Continental Basalts Research Articles

Deciphering the origin of a basanite-alkali basalt-tholeiite suite using Zn isotopes

Basanite-alkali basalt-tholeiite suites have unique chemical and isotopic signatures associated with recycled crustal materials. However, the relative contribution of recycled carbonates and silicates is unclear owing to the inability of traditional isotopic approaches (e.g., Sr, Nd, Pb, and O isotopes) to fully identify trace amounts of recycled crustal materials. On the other hand, the identification of the types of recycled components would help decipher origin of these magmatic suites. Since the zinc isotopic ratios (reported as the per mil deviation of the 66Zn/64Zn ratio from the JMC-Lyon standard, δ66Zn) are distinct between marine carbonates and silicate reservoirs (e.g., mantle, oceanic mafic crust, and siliciclastic sediments), they could be used to identify the different source endmembers in magmatic rocks. In this work, we used the Zn and Sr-Nd isotopic ratios of Cenozoic continental basalts from the Central North China Craton (also known as Trans-North China Orogen, TNCO) to identify the nature of the recycled materials present in the mantle source of these basanite-alkali basalt-tholeiite suite. We found that, basanites and alkali basalts have high δ66Zn values (ranging from 0.32‰ to 0.46‰), while tholeiites have low δ66Zn values (0.28 ± 0.04‰, 2sd) similar to mid-oceanic ridge basalts (MORB) (δ66Zn = 0.27 ± 0.05‰, 2sd). Given the limited extent of Zn isotopic fractionation (<0.1‰) during crystallization and <20% partial melting, the elevated δ66Zn values observed in the basanites and alkali basalts reflect the involvement of recycled marine carbonates (which have an average δ66Zn of ∼0.91‰) in the mantle sources. Moreover, the δ66Zn values of these basalts show clear correlations with carbonate metasomatism indexes (such as, Ti/Eu and Zr/Hf ratios). Mixing models for δ66Zn values vs. 87Sr/86Sr ratios and εNd(t) values demonstrate that the mantle sources of basanites, alkali basalts, and tholeiites were metasomatized by different proportions of carbonates and siliceous sediments. Furthermore, both isotopic and geochemical compositions exhibit symmetrical variations from the central zone towards the south and the north along the TNCO, that is, δ66Zn values and 143Nd/144Nd and Ce/Pb ratios decrease, while SiO2 content and 87Sr/86Sr ratios increase. We suggest that more carbonates and fewer siliceous sediments are released from the subducted slab, with an increase in the depth of subduction. We further propose that this process is one of the most important causes of compositional variations in the basanite-alkali basalt-tholeiite suite.

Low H2O/Ce ratios and δ18O values for continental basalts in eastern China: Geochemical evidence for involvement of the dehydrated crustal component in the mantle source

Crustal components derived from subducted oceanic crust have been increasingly identified in the mantle sources of intraplate basalts. The geochemical processes in transferring the crustal signatures into the mantle sources of these basalts has been documented to be similar to those for the origin of island arc basalts. However, the dehydration melting of subducting oceanic slabs is assumed to occur at postarc depths, releasing less water than the dehydration that occurs at subarc depths. This study examines this assumption using a combination of mineral H2O/Ce and O isotope ratios for Cenozoic continental basalts in eastern China, an area where the western Pacific plate is subducting beneath its continental margin. Clinopyroxene phenocrysts from these continental basalts show H2O/Ce ratios of 108–204 for their magma and δ18O values of 3.9‰-5.0‰ for the clinopyroxene, both of which are lower than the values of the depleted MORB mantle. This suggests that the basalts were derived from a mantle source enriched by the dehydrated lower oceanic crust-derived component. Mineral H2O/Ce thermometry yields high temperatures of 1055 °C–1267 °C that represent the conditions of chemical metasomatism at the slab-mantle interface, significantly higher than those in generating the mantle sources of island arc basalts. On the other hand, lithochemical thermometry of the basalts yields much higher temperatures of 1456 °C–1522 °C that are indicative of the condition of mantle melting. While the substantial differences between these two temperatures indicates two-stage processes for origin of the continental basalts, the considerably higher metasomatic temperatures testify the dehydration melting of subducting oceanic slabs at postarc depths. These interpretations are verified by quantitative modeling of geochemical differentiation and mixing in an oceanic subduction zone. Therefore, the origin of intraplate basalts generally involves dehydration melting of subducting oceanic slabs at much greater depths than that of island arc basalts.

