东秦岭二郎坪地区西庄河花岗岩体的成因及形成机制

【作者】 郭彩莲；

【导师】 陈丹玲；

【作者基本信息】 西北大学 ， 矿物学、岩石学、矿床学， 2010， 硕士

【摘要】 出露于东秦岭二郎坪地区的西庄河花岗岩一直被认为是与二郎坪弧后盆地蛇绿岩有关的幔源型花岗岩,并利用其Pb-Pb蒸发法定年结果来限定二郎坪蛇绿岩的形成时代。本文在详细的野外地质调研基础上,通过对该花岗岩以及新近在其中发现的暗色包体系统的岩相学、地球化学、年代学及锆石Hf同位素研究,在以下方面取得重要进展：1、详细的岩相学研究,在暗色包体中发现大量反映岩浆混合作用的显微结构证据,如：(1)寄主花岗岩呈粗粒花岗质结构,但暗色包体为斑状-似斑状结构或辉绿结构；(2)包体中发育反映岩浆快速冷凝的针状磷灰石；(3)包体中的斜长石呈明显的成分环带结构；(4)包体中存在由黑云母、角闪石等暗色矿物环绕眼球状石英斑晶所构成的眼斑结构；(5)包体中的斜长石斑晶骑跨寄主花岗岩和包体的边界,等等。2、地球化学研究显示,寄主花岗岩具高硅(SiO2=66.84%～71.57)、富碱(K2O+Na2O=6.08～7.41%)的特点,同时显示高Sr(327.04×10-6～613.97×10-6)、低Y(12.30×10-6～17.17×10-6)和Yb(1.36×10-6～1.88×10-6)及高Sr／Y(介于24～38)比值的特征；岩石的稀土总量中等,轻、重稀土分异强烈((La／Yb)N=12.35～23.28),Eu呈弱负异常-明显的正异常(δEu介于0.97～1.34)。在微量元素的原始地幔标准化蛛网图上,明显富集Rb、Ba、Th、U等大离子亲石元素和轻稀土,相对亏损Nb、Ta、Ti等高场强元素和重稀土,显示出与大洋俯冲有关的O型埃达克岩的地球化学特征。暗色包体Si02含量较低(53.38%～56.39%),同时显示高铝(Al2O3=14.11%～16.78%)、高镁(MgO=3.24%～3.9%,Mg#值=45.1～51.0)和高碱(K20+Na2O=5.92%～7.43%)的特征,属于钾玄岩-高钾钙碱性系列岩石。包体的稀土总量较高(∑REE=120.91×10-6～220.87×10-6),具轻稀土富集型的稀土配分模式,具有中等-弱的负铕异常(δEu介于0.65～0.86)。在原始地幔标准化蛛网图也表现出富集Rb、Ba、Th、U等大离子亲石元素,相对亏损Nb、Ta、Ti等高场强元素,总体显示出岛弧火山岩的地球化学特征。与寄主花岗岩相比,暗色包体具有明显高于寄主花岗岩的稀土含量和明显低的轻重稀土分异程度排出了暗色包体为岩浆早期结晶集合体的可能性；在微量元素原始地幔标准化蛛网图上,包体和花岗岩显示出相似的配分曲线特征,而且在全岩组分的哈克图解及其它主微量多元素协变图解上,寄主花岗岩及暗色包体均显示线性相关的特点,说明两者岩石成分的变异与岩浆混合作用有关。3、CL图像和原位微量元素分析结果表明,花岗岩和暗色包体中的锆石皆为典型岩浆结晶锆石的特征。LA-ICP-MS锆石原位U-Pb定年结果得到寄主花岗岩和暗色包体的结晶年龄分别为460±0.9 Ma和461±1.4 Ma,两者在误差范围内完全一致。4、锆石Hf同位素研究揭示寄主花岗岩与暗色包体存在明显差异,其中,花岗岩锆石的εHf(t)值变化于8.37～11.13之间,包体的锆石εHf(t)值变化于11.65～15.16之间,指示两者来自不同的岩浆源区。5、综合岩相学、地球化学、年代学和锆石Hf同位素研究结果,论证指出东秦岭二郎坪地区的西庄河花岗岩形成于岛弧构造环境,是早古生代俯冲洋壳熔融的中酸性岩浆与楔形地幔部分熔融来源的基性岩浆混合的产物,其中的暗色包体可能是岩浆混合过程中基性岩浆的残余。更多还原

