云南澜沧老厂隐伏花岗斑岩体地球化学特征及构造环境

详细阐述了老厂隐伏花岗斑岩的地球化学特征,利用图解判别了其形成的构造环境,并初步探讨其成因机制。老厂花岗斑岩SiO2含量(68.09%~73.48%)与中国花岗岩平均值基本相当,富碱(Na2O+K2O为7.73%~8.51%),富钾(K2O/Na2O为1.85~25.8),属酸性偏铝质高钾钙碱性系列岩浆岩。轻稀土元素中等富集,重稀土元素相对亏损,(La/Yb)N=12.01~24.85,δEu=0.79~0.99(为弱负异常),稀土元素分布曲线为平滑的右倾曲线。通过花岗斑岩地球化学特征、图解判别、锆石标型特征等的综合研究,认为老厂隐伏花岗斑岩体是以壳源为主的壳幔源混合成因花岗斑岩,形成于青藏高原新生代碰撞造山主碰撞阶段区域挤压机制下的主碰撞构造环境。碰撞激发深部地幔局部熔融,使熔融体沿穿透性构造发育的早期裂谷带轴部上涌,成为构造-岩浆-热事件的主要驱动力;地幔物质涌入,向下地壳注入新生物质,诱发下地壳物质熔融,形成壳幔混合源富钾含矿岩浆。

Geochemistry and tectonic setting of the Laochang concealed granite porphyry in Lancang, Yunnan Province

Located in the Sangjiang metallogenic belt,Laochang is one of the important noble metal and nonferrous metal ore-forming belts of southwestern China,whose geotectonic location is very unique with rich ore resources and lots of metal deposits.Laochang is one of the areas with significant breakthrough for new discovery of great thick porphyry molybdenum ore bodies in the depth since 2008.Geochemical characteristics of the concealed granite porphyry in the Laochang ore deposit were studied in this paper.Tectonic setting and genetic mechanism of the granite porphyry were investigated by using various discrimination diagrams.Petrologic research indicates that granite porphyry of this area is light gray and grayish white in color and has porphyritic texture and massive structure.Phenocrysts are mainly composed of orthoclase,quartz,plagioclase and biotite,whereas the matrixes are composed of equiaxial fine-grained orthoclase and quartz.It is concluded that chemical compositions of the concealed granite porphyry in Laochang are characterized by the similarity to China’s granite porphyry in average SiO2(68.09%~73.48%),alkali enrichment(Na2O+K2O=7.73%~8.51%),K enrichment(K2O/Na2O=1.85~25.8),LREE enrichment,HREE depletion,(La/Yb)N varying from 12.01 to 24.85,weak negative Eu anomaly(δEu varying from 0.79 to 0.99),and smoothly right-declined REE distribution pattern.Thus,the granite porphyry belongs to acid,aluminum medium-saturated,high-K calc-alkalic series of magmatic rock.A comprehensive study of geochemical characteristics by using discrimination diagrams and zircon features of concealed granite porphyry in Laochang indicates that the granite body was mainly derived from crust source mixed with minor mantle materials and formed at the main collision stage of the Cenozoic collisional oregeny of the Tibetan Plateau under the regional compressional environment.Zircon SHRIMP U-Pb dating of the Laochang granite porphyry yielded 44.6±1.1 Ma.The chronologic age shows that the granite porphyry was formed in Eocene.The petrogenesis and mineralization of the Laochang granite porphyry took place during the main collisional stage of the Cenozoic intracontinental collision orogeny,and this is consistent well with metallogenic peak period of porphyry type deposits in the Sangjiang region.Based on the results from Sr-Yb classification of granite-porphyry diagram,R1-R2 discriminating tectonic setting of granite-porphyry diagram and regional tectonic development,it is suggested that the Laochang granite porphyry was formed in the collisional setting.The magma,which was derived from partial melting of deep mantle initiated by collision,experienced upwelling through the core part of early developed rift,and became a major driving force for structural,magmatic and thermal events.The input of mantle material into the crust induced partial melting of the lower crust to form a potassium-rich ore-bearing magma with mixed sources of the crust and the mantle.