Early Paleozoic Magmatism and Crustal Evolution in the North Qinling Orogen

1 Introduction The North Qinling orogenic belt is characterized by diverse rocks,multi-phase tectonic-magmatic events,which is composed of not only basement rocks of the Qinling Group,but also numerous magmatic rocks in

Magmatic Evolution and Mineralization Process of the Super-Large Shihuiyao Rb–Nb–Ta Deposit,Southern Great Xing'an Range,China

Rare metal ore reserves are an important strategic resource, and their metallogenic mechanism and mineralization studies have also been received widespread international attention.

Magmatic Evolution and Mineralization of Porphyry Copper-Gold Deposits in the Duolong Ore Concentration Area, Tibet

The Bangong Lake-Nujiang River metallogenic belt is located between the Qiangtang Block and Lhasa Block, and the Duolong ore concentration area is located in the western section of the Bangong Lake-Nujiang River metallogenic belt. Till now, several large and super large copper-gold deposits, such as Duobuza, Bolong, Dibaonamugang, Naruo and Rongna deposits have been discovered in this area, mainly porphyry copper-gold ones.

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.

Magma source and evolution law in Yueshan ore field, Anhui Province, China

Major elements, trace elements and sulfur, oxygen isotopic compositions of the main intrusions were studied in Yueshan area. The fact that intrusions enriched in Th and Sr, and depleted in Rb and Ba in this area, suggests that the original magma roots in alkali basalt magma of upper mantle, with deep characteristics. It can be seen that the diagenesis environments are the island arc and active continental margin areas from the lg τ to lg σ diagram of intrusions. With the increase of SiO2, Fe2O3, MnO and P2O5 decrease, which shows that the magma of Yueshan area endured crystal fractionation of ferromanganese mineral and apatite in early evolution stage. With the further rise and evolution of magma, magma composition of calcium increased, meanwhile enriched in Zr, and depleted in Nb and Ta. This indicates that crustal component is gradually added, the assimilation and contamination occur between magma and crustal material, which includes the magma evolving, from calc-alkaline series to alkaline series. The results show that crystal fractionation, assimilation and contamination are the main evolution law of magma in this area.

Granitoid magmatism in the Qinling orogen, central China and its bearing on orogenic evolution

The Qinling orogen is a typical composite orogen for understanding multi-stages of magmatism and orogenic processes. Many studies have been carried out on the magmatic rocks in the Qinling orogen but their petrogenesis is still controversial. This pa- per presents a review of all granitoid rocks based on previous and new studies of geochronology and geochemistry. Four dis- tinct periods of granitoid magmatism, Neoproterozoic （979-711 Ma）, Paleozoic （507-400 Ma）, Early Mesozoic （250-185 Ma） and Late Mesozoic （160-100 Ma）, have been recognized from the Qinling orogen according to zircon U-Pb ages, intrusion as- sociations and deformation, as well as regional geology. The Neoproterozoic granitic rocks consist of three stages at 979-911, 894-815 and 759-711 Ma, respectively, corresponding to strongly deformed S-type, weakly deformed I-type and A-type gran- itoids. They can be interpreted as magmatic occurrences in syn-collisional, post-collisional and extensional settings, respec- tively, in response to old continental terranes of the Neoproterozoic tectonomagmatic events in the old continents of China, such as South China and Tarim cratons. Although this continental terrane would be involved in the Phanerozoic Qinling orog- eny, the Neoproterozoic magmatic rocks are not the products of the Qinling orogenic processes. The Paleozoic magmatic rocks can be classified into three stages at 507-470, 460-422 and 415-400 Ma, respectively. The first-stage magmatism is temporal- ly associated with ultra-high pressure metamorphism in the North Qinling terrane. These magmatic rocks are interpreted as magmatic occurrences in subductional, syn-collisional and post-collisional settings, respectively. The Early Mesozoic mag- matic rocks occur in two stages at 252-185 and 225-200 Ma, respectively. The first-stage granitoids are mainly represented by I-type quartz diorites and granodiorites, and the second stage by granodiorites and monzogranites with the 1- to A-type charac- teristics and some with rapakivi textures. Their emplacement ages and geochemical parameters such as A/CNK, K2O/Na2O ra- tios and εNd（t） values do not show any polarity change in perpendicular to subduction/collision zone. Therefore, all these Early Mesozoic granitoids are unlikely the product of continental subduction as some researchers suggested. Instead, they are plausi- bly related to the subduction of the Mianlue Ocean and the subsequent collision between the South China Craton and the South Qinling terrane. The Late Mesozoic granitoids were emplaced mainly at two stages of 160-130 and 120-100 Ma, and charac- terized by the evolution from I- to I-A- and A-type granitoids. These characteristics are consistent with the granitoid magmatic evolution from contractional to extensional settings during the Jurassic/Cretaceous in eastern China. Accordingly, the Late Mesozoic granitoid rocks in the Qinling orogen probably have a similar petrogenetic mechanism to those of the huge magmatic belt along the western Pacific margin, i.e., intra-continent magmatism related to a far-field effect of the subduction of Paleo-Pacific plate.

