广东三水盆地玄武岩源区特征与南海早期演化  

Source characteristics of basalts in Sanshui Basin and the early tectonic evolution stage of the South China Sea

作　　者：张煜 方念乔[1] ZHANG Yu;FANG Nianqiao(School of Ocean Sciences,China University of Geosciences,Beijing 100083,China)

机构地区：[1]中国地质大学海洋学院,北京100083

出　　处：《海洋地质与第四纪地质》2021年第3期95-113,共19页Marine Geology & Quaternary Geology

基　　金：国家自然科学基金面上项目“三水盆地及周边地区古近纪火山-沉积组合与南海早期演化”(41572207)。

摘　　要：南海在扩张前是否经历了陆内裂谷阶段是南海成因研究中一个重要的问题。三水盆地位于南海北部陆缘,其新生代以来喷发的双峰式火山岩具备大陆裂谷的岩石组合特征。通过对其中玄武岩主微量元素分析认为三水盆地玄武岩可以分为亚碱性和碱性玄武岩系列,两者均显示出明显的Nb、Ta正异常,相对于大陆地壳具有较低的Th/Sc、La/Nb和U/Al×1000,陆壳混染程度低;首次对盆地内玄武岩进行^(40)Ar-^(39)Ar测年,结合前人年代学结果表明玄武质岩浆强烈喷发的时段为61~54 Ma,其中亚碱性玄武岩喷发时间(60 Ma)早于碱性玄武岩(56 Ma);通过熔融柱模型反演得到亚碱性岩浆源区起止熔融温压分别为1517℃(3.03 GPa)和1471℃(2.25 GPa),深度为101~76 km,碱性岩浆源区起止熔融温压分别为1555℃(3.33 GPa)和1506℃(2.48 GPa),深度为110~84 km,整体为石榴石-尖晶石橄榄岩过渡区且呈逐渐变深的趋势。综合岩浆源区特征以及岩石组合特征认为三水盆地在古新世具备大陆裂谷特征。通过对比三水盆地与南海扩张期岩浆活动的分布时段及源区特征,发现三水盆地与南海扩张期岩浆活动时间分布存在较长间隔,深部过程差异较大,三水盆地岩浆活动与南海扩张并无直接因果联系。It is an important issue in the study of the origin of the South China Sea(SCS)whether the SCS evolved in an intracontinental rift stage before its opening up.The Sanshui Basin is located on the northern margin of the SCS.The Cenozoic eruption of the bimodal volcanic suggest that the basin was a continental rift.The major and trace elements of the basalts indicate that the basalts in Sanshui Basin can be divided into sub-alkaline and alkaline basalts series.Both of them show obvious Nb and Ta positive anomalies.Compared with the continental crust,they have lower Th/Sc,La/Nb and U/Al×1000.^(40)Ar-^(39)Ar age and previous chronology results suggests that the eruption of basaltic magma was intense in 61~54 Ma,while the eruption of sub-alkaline basalt(60 Ma)is earlier than alkaline basalt(56 Ma).Based on the mantle melting column model,the temperature and pressure of the subalkaline magma source range from 1517℃(3.03 GPa)to 1471℃(2.25 GPa),in a depth of 101~76 km,while the alkaline magma source range from 1555℃(3.33 GPa)to 1506℃(2.48 GPa)in a depth of 110~84 km.According to the source of magmatism and rock assemblage,it is concluded that the Sanshui Basin was a Paleocene continental rift.Comparing the temporal distribution and source magmatic activity in the spreading period between the Sanshui Basin and the South China Sea,it is inferred that there is a long time interval between the magmatic activity of the Sanshui Basin and the spreading of SCS,and the deep process is quite different.There is no direct relationship between magmatism in the Sanshui Basin and the spreading of SCS.

关 键 词：玄武岩 三水盆地 南海演化 岩浆源区 熔融柱模型 

分 类 号：P736.1[天文地球—海洋地质]