Tectonic Implications of the Combined Use of Tectonomagmatic Geochemical Discrimination Diagrams and Indicators of Magma Flow Sense in Mafic Dykes

Geochemistry is a powerful tool to help characterize the tectonic setting of igneous rocks associations.However,when continental mafic dykes and flood basalts are the target most of the proposed geochemical discrimination diagrams fail to correctly classify them,i.e.many mafic

Identification of the Original Tectonic Setting for Oceanic Andesite Using Discrimination Diagrams:An Approach Based on Global Geochemical Data Synthesis

Systematical analyses of data from GEOROC and PetDB database show that large amount of Cenozoic andesites occurred in the various oceanic environments such as mid-oceanic ridge,plumerelated island and oceanic arc.In this study,we employed the geochemical data of 351 mid-ocean ridge andesites(MORA),2539 plume-related andesites(PRA)and 3488 oceanic arc andesites(OAA)from the database to discuss the relationship between andesite tectonic settings and their geochemical features,thereby making an attempt to construct tectonic discrimination diagrams.Based on the data-driven pattern,all available elements were employed to derive logratios for the possible coordinates,and the overlap-rate calculation was adopted to evaluate the discrimination effect of more than 330000 prospective diagrams.Finally,four tectonic discrimination diagrams have been successfully established to identify MORA,PRA and OAA,which can be utilized to identify the original settings of andesite with an age range from Cenozoic to Archean a certain extent.Of these diagrams,PRA is mainly distinguished by high LREE/HREE ratio due to enriched mantle source.Whereas,OAA is mainly characterized by high LILE/HFSE ratio,which reveals that fluids derived from subducted slab play an important role in forming oceanic arc andesites.Consequently,the petrogenesis of andesites is closely related to their tectonic settings.However,it should be noted that those andesites formed in both continental and oceanic environments cannot be effectively distinguished using these diagrams.We strongly recommend integrating the discrimination diagrams result with other geological information to reach a comprehensive interpretation of evolution history with those ancient andesites.This paper presents a case study which suggests that data-driven method is a powerful tool for solving geological problems in this’big data’era.

New discrimination diagrams for basalts based on big data research

In recent days,discrimination diagrams have been widely used for tracing the tectonic settings and origins of basalts from orogenic belts.However,conventional discrimination diagrams are not accurate enough.Here,we reported six new discrimination diagrams obtained from the global database using data mining methods.For most individual diagrams,island arc basalt can be nearly 100%was identified,whereas ocean island basalt and midocean ridge basalt can be discriminated from each other with less than 10%of overlap,under a confidence coefficient of 85%.Using the six new discrimination diagrams together,basalts of different origins can be efficiently identified.

Applicability of large-ion lithophile and high field strength element basalt discrimination diagrams

Basalt discriminant diagrams have been used to identify the tectonic setting of basaltic magmatism since the 1970s and have played an important role in reconstructing paleotectonic environments.However,the significant increase in the availability of geochemical data has led to a reassessment of these diagrams,suggesting that some of the tectonic settings indicated by these diagrams are not accurate.Here,we use a database of global ocean island basalt(OIB),mid-ocean ridge basalt(MORB),and island arc basalt(IAB)geochemistry to propose a series of new tectonic discriminant diagrams based on the ratios of large-ion lithophile elements(LILEs)to high field strength elements(HFSEs).These new diagrams indicate that the LILE can be used to differentiate OIB,MORB,and IAB samples,meaning that LILE/HFSE ratios can discriminate between these basalts that form in different tectonic settings.Our new diagrams can correctly assign samples to OIB,MORB,and IAB categories more than 85%of the time,with the discrimination between OIB and MORB having an accuracy of slightly less than 85%.

Practical applications and limitations of basalt discrimination diagrams

Determining the tectonic setting of unknown volcanic rocks continues to be one of the key challenges in geoscience.While discrimination diagrams have been successfully employed due to their ease of use,recently,validation with big data has raised questions about their performance.In this study,the discrimination boundaries of Th/Yb versus(vs.)Nb/Yb and TiO2/Yb vs.Nb/Yb diagrams,which are the most used types of discrimination diagrams,were redefined based on a large amount of compiled data and support vector machine,a machine learning method.The effectiveness of discrimination diagrams was verified,and the limitations and conditions when using them were clarified.The results show that when using the Th/Yb vs.Nb/Yb diagram,only basalts with Th/Yb ratios higher than the discrimination boundary can be identified as volcanic arcs in origin.In contrast,a significant overlap occurs across boundaries in other cases when using these diagrams,particularly for enriched samples with Nb/Yb ratios higher than five.Therefore,when using these diagrams to determine the tectonic setting of unknown samples,their limitations must be considered when interpreting their results.