The Influence of Trap Magmatism on the Geochemical Composition of Brines of Petroliferous Deposits in the Western Areas of the Kureika Syneclise (Siberian Platform)

Abstract —We present the results of study of the influence of trap magmatism on the geochemical composition of brines and on the geothermal regime of the Earth’s interior in the western areas of the Kureika syneclise. The Siberian trap province, which unites all cutting and layered tholeiite–basic magmatic intrusions and erupted basaltic lava, is the world’s largest Phanerozoic continental basalt province. Brines, hydrocarbon deposits, and organic matter of the sedimentary cover were subjected to a significant thermal impact as a result of the Permo-Triassic trap magmatism. During the trap intrusion, the maximum paleotemperatures in major Silurian (D’yavolskii), Ordovician (Baikit), and Cambrian (Deltula–Tanachi, Abakun, and Moktakon) productive horizons reached 650 °C. The Paleozoic and Proterozoic deposits of the study area contain brines with TDS = 50–470 g/dm3. By chemical composition, they are of Na, Na–Ca, Ca–Na, Ca–Mg, and Ca chloride types (according to the classification by S.A. Shchukarev), with mixed Ca–Na and Na–Ca chloride brines dominating. The studied brines can be divided into three groups according to the degree of metamorphism: low (S1), medium (S2), and high (S3). The first group includes mainly sodium chloride brines with TDS = 50–370 g/dm3 (rNa/rCl = 0.60–0.95; S ≤ 100). The second (dominating) group comprises Na–Ca, Ca–Na, Ca, and Ca–Mg chloride brines with TDS = 150–470 g/dm3 (rNa/rCl = 0.10–0.87; 100 ≤ S ≤ 300). The third group is Ca–Na and Ca chloride brines with TDS = 223–381 g/dm3 (rNa/rCl = 0.12–0.45; S ≥ 300). We have first established changes in the hydrogeochemical field (major- and trace-component and gas compositions) with distance from the contacts of intruded dolerite sills and dikes. Hydrocarbons (CH4, C2H6, C3H8, i-C4H10, n-C4H10, i-C5H12, n-C5H12, and C6H14) and water-soluble components I, B, and NH4 were most actively subjected to destruction. For example, at a distance of 100 m from the intrusion zone, the water-dissolved gases are dominated by CO2 (&amp;gt;90 vol.%), and CH4 amounts to 5 vol.%, whereas at a distance of 250 m, the concentration of CO2 decreases to 30 vol.%, and that of CH4 increases to 60–70 vol.%. In addition to the negative effect on the hydrocarbon preservation in the contact zone (≤400 m), the intrusive trap magmatism favored the formation of hydrocarbons in remote horizons. The reaction of intruding traps with brines of the sedimentary cover led to the saturation of the latter with iron, aluminum, and silica, which suggests extraction of metals in the form of salts from magmatic melts into an ore-bearing fluid.

Metamorphic gabbro and basalt in ophiolitic and continental nappes of the Zermatt region (Western Alps)

The composition of meta-gabbro and meta-basalt occurring abundant and widespread in all nappes of the nappe stack exposed in the Zermatt region of the Western Alp shows distinct patterns related to the geodynamic origin of metamorphic basic rocks. Eclogitic meta-basalts of the ophiolitic Zermatt-Saas Unit (ZSU) show enriched MORB signatures. The meta-basalts (eclogites) of the continental fragment of the Theodul Glacier Unit (TGU) derive from pre-Alpine metamorphic continental intraplate basalts. Meta-basalts (eclogites) from the continental basement of the Siviez-Mischabel nappe (SMN) derive from MORB thus a genetic relation to the TGU eclogites can be excluded. All basic igneous rocks experienced post-magmatic alteration by fluid-rock interaction ranging from processes at the seafloor, in the shallow crust, during subduction zone hydration, in the exhumation channel and late Alpine regional metamorphisms. The consequences of these alteration processes can be identified at various levels in the rock composition data. It was found that the REE data are little affected by fluid-rock alteration. Some trace elements, notably Cs, Rb, and Ba are typically massively altered relative to igneous compositions in all three groups of meta-basalts. Generally, meta-basalts from the TGU and the SMN preserved the features of the original composition whilst the ZSU meta-volcanic rocks experienced massive alteration. For the ZSU meta-volcanic rocks it is evident that Zr was gained and Y lost during high-pressure fluid-rock interaction indicating a mobile behavior of the two elements during HP-metamorphism in contrast to their behavior in hydrothermal near-surface fluid-rock interaction.

Barium isotope evidence for recycled crustal materials in the mantle source of continental basalts

To investigate the potential of using Barium (Ba) isotopes to trace recycled crustal materials, we report high-precision Ba isotopic compositions for 22 well-characterized samples of Cenozoic basalts from the Taihang Mountains, North China Craton. The basalts have δ138/134Ba [= (138Ba/134Basample/138Ba/134BaSRM3104a – 1) × 1000] of −0.06‰ to +0.03‰(±0.05‰，2SD), lower than the depleted upper mantle (+0.03‰ to +0.15‰). Because Ba isotope fractionation during basaltic magmatic differentiation is negligible, the light Ba isotopic compositions likely reflect the involvement of isotopically light crustal materials in the mantle source. The Ba isotopic variations, combined with the data of elements and SrNd isotopes of the basalts, indicate that the light Ba isotopic compositions mainly resulted from marine sediments rather than altered oceanic crust. The quantitative modeling using Sr-Nd-Ba isotopic compositions suggests that about 0.5% to 3% of marine sediments were involved in generating the continental basalts. Our study here highlights the potential of using Ba isotopes to trace recycled materials in the mantle.

