海南岛中三叠世正长岩-花岗岩套分布于陵水-龙滚深断裂两侧及白沙深断裂南东侧.出露总面积516km2.由石英二长岩、石英正长岩及正长花岗岩组成.该岩套SiO2含量变化大,为58.40%-76.14%,富碱富钾,Na2O+K2O为7.10%-11.49%,K2O含量为3.78%-7.91%,属钾玄质碱性系列;A/CNK为0.88-1.12,绝大多数属准铝质(<1).FeO+/M9O值普遍较高,为3.14-87.13;稀土元素含量普遍较高且变化较大.为125.5×10(-6)-980×10(-6);稀土配分曲线显示右倾特征,轻重稀土分异中等-明显;δEu为1.80-0.06,早期的石英二长岩、石英正长岩具正铕异常或铕异常不明显,而晚期的正长花岗岩类具明显的负铕异常.该岩套的Zr、Hf、Zn、F含量高,(Ga×10(4))/Al高,Nb/Ta高(平均20.36),早期单元Ba含量较高而晚期单元Ba含量较低.在微量元素蛛网图上,从早期单元往晚期单元,Sr、P及Ti负异常逐渐增强.岩石的铅同位素组成较一致,n(200Pb)/n(204Pb)值为18.487-18.586,,n(207Pb)/n(204Pb)值为15.606-15.655,n(208pb)/n(204pb)值为38.503-38.635;初始锶比值、εNd(t)和T2DM值也比较一致,平均值分别为0.71067、-6.88、1575Ma.其Pb、sr、Nd同位素组成与海南中-晚三叠世镁铁质侵入岩相一致.该岩套属于A型花岗岩类,其中的花岗岩类由石英正长质岩浆通过长石、角闪石、磷灰石、钛铁矿、榍石等矿物的结晶分异形成;而石英正长质岩浆可能是源自EM Ⅱ的碱性玄武质岩浆在地壳深部再分异派生的产物.该岩套形成于后造山伸展构造环境.%In the Hainan Island, the Middle Triassic syenite-granite suite, with outcropped total area of 516km2,was distributed along the Lingshui-Longgnn deep fault and on the southeast side of the Baisha deep fault. It is composed of quartz monzonite, quartz syenite and syenogranite. The suite shows a large variation in their SiO2contents (SiO2 =58.40％ -76.14％ ), and is riched in alkali and K20, with Na2o + K20 =7.10％ - 11.49％and K2O = 3.78％ ～ 7.91％, It belongs to shoshonitic alkaline series. Their A/CNK values range from O. 88 to 1.12, most rocks of the suite belong to sub-aluminous series(A/CNK mostly ＜ 1). Their FeO*/MgO ratios are generally high (3.14 ～ 87. 13 ). Their abundances of rare earth elements change greatly and most of them are relatively high, with REE = 125.5 × 10-6 -980 × 10-6. The REE patterns display right-inclined, with mediumdistinct differentiations between light and heavy rare earth elements. The δEu values are 1.80 ～ O. 06, with positive or insignificant Eu anomaly for the early units, such as the quartz monzonites and quartz syenites, but obvious negative Eu anomaly for the late syenogranites. Rocks of the suite have high Zr, Hf, Zn, F concentrations and (Ga × 104)/Al ,Nb/Ta ratios(20.36 on average). The Ba contents of the early unit rocks are higher but which of the late rocks are lower. Normalized trace element spidergrams demonstrate that negative Sr, P and Ti anomalies are gradually enhanced from early units to late units. Their isotopic compositions of Pb, Sr and Nd are relatively homogenous and are consistent with which of the Middle-Late Triassic mafic intrusions, with n( 206Pb)/n(204Pb) = 18. 487 ～ 18. 586,n(207Pb)/n(204Pb) = 15. 606 - 15. 655, n(208Pb)/n(204Pb) = 38. 503 ～ 38. 635,mean [ n(87Sr)/n(86Sr) ]i, εNd (t) and T2DMis 0.71067, -6.88 and 1575Ma respectively. The suite is classified in A-type granitoids, In which the granites were probably generated from quartz syenitic melt through crystallization differentiations of minerals, such as feldspar, hornblende, apatite, ilmenite, sphene, etc. And the quartz syenitic melt may be derived in the deep crust mainly by differentiation of alkali basaltoid magma which was originated from the EM Ⅱ-type enriched mantle. The suite is considered formed in a post-orogenic extensional tectonic setting.
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