| The “Erkeshan-Wudalianchi-Keluo-Xiaogulihe” potassic volcanic belt is located in the northmargin of the Songliao basin, NE China, which is the most important and youngest potassic volcanics in this area. In view of its particular geochemical properties(high K, low Al,incompatible elements enriched, LREE and LILE strongly enriched, light-heavy REE intensively differentiated, high 87Sr/86 Sr, 207Pb/206 Pb, and low 143Nd/144 Nd and 208Pb/204Pb), this volcanic belt has been the focus of research. A series of petrogenetic models for the belt were sequentially proposed in last several decades. The controversial issues of these models are related to magmatic source temperature and depth, the partial melting degree, metasomatic agent and mechanism, and the water content in mantle source. Among them, the issue about the source location, whether it originated from the transition zone in asthenosphere as a result of hydrous mantle plume or subcontinental lithospheric mantle, is of the biggest conflicts.In consideration of that, this study we introduce and employ a new olivine trace element tracing system, combining the traditional whole-rock geochemical data, to investigate the unsolved issues above and the detail of potassic volcanic petrogenesis will be reported. Meanwhile,given that the volcanic belt is located in a special tectonic setting(Songliao basin), where the lithospheric thickness changes from thin centre(80km) to thick margin(>120km), it provides an ideal natural laboratory to study the effects of lithospheric thickness and composition on the recent basaltic lavas and crystalizing olivines. We will also report a comprehensive set of whole-rock and mineral chemical data for the lavas from the volcanic belt to discuss this topic.1. Methodology and Theoretical FundamentalWith the development of high-precision electron microprobe(EMPA) and Laser-ICP-MS in-situ analyses, the measurement of low concentration trace elements in olivine is nicely accessible. The trace elements in olivine are used as a new geochemical indicator of mantle partition melting, mantle metasomatism and magma fractionation in recent years. A series of pioneering research papers is released continually to refer to Ni, Co, Al, Cr, Zn, Ti, Li, V, Sc, Mn,Ca and P in olivine. In this study, we apply these geochemical indicators to constraint the petrogenesis of potassic volcanic belt. At first, we summarize the latest achievements in this area and try to introduce the latest research progress and developing tendency of the issues as follows:the occurrence state of trace elements in olivine; the measurement, data reliability and the key point of measurement process of trace element in olivine; the tradition tracing indicator and the latest applications; the precautions for using the trace elements as indicator. And then we put these principles into the practice, to conduct the research proposal.2. Research ContentsDistribution of the volcanic belts is consistent with the lithospheric thinning along the northern part of the Songliao basin. In order to constrain the relationship between the geochemistry of the lavas and olivine composition, and the lithospheric thickness variation and composition, we collected basaltic samples from the whole volcanic belt and conducted a petrological study by whole-rock major, trace and Sr-Nd-Pb isotope analyses and detailed petrographical and geochemical study on olivine macrocrysts. The samples are divided into three groups: Group 1- Nanjianshan, Erkeshan and Lianhuashan; Group 2- Wudalianchi, Beijianshan,and Keluo; Group 3- Menluhe and Xiaogulihe. The grouping is based on the thickness of the lithosphere(L), as follows: 100 km < LGroup1 < 120 km < LGroup2 < 150 km < LGroup3.Basing on the whole-rock major, trace and Sr-Nd-Pb isotope analyses, we discussed the lava geochemical properties related to volcanic classification, magmatic affinity, characteristics of source metasomatism, and mantle end-members, etc.Basing on the major and trace elements study of olivine macrocrysts, we investigated the magmatism and mantle source nature in detail as follows:(1) we divided the olivine trace elements into several groups( i) Ni, Mn, Co, Cu, Zn and Li,ii) Cr, Al, V, Sc, Ca and Na, iii) P, Ti,Li and Zn), according to their occurrence characteristics, partition behaviour and tracing features in olivine;(2) we distinguished the different populations in olivine phenocrysts and xenocrysts,basing on the Ti, Ca, Zr, Sc and Mn content in olivine;(3) we inferred the depleted degree of magma source by olivine phenocrysts Mg# value, spinel Cr# value and olivine-spinel mantle