Large scale of Early Devonian magmatism was found in both east and west parts of Eastern Kunlun Orogen. But few studies were focused on its origin, geodynamic mechanism and relationship with Proto-tethys. This papeT reports a typical Early Devonian intrusion from the east part of the Eastern Kunlun Orogen, which named Yuejinshan intrusion. The intrusion is mainly composed of monzogranite, granodiorite, some gabbro and cordierite-bearing granite, in which the nature of the first three rock types is the study object of this paper. Monzogranite and granodiorite samples are absence of hornblende and cordierite, and are characterized by relatively high contents of silicon and potassium, low contents of iron, magnesium and calcium, with moderate alumina saturation index (A/NCK = 1. 0 ~ 1. 1) and enrichment of large ion lithophile elements (LILE: Rb, Th and K) and distinct depletion of Ba, Sr, Nb, Ta, Ti, P, Eu, etc. , belonging to peraluminous I-S transition type granites. Gabbros are characterized by significantly high contents of iron and titanium (the contents of FeO and TiO2 are 8. 17% -12. 70% and 4. 50% ~ fl 54% , respectively) , relatively high Cu contents (11. 5 xl0-6 -30. 6 xl0-6) and Cu/Ni values (1.41-6.41), enriched in LREE (the values of (La/Yb) ?and LREE/HREE are 1.94-3.15 and 2. 65 - 3. 48, respectively), relatively low Mg# (48 ~ 50), Cr (3. 8 x 10-6 -60.4 x 10-6) and Ni (1. 8 X 10-6~12. 5x10-6) , Nb-Ta-Ti anomalies relative to primitive mantle. Monzogranites and granodiorites have relatively high /Sr values (0.710-0.740 and 0.710, respectively), low ----------Nd(t) values ( -4.05~ -5.80 and -3.54~ -3.71, respectively) and old Nd istope model ages (the t2DM values are 1.47 - 1.62Ga and 1.43 ~ 1.45Ga, respectively). But the granodiorites have relatively higherεHf(t) values ( -3.00-0.86). Gabbros have relatively high/Sr values (0.711 -0.714), low εNd(t) ( -3.44~ -6.82) and more concentrated εHf(t) values ( -2. 19 -1.05) , which show the characteristics of an enriched mantle. The LA-ICP-MS zircon U-Pb analyses suggest that the formation ages of granodiorites and gabbros are 407 ±3 Ma and 406 ?3Ma, respectively. With a comprehensive consideration on petrology, geochemistry, whole rock Sr-Nd isotopes and zircon Lu-Hf isotope analyses, and so on, we consider the formation of Yuejinshan pluton had experienced the following processes. Mantle derived magmas rose to the crust and provided enough heat for partial melting of crust materials ( meta-greywaeke) which produced S-type cordierite-bearing granites. Meanwhile, they mixed with crust derived magmas and produced I-S transition type uiunzogranite and granodiorite. The Yuejinshan and other Early Devonian magmatisms in ihe Eastern Kunlun orogeny have the same rock assemblages as typical post-collisional magmatisms and show obvious marks of mantle magmatism, which suggests the Eastern Kunlun orogeny at least begun its post-collisional extension in Early Devonian. Combining with regional studies, this paper suggests that Early Devonian is a critical period for the tectonicrntransformation of the Central orogenic belt (especially for Eastern Kunlun, North Qinling and Northern margin of Qaidam basin).rnDuring this period, the merging of terranes had finished and the tectonic framework begun to transfer from compression tornextension.%研究发现,早泥盆世东昆仑造山带东段和西段均发育较大规模的岩浆活动.然而对于其成因、深部动力学机制以及与始特提斯构造演化的关联等问题一直缺乏系统的研究.本文以东昆仑东段地区跃进山岩体为例,探讨早泥盆世岩浆活动机理及其构造意义.该岩体主体由二长花岗岩和花岗闪长岩组成,含少量的辉长岩和含堇青石花岗岩,前三种岩性是本文的重点.其中,二长花岗岩和花岗闪长岩无角闪石和堇青石,具有高硅和钾,低铁、镁和钙,铝饱和指数(A/CNK)多数在1.0～1.1之间,富集大离子亲石元素(LILE:Rb、Th和K)和轻稀土(LREE),明显亏损Ba、Sr、Nb、Ta、Ti、P和Eu等元素,属过铝质I-S过渡型花岗岩；辉长岩具有显著高的Fe和Ti(FeOT为8.17％ ～ 12.70％,TiO2为4.50％ ～6.54％)含量,高Cu(11.5×10-6～30.6×10-6)和Cu/Ni值(1.41 ～6.41),相对富集的LREE( (La/Yb)N为1.94 ～3.15,LREE/HREE为2.65 ～3.48),相对低的Mg#(48 ～50)、Cr(3.8×10-6 ～60.4×10-6)、Ni(1.8 ×10-6 ～12.5×10-6)值,相对于原始地幔具有明显的Nb-Ta-Ti正异常.二长花岗岩和花岗闪长岩均具有相对高的Isr值(分别为0.710～ 0.740和0.710)、相对低的εNd(t)值(分别为-4.05～-5.80和-3.54～ -3.71)和偏古老的t2DM(分别为1.47～ 1.62Ga和1.43 ～ 1.45Ga),但花岗闪长岩具有相对高的εHf(t),其变化在-3.00 ～0.86.跃进山辉长岩具有相对高的Isr(0.711 ～0.714)、相对低的εNd(t)值(-3.44～ -6.82)和较为集中的εHf(t)值(-2.19～ 1.05),显示富集地慢的特征.本次利用LA-ICP-MS锆石U-Pb定年方法获得花岗闪长岩的形成年龄为407±3Ma,辉长岩的形成年龄为406±3Ma.综合岩石学、地球化学和Sr-Nd-Hf同位素等多方面的指标,可以判断该岩体的形成为:幔源岩浆上侵至地壳,供热诱发古老的地壳物质部分熔融产生S型岩浆最终形成含堇青石花岗岩,同时与壳源熔体发生混合产生I-S过渡型岩浆并经历较高程度的分异最终形成二长花岗岩和花岗闪长岩.跃进山与东昆仑造山带其他地区早泥盆世岩浆活动均具有与典型后碰撞岩浆作用类似的岩石组合,并且显示很强的幔源岩浆作用的印记.这表明,至少从早泥盆世开始,东昆仑地区已经进入后碰撞的伸展阶段.综合区域上的研究成果,本文认为早泥盆世应为中央造山带(特别是东昆仑、北秦岭和柴北缘)始特斯构造体制转换的关键时期,这一时期相关地体的碰撞拼合已基本完成,区域构造体制由挤压开始转向伸展.
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