Basic dikes occurring within the Dongfangzhongsha farmland in SW Hainan Island are mainly composed of plagioclase,monoclinic pyroxene and a small amount of hornblende.These basic dikes are charactered by such geochemical features as SiO2=52.39% 53.44%,K2 O+Na2 O=5.02%5.71%,K2 O=2.45% 3.29% and high K2 O/Na2 O value of 0.91 1.63.These rocks are also featured by enrichment in LREEs and LILEs,slight Eu negative anomaly (δEu=0.76 0.84),insignificant Sr and P negative anomalies as well as significant Nb Ta and Ti negative anomalies.These signatures show these basic dikes may come from fluid metasomatic mantle resource.The representative basic dike (11HN31A)yielded the LA ICP MS zircon U Pb ages of (242.2±2.6)Ma with MSWD=0.074 (n=14),interpreted as the crystallization age,which is consistent with those in east Hainan Island.Our new data,together with the available data,indicate the Indosinian basic dikes formed under the post-orogenic extensional tectonic setting in Hainan Island,in Early Triassic ( 240 Ma).%琼西南东方中沙农场发育有基性岩脉，主要由斜长石、单斜辉石和少量角闪石组成。其 LA ICP MS锆石 U Pb年代学测试表明，该岩脉形成年龄为（242．2±2．6）Ma，和东部地区辉绿岩脉的形成年龄一致。该基性岩脉 w（SiO2）＝52．39％～53．44％，w（K2 O＋Na2 O）＝5．02％～5．71％，具富钾（w（K2 O）＝2．45％～3．29％）和高K2 O/Na2 O值（0．91～1．63）特征。该基性岩脉富集轻稀土元素和大离子亲石元素（LIL），轻微 Eu负异常（δEu＝0．76～0．84），Sr、P弱负异常，明显亏损高场强元素 Nb、Ta、Ti。海南约240 Ma的基性岩脉可能来源于被流体交代的地幔源区，形成于华南印支陆块碰撞后伸展构造环境。
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