皖南新元古代花岗闪长岩体锆石U-Pb定年以及元素和氧同位素地球化学研究

Zircon U-Pb age, element and oxygen isotope geochemisty of Neoproterozoic granodiorites in South Anhui.

对皖南新元古代花岗闪长岩样品进行了锆石LA-ICPMS微区U-Pb定年、全岩主微量元素分析和Sr-Nd同位素分析以及矿物氧同位素分析。结果表明,皖南花岗闪长岩中存在两个时代的岩浆锆石,在阴极发光结构图像上没有明显区别, 对应的U-Pb年龄分别为821±7Ma和881±9Ma,其中较老年龄的分析点多在锆石核部。这些岩石以富Al2O3为特点,为典型的过铝花岗岩;其全岩δ18O值高达11．1-13．6‰,显示了突出的壳源S型花岗岩的特点。但是,这些岩石具有与地幔相似的 Nd-Sr同位素组成,εNd(t)值为-2．06-0．02,初始87Sr／86Sr比值是0．7033-0．7087,指示其岩浆源区含有显著的初生地壳组分, 类似于Ⅰ型花岗岩的特点。石英、锆石、石榴石等氧同位素封闭温度高的矿物保存了岩浆结晶时的δ18O值,而钾长石、斜长石等矿物在岩浆冷却中经受不同程度地中低温热液蚀变。根据这些岩石的同位素年代学和地球化学特征,我们将U-Pb年龄 881±9Ma左右的锆石解释为继承来源,而年龄821±7Ma的锆石解释为同时代岩浆成因。因此在约900-880Ma,在扬子板块东南缘的皖南地区出现过大规模的幔源岩浆活动。这些新生岩浆岩很快遭受风化沉积作用后,形成低成熟度的沉积岩。大约820Ma热事件(地幔超柱活动?)使岩石圈地幔及其上覆地壳加热,导致拉伸加厚地壳内部的富水沉积岩重熔形成典型S型花岗岩。在花岗岩浆冷却过程中,出现的是中低温热液蚀变而不是高温超固相热液蚀变,指示皖南花岗闪长岩体形成于大陆边缘前裂谷期而不是裂谷活动高峰期。由于前裂谷期阶段没有软流圈地幔物质的直接加入,只有热源引起地壳加厚部位岩石部分熔融,因此由这种年青沉积物重熔所形成的花岗闪长岩兼具S型花岗岩和初生地壳的岩石学和地球化学特点,从而见证了初生地壳的短周期再循环。

Zircon U-Pb dating, whole-rock elements and Sr-Nd isotopes, and mineral O isotope analyses were carried out for three plutons of Neoproterozoic granodiorite in South Anhui. LA-ICPMS zircon U-Pb dating indieates two age phases of magmatic zircons,formed at 821 ±7 Ma and 881± 9Ma, respectively. From CL imaging no apparent distinction is found between the two groups of zircon, but most spots of the old age occur in cores. The granodiorite has high Al2O3 contents and high whole-rock δ^18O values of 11.1 to 13.6, pointing to a supracrustal origin characteristic of S-type granite, On the other hand, it has mantle-like Nd-Sr isotopic compositions with εNd （t） values of -2.06 to 0.02 and initial ^87 Sr/^86Sr ratios of 0. 7033 to 0. 7087, indicating a magmatic source with significant proportions of juvenile crust in affinity to I-type granite. Refractory minerals like zircon, garuet and quartz retain the magmatic O isotope ratios; the other minerals such as K-feldspar, plagioclase and biotite are obviously at isotopic disequilibrium when paired with quartz, and thus suffered different degrees of post-magmatic alteration at medium to low temperatures. On the basis of element and isotope results, we interpret the zircons of 881±Ma age as the inherited one, and those of 821±7Ma age as the coeval magmatic genesis. It is inferred that an extensive mantle-derived magmatic activity took place at ca. 900 to 880Ma along the southeastern margin of the Yangtze plate in South Anhui. After the rapid weathering and deposition of juvenile crust, the low-mature sedimentary rocks were formed in the southeastern margin of the Yangtze craton. It is assumed that due to anomalously thermal pulse by a mantle superplume event at ca. 820 Ma that heated the overlying lithosphere, the S-type granodiorite was generated by remelting of the water-rich sedimentary rocks in the thickened orogenic crust. During emplacement and cooling of granitoid magma,only medium to low temperature hydrothermal alteration occurred without supersoildus water-rock reaction, indicating that the granodiorite was formed in the pre-rift phase of continental margin rather than at the peak of rift magmatism, Although no coeval mantlederived magma was added to the granodiorite, the heat is appealed to cause the remelting of juvenile crust that experienced weathering and sedimentation. As a result, the granodiorite has the petrological and geochemical features of both S-type granite and juvenile crust,and thus witnesses the short-term recycling of juvenile crust.