纸质出版日期:2016,
网络出版日期:2016-7-25
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海南岛地处印支陆块和华南陆块的交界地带,具有复杂的构造演化史,其构造属性及其拼合历史一直备受争议。通过野外调查发现,在海南岛琼中枫木地区斑状花岗岩内夹持有基性岩。该基性岩的主要组成矿物为斜长石、角闪石和辉石。LA-ICP-MS锆石定年结果显示该基性岩锆石U-Pb年龄为(240.3±2.6) Ma,代表了其形成年龄。主、微量元素和同位素地球化学结果显示,样品SiO2变化于48.99%~56.00%之间,大部分样品高K-2O(1.90%~3.94%)、Al2O3(14.56%~17.77%)含量和K2O/Na2O(0.58~1.32)比值。样品有较高的稀土元素总量(∑REE=214.7~522.7 μg/g),显示出右倾岛弧型稀土元素配分模,其(La/Yb)N=11.2~31.1,(Gd/Yb)N=2.0~4.0,δEu=0.7~1.1。样品富集大离子亲石元素、亏损Nb、Ta和Ti等高场强元素,具有较低的Nd同位素组成(εNd(t)=-1.05~-2.53)。综合分析表明,枫木地区基性岩源于受到俯冲沉积物和板片流体交代改造的富集地幔,形成于古特提斯洋俯冲格局下华南与印支陆块间碰撞后伸展构造背景,暗示240 Ma左右华南和印支陆块之间的拼合已趋结束。
Hainan Island, tectonically located between the Indochina and the South China Blocks, has undergone multiphased structural overprinting and complex tectonic evolution. Mafic rocks, newly discovered in the Fengmu area, Central Hainan, are coeval with Triassic porphyritic granite and mainly composed of plagioclase, clinopyroxene and hornblende. LA-ICP-MS zircon U-Pb dating results for the representative sample yield the weighted mean206Pb/238U age of (240.3±2.6) Ma, interpreted as the crystallization age of the mafic rocks. They have SiO2ranging from 48.99% to 56.00%, K2O between 0.63%~3.94% and Al2O3between 14.56%~17.77%, and have a K2O/Na2O ratio of 0.58~1.32. These rocks show high REE concentrations with ∑REE=214.7~522.7 μg/g, and (La/Yb)N=11.17~31.10, (Gd/Yb)N=2.00~4.50, δEu=0.74~1.. They are characterized by subparallel right-sloping REE pattern, similar to those of island arc volcanic rocks, and by enrichment in LREEs and LILEs and depletion in HFSEs with significant Nb-Ta and Ti negative anomalies. The εNd(t) values ranges from -1.05 to -2.53. Our data indicate that these rocks originated from the enriched mantle source modified by subduction-related fluid/melt. In combination with other available data, it is proposed that the Fengmu mafic rocks at 240 Ma generated at the post-collision extensional setting of the convergence of the South China with Indochina Block in response to the subduction of the Paleotethyan Ocean.
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