摘要
对大别造山带北麓的北淮阳新开岭地区岩浆岩进行了锆石阴极发光显微结构观察和SHRIMP法锆石微区UPb定年.在锆石阴极发光图像中,一个花岗岩样品中的大部分锆石颗粒具有明显的初始岩浆振荡环带,为典型的岩浆锆石,少有蚀变的颗粒和/或区域;而另一个花岗岩样品中的锆石虽然同样具有振荡环带,但是大部分颗粒中心的初始岩浆环带被扰动,指示这些锆石为岩浆锆石,受到了较强的后期热液蚀变的改造.对锆石具有初始岩浆环带和溶蚀结构的区域分别进行SHRIMP法UPb微区定年结果表明,这些岩浆岩的形成年龄为(820±4)Ma,热液蚀变作用发生的时间为(780±4)Ma.新开岭地区新元古代花岗质岩石的形成和后期超固相热液蚀变作用分别对应于超大陆裂解之前的约830～795Ma岩浆活动和裂解过程中约780～745Ma的岩浆作用.单矿物激光氟化氧同位素分析结果表明,这些岩浆岩具有非常低的δ18O值,其中锆石为1.90‰～5.78‰,石英为-2.88‰～-7.67‰,斜长石为-4.01‰～-11.40‰.锆石和其他矿物之间表现出强烈的氧同位素不平衡,而其他矿物之间则达到了氧同位素的再平衡.结合不同δ18O值锆石的内部结构特征,认为该地区的热液蚀变作用为超固相条件下的高温热液蚀变.这一过程不但改变了石英等矿物的氧同位素组成,同时也不同程度地改变了锆石的氧同位素组成,所以这些样品中低δ18O值锆石可能是超固相条件下热液蚀变的结果.石英中具有异常低的δ18O值表明蚀变流体来源应为寒冷气候大气降水.所以,新开岭地区亏损18O蚀变岩石的形成与裂谷岩浆作用和雪球地球事件相耦合的高温大气降水热液蚀变有关.
SHRIMP zircon U-Pb dating and oxygen isotope analysis were carried out for granites at Xinkailing in the Beihuaiyang zone. In cathodoluminescence （CL） images, most zircons in one granite sample exhibit primary oscillatory zoning, which is typical for magmatic zircon, but some grains have internal structure altered by hydrothermal fluid; whereas the oscillatory zonations of most zircons in the other granite have been significantly disturbed, reflecting much stronger modification by hydrothermal alteration. The zircon U-Pb dating for the oscillatory zoned and hydrotherrnally altered domains yields two groups of weighted mean ^205 pb/^238 U ages at （820±4） Ma and （780±4） Ma, respectively, which are interpreted as timing of granite emplacement and hydrothermal alteration. According to the obtained ages, the formation and later supersolidus hydrothermal alteration of the Xinkailing granite correspond to the two stages of bimodal magmatism with ages of ca. 830 to 795 Ma and ca. 780 to 745 Ma, respectively, for pre-rift and syn-rift during the breakup of the supercontinent Rodinia. They have very low δ^180 values, with 1.90‰ to 5.78‰ for zircon, -2.88‰ to -7. 67‰ for quartz, and -4. 01‰ to -11.40‰ for plagioclase. Zircon and other minerals display obvious O isotope disequilibrium, while the other minerals approached O isotope re-equilibration at different temperatures during the hydrothermal alteration. Zircons with a more strongly altered internal structure have lower relative δ^18O values. Therefore, the high-T hydrothermal alteration at Xinkailing may have occurred under supersolidus conditions, which not only completely reset the O isotope composition of quartz, but also partly modified the O isotope composition of zircon. The low δ^18O values of zircon in some samples may be partly acquired from the low δ^18O hydrothermal fluid. The anomalously low δ^18O values of -2. 88‰ to -7. 67‰ for quartz indicate that the hydrothermal fluid may be derived from the meteoric water of a cold paleoclimate. Therefore, the formation of low δ^180 granite during the rnid-Neoproterozoic at Xinkailing is correlated with rifting magmatism and the snowball earth event.
出处
《地球科学（中国地质大学学报）》
EI
CAS
CSCD
北大核心
2005年第6期659-672,共14页
Earth Science-Journal of China University of Geosciences
基金
国家自然科学基金项目(No.40334036).
