Geochronology, geochemistry and tectonic significance of two Early Cretaceous A-type granites in the Gan-Hang Belt, Southeast China
Geochronology, geochemistry and tectonic significance of two Early Cretaceous A-type granites in the Gan-Hang Belt, Southeast China
复制标题
DOI：
10.1016/j.lithos.2012.01.028
复制
发表时间：
2012-10
期刊：
影响因子：
3.5
通讯作者：
Shuiyuan Yang;Shaoyong Jiang;K. Zhao;Yao-Hui Jiang;H. Ling;L. Luo
中科院分区：
文献类型：
--
作者：
Shuiyuan Yang;Shaoyong Jiang;K. Zhao;Yao-Hui Jiang;H. Ling;L. Luo
中国东南赣杭带中生代花岗岩的广泛赋存与同样广泛的、具有重要经济意义的成矿作用有关，但其确切的成矿时间、成因及其板块构造意义尚不清楚。通过锆石U-Pb年代学、主微量元素分析和Nd-Hf同位素分析，对浙西赣杭带东北部的杨梅湾花岗斑岩和大桥坞花岗斑岩进行了研究。LA-ICP-MS和SHRIMP U-Pb定年结果表明，这两块花岗岩的锆石年龄为133 ~ 136ma，代表了早白垩世岩浆事件。这些花岗质岩石具有明显的a型地球化学特征，具有较高的Na2O+K2O、Fe2O3*/MgO和Ga/Al比值。它们的CaO、MgO和tio2含量低，部分LILEs（如Rb和Th）和HFSEs（如Zr、Y）富集，Sr、Ba、P、Eu和Ti富集。它们也表现出a2亚型亲和，可能形成于高温（杨梅湾花岗岩~810℃，大桥乌花岗斑岩~850℃）。a型花岗质岩石总体εNd(t)值在−6.5 ~−3.6之间，锆石εHf(t)值在−7.8 ~−0.9之间，Nd和Hf同位素均具有中元古代tdma特征。地球化学和同位素资料表明，这些a型花岗质岩石主要是由中元古代变质基岩（包括参变质岩和正变质岩）的粒状部分熔融形成的，可能有幔源物质的输入，随后发生了分离结晶。新资料结合前人资料表明，干杭带白垩系a型花岗岩年龄在137Ma ~ 122Ma之间，是中国东南部中生代一次重要的伸展活动。该事件为古太平洋板块回滚引起的弧后伸展或弧内裂谷，早在~137Ma就开始在香山盆地和新陆盆地发生。研究区早于前人提出的~130Ma，晚于石行带南段。随着古太平洋板块板块回滚过程的持续伸展，地壳和岩石圈地幔逐渐变薄。软流圈的上涌可能导致地壳岩石部分融化，产生花岗质岩浆。新的地球化学资料还表明，干杭带a型花岗质岩浆形成过程中的幔壳相互作用在约137 ~约122Ma之间由早到晚逐渐增强。
The widespread occurrence of Mesozoic granites in the Gan-Hang Belt in Southeast China is associated with similarly widespread, economically important mineralization, but the precise timing, origin, and plate tectonic significance of these granites are not well understood. We have studied two of these (Early Cretaceous) granite bodies, the Yangmeiwan granite and the Daqiaowu granitic porphyry, from the northeastern part of the Gan-Hang Belt in western Zhejiang Province by zircon U-Pb geochronology, major and trace element analyses, and Nd–Hf isotopic analyses. LA-ICP-MS and SHRIMP U–Pb dating of zircon grains from these two granites yield ages of 133–136Ma, representing a Early Cretaceous magmatic event. These granitic rocks are metaluminous to weakly peraluminous and have a pronounced A-type geochemical signature with high Na2O+K2O, Fe2O3*/MgO and Ga/Al ratios. They show low CaO, MgO and TiO2contents, enrichment in some LILEs (such as Rb and Th) and HFSEs (such as Zr, Y), depletion in Sr, Ba, P, Eu and Ti. They also show A2subtype affinity and were probably formed at a high temperature (~810°C for Yangmeiwan granite and ~850°C for Daqiaowu granitic porphyry). These A-type granitic rocks show bulk rock εNd(t) values in the range of −6.5 to −3.6 and zircon εHf(t) values from −7.8 to −0.9, with Mesoproterozoic TDMcages for both Nd and Hf isotopes. Geochemical and isotopic data suggest that these A-type granitic rocks were generated largely by partial melting of granulitized Mesoproterozoic metamorphic basement rocks (including parametamorphic and orthometamorphic rocks), with a possible input of mantle-derived materials and followed by subsequent fractional crystallization. Our new data together with previous published data suggest that the Cretaceous A-type granitic rocks with ages between 137Ma and 122Ma occurred along the Gan-Hang Belt, indicating an important Mesozoic extensional event in Southeast China. This event represents either a back-arc extension or an intra-arc rift due to the roll-back of the paleo-Pacific plate, and it started as early as ~137Ma in Xiangshan and Xinlu basins. It is earlier than ~130Ma as previously suggested in the studied region and later than that along the south part of the Shi-Hang Zone. With ongoing extension during slab roll-back of paleo-Pacific plate, the crust and lithospheric mantle became progressively thinned. The upwelling of asthenosphere would have triggered crustal rocks to partially melt, generating granitic magmas. Our new geochemical data also suggest that mantle-crust interaction in the formation of the A-type granitic magma along the Gan-Hang Belt gradually intensified from early to late stages between ca. 137 and ca. 122Ma.