grains and titanite rather than by LREE-rich monazite or allanite. Because contributions by ultradense minerals is subordinate, intrasample variability of Sr and Nd ratios is minor. In Blue Nile, Atbara and main Nile sediments of mixed provenance, however, the Nd ratio tends to be higher and t DM ages lower in largely volcaniclastic mud than in mixed volcaniclastic/metamorphiclastic sand. The complete geochemical database presented here, coupled with high-resolution bulk-petrography and heavy-mineral data, provides a key to reconstructing erosion patterns and detrital fluxes across the whole Nile basin, and to investigate and understand how sources of sediment have changed in the historical and pre-historical past in relation to shifting climatic zones across arid northern Africa.

Tectonic implications of the discovery of a Shawinigan ophiolite (Pyrites Complex) in the Adirondack Lowlands

The Pyrites Complex in the Adirondack Lowlands domain of the Grenville Province forms the core of a large NE-trending, elongate, winged structure ∼15 km long dominated by highly deformed metagabbro, amphibolite, and hornblende schist. A previously unrecognized km-scale boudin of metamorphosed ultramafic rocks associated with the belt is described. It is largely replaced by secondary hydrous minerals, but retains relict igneous textures and some primary minerals such as augite and chromite, and is cut by several 1–2-m-wide lamprophyre dikes. The ultramafic rocks are interpreted as part of an obducted ophiolite complex on the basis of its structural contact with a belt of rocks including marine metasedimentary rocks, pyritic gneisses, metagabbros, and amphibolites with mid-ocean ridge basalt chemistry and were emplaced within a collapsing backarc basin of Shawinigan age. Small (50–200 μ) zircon crystals separated from peridotite and pyroxenite yield minimum ages (1140 ± 7 and 1197 ± 5 Ma) and constrain the timing of metamorphic and thermal events associated with the Shawinigan orogenesis and anorthosite-mangerite-charnockite-granite (AMCG) plutonism. Neodymium T DM ages from the Pyrites Complex range from 1440 to 2650 Ma, are not compatible with derivation from a typical depleted mantle reservoir, and suggest, along with incompatible element concentrations, that these rocks record mantle enrichment, presumably during subduction beneath the leading edge of Laurentia. Rifting and development of oceanic crust between the southern Adirondack Highlands and Lowlands, coincident with a similar backarc rifting in the Central Metasedimentary Belt at ca. 1300 Ma, are proposed. Three mafic suites in the Adirondack Lowlands are distinguished by their field relations, age, geochemistry, and Nd isotopic systematics and reflect the various stages of evolution of the Trans-Adirondack backarc basin. Within the Lowlands interleaved evaporites, metasedimentary and possible metavolcanic rocks, calc-alkaline and transitional plutonic rocks, and ophiolitic rocks of the Pyrites Complex provide constraints on the tectonic processes and sedimentary response to development of a backarc basin, magmatic arc, foredeep sedimentation, and ophiolite obduction during Shawinigan convergence from ca. 1200–1160 Ma, which culminated in slab breakoff and plate delamination resulting in intrusion of the AMCG suite throughout the Frontenac-Adirondack terrane and beyond.