A Study On Melt-Lithosphere Interaction Beneath Hannuoba, North China

Since the1980s, the studies on Chinese Cenozoic volcanoes and mantle-derivedxenoliths within them have been started and a lot of achievements are obtained.According to the calculated results of equilibrium temperature and pressure ofmantle–derived peridotite xenoliths in Chinese Cenozoic basalts, Liu et al.（1981） firstproposed that the paleotemperature of upper mantle in North China is close to oceangeotherm and near rift geotherm and the lithospheric thickness of Cenozoic NorthChina is about50～80kilometers. This conclusion waved the previous traditionalunderstanding about the tectonic attribute of the North China Craton and laid afoundation for the major research project of the destruction of North China Craton.Up to now the study on the destruction of the North China Craton is still a focusedsubject.At present it is widely accepted that the lower lithospheric mantle of North ChinaCraton was destroyed in Phanerozoic time. Its concrete manifestation is the changefrom Paleozoic old cratonic mantle with cold, thick, high P wave velocity, low density,depleted in major elements, mainly harzburgite and enriched in radiogenic isotopes tonew oceanic mantle with hot, thin, low P wave velocity, relatively high density,enriched in major elements, mainly lherzolite and depleted in radiogenic isotopes.However the questions about the time, space and mechanism of destruction have notbeen solved yet and always debatable in academic circles. Most of people state thatthe occurrence time of North China lithospheric mantle’s destruction is limited inMesozoic, but some researchers think that the destructive effect lasted to Cenozoic.With respect to destructive range, previous studies think it was only confined to theeastern area. More and more evidence demonstrates that parts of the eastern area havealso experienced or are experiencing lithospheric mantle’s thinning andtransformation. Regarding the question about the destructive dynamic mechanisms ofNorth China lithospheric mantle, nowadays there are two representative models, oneis the delamination model and another is the erosion model. Although these two viewpoints can explain some issues respectively, they still have some limitations. Inrecent years, some scholars propose that melt-lithosphere interaction plays animportant role in lithospheric mantle’s destruction and transformation. The mostvisual evidence is the reaction phenomenon between melt and peridotites in manyareas of North China, which resulted in the changes of lithospheric mantle’scompositions and properties. It is concretely represented that reactions betweenperidotite xenoliths or peridotite mineral xenocrysts and basaltic magma are observedin many areas of North China. These minerals’ structures show perfectly round,embayed or zigzag, besides the compositions of minerals’ cores are quite differentfrom their rims. The longer the residence time of xenoliths or xenocrysts in magmalasts, the more thoroughly the reaction goes on. In that way xenoliths or xenocrystscan not really stand for mantle source origin.This thesis makes a deep research on peridotite xenoliths and carbonatites inCenozoic basalts from Hannuoba and its surrounding areas, which are located at thenorth margin of the western North China Craton. The purpose is to investigatewhether the lithospheric mantle below this region has experienced destruction andtransformation, what is the degree of destruction and transformation and what is thetime and way of destruction and transformation. The key point of this thesis is todiscuss the roles of melt in lithospheric mantle’s destruction and transformation.Through the detailed studies of petrography and geochemistry, it is found that therehave a large number of melt-rock reaction and metasomatism phenomena in mantlexenoliths. The melt types include silicate melt and carbonate melt. The results indicatethat melt-rock interaction occurs not only in destroyed and thinned eastern area, butalso in the western area. Owing to the melt’s transforming lithospheric mantle in thelate stage, the understanding that the western lithospheric mantle can represent oldmantle relics should be questioned.Regional geological conditionsThe forming process of North China Craton is very complex. In general, it is divided into three evolution stages. Firstly, Archean to Paleoproterozoic active stage, thenMeso-Neoproterozic to Mesozoic Triassic sustaining stable stage, finally Mesozoic toCenozoic active stage. Most researchers hold that the completion time of cratonizationof North China is roughly late Archean.The study area, Hannuoba