Mantle sources of Cenozoic volcanoes around the South China Sea revealed by geochemical and isotopic data using the principal component analysis

Principal component analysis(PCA)was employed to determine the implications of geochemical and isotopic data from Cenozoic volcanic activities in the Southeast Asian region,including China(South China Sea(SCS),Hainan Island,Fujian-Zhejiang coast,Taiwan Island),and parts of Vietnam and Thailand.We analyzed 15 trace element indicators and 5 isotopic indicators for 623 volcanic rock samples collected from the study region.Two principal components(PCs)were extracted by PCA based on the trace elements and Sr-Nd-Pb isotopic ratios,which probably indicate an enriched oceanic island basalt-type mantle plume and a depleted mid-ocean ridge basalt-type spreading ridge.The results show that the influence of the Hainan mantle plume on younger volcanic activities(<13 Ma)is stronger than that on older ones(>13 Ma)at the same location in the Southeast Asian region.PCA was employed to verify the mantle-plume-ridge interaction model of volcanic activities beneath the expansion center of SCS and refute the hypothesis that the tension of SCS is triggered by the Hainan plume.This study reveals the efficiency and applicability of PCA in discussing mantle sources of volcanic activities;thus,PCA is a suitable research method for analyzing geochemical data.

Geochemical constraints on CO2-rich mantle source for the Kocebu Seamount,Magellan Seamount chain in the western Pacific

The alkaline oceanic island basalts(OIBs)with under-saturated SiO2 and high contents of CaO and alkaline are usually attributed to mantle sources different from typical tholeiitic OIBs.Based on the results of high pressure and temperature experiment study,the genesis of silica under-saturated alkaline basaltic melts could be explained by the role of CO2,thus,the genetic relationship of alkaline basalts with CO2 has become a topic of relevance because it is closely related to the deep carbon cycle.The Magellan Seamount chain in the West Pacific Seamount Province has wide distribution of alkali basalts.For the first time,we collected alkaline basalt samples from the Kocebu Seamount of the Magellan Seamount chain and found that magmatic apatites widely occur in the less evolved volcanic rock samples,and the high contents of phosphorus should be a feature of the alkaline OIBs of the Magellan Seamounts.Compared with typical OIBs,these alkaline volcanic rocks have higher CaO and P2O5,lower SiO2 content,negative anomaly of high field strength elements(HFSEs),more distinctly negative anomaly of potassium(K)and the ubiquity of titanaugite,indicating a CO2-rich mantle source.Based on the relatively high K2O and TiO2 contents and La/Yb ratio and low MgO content of these alkaline rocks,we suggest that the volcanic rocks of the Magellan Seamounts are originated from carbonated eclogites derived possibly from ancient subducted altered oceanic crust.

Chemical Composition of the Primitive Mantle Source of Peridotite Inclusions at Mount Lianshan, Liuhe County,Jiangsu Province

: This paper presents a study of the major and trace element compositions of fresh mantle-derived spinel lherzolite and harzburgite inclusions from Cenozoic alkaline basalt in Mount Lianshan and Mount Panshi, Liuhe County, Jiangsu Province. An estimation is made of the contents of the major elements and some of the trace elements in the primitive mantle source region of the area, from which the authors have obtained MgO/Al2O3= 7.86. The contents of MgO and Al2O3 are also obtained as 37.58% and 4.78% respectively based on the correlation of MgO-Al2O3. Then, the contents of various elements in the primitive mantle are calculated using their regression equations with MgO, and the compositon of the primitive mantle, a basic issue in geochemistry study, is discussed on that basis.

Major,trace element and Sr-Nd isotope evidence for a sublithospheric mantle source for the Umkondo large igneous province