Early–Middle Permian plutons in the Langshan area, western Inner Mongolia, China, and their tectonic implications

Whether the southward subduction of the Paleo-Asian Ocean (PAO) beneath the northern North China Craton (NCC) was still on-going during Permian is a key issue for understanding the tectonic evolution of the southeastern Central Asian Orogenic Belt (Xing'an–Mongolia Orogenic Belt), and the voluminous Permian plutons distributed along the northern margin of the NCC can provide important insights into this controversy. New LA-ICP-MS zircon U–Pb ages from 4 basic, 6 intermediate and 11 acidic plutons in the Langshan area in northwestern NCC indicate that the plutons were formed during the Early–Middle Permian (290–265 Ma). Four basic diorites show affinities with continental basalts and probably crystallized from mantle-derived melts, whereas the intermediate and most acidic plutons are calc-alkaline I-type granitoids and four acidic plutons have the features of aluminous A2-type granites, and all of them probably originated from partial melting of various crustal materials. The Early–Middle Permian plutons and coeval bimodal volcanic rocks and continental basaltic intrusions occur together in both the northern NCC and adjacent Xing'an–Mongolia Orogenic Belt, implying a uniform extensional setting at that time. In addition to the intense magmatism, the regional extension also resulted in the development of a Red Sea-like limited oceanic basin within the Xing'an–Mongolia Orogenic Belt, and its subsequent closure may have created the Solonker suture. Such a tectonic scenario is in agreement with the Late Permian–Early Triassic blueschist-facies metamorphism induced by warm subduction of young oceanic slabs, Permian stratigraphic and paleontological data, and paleomagnetic record.

Geochemical Characteristics and Genesis Analysis of Deju Basalt in Midu County, Western Yunnan

Deju basalts is a characteristic volcanic association and was first founded in the 1 : 50 000 regional geological survey. There is no previous study. The basalts’ characteristics and genesis are systematically studied from the geochemistry. It is showed that the basalts’s w(SiO2) = 50.61‒54.54%, w(K2O + Na2O) = 0.15‒5.96%, w(P2O5) = 0.18‒0.45%, w(TiO2) = 1.82‒3.58%, belong to sub alkaline basalt series. Chondrite-normalized REE patterns indicate the basalts are relatively rich in LREE but deficient in HREE (LREE/HREE = 5.81‒6.51), and have a similar mantle source with WPB and OIB. The mantle source have a characteristics of GP. The discrimination diagrams of basalt tectonic setting reflect the basalts in Deju belongs to continental intraplate basalt, formed in a continental rift stretching environment, and during the diagenesis process, the magma source was contaminated by the crust and oceanic crust subducted materials.

Lithostratigraphy, sedimentary petrography and geochemistry of the Upper Karoo Supergroup in the Central Kalahari Karoo Sub-Basin, Botswana

The Kalahari Karoo Basin of Botswana is an intracratonic basin developed in south-central Gondwana and is filled with Upper Carboniferous–Lower Jurassic Karoo Supergroup volcano-sedimentary units. The Karoo Supergroup units are unconformably covered by the Cretaceous to recent Kalahari Group. The Upper Karoo Supergroup in the Central Kalahari Karoo Sub-Basin of Botswana consists of the lacustrine sediments of the Beaufort Group (Tlhabala Formation), overlaid unconformably by the continental sediments of the Lebung Group, and by the continental basalts of the Stormberg Group. The Late Triassic to Early Jurassic Lebung Group is subdivided into the Mosolotsane Formation and the Ntane Formation. This sedimentary sequence is poorly exposed, and formation boundaries are tentatively identified by lithological changes in boreholes. Here, new sedimentological and geochemical data are used for the refinement of the stratigraphy of the Upper Karoo Supergroup in the Central Kalahari Karoo Sub-Basin of Botswana, new criteria for the identification of unit boundaries are set and provenance of the Lebung Group sediments has been defined. The geochemical investigation of the Upper Karoo Supergroup in the Central Kalahari Karoo Sub-Basin of Botswana revealed clear geochemical markers that have allowed the pinpointing of unit boundaries between the Tlhabala Formation (Beaufort Group), Mosolotsane Formation (lowermost Lebung Group). The sediments from the Tlhabala Formation have a geochemical signature typical of wackes, while the sandstones from the overlying Mosolotsane Formation and the Ntane Formation are dominated by arkose to subarkose. The transition from wacke to subarkose pinpoints the boundary between the Beaufort Group and the base of the Lebung Group. The Tlhabala Formation is characterized by increasing SiO2/Al2O3 ratio, with a maximum at the base of the Lebung Group, and by an upward decrease in the abundance of Na, K and transition metals such as Cr, Mn and Fe. The SiO2/Al2O3 ratio decreases upward in the Lebung Group. Similarly, the abundances of Na, K, Cr, Mn and Fe slightly increase upward only to show a negative peak at the Lebung Group and the Kalahari Group boundary. This contact is marked by a clear increase in the total REE content.The chemical composition of the Lebung Group sandstones also indicates a quartzose sedimentary provenance. The CIA (chemical index of alteration) values for these sandstones ranged from 63 to 73, indicating a high degree of chemical weathering. This type of intense weathering should produce high volumes of clay minerals, which are rather scarce in the studied samples. This indicates that the large amount of quartz in the Lebung Group has been recycled from the Paleoproterozoic sedimentary sequences of the Palapye Group, located southeast from the study area.

