西天山艾肯达坂组火山岩系的元素地球化学特征和构造环境

TECTONIC SETTING AND ELEMENT GEOCHEMISTRY OF THE AIKENDABAN FORMATION VOLCANIC ROCKS IN WEST TIANSHAN

西天山二叠纪艾肯达坂组红色陆相火山岩建造不整合在下石炭统大哈拉军山组之上,未经变形和变质。主要岩石类型有粗面玄武岩、玄武粗安岩、粗安岩、粗面岩和粗面英安岩,w(SiO2)介于41.69%～65.99%,低于上陆壳平均成分(66%)。w(Na2O+K2O)随SiO2增加而增加;SI随SiO2增高而变小;w(TiO2)一般小于1.3%;w(Al2O3)较高(12.82%～18.37%),由此显示艾肯达坂组属于典型的橄榄安粗岩系。其中,玄武岩和玄武粗安岩的w(SiO2)低于54.4%(下陆壳平均值),表明它们应源于地幔,而非陆壳;相反,w(SiO2)>54.4%的粗安岩、粗面岩和粗面英安岩可能来自陆壳或经历了壳内分异作用。玄武岩类和玄武粗安岩类ΣREE,LREE,Zr,Hf,Nb,Ta,Ba,Sr,Pb,Y等的质量分数和LaN/YbN均高于世界同类岩石平均值,而Cr,Co,Ni等的质量分数低于同类岩石,指示源区地幔富集大离子亲石元素(LILE)和不相容元素;玄武岩类和玄武粗安岩类的Eu/Eu*<0.94,Sr由亏损变化到正异常,显示地幔源区成分不均一,此不均一性可能由上壳物质返回地幔所致。粗面岩和粗面英安岩Eu/Eu*平均0.59,低于上陆壳平均值0.65;Sr亏损显著;但ΣEEE,LREE,LREE/HREE,LaN/YbN,Ce/Ce*反而低于粗安岩;Sm/Nd平均值为0.25,与下陆壳(Sm/Nd=0.25)相一致;La,Ce,Nd,Sm等相对于Y和Yb富集;Ba正异常,Nb,Ta,Hf等?

The Aikendaban Formation is a red-colored, continental volcanic sequence,unconformably overlying on the Dahalajunshan Formation of Early Carboniferous age.It shows no regional deformation and metamorphism. Its main lithologies include trachy basalt,basaltic trachyandesite, trachyandesite and trachyte. The SiO_2 contents range from 41.69 to 65.99%, lower than the average of upper continental crust (66%). Along with increasing SiO_2 contents,w(Na_2O+K_2O) values increase, whereas SI values decrease, with TiO_2 generally lower than 1.3% and Al_2O_3 ranging from 12.82 to 18.37%. These petrochemical characters show that the Aikendaban Formation is a typical volcanic sequence of shoshonite series. SiO_2 contents in basalts and basaltic trachyandesites are lower than the average of lower continental crust (54.4%), suggesting that basalts and basaltic trachyandesites were originated from the mantle rather than the continental crust. On the contrary, the trachytes and trachyandesites, with SiO_2>54.4%, were likely to be resulted from the crust or related to intracrustal differentiation. ΣREE,LREE,Zr,Hf,Nb,Ta,Ba,Sr,Pb and Y contents and La_N/Yb_N ratios in basalts and basaltic trachyandesites are higher than their averages in rocks of the same kinds worldwide, whereas Cr, Co and Ni contents are lower than their averages in same kinds of rocks worldwide. This indicates that mantle source was enriched in large ion lithosphile elements (LILE) and incompatible elements. With Eu/Eu~*<0.94 and variable Sr contents (from depletion to enrichment), the basalts and basaltic trachyandesites were probably derived from a heterogeneous mantle source resulted from return of upper crustal compositions related to subduction. Eu/Eu~* values in trachytes average 0.59, lower than 0.65 which is supposed as average of upper continental crust. Sr is strongly depleted in trachytes. Meanwhile, the ΣREE,LREE,LREE/HREE,La_N/Yb_N and Ce/Ce~* values in trachytes are lower than those in trachyandesites. Sm/Nd ratios average (0.25),same as the average of lower continental crust (0.25). La, Ce, Nd and Sm are enriched relative to Y and Yb. Ba shows positive anomaly, while Nb, Ta and Hf show depletion relatively. All these characteristics indicate that the trachytes are originated from an intracrustal differentiation with plagioclase-crystalline fractionation. Among various lithologies, trachyandesites have highest ΣREE,LREE,Ba,Sr,Zr and Nb contents,highest LREE/HREE,La_N/Yb_N,Ce/Ce~*,Zr/Y and Hf/Yb ratios; and lowest Sm/Nd ratios. This shows that LILE and HFSE have enriched in trachyandesites, suggesting that magma has experienced intense crystalline fractionation. With Eu/Eu~*=0.63, lower than the upper continental crust (0.65), the trachyandesites have higher HREE contents than basalts and basaltic trachyandesites, suggesting that trachyandesites have evolved from basaltic and/or basaltic-trachyandesitic magmas by intracrustal differentiation. Study on lithologic sequence and geological synthesis shows that the Aikendaban Formation developed in extension-collapse stage of late collision, i.e. Early Permian in Tianshan Orogen.Extension of a collisional-orogenic lithosphere generally evolved from shallow crust-level to deep root-level (lower crust and lithospheric mantle). Eruption of basalts, probably coeval with underplating of mantle-derived magmas, can be used as indicator of the end of a collisional-orogenic event.