南海北部陆缘古新世埃达克质岩的岩石成因

Petrogenesis of Paleocene Adakite-Like Rocks in Northern Margin of the South China Sea

由于前人对南海北部陆缘新生代早期形成的火山岩岩石类型、成因和地球动力学背景存在不同的认识,通过对珠江口盆地惠州凹陷钻井揭示的粗面质集块/角砾熔岩、粗面岩、流纹岩和粗面英安岩开展了相关锆石U-Pb年代学和全岩地球化学研究.结果表明研究区新生代火山岩形成年代介于57.53~59.15 Ma,靠近古新世(Paleocene)和始新世(Eocene)之间的界限,落入坦尼特阶(Thanetian).岩石具有较高的SiO2(64.60%~73.22%)、Al2O3(11.86%~16.57%)含量,低MgO(1.16%~1.80%),高Sr(305.20×10-6~465.50×10-6,平均值为371.15×10-6)含量,低Y(7.47×10-6~9.84×10-6)、Yb(0.90×10-6~1.43×10-6)含量及高Sr/Y比值(35.19~54.55),富集轻稀土(LREE)、亏损重稀土(HREE),富集大离子亲石元素(LILE)Ba、K等、亏损高场强元素(HFSE)Nb、Ta、Ti和P元素,具有轻微的Eu正异常,显示埃达克质岩石的地球化学特征,属于钙碱性-高钾钙碱性-钾玄岩系列,准铝质岩石(A/CNK=0.85~1.04).这些早古近纪埃达克质岩可能是由先期底侵的镁铁质下地壳在石榴石-角闪岩岩相条件下发生部分熔融作用的产物,认为南海北部陆缘新生代时期发生的拆沉作用在岩浆形成演化过程中可能起到了重要作用.

There are different understandings on the types,genesis and geodynamic mechanism of the volcanic rocks formed in the Early Cenozoic in the northern margin of the South China Sea.In this paper,it presents zircon U-Pb data and geochemical data for the trachytic agglomerate or breccia lava,trachyte,rhyolite and trachydacite revealed by drilling in Huizhou sag of the Pearl River Mouth basin.The results show that the Cenozoic volcanic rocks in the study area were formed at 57.53-59.15 Ma,near the boundary between Paleocene and Eocene,and fell into the Thanetian stage.These rocks are characterized by high SiO2(64.60%-73.22%),Al2 O3(11.86%-16.57%),low MgO(1.16%-1.80%),high Sr(305.20×10-6-465.50×10-6,371.15×10-6 on average),low Y(7.47×10-6-9.84×10-6),Yb(0.90×10-6-1.43×10-6)and high Sr/Y ratios(35.19-54.55).They are enriched in LREE and depleted in HREE,together with enrichment in large-ion lithophile elements(LILE)Ba,K and depletion in high field strength elements(HFSE)Nb,Ta,Ti and P,as well as small positive Eu anomalies,similar to those of adakite.These rocks are calc-alkaline,high-K calc-alkaline and shoshonite metalumious(A/CNK=0.85-1.04).The Early Paleogene adakites are interpreted as the products of partial melting of newly underplated mafic lower crust under garnet-amphibolite facies conditions.It is concluded that the Cenozoic delamination in the northern margin of the South China Sea plays an important role in the formation and evolution of magma.