Contrasting compositions between phenocrystic and xenocrystic olivines in the Cenozoic basalts from central Mongolia: Constraints on source lithology and regional uplift

AbstractTwo Cenozoic prominent features are spatio-temporally associated in central Mongolia, i.e., the continental basalts and regional uplift, but their genesis and relationship remain unclear. This study presents major- and trace-element compositions for olivine phenocrysts and xenocrysts, as well as data of bulk-rock geochemistry and Sr-Nd-Hf isotopes for the host basalts. The studied basalts mostly have trachybasalt compositions with high total alkali (Na2O + K2O = 5.1–8.2 wt%) contents and all display OIB-like trace element patterns (e.g., spikes of Ba, Nb, and Ta and troughs of Th and U) and EM1-like Sr-Nd-Hf isotopic compositions. Compared to the partial melting products of mantle peridotite, these basaltic samples have higher FeO/MnO, Zn/Mn, and Zn/Fe ratios. Meanwhile, phenocrystic olivines are characterized by lower Ca, Mn, Mn/Zn, and Mn/Fe but higher Ni than their counterparts in the peridotitic melts, indicating a pyroxenite-rich mantle source. The above geochemical data suggest that the source of the studied basalts was mainly made up of secondary pyroxenite produced by the reaction of recycled oceanic crust with its ambient mantle peridotite. The calculated magma oxygen fugacities (ΔFMQ-0.26 to +0.42) and mantle melting temperatures (1343–1430 °C) do not support a genetic link with the stagnant Pacific slab or with a deep mantle plume. Instead, the far-field effect of India-Eurasia convergence possibly tapped the upper asthenospheric mantle, subsequent melting of which gave rise to the dispersive Cenozoic basalts. On the other hand, the xenocrystic olivines exhibit zoned textures with high-Fo (up to 92) cores and low-Fo (down to 76) rims, reflecting the melt-rock interaction. Preservation of zoned olivine xenocrysts indicates rapid magma ascent and widespread melt-rock reaction in the mantle lithosphere, which may modify the rheology and accelerate the mechanical erosion of mantle lithosphere. Consequently, mass deficit in the lithosphere could have caused isostatic uplift of central Mongolia in the Cenozoic.

LA–ICP–MS zircon U[sbnd]Pb dating, Lu[sbnd]Hf, Sm[sbnd]Nd geochronology and tectonic setting of the Mesoarchean mafic and felsic magmatic rocks in the Sangmelima granite-greenstone terrane, Ntem Complex (South Cameroon)

The Ntem Complex represents the northwestern margin of Congo Craton in southern Cameroon. It consists mainly of charnockites, TTG suites, potassic grantitoids, minor supracrustal and mafic rocks. The latter include gabbronorite enclave or sill, gabbro and dolerite dykes. The metagabbro are basaltic to andesitic in composition and show geochemical features of low-La Archean tholeiitic basalts, and LREE-depleted (LaN/SmN < 1) akin to modern N-MORB. The doleritic dykes show high TiO2/P2O5 (7.50–11.47) and (La/Yb)N ratios, and low NbTa values similar to modern intraplate continental basalts and intermediate-La Archean tholeiitic basalt, least affected by contamination from granitic continental crust. The partial melting of low-to high-K mafic rocks of oceanic and continental arc affinity generated the charnockites and low−/medium-pressure TTGs. The potassic granites were formed by partial melting of charnockite or TTG, and inherited many geochemical features of their sources. New LA–ICP–MS UPb zircon dates suggest that the charnockites and TTG suites formed at ~3155–2850 Ma. The gabbro intrusion was dated at 2866 ± 6 Ma. Migmatisation of TTG-gneiss was coeval whit the regional high–grade metamorphism event at 2843 ± 7 Ma. The potassic granites were probably crystallized at ~2758 Ma. The garnet-whole rock SmNd age of 2744 ± 31 Ma may represent the age of the second anatexis event coeval with D2 deformation. Literature data indicate the disruption or open isotopic systems in the Ntem Complex around 2500–2600 Ma.