Ages, Sources and Tectonic Settings of the Triassic Igneous Rocks in the Easternmost Segment of the East Kunlun Orogen, Central China

U–Pb analysis of zircons from igneous rocks in the Elashan Mountain, easternmost segment of the East Kunlun Orogen yielded 252–232 Ma. Geochemically, these rocks are mainly high in SiO_2, K_2O and K_2O＋Na_2O contents, low in P_2O_5 and TiO_2 contents, depleted in Ba, Sr, P, Ti and enriched in U, Hf, Zr, showing features of I–type granite. The zircon εHf（t） values of the Early Triassic Jiamuge＇er rhyolite porphyry（252±3 Ma） are positive（＋1.6 to ＋12.1）, suggesting a juvenile crustal source mixing with little old crustal component, and the zircon εHf（t） values of the Middle Triassic Manzhang＇gang granodiorite（244±3 Ma） and Dehailong diorite（237±3 Ma） are predominately negative（-8.4 to ＋1.0）, indicating an older crustal source. In comparison, the zircon εHf（t） values of the Late Triassic syenogranites from Suigen＇ergang（234±2Ma）, Ge＇ermugang（233±2 Ma） and Yue＇ergen（232±3 Ma） plutons vary from-3.8 to ＋5.0, suggesting a crust-mantle mixing source. From Early–Middle Triassic（252–237 Ma） to Late Triassic（234–232 Ma）, the geochemical characteristics of these rocks show the change from a subduction–collision setting to a post-collision or within-plate setting. By comparing of these new age data with 77 zircon U–Pb ages of igneous rocks of the eastern part of East Kunlun orogen from published literatures, we conclude that the igneous rocks of Elashan Mountain and these of the eastern part of East Kunlun Orogen belong to one magmatic belt. All these data indicate that the Triassic magmatic events of the eastern part of East Kunlun Orogen can be divided into three stages： 252–238 Ma, 238–226 Ma and 226–212 Ma. Statistically, the average εHf（t） values of the threestage igneous rocks show a tendency, from the old to young, from-0.75±0.25 to lower-2.65±0.52 and then to-1.22±0.25, respectively, which reveal the change of their sources. These characteristics can be explained as a crust-mantle mixing source generated in a subductional stage, mainly crust source in a syn–collisional stage and a crust-mantle mixing source（lower crust with mantle-derived underplating magma） in a post-collisional stage. The identification of these three magmatic events in the Elashan Mountain, including all the eastern part of East Kunlun Orogen, provides new evidence for better understanding of the tectonic evolution of the northward subduction and closure of the Paleo-Tethyan（252–238 Ma）, the collision of the Songpan–Ganzi block with the southern margin of Qaidam block（238–226 Ma）, and the post–collisional setting（226–212 Ma） during the Early Mesozoic period.

Mantle Source Components and Magmatic Evolution for the Comei Large Igneous Province:Evidence from the Early Cretaceous Niangzhong Mafic Magmatism in Tethyan Himalaya

