Intracontinental Rift Research Articles

Mechanism of Paleo-Mesoproterozoic rifts related to breakup of Columbia supercontinent: A paleostress field modeling

The Paleo-Mesoproterozoic Zhongtiao aulacogen in the North China Craton and Cuddapah basin in the Indian Craton, have both been interpreted as intra-continental rift formed by a mantle plume that led to the breakup of Columbia supercontinent, but the mechanism has not been completely deciphered. In this paper, the mechanism of the Zhongtiao aulacogen and Cuddapah basin related to initial breakup of Columbia has been evaluated with 2D elastic finite element models of the North China Craton and the Indian Craton. The trajectories of the horizontal maximum principal compressive stress of the best-fit model fit well with the trends of dyke swarms in the North China Craton and the Indian Craton. When the other three models generated were compared with the best-fit model, it can be found that a mantle plume beneath the Zhongtiao and Cuddapah areas played the most vital role in developing the Zhongtiao aulacogen, Cuddapah basin and initial breakup of Columbia supercontinent. The boundary subduction forces, including the northern margin of the NCC, the northwest and southwest margins of the Indian Craton are indispensable factors for the rifting and breakup, whereas the mechanical properties have little influence on these modeling results. The initial breakup of Columbia supercontinent might have been resulted from the coupling between a mantle plume upwelling and some plate tectonic forces.

New U-Pb constraints on the age of the Little Dal Basalts and Gunbarrel-related volcanism in Rodinia

Diabase exposed amongst outcrops of the Little Dal Basalts (LDB) between the Coates Lake Group and overlying the Little Dal Group in the Mackenzie Mountains of northern Canada has a geochemical signature that is characteristic of the LDB and associated Tsezotene sills. Marginally concordant magmatic zircons from a sample of the diabase yielded CA-ID-TIMS weighted mean 207Pb/206Pb and 206Pb/238U dates of 778.4±1.8Ma (2σ) and 775.10±0.54Ma (2σ), respectively. These dates provide the first direct constraint on the age of the LDB and confirm a temporal relationship to the widespread Gunbarrel magmatic event that affected western North America during the protracted breakup of the supercontinent Rodinia. Hafnium isotopes were measured on three grains and calculated values of εHf at 775Ma are 9.5±0.4, 9.7±0.2 and 9.3±0.2 (2σ), consistent with a mantle plume genesis.Gunbarrel magmatism was not temporally related to the final fragmentation of Rodinia and the establishment of the west Laurentian continental margin (rift-drift transition), which did not occur until after 720Ma. However, the age and stratigraphic position of the LDB suggest the Gunbarrel event may have thermally weakened the lithosphere leading to a period of intracontinental rifting and the deposition of the Coates Lake Group rift-sediments prior to 732Ma.The sample of diabase also yielded a single 207Pb/206Pb 1565.7±1.5Ma xenocrystic zircon with a model Th/U ratio of 0.47 indicative of a crustal origin. Although there is limited evidence for felsic magmatism, metamorphism, and orogenesis of this age in western Laurentia, a range of felsic volcanic rocks and/or intrusive suites of this age do occur in southern, central and eastern Australia, supporting existing reconstructions of the Nuna/Columbia supercontinent that place the northwest margin of Laurentia adjacent to Australia.

A tale of two basins? Stratigraphy and detrital zircon provenance of the Palaeoproterozoic Turee Creek and Horseshoe basins of Western Australia

The 2445–2010Ma tectonic evolution of the southwestern Pilbara Craton is recorded by two depositional basins, but considering the time span involved it is pertinent to question whether only two are preserved and others have vanished. The first, the Turee Creek Basin, developed conformably on an ∼2450Ma back-arc volcanic province, and was closed long before ∼2208Ma. It was a retroarc basin sited in front of a magmatic fold-thrust belt on the southwestern margin of the craton. Its folded strata were intruded by ∼2208Ma dolerite sills that are the preserved rock record of a large igneous province. The basin and dolerites were deformed by the N-verging ∼2195–2145Ma Ophthalmia Fold-Thrust Belt. The second, the Horseshoe Basin, formed at ≤2050Ma, unconformably truncating folded strata of the Turee Creek Basin and the ∼2208Ma dolerites, and was closed prior to intrusion of ∼2008Ma dolerite dykes. It was an intracontinental rift basin sited on the inverted fold-thrust belt. Detrital zircon age-spectra for the basins have poor fit to the zircon age-spectrum of the Pilbara, and differ significantly in their youngest modes. The youngest mode for the Turee Creek Basin is ∼2442Ma, whereas the youngest mode for the Horseshoe Basin decreases upwards from ∼2276Ma to ∼2206Ma. Although differing in youngest detrital zircon age-modes, zircon age-spectra of the basins are otherwise similar, and broadly match the Glenburgh Terrane on the southwestern margin of the craton. While Turee Creek sediments were derived from the southwest, Horseshoe sediments were derived from the north and northeast. This conundrum is resolved by the Ophthalmia Fold-Thrust Belt, which tectonically loaded the craton to the north and northeast to form a now-vanished Ophthalmia foreland basin that derived sediment from the south and southwest, thereby emplacing zircon with an exotic age-spectrum on the North Pilbara. Inversion of tectonic elements occurred during formation of the Horseshoe Basin, with the fold-thrust belt subsiding beneath the rift while the foreland basin was uplifted. Recycling of the foreland basin inverted its detrital zircon age-spectrum, explaining why the youngest detrital zircon age-mode of the Horseshoe Basin decreases in age upwards.

