华夏地块南缘凤凰山岩体形成时代及成因

The petrogenesis of the Fenghuangshan pluton at the southern margin of the Cathaysia block

中生代以来的岩浆事件记录一直是华夏地块中新生代大地构造演化的研究热点。近年来关于华夏中生代花岗岩的研究主要集中于南岭及闽浙沿海地区,对于粤东南地区的研究程度较低。通过地球化学、锆石U-Pb年代学及Hf同位素等手段,旨在探讨华夏地块南缘凤凰山岩体的形成时代、岩石成因机制及其大地构造环境。野外调查及岩相学研究表明,凤凰山岩体的岩石组合为黑云母二长花岗岩和黑云母钾长花岗岩,锆石U-Pb年龄分别为(156±2) Ma和(158±3) Ma,为晚侏罗纪岩浆活动的产物。两组花岗岩均具有高硅(SiO 2=72.66%~ 76.63%)、富碱(Na 2O+K 2O =7.89%~ 9.43%)、低镁(MgO=0.05%~ 0.54%)和弱过铝质(ASI=1.02 ~ 1.04)的特征,富集Rb、Th、U等元素,亏损Ba、Sr、P、Ti等元素,且P 2O 5含量与SiO 2含量呈明显的负相关关系。岩相学和地球化学显示,黑云母钾长花岗岩应属于弱分异I型花岗岩,而黑云母二长花岗岩分异指数较大(DI=93.91 ~ 95.74)、固结指数较小(SI=0.48 ~ 0.66),Rb/Sr(=99.12 ~ 243.75)和Rb/Ba(=67.45 ~ 203.80)比值较高、Eu负异常强烈(δ Eu=0.01 ~ 0.02),稀土元素配分图呈“海鸥式”分布,应属于高分异I型花岗岩。黑云母二长花岗岩和黑云母钾长花岗岩ε Hf ( t )分别变化在-8.4 ^-5.4和-9.4 ^-7.0;二阶段Hf模式年龄 T DM2 变化范围分别在1 545 ~ 1 741 Ma和1 648 ~ 1 804 Ma,表明两者都是中-古元古代古老地壳物质重熔的产物。结合该时期区域构造背景,推断凤凰山岩体可能发育在与大洋俯冲板片后撤断离相关的岩石圈伸展的构造环境下,是地壳拉张减薄引起幔源岩浆底侵,带来大量热量,导致中-古元古代古老地壳物质发生部分熔融形成的产物。

Researching on the Mesozoic magmatism is critical to understanding the tectonic evolution of the Cathaysia Block, and therefore has become a hot topic in the past decade. Numerous petrological and geochemical studies were implemented in the eastern and middle parts of the block, but few studies focused the southernmost area in western Guandong Province. Therefore, the geochronological, geochemical, and Hf isotopic data from the Fenghuangshan granitoids, which are located at the southern margin of the Cathaysia Block, were used to constrain the crystallization age, petrogenesis and tectonic environment. The Fenghuangshan granitoids are dominated by biotite-bearing monzogranites and biotite-bearing syenogranites. LA-ICP-MS zircon U-Pb data show that the former is (156±2) Ma and the latter is (158±3) Ma, respectively. All of the samples contain high silicon (SiO 2=72.66%-76.63%), high alkali (Na 2O+K 2O =7.89 %-9.43%), and low magnesium (MgO=0.05%-0.54%) concentrations. They show weakly peraluminous characteristics (A/CNK=1.02-1.04). The samples are enriched in Rb, Th, K and depleted in Ba, P, Sr, Ti, with a negative correlation between SiO 2 and P 2O 5. Mineralogical and geochemical features suggest that the biotite-bearing syenogranite is I-type granite. By contrast, the biotite-bearing monzogranite falls into the area of highly fractionated I-type granite, with high differentiation index (DI=93.91-95.74), high Rb/Sr (=99.12-243.75) and Rb/Ba (=67.45-203.80) ratio, low consolidation index (SI=0.48-0.66) as well as significant negative Eu anomalies (δ Eu=0.01-0.02). The samples of monzogranite and syenogranite have relatively homogeneous Hf isotopic compositions, with ε Hf (t) values and two-stage model ages ( T DM2 ) of -9.4 to-5.4 and 1 545-1 804 Ma, respectively. The Hf isotopic features indicated that the monzogranites and syenogranites might share a same magma source, which was sourced from reworking of the Paleoproterozoic-Mesoproterozoic crust in the Jurassic. Combined with the previous studies, the Fenghuangshan pluton was suggested to be formed by underplating of the mantle-derived magma in an extensional tectonic setting, which was related to the roll-back or breakup of the subducting oceanic lithosphere.