沙捞越古晋地区伦杜基性岩的形成时代、地球化学特征及其地质意义

Geochronoloy and Geochemical Characteristics of Lundu Mafic Rocks in Kuching Area,Sarawak

加里曼丹岛作为南海南部最大的岛屿,处于印度-澳大利亚板块、欧亚板块和菲律宾海板块的汇聚带,具有复杂的构造演化史.西加里曼丹岛古晋地区晚白垩世岩浆作用强烈,虽然该期基性岩分布少,但其成因研究对探讨西加里曼丹晚白垩纪构造演化过程具有重要意义.对古晋地区伦杜基性岩开展了详细的岩石学、年代学和地球化学研究,结果表明,该地区基性岩的岩石组成主要为辉绿岩和含橄榄石辉长岩,其中辉绿岩锆石定年显示其结晶年龄为83.4±0.9 Ma,说明岩体侵位于晚白垩世.岩石具有较为均一的SiO_(2)(52.01%~52.38%),大部分样品具有较低的TiO_(2)含量(0.81%~0.92%),K_(2)O含量(0.37%~0.53%)和较高的Al_(2)O(314.00%~14.54%)、MgO(7.40%~7.86%).微量元素分析结果显示具有较低的稀土元素总量(∑REE=43.96×10^(-6)~48.19×10^(-6)),呈LREE轻度富集的平坦型配分模式,富集大离子亲石元素,亏损Nb、Ta和Ti等高场强元素.(^(87)Sr/^(86)Sr)i=0.7051~0.7053,ε_(Nd)(t)=2.1~3.3.综合分析表明,伦杜基性岩来源于受到俯冲沉积物和板片流体交代改造影响的地幔源区,并结合前人数据推测伦杜基性侵入岩形成于古太平洋俯冲格局下的弧后盆地构造背景,可能与中国东南沿海海南-越南构成一条俯冲带.

Kalimantan Island is the largest island in the south of South China Sea that has undergone complex tectonic evolution,resulting from convergence of the Indian-Australian,Pacific and Philippine Sea plates.The Kuching area of Kalimantan Island has a series of Late Cretaceous magmatic rocks,but the distribution of basic rocks is less.The petrogenesis of basic rocks is significant to reveal the tectonic framework and evolution history in that period.In this study,it presents new petrographic,geochronological and geochemical data for the Lundu gabbroic pluton in the Kuching area.The basic rocks are predominantly made of dolerite and gabbro.Zircon U-Pb dating result shows that the crystallization age of the gabbro is 83.4±0.9 Ma,suggesting that the basic rocks intruded in the Late Cretaceous.These samples have low SiO_(2) ranging from 52.01%to 52.38%,K_(2)O from 0.37%to 0.53%and TiO_(2) ranging from 0.81%to 0.92%with high Al_(2)O_(3) of 14.00%-14.54%and MgO of 7.40%-7.86%.These samples are geochemically characterized by enrichment of light rare earth elements(LREE)and large-ion lithophile elements(LILE)and flat distribution of heavy rare earth elements(HREE)with light low REE abundance(∑REE=43.96×10^(-6)-48.19×10^(-6)).The representative samples show low initial ^(87)Sr/^(86)Sr ratios(0.7051 to 0.7053)and positiveε_(Nd)(t)values(2.1 to 3.3).Combination of trace elemental and isotopic results suggest that the parental magmas were likely derived from the mantle source modified by subduction-related fluid and sediments in the back-arc basin tectonic setting in the response of the subduction of the Paleopacific plate,and may link with Southeast China,Hainan island and Vietnam.