Petrogenesis of the 2090 Ma Zanhuang ring and sill complexes in North China: A bimodal magmatism related to intra-continental process

It is widely debated as to whether the ∼2100Ma bimodal magmatic events in the Eastern North China craton represent intra-continental rifting or marginal processes. The genetic relationship between the two components of bimodal suites is the key solution to this issue; and this is investigated on a group of newly identified ring complexes, as well as the apparently associated sills in Zanhuang (Southern Taihang Mts.). These ring complexes comprise varied components: pyroxenite, gabbro, anorthosite, K-rich A-type granite and minor Na-rich A-type granite. U-Pb analyses on baddeleyite and zircon grains liberated from a gabbro sample both yield a ∼2090Ma age, which represents crystallization timing and is contemporaneous with A-type granites of the ring complexes, as well as the sills in the Gantaohe Group. The mafic rocks of the ring complexes and sills show similar chemical features: they were characterized by enriched light rare earth elements compared to middle and heavy ones (La/Ybn=1.4−2.8), enriched large lithosphile elements but depleted high field strength elements, and enriched Sr-Nd isotopes (εNdt (t=2090Ma)=−1.9–0.6, Nd TDM=∼2.7Ga). They were most likely originated from the same magma source resided in the sub-continental lithospheric mantle, which formed in the Late Archean. The compositional variation of the mafic sills may be resulted from the fractionation of olivine and plagioclase. The K-rich A-type granite, as well as the dacitic sills, could be originated from this same source with firstly olivine+plagioclase-dominated fractionation in the lower magma chamber to more felsic compositions, and then with significant removal of clinopyroxene±plagioclase in the upper magma chamber, which at the same time produced pyroxenite-gabbro and minor anorthosite cumulates in the ring complexes. Na-rich A-type granite was likely formed locally under relatively high-water fugacity with fractionation of K-feldspar+plagioclase±clinopyroxene from a parental magma similar to the K-rich A-type granite. This bimodal Zanhuang ring and sill complexes, and other comparable bimodal suites widely distributed in the Eastern North China craton, were likely intra-continental, representing a craton-scale extension at ∼2100Ma.

Igneous activity in the Bornu Basin, onshore NE Nigeria; implications for opening of the South Atlantic

The structure of igneous plumbing systems in circum-South Atlantic, intra-continental rift basins (e.g. the West and Central African Rift Systems) remains enigmatic owing to poor subsurface data coverage and quality. How magmatism in these basins related to the opening of the South Atlantic is thus poorly understood. We integrate 2D and 3D seismic reflection data ( c. 27 600 km 2 ), data from 23 boreholes, and field observations from the Bornu Basin and Upper Benue Trough, onshore NE Nigeria, to examine the timing and development of igneous bodies possibly related to opening of the South Atlantic. We identify numerous sills, which typically have saucer-shaped and en echelon morphologies, and extrusive volcanic cones. The igneous rocks are alkali basalts and dolerites. Seismic-stratigraphic relationships indicate that emplacement occurred in the Early Cretaceous (Albian to Cenomanian; c. 120 Ma), Late Cretaceous (Santonian to early Campanian; c. 83 Ma) and Cenozoic (Miocene; c. 22 Ma). Magmatism was broadly coeval with major plate boundary interactions, characterized by major azimuthal changes in fracture zones in the developing South Atlantic Ocean. The broad temporal correlation between intra-continental rift basin magmatism and plate boundary interactions suggests that periods of magma emplacement may have, in some way, been instigated by stress dissipation into intra-continental rift basins.

