西天山艾肯达坂二叠纪钾质火山岩的地球化学特征及岩石成因

西天山艾肯达坂组中粗安岩和粗面岩富碱(Na2O+K2O8%)、高铝(Al2O313.5%)和低钛(TiO21.31%),具有典型橄榄安粗岩系火山岩的岩石化学特征。两类钾质火山岩的微量元素和同位素地球化学特征差异显著:粗安岩明显富集大离子亲石元素和轻稀土元素,具有较高的Sr/Y(29～49)比值和相对较低的(87Sr/86Sr)i(0.7044～0.7045);而粗面岩大离子亲石元素和轻稀土元素的富集程度明显减弱,甚至出现了Sr强烈亏损的现象,它具有较低的Sr/Y(1.74～2.50)比值和相对较高的(87Sr/86Sr)i(0.7060～0.7080)。研究结果显示,艾肯达坂组中粗安岩和粗面岩均是后俯冲岩浆作用的产物,但它们分别形成于俯冲演化的不同阶段:粗安岩是后俯冲伸展阶段形成的典型橄榄安粗质碱性基性火山岩,它是富集地幔低程度(熔融程度低于5%)部分熔融的产物,其熔融相以角闪石为主,在残留相中存在着大量的石榴石,因此它具有较高Sr/Y比值;而粗面岩则是后俯冲挤压阶段形成的长英质的碱性火山岩,它是年青下地壳部分熔融的产物,其形成时间早于粗安岩。

Geochemical characteristics and petrogenesis of the Aikendaban Permian shoshonite in the western Tianshan.

Trachyandesite and trachyte from the Aikendaban Formation in the western Tianshan, are alkali-rich (Na₂O+K₂O>8%), of high-aluminium (Al₂O₃>13.5%) and low-titanium (TiO₂<1.31%), and have typical petro-geochemical characteristics of shoshonitic volcanics. Compared with classic shoshonite association, the trachyandesite is obviously high in TiO₂, Al₂O₃ and Na₂O, and relatively low in Fe₂O₃, CaO and MgO, and the trachyte is obviously high in TiO₂, Fe₂O₃ and Na₂O, comparatively low in Al₂O₃ and extremely low in CaO. The REE geochemical characteristics of trachyandesite show obvious enrichment of LREE, obvious depletion of HREE and negative Eu anomalies, high Sr/Y(29~49)ratio and low (⁸⁷Sr/⁸⁶Sr)_i(0.7044~0.7045). Trachyte show less enrichment of LREE and depletion of HREE, stronger negative Eu anomalies, even appears strong depletion phenomenon of Sr, low Sr/Y(1.74~2.50)ratio and high (⁸⁷Sr/⁸⁶Sr)_i(0.7060~0.7080). The results showed that both trachyandesite and trachyte from Aikendaban Formation as a product of post-subduction magmatism, but they were formed in different stages of subduction: the trachyandesite is a typical shoshonitic alkaline volcanics which originated by low-degree (less than 5%) partial melting of the EM in the post-subduction extension stage, with melting phase of amphibole and residual phase of abundance garnet for their high Sr/Y ratio; and the trachyte is felsic-alkaline volcanics originated by partial melting of the juvenile lower crust in the post-subduction compression stage, which formed earlier than the trachyandesite.