东昆仑早古生代洋壳俯冲与碰撞造山作用的转换:来自胡晓钦镁铁质岩石的证据

Early Paleozoic tectonic transition from ocean subduction to collisional orogeny in the Eastern Kunlun region: Evidence from Huxiaoqin mafic rocks

位于东昆仑昆中缝合带内的胡晓钦镁铁质岩石主要以角闪辉绿岩为主。利用锆石LA-ICP-MSU-Pb定年方法获得其结晶年龄为438±2Ma(MSWD=1.06,n=15),表明该岩石应为早志留世岩浆活动的产物。岩石样品均具有相对低的TiO2含量(0.43%～1.58%)和变化较高的MgO值(2.83%～8.22%)和Mg#(45～74),相对于原始地幔富集大离子亲石元素(LILE:Rb、Ba、Th和U等)和轻稀土(LREE),明显亏损高场强元素(HFSE:Nb、Ta和Ti),并且具有略微富集的Hf同位素组成(εHf(t)为3.68～8.20,tDM2为0.90～1.19Ga)。以Mg#作为横坐标的二元图解和(2CaO+Na2O)/TiO2-Al2O3/TiO2图解揭示其形成过程中应主要经历了单斜辉石、橄榄石和斜长石的分离结晶。岩石样品均具有相对低的Nb/La和Nb/Ce比值(分别为0.15～0.28和0.07～0.13)以及较高的Nb/Ta和Zr/Hf比值(分别为13.15～17.38和36.14～43.88),指示岩石的形成过程受到地壳混染的影响非常小。地球化学和锆石Lu-Hf同位素研究揭示其形成可能与受板片流体交代含尖晶石橄榄岩的部分熔融有关。胡晓钦镁铁质岩石具有类似岛弧玄武岩特征的地球化学组成,并且其年龄明显早于东昆仑与碰撞相关的榴辉岩相变质年龄(428Ma)和中压(绿帘)角闪岩相变质峰期年龄(427Ma),表明其形成仍与东昆仑洋壳俯冲关系密切。综合区域资料可以判断,胡晓钦镁铁质岩石可能代表了东昆仑早古生代洋壳俯冲最晚期的岩浆记录。这样可以确定,东昆仑早古生代洋盆最终关闭和碰撞造山开始的时间为早志留世,洋壳俯冲持续的时间至少为79Myr,碰撞造山持续时间至少为8Myr。

The Huxiaoqin mafic rocks which distribute in the Central Kunlun Suture Zone,are mainly composed of hornblen dediabases. LA-ICP-MS zircon U-Pb analyses gave 438 ±2Ma （ MSWD = 1. 06,n = 15） of the crystallization age,suggesting Huxiaoqin mafic rocks are the products of the Early Silurian magmatism. All the rock samples are characterized by relatively low TiO2 （ 0.43%～1. 58% ） ,variable MgO （2. 83%～8.22% ） and Mg # （45 ～74） ,slight enriched Hf isotopic composition （ ε Hf （ t） =3. 68 ～8.20,t DM2 = 0. 90～1. 19Ga） ,enrichment of large ion lithophile elements （ LILE： eg. Rb,Ba,Th,U and LREE） and marked depletion of high field strength elements （ HFSE： eg. Nb,Ta,Ti） . Various binary diagrams taking Mg # as the abscissa,and adiagram plotting （2CaO ＋ Na20）/TiO2 vs. Al203/TiO2 reveal fractional crystallization of clinopyroxene,olivine,and plagioclase. Crustal contamination was insignificant,as reflected by whole-rock relatively low Nb/La and Nb/Ce ratios,high Nb/Ta and Zr/Hf ratios. Based on our geochemical and Lu-Hf isotopic studies,we suggest the maficdykes likely were generated by partial melting of the spinel peridotites metasomatised by slab-derived fluids. Huxiaoqin mafic rocks have similar chemical composition to island arc basalts,and they formed earlier than metamorphic peaks of eclogite and epidote-amphibolite facies （428Ma and427Ma,respectively） ,indicating their formationwere closely related to the subduction of Early Paleozoic Eastern Kunlun 0ceanic lithosphere. Combining with regional studies,we suggest that Huxiaoqin mafic rocks may represent the latest magmatism which related to Early Paleozoic ocean subduction. Finally,we infer the tectonic transition fromocean subduction to collisional orogeny commenced at Early Silurian,the duration ofocean subduction is more than 79Myr and that of collisional orogeny is more than 8Myr.