Provenance and depositional conditions of fluvial conglomerates and sandstones and their controlling processes in a rift setting, mid-Permian lower and upper Quanzijie low order cycles, Bogda Mountains, NW China

This study investigates how provenance lithology, transport distance, and depositional and paleoclimatic conditions had affected the composition and texture of fluvial conglomerates and sandstones in the Middle Permian lower and upper Quanzijie low-order cycles, Bogda Mountains, greater Turpan-Junggar intracontinental rift basin, NW China. Composition and texture of gravels in conglomerates and sandstones define two conglomerate and two sandstone petrofacies, respectively, in Tarlong-Taodonggou area in southern Bogda and Dalongkou area 70km to the north in northern Bogda. Conglomerate Petrofacies I occurs in Tarlong-Taodonggou and is dominated by basaltic gravels, whereas Conglomerate Petrofacies II occurs mainly in Dalongkou and is dominated by sedimentary gravels. Sandstone Petrofacies I occurs in Tarlong-Taodonggou and contains dominantly basaltic lithics, few sedimentary lithics, and minimal quartz. It is compositionally immature and texturally immature to submature. The compositional maturity increases upsection. Conglomerate Petrofacies I and sandstone Petrofacies I both indicate the young Tian Shan arc to the south as the main sediment source and rift shoulders as a local source. Sandstone Petrofacies II occurs in Dalongkou and contains dominantly sedimentary lithics and abundant and well-developed pedogenic clay coats. The petrofacies in Dalongkou indicate older rift sedimentary rocks exposed at rift shoulders as the main sediment source and distant suture zones as a minor source. In summary, the composition and texture of Quanzijie conglomerates and sandstones are mainly controlled by provenance lithology and, secondarily, depositional and paleoclimatic conditions and transport distance. The results substantiate the rift origin of the Turpan-Junggar basin in Middle Permian, which had many half-grabens and grabens and was bordered by a volcanic arc at the southern basin margin.

Late Mesozoic rifting at the flanks of the Dzhagda segment of the Mongolia-Okhotsk collisional orogen: global and regional aspects

The role of rifting in the formation of the recent structure of the Mongolia-Okhotsk orogen is extremely high, but it is still underestimated with regard to flanks of the Dzhagda segment of this orogen. Current researches refer to a combination of physical and chemical processes in the depth of the lithosphere, as well as interactions between the Izanagi, Eurasian and Pacific plates as explanations of repeated rifting events in East Asia. Upwelling of the asthenosphere due to significant differences in the lithosphere thickness (150–200 km under cratons, and only 100 km under orogenic belts) was viewed as a cause of rifting. It was assumed that rifting was controlled by mantle plumes, volcanism and heat regime. Structures bordering the Mongolia-Okhotsk orogen from north and south were considered as superimposed or marginal troughs. Recent studies have revealed numerous riftogenic Late Mesozoic structures in the Central Asian orogenic belt, which resulted from the collision of the Siberian and North Chinese cratons. New geological survey and geochemical data on volcanites confirmed the riftogenic origin of the Zeya-Uda (or Uda) and Nora-Selemdzha troughs bordering the Mongolia-Okhotsk orogen from north and south, respectively (Fig. 1, and 2). Geology and geophysics of those troughs has been described. It is noted that riftogenic volcanites formed later in the east than those in the west. The Late Mesozoic rifting is widely manifested in North Eastern Asia across the area exceeding two million square kilometers, from Lake Baikal to the Sikhote-Alin region (west to east) and from the Southern Yakutia basins to North China (north to south). It is evidenced by intra-continental rifts of various trends, volcanic provinces and extension structures along large strike-slip faults [ Ren et al., 2002 ]. The Uda and Nora-Selemdzha marginal troughs located along the Dzhagda segment of the Mongolia-Okhotsk orogen give evidence that compression was replaced by extension in the study area. Rifting structures may be due to physical and chemical processes, the development of plumes [ Yarmolyuk et al., 2000 ], as well as the interaction between the Pacific and Eurasian lithospheric plates. Volcanic activity took place earlier in the west and then propagated to the east due to the shifting of the subduction zone in this direction. This paper analyzes regional and global geological events on the basis of new drilling data and the geochronological dating of volcanites. It describes the Late Mesozoic stage of rifting at the flanks of the Dzhagda segment of the Mongolia-Okhotsk collisional orogen.