【Abstract】 The Xizhuanghe granite locates in Erlangping area of Xixia County, western of Henan Provinces. It has been considered as mantle-derived granite which was related to the Erlangping Ophiolite. And zircon Pb-Pb evaporation dating of this granite had been employed to constrain the formation age of the Erlangping Ophiolite. Based on detailed field investigation, the thesis focuses its studies on petrography, geochemistry, geochronology and zircon Hf isotope of the Xizhuanghe granite and mafic enclaves within it. The main achieves of the thesis can sum up in the following aspects:1. Petrographic studies found abundant microstructure of magmatic mixing within the mafic enclaves, for instance:(1) the enclaves show porphyritic-porphyaceous texture or diabasic texture while the host granite shows coarse-grained granitic texture; (2) acicular prismatic apatites occurring in the enclaves reflecting rapid cooling of magma; (3) plagioclase phenocrysts from the enclaves display composition zoning; (4) quartz phenocrysts within the enclaves are surrounded by dark colored minerals like biotite and amphibole; (5) plagioclase phenocrysts lie on the boundary of granite and the mafic enclaves.2. Geochemical studies indicate that the host granite has high SiO2 (66.84%～71.57), alkali (K2O+Na2O=6.08～7.41%), Sr (327.04×10-6～613.97×10-6) contents, low Y (12.30×10-6～17.17×10-6) and Yb (1.36×10-6～1.88×10-6) contents, with high Sr/Y ratios (24～38). LREE and HREE of the granite are markedly fractionated.δEu shows slightly negative-obviously positive anomaly (8Eu=0.97-1.34). Primitive mantle normalized trace element spider pattern shows obvious enrichment in LILE and LREE such as Rb, Ba, Th, U, etc. and relative depletion in HSFE and HREE such as Nb, Ta, Ti, etc., displaying geochemical features of O-type adakites which are related to subduction of oceanic slab.Mafic enclaves have relative lower SiO2 (53.38%-56.39%) but high Al2O3 (14.11%～16.78%), MgO(MgO=3.24%～3.9% and Mg number is (45.1～51.0) and alkali (K2O+Na2O=5.92%-7.43%) contents, belong to the shoshonite-high-K calc-alkalic series. The enclaves have high∑REE (120.91×10-6～220.87×10-6) abundance and LREE-enriched Chondrite-normalized REE patterns with moderate-slightly negative anomaly (δEu=0.65-0.86). Primitive mantle normalized trace element spider pattern shows obvious enrichment in LILE and LREE such as Rb, Ba, Th, U, etc. and relative depletion in HSFE and HREE such as Nb, Ta, Ti, etc., displaying geochemical features of island-arc volcanics.Comparing with the host granite, the mafic enclaves show obviously high REE contents but lower LREE-HREE fractionating, excluding the possibility of mafic enclaves resulted from crystal fractionation. Meanwhile, the granite and the mafic enclaves show similar variation trend of trace elemental compositions on the primitive mantle normalized spider patterns and linear correlation on the covariogram of Harker and other major and trace elements, indicating the magmatic mixing.3. CL images and trace element analysis show that zircons from the host granite and mafic enclaves show the characteristics of magma crystallization. In situ LA-ICP-MS zircon U-Pb dating yield the crystallization ages of 460±0.9 Ma for the host granite and 461±1.4 Ma for the enclaves.4. Zircon Hf isotopic compositions of the host granite are different from those of the mafic enclaves. ZirconεHf(t) values range from 8.37-11.13 for host granite and 11.65-15.16 for enclaves.5. Studies of petrography, geochemical, geochronology and zircon Hf isotopes show that, the Xizhuanghe granite which located in Erlangping area of the western Henan Province was formed in island arc setting. Mixing of the medium-acid magma produced by partial melting of the early Paleozoic subducting oceanic slab with the basaltic magma derived from partial melting of the mantle wedge resulted in the formation of the Xizhuanghe granite. The mafic enclaves within the granitic pluton may be the enclosed basaltic magma during the magmatic mixing process.更多还原