江南造山带西南段宰便辉绿岩年代学、地球化学及其构造意义

Geochronology,geochemistry and isotopes of Zaibian diabase in the western margin of Jiangnan orogenic belt,China:Implications for tectonic evolution

宰便地区位于江南造山带西南段,发育有一系列新元古代镁铁质-超镁铁质火成岩。然而,其成因与构造背景仍存在广泛争议。因此,本文选择江南造山带西南段宰便辉绿岩为研究对象,系统开展岩石学、地球化学、Nd-Hf同位素和锆石U-Pb年代学研究。研究结果表明,辉绿岩具有高Mg^(#)(50~69)、高Al_(2)O_(3)(13.94%~18.63%)、低钾(0.03%~0.33%)、低磷(0.04%~0.28%)特征,显示为低钾拉斑系列岩石特点。同时,岩石整体相对富集Th、U等大离子亲石元素,亏损Nb和Y等高场强元素,Eu具有微弱负异常特征。辉绿岩的Nb/Ta、Zr/Nb比值揭示了母岩浆在岩浆侵位过程中未发生地壳混染。3件样品的锆石U-Pb年龄分别为820.7±3.0Ma、821.1±9.2Ma和823.1±9.5Ma,表明岩浆侵位时代为新元古代早期。锆石ε_(Hf)(t)值介于-13.6~7.7之间,Hf的t_(DM2)值介于1214~2547Ma之间,结合全岩地球化学特征,揭示其原始岩浆可能源于富集地幔,并伴有一定的亏损地幔参与。综合前人和本次研究的地质、地球化学和年代学数据,本文认为江南造山带西南段宰便地区在新元古代早期处于板内构造环境。

The Neoproterozoic mafic-ultramafic rocks are widely developed in the Zaibian area,the southwestern margin of the Jiangnan orogenic belt(JOB),South China.The petrogenesis and tectonic setting of these maficultramafic rocks remain controversial.In this work,we present petrology,geochemistry,Nd-Hf isotopes and zircon LA-ICP-MS U-Pb geochronology of Zaibian diabase,with the aim to reveal their petrogenesis and tectonic setting.Geochemical data suggest the diabase are characterized by high Mg^(#)(50~69),high Al_(2)O_(3)(13.94%~18.63%),low K_(2)O(0.03%~0.33%)and low P_(2)O_(5)(0.04%~0.28%),indicating the characteristics of the low potassium tholeiite series.The rocks are also relatively enriched in Th,U elements and depleted in Nb and Y elements,with a weakly negative Eu anomaly.Their relatively constant Nb/Ta,Zr/Nb ratios suggest that these rocks have not underwent contamination during magma emplacement.Zircon U-Pb dating results yielded U-Pb concordant ages of 823.1±9.5Ma,821.1±9.2Ma and 820.7±3.0Ma,respectively,suggesting that Zaibian diabases were formed in the Early Neoproterozoic.Zircon Lu-Hf isotopes show theε_(Hf)(t)values varying from-13.6 to 7.7,with t_(DM2) model ages in the range of 1214~2547Ma.Combined with the whole rock geochemical characteristics,these isotopic features further suggest that the magmas are derived mainly from E-mantle involved some depleted mantle components.Based on geological,geochemical and chronological characteristics from this and previous studies,we propose that the Zaibian area within the southwestern part of JOB was under an intraplate tectonic setting during the Early Neoproterozoic.