Whole-rock geochemical, U-Pb and Sm-Nd isotope characteristics of the Dongueni Mont nepheline syenite intrusion, Mozambique

The Dongueni Mont nepheline syenite intrudes migmatitic paragneisses and siliciclastic metasediments of the Barue Complex, Mozambique. This study reports the whole-rock geochemical, U-Pb and Nd isotopic data of the nepheline syenite. The ferroan and alkalic geochemical characteristics are typical of alkaline rocks formed in a within-plate setting. The strong depletion in high field strength elements (HFSEs) (e.g. Ba, Nb, P, and Ti) and enrichment in large ion lithophile elements (LILEs) (e.g. Rb, Th, K, and Pb) are consistent with magmatism in a continental alkaline magmatic province associated with intracontinental rifting. Zircon U-Pb data yielded crystallization ages from 498 ± 19 to 562 ± 14 Ma, consistent with the Pan-African Orogeny and the inherited zircons yield an age of 1040 Ma, which supports the presence of a Mesoproterozoic crust. The εNd (t) values from the nepheline syenite samples range from −15.1 to −16.1 and the TDM values from 1.77 to 1.67 Ga, which indicate that the initial nepheline syenite magma formed from a tholeiitic or mantle source in a within-plate setting with crustal assimilation.

Isotopic evidence for a lithospheric origin of alkaline rocks and carbonatites: an example from southern Africa

Alkaline rocks and carbonatites, including nepheline syenites, are well established as mantle-derived magmatic products, but the nature and location of their mantle sources is debated. Some workers have used isotopic compositions to infer mixed mantle plume type sources such as EM1, HIMU, and FOZO, implying derivation from the subcontinental asthenosphere. Other models favour an entirely lithospheric source, whereby the magmas, originally formed during intracontinental rifting, became deformed and subducted into the mantle lithosphere during later continental collisions, and constituted part of a source component for later rift-related alkaline and carbonatite magmatism. We tested this model using Sr, Nd, and Hf isotopic compositions of deformed and undeformed nepheline syenites and carbonatites from three occurrences in southern Africa, representing emplacement over a ∼1 Ga time span. These include Bull’s Run, South Africa (1134 Ma); Tambani, Malawi (726 Ma); and the Chilwa Alkaline Province, Malawi (130 Ma). Mixing modelling indicates that the isotopic compositions of the early Cretaceous Chilwa samples can be accounted for if their source consisted of a blend of ∼99% depleted subcontinental mantle lithosphere and ∼0.5%–1% of a subducted component similar to the Neoproterozoic Bull’s Run nepheline syenites. We do not consider the Bull’s Run material specifically as the component involved in the Chilwa source, but our model illustrates an example of how recycled, older, alkaline magmatic rocks can contribute to the mantle sources of younger alkaline rock and carbonatite magmatism. This model accounts for the observation of recurrent alkaline rock and carbonatite magmatism over hundreds of millions of years in spatially restricted areas like southern Africa. Carbonatite and related alkaline magmatic rocks, therefore, need not owe their origin to deep, sublithospheric melting processes.

3.30 Ga high-silica intraplate volcanic–plutonic system of the Gavião Block, São Francisco Craton, Brazil: Evidence of an intracontinental rift following the creation of insulating continental crust

High-silica rhyolites having U–Pb zircon ages of 3303±11Ma occur along the eastern border of the Gavião Block (Brazil) associated with the Contendas-Mirante and Mundo Novo supracrustal belts. Unlike many Archean greenstone sequences, they are not interlayered with mafic to intermediate units. Instead, they belong to an inter-related plutonic–volcanic system, together with granitic massifs having similar zircon crystallization ages of ca. 3293±3Ma and 3328±3Ma and plotting along the same geochemical trends as the rhyolites. The rhyolites show well-preserved primary volcanic features such as magma flow textures and euhedral phenocrysts. High emplacement temperatures are indicated by petrographic evidence (β-quartz phenocrysts), zircon saturation temperatures (915–820°C) and geochemical data, especially high SiO2 (74–79wt.%) together with elevated Fe2O3(T) (~3wt.%), MgO (0.5–1.5wt.%) and low Al2O3 (<11wt.%). The rhyolites show homogeneous trace element ratios (La/YbN 4.8±1.8; EuN/Eu* ~0.55; Sr/Y ~0.7) and negative ԐHf(3.3Ga) from 0 to −7, indicating derivation from a single crustal source for both occurrences. Specifically, the rhyolites would have derived from extraction and eruption of highly silicic residual liquid formed by crystallization of granitic magma in a relatively shallow (<10km) reservoir, now represented by the granite massifs. The granite magma was formed by melting or differentiation of material similar to the diorite gneiss that occurs regionally. The 3.30Ga volcanic–plutonic systems formed after a period of crustal growth and stabilization of a thick continental lithosphere, represented by massive 3.40–3.33Ga TTG and medium to high-K calk-alkaline magmatism in the Gavião Block. The 3.30Ga-old rhyolites and granites would therefore have formed in an intracontinental tectonic setting after the formation and stabilization of new continental crust, and accordingly would represent the first stages of rifting and continental break-up. Intraplate magmatism and intracrustal differentiation processes took place on Earth at 3.3Ga and produced magmas that were distinct from Archean TTGs, questioning the reliability (or at least the uniqueness) of “intraplate models” to explain the origin of the latter.