Magnesium and zinc isotope evidence for recycled sediments and oceanic crust in the mantle sources of continental basalts from eastern China

To investigate the origin of compositional variations in intraplate alkaline and tholeiitic basalts, this study reports a combined study of metal stable (i.e., Mg and Zn) and radiogenic (i.e., Sr and Nd) isotopes of 50 Cenozoic continental basalts from the South China Block, eastern China. These basalts are classified into high- and low-CaO basanites, trachybasalts, and tholeiites, and have variable Mg–Zn–Sr–Nd isotopic compositions (δ26Mg = −0.29 to −0.46‰, δ66Zn = 0.27 to 0.50‰, 87Sr/86Sr = 0.703233 to 0.704175, and εNd = 3.8 to 8.0), reflecting mantle heterogeneity. In detail, the high-CaO basanites and some trachybasalts have high Ca/Al and δ66Zn, low Hf/Hf* and δ26Mg, and depleted SrNd isotopic compositions, consistent with partial melts of carbonated mantle formed by reaction of the mantle with slab-derived carbonate melts. The low-CaO basanites, some trachybasalts, and tholeiites have low Ca/Al and δ26Mg, high Hf/Hf* and δ66Zn, and depleted SrNd isotopic compositions. These characteristics are related to recycled carbonated eclogites, rather than carbonates, which is consistent with the correlations between δ26Mg, δ66Zn, and Zn/Fe values. In addition, previously published data for the late Mesozoic to Cenozoic basalts from eastern China reveal another group of basalts with high Ba/Th ratios and enriched SrNd isotopic compositions, indicative of significant contributions from siliceous sediments. Our study demonstrates that the compositional variations of the late Mesozoic to Cenozoic alkaline and tholeiitic basalts from eastern China reflect variable proportions of oceanic slab components in their mantle sources.

Generation of Late Cretaceous Ji’an basalts through asthenosphere-slab interaction in South China

Abstract Recycled crustal components have been widely identified in the source of continental basalts with geochemical features similar to oceanic island basalts (OIBs). However, the mechanism of how these recycled materials are involved remains highly debatable. Here we conduct comprehensive geochemical analyses (including whole-rock, olivine, and melt inclusion) and numerical modeling on Late Cretaceous Ji’an basalts from South China interior, aiming to investigate the possible role of recycled crustal components in basalt petrogenesis driven by the subducted paleo-Pacific oceanic plate. The Ji’an basalts show geochemical characteristics akin to OIBs and have depleted asthenospheric mantle-like Sr-Nd-Pb-Hf isotopic compositions with moderately radiogenic Os. Their olivine-hosted melt inclusions have low H2O and highly negative δD values and olivine phenocrysts are mainly characterized by depletion of 18O with δ18O values lowering to 3.9‰. These features are consistent with positive Sr and Eu anomalies in some whole-rock samples. The combined geochemical data suggest that the primary magmas were derived from an asthenospheric mantle enriched by melts from an altered gabbroic oceanic crust, which had experienced intensive dehydration. Further numerical modeling shows that melting of the dehydrated oceanic crust can occur along the torn flank of the subducting lithosphere, in the case that the slab is strongly thinned and fractured. The low δ18O preserved in olivine and the estimated slab age (&amp;lt;300 Ma) from the radiogenic whole-rock Os and Pb compositions also require the involvement of a recently recycled slab, probably represented by the subducted paleo-Pacific oceanic plate. Rollback of the subducting paleo-Pacific slab might create a slab window, in which melt from the torn/fractured slab reacted with the upwelling asthenosphere to form an enriched mantle source for the Ji’an basalts and similar counterparts.

The Theodul Glacier Unit, a slab of pre-Alpine rocks in the Alpine meta-ophiolite of Zermatt-Saas, Western Alps

The Theodul-Glacier-Unit (TGU) is a 100 m thick and 2 km long slab of pre-Alpine schist, gneiss and mafic rocks tectonically emplaced in the eclogite-facies Zermatt-Saas meta-ophiolite nappe (ZSU). The meta-sedimentary rocks occur mostly as garnet-phengite schists with locally cm-sized garnet porphyroblasts. The metavolcanic basic rocks are present as variably retrogressed eclogites showing a continental basalt signature and contain abundant zircon, which is unusual for basalts. The zircons dated with the U–Pb system yield an upper intercept age of 295 ± 16 Ma and a lower intercept age of 145 ± 34 Ma. The early Permian age is interpreted to represent the age of high-grade granulite facies metamorphism, evidence of which is also preserved in the cores of garnet porphyroblasts of the Grt-Ph schists. The lower intercept age corresponds to the time of continental breakup and the initiation of the Tethys in the Mid-Jurassic; these events may have created the TGU as an extensional allochton. Eclogite facies metamorphism recorded by the TGU rocks occurred during Alpine subduction at 57 Ma, the Lu–Hf age of TGU eclogite garnets. The TGU reached a depth of about 53 km at P–T conditions of 1.7 GPa and 520 °C derived from both, eclogite and Grt-Ph schist. This is in contrast to the ZSU surrounding the TGU with a reported subduction depth of more than 80 km at 43 Ma. It is proposed here that TGU and ZSU were subducted separately out of sequence. After juxtaposition of the two units during late Alpine thrusting and folding forming the present day geometry of nappes in the Zermatt-Saas region both units were progressively metamorphosed to about 650 MPa and 470 °C. This late prograde metamorphism at 34 Ma produced oligoclase + magnesio-hornblende in the matrix of Grt-Ph schists and eclogites. The derived TGU data document a complete Wilson Cycle.

