北秦岭武关岩体LA-ICPMS锆石U-Pb定年及岩石成因研究

摘要

武关岩体位于北秦岭武关镇,侵入于丹凤岩群之中,主要岩石类型为闪长岩和花岗闪长岩,属低钾拉斑-中钾钙碱性系列.微量元素地球化学特征为富集大离子亲石元素而亏损高场强元素、具有较高的LILE/HFSE比值,且显示出有俯冲沉积物的地球化学信息,因而其形成与大洋俯冲消减作用有关.稀土元素球粒陨石标准化分配型式为LREE富集、HREE平坦的右倾型且无明显的Eu负异常.此外,其明显富铝、富钠,有较高的Sr/Yb比值和CaO/Na2O比值,因而极有可能为角闪石作为主要残余条件下玄武质岩石部分熔融的产物.在假定有角闪石为稳定残留的中下地壳压力条件下,通过对稀土元素的模拟计算表明:以丹凤岩群中形成于岛弧环境下的斜长角闪岩为源岩在部分熔融程度为30％ ～40％,残余矿物相以角闪石为主并伴有少量的斜长石和单斜辉石(石榴子石极有可能无残余)的条件下,可以形成该类岩体.经锆石阴极发光(CL)研究及LA-ICPMS同位素测年技术分析后可知,武关岩体的锆石U-Pb同位素年龄为438.6±2.2Ma.该年龄晚于商丹洋的俯冲消减阶段,但仍明显保留有俯冲消减环境所具有的地球化学特征,因而其可能继承有俯冲消减源区的地球化学特征.以岛弧成因的斜长角闪岩为源岩进行的稀土元素部分熔融模拟计算,可以证明武关岩体为北秦岭加里东造山期中下地壳源区岩石在碰撞造山过程中由“滞后的消减源区”部分熔融的产物.%Located in North Qinling, the Wuguan intrusive body intruded into the Danfeng Group and developed gneissosity in some regions. It mainly includes diorites and granodiorites and belongs to low-K tholeiitic and medium-K calc-alkaline series, characterized by enrichment of large ion lithophile elements and depletion of high field strength elements, with a high LILE/HFSE ratio. Geochemical data also suggest the involvement of a sub-duction sedimentary source. All the evidence indicates that the formation of the rock was associated with the o-cean subduction. Chondrite-normalized REE patterns show an enriched LREE distribution and a flat HREE distribution, with no obvious negative Eu anomaly. Moreover, the Wuguan intrusive rocks are rich in sodium and aluminum, and have high Sr/Yb and CaO/Na2O ratios. These geochemical features suggest that these rocks were formed by the remelting of the basaltic rocks with amphibole as a stable residual phase. The result of simu-lation calculation suggests that plagioclase amphibolite produced in an island-arc environment in the Danfeng Group could form the Wuguan intrusive body through partial melting with a degree of 30% ~ 40%. Besides, the residual mineral assemblage is mainly hornblende with a small amount of plagioclase and clinopyroxene, and garnet-bearing residues may be negligible. CL images and LA-ICPMS dating show that zircon U-Pb isotopic age of the Wuguan intrusive body is 438.6±2.2 Ma, and the formation of this intrusive body might be attributed to Caledonian collision event of the North Qinling. This zircon U-Pb isotopic age is later than the period of the sub-duction of Shangdan Ocean, but the chemical characteristics of this intrusive body are closely related to the island-arc environments. It is therefore inferred that the geochemical characteristics reflected in the Wuguan intrusive body might have inherited some chemical features of its sources in an arc environment. Combined with the REE simulation calculation by the melting of the amphibolites formed in an arc environment, it is proved that the Wuguan intrusive body was formed in a collision orogenic process and it might have been a partial melting product of the pre-existing source in a subduction environment.