Classification and hydrocarbon distribution of passive continental margin basins

Abstract Sixty-six passive continental margin basins around the world were compared comprehensively from the aspect of seismogeology on the basis of plate tectonics. According to their textural and structural differences, passive continental margin basins were classified into seven subdivisions, i.e., rifted basin, non-saline faulted depression basin, saline faulted depression basin, non-saline depression basin, saline depression basin, delta reformed basin and positive reverse deformed basin. The passive continental margin basins around the world have been generated with the formation of Mesozoic and Cenozoic Atlantic and Indian Oceans and they have experienced superimposition of three prototypes, including intra-continental rift in rifting period, intercontinental rift in transitional period and passive continental margin in drifting period. In rifted basins, the petroleum systems are mainly located in the lower lacustrine/marine rift series of strata and the thinner depression series of strata at the upper part are only regional cap-rocks. Large oil and gas fields are mainly concentrated in structural traps of rift series of strata. In non-saline faulted depression basins, hydrocarbon generation and expulsion peaks occur in both upper thicker marine depression series of strata and lower rift series of strata. Reservoirs are formed in the structures of rift series of strata, and oil and gas are highly concentrated at slope fans in depression series of strata. In saline faulted depression basins, large oil and gas fields are mainly distributed in the lagoon carbonate rocks of subsalt series of strata and the deepwater slope fans of suprasalt depression series of strata. In saline depression basins, only petroleum systems in depression series of strata are active, and various traps are developed, such as slope fan, salt structure and bioherm. In non-saline depression basins, large oil and gas fields are mainly located in submarine fan groups of depression series of strata because this type of basins are of narrow continental shelf and steep continental slope. In delta reformed basins, four major ring-like structure belts (i.e., growth faulting-mud diapir-thrust nappe-foredeep gentle slope) are formed from the shore to the deepwater and large oil and gas fields can be formed in each belt. Positive reverse reformed basins are the passive continental margin basins which are influenced by global orogenesis since the Miocene. In this type of basins, oil and gas are concentrated in compressional anticlines of reverse series of strata.

Magma source feature and eruption age of volcanic rocks in the Tram Tau district, Tu Le Basin

Magma source feature and eruption age of volcanic rocks in the Tram Tau district, Tu Le Basin

Structural Characteristics and Formation Dynamics: A Review of the Main Sedimentary Basins in the Continent of China

AbstractThe formation and evolution of basins in the China continent are closely related to the collages of many blocks and orogenic belts. Based on a large amount of the geological, geophysical, petroleum exploration data and a large number of published research results, the basement constitutions and evolutions of tectonic–sedimentary of sedimentary basins, the main border fault belts and the orogenesis of their peripheries of the basins are analyzed. Especially, the main typical basins in the eight divisions in the continent of China are analyzed in detail, including the Tarim, Ordos, Sichuan, Songliao, Bohai Bay, Junggar, Qiadam and Qiangtang basins. The main five stages of superimposed evolutions processes of basins revealed, which accompanied with the tectonic processes of the Paleo–Asian Ocean, Tethyan and Western Pacific domains. They contained the formations of main Cratons (1850–800 Ma), developments of marine basins (800–386 Ma), developments of Marine–continental transition basins and super mantle plumes (386–252 Ma), amalgamation of China Continent and developments of continental basins (252–205 Ma) and development of the foreland basins in the western and extensional faulted basin in the eastern of China (205–0 Ma). Therefore, large scale marine sedimentary basins existed in the relatively stable continental blocks of the Proterozoic, developed during the Neoproterozoic to Paleozoic, with the property of the intracontinental cratons and peripheral foreland basins, the multistage superimposing and late reformations of basins. The continental basins developed on the weak or preexisting divisional basements, or the remnant and reformed marine basins in the Meso–Cenozoic, are mainly the continental margins, back–arc basins, retroarc foreland basins, intracontinental rifts and pull–apart basins. The styles and intensity deformation containing the faults, folds and the structural architecture of regional unconformities of the basins, responded to the openings, subductions, closures of oceans, the continent–continent collisions and reactivation of orogenies near the basins in different periods. The evolutions of the Tianshan–Mongol–Hinggan, Kunlun–Qilian–Qinling–Dabie–Sulu, Jiangshao–Shiwandashan, Helanshan–Longmengshan, Taihang–Wuling orogenic belts, the Tibet Plateau and the Altun and Tan–Lu Fault belts have importantly influenced on the tectonic–sedimentary developments, mineralization and hydrocarbon reservoir conditions of their adjacent basins in different times. The evolutions of basins also rely on the deep structures of lithosphère and the rheological properties of the mantle. The mosaic and mirroring geological structures of the deep lithosphère reflect the pre–existed divisions and hot mantle upwelling, constrain to the origins and transforms dynamics of the basins. The leading edges of the basin tectonic dynamics will focus on the basin and mountain coupling, reconstruction of the paleotectonic–paleogeography, establishing relationship between the structural deformations of shallow surface to the deep lithosphère or asthenosphere, as well as the restoring proto–basin and depicting residual basin of the Paleozoic basin, the effects of multiple stages of volcanism and paleo–earthquake events in China.

