西秦岭西段塔洞花岗闪长岩体年代学、地球化学特征及其地质意义

Geochronological,Geochemical Characteristics and Geological Significance of Tadong Granodiorite Pluton in the Western Section of West Qinling

塔洞花岗闪长岩体位于西秦岭造山带西段,主体岩性为灰白色中细粒块状花岗闪长岩,岩体中未见暗色闪长质包体。对该岩体进行LA-ICP-MS锆石U-Pb同位素年代学测试,所选锆石均为具岩浆韵律环带结构的岩浆锆石,锆石w（Th）/w（U）值主体大于0.4,锆石U-Pb加权平均年龄为（219.0±5.3）Ma,代表塔洞花岗闪长岩体的结晶年龄。地球化学特征显示,该岩体具有高SiO2质量分数（66.88%~67.46%）、高Al2O3质量分数（15.39%~16.54%）、富K（质量分数为2.83%~4.12%）、准铝质（A/CNK值介于0.97~1.0之间）的高钾钙碱性花岗岩特征。该岩体轻、重稀土元素分异强烈,Eu异常不明显,球粒陨石标准化稀土元素配分模式为右倾型,原始地幔标准化微量元素蛛网图具明显的富集大离子亲石元素（Cs、Rb、Ba等）,亏损重稀土元素与高场强元素（如Nb、Ta等）的特征。该岩体具有与埃达克质花岗岩相似的特征,如高Sr质量分数（主体大于400×10-6）、低Y质量分数（（5.49~12.8）×10-6）、低Yb质量分数（（0.34~0.91）×10-6）、高w（Sr）/w（Y）值（大于40,多介于50~90之间）。岩石成因研究表明,该岩体为加厚下地壳石榴子石角闪岩（石榴子石体积分数为10%~20%）部分熔融并经历分离结晶作用形成的。综合区域资料认为,西秦岭造山带西段古特提斯洋于印支晚期已经关闭,南北陆块完成同碰撞造山作用,并进入后碰撞构造演化阶段。

Tadong granodiorite pluton,which is located in the western section of West Qinling orogenic belt,is mainly composed of gray-white fine-to-medium grained massive granodiorite without any dioritic enclaves,and several schist xenoliths are found at the margin of the pluton. LA-ICP-MS zircon U-Pb isotope chronology of the pluton was analyzed.The zircons have the characteristics of magmatic oscillatory zoning.The ratios of mass fractions of Th and U are mainly more than 0.4,and the U-Pb weighted average age of zircon is （21 9.0±5.3）Ma,which represents the crystallization age of Tadong granodiorite pluton.The geochemical characteristics show that the pluton has the characteristics of high-K calc-alkaline granite,including high mass fraction of SiO2 （66.88%-67.46%）,high mass fraction of Al2 O3 （15.39%-1 6.54%）,K-rich （the mass fraction is 2.83%-4.12%） and metaluminous （the value of A/CNK is 0.97-1.0 ）. Chondrite-normalized REE pattern of the pluton is right-inclined, which displays obvious differentiation of light and heavy rare earth elements and weak Eu anomaly.Primitive mantle-normalized trace element spider diagram of the pluton shows that large ion lithophile elements （Cs,Rb,Ba,etc.）are obviously enriched,heavy rare earth elements and high field strength elements （Nb,Ta,etc.）are depleted.The characteristics of Tadong granodiorite pluton are similar to adakite-like granite,which is typical of high mass fraction of Sr（mainly more than 400 × 10 -6 ）,low mass fraction of Y （（5.49-12.8）× 10 -6 ）,low mass fraction of Yb （（0.34-0.91 ）× 10 -6 ）and high ratio of mass fractions between Sr and Y （more than 40,mostly 50-90 ）. Petrogenesis results show that Tadong granodiorite pluton may result from the partly melt of garnet amphibolite （volume fraction of garnet is 10%-20%）from the thickening lower crust,following the process of fractionation and crystallization.In general,Paleotethys has closed at the later stage of Indosinian in the western section of West Qinling,and the syn-collision orogenesis between the north and south blocks finishes,following the tectonic evolution stage of post-collision.