东天山卡拉塔格泥盆纪岩浆岩地球化学特征及成因

Geochemical characteristics and petrogenesis of Devonian magmatic rocks in the Kalatag district,eastern Tianshan,NW China

文中对卡拉塔格地区泥盆纪岩浆岩进行了系统的岩相学、岩石地球化学研究,探讨其成因及形成构造背景。卡拉塔格地区泥盆纪火山岩主要为安山岩,并被同期一套成分连续的侵入体,包括辉长岩、闪长岩、石英闪长岩、二长岩等侵入。其中,辉长岩为钙碱性-低钾系列准铝质岩石,具有较高的Al2O3含量(18.76%~19.06%)和Mg#(57.28~69.37),具有正Eu异常;闪长岩、石英闪长岩、二长岩和安山岩为钙碱性-高钾钙碱性系列的准铝质-弱过铝质岩石,也具有较高的Al_2O_3含量(15.02%～17.43%)和Mg#(53.67～72.91),具有较弱的Eu负异常或无Eu异常。这些岩浆岩都具有较低的初始Sr值(0.703 11～0.705 18)以及较高的ε值Nd(t)(5.36～7.72),均富集轻稀土和大离子亲石元素Rb、Ba、K等,亏损重稀土和高场强元素Ta、Nb、Ti等,都具有岛弧岩浆岩的特征。相似的岩石地球化学特征显示它们可能为同源岩浆活动的产物,可能是由俯冲洋壳形成的流体交代地幔楔岩石发生部分熔融而形成。泥盆纪岩浆岩形成于康古尔洋向卡拉麦里洋俯冲的岛弧背景。

We conducted petrologic investigation and performed major and trace elemental and Sr-Nd isotopic analysis on magmatic rocks from the Kalatag district,eastern Tianshan in order to understand the petrogenesis and geodynamic setting of Devonian magmatic rocks.The results show that Devonian magmatic rocks were mainly composed of intermediate-basic rocks including gabbro,diorite,quartz diorite,monzonite and andesite.Gabbro was calc-alkaline to tholeiitic with high Al2O3 content（18.96%-19.06%）and Mg#value（57.28-69.37）,yielding A/CNK value of 0.73-0.88.Diorite,quartz diorite,monzonite and andesite were calc-alkaline to high-K calc-alkaline with high Al2O3 contents（15.02%-17.43%）and Mg#values（53.67-72.91）,and A/CNK values of 0.73-1.17.Sr-Nd isotopic analysis yielded positiveεNd（t）values ranging from 5.36 to 7.72 with low initial 87 Sr/86 Sr ratios of 0.70311 to 0.70518.Devonian magmatic rocks were enriched in large ion lithophile elements（Rb,Ba and K）while depleted in high field strength elements（Ta,Nb and Ti）.These geochemical characteristics indicate that Devonian magmatic rocks were derived from the same magma chamber with an enriched mantle wedge metasomatized by subduction slab fluid and emplaced in an island arc setting during the northward subduction of the Kanggur Ocean.