The Mesoproterozoic(1.11 Ga)Umkondo large igneous province(LIP)in southern Africa and Antarctica was emplaced in<5 Myr and is dominated by low-Ti tholeiitic doleritic-gabbroic sills.It is of particular interest because it is the least studied LIP in southern Africa with both sublithospheric and lithospheric mantle sources proposed and it coincides with the early assembly of Rodinia,so it has importance in understanding the nature of magmatism and tectonics in and around the Kalahari craton during the Mesoproterozoic.In this study,we compiled a large database of existing(~750)and new(~100)major and trace element data for the Umkondo province,as well as 42 new Sr-Nd isotopic measurements,to provide constraints on its magma sources and geochemical evolution.Major element compositional variations in the low-Ti tholeiites are explained by low-pressure(1 kbar)three-phase fractional crystallisation(olivine,clinopyroxene and plagioclase)of a parent magma with~10 wt.%MgO in oxidising conditions(QFM+1).Inverse models show that the low-Ti tholeiitic magmas were derived as residual melts after the crystallization of 12%-33%olivine from primary komatiitic-basaltic magmas(up to~20 wt.%MgO)in equilibrium with mantle olivine(Fo90).Low Sm/Yb and TiO2/Yb-Nb/Yb indicate that the primary magmas were derived by 2%-20%shallow(40-50 km)partial melting of spinel lherzolite.High Sm/Yb is restricted to dyke swarms and may imply limited magma production from deeper(up to~70 km)garnet lherzolite-like sources.The low-Ti tholeiites of the Umkondo province are enriched in large ion lithophile elements(Rb-Sr-Cs-K)and depleted in high-field strength elements(Zr-Hf-Nb-Ta),indicating the involvement of crustal material and/or the subcontinental lithospheric mantle.This is supported by covariations in Th/Nb,Nb/Yb,Nb/La and Ce/Sm with generally negativeΔNb.Sr-Nd isotopes lend support to the notion that the Umkondo magmas were derived from depleted and/or enriched sublithospheric mantle sources and subsequently contaminated by enriched lithospheric material during emplacement(initial(at 1.11 Ga)^(87)Sr/^(86)Sr between 0.704820 and 0.737464 andεNd between-8.9 and+5.3).The Vredefort sills are significant as they display the most depleted Sr-Nd isotopic signatures(average initial ^(87)Sr/^(86)Sr of 0.705342 and averageεNd of 0.4)and are the least contaminated magma suite in the Umkondo province.Because of(i)the large volume of low-Ti magmas,(ii)evidence of a primary hot and MgO-rich(komatiitic)magma,and(iii)the short duration of magmatism,we suggest that the Umkondo province was formed by plume-induced melting of the sublithospheric mantle beneath the Kalahari craton in an extensional setting.This contrasts with previous suggestions that the heat source developed in response to the“thermal insulation”of the mantle beneath a thickened Kalahari craton in the absence of a mantle plume.There is further evidence from the elevated Zn/Fe that the sublithospheric mantle was lithologically heterogeneous and consisted of mixed peridotite and pyroxenite domains.There is a general lack of ultramafic cumulates in the low-Ti magma suite that may imply there was deeper ponding and storage of the primary magmas that fractionated large quantities of ultramafic rocks.There is also a paucity of high-Ti rocks in the Umkondo province that may reflect limited direct melting of the lithospheric mantle or that they are simply not as well-preserved in this province compared to the Karoo province.The similar trace element and Sr-Nd isotopic compositions of the Umkondo sills in southern Africa with the Borgmassivet sills in Antarctica support the concept that the Kalahari craton and Grunehogna terrane were adjoined at 1.11 Ga.The timing of the Umkondo province indicates there was localised lithospheric extension and upwelling asthenospheric mantle during a time of dominantly compressional tectonics on Earth at the end of the‘boring billion’.

S, C, O, H Isotope Data and Noble Gas Studies of the Maoniuping LREE Deposit, Sichuan Province, China: A Mantle Connection for Mineralization

The Maoniuping REE deposit, located about 22 km to the southwest of Mianning, Sichuan Province, is the second largest light REE deposit in China, subsequent to the Bayan Obo Fe-Nb-REE deposit in the Inner Mongolia Autonomous Region. Tectonically, it is located in the transitional zone between the Panxi rift and the Longmenshan-Jinpingshan orogenic zone. It is a carbonatite vein-type deposit hosted in alkaline complex rocks. The bastnaesite-barite, bastnaesite-calcite, and bastnaesite- microcline lodes are the main three types of REE ore lodes. Among these, the first lode is distributed most extensively and its REE mineralization is the strongest. The δ^34Sv.cDT values of the barites in the ore of the deposit vary in a narrow range of ＋5.0 to ＋5.1‰ in the bastnaesite-calcite lode and ＋3.3 to ＋5.9‰ in the bastnaesite-barite lode, showing the isotopic characteristics of magma-derived sulfur. The δ^13Cv-PDB values and the δ^Ov.SMOW values in the bastnaesite-calcite lode range from -3.9 to -6.9‰ and from ＋7.3 to ＋9.7 ‰, respectively, which fall into the range of ＂primary carbonatltes＂, showing that carbon and oxygen in the ores of the Maoniuping deposit were derived mainly from a deep source. The δ^13Cv.PDB values of fluid inclusions vary from -3.0 to -5.6‰, with -3.0 to -4.0‰ in the bastnaesitecalcite lode and -3.0 to -5.6‰ in the bastnaesite-barite lode, which show characteristics of mantle- derived carbon. The δDv-SMOW values of fluid inclusions range from -57 to -88‰, with -63 to -86‰ in the bastnaesite-calcite lode and -57 to -88‰ in the bastnaesite-barite lode, which show characteristics of mantle-derived hydrogen. The δ^18OH2OV.SMOW values vary from ＋7.4 to ＋8.6‰ in the bastnaesitecalcite lode, and ＋6.7 to ＋7.8‰ in the bastnaesite-barite lode, almost overlapping the range of ＋5.5 to ＋9.5‰ for magmatic water. The 4He content, R/Ra ratios are （13.95 to 119.58）×10^-6 （cm^3/g）STP and 0.02 to 0.11, respectively, and ^40Ar/^36Ar is 313± 1 to 437 ± 2. Considering the 4He increase caused by high contents of radioactive elements, a mantle-derived fluid probably exists in the inclusions in the fluorite, calcite and bastnaesite samples. The Maoniuping deposit and its associated carbonatite-alkaline complex were formed in 40.3 to 12.2 Ma according to K-Ar and U-Pb data. All these data suggest that large quantities of mantle fluids were involved in the metallogenic process of the Maoniuping REE deposit through a fault system.

