Late Early Cretaceous adakitic granitoids and associated magnesian and potassium‐rich mafic enclaves and dikes in the Tunchang–Fengmu area, Hainan Province (South China): Partial melting of lower crust and mantle, and magma hybridization
Late Early Cretaceous adakitic granitoids and associated magnesian and potassium‐rich mafic enclaves and dikes in the Tunchang–Fengmu area, Hainan Province (South China): Partial melting of lower crust and mantle, and magma hybridization
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DOI：
10.1016/j.chemgeo.2012.04.029
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发表时间：
2012-10
期刊：
影响因子：
3.9
通讯作者：
Qiang Wang;Xian‐Hua Li;Xiaoliang Jia;D. Wyman;G. Tang;Zheng‐Xiang Li;Lin Ma;Yue-heng Yang
中科院分区：
文献类型：
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作者：
Qiang Wang;Xian‐Hua Li;Xiaoliang Jia;D. Wyman;G. Tang;Zheng‐Xiang Li;Lin Ma;Yue-heng Yang
本文报道了海南岛（中国东南部）屯昌-丰母地区罕见的埃达克岩岩浆套以及相关的富镁、富钾岩浆包体和岩脉。 LA-ICP-MS 锆石 U-Pb 年龄数据显示它们生成于早白垩世晚期（~107Ma）。埃达克质岩石主要由花岗闪长岩和黑云母花岗岩组成，属于高钾钙碱性岩石，镁值较低（0.27-0.50）。它们在地球化学上与板状埃达克岩相似，例如，具有高 SiO2、Al2O3、Sr、Sr/Y 和 La/Yb 值，低 Y 和 Yb 含量，以及可忽略不计的 Eu 和正 Sr 异常。它们还具有相对均匀的 (87Sr/86Sr)i(0.7086–0.7096)、(206Pb/204Pb)i(18.50–18.61)、(207Pb/204Pb)i(15.56–15.64) 和 (208Pb/204Pb)i(38.17–38.44)同位素比率，略有变化 εNd(t)（-3.85 至 -6.55）和原位锆石 εHf(t)（-4.7 至 +1.7）值。镁铁质包体和岩脉显示出不平衡的结构（例如，具有低 MgO 边缘的多区单斜辉石与长辉石和石英微晶接触）。它们是高 K 钙碱性和钾玄铁矿，除了一个样品外，所有样品都具有高 Mg#(0.63–0.72) 值。这些镁铁质岩石的特点是轻稀土元素富集和重稀土元素（REE）贫化，Eu和Sr可忽略不计，Pb正异常，以及Nb和Ta贫化。它们的初始 87Sr/86Sr 同位素比 (0.7064–0.7086)、εNd(t) (−5.1 至 +0.1) 值以及 (206Pb/204Pb)i(18.35–18.50)、(207Pb/204Pb)i(15.45–15.59) 和 (208Pb/204Pb)i(38.18–38.70) 比率。一份镁铁质岩脉样本的原位锆石 εHf(t) 值（-4.94 至 -2.42）与该地区埃达克岩的值（-4.7 至 +1.7）相似。我们认为，埃达克岩很可能是由具有弧状地球化学特征的新俯冲玄武岩下地壳部分熔融而生成的，镁铁质包体和岩脉的原始成分可能源自俯冲大洋沉积物交代的岩石圈+软流圈地幔源区或相对年轻的岩石圈地幔源区。源自地幔的原始岩浆在注入埃达克质岩浆室之前，可能在深处与少量源自地壳的长英质岩浆进行了混合。这种注入可能将岩浆破碎成离散的小球，对流运动将飞地分布在埃达克岩主体中。古太平洋板块俯冲引起的软流圈上涌可能引发了同时期的早白垩世晚期壳幔岩浆作用，从而导致了海南岛的岩浆混合现象。
This paper reports on a rare magmatic suite of adakitic rocks and associated magnesian and potassium-rich magmatic enclaves and dikes, which occur in the Tunchang–Fengmu area, Hainan Island (Southeast China). LA-ICP‐MS zircon U–Pb age data show that they were generated in the late Early Cretaceous (~107Ma). The adakitic rocks, consisting mainly of granodiorites and biotite granites, are high-K calc-alkaline and have low Mg#values (0.27–0.50). They are geochemically similar to slab-derived adakites, e.g., with high SiO2, Al2O3, Sr, Sr/Y and La/Yb values, low Y and Yb contents, and negligible Eu and positive Sr anomalies. They also have relatively uniform (87Sr/86Sr)i(0.7086–0.7096), (206Pb/204Pb)i(18.50–18.61), (207Pb/204Pb)i(15.56–15.64) and (208Pb/204Pb)i(38.17–38.44) isotope ratios, with slightly variable εNd(t) (−3.85 to −6.55) and zircon in situ εHf(t) (−4.7 to +1.7) values. The mafic enclaves and dikes display disequilibrium textures (e.g., multiple-zoned clinopyroxene with low-MgO rims in contact with perthite and quartz microcrystals). They are high-K calc-alkaline and shoshonitic, and all but one sample have high Mg#(0.63–0.72) values. These mafic rocks are characterized by light rare earth element enrichment and heavy rare earth element (REE) depletion, negligible Eu and Sr and positive Pb anomalies, and Nb and Ta depletion. They have slightly more variable initial87Sr/86Sr isotope ratios (0.7064–0.7086), εNd(t) (−5.1 to +0.1) values, and (206Pb/204Pb)i(18.35–18.50), (207Pb/204Pb)i(15.45–15.59) and (208Pb/204Pb)i(38.18–38.70) ratios. One mafic dike sample has zircon in situ εHf(t) values (−4.94 to −2.42) similar to those of adakitic rocks (−4.7 to +1.7) in the area. We suggest that the adakitic rocks were most likely generated by partial melting of newly underplated basaltic lower crust with arc-like geochemical characteristics, and the primitive compositions of the mafic enclaves and dikes likely originated from lithospheric+asthenospheric mantle sources metasomatized by subducted oceanic sediments or a relatively juvenile lithospheric mantle source. Mantle-derived primitive magmas likely underwent mixing at depth with minor crustally-derived felsic magmas before being injected into the adakitic magma chamber. Such injections may have broken up the magma into discrete globules and convective motion distributed the enclaves through the adakitic host. Asthenosphere upwelling due to the roll-back of the subducted Paleo-Pacific plate likely triggered the coeval late Early Cretaceous crust- and mantle-derived magmatism, resulting in the magma hybridization observed on Hainan Island.