The Niangzhong diabase dikes,dated at 138.1±0.4 Ma,are located within the outcrop area of the Comei large igneous province(LIP).These diabase samples can be divided into two groups:samples in Group 1 show varying MgO(1.50 wt.%-10.25 wt.%)and TiO_(2)(0.85 wt.%-4.63 wt.%)contents,and enriched initial isotope compositions(^(87)Sr/^(86)Sr(t)=0.7056-0.7112,ε_(Nd)(t)=-0.3-+3.8),with OIB-like REEs and trace elements patterns,resulting from low degree melting of garnet-bearing lherzolite mantle sources;in contrast,samples in Group 2 show limited MgO(4.14 wt.%-7.75 wt.%)and TiO_(2)(0.98 wt.%-1.69 wt.%)contents,and depleted initial isotope compositions(^(87)Sr/^(86)Sr(t)=0.7075-0.7112,ε_(Nd)(t)=+5.5-+6.2),with N-MORB-like REEs and trace elements patterns,resulting from relatively high degree melting of spinel-bearing lherzolite mantle source.Combined with the published representative data about Comei LIP,we summarize that the source components for Comei LIP products include OIB end-member,enriched OIB end-member,and N-MORB end-member,respectively.Melts modeling suggests that magmas in the Comei LIP evolve in a relatively high oxygen fugacity condition,which influenced their fractionation sequences and led to systematic changes of TiO_(2)contents,Ti/Y and Ti/Ti*ratios.From the spatial and temporal distribution of above three end-member samples,deep process of Kerguelen plume during the Comei LIP formation can be interpreted as the interaction among the Kerguelen plume,the overlying lithospheric mantle,and the upwelling asthenosphere.The magmatism of Comei LIP began at~140 Ma and then lasted and peaked at~132 Ma with the progressively lithospheric thinning of eastern Gondwana upon the impact of Kerguelen plume.

Geochemical characteristics of major and trace elements in the Okinawa Trough basaltic glass

The Okinawa Trough（OT） is a back-arc basin at an initial spreading stage that is under the influence of subduction of the Philippine Sea Plate. In this study, we analyzed the geochemical compositions of basaltic glass in the OT and discussed the effects of different magmatic sources, evolution, and subducted components in basalts. Our results showed that the middle and southern regions of the OT exhibit characteristics consistent with an iron-rich tholeiite series. Trace element proportions conform to the typical spider diagram pattern characteristic of back-arc basin basalts, rich in large ion lithophile elements（LILEs） including Rb, Ba, Pb, U, and Th, while depleted in high field-strength elements（HFSEs） including Nb, Ta, Zr, Hf, and Ti. The distribution of rare earth elements（REEs） is also consistent with enrichment by right-leaning light rare earth elements（LREEs）.The addition of enriched mantle type I（EMI） materials as well as mantle heterogeneity may have led to variable degrees of enrichment in different regions. The magma source of the middle trough has undergone crystallization towards pyroxene, while development of plagioclase was restricted partly, and the crystallization of spinel and olivine ceased altogether. At the same time, crystallization of the southern OT magma source was dominated by olivine and including the formation of plagioclase, pyroxene, and magnetite（or titanomagnetite）. Finally, the results of this study showed that 90% Th, 95% Ba in the southern basalt, 50%–70% Th and 70%–90% Ba in the middle basalt originated from subducted component. Different subducted component influence may be due to different subduction zone structural feature.

Chemical composition and petrogenesis of plagioclases in plagioclase-phyric basalts from the Southwest Indian Ridge(51°E)

Electron microprobe analysis was conducted on plagioclase from the plagioclase ultraphyric basalts(PUBs)erupted on the Southwest Indian Ridge(SWIR)(51°E)to investigate the geochemical changes in order to better understand the magmatic processes occurring under ultraslow spreading ridges and to provide insights into the thermal and dynamic regimes of the magmatic reservoirs and conduit systems.The phenocryst cores are generally calcic(An74–82)and are depleted in FeO and MgO.Whereas the phenocryst rims(An67–71)and the plagioclase in the groundmass(An58–63)are more sodic and have higher FeO and MgO contents than the phenocryst cores.The crystallization temperatures of the phenocryst cores and the calculation of the equilibrium between the phenocrysts and the matrix suggest that the plagioclase cores are unlikely to have crystallized from the host basaltic melt,but are likely to have crystallized from a more calcic melt.The enrichment in incompatible elements(FeO and MgO),as well as the higher FeO/MgO ratios of the outermost phenocryst rims and the groundmass,are the result of plagioclase-melt disequilibrium diffusion during the short residence time in which the plagioclase crystallized.Our results indicate that an evolved melt replenishing under the SWIR(51°E)drives the eruption over a short period of time.