Lithosphere thickness controls continental basalt compositions: An illustration using Cenozoic basalts from eastern China

Abstract Recent studies demonstrate that lithosphere thickness variation exerts the primary control on global seafloor basalt compositions. If the mechanism of such control, i.e., the lid effect, is indeed at work, lithosphere thickness variation must also influence basaltic compositions in continental settings. To test this hypothesis, we chose to study Cenozoic basalts in eastern continental China over a distance of ∼260 km along a southeast-to-northwest traverse with a steep topographic gradient (∼500 to ∼1500 m above sea level) mirrored with a steep lithospheric thickness gradient (∼80 to ∼120 km). The basalts erupted on the thinned lithosphere to the east are characterized by lower pressure (e.g., higher Si72, lower Mg72, Fe72, and [Sm/Yb]N; subscript “72” refers to corresponding oxides corrected for fractionation effect to Mg# = 72; N—primitive mantle normalized) and higher extent (e.g., low Ti72, P72, K72, Rb, Ba, Th, and ratios of more- to less-incompatible elements such as [La/Sm]N, Ba/Zr, and Zr/Yb) of melting than basalts erupted on the thickened lithosphere to the west. Importantly, these geochemical parameters all show significant correlations with both lithosphere thickness and topographic elevation. These first-order observations are a straightforward manifestation of the lid effect. Lithospheric contamination and mantle-source compositional variation can indeed contribute to the compositional variability of these continental basalts, but these latter effects are averaged out and are overshadowed by the lid effect. This finding emphasizes the importance of evaluating the lid effect before interpreting the petrogenesis of continental basalts and mantle dynamics. Our results also indicate that the continental surface elevation is isostatically balanced above a mantle depth that is deeper than the lithosphere-asthenosphere boundary.

Recognition of Early Paleozoic Magmatisms in the Supposed Proterozoic Basements of Zhalantun, Great Xing'an Range, NE China

The Zhalantun terrane from the Xing'an massif, northeast China, was used to be considered as Proterozoic basements. However, amounts of detrital zircon ages from the meta‐sedimentary rocks deny the existence of Precambrian basements recently. Notably, magmatic rocks were barely reported to limit the exact ages of the Zhalantun basements. In this study, we collected rhyolite, gabbro and quartz diorite for zircon in‐situ U‐Pb isotopic dating, which yield crystallization ages of ∼505 Ma, ∼447 Ma and ∼125 Ma, respectively. Muscovite schist and siltstone define maximum depositional ages of ∼499 Ma and ∼489 Ma, respectively. Additionally, these dated supracrustal rocks and plutons also yield ancient detrital/xenocryst zircon ages of ∼600–1000 Ma, ∼1600–2220 Ma, ∼2400 Ma, ∼2600–2860 Ma. Based on the whole‐rock major and trace element compositions, the ∼505 Ma rhyolites display high SiO2 and alkaline contents, low Fe2O3T, TiO2 and Al2O3, and relatively high MgO and Mg#, which exhibit calc‐alkaline characteristics. These rhyolites yield fractionated REE patterns and negative Nb, Ta, Ti, Sr, P and Eu anomalies and positive Zr anomalies. The geochemistry, petrology and Lu‐Hf isotopes imply that rhyolites were derived from the partial melting of continental basalt induced by upwelling of sub‐arc mantle magmas, and then experienced fractional crystallization of plagioclase, which points to a continental arc regime. The ∼447 Ma gabbros exhibit low SiO2 and alkaline contents, high Fe2O3T, TiO2, MgO and Mg#. They show minor depletions of La and Ce, flat MREE and HREE patterns, and negative Nb, Ta, Zr and Hf anomalies. Both sub‐arc mantle and N‐MORB‐like mantle were involved in the formation of the gabbros, indicative of a probable back‐arc basin tectonic setting. Given that, the previously believed Proterozoic supracrustal rocks and several plutons from the Zhalantun Precambrian basements were proved to be Paleozoic to Mesozoic rocks, among which these Paleozoic magmatic rocks were generally related to subduction regime. So far, none Proterozoic rocks have been identified from the Zhalantun Precambrian basement, though some ∼600–3210 Ma ancient detrital/xenocryst zircons were reported. Combined with ancient zircon ages and newly reported ∼2.5 Ga and ∼1.8 Ga granites from the south of the Zhalantun, therefore, the Precambrian rocks probably once exposed in the Zhalantun while they were re‐worked and consumed during later long tectonic evolutionary history, resulting in absence of Precambrian rocks in the Zhalantun.

