青藏高原西北缘新生代超基性-基性岩脉锆石U-Pb年代学及地球化学特征

Zircon U-Pb dating and geochemical characteristics of Cenozoic ultrabasicbasic dyke swarms in northwestern Tibetan Plateau

青藏高原西北缘米提孜北新发现多条新生代超基性-基性岩脉。岩脉侵位于长城纪赛拉加兹塔格岩群中,整体位于柯岗断裂北侧附近,近东西向展布。岩脉岩性为闪斜煌岩和云斜煌岩,分别具钠质碱性煌斑岩和钠质钙碱性煌斑岩地球化学特征。LA-ICP-MS锆石U-Pb定年获得闪斜煌岩年龄为38.24±0.54 Ma,表明岩脉形成于新生代古近纪。岩脉稀土元素总量偏低(64.28×10^-6~253.41×10^-6),Eu(δEu=1.04~1.21)和Ce(δCe=0.99~1.03)异常均不明显,稀土元素配分模式呈LREE富集(LREE/HREE=4.15~6.45)的右倾型,大离子亲石元素(LILE)相对富集,高场强元素(HFSE)相对亏损。研究结果显示,岩脉是岩石圈地幔低程度部分熔融的产物,岩浆活动持续时间短,岩脉侵位后冷却速度快;岩脉产出于走滑构造体制下,大型走滑断裂控制了岩浆活动的发生和岩脉的分布。

Cenozoic ultrabasic-basic dyke swarms were newly discovered in northern Mitizi area in northwest Tibetan Plateau. The dyke swarms intruded into the Changchengian Sailajiazitage Group. The nearly E-W-trending dyke swarms are located on the northern margin of Kogan Fault. The rocks are mainly composed of spessartite and kersantite,which have features of ALNaand CALNarespectively,as shown by geochemical analyses. The LA-ICP-MS zircon U-Pb age of spessartite yielded an age of 38. 24 ± 0. 54 Ma,suggesting that these dyke swarms were formed in Paleogene. The dyke swarms have low REE( 64. 28 × 10^-6~ 253. 41 × 10^-6),and the anomalies of Eu( δEu= 1. 04 ~ 1. 21) and Ce( δCe = 0. 99 ~ 1. 03) are not obvious. REE distribution patterns are of the right-oblique type( LREE/HREE = 4. 15 ~ 6. 45). All samples are characterized by the relative enrichment of LILE and depletion of HFSE. The dyke swarms are products of low degree partial melting of lithospheric mantle. The magmatic activity lasted for a short time and the dyke swarms had quick cooling rate after intruding into the surrounding rocks.The tectonic setting of the dyke swarms was strike-slip fault,and the magmatism occurred along the large scale strike-slip faults.