湘东北早白垩世桃花山--小墨山花岗岩体岩石地球化学特征及成因

Petrological and Geochemical Characteristics and Petrogenesis of the Early Cretaceous Taohuashan-Xiaomoshan Granites in Northeastern Hunan Province

出露于湖南东北部华容县附近的桃花山—小墨山岩体侵位于中元古代冷家溪群。通过锆石LA-ICP-MSU-Pb法测得两岩体侵位时代分别为129±1Ma和117±1Ma。桃花山主体岩性为(片麻状)二云母二长花岗岩,SiO2=71.75%~73.81%,K2O/Na2O=0.84~1.11,属弱过铝质高钾钙碱性系列;岩石明显富集大离子亲石元素,亏损高场强元素;Eu为中等负异常,∑REE较高,Rb/Sr=0.9~2.7,(La/Yb)N=26.92~86.02;高ISr(0.714~0.723),低εNd(-9.76^-10.6),高t2DM(1.72~1.79Ga)。小墨山黑云母二长花岗岩SiO2=69.64%~72.73%,K2O/Na2O=0.62~0.7,准铝质至弱过铝质,Rb/Sr=0.26~0.88,(La/Yb)N=11.97~12.67;低ISr(0.707~0.709),高εNd(-6.38^-6.73),低t2DM(1.43~1.46Ga)。综合分析表明,桃花山二云母二长花岗岩为壳源含白云母过铝花岗岩类(GPG),源岩为华南古元古代基底;小墨山黑云母二长花岗岩类似含堇青石及富黑云母过铝花岗岩类(CPG),源岩为低成熟度的变杂砂岩。桃花山、小墨山岩体形成于华南早白垩世伸展背景下的局部挤压增厚环境。江南断裂晚燕山期的逆冲推覆构造造成了华容地区的小范围地壳增厚,并为桃花山源岩的"湿"深熔作用提供了流体聚集通道;小墨山花岗岩的形成则与幔源岩浆的底侵有关,热的幔源岩浆不仅为地壳的部分熔融提供了热量,而且与熔融的壳源岩浆发生了混合作用。

The Taohuashan and Xiaomoshan granites occur in Huarong county, Northeastern Hunan Province, and intruded into the Mesoproterozoic Lengjiaxi Group. Using the method of single zircon LA-ICP-MS U-Pb dating, the emplacement ages of 129±1 Ma and 117 ± 1 Ma were yielded for these two plutons. The main rock type of Taohuashan pluton is （gneissie） two-mica monzonitic granite, with SiO2 = 71.75-73.81%, K2O/Na2O = 0.84-1.11,and belonging to weak peraluminous highK calc-alkaline series. It is obvious that the Taohuashan rocks are enriched in LILE and depleted in HFSE; with moderately negatively Eu anomalies, relatively high ∑REE contents, Rb/Sr = 0.9-2.7, （ La/Yb）N = 26.92-86.02, high ISr （0.714-0.723）, low and （--9.76 ～ --10.6 ）, and high t2DM （1.72-1.79Ga）. The rock type of Xiaomoshan pluton is biotite monzonitic granite. SiO2= 69.64-72.73%, K2O/Na2O= 0.62～0.7, metaluminous to weakly peraluminous, Rb/Sr = 0.26-0.88, （ La/Yb）N = 11.97-12.67, low Isr （0.707--0.709）,high and （-6.38～--6.73）, and low t2DM （1.43-1.46 Ga ）. The analysis shows that the Taohuashan two mica monzonitic granite is crustal-derived CPG. Its source rocks are the Palaeo-Proterozoic basement of south China. The Xiaomoshan biotite monzonitic granite is similar with CPG type, with low maturity metagreywackes as its source rocks. The Taohuashan and Xiaomoshan plutons were formed in a local compressional environment under the extensional geological setting of South China. The thrust of Jiangnan Fault caused a small scope of crust thickening in Huarong region, and provided a fluid accumulation channel for the wet anatexis of Taohuashan source rocks. The formation of Xiaomoshan pluton is related to magmatie underplating. The hot mantle magma not only provided heat for the partial melting of crust, but also caused magma mixing with the crustalderived melts.