桂南钦防构造带西南段印支早期花岗岩的成因:年代学和地球化学约束

Petrogenesis of Early Indosinian Granites from the Southwestern Segment of Qinfang Tectonic Belt,Southern Guangxi:Constraints from Zircon U-Pb Chronology and Geochemistry

对桂南钦防构造带西南段印支早期花岗岩的地质调查发现,该区存在含变质矿物和不合变质矿物两类花岗岩。岩石学、地球化学和Sr-Nd同位素的分析结果表明:这些花岗岩均属于高钾钙碱性系列岩石,在岩石化学成分上明显富集大离子亲石元素(LILE,包括Rb、Th、U、K和Pb等),而相对亏损高场强元素(HFSE,包括Nb、Ta、P和Ti等),并具有较高的锶同位素初始比值((^(87)Sr/^(86)Sr)_i=0.715 250~0.716 316)和较低的ε_(Nd)(t)值(-10.80^-9.92),反映其具有俯冲消减作用形成的岛弧岩浆岩地球化学特征。锆石SHRIMP U-Pb定年结果显示,含紫苏辉石花岗斑岩和黑云母二长花岗岩的锆石^(206)Pb/^(238)U加权平均年龄分别为(248.8±1.9)Ma(MSWD=1.5)和(246.0±3.4)Ma(MSWD=2.2)。结合本区存在有早古生代MORB型变质基性火山岩和二叠纪弧后扩张中心环境形成的E-MORB型玄武岩的资料,认为扬子板块和华夏板块结合带(称之为钦—杭结合带)西南段有古生代洋盆的存在。本区印支早期花岗岩很可能是在扬子板块和华夏板块之间的洋壳岩石圈向北俯冲的地球动力学背景下,幔源岩浆诱发俯冲板片中古老地壳物质重熔并受到地幔源区岩浆不同比例的交代作用的产物。

Based on geological survey of the Early Indosinian granites in southern Guangxi, two types of granites are recognized in the southwestern segment of the Qinfang tectonic belt, namely, metamorphic minerals-bearing granite and not metamorphic minerals-bearing granite. The petrological,geochemical and Sr- Nd isotope data reveal that these granites chemically belong to high K ealc-alkaline series, and are characterized by enrichment in large ion lithophile elements （LILEs, e. g. , Rb, Th, U, K and Pb） and relative depletion in high field strength elements （HFSEs, e. g. , Nb, Ta, P and Ti）. They have high values of （^87Sr/^86Sr）i=0. 715 25 -0. 716 32 and low values of εNd（t）=-10. 80-9. 92. Such signatures display geochemical characteristics of typical subduction-related arc magmatic rocks. Zircon SHRIMP U- Pb dating shows that the ^206pb/^238 U weighted average ages of zircons from hypersthene-bearing granitic porphyry and biotite monzogranite are （248.8±1.9） Ma （MSWD = 1.5） and （246.0±3.4） Ma （MSWD=2. 2）, respectively. Together with the Early Paleozoic MORB-type metabasic volcanic rocks and Permian E-MORB type basalts formed in a back-arc spreading center setting in the region, we conclude that a Paleozoic oceanic basin could occur within the southwestern segment of the joint belt between the Yangtze and Cathaysian blocks （so-called Qinzhou-Hangzhou joint belt）, and that the Early Indosinian granites in the region were most likely generated in the setting associated with northward subduction of the oceanic plate between the Yangtze and Cathaysian blocks, where the mantle-derived magma caused the remelting of the ancient crust within subducted slab, and the melts was subjected to modification of the mantle-derived magma in different proportions.