Recycled oceanic crust in the form of pyroxenite contributing to the Cenozoic continental basalts in central Asia: new perspectives from olivine chemistry and whole-rock B–Mo isotopes

Cenozoic continental basalts are widespread in central Asia. To explore their source nature and petrogenesis, this study presents an integrated study of olivine chemistry, bulk-rock 40Ar/39Ar age and geochemistry as well as Sr–Nd–Pb–B–Mo isotopes for the Miocene (ca. 15.5 Ma) Halaqiaola basalts in the Chinese Altai, central Asia. The Halaqiaola basalts mostly have basanite compositions with high total alkali (Na2O + K2O = 6.89–8.01 wt%) contents and high K2O/Na2O (0.87–1.39) ratios. Compared with partial melting products of mantle peridotite, the basaltic samples possess lower CaO and CaO/Al2O3 but higher TiO2, Zn/Mn and Zn/Fe values. Meanwhile, olivine phenocrysts from these basalts are characterized by lower Ca, Ni and Mn contents but higher Fe/Mn ratios than their counterparts in the peridotitic melts, suggesting a pyroxenite-rich source. Moreover, these rocks show OIB-like trace element patterns (e.g., spikes of Ba, Sr, Nb and Ta and troughs of Th and U), and constant Nd but variable Sr and EM1-like Pb isotopic compositions, and yield light δ11B (– 11.0 to – 8.1‰) and δ98Mo (– 0.40 to – 0.06‰) values. The above geochemical data suggest that secondary pyroxenite was likely produced by reaction of recycled oceanic crust with its ambient peridotite and subsequently became the main source for the basanite. Furthermore, their light and variable δ98Mo values probably reflect that recycled oceanic crust involved in such pyroxenite was altered with different degrees. In combination with available data from adjacent regions, we propose that the far-field effect of India–Eurasia collision was the first-order factor for the upwelling of dispersive asthenospheric mantle beneath central Asia, subsequent melting of which gave rise to the widespread Cenozoic volcanism.

Geochemical features and formation conditions of Early-Devonian cherty-argillaceous shales and the underlying basalts in the Ishkildino section (eastern slope of the Southern Urals)

Research subject.The West Magnitogorsk zone of the Southern Urals in the vicinity of the Ishkildino village features a subaerially exposed basaltic sequence superposed by cherts and siliceous-clay shales. The basalts and the overlying shales are assumed to have formed during the Ordovician and Silurian (?)–Early Devonian (up to the conodont zone excavates inclusive) periods, respectively. The aim of this research was to reconstruct, using geochemical data, the conditions under which the rocks present in this geological location were formed.Materials and methods. Five samples of the basalts (XRD and ICP-MS methods), 27 samples of the siliceous-clay shales and 10 samples of the cherts (XRD and ICP-AES methods) were analyzed.Results.According to the ratio of SiO2, Na2O and K2O, the volcanic rocks from the lower part of the section are represented by basalts and trachybasalts. Their geochemical composition corresponds to the N-MORB and is established to be similar to that of the basalts in the Polyakovskaya formation (the Middle–Upper Ordovician). In terms of main elements, the shales under study consist of quartz and illite with a slight admixture of organic matter, goethite, quartzfeldspar fragments, etc. The degree of the sedimentary material weathering according to the CIA, CIW and ICV index values is shown to be moderate. The values of Strakhov’s and Boström’s moduli correspond to sediments without the admixture of underwater hydrothermal vent products. The values of Cr/Al, V/Al and Zr/Al correspond to those characteristic of deposits in deep-water zones remote from the coasts of passive and active continental margins, basalt islands and areas adjacent to mid-ocean ridges. For most samples, the values of Ni/Co, V/Cr, Mo/Mn are typical of deposits formed under oxidative conditions. However, several samples from the upper part of the section, which is comparable to the kitabicus and excavatus conodont zones, demonstrate the Ni/Co, V/Cr, and Mo/Mn values corresponding to deposits formed under reducing atmospheres. An assumption is made that the existence of these deposits can be associated with the Bazal Zlichov event.Conclusion.The investigated pre-Emsian shales have shown no signs of volcanic activity in the adjacent areas. The studied deposits are established to correspond to the central part of the Ural Paleoocean.

Evaluating the effect of leaching on trace element and Nd-Pb isotopic systematics in continental basalts