Upwelling of Mantle-derived Material in Southeast China:Evidence from Noble Gas Isotopes

Shallow groundwater collected in Chaozhou,Huizhou,and Guangzhou allowed testing of concentrations and the isotope ratios of noble gases.Based on the calculated noble gas temperature(NGT)and the ratio of noble gas isotopes,the recharge temperature,recharge source,and residence time of groundwater can be calculated.In addition,the contribution of noble gas components from different sources to the sample components can be assessed.In the Huizhou area,according to the 1/Xe vs.Ne/Xe and NGT data,the shallow sandstone-confined water samples in the Shiba area and the unconfined water samples of the Huangshadong are in different temperature ranges,indicating that they have different recharge sources,both in time or space.The He components in the samples are calculated to obtain the content of radiogenic ^(4)He in the crust and to simulate the groundwater ages.The noble gas isotope ratios show the addition of mantle components into the basalt aquifers and sandstone aquifers in Chaozhou and Huizhou.Except for atmospheric and crustal sources,there is a certain proportion of mantle-derived components in the shallow underground cold water in Huizhou and Chaozhou.The noble gases in the Chaozhou groundwater have an obvious mantle signature,allowing speculation that there is a deep fluid carrying mantle characteristics.This upwelling of mantle-derived material might be caused by the India-Eurasia collision or that between the Philippine Sea Plate and the Eurasian Plate.

Extreme Mantle Heterogeneity beneath the Jingpohu Area,Northeastern China-Geochemical Evidence of Holocene Basaltic Rock

Holocene basaltic rocks of the Jingpohu area are located in the “Crater Forest” and Hamatang districts to the northwest of the Jingpohu Lake. Although there is only a distance of 15 km between the two districts, their petrological characteristics are very different: alkaline olivine basalt without any megacrysts in the former, and leucite tephrite with Ti-amphibole, phlogopite and anorthoclasite megacrysts in the latter. On the basis of their geochemical characteristics, the two types of basaltic rocks should belong to weakly sodian alkaline basalts. But leucite tephrite is characterized by higher Al2O3, Na2O and K2O, higher enrichment in light rare earth elements (LREE) and large ion lithophile elements (LILE), lower MgO and CaO, compatible elements and moderately compatible elements and lower Mg*values and Na/K ratios in comparison with alkaline olivine basalt. However, the two types of basaltic rocks have similar Sr, Nd, Pb isotopic compositions, which suggests that the mantle beneath the Jingpohu area was homogeneous before undergoing some geological processes about 3490 years ago. As the activity of the mantle plume led to different degrees of metasomatism, extreme mantle source heterogeneities occurred beneath the Jingpohu area. In comparison with alkaline olivine basalt, the leucite tephrite was derived from the more enriched mantle source region and resulted from strong metasomatism.

