闽西南古田-小陶花岗质杂岩体的形成时代和成因

Formation age and origin of the Gutian-Xiaotao granitic complex in the southweslern Fujian province, China

古田-小陶花岗质杂岩体是由小陶、古田、华家、蛟洋和姑田等单元组成。小陶、古田和华家单元为含黑云母二长花岗岩,蛟洋和姑田单元是含角闪石黑云母石英二长岩。小陶和古田花岗岩以高 SiO_2(>71.77%)、K_2O(>4.67%)和低 CaO(<1.71%)为特征,属于弱过铝-强过铝花岗岩;它们含高的 Rb、Nh、Ta、Th、U 和低的 Sr、Ba 含量;稀土(REE)含量和轻重稀土分异((La/Yb)_N)变化大,分别变化于143×10^(-6)～548×10^(-6),和3.3～26.5,Eu 亏损中等到强。姑田和蛟洋单元都以低SiO_2(62.88%～66.38%)和高 Al_2O_3(15.44%～16.51%)为特征。姑田单元的 REE 总量低而蛟洋单元的 REE 总量高,但两者的 Eu 负异常都不明显。华家单元花岗岩的地球化学特征介于上述两组花岗岩之间。LA-ICPMS 锆石 U-Pb 定年显示小陶花岗岩形成于222±3Ma,而古田花岗岩形成于161±1Ma。小陶、古田和姑田花岗岩具有相似的^(87)Sr/^(86)Sr 初始比值(0.7089～0.7099)和ε_(Nd)(t)值(-11.1～-11.5),而且古田和小陶花岗岩的锆石 Hf 同位素组成也相似,表明它们起源于相同的源区。地球化学和 Sr-Nd-Hf 同位素指示它们的源区物质是具有1.73～1.5Ga 平均地壳存留年龄的变质沉积岩。而位于杂岩体南部的华家花岗岩显示明显高的^(87)Sr/^(86)Sr 初始比值(0.7310)和更低的ε_(Nd)(t)值(-16.4),指示它的母岩浆起源于古元古代壳源物质,暗示杂岩体南北基底组成可能不同。古田花岗岩形成于燕山早期的板内伸展构造背景,而小陶花岗岩形成于印支晚期,地球化学特征以及同时代岩石组合以及构造应力场分析表明其最可能形成于后造山构造背景。

The Gutian-Xiaotao granitic Complex is composed of the Xiaotao, Gutian, Huajia, Jiaoyang and Gutian units. The Xiaotao, Gutian and Huajia units are biotite-bearing monzogranite, and the Jiaoyang and Gutian units are hornblende-bearing biotite quartz-monzonite. The Xiaotao and Gutian granites are characterized by high SiO2 （ 〉 71.77% ） , K2O （ 〉 4. 67% ） and low CaO （ 〈 1.71% ） , belonging to weakly to strongly peraluminous. They have high Rb, Nb, Ta, Th, U contents and low Sr, Ba. Their REE concentrations and differentiation extents are variable with middle to strong Eu depletion （ Eu/Eu^＊ = 0. 52 - 0. 08 ）. The Jiaoyang and Gutian quartz monzonites have low SiO2 （62. 88% - 66. 38% ） and high Al2O3 contents （ 15.44% - 16.51% ）. The former has high REE content and later low REE contents, both have weak Eu depletion （ 0. 65 - 0. 80 ） and high [a/Yb ratios. The Huajia granites have geochemical characteristics ranging between above-mentioned two suites of granites. LA-ICPMS zircon U-Pb dating indicates that the Xiaotao granite formed at 222± 3Ma and Gutian granite at 161 ±Ma. The Xiaotao and Gutian granites and Gutian monzonite have similar initial ^87 Sr/^86 Sr ratios （0. 7089 -0. 7099） and εNd （t）values （ - 11. 1 - - 11.5 ） , and zircons from the Xiaotao and Gutian granites show similar Hf isotopic compositions, suggesting that they originated from same source, although they were generated at different time. Geochemistry and Sr-Nd-Hf isotopes indicate that that the source consists of metasedimentary rocks with 1.7 - 1.5Ga model age. The Huajia granite in the southern Complex has marked high initial ^87 Sr/^86 Sr ratio （0. 7310） and lower εNd （t） values （ - 16. 4）, indicating that its parental magma derived from Paleoproterozoic crustal materials, which implies different basements under the northern and southern parts of the Complex. The Gutian granite formed in early Yanshanian and the Xiaotao granite formed in late Indosinian. Geochemical features, rock assemblage and structure stress analyses indicate that early Yanshanian granitic magmas generated in the extensional tectonic setting within plate, and late Indosinian ones in post-orogeny tectonic setting.