大别山中生代中酸性岩浆岩锆石U-Pb定年、元素和氧同位素地球化学研究  

Zircon U-Pb age, element and oxygen isotope geochemistry of Mesozoic intermediate-felsic rocks in the Dabie Mountains.

作　　者：赵子福[1] 郑永飞[1] 魏春生[1] 吴元保[1] 

机构地区：[1]中国科学技术大学地球和空间科学学院,合肥230026

出　　处：《岩石学报》2004年第5期1151-1174,共24页Acta Petrologica Sinica

基　　金：国家自然科学基金(40303002和40334036);中国科学院知识创新工程项目(KZCX2-107);中国科学技术大学青年基金

摘　　要：对大别山中生代主簿源、天柱山和团岭中酸性岩浆岩进行了锆石U-Pb定年、全岩主量和微量元素分析以及全岩和单矿物的氧同位素分析。结果表明,这些富钾的中酸性侵入岩表现出明显的轻稀土富集和高场强元素(Nb、P和Ti)负异常,与围岩片麻岩之间具有类似的微量元素分布特征和初始Sr-Nd-Ph同位素组成。锆石U-Pb年龄指示了早白垩世(121～131Ma)的岩浆结晶年龄。通过CL照相和SHRIMP定年在某些锆石颗粒中发现了老的继承核,年龄分别为742～815Ma和222Ma。这些新元古代和三叠纪核年龄分别与大别-苏鲁造山带超高压变火成岩原岩年龄和超高压变质年龄一致。尽管石英和锆石具有较小的δ^(18)O值变化范围(石英:6.30‰～8.66‰,锆石:4.14‰～6.11‰),全岩和其它单矿物氧同位素比值变化较大(全岩:0.07‰～7.13‰,钾长石:0.55‰～7.40‰,斜长石:-4.88‰～6.96‰),大多数锆石具有与正常地幔锆石(5.3±0.3‰)一致的δ^(18)O值。大多数样品的石英-锆石之间保存了氧同位素平衡分馏,而其它矿物(如,钾长石、斜长石、黑云母和角闪石)与锆石之间则大多表现出明显的氧同位素不平衡分馏,指示它们受到了岩浆期后亚固相水-岩相互作用的扰动。元素和同位素特征表明,大别山中生代中性岩起源于加厚基性下地壳的脱水部分熔融,在岩浆侵位过程中伴随有结晶分异作用;而花岗岩则起源于与北大别TTG正片麻岩具有相似化学组成的中性地壳的部分熔融。因此,大别山中生代中酸性岩岩浆源区是三叠纪扬子陆块俯冲产生的加厚地壳,形成机制可能与早白垩世地幔超柱事件热扰动所引起的部分熔融有关。Zircon SHRIMP U-Pb ages, major and trace elements of whole-rock, oxygen isotope composition of mineral separates were determined for post-collisional intermediate-felsic rocks at Zhubuyuan, Tianzhushan and Tuanling in the Dabie Mountains. The results show that these intermediate-felsic rocks are characterized by strong LREE enrichment and negative HFSE anomalies (Nb, Ti and P), resembling surrounding gneisses in both trace element and Nd-Sr-Pb isotope compositions. Zircon U-Pb dating yields ages of 121 to 131 Ma for magma crystallization. Inherited cores were observed by CL images and SHRIMP dating in some zircon grains, which yield older ages of 742 to 815 Ma and 222 Ma, respectively, in agreement with protolith ages of UHP metaigneous rocks and UHP metamorphic event in the Dabie-Sulu orogen. Although quartz and zircon have small variation in delta(18)O values ( quartz: 6.30parts per thousand to 8.66parts per thousand, zircon: 4.14parts per thousand to 6.11parts per thousand), there is a large variation in oxygen isotope ratios of the other minerals and their host rocks (whole-rock: 0.07parts per thousand to 7.13parts per thousand, K-feldspar: 0.55parts per thousand to 7.71parts per thousand, plagioclase: -4.88parts per thousand to 6.96parts per thousand). Most of the zircons have delta(18)O values similar to the normal mantle zircon (5.3 +/- 0.3parts per thousand). Equilibrium fractionations of oxygen isotopes between quartz and zircon have been preserved for most of the samples; the other minerals such as K-feldspar, plagioclase, biotite and amphibole show obvious disequilibria when paired with zircon, indicating alteration by post-magmatic subsolidus water-rock interaction. The element and oxygen isotope results suggest that the Early Cretaceous intermediate rocks were generated by dehydration melting of mafic rocks in the thickened lower crust, coupled with fractional crystallization during magma emplacement; the granites were derived from partial melting of middle crust which is similar in chemical composit

关 键 词：中酸性岩 氧同位素 锆石 U-PB 中生代 大别山 

分 类 号：P588.121[天文地球—岩石学] P597.2[天文地球—地质学]