Geochemistry and geochronology of mafic rocks from the Spanish Central System:Constraints on the mantle evolution beneath central Spain

The Spanish Central System(SCS)contains several suites of Palaeozoic mafic igneous intrusions with contrasting geochemical affinity:Ordovician tholeiitic metabasites,Variscan calc-alkaline gabbros(Gb1)and microdiorites(Gb2),shoshonitic monzogabbros(Gb3)and alkaline diabases and lamprophyres(Gb4).Not all of these rocks are accurately dated,and several aspects of their genesis are still poorly understood.We present new whole-rock geochemical data(major and trace elements,and Sr-Nd isotopes),U-Pb and Lu-Hf isotopic ratios on magmatic zircons and 40 Ar/39Ar amphibole geochronology results in order to establish a precise chronology for the successive events of magmatism in the SCS,and discuss the nature of their mantle sources.Accurate ages have been determined for the Variscan gabbros(305-294 Ma),the microdiorites(299 Ma)and the accompanying felsic porphyries(292 Ma),the shoshonitic monzogabbros(285 Ma),and the alkaline diabases(274 Ma)and monzosyenites(271-264 Ma).According to this information,the Variscan mafic magmatism would be mainly concentrated in the range of 305-294 Ma,with a final manifestation represented by the minor shoshonitic dykes.The alkaline magmatism proved to be slightly older than previously thought and yielded at least two distinct pulses:diabases and lamprophyres-monzosyenites.Zircon Hf isotopes evidence the involvement of depleted and slightly enriched mantle sources.The bulk of the eHf values are in the broad range of-8 to+11,indicative of melting both depleted and enriched mantle regions.The high within-sample Hf isotope variation(up to-11 epsilon units)shown by samples from the Variscan series(gabbros,microdiorites and monzogabbros)could be explained mainly by hybridisation of magmas derived from heterogeneous lithospheric mantle sources.Pressure estimates indicate that the Variscan mafic magmas were extracted from the lithosphere.The Nd-Hf isotopic composition of these suites of rocks suggests the recycling of pelitic sediments during the Cadomian orogeny.Deeper(asthenospheric)mantle levels were involved in the generation of the alkaline suite,whose anomalous negative eHf values(moderately decoupled with respect to radiogenic Nd)could be associated with subducted oceanic components raised by mantle upwelling associated with lithosphere thinning and extension during the Permian.

Source and magmatic evolution of ocean island basalts from the Pohnpei Island,Northwest Pacific Ocean:Insights from olivine geochemistry

The compositional variability of ocean island basalts(OIBs)is thought to reflect partial melting of a lithologicallyheterogeneous mantle source dominated by either pyroxenite or peridotite.The Pohnpei Island in Micronesia,which is associated with the Caroline hotspot,is suggested to have been generated from partial melting of a pyroxenite-rich mantle.To examine this hypothesis,we present new major-and trace-element compositions of olivine phenocrysts in basalts from the island.The olivines exhibit large systematic inter-and intra-crystalline compositional variability.In Sample DS1,olivines record compositional zonation,in which cores have relatively high Fo(77–85),Ni(550×10^(-6 )–2392×10^(-6 )),and Fe/Mn ratios(66–82),whereas rims have lower Fo(71–78),Ni(526×10^(-6 )–1537×10^(-6 )),and Fe/Mn ratios(51–62).By contrast,olivines within other samples preserve no clear compositional zonation,exhibiting similar or slightly lower Fo values(66–78),Ni contents(401×10^(-6 )–1268×10^(-6 )),and Fe/Mn ratios(53–69)as the rims of zoned crystals.The distinct chemical contrast between the two different types of olivine suggests they formed in magma chambers at different depths.Analysis using forward petrological modeling and multi-element indicators(Fe/Mn,Zn/Fe,FC3MS(FeO^(T)/CaO-(3×MgO/SiO^(2))),Mn/Zn,and Ni/(Mg/Fe))of whole-rock samples and high-Fo olivines is inconsistent with a pyroxenite-rich mantle source.We suggest these inconsistencies reflect an influence on the partition coefficients of Ni and Mn between olivine and liquid during melting at variable pressures and temperatures.In addition,magma recharge and mixing within the magmatic plumbing system can change the composition of olivine.We suggest that identification of the mantle source of OIBs in volcanic islands such as the Pohnpei Island using olivine geochemistry should be treated with caution.

Lamprophyre or Lamproite Dyke in the S WTarim Block? —Discussion on the Petrogenesis of These Rocks and Their Source Region