The petrogenesis of modern and ophiolitic lavas reconsidered:Ti-V and Nb-Th

Tectonic discrimination diagrams are a key tool for understanding ancient volcanic rock origins.In this contribution we compile over 15,000 whole rock compositions to re-evaluate the Ti-V discrimination diagram and compare it to another commonly used tool,the Nb/Yb-Th/Yb diagram.We have reformulated the Ti-V diagram into a log-log plot to distinguish samples more clearly at lower concentrations.The compilation shows that MORB are dominated by Ti/V=20-43,whereas juvenile arc tholeiites and boninites are characterized by Ti/V<20 generally,although there is minor overlap at the boundary(Ti/V=20-22).Plume-related volcanic rocks(ocean island basalts,oceanic plateaux,and continental flood basalts)generally have Ti/V>43,although there may be significant overlap with MORB-like ratios for ridgecentered OIB and for some oceanic plateaux.About 56%of alkaline OIB have Ti/V>70.Back-arc basins are dominantly MORB-like.Melt models show that MORB and juvenile arc volcanics most likely formed under different fO;conditions,but are permissive of similar fO_(2)if the arc rocks form by much higher melt fractions.The Nb/Yb vs.Th/Yb plot clearly distinguishes most oceanic basalts(MORB,plateaux,OIB)from subduction-related volcanic rocks(boninite,juvenile arc tholeiite,calc-alkaline)and from flood basalts.We propose here a new two-proxy diagram of Ti/V vs.Th/Nb,which incorporates the advantages of both.

A new method of discriminating different types of post-Archean ophiolitic basalts and their tectonic significance using Th-Nb and Ce-Dy-Yb systematics

In this paper, a new discrimination diagram using absolute measures of Th and Nb is applied to postArchean ophiolites to best discriminate a large number of different ophiolitic basalts. This diagram was obtained using 〉2000 known ophiolitic basalts and was tested using -560 modern rocks from known tectonic settings. Ten different basaltic varieties from worldwide ophiolitic complexes have been examined. They include two basaltic types that have never been considered before, which are： （1） medium-Ti basalts （MTB） generated at nascent forearc settings; （2） a type of mid-ocean ridge basalts showing garnet signature （G-MORB） that characterizes Alpine-type （i,e., non volcanic） rifted margins and ocean-continent transition zones （OCTZ）. In the Th-Nb diagram, basalts generated in oceanic subductionunrelated settings, rifted margins, and OCTZ can be distinguished from subduction-related basalts with a misclassification rate 〈 1%. This diagram highlights the chemical variation of oceanic, rifted margin, and OCTZ basalts from depleted compositions to progressively more enriched compositions reflecting, in turn, the variance of source composition and degree of melting within the MORB-OIB array. It also highlights the chemical contributions of enriched （OIB-type） components to mantle sources. Enrichment of Th relative to Nb is particularly effective for highlighting crustal input via subduction or crustal contamination. Basalts formed at continental margin arcs and island arc with a complex polygenetic crust can be distinguished from those generated in intra-oceanic arcs in supra-subducrion zones （SSZ） with a misclassification rate 〈1%. Within the SSZ group, two sub-settings can be recognized with a misclassification rate 〈0.5%. They are： （1） SSZ influenced by chemical contribution from subduction- derived components （forearc and intra-arc sub-settings） characterized by island arc tholeiitic （IAT） and boninitic basalts; （2） SSZ with no contribution from subduction-derived components （nascent forearc sub-settings） characterized by MTBs and depleted-MORBs. Two additional discrimination diagrams are proposed： （1） a Dy-Yb diagram is used for discriminating boninite and IAT basalts; （2） a Ce/Yb-Dy/Yb diagram is used for discriminating G-MORBs and normal MORBs. The proposed method may effectively assist in recovering the tectonic affinity of ancient ophiolites, which is fundamental for establishing the geodvnamic evolution of ancient oceanic and continental domains, as well as orogenic belts.

Geochemistry of lower Silurian shale of Longmaxi Formation,southeastern Sichuan Basin,China:Implications for provenance and source weathering

Lower Silurian Longmaxi Shale(SLS) in southeastern Sichuan Basin, China, was analyzed for major and selected trace elements, and their provenance, intensity of palaeoweathering of the source rocks were analyzed based on these elements. The results show that SiO_2, Al_2O_3 and Fe_2O_3, are dominant major elements with average contents of 60.59%, 15.91% and 5.87% in Upper Silurian Longmaxi Shale(USLS), and 65.14%, 13.24% and 4.68% in Lower Silurian Longmaxi Shale(LSLS). The TiO_2-Zr plot, Hf(ppm) versus La/Th discriminant diagram, and abundance of Cr and Ni suggest a dominantly felsic source for the Longmaxi sediments. Average chemical index of alteration(CIA), plagioclase index of alteration(PIA) values(64.05% and 72.86%, respectively) imply low-degree chemical weathering of the source material in early Longmaxi time, and average CIA, PIA values(68.44% and 80.35%, respectively) imply moderate chemical weathering of the source material in late Longmaxi time.