and its surrounding area, is located in the west of theNorth-South gravity gradient lineaments in North China Craton. This area hostslarge-scale basalts with several cycles in which both alkaline basalts and tholeiiteswere produced. Alkaline basalts usually contain plenty of mantle and crust xenolithsand megacrysts which are all non-directional. The content of xenoliths can reach70～80%at most. The xenoliths are not of uniform size. Their diameters are several todozens of centimeters, generally10～20cm. The xenoliths are mostly spherical toellipsoidal without edge. Lherzolite is the most common rock type （～90%）. Besides,other types are relatively common, such as harzburgite, dunite, pyroxenite andgranulite xenoliths and pyroxene and feldspar megacrysts. In some areas there aresome carbonatite veins which cross alkaline basalts and peridotite xenoliths withinbasalts. One carbonatite pipe produced by magma explosion was also found.Experimental analysis methodsWe use many different analytical methods to achieve different research objectives. Atfirst we have representative samples made to thin sections, then through microscopepetrographical observation, we learned the mineral compositions and structuralfeatures of samples. For different samples we chose different analytical methods asfollows. Minerals in peridotites reacted with basaltic magma as well as their reactionrims and melt inclusions in peridotites were used electron probe analysis to test majorelements. Peridotites of different types were used for Re-Os isotope and PGE elementanalysis to explore peridotites’ genesis and evolution, for example whether theyexperienced metasomatism. For the genesis of carbonatites, we used XRF analysis ofmajor elements, ICPMS analysis of trace elements, C-O isotopes analysis andminerals’ electron probe analysis. For pyroxenites and granulites, minerals’ electron probe analysis, trace elements’ laser probe analysis of minerals and Sr-Nd isotopesanalysis were used to investigate their origins.Silicate melt-peridotite interaction1Basaltic magma-peridotite interactionMantle peridotite and its disaggregated mineral xenocrysts, which were captured bythe Cenozoic basalts in Hannuoba and its surrounding area, usually have reaction rimstructure, which provides important information of melt-peridotite interaction whenbasaltic magma passing through lithospheric mantle. Based on the petrographicalobservation of mantle-derived peridotite xenoliths, further electron probe analysis ofmantle minerals and their reaction rims have been made to reveal the significance ofmelt-peridotite interaction in North China lithospheric evolution. The rims of olivine（Ol）, clinopyroxene （Cpx） and spinel （Sp） show similar composition changes from theMg-rich cores to the Fe-rich rims. The compositions of Ol and Cpx reaction rimsapproach to those of the phenocrysts in basalt. The reaction rim of orthopyroxene（Opx） usually consists of Ol+Cpx+Glass. The compositions of Ol and Cpx in Opxreaction rims are rich in iron relative to their mantle counterparts. The glass in Opxreaction rim is silicon-and alkaline-rich intermediate-acid glass. Simultaneously thevariations of colour and composition of spinels in peridotite xenoliths are related totemperature. The reservation of reaction rim structures in mantle minerals impliesrapid uprising of the host basaltic magma. The reaction between basaltic magma andmantle peridotites during magma ascending occured not only in Mesozoic but also inCenozoic. The final results of reactions translated the lithospheric mantle frommagnesium-rich to iron-rich and from major elements-depleted to majorelements-enriched. This kind of melt-peridotite reaction has already become animportant chemical kinetics mechanism for lithospheric mantle’s destruction andtransformation in eastern and western North China Craton. 2Si-rich melt-peridotite interactionA comparative study on Si-rich melt of different occurrence features in Hannuoba andits surrounding area has been carried out in this work. The main existing forms ofSi-rich melt are melt inclusions in mantle minerals and glasses in orthopyroxenereaction rims. Through detailed petrographical observation and electron microprobeanalysis, it is found that although these two melts are both rich in silica, because ofother component differences they have different origins. The glass composition inmelt inclusions is rich in silica （SiO261～65%） and volatile （about3～6%）, andrelatively poor in Na2O （1～3%） and K2O （<1%）. This kind of glass was produced bymantle metasomatism. The glass in orthopyroxene reaction rims enriches in silica（SiO264～67%） and alkaline elements （Na2O5～7%and K2O6～9%）, and rarelyhas volatile. It is the product of reaction between orthopyroxene in mantle xenolithsand host basaltic magma when magma carrying mantle xenoliths was ascending toearth’s surface quickly. Si-rich melt inclusions which were produced by mantlemetasomatism highly influenced geochemical properties of lithospheric mantle. Theirexistence brought about trace element composition’s change of mantle rocks andminerals, so as to cause geochemical heterogeneity of lithospheric mantle. Meanwhilethe reaction between orthopyroxene within mantle xenoliths and basaltic magmaresulted in lithospheric mantle’s transformation from Si-rich mantle to Si-poor mantle,and provided a new evidence for the destruction of western North China lithosphere.3Metasomatic peridotiteIn this work we chose Re-Os isotopes and PGE elements of harzburgites andlherzolites to investigate these two types of rocks’ petrogenesis and evolution. Theresearch results of Re-Os isotopes show that Os contents of peridotites are depletedrelative to those of primitive mantle. The reason may be related to the transformationof late stage after the rocks’ formation by partial melting, which led to Os element’smigration and decrease of Os content. The ratios of187Re/188Os and187Os/188Os ofperidotites are depleted compared with those of primitive mantle. Nevertheless thereis no linear correlation between187Os/188Os and187Re/188Os. It may be because the peridotites underwent recent disturbance. Although the peridotites have relativelyyoung Re depletion ages （t=0.20～2.19Ga）, it does not mean that the lithosphericmantle was newly accreted during Phanerozoic period. The ages may indicate that it isthe result of interaction between old lithospheric mantle and melt. The PGE resultsshow that the harzburgites and lherzolites are the residuals of different degrees’ partialmelting of mantle and a part of them underwent later melt/fluid metasomatism so as tocause the relative enrichment of Pt and Pd contents. The range of this metasomatismis relatively small and different from that of the above metasomatism, which broughtabout the general reduction of Os content. It is concluded that the peridotites in thestudy area at least experienced two stages of metasomatism.Carbonate melt-peridotite interactionIt is found that carbonatites occur as veins and pipes in Cenozoic alkaline basalts fromHannuoba and its surrounding area. We used field geological, petrographical andgeochemical methods （including major and trace element compositions, C–O isotopiccompositions and mineral chemical compositions） to constrain the origin ofcarbonatites. Based on the above results, we come to the conclusion that thecarbonatites are mantle-derived magmatogenic origin. Field observation shows thatcarbonatite veins cross the basalts and peridotite xenoliths within basalts. Peridotitexenoliths which are adjacent to carbonatite veins experienced stronger carbonatizationthan those far away from carbonatite veins. Petrographical observation displays thatthe calcites in the carbonatites have columnar and granular structures. Thecarbonatites usually contain mantle-derived mineral xenocrysts, such as olivine,orthopyroxene, clinopyroxene, and spinel. The carbonatites are calcite carbonatitescharacterized by high CaO content. The close spatial-temporal relationships, similarREE patterns and C-O isotopic compositions for both carbonatites and alkaline basaltsindicate that they are originated from the asthenosphere mantle and generated bysilicate-carbonate liquid immiscibility. It is speculated that CO2-rich silicate melt fromasthenosphere experienced immiscibility at the relatively low pressure during its ascending through lithospheric mantle. Alkaline basaltic magma with relatively hightemperature firstly erupted to the surface, and later CO2-rich carbonate magma withlower temperature erupted to produce the pipe or the veins injecting into solidifiedalkaline basalt and peridotite xenoliths. Carbonate melts interacted with capturedMg-rich peridotite minerals during ascending of carbonate melt, causing increase ofthe magnesium content of the calcite.Discussion on petrogenesis of pyroxenite and granulite xenolithsThe Cenozoic basalts from Hannuoba contain a certain amount of pyroxenite andgranulite xenoliths, besides abundant peridotite xenoliths. As pyroxenite and granulitexenoliths appear very similar by eye observation sometimes, it is easy to confound thetwo kinds of rocks. Here we present the research on the petrogenesis of pyroxeniteand granulite through rock texture’s and petrographical observation, mineralchemistry and trace element and Sr-Nd isotopic