| As we all know,granite or granitoid are the main components of continental crust.Studying the formation and evolution of granitoid can explore the growth process of continental crust.Tibetan Plateau is the world's largest Cenozoic plateau,where numerous large granite batholith and granitic pluton are exposed.Early Cretaceous magmatism is frequent in the Lhasa terrane,but there are few reports in the central and eastern regions of the North Lhasa terrane.This paper takes the Namtso Cretaceous granitic pluton as the research object to find out the genetic types and source characteristics of the MMEs and host granitic plutons,to provide petrology and geochemical study instance for discussing the genetic mechanism of large-scale intermediate-acid rocks in the Lhasa terrane,and enrich basic research data in the central-eastern part of the northern Lhasa terrane.The Namtso Pluton is located on the northeast shore of the Namtso Lake,in the northern Lhasa terrane,near the junction of the central and northern terrane.Field observations have revealed that the Namtso Pluton is mainly composed of intermediate-acid granitic intrusions,mainly including granodiorite and monzogranite,in which gabbroic MMEs are developed.Most of MMEs are elliptical,oval,different in size,and have a clear boundary with the host granitic rocks.Zircon U-Pb geochronology shows that granodiorite and monzogranite are the products of Early Cretaceous magmatism formed at?113 Ma.Granitic rocks have high Si O2 content(65.43-70.22%),high K2O and total alkali contents(K2O 2.96-4.52%,Na2O+K2O 5.78-8.23%),low Fe2O3T(3.43-5.52%)and Mg O(0.83-1.97%)contents,Mg#value(33-53).The characteristics of trace elements are that they are enriched in light rare earths and depleted in heavy rare earths,and the light and heavy rare earth elements are clearly fractionated,the(La/Yb)N ratio is 8.2-12.7.Granitic rocks have a significant negative Eu anomaly,the?Eu value is 0.41-0.77,and they are enriched in large ion lithophilic elements(such as Rb,Th,U,etc.)and depleted high-field-strength elements(Nb,Ta,etc.).The granitic rocks have low Zr+Nb+Ce+Y content and(Na2O+K2O)/Ca O ratio,A/CNK<1.1,are high-potassium calc-alkaline series rock and I type granites.The granitic rocks have an enriched and uniform Sr-Nd isotopic composition.The 87Sr/86Sr(i)of the granodiorite is 0.70970-0.71436,?Nd(t)=-9.78?-8.55,the second-stage Nd model age T2DM=1.66-1.69Ga,?Hf(t)=-11.99?-0.33,monzogranite 87Sr/86Sr(i)=0.71081-0.71974,?Nd(t)=-9.52?-9.09,T2DM=1.66-1.69Ga,?Hf(t)=-9.01?-0.20,indicating that the source area of the granitic rocks is mainly the partial melting of ancient crustal materials,accompanied by the process of crust-mantle interaction.In addition,during the formation of intermediate-acid granitic rocks,obvious fractional crystallization of plagioclase and potash feldspar has occurred.The MMEs have significantly low Si O2content(45.86-60.48%)and low K2O content(1.38-2.98%),high Ca O content(5.47-7.68%),and Al2O3 is 13.93-16.62%.They have obvious higher contents of the Ti O2,Fe2O3T and Mg O than the granitic rocks(Ti O2=0.54-0.91%,Fe2O3T=8.67-19.91%,Mg O=3.07-5.78%),the Mg#value is 40.35-55.55.In terms of trace elements,the MMEs have higher total rare earth content and lower light and heavy rare earth elements fractionation characteristics.The total rare earth content?REE=200.4×10-6?245.3×10-6,(La/Yb)N ratio is 2.4-6.6.MMEs show negative Eu anomaly,?Eu 0.35-0.50,rich in large ion lithophile elements and depleted high field strength elements,with obvious Nb,Ta,Sr and Ba negative abnormality.The petrography shows that the MMEs have obvious characteristics of magma mixing,and their Sr-Nd isotopic composition is similar to that of the host granitic rocks.The MMEs87Sr/86Sr(i)=0.70972-0.70999,?Nd(t)=-9.08?-8.90,T1DM=1.81-3.16 Ga.Combined with the characteristics of trace elements,the source area of the MMEs is mainly enriched lithospheric mantle,and the formation process of MMEs has experienced mixing of crust-mantle magma,and the formation of quenching occurred.Sandstone enclaves were formed from the late Jurassic to early Cretaceous,with Si O2 content of 58.72-62.86%,K2O content of 2.56-3.29%,Na2O content of 2.93-4.15%,Al2O3 content of13.24-14.41%,and Ca O content of 3.35-5.15%.Sandstone 87Sr/86Sr(i)=0.70928-0.714564,?Nd(t)=-6.85?-5.94,T2DM=1.40-1.48Ga,?Hf(t)=-9.18?-1.60,concentrated in?Hf(t)=-3.This sandstone enclaves shows southward provenance characteristics.The formation of the Namtso Pluton is controlled by the low-angle northward or flat subduction of the Neo-Tethys Ocean. |
PDF Full Text Request