Estimating the Parental Magma Composition and Temperature of the Xiaohaizi Cumulate-Bearing Ultramafic Rock: Implication for Magma Evolution of the Tarim Large Igneous Province, Northwestern China

The ultramafic dikes in the Tarim large igneous province（Tarim LIP）, exposed in the Xiaohaizi area in the northwestern Tarim Basin of northwestern China, have porphyritic textures, and the olivine and clinopyroxene are as the major phenocryst phases. The groundmass therein consists of clinopyroxene, plagioclase and Fe-Ti oxides, with the cryptocrystalline texture. The olivine phenocrysts in one typical ultramafic dike have Fo（Mg/（Mg＋Fe）） numbers ranging from 73 to 85, which are not in equilibrium with the olivine（Mg# of 89） from the host rock crystalized. Combined with microscope observation, both the olivine and clinopyroxene phenocrysts as well as some Fe-Ti oxides in the ultramafic rock are accounted as cumulates. The liquid（parental magma） composition of SiO2 of 45.00 wt.%–48.82 wt.%, MgO of 9.93 wt.%– 18.56 wt.%, FeO of 5.85 wt.%–14.17 wt.%, CaO of 7.54 wt.%–11.52 wt.%, Al2O3 of 8.70 wt.%–11.62 wt.% and TiO2 of 0.00 wt.%–3.43 wt.% in the Xiaohaizi ultramafic rock was estimated by mass balance, and the results show a reasonable liquid proportion in the cumulate-bearing ultramafic dike（ca. 45%–60% in the whole rock）. The estimated parental magma composition corresponds to a melting temperature of 1 300–1 550 oC, which is equal or higher than those of a normal asthenosphere mantle, supporting the involvement of a mantle plume. Combined with other previous studies, an evolution model for the formation processes of the Xiaohaizi ultramafic dike of the Tarim LIP is proposed.

Zircon Trace Element Constraints on the Evolution of the Paleoproterozoic Birimian Granitoids of the West African Craton(Ghana)

The Paleoproterozoie Birimian granitoids of the West African Craton （WAC） in the northwestern part of Ghana, have been studied for their zircon trace elements concentrations to infer the source characteristics, origin, and magmatic evolution. The zircons in the granitoids have Th/U ratios ranging from 0.03 to 1.55, and display depleted light rare earth elements 0LREE） and enriched heavy rare earth elements （HREE） contents, characterized by pronounced positive to negative anomalies of Eu （Eu/Eu＊=0.14-0.98 and 1.01-6.06, respectively） and Ce （Ce/Ce＊=0.08-0.98 and 1.02-116, respectively）, which may imply that they were derived from both magmatic and hydrothermal sources. The geochemical plots of U/Yb vs. Y and Hf, the positive correlation between Hf and the other high field strength elements 0tFSE） and high rare earth elements （REE） contents, with enrichment in Ce and depletion in Eu, indicate that the granitoids possibly formed from partial melting of the crust. The trace elements characteristics （i.e., wide range of Hf, Ce/Ce＊, Th/U and Zr/Hf values） of the zircons suggest that crystallization of the magma occurred under variable oxidation states, which spanned over a longer period, implying that our data corroborate interpretations from studies of whole-rock geochemistry and geochronology on the granitoids of northwestern Ghana. This further indicates that the evolution of the Birimian granitoids in this part of the WAC occurred earlier than what had been reported in the literature.

The Geochemical Effect of Lanthanides: Its Types and Application for Magmatic Rocks—A New Method to Semi-Quantitatively Determine Strength of Magmatic Fluid Complexation and Fractional Crystallization

In this paper, the relationship between rare earth elements （REEs） geochemical behavior and their ionic radii was studied. According to the basic law called the effect on geochemistry of lantha- nides, five types and some of subtypes of REEs occurrence, both in magmatic rocks and their minerals, were found, which may correspond to the evolutionary way of magmatic rocks. Analysis of REEs data for an amazonite-topaz granite pluton in East Xinjiang, one of the administrative regions of West China, and Cenozoic alkaline volcanic-intrusive rocks in the Xialiaohe depression of Bohai Bay Basin, China show that types II1, III and IV of effect on geochemistry of lanthanides not only reflect fluid complexa- tion and fractional crystallization existed in magmatic evolution directly, but also can determine semi-quantitatively the strength of fluid complexation and fractional crystallization. Therefore, the ef- fect on geochemistry of lanthanides, a new way to study semi-quantitatively evolution of magmatic rocks, is probably practicable. Moreover, the mafic lavas of different sources from the margin of Gonghe Basin, Qinghai Province, China, can be effectively distinguished in the diagram which can be drawn with some parameters of linear equation of LREEs and its ion radius.