compositions of clinopyroxene. Theresults indicate that the two kinds of xenoliths are remarkably different. Granulitexenoliths have layered cumulate structure and two kinds of pyroxenes （salite andbronzite） are relatively rich in FeO and poor in MgO. Clinopyroxene in granulitexenoliths is relatively rich in REE and characterized by high⁸⁷Sr/⁸⁶Sr ratios and low¹⁴³Nd/¹⁴⁴Nd ratios. In contrast, pyroxenite xenoliths generally show massive structure.Diopside and bronzite-enstatite of the pyroxenite xenoliths are relatively rich in MgOand poor in FeO. Clinopyroxene in pyroxenite xenoliths has very low REE contentand a REE-depleted pattern. Isotopic compositions of pyroxenite xenoliths arebetween those of peridotite and granulite xenoliths. The above features as a wholesuggest that the granulite xenoliths were resulted from magma underplating, whichwere subjected to lower crustal contamination. However, the pyroxenite xenoliths,distinguished from peridotite and granulite xenoliths, were derived from enrichedmantle. Conclusions1. From field investigation and petrographical observation on Cenozoic basalts andmantle-derived peridotite xenoliths within them, it is found that there are corrodedperidotite xenoliths and mineral xenocrysts and many reaction structures in basalts,and the peridotites crossed by carbonatite veins are subjected to carbonatization.These phenomena reveal that the interactions between silicate or carbonate melt andlithospheric mantle in Hannuoba and its surrounding area exist widely.2. Basaltic magma reacted with peridotite xenoliths when passing through lithosphericmantle. The reaction results caused the composition changes of Ol, Cpx and Sp fromthe Mg-rich and Fe-poor cores to the Fe-rich and Mg-poor rims and the generation ofOpx reaction rims with Ol+Cpx+Si-rich Glass （almost no volatile）. The basalticmelt-peridotite reactions transformed the lithospheric mantle from magnesium-rich toiron-rich and from major elements-depleted to major elements-enriched. This kind ofreaction is considered to be an important chemical kinetics mechanism for thedestruction and transformation of Cenozoic lithospheric mantle in western NorthChina.3. The composition of Si-rich melt inclusions in mantle minerals is rich in silica andvolatile, and relatively poor in alkaline elements. This kind of glass is different fromthe melt in orthopyroxene reaction rims, which enriches in silica and alkalineelements, but rarely has volatile. The existence of Si-rich melt inclusions broughtabout geochemical heterogeneity of lithospheric mantle.4. The relative depletion of Os contents in peridotites indicates that the rocks weresubjected to metasomatism, so that leading to Os element’s migration and reduction.The relatively young Re depletion ages （t=0.20～2.19Ga） of peridotites really doesnot suggest that the lithospheric mantle was newly accreted during Phanerozoicperiod. It may indicate that it is the result of interaction between old lithosphericmantle and melt. The PGE results manifest that the harzburgites and lherzolits are theresiduals of partial melting of different degrees in the mantle and a part of samplesexperienced later melt/fluid metasomatism which caused the relative enrichment of Pt and Pd contents.5. The calcites in carbonatites have columnar and granular structures and thecarbonatites usually contain mantle-derived mineral xenocrysts. These twopetrographical features indicate that the carbonatites are of mantle-derivedmagmatogenic origin. The close spatial-temporal relationships, similar REE patternsand C-O isotopic compositions for both carbonatites and alkaline basalts suggest thatthey are homologous and generated by silicate-carbonate liquid immiscibility duringmagma ascending through lithospheric mantle.6. Pyroxenite xenoliths and granulite xenoliths show massive structure and layeredcumulate structure, respectively. Diopside and bronzite-enstatite of the pyroxenitexenoliths are relatively rich in MgO and poor in FeO. Clinopyroxene in pyroxenitexenoliths has very low REE content and a REE-depleted pattern. Isotopiccompositions of pyroxenite xenoliths are between those of peridotite and granulitexenoliths. The above features as a whole suggest that pyroxenite xenoliths are derivedfrom enriched mantle. In granulite xenoliths the two kinds of pyroxenes （salite andbronzite） are relatively rich in FeO and poor in MgO. Clinopyroxene in granulitexenoliths is relatively rich in REE and characterized by high⁸⁷Sr/⁸⁶Sr ratios and low¹⁴³Nd/¹⁴⁴Nd ratios. The above results demonstrate that granulite xenoliths wereresulted from magma underplating, which was subjected to contamination of lowercrust.