酒西盆地南缘旱峡早白垩世火山岩地球化学特征及其构造意义

Geochemical Characteristics and Tectonic Significance of the Early Cretaceous Volcanic Rocks in the Southern Margin of Jiuxi Basin

酒西盆地旱峡和红柳峡等地有基性火山岩产出 ,它们曾一度被认为是新生代构造运动的产物。旱峡火山岩作为夹层产于早白垩世新民堡群下沟组地层中 ,火山岩气孔、杏仁构造发育 ,局部发育枕状构造 ,具明显的水下喷发特征。旱峡火山岩SiO2 相对较低 ,K2 O +Na2 O不高 ,但钾相对较高。稀土总量为 (2 11.875～ 2 37.4 5 4)× 10 - 6 ,轻稀土富集 ,(La/Yb) N 为 12 .4 7～ 13.79;稀土配分曲线右倾、平滑 ,Ce为弱的负有异常 ,无Eu异常。在原始地幔标准化的微量元素蛛网图上 ,各元素均表现为相对富集 ,大离子亲石元素富集程度最高 ,有Nb ,Ta槽 ,但相对于原始地幔不亏损 ;曲线表现为整体上隆的特征 ,表明其源区为富集地幔。旱峡火山岩与红柳峡粗玄质熔岩的稀土和微量元素的配分模式几乎完全相同 ,表明它们同源同期。运用多个玄武岩大地构造环境判别图解 ,最终确定酒西盆地早白垩世火山岩形成于大陆拉张环境。这与酒西盆地及其周围地区 (酒泉盆地群 )下白垩统构造特征和盆地群热演化特征一致。对酒西盆地火山岩形成构造背景的研究 。

The volcanic rocks from Hanxia and Hongliuxia in Jiuxi basin are considered to be resulted from Cenozoic tectonic events. Hanxia volcanic rocks occurr among the layers of Xiagou Formation, Xinminbao Group of the Early Cretaceous age, and have vesicular and amygdaloidal structures and distinct characteristics of underwater extrusion. These volcanic rocks are characterized by lower SiO_2 and K_2O+Na_2O, but relatively higher K. ∑REE in these rocks are (212～237)×10^(-6). The chondrite-normalized REE patterns are smooth and right-inclined, with weak Ce anomaly and without Eu anomaly. On the primive mantle-normalized spidergrams, all the incompartible trace elements are enriched, especially LILE. There occur Nb and Ta negative anomalies, but still, compared with the primitive mantle, Nb and Ta are not depleted. These characteristics indicate that the source of the volcanic rocks is the enriched mantle. The REE and trace elements characteristics of the Hongliuxia trachybasalts are very similar to those of Hanxia volcanics, which implies a possible common source. The tectonic setting of the volcanic rocks in Jiuxi basin is confirmed to be continental rifting by using several geochemical diagrams for basalts. This is in accordance with the structural characteristics of lower Cretaceous and the thermal history of Jiuquan basin group. Study on volcanic rocks in Jiuxi basin is significant to deepen the understanding of tectonic evolution of the basin and make clear the tectonic framework of the western China in the Mesozoic.