甘孜—理塘蛇绿混杂岩带中三叠世洋岛型岩石组合的识别及其构造意义——来自岩石学、地球化学和年代学证据

Identification of the Middle Triassic Ocean Island Rock Association in the Garze–Litang Ophiolite Mélange Zone and Its Tectonic Significance:Constraints from Petrology,Geochemistry and Geochronology

甘孜—理塘蛇绿混杂岩带为西南“三江”多岛弧系的重要组成部分,为区域上义敦岛弧带与扬子板块间的重要界线。研究区洋岛型岩石组合主要为晶屑熔结凝灰岩、泥晶灰岩,局部夹少量的硅质岩、玄武岩,各端元之间均呈整合接触。对基性火山岩进行详细的岩石地球化学研究表明,SiO2含量为47.16%~54.30%,TiO2含量为2.76%~5.66%,MgO含量为5.29%~6.65%,K2O含量介于0.82%~3.66%之间,Na2O含量在2.52%~3.66%之间变化,属于碱性玄武岩系列。稀土总量ΣREE=195.21×10^–6~341.81×10^–6,(La/Yb)N=11.75~13.62,无明显铕异常,相对富集大离子亲石元素Rb、Ba、Th、K和高场强元素Nb、Ta、Zr、Hf,稀土和微量元素配分模式类似于OIB。基性火山岩微量元素判别表明其形成于洋岛构造环境,岩石成因研究表明其起源于地幔源区而没有遭受明显的陆壳物质混染。晶屑熔结凝灰岩的锆石U-Pb定年结果为(245.1±1.5)Ma,这表明该洋岛型基性火山岩形成时代为中三叠世早期。研究区洋岛型岩石组合的发现丰富了甘孜—理塘蛇绿混杂岩带的研究内容,为区域上甘孜—理塘洋盆中三叠世时期构造演化提供了直接的物质证据。

The Garze–Litang ophiolite mélange zone is an important part of the‘Three-River’archipelagic arc basin system in Southwest China,and is also an important boundary between the regional Yidun island arc belt and Yangtze plate.The oceanic island rock assemblage in the study area is mainly composed of crystal ignimbrite and micrite,with a small amount of siliceous rocks and basalt in the local part,and the terminal elements are in contact with each other.Detailed rock geochemical studies of the basic volcanic rocks show that SiO2 content is 47.16%~54.30%,TiO2 content is 2.76%~5.66%,MgO content is 5.29%~6.65%,K2O content is 0.82%~3.66%and Na2O content is 2.52%~3.66%,suggesting alkaline basalt series.Geochemical analysis reveals that the features of basalt in the REE patterns and spider diagrams are similar to features of typical OIB,with REE values being(195.21×10^–6~341.81×10^–6)and REE being[(La/Yb)N=11.75~13.62],no obvious europium anomaly,and significant enrichment of large-ion lithophile elements(Rb,Ba,Th and K)as well as high strength field elements(Nb,Ta,Zr and Hf).The identification of trace elements in basic volcanic rocks indicates that they were formed in the oceanic island tectonic environment,and studies of the genesis of the rocks show that they originated from the mantle source region and did not suffer obvious contamination of continental crust materials.The zircon U-Pb age of the crystal ignimbrite is(245.1±1.5)Ma,indicating that the formation of the oceanic island basic volcanic rock took place in the early middle Triassic.The discovery of the ocean island rock association in the study area has enriched the research content of the Garze–Litang ophiolite mélange belt and provided direct material evidence for the tectonic evolution of the Garze–Litang mid-Triassic period.