Research On Petrological Microtexture,Physicochemical Conditions And Deep Magmatism Of Mafic Intrusions In Emeishan Large Igneous Province

This paper carry out research work on the mafic roks formed in Panxi area of the Emeishan Large Igneous Province,mainly based on systematic macroscopic petrology,micropetrography,cathodoluminescence,quantitative texture analysis,mineral geochemistry,major and trace elements,situ zircon U-Pb-O isotope geochemistry and other research methods.Also this paper takes comprehensive study on the Panzhihua and Panlian intrusions combined with the crystallization thermodynamics theory of modern experimental petrology,mainly including mineral-melt equilibrium thermobarometers and barometers,hygrometers,magmatic condensation speedometers,diffusion law of crystal element,distribution law of elements in crystal,and other experimental petrological theoretical models.This paper considers it as a comprehensive magmatic process and gives quantitative calculation of physical chemistry parameters and the following progress has been made:(1)The microstructure characteristics of the Panzhihua intrusions indicate that the plagioclase can be divided into three types,including original euhedral plagioclase,partially dissolved plagioclase,and isolated island shaped residual plagioclase:?The original euhedral plagioclase has a perfect crystal shape,the twin crystal plane bond surface is straight and tidy,and the twin crystal slit is very fine.The crystal structure shows a stable and slow crystallization environment,belonging to the magma chamber crystal subgroups of the melt crystal group type;?Most of outer edges of the partially dissolved plagioclase show flame-like shape dissolution,while the plagioclase and ferrotitanium oxide react in the dissolution zone.The partially dissolved plagioclase belongs to volcanic conduit crystal subgroup of the melt crystal group type;?The isolated island shaped residual plagioclase has a sub-circular shape,the crystal form is poorly developed,and the degree of dissolution is very strong.It belongs to the volcanic conduit crystal subgroup which is strongly dissolved under the melt crystal group type.The microstructure shows that the plagioclase crystallization process is very slowly,the gravity has an effect on the seating of the plagioclase in the crystal mush.(2)The crystal size distribution curve of plagioclase and pyroxene in the Panzhihua intrusions shows a slope fluctuation,and then the curve become concave,which showed a sudden decrease of large particles,suggesting that the pyroxene experienced accumulation and deviated from the magma system.Generally,for all particles in the Panzhihua instrusions,when the particle size is less than 2 mm,.Pyroxene has higher population density than plagioclase,and when the grain size larger than 2 mm,plagioclase has higher population density than pyroxene,2 mm particle size is the demarcation point,suggesting that when the crystals grows to 2 mm in this magma system,a large number of pyroxene cumulated.The fractal dimension of pyroxene is between 0.93-1.13,which indicates that there is a strong force between the crystals during the crystallization of pyroxene,which results in the growth rate of pyroxene is less than plagioclase.The particle size of plagioclase is between 0.54-0.96mm.At the end of final nucleation and growth,the crystal volume content is 21.86-42.04%,which may be derived from deeper lithosphere depth and crystallizate in a approximately stable magma barely disturbed by external geological factors.The residence time scale of plagioclase constrained by characteristic particle size is 171.223-40.41 years,of which the length of time of 304.41 may represent the crystallization time scale of the more uniform central part of the melt,which is close to the result by Witter et al.(2016),with the calculated residence time of 63-171 years for plagioclase in the Telica volcano in the Nicaraguan region of Latin America,This residence time scale indicates that the crystals in magma chamber have a residence time of tens to hundreds of years during magmatism.