新疆北部阿尔泰南缘晚古生代高钾高硅熔结凝灰岩的地球化学、岩浆成因及构造背景

Geochemical characteristics, magmtic genesis and tectonic background of the late paleozoic high potassium and high silicon ignimbrite on the southern margin of Altaid, north Xinjiang.

新疆北部阿尔泰南缘分布着一条晚古生代的火山岩带。通过岩石学和地球化学的研究,我们从克朗和麦兹火山-沉积盆地的下泥盆统康布铁堡组地层的火山岩中,厘定出一种高钾高硅熔结凝灰岩(Ignimbrite),前人称之为钾质流纹岩。高钾高硅熔结凝灰岩主要由钾长石(微斜长石)、石英和黑云母,以及少量白云母组成。其岩石化学成分的特征为过铝质的(A/CNK=1.01～1.36)、高硅(SiO_2=69%～80%)、高钾(K_2O=5%～11%)、高钾钠比值,并富集大离子亲石元素(Rb,Ba,K,La),亏损高场强元素(Nb,Ta,Ti,P)和低的 Nb/Y 比值,以及富 LREE 和亏损 Eu 的 REE 分布模式。以上这些特征体现它们继承了形成硅质岩浆的大陆地壳源区特点。微量元素的构造环境判别图解显示,本区高钾高硅熔结凝灰岩形成于活动大陆边缘的岛弧构造环境。综合岩石学和地球化学研究的结果,作者提出高钾高硅熔结凝灰岩岩浆的二阶段成因模型:1)地幔楔二辉橄榄岩部分熔融产生的大体积玄武岩岩浆侵入导致其上部地壳发生部分熔融形成了高硅的熔结凝灰岩岩浆;2)高硅岩浆经富钠斜长石的分离结晶作用最终形成高钾高硅的熔结凝灰岩浆。

A suite of high-K and high-Si ignimbrite has been identified in the Late Paleozoic volcanic rock belt on the southern margin of Ahaid. It is suggested from detailed studies on the petrology and geochemistry of volcanic rocks of Lower Devonian Kangbutiebao Formation in the Kelang and Maizi volcanic-sedimentary basin. This suite was previously named as the potassic rhyolite. These High-K and high-Si ignimbrites are mainly composed of K-feldspar （microcline）, quartz, biotite, and minor muscovite. They are characterized by peraluminous （A/CNK = 1.01 - 1.36）, high SiO2 （69% - 80% ）, high K2O （5% - 11% ）, and high K2O/Na2 O ratios; as well as enriched in LILEs （Rb, Ba and K）, and depleted in HFSEs （Nb, Ta and Ti） with low Nb/Y ratios. Their REE patterns are of LREE enrichment with Eu depletion. Above-mentioned features imply that they inherited the characteristics of continental crust, the source region of siliceous magma. The trace element tectonic setting discriminations show that the high-K and high-Si ignimbrites were formed in a tectonic setting of active continental margin arc related with the subduction. Combined the petrologic and geochemical studies, a two-stage formation model of high-K and high-Si ignimbritic magma has been suggested： 1 ） a high-Si ignimbritic magma was formed by the partial melting of crust, resulted from the intrusion of large volume basaltic magma derived from the partial melting of lherzolite in the mantle wedge; 2 ） the fractional crystallization of Na-rich plagioclase resulted in the formation of high-K and high-Si ignimbritic magma.