Ages and geochemistry of granites in the Pingtan–Dongshan Metamorphic Belt, Coastal South China: New constraints on Late Mesozoic magmatic evolution
Ages and geochemistry of granites in the Pingtan–Dongshan Metamorphic Belt, Coastal South China: New constraints on Late Mesozoic magmatic evolution
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DOI：
10.1016/j.lithos.2012.06.031
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发表时间：
2012-10
期刊：
影响因子：
3.5
通讯作者：
Qian Liu;Jinhai Yu;Qin Wang;Bingkai Su;Mei‐Fu Zhou;Hailong Xu;X. Cui
中科院分区：
文献类型：
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作者：
Qian Liu;Jinhai Yu;Qin Wang;Bingkai Su;Mei‐Fu Zhou;Hailong Xu;X. Cui
通过对华南沿海北东向平潭-东山变质带花岗岩和变质沉积岩的锆石U-Pb定年、Hf同位素组成和体相地球化学分析，确定了花岗岩和变质沉积岩的形成时代和构造环境。LA-ICPMS锆石U-Pb定年结果揭示了沿着PDMB带的4期岩浆/变质作用，其中北方的~ 187 Ma、133- 130 Ma和108- 100 Ma的花岗质岩浆作用，以及南部带的147- 140 Ma的花岗岩和变质岩。在144- 140 Ma之间存在一次低P/T变质事件，在130- 125 Ma和108- 100 Ma之间存在两个变形阶段。燕山早期高钾钙碱性花岗岩（187- 140 Ma）以高SiO_2、高碱金属含量和低轻稀土/重稀土分馏为特征（LREE/HREE=4.82-11.54，（La/Yb）N=3.99-13.53）和弱Eu亏损燕山晚期（140- 100 Ma）钙碱性花岗岩具有低的A/CNK、中等的LREE/HREE比值（10.13-19.18）和不明显的Eu正异常（0.79-1.95）。两期花岗岩均亏损Nb、Ta、Ti等高场强元素（HFSE），富集LREE和大离子亲石元素（LILE），Rb/Ba、Rb/Sr比值和A/CNK值均较低，为典型的古太平洋板块俯冲弧相关构造背景下的I型花岗岩。但燕山早期和晚期花岗岩的Sr、Nb、K2 O含量及La/Yb、Sr/Y和Eu/Eu* 比值明显不同，反映了不同的P-T条件。北方PDMB花岗岩中的锆石具有较高的Hf同位素组成，εHf（t）=+0.01 ~+6.47，Hf模式年龄（TCDM）=0.82-1.16Ga;而南方PDMB花岗岩中的锆石具有较低的Hf同位素组成，εHf（t）=-6.63 ~+0.07，Hf模式年龄（TCDM）=1.19-1.61Ga。与研究区附近的变质基底相比，北方PDMB的花岗岩可能来自新元古代地壳，而南部PDMB的花岗岩可能来自新元古代和少量更古老的地壳物质的混合源。提出本区早燕山期花岗岩是南岭东西向花岗岩-火山岩带的东延，晚燕山期火成岩主要分布于东南沿海，走向NE。燕山早、晚两幕岩浆事件的明显分布和地球化学特征可能与俯冲到欧亚大陆东部之下的古太平洋板块的方向和倾角的变化有关。
Zircon U–Pb dating and Hf-isotope compositions and bulk geochemical analyses of granites and metasedimentary rocks from the NE-striking Pingtan–Dongshan Metamorphic Belt (PDMB), coastal South China, are used to constrain their formation ages and tectonic settings. LA-ICPMS zircon U–Pb dating results reveal four periods of magmatism/metamorphism along the PDMB, including ~187Ma, 133–130Ma and 108–100Ma granitic magmatism in the northern PDMB and 147–140Ma granites and metamorphic rocks in the southern belt. A low P/T metamorphic event at 144–140Ma and two deformation phases at 130–125Ma and 108–100Ma are also recognized. The Early Yanshanian high-K calc-alkaline granites (187–140Ma) are characterized by high contents of SiO2, alkali and low light rare earth element (LREE)/ heavy rare earth element (HREE) fractionation (LREE/HREE=4.82–11.54, (La/Yb)N=3.99–13.53) and weak Eu depletion (0.54–0.61), whereas the Late Yanshanian calc-alkaline granites (140–100Ma) have low A/CNK, middle LREE/HREE ratios (10.13–19.18) and positive to inconspicuous Eu anomalies (0.79–1.95). Granites in both periods are depleted in high field strength elements (HFSE, e.g. Nb, Ta, Ti) but enriched in LREE and large ionic lithophile elements (LILE), with low Rb/Ba and Rb/Sr ratios and A/CNK values, indicating that they are typical I-type granites in the arc-related tectonic setting due to subduction of the Paleo-Pacific plate. However, both the Early and Late Yanshanian granites have distinct Sr, Nb and K2O contents and La/Yb, Sr/Y and Eu/Eu* ratios, suggesting different P–T conditions. Zircon grains from the granites in the northern PDMB have higher Hf-isotope compositions with εHf(t)=+0.01 to +6.47 and Hf model ages (TCDM)=0.82–1.16Ga, whereas those from the granites in the southern PDMB show slightly lower Hf-isotope compositions with εHf(t)=−6.63 to +0.07 and older Hf model ages (TCDM)=1.19–1.61Ga. Compared with the metamorphic basement near the study area, the granites in the northern PDMB were probably derived from a Neoproterozoic crust, whereas those in the southern PDMB may have originated from a mixed source of Neoproterozoic and minor older crustal materials. We propose that the Early Yanshanian granites in the PDMB are the eastern extension of the E–W-striking granitoid–volcanic belt in the Nanling Range, whereas the Late Yanshanian igneous rocks mostly distribute in coastal Southeast China and strike NE. The distinct distribution and geochemical features between the Early and Late Yanshanian magmatic events can be related to the change of direction and dip angle of the Paleo-Pacific plate that was subducted underneath the Eastern Eurasian continent in the two episodes.