array(OSMA) diagram;(4) we indicated if the mantle metasomatism involve of crust-derived material happened by P, Li, Ti content in olivine;(5) we calculated the mantle potential temperature, using olivine-Al thermometer;(6) we figured out the mineral assembly in mantle source by Al and Cr characteristics in olivine phenocrysts;(7) we inferred the magma mixing event and carbonatization in source region;(8) we discriminated the origin of EM-1 endmember in study area; etc.With regard to the topic about the effects of lithospheric thickness and composition on the recent basaltic lavas and crystalizing olivines, we took advantage of following approaches: we related a comprehensive set of whole-rock chemical data for the lavas from the volcanic belt to subcontinental lithospheric mantle thickness variation to test the “Lid effect”; we adopted a serial of selected element indexes(e.g. Ni, Ni/Mg, Ni/(Mg/Fe), Mn/ Fe and Ca/ Fe) in olivine to weigh the effect by Lid effect and lithospheric mantle composition on mineral composition; etc.3. Research Conclusions(1) The olivines in potassic volcanics from study area can be divided into four populations.Three of them are igneous phenocrysts and the rest is mantle xenocrysts. We name them: normal igneous phenocryst; evolved igneous phenocryst; Low-Ni igneous phenocryst; mantle xenocryst.(2) The olivine trace elements characteristic indicated that: the depth of the magma source is at least equal to the garnet peridotite region. The source experienced high degree depletion and the extent of melt extraction lead to the composition differences of three groups. The harzburgite rather than lherzolite is envisaged as mantle source and the lithospheric mantle beneath the Songliao basin could be layered. Our new olivine data offer additional support for the SCLM origin of the Songliao basaltic lavas, with clear affinity towards EM-1 component, which usually experienced one or more episodes of mantle depletion and subsequent metasomatism. The low Ni olivine may be a consequence of orthopyroxene digestion into the melt during carbonation of the lithospheric mantle, which includes interaction of carbonatitic melt with the lithospheric mantle involving assimilation of orthopyroxene and instantaneous crystallization of low-Ni olivine. The partial melting of “hybrid” ultramafic sources comprising wehrlitic metasomes and surrounding lithospheric mantle may produce basaltic primary melts in the Xiaogulihe area(Group 3).(3) The average maximum crystallization temperature of olivines from three groups are1258℃, 1224℃ and 1203℃, respectively, which indicates that there is no obvious regional thermal anomaly between them(the temperature difference is around 20℃). All the maximum crystallization temperature of olivines from study area are lower than 1300℃, much lower than the thermal anomaly of typical mantle plume, so we inferred that there is no mantle plume proposed by former research in this region.(4) Basing on the whole-rock geochemical data and the trace elements characteristic in olivine, our study infered the formation process of the “Erkeshan-Wudalianchi-Keluo-Xiaogulihe”potassic volcanic belt as follows: melting across the Songliao basin occurs partially within the asthenosphere and generally stops at the base of the mantle lithosphere. The interaction of the asthenosphere with heavily metasomatised and most probably stratified SCLM leaded to the material mixing with different proportion between the asthenosphere and lithosphere in different location with different lithosphere thickness, which produced the composition variation among three volcanic groups.(5) The correlationship between the major elements(e.g. Si72, Al72, Mg72, Fe72, Ti72 and P72),trace elements(e.g. Sm/Yb, La/Sm), several selected parameter(e.g. Ni, Ni/Mg, Ni/(Mg/Fe), Mn/Fe and Ca/ Fe) in melt-related olivine and lithospheric thickness demonstrates that the continental lithosphere thickness variation affect the geochemistry of the Songliao lavas.4. Innovation Points(1) We systematically introduced methology of olivine trace elements as indicators for magma process and petrogenesis.(2) We firstly applied the trace elements in olivines as geochemical indicators to petrogenesis study of potassic volcanic belt in Songliao basin, NE China and made some effective constraints to petrogenesis information, such as mantle source lithology assembly, source depleted degree, extent of partial melting, etc.(3) We investigated the effect of subcontinental lithospheric mantle thickness and composition on the recent basaltic lavas and crystalizing olivines. |
PDF Full Text Request