The Study On Qinlingliang Intrusion And Its Mafic Microgranular Enclaves In Baoji Area,Northern Qinlin

The study area is located in the north of the Shangdan suture belt,Qinling orogenie belt.Triassic granite is widespread in this area,which directly records the important information of the evolution process of the Qinling orogenie belt.The Late Triassic Qinlingliang intrusion is the second stage intrusion of Baoji pluton,and has amounts of mafic microgranular enclaves,which is the most important symbol of crust-mantle mixing,while the predecessors are lack of deep study about the crust-mantle magma mixing mechanism and the cause of mafic microgranular enclaves.Based on detailed field geological survey,petrography,petrology,major elements and trace elements,this paper has done detailed exploration and discussion about the petrogenesis and tectonic significances of Qinlingliang intrusion.According to the research,we get the main points as follows:1.The Qinlingliang intrusion is mainly made up host-rock porphyaceous monzonitic granite and mafic microgranular enclaves.The mafic microgranular enclaves are mainly quartz monzonite and monzodiorite.2.The Qinlingliang intrusion is quasi aluminum to weakly peraluminous cal-alkaline I and A type granites.The minerals of the Qinlingliang intrusion include hornblende and biotite,without rich aluminium mineral like muscovite and cordierite.Geochemistry researches show that the rocks are quasi aluminum to weakly peraluminous cal-alkaline series with enrichment of Si,Al,K,Mg,the corundum content in CIPW is less than 1%,P2O5 decreased with the increase of SiO2 content,Rb/Sr is in obviously less than 0.9.The primitive mantle-normalized trace element spider diagram shows right deviation and negative slope characteristics,and indicate the valley of Ba,Sr,Nb,P,Ti.It shows strong LREE enrichment and fractionation opposite to HREE.The REE patterns show rightward incline and moderate negative of Eu anomaly.3.The Qinlingliang intrusion has amount of mafic microgranular enclaves,the inclusions have varial shape.Within the inclusions,there develop lopsided texture of minerals,such as quartz phenocryst included by microgranular enclaves,acicular hollow apatite,and the rapakivi texture of piagioclase in potassium feldspar.The mafic microgranular enclaves and host-granite presents a gap and display a well linear trend correlation in Harker diagram and magmatic mixing trend in the TFe2O3-MgOdiagram.The above-mentioned phenomenon proved that the mafic microgranular enclaves in Qinlingliang intrusion were formed by magma mixing.4.Based on the research of higher Na clinopyroxene wrapped by amphibole that developed in mafic microgranular enclaves of Qinlingliang intrusion,speculated that the mafic microgranular enclaves formation process are: the mixture of the mantle and lower crust magma in the crust-mantle boundary caused rapid crystallization differentiation of high temperature basic magma,in this stage,some basic mineral assemblage like monoclinic pyroxene starts to crystallize,after the magma experienced crystallization differentiation,it invaded upward with residual partial basic and acidic magma,at the same time,the monoclinic pyroxene starte to crystal.After then,the rest of basic magma more rapid coolling and crystallization in one location,which resulted that part of the fine clinopyroxenee are encapsulated by amphibole and other minerals packed,thus formed the mafic microgranular enclaves of granite.And this phenomenon provides a new basis for the crust-mantle mixed source.5.According to the clinopyroxene geobarometer which applied to mafic microgranular enclaves in granite originated from crust-mantle mixed source,and use the contnets of Na2 O and the mole fraction jadeite relatedto Na in Clinopyroxene from the MMEs to calculate the formation pressure and depth of the Clinopyroxene,the formation depth of the granitic magma originated from crust-mantle mixed source is 35km-48 km,this thickness represents the minimum thickness of crust in the study area of the late Triassic period.From this,the Study area has experienced collision orogenic activity during late Triassic.6.From Comprehensive analysis of the regional geological evolution,sedimentology,petrology,the tectonic environment diagram and the petrotectonic assemblage,we can get the Late Triassic Qinlingliang intrusion should be formed in the post-collisional tectonic setting,indicates that the Mian-Lüe ocean has been closed before the Late Triassic,and Yangtze and North China plates should already under the post-collision stage.