粤西连阳复式岩体的地球化学特征及其成因研究

Geochemistry and petrogenesis of Lianyang granite composite,west Guangdong province.

粤西连阳复式岩体由燕山早期连阳花岗岩岩体(锆石 U-Pb 年龄为144.3±0.8Ma)和燕山晚期白浆花岗岩岩体(锆石 U-Pb 年龄为106.4±0.7Ma)组成。这两个岩体在岩石化学上均富硅,属准铝质,并相对富钾。在微量元素组成上,连阳岩体和白浆岩体都强烈亏损 Ba、Sr 和 Ti,轻微亏损 Nb 和 Ta。两岩体的稀土元素特征类似,总量比较高,分别为187.6～300.6×10^(-6)和185.5～294.3×10^(-6),轻稀土富集,LREE/HREE 分别为4.17～9.28和4.13～7.08,(La/Yb)_N 分别为3.66～9.03和3.29～5.16,Eu 明显亏损,δEu 值分别为0.04～0.53和0.04～0.16。连阳岩体的ε_(Nd)(t)为-9.1～-7.6,白浆岩体的ε_(Nd)(t)为-9.8～-7.8,两者具有相似的 Nd 同位素组成。钕模式年龄分别为1.56～1.68Ga 和1.54～1.71Ga。研究表明,连阳复式岩体燕山期花岗岩形成于地壳剪切-拉张的构造背景下,是中元古地壳部分熔融的产物。

Zircon U-Pb dating of the Yanshanian Lianyang composite granites yields ages of 144.3 ± 0.8Ma （ the Lianyang granite） and 106.4±0.7Ma （the Baijiang granite） , which suggests that the Lianyang granite intruded during the early Yanshanian period and the Baijiang granite intruded during the later Yanshanian period. The Yanshanian Lianyang composite granites is characterized by metaluminous, high SiO2 and K20 in main elements. It is highly depleted in Ba, Sr, Ti, but slightly depleted in Nb and Ta. The Lianyang and Baijiang granites show similar REE characteristics. They have high concentrations of REE （ 187.60 - 300.59 × 10^-6 and 185.49 -294.27 × 10^-6） , respectively. They are enriched in LREE relative to HREE （LREE/HREE） ratios = 4.17 -9.28 and 4. 13 - 7.08 and （La/Yb） N = 3.66 - 9.03 and 3.29 - 5.16, respectively. Both of the granites show prominent negative Eu anomaly with 8Eu value of 0.04 - 0.53 and 0.04 - 0.16, respectively. Isotope geochemistry study reveals moderate end （t） values of the Lianyang granite （ -9.1- -7.6） and the Baijiang granite （ -9.8 - -7.8）. They have low Nd model ages （1.54 - 1.71Ga） relative to other granites in southern China. All these features suggest that the Lianyang granite composite was derived from partial melting of the Mesoproterozioc continental crust under a tectonic setting of crust shear-extension system during the Yanshanian period.