The Lithogeochemical Characteristics And Causes Of Granodiorite In Lincang,Yunnan Province

Lingchang granite batholith is strictly controlled by Lancangjiang fault,which located in the central part of Sanjiang Region of Yunnan.This thesis studies at granodiorite in Lincang region,lying at central zone of this batholith,which shows gradational relationships with monzonitic granite country rock.On the basis of a comprehensive collecting and studying the previous work,this article adopts means field geological survey,microscopic identification for rocks,isotope analysis,characteristics of whole rock geochemistry,etc.It researches on the petrography characteristics,geochemical characteristics,spatial and temporal distribution pattern of granodiorite in Lincang,Yunnan province.It will discuss granodiorite's property,source oregion,petrogenesis and tectonic dynamic setting,in order to provide the scientific evidences for understanding paleotethys tectonic evolution process and revealing the magmatism in the Tethys tectonic evolution in Sanjiang area,western Yunnan.The silica content of the granodiorite is between 53.09% and 74.17%,and the average is 66.37%.The aluminum saturation index A/CNK ranges from 0.984 to 1.782.The content of potassium dioxide and sodium oxide ranges from 4.21% to 6.68%,mostly closing to 6%.Litman index ranges from 0.61% to 1.95%.The granodiorite belongs to intermediate-acid rock,silica and aluminum oversaturation types,Calc-alkaline series to high kalium calc alkalic series.The pluton is rich in Rb,Ba,Th,U,K,etc.LILE elements,lossing Nb,Ta,Zr,Hf,Ti,etc.high field strength elements and being higher Pb positive anomaly.The ratio of light to heavy rare earth and La/Yb ratio are from 3.03 to 13.48 and 2.01 to 21.42,respectively.The granodiorite belongs to LREE-enriched pattern and strong Eu negative anomaly.Its country rock monzonitic granite also belongs to Calc-alkaline series to high kalium calc alkalic series,aluminum oversaturation types.It has similar trace and rare earth elements distribution characteristics,similar separation crystallization process to granodiorite.They all have evolution characteristics of typical Calc-alkaline rock series.The granodiorite plution's U-Pb isotopic age of zircon is complex,whose accretion edge gives age of 247 Ma.Combining dating results of 1:250000 Lincang range(~243Ma),1:50000 Mangnuo range(234±1.8Ma)and mica-rich dioritic inclusion captured by monzonitic granite(237.7±0.72Ma),the crystalline age of the granodiorite intrusion probably goes for middle Triassic,rather than permian.The granodiorite and its country rock monzonitic granite all belong to S-type granite and form in post collision environment.During the collision process,the pressure increases rapidly in the thickened continental crust.After a short period of plate collision,it goes into stretch environment with hysteretic decompression and warming effect.In addition,subducted slab slip-off and delamination cause underplating of the basic magma.The heat energy from the underplating magma leads to the deep melting action of the crust surrounding rock.These process make thickened upper crust psammitolite material partial melting and form acidic magma.Basic magma formed from metasomatic mantle wedge material partial melting mixes with shell source acidic magma during rising.This prosess can form magma chamber with crust mantle mixing characteristics.After the process of fractional crystallization evolution from high temperature to low,the mixed magma fanally take their place forming ganodiorite and monzonitic granite through consolidation.Based on the data in the area of Paleotethys evolution,the Paleotethys evolution model in the southern belt of Lancang River(Changning Menglian oceanic basin)can be summarized as follows.Oceanic crust has formed in the Lower Devonian to Early Carboniferous,and formed the wide Changning Menglian oceanic basin.Subduction of oceanic crust has occurred in the Late Carboniferous to Early Permian,at the same time,along with the subduction sediments has been added to the overlying lithospheric mantle.Continental-continental or continental-acr collision have accured from the Late Permian,and it has comed into post collision stage in the Middle and Late Triassic.The Triassic magmatism in this area is the product of asthenosphere and enriched lithosphere interplay afer the collision and subducted slab slip-off.