The mineralogical, petrological and geochemical studies on Keliyang （克里阳） potassic dykes have been carried out to understand their rock types, the petrogenesis and the nature of their mantle sources. They arc potassic lamprophyre, not lamproites as the previous researchers believed. In this study, the whole-rock major and trace element compositions of another 6 lamproite dykes recently discovered are reported. Major elements were determined by X-ray fluorescence spectrometry （XRF） techniques, while REE and trace elements were determined by inductively coupled plasma mass spectrometry （ICP-MS）. They can be classified into phlogopite-diopside lamprophyre, leucite-diopside lamprophyre and granular carbonatite-bearing diopside lamprophyre on the basis of their mineral components. They are all characterized by relatively low SiO2 （41.31% --44. 84% ）, TIO2 （0.75%-0.86% ） and high MgO （7.30%-11.33%）, K2O （4.01%-6.01%） concentrations with K2O/Na2O ratios of 2.77-12.49. In addition, they display enrichment in large-ion lithophile elements （LILEs, e. g., Rb, Sr, Ba） and LREE, but a relative depletion in high-field-strength elements （HFSEs, e.g. , Nb, Ta, Zr, Hf and Ti）. They display similar chondrite-normalized REE patterns with slight negative Eu anomalies （δEu=0.64-0.82）, and high initial ^87 Sr/^86Sr ratios, which resemble those of high K/Ti and Iow-Ti potassic magmas formed in subduction-related settings. Consequently, we suggest that the parental magma was generated by partial melting of the phlogopite-amphibole-bearing garnet lherzolite within the lithospheric mantle that might have been metasomatized by a potassium-bearing fluid released from a sabdaction oceanic crust.

Compositional and temperature variations of the Pacific upper mantle since the Cretaceous

The geological evolution of the Earth during the mid-Cretaceous were shown to be anomalous, e.g., the pause of the geomagnetic field, the global sea level rise, and increased intra-plate volcanic activities, which could be attributed to deep mantle processes. As the anomalous volcanic activities occurred mainly in the Cretaceous Pacific, here we use basalt chemical compositions from the oceanic drilling（DSDP/ODP/IODP） sites to investigate their mantle sources and melting conditions. Based on locations relative to the Pacific plateaus, we classified these sites as oceanic plateau basalts, normal mid-ocean ridge basalts, and near-plateau seafloor basalts. This study shows that those normal mid-ocean ridge basalts formed during mid-Cretaceous are broadly similar in average Na8, La/Sm and Sm/Yb ratios and Sr-Nd isotopic compositions to modern Pacific spreading ridge（the East Pacific Rise）. The Ontong Java plateau（125–90 Ma） basalts have distinctly lower Na8 and143Nd/144 Nd, and higher La/Sm and 87Sr/86 Sr than normal seafloor basalts, whereas those for the near-plateau seafloor basalts are similar to the plateau basalts, indicating influences from the Ontong Java mantle source. The super mantle plume activity that might have formed the Ontong Java plateau influenced the mantle source of the simultaneously formed large areas of seafloor basalts. Based on the chemical data from normal seafloor basalts, I propose that the mantle compositions and melting conditions of the normal mid-ocean ridges during the Cretaceous are similar to the fast spreading East Pacific Rise. Slight variations of mid-Cretaceous normal seafloor basalts in melting conditions could be related to the local mantle source and spreading rate.

Geochemistry and petrogenesis of Quaternary volcanism from the islets in the eastern Beibu Gulf: evidence for Hainan plume

Some of the islets in the eastern Beibu Gulf are covered by Quaternary volcano strata. The rock samples from these islets mainly consist of quartz tholeiites （at Shenjiandao）, olivine tholeiites （at Linshidao and Xieyang- dao） and alkali basalts （at Yangpubi and Jianshidao）, and basically represent four periods of the Quaternary volcanism of Hainan Island and its adjacent regions. Except for the samples from Shenjiandao, most of the Quaternary volcanics of these islets belong to alkali magma series. The trace element characteristics of all of these samples show they are OIB （oceanic island basalt） -like, which implies that their deep geodynamic setting may be related to a mantle plume. The Sr-Nd-Pb isotopic compositions show that the mantle source beneath the Quaternary strata can be regarded as a result of binary mixing between a depleted, DMM （de- pleted MORB mantle）-like source and an enriched mantle type 2 （EM2）. The EM2 may be originated from the Hainan mantle plume, and has been metasomatized by carbonaceous fluids released from ancient re- cycled oceanic crust at an asthenospheric mantle level. These features, together with typical trace element ratios, reflect that the parent magma was not subjected to crustal contamination during its ascent to the surface. This study provides further petrological and geochemical evidence for the existence of the Hainan mantle plume.

