Petrogenesis of Early Devonian intrusive rocks in the east part of Eastern Kunlun Orogen and implication for Early Palaeozoic orogenic processes
Petrogenesis of Early Devonian intrusive rocks in the east part of Eastern Kunlun Orogen and implication for Early Palaeozoic orogenic processes
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作者：
Ma Chang
东昆仑造山带东部和西部都发现了大规模的早泥盆世岩浆活动，但对其成因、地球动力学机制及其与原岩关系的研究较少。本文报道了东昆仑造山带东部一个典型的早泥盆世侵入岩--跃金山侵入岩。该侵入岩主要由二长花岗岩、花岗闪长岩、少量辉长岩和含堇青石花岗岩组成，其中前三种岩石类型的性质是本文研究的对象。二长花岗岩和花岗闪长岩样品中不含角闪石和堇青石，具有硅、钾含量较高，铁、镁、钙、钙含量较低的特点辉长岩具有中等的氧化铝饱和指数(A/Nck=1.0~1.1)和大离子亲石元素(LILE：Rb、Th、K)的富集性，而Ba、Sr、Nb、Ta、Ti、P、Eu等元素明显亏损，属于过铝I-S过渡型花岗岩。辉长岩具有明显的铁和钛含量(FeOT8.17%~12.70%，TiO24.50%~6.54%)的特征。相对较高的铜含量(11.5×10-6~30.6×10-6)和铜/镍比值(1.41~6.41)，富含轻稀土((La/Yb)N和LREE/HREE分别为1.94~3.15和2.65~3.48)，相对较低的Mg#(48~50)、Cr(3.8×10-6~60.4×10-6)和Ni(1.8×10-6~12.5×10-6)；二长花岗岩和花岗闪长岩具有较高的ISR值(分别为0.710~0.740和0.710)，较低的端面(T)值(分别为-4.05~-5.80和-3.54~-3.71)和古老的ND同位素模式年龄(T2 DM值分别为1.47~1.62Ga和1.43~1.45Ga)，而花岗闪长岩的ISR值相对较高(-3.00~0.86)，而辉长岩的ISR值相对较高(0.711~0.714)。低端(T)值(-3.44~-6.82)和较集中的EHF(T)值(-2.19~1.05)，显示出富集地幔的特征。LA-ICPMS锆石U-Pb分析表明，花岗闪长岩和辉长岩的形成年龄分别为407±3 Ma和406±3 Ma。综合岩石学、地球化学、全岩锶-钕同位素和锆石Lu-Hf同位素分析等，确定了花岗闪长岩和辉长岩的形成年龄。我们认为，越金山岩体的形成经历了以下几个过程。地幔岩浆上升到地壳，为地壳物质(变质灰岩)的部分熔融提供了足够的热量，形成了S型含堇青石花岗岩。同时，它们与壳源岩浆混合，产生了I-S过渡型二长花岗岩和花岗闪长岩。结合区域研究，东昆仑造山的跃金山等早泥盆世岩浆与典型的后碰撞后岩浆具有相同的岩石组合，并显示出明显的地幔岩浆活动标志，这表明东昆仑运动至少在泥盆纪早期开始了后碰撞伸展作用。结合区域研究，东昆仑造山带和其他早泥盆世岩浆与典型的后碰撞岩浆具有相同的岩石组合，并显示出明显的地幔岩浆活动标志，这表明东昆仑运动至少在泥盆纪早期开始了后碰撞伸展本文认为，早泥盆世是中央造山带(特别是东昆仑、北秦岭和柴达木盆地北缘)构造转换的关键时期，在此期间，地体合并完成，构造格架开始由挤压向伸展转变。
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 paper 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 FeOT and TiO2 are 8.17% ～ 12.70% and 4.50% ～ 6.54%,respectively),relatively high Cu contents(11.5 × 10-6 ～ 30.6 × 10-6) and Cu/Ni values(1.41 ～ 6.41),enriched in LREE(the values of(La/Yb) N and LREE/HREE are 1.94 ～ 3.15 and 2.65 ～ 3.48,respectively),relatively low Mg#(48 ～ 50),Cr(3.8 × 10-6 ～ 60.4 × 10-6) and Ni(1.8 ×10-6 ～12.5 ×10-6),Nb-Ta-Ti anomalies relative to primitive mantle.Monzogranites and granodiorites have relatively high ISr values(0.710 ～ 0.740 and 0.710,respectively),low eNd(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 higher eHf(t) values(-3.00 ～ 0.86).Gabbros have relatively high ISr values(0.711 ～ 0.714),low eNd(t)(-3.44 ～-6.82) and more concentrated eHf(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 ± 3Ma 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-greywacke) which produced S-type cordierite-bearing granites.Meanwhile,they mixed with crust derived magmas and produced I-S transition type monzogranite and granodiorite.The Yuejinshan and other Early Devonian magmatisms in the 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 extensionin Early Devonian.Combining with regional studies,this paper suggests that Early Devonian is a critical period for the tectonic transformation of the Central orogenic belt(especially for Eastern Kunlun,North Qinling and Northern margin of Qaidam basin).During this period,the merging of terranes had finished and the tectonic framework begun to transfer from compression to extension.