桐柏造山带深熔作用:混合岩LA-ICPNIS锆石U-Pb年代学证据

摘要

桐柏造山带是研究秦岭-桐柏-大别-苏鲁变质带演化的关健地区.由于桐柏高级变质杂岩深熔作用发生时间还缺乏准确的限定,这一区城的构造演化仍存在较大的争议.本文对桐柏杂岩中的一个混合岩的中色体和两个混合岩浅色体样品中的锆石进行了LA-ICPMS年代学测定.中色体中锆石分析点获得的上下交点分别为859±73Ma和135±250Ma.接近上交.4的6个谐和分析点给出的206 Pb/238 U加权平均结果为828±7Ma(MSWD=0.57).这一年龄结果同上交点在误差范围内一致,代表混合岩原岩结晶年龄,对应扬子板块北缘出现的中-新元古代的岩浆事件.另一方面,混合岩浅色体中的新生锆石具有面状分带或是弱的振荡环带,低的Tb/U比值,锆石形态学和内部结构也表明新生锆石结晶于与深熔作用有关的熔体中,它们的206 Pb/238 U加权平均值分别为135±4Ma和131±3Ma.这一年龄范围代表桐柏高级变质地体发生深熔作用时间,区城上与桐柏-大别变质带广泛出现的碰撞后岩浆事件的时代相同.桐柏造山带出现造山后伸展的时间应不晚于135Ma.%The Tongbai orogen is a key region to investigate the evolution of the Qingling-Tongbai-Dabie-Sulu metamorphic belt However, the age of the crustal anatexis in the Tongbai high-grade metamorphic complex has not been well constrained, and thus hinders our understanding of the tectonic evolution of this area. Three migmatite samples from the Tongbai complex, including one melanosome and two leucosomes, were selected for LA-ICPMS zircon U-Pb dating. Zircon from the melanosome defines upper and lower intercept U-Pb ages of 859 ±73Ma and 135 ±250Ma, respectively. Six spot analyses near the upper intercept yield a weighted mean 206Pb/238U age of 828 ±7Ma (MSWD=0.57). This age consists with the upper intercept age, interpreting as the age of the protolith crystallization. The protolith formation age might correspond to magmatism during the Mid-Neoproterozoic in the northern margin of the South China Block.On the other hand, new growth zircons in the leucosome samples show planar or weakly oscillatory zoning, low Th/U ratios. The morphological and internal structure features of zircons suggest that their growths were associated with partial melting. They yielded 206pb/238U weighted mean ages of 135Ma and 131Ma, which were interpreted to register the ages of migmatization. The timing of the anatexis in the Tongbai high-grade metamorphic complex is coeval with the widespread post-collision magmatism in the Tongbai-Dabie orogen. It suggests that the onset of extensional tectonism in the Tongbai orogen is no later than 135Ma.