扎河坝蛇绿混杂岩内富铌玄武(安山)岩的地球化学特征及其地质意义

Geochemistry of the Nb-enriched basalt and its significances in Zaheba ophiolite melange.

在新疆北部东准噶尔的扎河坝蛇绿混杂岩中发现的富铌玄武(安山)岩,其SiO_2含量介于46.71%～57.65%,TiO_2含量为1.00%～1.76%,与太古代绿岩带内富铌玄武(安山)岩相似,Na_2O含量为3.86%～6.64%,P_2O_5为0.34%～0.82%,明显高于太古代绿岩带富铌玄武(安山)岩。扎河坝富铌玄武岩铌含量介于7.22×10^(-6)～21.91×10^(-6)之间,大于7×10^(-6),与典型的富铌玄武岩相同。该岩石轻重稀土元素分馏较明显,其分布模式为无明显铕至弱负铕异常的右倾曲线。尽管铌的绝对含量较高,但由于钍和轻稀土元素更加富集,在微量元素蛛网图中扎河坝富铌玄武(安山)岩仍表现为铌的明显亏损,同时高场强元素Zr、Hf及Ti也表现出一定程度的亏损。微量元素地球化学特征显示,扎河坝蛇绿混杂岩内富铌玄武(安山)岩形成于古亚洲洋的洋内弧,它是被埃达克质岩浆交代的地幔楔橄榄岩部分熔融的产物,同时大洋沉积物及俯冲板块释放的流体对成岩作用也有一定的贡献。富铌玄武(安山)岩作为弧前增生楔定位在扎河坝蛇绿混杂岩体内,与早前报道的超高压变质岩共存表明,该蛇绿混杂岩体至少记录了两次性质不同的古亚洲洋洋壳俯冲,这更进一步证实新疆北部晚古生代新增陆壳是古亚洲洋多次俯冲作用的产物。

Nb-enriched basaltic rocks are identified within the Zaheba ophiolite melange in the eastern Junggar, northern Xinjiang. They show high Nb contents of 7.22 x 10(-6) to 21.9 1 x 10(-6), SiO(2) contents of 46.71% to 57.65% and TiO(2) contents of 1.00% to 1.76%, similar to Nb-enriched basalts from Archaean greenstone belt. The REE patterns are of LREE enrichment with weak Eu anomalies, suggesting obvious fractionation. Moreover, they are characterized by obviously negative Nb anomalies and negative Ti anomalies, suggesting that the Nb-enriched basalts were probably derived from partial melting of mantle peridotite that interacted with slab melt under high geothermal gradient in the intra-ocean arc setting of the Palco-Asian Ocean. Moreover, Nb-enriched basalts occurred in the Zaheba ophilite as a fore-arc accretionary prism. The association of the Nb-enriched basalts with ultra-high pressure metamorphic rocks found previously by the authors implies that more than two times of the Paleo-Asian oceanic crust subduction with different characteristics were recorded in the ophiolite. Therefore, multiple partial melting of the subducting oceanic crust and ocean sediments, and the succedent interactions between the slab melt and peridotite in the mantle wedge was probably responsible for the crust growth and the formation of central Asian orogenic belts.