Characteristics of Hydrogen and Oxygen Isotopes and Noble Gas Isotopes in the Groundwater of Weishan, Wudalianchi, Northeast China

According to the hydrochemical characteristics, hydrogen and oxygen isotope characteristics and the ratio of noble gas isotopes of the sandstone aquifer and basalt aquifer, this study calculated the recharge temperature and residence time of groundwater in the Weishan area of Wudalianchi, also calculating the contribution of noble gas components from different sources to the samples. Based on the characteristics of hydrogen and oxygen isotopes and noble gases Xe and Ne, the recharge altitude and recharge temperature of the two aquifers were estimated, and the recharge temperature fitting with the NGT model as verified, the results showing that the main recharge altitude of groundwater in the region was 500–600 m, the recharge temperature being 2–7°C. He_(eq) and He_(ea) of the samples have been simulated using the OD model, the content of radioactive ~4He in the crust being obtained, the groundwater ages under the two conditions(closed condition and open condition) both being simulated. The results show that groundwater from the sandstone layer water is older than groundwater from the basalt layer. Hydrochemical characteristics and noble gas isotope ratios indicate that in the basalt aquifer and sandstone aquifer in the Weishan area, in addition to atmospheric and crustal helium, there is also an input of mantle-derived helium. The fault constitutes the uplift channel for groundwater containings mantle components, which results in the mantle source composition in water samples near the fault being much higher than those form non-fault areas.

Petrogenesis of basaltic shergottite NWA 8656

Most basaltic shergottites are too Mg-rich to represent parent melt compositions because they contain some cumulus pyroxenes. However, basaltic shergottite Northwest Africa(NWA) 8656 with subophitic texture can be used as the parent melt composition in petrogenetic studies because it contains no or rare cumulus pyroxenes. Its pyroxene cores(Mg# 66-68, the most magnesian) are in equilibrium with the bulk rock composition based on major(Fe-Mg) and trace elements(REE—rare earth elements).The patchy zoning of pyroxenes has been interpreted as reflecting a two-stage crystallization history: 1) crystallization of Mg-rich pyroxene cores at depth(50 km, the base of Martian crust), 2) crystallization of Fe-rich pyroxene rims at the shallow depth near the Martian surface with a fast cooling history. The crystallization of Fe-rich pyroxenes and the existence of different symplectites indicate that NWA 8656 underwent eruption. The oxygen fugacity of NWA 8656(QFM –0.9±0.5) suggests an oxidized condition at the late-stage crystallization process, and the CI-normalized REE patterns of different minerals show enrichment in LREE, compared to that of depleted shergottites. Both of these observations suggest a relatively ITE(incompatible trace elements)-enriched signature of NWA 8656, similar to those of other enriched shergottites. The REE compositions of augite core and rim and plagioclase can be successfully reproduced by progressive crystallization without exogenous components, which indicates a closed magmatic system for NWA 8656. Consequently, we conclude that the ITE-enriched signature of NWA 8656 is inherited from an enriched mantle source rather than caused by crustal assimilation. Moreover, partial melting of depleted Martian mantle could not directly yield magmas that have geochemical characteristics similar to enriched shergottite parent magmas, so the enriched and depleted shergottites are derived from distinct mantle sources, and the mantle source of enriched shergottites would be expected to contain ilmenite.

Geochemistry and petrogenesis of Rajahmundry trap basalts of Krishna-Godavari Basin,India

The Rajahmundry Trap Basalts （RTB） are erupted through fault-controlled fissures in the Krishna-Godavari Basin （K-G Basin） of Godavari Triple Junction, occurring as a unique outcrop sandwiched between Cretaceous and Tertiary sediments along the east coast of India. Detailed geochemical studies have revealed that RTB are mid-Ti （1.74-1.92） to high-Ti （2.04-2.81） basalts with a distinct quartz tholeiitic parentage. MgO （6.2-13.12 wt.%）, Mg# （29-50） and Zr （109-202 ppm） suggest that these basalts evolved by fractional crystallization during the ascent of the parent magma along deep-seated fractures. Mod- erate to high fractionation of HREE, as indicated by （Gd/Yb）N ratios （1.71-2.31） of RTB, suggest their generation through 3-5% melting of a Fe-rich mantle corresponding to the stability fields of spinel and garnet peridotite at depths of 60-100 km. Low K2O/P2O5 （0.26-1.26）, high TiO2/P2O5 （6.74-16.79）, La/Nb （0.89-1.45）, Nb/Th 〉 8 （8.35-13）, negative anomalies at Rb reflect minimum contamination by granitic continental crust. （Nb/La）PM ratios （0.66-1.1） of RTB are attributed to endogenic contamination resulted through recycling of subducted oceanic slab into the mantle. Pronounced Ba enrichment with relative depletion in Rb indicates assimilation of Infra- and Inter-trappean sediments of estuarine to shallow marine character. Geochemical compositions such as A1203/TiO2 （3.88-6.83）, medium to high TiO2 （1.74 -2.81 wt.%）, positive Nb anomalies and LREE enrichment of these RTB attest to their mantle plume origin and indicate the generation of parent magma from a plume-related enriched mantle source with EM 1 signature. Ba/Th （46-247）, Ba/La （3.96-28.51） and Th/Nb （0.08-0.13） ratios suggest that the source enrichment process was marked by recycling of subduction-processed oceanic crust and lithospheric components into the mantle. Zr/Hf （37-41） and Zr/Ba （0.51-3.24） indicate involvement of an asthenospheric mantle source. The Rajahmundry basalts show affinity towards FOZO （focal zone mantle） and PSCL （post-Archaean subcontinental lithosphere） which reflect mixing between asthenospheric and lithospheric mantle components in their source. Origin of RTB magma is attributed to plume-lithosphere interaction and the upward movement of melt is facilitated by intrabasinal deep-seated faults in the K-G Basin.

