福建平潭岛花岗质岩石成因:来自锆石U-Pb定年、O-Hf同位素及黑云母矿物化学的约束

东南沿海地区发育具有"等同位素组成"特征的晚中生代双峰式火成岩,其成因备受争论。本文选择了福建省平潭岛双峰式杂岩体中的花岗闪长岩、英云闪长岩与花岗岩开展了高精度锆石U-Pb定年、原位O-Hf同位素和黑云母矿物化学方面的研究,以探讨这些花岗质岩石的成因。分析结果显示,花岗闪长岩、英云闪长岩与花岗岩的锆石U-Pb年龄分别为116.8±1.0Ma、116.3±1.0Ma与117.4±1.0Ma,表明它们均侵位于早白垩世。这三类岩石也有较为相似的锆石O-Hf同位素组成。其中,锆石的δ18O分别变化为4.6%~5.3‰、4.8%~5.3‰及5.0%~5.6‰,与地幔的O同位素值基本一致。锆石的εHf(t)变化范围分别为+2.0^+7.1、+2.8^+6.5及+1.8^+5.6,相应的两阶段模式年龄tDM2分别为741~1046Ma、754~995Ma及815~1058Ma。锆石O-Hf同位素数据反映其熔融源区主要为相对较年轻的地壳物质,来自华夏地块古老基底的贡献较少。另外,黑云母矿物化学暗示这三类岩石具有Ⅰ型花岗岩的特征。但是,黑云母种属在不同岩性间存在差异,其中花岗闪长岩与英云闪长岩中的黑云母为镁质类型,花岗岩中的黑云母为铁质类型。从花岗闪长岩、英云闪长岩到花岗岩,黑云母的结晶温度和岩浆体系的氧逸度均逐渐降低,这与钙碱性岩浆分异演化的趋势基本一致。结合前人研究资料及区域地质演化历程,我们认为平潭岛杂岩体中的花岗质岩石形成于古太平洋俯冲背景,其熔融源区主要为新增生地壳物质。花岗质岩石内部岩性的差别主要是母岩浆经历不同程度分异演化的结果。我们的研究结果暗示壳幔岩浆混合在东南沿海早白垩世长英质岩石形成过程中所具有的作用可能较为有限。

Petrogenesis of granitic rocks in the Pingtan Island, Fujian Province: Constraints from zircon U-Pb dating, O-Hf isotopes and biotite mineral chemistry

The coastal SE China is characterized by emplacement of Late Mesozoic bimodal igneous rocks that show an equalisotopic feature across a wide spectrum of rock types. The genetic relationships among these different lithologies remain highly debated. In this paper, we present high-precision zircon U-Pb ages and O-Hf isotope data and chemical compositions of biotites in granodiorite,tonalite and granite from the Pingtan Igneous Complex, Fujian Province, aiming to investigate the genesis of these granitic rocks. Zircon U-Pb dating results of granodiorite, tonalite and granite respectively yield U-Pb ages of 116. 8 ± 1. 0 Ma, 116. 3 ± 1. 0 Ma and 117. 4 ± 1. 0 Ma, indicating their emplacement during Early Cretaceous. The three rock types also show similar zircon O-Hf isotope compositions. The zircon populations respectively from the granodiorite, tonalite and granite have δ18O ranges of 4. 6% ~ 5. 3‰, 4. 8%~ 5. 3‰ and 5. 0% ~ 5. 6‰, which are consistent with that of mantle origin. The three rock types respectively span zircon εHf(t) ranges of + 2. 0 ~ + 7. 1, + 2. 8 ~ + 6. 5 and + 1. 8 ~ + 5. 6 with corresponding tDM2 ranges of 741 ~ 1046 Ma, 754 ~ 995 Ma and 815~ 1058 Ma. The combined zircon O-Hf isotope data indicate their derivation from a relatively juvenile crust with an insignificant contribution from the ancient crust of the Cathaysia Block. Besides, compositions of biotites from the three rock types suggest that they belong to I-type granitoids. Nevertheless, species of biotite among these granitic rocks are different, i. e., biotite in granodiorite and tonalite belongs to Mg-biotite, whereas that in granite belongs to Fe-biotite. The crystallization temperatures of the biotite and oxygen fugacity of the magmatic system gradually decrease from granodiorite, tonalite to granite, being consistent with the differentiation trend of the calc-alkaline magma. In combination with previous studies upon regional geological characteristics and tectonic evolution histories, it can be concluded that the parental magmas were mainly derived from the newly accreted crust in response to subduction ofthe paleo-Pacific Ocean, and experienced various degrees of differentiation to form the multiple granitic lithologies of the Pingtan Complex. Our results indicate mixing between the crust-and mantle-derived magmas might play a limited role in formation of the Early Cretaceous felsic rocks in the coastal region of SE China.