Gallium concentration in natural melts and fluids

We generalize, for the first time, published and original data on the gallium concentrations in natural magmatic melts and fluids obtained by studying quenched glasses in volcanic rocks and inclusions in minerals. Based on 2688 determinations, gallium concentrations in magmatic melts vary between 0.47 and 495 ppm at average content of 18.0 ppm (+4.2/–3.4). Gallium concentrations in magmatic melts generated in different geodynamic settings show different distribution. Minimum concentrations (on average, 16.0 ppm, +3.6/–2.9) are typical of the island-arc melts, while maximum contents were determined in melts of oceanic islands (on average, 29.1 ppm, +13.4/–9.2) and intracontinental rifts and hot spots (26.5 ppm, +25.4/–13.0). Published and new 339 determinations of gallium concentrations in natural fluids indicate the wider range of their variations as compared to those of melts: from 0.02 to 11260 ppm, at average 1.6 ppm (+10.8–1.4). The possible gallium fractionation in fluid—magmatic systems is discussed.

Petrogenesis of the Dalongkai ultramafic-mafic intrusion and its tectonic implication for the Paleotethyan evolution along the Ailaoshan tectonic zone (SW China)

Layered ultramafic-mafic intrusions are usually formed in an arc/back-arc or intra-plate tectonic environment, or genetically related to a mantle plume. In this paper, we report on an ultramafic-mafic intrusion, the Dalongkai intrusion in the Ailaoshan tectonic zone (SW China), whose occurrence is closely associated with arc/back-arc magmatic rocks. The Dalongkai intrusion is composed of plagioclase-lherzolite, hornblende-peridotite, lherzolite and wehrlite at the bottom, cumulate plagioclase-pyroxenite at the middle part, changing to fine-grained gabbro towards the upper part of the intrusion, forming layering structure. Zircons from the plagioclase-pyroxenites and gabbros yielded U-Pb ages of 272.1±1.7Ma and 266.4±5.8Ma, respectively. The plagioclase-pyroxenites show cumulate textures, and are characterized by high MgO (25.0–28.0wt.%; mg#=80.6–82.3), Cr (1606–2089ppm) and Ni (893–1203ppm) contents, interpreted as early cumulate phases. By contrast, the gabbros have relatively lower mg# values (56.3–62.7), and Cr (157–218ppm) and Ni (73–114ppm) concentrations, and may represent frozen liquids. The plagioclase-pyroxenites and gabbros share similar chondrite-normalized REE patterns and primitive mantle-normalized trace element profiles which are analogous to those of typical back-arc basin basalts. The εNd(t) values for both rock types range from +2.20 to +4.22. These geochemical and isotopic signatures suggest that the Dalongkai ultramafic-mafic rocks originated from a MORB-like mantle source metasomatized by subduction-related, sediment-derived fluids. Our data, together with other geological evidence, indicate that the emplacement of the Dalongkai ultramafic-mafic intrusion most likely occurred in a back-arc extensional setting associated with subduction of the Ailaoshan Paleotethyan branch ocean during the Middle Permian, thus ruling out the previously speculated linkage to the Emeishan mantle plume, or to an intra-continental rift.

Mesozoic rift to post-rift tectonostratigraphy of the Sverdrup Basin, Canadian Arctic

