海南岛至今未见早白垩世初期 A 型花岗岩报道。本文首次在石碌铁矿床中部钻孔中发现了隐伏的 A 型花岗岩,岩性为黑云母二长花岗岩。对该隐伏岩体开展了年代学、地球化学和Nd-Hf同位素分析,探讨岩石成因及成岩构造背景。LA-ICP-MS锆石U-Pb定年结果为133±3 Ma(MSWD=0.33),为早白垩世初期岩浆活动产物。岩体属高钾钙碱性岩系,具有高硅(SiO2=75.22%~76.63%)、富碱(K2O+Na2O=8.01%~8.56%)和铁(FeOT=1.62%~2.01%)、贫钙(CaO=0.55%~0.69%)和镁(MgO=0.03%~0.15%)特征。铝饱和指数 A/CNK=0.99~1.04,为准铝–弱过铝质花岗岩。岩石整体富集 K、Rb 等大离子亲石元素(LILE)和U、Th等高场强元素(HFSE),明显亏损Ba、Sr。LREE富集, HREE相对亏损,且具强烈的Eu负异常(δEu=0.02~0.08)。岩相学和地球化学显示其为高分异铝质A型花岗岩。Nd同位素组成εNd(t)=–6.3~–7.1,二阶段模式年龄tDM2(Nd)=1.44~1.47 Ga, Hf同位素组成εHf(t)=–18.5~–21.7,二阶段模式年龄tDM2(Hf)=1.93~2.09 Ga,揭示该隐伏花岗岩体源区主要为古老地壳基底,并混合了少量富集地幔物质。黑云母二长花岗岩具有后碰撞花岗岩的特征,形成于伸展构造背景,与早白垩世初期印度板块北向漂移挤压及太平洋板块俯冲作用减弱造成的局部伸展有关。%Mesozoic granites widely distribute in Hainan island, South China. However, U-Pb age, geochemistry and Nd-Hf isotopes of the Early Cretaceous A-type granites have rarely been reported so far. This paper reports a concealed A-type granite, mainly composed of biotite monzogranite, found in drill cores from the Shilu iron deposit (it is called the Shilu A-type granite). LA-ICP-MS zircon U-Pb dating of the A-type granite yields a weighted 206Pb/238U mean age of 133±3 Ma (MSWD=0.33), indicating that it was emplaced in the Early Cretaceous. Petrography and geochemical compositions show that the granite belongs to high-K cal-alkaline series and is characterized by high silicon (SiO2=75.22%–76.63%), alkali (K2O+Na2O=8.01%–8.56%) and iron (FeOT=1.62%–2.01%), but low calcium (CaO=0.55%–0.69%) and magnesium (MgO=0.03%–0.15%). The granite is metaluminous to weak peraluminous with A/CNK values in the range of 0.99 to 1.04. The granitic rocks are enriched in large ion lithophile element (e.g. K and Rb) and high field strength elements (e.g. U and Th) but depleted in Ba, Sr. The chondrite normalized REE patterns of the granitic rocks are right dipping with significant negative Eu anomalies (δEu=0.02‒0.08). The evidence from mineralogy and geochemistry suggests that the concealed granite is a highly fractionated aluminous A-type granite. TheεNd(t)values range from–6.3 to–7.1 with two-stage model agestDM2(Nd) from 1.44 to 1.47 Ga. TheεHf(t) values of zircon from the A-type granite range from–18.5 to–21.7 and the two-stage model agestDM2(Hf) range from 1.93 Ga to 2.09 Ga, which suggests that the granite was dominantly derived from the basement with minor enriched mantle materials. The trace element features show that the A-type granite was formed in an extensional tectonic setting in the Early Cretaceous, which was related to the northward movement of the Indian plate and the atteunuation of the subduction of the Pacific plate.
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