Neoproterozoic tonalite and trondhjemite in the Huangling complex, South China: Crustal growth and reworking in a continental arc environment
Neoproterozoic tonalite and trondhjemite in the Huangling complex, South China: Crustal growth and reworking in a continental arc environment
复制标题
DOI：
10.2475/06.2013.02
复制
发表时间：
2013-06
影响因子：
2.9
通讯作者：
J. Zhao;Mei‐Fu Zhou;Jianping Zheng;W. Griffin
中科院分区：
文献类型：
--
作者：
J. Zhao;Mei‐Fu Zhou;Jianping Zheng;W. Griffin
南中国扬子地块中部黄陵火成杂岩由∼850 Ma英云闪长岩和奥长花岗岩组成，受∼820 Ma花岗岩侵位。英云母主要由斜长石和石英组成，含有丰富的角闪石和黑云母，并含有细粒到粗粒的富角闪石的火成岩包体。与太古宙英云闪长岩不同，它们具有不同程度的低SiO_2(56.4~67.5 wt%)和不同程度的Sr(230~517ppm)，高MgO(1.47~4.02wt%)和高Y(8.87~15.7ppm)，低Sr/Y(17~43)。其全岩εNd值为−8.3~−10.3，锆石εHf值为−3.9~−12.6，与空间伴生的新元古代镁铁质岩相似。英云闪长岩的形成经历了俯冲变质岩石圈地幔的部分熔融、中下地壳含水玄武岩岩浆5%~20%的地壳混染和角闪石+斜长石+黑云母±磁铁矿的分离结晶三个阶段。辉绿岩由斜长石、石英、碱性长石和少量黑云母、角闪石组成。它们具有高SiO_2(67.3~75.6 wt%)和高Sr(340~840 ppm)，低MgO(0.09~0.78 wt%)和Y(4.8~11.2ppm)，高Sr/Y(32~162)，类似于太古代奥长花岗岩和镁铁质岩石高压部分熔体。这些岩石具有极负的εNd值(−18.7~−19.9)和锆石εHf值(−18.9~−49.2)，两阶段Hf同位素模式年龄为2.46~3.90Ga，表明它们是由可能广泛分布于扬子地块以下的太古代角闪岩和麻粒岩熔融形成的。它们的化学成分也是斜长石+角闪石+黑云母分离结晶的结果。因此，英云闪长岩和奥长花岗岩的组合不同于太古代TTG系列。这种差异可能反映了不同的地温梯度。然而，利用黄陵杂岩的英云闪长岩和奥长花岗岩样品进行的模拟表明，两种岩浆的混合可以产生典型的TTG亲和系列，这表明某些TTG系列的起源可能既涉及壳源熔体，也涉及地幔熔体。黄陵火成杂岩的形成与新元古代扬子地块北缘之下的板片俯冲作用有关，类似于安第斯山脉的新生代弧形岩浆作用。因此，交代的含水地幔在弧形背景下熔融可以产生镁铁质岩浆，这些基性岩浆不仅在底板之下促进了大陆地壳的生长，而且还导致了增厚的古壳和新形成的下基性壳的熔融。
The Huangling igneous complex in the central Yangtze Block of South China is composed of ∼850 Ma tonalite and trondhjemite, intruded by ∼820 Ma granite. Tonalites mainly consist of plagioclase and quartz with abundant hornblende and biotite, and contain fine- to coarse-grained hornblende-rich igneous enclaves. They have variably low SiO2 (56.4-67.5 wt%) and Sr (230-517 ppm), and high MgO (1.47-4.02 wt%) and high Y (8.87-15.7 ppm) with low Sr/Y (17-43), unlike Archean tonalites. Their whole-rock εNd values range from −8.3 to −10.3 and zircon εHf values from −3.9 to −12.6, similar to those of spatially associated Neoproterozoic mafic rocks. The tonalites are thus proposed to have been produced by three stages, involving partial melting of subduction-modified lithospheric mantle, 5 to 20 percent crustal contamination of hydrous basaltic magmas in the middle to lower crust, and fractional crystallization of amphibole + plagioclase + biotite ± magnetite. Trondhjemites are composed of plagioclase, quartz, and alkali feldspar with minor biotite and hornblende. They have high SiO2 (67.3-75.6 wt%) and Sr (340-840 ppm), and low MgO (0.09-0.78 wt%) and Y (4.8-11.2 ppm) with high Sr/Y (32-162), similar to Archean trondhjemites and high-pressure partial melts of mafic rocks. These rocks have extremely negative εNd (−18.7 to −19.9) and zircon εHf values (−18.9 to −49.2) with two-stage Hf-isotope model ages of 2.46 to 3.90 Ga, indicating their formation in the lower crust by melting of Archean amphibolites and granulites which were probably widely distributed beneath the Yangtze Block. Their highly variable chemical compositions also resulted from fractional crystallization of plagioclase + hornblende + biotite. Thus, the association of the tonalite and trondhjemite is different from the Archean TTG series. This difference may reflect different geothermal gradients. However, modeling using tonalitic and trondhjemitic samples of the Huangling complex suggests that mixing of the two magmas can generate a typical TTG-affinity series, suggesting that the origin of some TTG series may have involved both crustally-derived and mantle-derived melts. The generation of the Huangling igneous complex was linked to slab subduction beneath the northern margin of the Yangtze Block during the Neoproterozoic, similar to the Cenozoic arc magmatism in the Andes. Therefore, melting of metasomatized, hydrous mantle in arc settings can generate mafic magmas that not only under-plated and contributed to continental crustal growth, but also caused melting of thickened ancient crust and newly formed lower mafic crust.