Jurassic-Cretaceous rift successions and basin geometries of the Sverdrup Basin are reconstructed from a review and integration of stratigraphy, igneous records, outcrop maps, and subsurface data. The rift onset unconformity is in the Lower Jurassic portion of the Heiberg Group (approximately 200–190 Ma). Facies transgress from early syn-rift sandstones of the King Christian Formation to marine mudstones of the Jameson Bay Formation. The syn-rift succession of marine mudstones in the basin centre, Jameson Bay to Deer Bay formations, ranges from Early Jurassic (Pleinsbachian) to Early Cretaceous (Valanginian). Early post-rift deposits of the lower Isachsen Formation are truncated by the sub-Hauterivian unconformity, which is interpreted as a break up unconformity at approximately 135–130 Ma. Cessation of rift subsidence allowed for late post-rift sandstone deposits of the Isachsen Formation to be distributed across the entire basin. Marine deposition to form mudstone of the Christopher Formation throughout the Canadian Arctic Islands and outside of the rift basin records establishment of a broad marine shelf during post-rift thermal subsidence at the start of a passive margin stage. The onset of the High Arctic Large Igneous Province at approximately 130 Ma appears to coincide with the breakup unconformity, and it is quite typical that magma-poor rifted margins have mainly post-rift igneous rocks. We extend the magma-poor characterization where rifting is driven by lithospheric extension, to speculatively consider that the records from Sverdrup Basin are consistent with tectonic models of retro-arc extension and intra-continental rifting that have previously been proposed for the Amerasia Basin under the Arctic Ocean.

Multiple rifting and alkaline magmatism in southern India during Paleoproterozoic and Neoproterozoic

The Southern Granulite Terrane (SGT) in India preserves the history of tectonothermal events ranging from Paleoarchean to latest Neoproterozoic–Cambrian. Here we investigate alkaline magmatism possibly associated with rifting events in Paleoproterozoic and Neoproterozoic based on petrological, geochemical and zircon U–Pb and Lu–Hf isotopic studies on the alkaline complexes of Korangani (KGAC) and Kambamettu (KAC) in the Madurai Block of SGT. The mica pyroxenite which represents the first intrusive phase at KGAC crystallized from a mildly alkaline hydrous magma derived from a metasomatized mantle. The younger shoshonitic syenite was emplaced at 2533±16Ma, carries mafic microgranular enclaves, and shows trace-elements ratios consistent with magma mixing trend, and zircon εHf(t) values display mixed positive and negative values −2.6 to 3.6 suggesting the mixing of adakite-like felsic crustal melt and non-adakitic mantle derived melt. In KAC, four distinct magmatic intrusions are identified: i) quartz-monzonite (emplaced at 2498±16Ma), an ultrapotassic adakitic rock derived from a carbonated alkali-rich lower crustal source with negative zircons εHf(t) values in zircon (−8.0 to −0.8); Y/Nb (>1.2) and Th/Ce (0.03–0.8) ratios; lower Ni (<30ppm) and Cr (<14ppm) contents; ii) phlogopite-rich pyroxenite, crystallized from an alkali-rich basaltic parental magma derived from carbonate metasomatized mantle; iii) mantle derived high Ba–Sr carbonatite (emplaced at 2470±15Ma); and iv) shoshonitic peralkaline syenite rock (emplaced at 608±6Ma) with strong adakitic signature, low MgO (<1wt.%), Ni (12–5ppm) and Cr (49–35ppm) contents and negative zircon εHf(t) values (−30.3 to −27.3) and trough of Zr–Hf in spidergrams suggesting a carbonated alkali-rich garnet-bearing crustal source.The geochemical features and petrogenetic considerations of the felsic shoshonitic-ultrapotassic adakite-like rocks (syenite, quartz monzonite), mica-pyroxenites and carbonatite reveal rift-related extensional tectonic settings with magma sources involving both mantle and crustal components during Paleoproterozoic. The late Neoproterozoic shoshonitic adakite-like syenite from KAC also formed in a rift-related setting. In both cases, intracontinental rifting associated with post-collisional extension is proposed.

Is the Paleoproterozoic Jiao-Liao-Ji Belt (North China Craton) a rift?

As a typical example of the Paleoproterozoic crust in the Eastern Block of the North China Craton, the Paleoproterozoic Jiao-Liao-Ji Belt consists principally of the Liaohe Group (and its equivalents), Liaoji granites and mafic intrusions. Previous studies indicate that the evolution of the Jiao-Liao-Ji Belt has been mainly attributed to the opening and closing of an intracontinental rift along the eastern continental margin of the North China Craton. Here we synthesize the Paleoproterozoic magmatism, sedimentation, metamorphism and metallogeny against the rift model and propose a process of arc-continent collision between the northern Longgang and the southern Nangrim Blocks. This conclusion is consistent with the observations, including that (a) the 2.0- to 2.2-Ga magmatism shows a typical subalkaline series, rather than a bimodal distribution, since the mafic rocks mostly have arc affinities and the acidic–intermediate rocks belong to the calc-alkaline series; (b) the main source of the 1.9- to 2.0-Ga sedimentary rocks is the Paleoproterozoic arc materials, indicating a fore-arc or back-arc basin setting; (c) a couple of big borate deposits occur in the boron-rich volcanic rocks that were formed in convergent continental margins; (d) the North and South Liaohe Groups show different rock associations and metamorphic histories (P–T paths); and (e) the Nangrim and Longgang Blocks vary in lithological units, geochronology and metamorphic features. Thus, an arc-continent collision tectonic scenario for the Paleoproterozoic Jiao-Liao-Ji Belt is involved: (a) a southward subduction in the period 2.0–2.2 Ga; (b) sedimentation during the period 1.9–2.0 Ga; (c) arc-continent collision at ca. 1.9 Ga; and (d) post-collisional extension at 1.82–1.87 Ga, marking the end of the Paleoproterozoic tectonothermal event.

