General Quantitative Texture Analysis In Igneous Petrological Systems And Its Implications

Based on the special quantitative textural analysis theory and techniques of igneousrocks, a general quantitative texture analysis method in igneous petrological systems isproposed and explored in this PhD dissertation. The fundamental starting points of thegeneral quantitative texture analysis method are complex magmatic systems andcomplex crystal populations of igneous rocks. This method takes rocks’ texture as thecore and ties in studies of igneous petrological systems, and connects field geology,petrology, mineralogy, phase equilibrium and geochemistry, and with special emphasison quantitative integrations between these and on connections between five basicvariables of time, space, temperature, pressure and compostion, and as far as seeks toclarify some basic issues. Through exploration of the general quantitative textureanalysis, the dissertation made some major achievements and insights as follows.1,The special quantitative texture analysis of igneous rocks is systematically, for thefirst time, introduced to our country. A quantitative texture analysis laboratory wasfounded through the financial support from “985” discipline platform. An effective andpractical method was developed for the special quantitative texture analysis.2,In order to obtain the size of representative samples and its constraints, therelationships of the representative samples with crystal contents and crystal size arepreliminarily discussed by statistical simulations of the holocrystalline rocks’ textureand the idealized granitic compositions. The maximum possible difference between the original and measured crystal contents of samples, and the size of representativesamples are determined under the variations of crystal contents and size. Thedifference between the original and measured crystal contents of samples can beobtained by counting the number of phenocrysts across the sections of samples. Thesize of representative samples can be acquired by using the average crystal size. Thedeviation between the measured and original crystal contents are much more higher forcoarse-grained samples with the same sample size. The largest representative samplesare needed when the crystal content reach20%for phenocrysts or coarse-grainedminerals in equigranular texture. Finally, two examples including the Mapeng andFangshan intrusions are used to examine the validity and practicability. The chemicalanalytical results of representative and non-representative samples of Fangshangranites suggest that non-representative samples are significantly affected by excessiveK-feldspar phenocrysts, resulting in K₂O and AI₂O₃contents increase and limitedvariation of SiO] content, suggesting that some K-rich adakitic rocks in Eastern Chinamay result from sample representativeness.3,744pieces of molybdenite Re-Os isotopic dating data published in China in recentyears were collected and the authors found that the Re content of all the samples ischaracterized by mixed distribution. Classification and statistical analysis of all thedata were based on the lithology and associated mineral types. The results show thatthe Re content of molybdenite has a close relationship with the lithology and associateminerals, i.e., pure molybdenite in the felsic veins and granite has the lowest Re content with the geometric mean of7.41x110^-6and7’.99x10^-6 respectively, and most ofth^e values range from10’ to110^-6; the Re content of pure molybdenite in the skam ismedium with the geometric mean of58.1x110^-6, and most of the values range from110^-6to10.4; molybdenite in the carbonatite has the highest Re content with the geometric4mean of231x110^-6, and most of the values are?10—. The Re content of molybdenite isalso affected by its associated mineral type: the Re content of molybdenite has thelowest value when molybdenite is only associated with scheelite （or wolframite） and/orgalena, sphalerite, native gold and native silver with the geometric mean of110^-6, andmost of the values range from110^-6 to110^-6; when it is only associated with chalcopyriteand/or magnetite （or pyrrhotine）, the molybdenite has the highest Re content with thegeometric mean of10.4, and most of the values range from110^-6 to110^-6; the medium Recontent occurs when molybdenite is associated with chalcopyrite （magnetite orpyrrhotine） and scheelite （or wolframite, galena, sphalerite, native gold and native^silver） with the geometric mean of10.?and most of the values ran"^ge from10" to10.A comprehensive analysis shows that molybdenite associated with scheelite （orwolframite, galena, sphalerite, native gold and native silver） or produced in the felsicveins and granite may reduce the Re content, whereas molybdenite associated withchalcopyrite and/or magnetite （or pyrrhotine） or produced in the skam and carbonatitemay increase the Re content. The magnitude changes of Re content may be related tothe modes of occurrence of molybdenite in combination with the contradiction ofisotopic tracing between the Re content of molybdenite and other isotopic methods published in recent years. It seems that the magnitude changes of Re content ofmolybdenite could not effectively represent the source of metallogenic material.4, It is well recognised that there are variable scales of pyroxenite present within theupper mantle peridotite, and the pyroxenite typically firstly partial melted to producebasaltic magmas because of its lower solidus temperature than that of mantle peridotite.However, source lithology identification of basalt remains uncertainty and is