(3)The plagioclase-melt equilibrium thermometer and the pyroxene-melt equilibrium thermometer indicate that the magma temperature range is between 1025-1245?,and the average pressure of the Panzhihua intrusions from the bottom to the top is calculated by the monoclinic pyroxene,for a 6.61 kbar.This pressure of Panzhihua intrusions is converted to depth yielded a depth of 24.83 km.The calculation results of the plagioclase hygrometer show that the average water content of the Panzhihua intrusions magma is 2.12%,which is derived from a wet magma system.The calculated values of the parent magma v iscosity of the Panzhihua intrusion ranged from 0.914 to 6.509 Pa-s.with an average of 2.408 Pa s.and the magma density range was between 2.571-3.15 g/cm3 with an average of 2.9g/cm3.There were two sharply downtrends in magma viscosity and density of the Panzhihua intrusions section.indicating that two large disturbances occurred in the magma process.which reduced the magma viscosity.The combination of crystal size distribution results was inferred to be due to two large replenishments of deeper magma.The genetic mechanism is most likely due to the addition of volatile liquids at the top of the magma to the magma system,thereby greatly reducing the viscosity and density of the magma system.Based on the data collected from literature about Panzhihua intrusions,this paper analyzes the carefully selected data and calculates the FC3MS numerical results ranging from-1.519 to 3.444 with an average of 0.926.which is higher than the the cutoff value of 0.65.which is used to distinguish pyroxene and peridotite sources,and some values are even greater than 1.5.Anyway the FC3MS value suggests that the Panzhihua intrusions may be derived from the pyroxene source area.(4)The Panlian and the Klang intrusions was considered as the southern extension branch of the Baima intrusions in Panxi,which is a direct product of large-scale magmatic activity in the Emeishan large igneous province of the Permian,and has similar geochemical composition content to the high-titanium basalt.such as typical continental OIB enrichment characteristics.The simulation results show that the parent magma density of Panlian gabbro ranges from 2.4029 to 2.5986 g/cm3 with an average of 2.5566 g/cm3.The parent magma density of Pandan basalt ranges from 2.4415 to 2.5259 g/cm3 with an average of 2.4953 g/cm3.The viscosity log ? of the Panlian gabbros is between 2.9960 and 6.1951(logPa*s).and the log ? average is 3.5511(logPa*s).The viscosity log ? value of the Pandan basalt is between 3.8483-4.6671(logPa*s),and the log ? average is 4.2013(logPa*s).The simulation results of the monoclinic pyroxene-melt thermometer and barometer show that the parent magma temperature of the Panlian gabbro is-1071.6?,and the parent magma pressure is-1.1kbar.The calculated pressure value is lower than the estimated 5kbar of the Panzhihua layered intrusions by Pang et al.(2008b),which is consistent with the pressure of the Panzhihua intrusions estimated by Song et al.(2013).The settlement results of the monoclinic pyroxene hygrometer show that the water content of the parent magma of the Panlian intrusions is between 2.7529 and 3.2167%,with an average of 3.0262%,indicating that the Panlian formation is formed in a water-rich parent Magma,which coincides with the large amount of volatile fluids observed under the polarizing microscope.Panlian gabbros is considered as one of direct material record of the strong magmatism of the Emeishan large igneous province in the Miyi area,yield a SHRIMP U-Pb weighted average age by zircon,is 258.7±1.1Ma(n=17,1?,MSWD=0.71).This age is consistent with the formation age of several major layered intrusions and Emeishan basalts in Panxi area,also is consistent with the U-Pb age of 261 ±2 Ma measured by the zircon SHRIMP of the Baima layered intrusions(Shellnutt et al.,2009).Generally this age represents the main peak period of large-scale intraplate magmatism in the Permian of Panxi area.(5)For the first time,a special metal ring structure was discovered and named as Luo's fluid metal corona.This metal corona is formed by volatilizing bubbles and metal migration.The rising bubbles is loaded by the metal materials in the form of a 45 degree liquid surface angle.It is a new prospecting mark.The FC3MS value of the Panlian formation is between 1.15 and 2.11,with an average of 1.25.The FC3MS value of this combined parameter is much larger than 0.65.The zircon-melt oxygen isotope diffusion equilibrium simulation results of the Panlian gabbros show that the oxygen isotope value of the parent magma of the Panlian intrusions is between 1.72‰-10.68‰,with an average value of 4.77‰,combined with the high water content of the melt.And the microstructure of the mineral,this paper finally recognizes that the Panlian intrutions is derived from a hydrous carbonate metamorphic mantle source with pyroxenite and peridotite mixed.