Leaching technique was mostly carried out on submarine rock powder to minimize or eliminate secondary effect of seawater interaction on Sr and Pb isotope magmatic signatures. However, the effect of leaching on trace element contents is comparatively poorly established. To investigate such effects, we have compared trace element and Nd-Pb isotopic data for acid leached ∼290 Ma Keping basalts in the Tarim Large Igneous Province (NW China) with unleached data on the same rocks. Rare earth elements (REE), Y, Th and U contents of leached sample grains (0.6–1.0 mm size) are 10–65% lower than those of unleached grains of the same sample, whereas other trace elements are not affected. Sm/Nd, Lu/Hf, U/Pb and Th/Pb ratios are fractionated by acid leaching between leached and unleached sample grains. Nd isotope values are unaffected by leaching, but 208Pb/204Pb and 206Pb/204Pb values of leached samples are significantly lower than those of unleached samples. X-ray diffraction results suggest that removal of secondary epidote during the acid leaching process could result in the fractionation of REE, Y, Th and U contents, and Sm/Nd, Lu/Hf, U/Pb and Th/Pb ratios, and 208Pb/204Pb and 206Pb/204Pb values. These results suggest that when the samples have been leached with strong acid (>1N HCl), their trace element cannot be used to trace the sources and petrogenesis. If leaching is applied to old (e.g., >100 Ma) continental basalts in order to eliminate contamination during sample collection, storage, sawing and crushing, Th/Pb and U/Pb ratios used to calculate the initial Pb isotope values, should be obtained on the same leached samples.

Characterization of Mineralogy, Petrography, Geochemistry and Petrogensis of Basaltic Outcrops in Jurf Ed Darawish Area, Central Jordan

This research was conducted to investigate the mineralogy, petrography, geochemistry and petrogensis of the basaltic flows in Jurf Ed Darawish (JDB) area of central Jordan. Sexton representative basalt rock samples were selected from the studied JDB outcrops. Modally, JDB consists of plagioclase, olivine, pyroxene (diopside), opaque’s, calcite and iddingsite minerals. Petrographically, basalt is holocrystalline, hypidiomorphic fine to medium grained and exhibited aphanitic to porphyritic texture. The common textures of the JDB rock samples were aphanitic, porphyritic, trachytic, glomeroporphyritic, sub ophitic, vesicular, and amygdaloidal. Geochemically, all of the inspected samples of JDB are located within Trachy basalt and plate alkaline basalt. The tectonic setting of JDB was plotted within the calcalkaline basalt and continental basaltic field. The rare-earth elements showed enrichment of the Ba and K, depletion of Ce relative to K, and enrichment of Nb and Pb with depletion of Y and positive Nb, Zr and Ti anomalies. Negative anomalies of Ba, Sr, Ti and P may be attributed to the fractionation of feldspar for Ba and Sr depletion apatite for P depletion. The positive Nb peak conforms to the tertiary as well as to recent continental alkali basalt provinces and acts as an indicator to the JDB product for the lithosphere from upwelling of the asthenosphere mantle.

The Sources of Paleoproterozoic Collisional Granitoids (Sharyzhalgai Uplift, Southwestern Siberian Craton): from Lithospheric Mantle to Upper Crust

Abstract —The paper presents the geochemical and isotope characteristics of rocks and the U–Pb age and Lu–Hf isotope composition of zircons from three plutons of Paleoproterozoic granitoids in the Sharyzhalgai uplift (southwestern Siberian craton). The age of granitoids of the Toisuk (1838 ± 6 and 1827 ± 9 Ma), Nizhnii Kitoi (1846 ± 7 Ma), and Malaya Belaya (1863 ± 16 Ma) plutons corresponds to the Late Paleoproterozoic collision stage and is correlated with the time of mafic magmatism. The studied rocks have a wide range of silica contents. The Toisuk pluton is composed of a range of rocks from monzodiorites to granodiorites (granosyenites) and granites; the Nizhnii Kitoi pluton, of granodiorites and granites; and the Malaya Belaya pluton, of leucogranites. The rocks of the three plutons are highly ferroan, enriched in LREE, Th, and HFSE, and correspond in composition to A-type granitoids. A characteristic feature of melanocratic granitoids of the Toisuk and Nizhnii Kitoi plutons is extremely high contents of Ba: 4080–1500 ppm and 1560–990 ppm, respectively. Based on analysis of experimental data on the melting of various substrates and the results of numerical simulation, it is assumed that monzodiorite–granodiorites of the Toisuk pluton and granodiorites of the Nizhnii Kitoi pluton resulted from the differentiation/melting of a mafic source similar in Ba and Sr contents to intraplate continental basalts. The isotope compositions of zircon and melanocratic granitoids of the Toisuk (εHf from –6.0 to –10.7 and εNd from –5.3 to –10.2) and Nizhnii Kitoi (εHf from –5.0 to –8.1 and εNd = –4.0 and –5.1) plutons argue for the generation of their mafic sources from the enriched lithospheric mantle formed as a result of Neoarchean subduction processes. Vein granites of the Toisuk pluton and leucogranites of the Malaya Belaya pluton formed through the melting of quartz–feldspar (granodiorite) substrate. The contrasting isotope parameters of the Toisuk vein granites (εHf from –6.7 to –10.1, zircons, and εNd = –5.5, rock) and Malaya Belaya leucogranites (εHf from 2.9 to 5.9, zircons, and εNd from +0.7 to –1.9, rocks) indicate melting of the Archean and Paleoproterozoic crust, respectively. The more radiogenic Hf isotope composition of zircons from vein granites as compared with rocks of the Archean crust of the Irkut terrane is evident of the contribution of juvenile material to the granite formation.