currentlyunder hot debate because of lacking valid criteria. Using a parameterization of meltingexperiments on peridotite and pyroxenite reported in the literature over the past threedecades, we show here a parameter call^ed FC3MS value （Fe0/Ca0-3*Mg0/SiO₂）which can effectively identify most pyroxenite-derived basalt although peridotite-andpyroxenite-derived melts cannot be distinguished in the commonly used simple plotsand phase diagrams. This parameter is pressure and temperature independent and takesinto accout compositional diversity and melting conditions of the two major lithologies.The FC3MS values of the peridotite-and pyroenite-derived melts are-0士.070.51（25,n=656） and0.46士0.96（25, n=494）, repectively. Coincidentally, the FC3MS value is0.68士0.34（25, n=525） for the continental oceanic island baslats-like volcanic rocks（C-OIB）（MgO>7.5wt%） in eastern China and Mongolia, which is too high to beproduced from peridotite source. Furthermore, the majority of these C-OIB in phasediagrams can be regarded as equilibrium with garnet and clinopyroxene. For these, it isproposed that garnet pyroxenite is the dominant source lithology for the C-OIB.Because there are significant differences for the petrogenesis of peridotite-and pyroxenite-derived basalts, many previous geological interpretations of the C-OIBbased on peridotite model need to be reconsidered. This study provides a new criterionto identify source lithology of basaltic magmas, and suggests that pyroxenite-derivedmelts may be a major contribution for continental mantle-derived magmas, and alsoprovides insight into intra-continetal magmatism.5, The Tianheyong alkaline basalt belongs to the north weatem part of Cenozoicbasalts in North China Craton. The “phenocrysts”(Length>300（im） in Tianheyongbasalts involve olivine, orthopyroxene, clinopyroxene and spinel. They aredisequilibrium with groundmass and remain core compositions as that of minerals inmantle peridotite. Olivine “phenocrysts” show F-type CSD. Groundmass olivine andmagnetite show S-type CSD. Chondrite-normalised REE and primitivemanlte-normalised trace element of the Tianheyong basalts show similar patterns astypical ocean island basalts, but more enriched in most incompatible elements andstronger fractionation between light and heavy rare earth elements. The heterogeneityof major and trace elements can be explained by0-15%contamination of the averagecompostion of Hannuoba peridotite. Sr-Nd-Pb isotopes show depleted manlte feature,similar as N-MORB. These features suggest that the “phenocrysts” in the Tianheyongbasalts are all manlte xenocrysts. The estimated primary Tianheyong basaltic magmashows extremely low magsuim number （Mg#<50） and MgO content, which areexperimentally equilibrium with garnet pyroxenite in the upper manlte condition. Iflithological heterogeneity exists in the upper mantle, the Tianheyong basalt was likely produced by partial melting of garnet pyroxenite. Therefore, the Tianheyong basaltmight represent mantle-derived low magnesium primary magma.6, By integrating the field geology, geochemistry, mineralogy, geochronology, andquantitative textural data of the Shanggusi granites, the following major conclusionscan be reached:（l）The Shanggusi granitic system consists of granite porphyry, granitedykes, and granitic pegmatite, which is one part of the Mesozoic porphyrymolybdenum deposits in east Qinling. The granitic pegmatite might be produced by theundercooled fluids-rich melts in front of the granite porphyry. The crystallization andmineralization ages of the Shanggusi granitic system are?126Ma.（2） Althoughaccumulation of feldspar and quartz occurs in the field, crystal size distribution ofquartz in thin section does not significantly affected by crystal accumulation. Mostgranite dykes samples and the groundmass of the granite porphyry samples showconcave-down CSDs, indicating textural coarsening.（3） Zircons in the Shanggusigranite mainly show cotectic relationships with hydrothermal minerals such as pyrite,fluorite, apatite, and albite, suggesting that they crystallized in late stage fluids. Mantlematerials’ contribution displayed by zircon Hf isotope and whole-rock Sr-Nd-Pb-H-Oisotopes might be the result of fluid-melt mixing in the Shanggusi granitic system,which may explain homogeneous major elements of the Shanggusi granite and raremafic microgranular enclaves in the field.（4） External hydrous fluids （possiblyenriched in CO2, F, and CI） in the intrusion not only controlled the fractionation oftrace elements and isotopic ratios but also affected its cooling history. Fluid migration-controlled undercooling can explain the solidification processes in theShanggusi intrusion, and may also be prevalent in other fluid-rich shallow intrusions.（5） Fluids-induced chemical fractionation can explain why chemical discriminationdiagrams of rock types and tectonic environment are invalid in the Shanggusi graniticsystem.（6） The mantle materials in granite may consist of manlte melt and mantlefluid, the latter night be the major component in the Shanggusi granite. The diversity offluids and melts in mantle-derived magmas may be one reason that there are variousforms of manifestation of mantle materails in granite.In summary, quantitative integration of textural and geochemical data for igneousrocks can contribute to our understanding of the relationships between physical andchemical processes in a magma system, and provide relatively comprehensive insightsinto the petrogenesis of igneous rocks. The general quantitative texture analysismethod is useful in understanding fundamental issues in igneous petrological systems.