Marine Mo biogeochemistry in the context of dynamically euxinic mid-depth waters: A case study of the lower Cambrian Niutitang shales, South China

Molybdenum (Mo) concentrations and Mo isotopes have been widely used as proxies for local and global redox conditions in early oceans (>520Ma) that were stratified and characterized by dynamically euxinic mid-depth waters. However, the nature of the Mo cycle and accompanying isotopic fractionations in such oceans remain poorly known. To fill this gap, we conducted an integrated study of Mo isotopes and redox-sensitive trace element (RSTE) abundances in the Lower Cambrian Niutitang Formation at Yangjiaping, South China. This section accumulated on the northern shelf margin of the Nanhua Basin, a failed intracontinental rift basin with a good connection to the open ocean during the early Cambrian. The Niutitang Formation contains a ∼18-m-thick lower black shale member, and a ∼56-m-thick upper gray silty shale member. The lower member (LM) is moderately to strongly enriched in Mo, U and V, and heterogeneous in Mo isotopic composition (δ98Mo=−0.65‰ to +2.14‰), indicative of dominantly euxinic depositional conditions punctuated by ferruginous episodes (as shown by previously reported Fe speciation data). The upper member (UM) shows lesser enrichment of Mo, U and V, higher Mo/U ratios, and intermediate and more uniform Mo-isotopic compositions (δ98Mo=+1.16‰ to +1.71‰), indicative of weakly oxic to anoxic-euxinic depositional conditions. Geochemical profiles suggest that the LM-to-UM transition reflects a shift of the O2/H2S chemocline from the water column to the sediment. Large δ98Mo fluctuations in the LM may record variations of H2S concentrations in the mid-depth euxinic waters. The intermediate and relatively uniform δ98Mo values of the UM are attributed to the effects of a local Fe–Mn particulate shuttle.In light of the observations at Yangjiaping and other contemporaneous sections in the Nanhua Basin, we propose a new marine Mo biogeochemical model for the early Cambrian ocean. In this model, an Fe–Mn reduction zone may have developed above mid-depth euxinic waters in nearshore areas of the Nanhua Basin due to relatively high availability of fluvial Fe–Mn oxides and their subsequent reduction below the chemocline. Within this redox framework, large sedimentary δ98Mo variations observed for the Niutitang shales can be explained through the combined effects of Mo isotopic fractionation associated with adsorption onto Fe–Mn-oxides and spatially variable H2S concentrations in the euxinic watermass. By compiling published data, we found that the large sedimentary δ98Mo variations previously observed in the early Cambrian Nanhua Basin and in other Neoproterozoic-Cambrian marine basins can be similarly explained. Our model provides a novel interpretation of the Mo biogeochemistry of early Earth oceans, within the framework of which Mo isotopes may serve to further our understanding of the oxygenation history of the Earth-surface system.

Petrology, geochemistry and tectonic setting of intrusives around omam (east of guilan)

Intrusives around Omam (East of Guilan) with gabbro, monzo-gabbro-porphyry, syenite and granite compositions have intruded to upper Cretaceous lavas, sedimentary and pyroclastic rocks. The geochemical data of the mentioned intrusives on the variation diagrams show that these rocks are categorized into mafic and felsic groups and in terms of magmatic series, they are often from alkaline one. Relative continuity between most intrusive mafic rocks data on the variation diagrams and the parallel feature of their rare earth elements patterns clearly show the derivation of this group from a common origin. Based on the severity of changes in strongly compatible elements versus the strongly incompatible elements, the dominant process governing the magmatic evolution for mafic rocks is fractional crystallization. REE patterns for different types of mafic rocks are in parallel with each other and have a uniform and downward slope, while the slope of felsic varieties patterns for LREE is steep and descending and flat for HREE. Based on the La/Sm ratio, it seems that the melts forming mafic rocks are derived from the enriched mantle with spinel lherzolite composition and partial melting degree of 4 to 10 percent. On the tectonic setting discrimination diagrams, the intrusive rocks of the studied area are categorized in the territory of intracontinental rifts.