Petrogenesis, LA-ICP-MS zircon U-Pb geochronology and geodynamic implications of the Kribi metavolcanic rocks,Nyong Group, Congo craton

Metavolcanic rocks are well-exposed in the Kribi area within the Nyong Group, Congo craton, but their origin, age, and tectonic significance are poorly known.Here we report integrated field mapping and petrography,geochemistry, and LA-ICP-MS zircon U-Pb ages of these metavolcanic rocks to constrain their petrogenesis and geodynamic implications. The studied rocks consist of mafic granulite, garnet-amphibole gneiss, and garnet-biotite gneiss, and occur interbanded with sharp contact and intruded by syenite dyke. These metavolcanic rocks are classified as MORB-like tholeiitic to calc-alkaline basalts,basaltic andesite, and rhyodacite rocks with within-plate setting geochemical signatures. The metabasite rocks(basalt to basaltic andesite protolith) are likely the equivalent of a spinel peridotite product representing ~ 2–5 %partial melting of metasomatized mantle source, while the metarhyodacite rocks are derived from the fractional crystallization of the same parental magma. Zircon U-Pb data revealed that the rhyodacite rocks initially formed at2671 ± 51 Ma and underwent later metamorphism at2065 ± 55 Ma. The Neoarchean protolith age is comparable to the ca. 2628 Ma tholeiitic magmatism and ca.2666 Ma high-K granites, suggesting bimodal Neoarchean magmatic event within the Ntem Complex, while the metamorphic ages fall within the ca. 2100–2000 Ma highgrade tectono-metamorphic event attributed to Eburnean/Trans-Amazonian orogeny. At the regional scale,metavolcanic rocks with similar origins and ages are documented in the Sa o Francisco Craton in Brazil, suggesting comparable geodynamic evolution on both sides of the south Atlantic during the Paleoproterozoic.

The Bafoussam volcanic series:origin and evolution of the volcanism along the Cameroon volcanic line

The Bafoussam area in western Cameroon is part of the central Cameroon Volcanic Ligne(CVL).This study presents the mineralogy,major and trace element compositions,Sr-Pb-Hf isotopes,and new K-Ar geochronological data about mafic and felsic volcanic rocks.These rocks belong to two different series:A transitional series made of basalts,basaltic andesite,and trachytes and an alkaline mafic series with basalts,hawaiites,and basanites.New age data show that the transitional series belongs to the oldest part of the CVL and was emplaced between 47 and 35 Ma.The alkaline volcanism is younger,with ages ranging from10 to 4.5 Ma.Magmatic evolution in both series is accomplished through a fractional crystallization process,with the removal of olivine and clinopyroxene,while plagioclase does not seem to be a major crystallizing phase.All the samples are enriched in incompatible trace elements,but the rocks from the alkaline series have more fractionated REE patterns and high Nb content compared to the transitional mafic lavas.Alkaline lavas have lower initial^(87)Sr/^(86)Sr and higher^(176)Hf/^(177)Hf and Pb isotopic ratios than the transitional lavas.Low La/Nb and high^(87)Sr/^(86)Sriratio are among chemical characteristics that show that some samples from the transitional series have interacted with a crustal component during their evolution in the crust.They cannot be used for discussing the mantle source of the volcanic rocks from this series.Trace elements show that primary magmas for both series formed in a garnet-bearing mantle source,with higher partial melting degrees(3-5%)for the transitional magmas than for the alkaline magmas(<2.5%).Combining trace elements and isotopic ratios,we show that the Bafoussam lavas formed from two different mantle sources.Transitional magmas formed from a pyroxenite-bearing enriched mantle with low Pb isotopic composition.This mantle source is present in all the oldest lavas from the CVL.Alkaline magmas formed from an HIMU-like mantle source,different from the Mt Cameroon HIMU mantle source.The depleted asthenospheric mantle is not involved in the Bafoussam magmatism and the two mantle sources are probably located in the lithospheric mantle,in agreement with recent geophysical models presenting the CVL as a consequence of the partial melting of the lithospheric mantle in response to edge convection along the margin of the Congo craton.

