湘中紫云山岩体的成因:锆石U-Pb年代学、元素地球化学及Hf-O同位素制约

Petrogenesis of the Ziyunshan pluton in central Hunan,South China:Constraints from zircon U-Pb dating,element geochemistry and Hf-O isotopes

位于湘中盆地周缘的紫云山岩体主要由似斑状石英二长岩(主体)和二云母花岗岩(补体)组成,前者分布于该岩体的周边,后者分布于其内部,岩体内可见有大量岩浆结构的镁铁质暗色包体。利用高精度SIMS锆石U-Pb定年方法得到主体岩体的年龄为225.2±1.7Ma和225.6±1.4Ma,补体岩体的年龄为227.0±2.2Ma,两者均形成于印支晚期,基本上是同时形成的。该区两类岩石均具有富SiO_2、Na_2O和K_2O,贫Ca O、Mg O和Al2O3的特征,A/CNK比值为0.85~1.05,固结指数较高,主体岩石为3.61~5.05,补体岩石为4.13~14.06;其微量元素均表现出富集Rb、U、La、Nd和Zr,亏损Ba、Nb、Ta、Sr、P和Ti的特征;稀土元素配分模式均呈明显的右倾"V"字型,轻重稀土分馏明显((La/Yb)N=7.35~11.7),Eu负异常较显著(δEu=0.32~0.70);该区花岗岩的主体和补体的锆石Hf-O同位素组成非常相似,εHf(t)值为-10.0^-1.6,δ18O值为7.8~11.4,两阶段模式年龄tDM2为1.22~1.79Ga;各类岩石主要氧化物之间表现出良好的线性协变关系,且明显沿I型花岗岩演化趋势线分布,反映主体和补体岩石具有密切的亲缘关系,均应归属于高分异的I型花岗岩。各类岩石样品沿岩浆混合趋势线分布、而远离结晶分异趋势线,结合Hf-O同位素分析,认为该岩体来源于扬子地块中元古代下地壳变质杂砂岩重熔,与部分幔源岩浆形成的壳幔混合岩浆源区。该岩体形成于碰撞后构造背景,暗示华南地块受太平洋板块及印支地块的双重影响,在印支晚期处于伸展-减薄的构造环境。通过与华南地块其它印支期花岗岩对比分析,提出扬子地块与华夏地块的拼合带在湖南境内应沿"攸县-双牌"一线展布。

The Ziyunshan pluton, located along the Xiangzhong basin in Hunan, consists mainly of porphyroid monzonitic granite and two-mica granite, the former is located in the surrounding of the pluton and constitutes its main intrusive granite, while the later occurs in its interior and belongs to late intrusive granite. The dark-colored marie enclave with magmatic textures is usually observed in the intrusive bodies. The emplacement time of the Ziyunshan pluton is determined by high-precision SIMS zircon U-Pb dating. The main intrusive granite is dated at 225.2 ± 1.7Ma and 225.6 ± 1.4Ma and the late intrusive granite is dated at 227. 0 ± 2. 2Ma, thus both granitic rocks were basically contemporary and emplaced during the Late Indosinian. Both two types of granite in this study are rich in SiO2 , Na20 and K20, and depleted in CaO, MgO and A12O3 , and its A/CNK index are 0. 85 - 1.05 with high consolidation index, 3.61 -5.05 for the main intrusive ganite and 4. 13 - 14. 06 for the late intrusive granite. Both granitic rocks are characterized by Rb, U, La, Nd, Zr enrichment, and Ba, Nb, Ta, Sr, P, Ti depletion; the chondrite-normalized distribution pattern of REEs is characterized by ＂V＂ -shaped pattern, obvious LREE-enrichment （ （La/Yb） N = 7.35 - 11.7 ） and negative Eu anomaly （ 8Eu = 0. 32- O. 70）. The zircon isotopes of both granites display similar signatures, the εHf （ t ） values of the Ziyunshan pluton display similar signatures - 10. 0 - - 1.6, and its δ18O values fall in the range of 7.8 - 11.4. The the model ages are estimated at 1.22 - 1.79Ga. The major oxides for all granite samples display good eovariant relationships, and are obviously distributed along I-type granite evolution line, indicating threr are a intrinsic genetic relationship between the main intrusive granite and the late intrusive granite, both are of highly-differentiated I-type granite. All samples are distributed along the magmatic taxing trend line but away from the crystallization differentiation trend line, combined with the Hf-O isotope resluts, it is shown that the Ziyunshan pluton was derived from the re-melting of Lower Proterozoic crustal meta-greywacke of Yangtze craton, and mixed with some mantle-sourced magma. The intrusive granite was formed under post-collision tectonic background, indicating South China block was subjected to the influences by both Pacific plate and Indosinian block under extension-thinning tectonic environment in the Late Indosinian. Compared with other Indosinian granites in South China block, it is considered that the suture region of Yangtze block and Cathaysia block lies along ＂Youxian-Shuangpai＂ region in Hunan.