Geochemistry and Petrogenesis of Tertiary Volcanic Rocks of the Eastern Roodbar, Alborz Mountain, North of Iran

In the Alborz Mountains of the eastern Roodbar (north of Iran), Tertiary volcanic rocks have a variety of composition between olivine basalt, basaltic andesite, pyroxene andesite and andesite. The presence of different xenoliths and xenocrysts is among the evidence of crustal contamination of these rocks. The positive correlations of CaO/Al2O3 vs to MgO and Al2O3 vs. SiO2 are of signs of the olivine and clinopyroxene fractionation in the variation trend of the area rocks. Positive correlations K2O/P2O5 vs with SiO2 and La/Sm vs. K2O/P2O5 demonstrate contamination of magma with the continental crust. The incompatible trace element patterns and their comparison with crustal contents indicate contamination of the rocks of the area with the lower and upper continental crust. Linear trends in the variation diagram of Nb/Y vs. Zr/Y, introduce two different source regions: a MORB source and the other continental crust for the rocks which are the genesis. The variations of Y/Nb vs. Zr/Nb and Rb/Y vs. Nb/Y reveal a crustal contamination of the magma originated from the MORB source. Geochemical studies represent that the area’s rocks were derived from the 15% melting of a mantle source of MORB type with spinel facies within a continental environment, which was contaminated by varying degrees of continental crustal rocks.

Sr-Nd-Pb isotopes of the Early Paleozoic maficultramafic dykes and basalts from South Qinling belt and their implications for mantle composition

Late Early Paleozoic mafic-ultramafic dykes and volcanic rocks from the South Qinling belt are char- acterized by εNd( t ) = +3.28― +5.02, (87Sr/86Sr)i= 0.70341― 0.70555, (206Pb/204Pb)i = 17.256― 18.993, (207Pb/204Pb)i= 15.505―15.642, (208Pb/204Pb)i=37.125―38.968, ?8/4=21.18―774.43, ?7/4=8.11―18.82. These charac- teristics suggest that they derived from a Middle Neoproterozoic mantle with isotopic compositions of mixed HIMU, EMII and minor EMI components. We interpret that these rocks were melting products of depleted mantle modified by subducted ancient oceanic crust and continental margin sediments along the northern margin of Yangtze block during Early Neoproterozoic.

Magma Generation of Magnetite-Rich Intermediate-Mafic Rocks and Its Mantle Processes in the Southwestern Alxa Block,NW China

Many studies focus on mineralization of huge magnetite ore deposits and petrogenesis of their large-volume host rocks.However,magma generation of those small-scale intrusions with enrichment of magnetite is poorly reported and paid attention to.We here carry out an integrated study of magnetite chemistry,U-Pb zircon dating,geochemistry,and Sr-Nd-Hf isotopes for the magnetite-rich intermediate-mafic rocks from the Helishan pluton in the southwestern Alxa Block,Northwest China.This,together with several previously reported magnetite/iron-rich intrusions nearby,is capable of providing some constraints on magma generation of magnetite/iron-rich intrusive rocks.The Helishan pluton,dated at ca.290 Ma,consists of hornblende gabbro,diorite,and quartz monzodiorite with~3%-5%magnetite in all the lithologies.Study on magnetite chemistry manifests a magmatic origin for them.All the lithologies display high TFeO/MgO ratio (1.71-1.89),weakly fractionated REE patterns((La/Yb)_(N)=1.82-10.17),enrichment of Rb,Sr,and Pb,and depletion of high field strength elements.They have (^(87)Sr/^(86)Sr)_(i) values of 0.705 2 to 0.705 8,ε_(Nd)(t) values of+0.03 to+0.64,and zircon ε_(Hf)(t) values of +6.5 to+12.0.We propose that they were derived from partial melting of iron-rich metasomatized lithospheric mantle.The systematic variations of Sr/Y ratios and Nd-Hf isotopic compositions with time for the Paleozoic igneous rocks at the southwestern Alxa Block indicate ever existence of thinning and rebirth of lithospheric mantle.This geodynamic process could be the potential mechanism to give rise to the iron-rich signature of the reborn mantle sources of the Helishan pluton.For intermediatemafic intrusions at subduction zones,they are unlikely to form considerable magnetite ore deposits since their modest magmatic flux and early fractional crystallization of magnetite at a high oxygen fugacity and H_(2)O condition.