大别造山带新太古代地壳岩石和古元古代混合岩化作用--来自锆石U-Pb年代学和Hf同位素证据

Neoarchean crustal rocks and Paleoproterozoic migmatization in the Dabie orogen:evidence from zircon U-Pb age and Hf isotopes

大别造山带北大别超高压变质带是研究秦岭-大别-苏鲁造山带古老基底演化过程的关键区域,其内广泛发育的混合岩长期被认为主要形成于中生代。本文对北大别团风一带新识别出的一套混合岩开展了锆石U-Pb定年和Hf同位素组成分析,结果显示,混合岩第一类锆石核部具有岩浆锆石特点,组成的不一致线上交点年龄为2850±86 Ma,该年龄代表了混合岩原岩年龄。第二类锆石具有变质深熔锆石特点,其加权平均207Pb/206Pb年龄为2011±12 Ma,代表了混合岩化的时间。岩浆锆石多数具有负的εHf(t)值(-8.1~2.2),对应两阶段Hf同位素模式年龄(TDM2)为3.6~3.0 Ga,表明原岩可能为大别造山带内古太古代地壳物质重熔形成,并可能在形成过程中伴有少量幔源物质加入。与之相比,变质锆石均具有正的εHf(t)值(0.3~8.2),对应TDM2为2.7~2.2 Ga,说明在混合岩化变质深熔过程中锆石Lu-Hf同位素体系完全开放,导致了锆石Hf同位素组成的升高。本文研究表明,大别造山带除了中生代混合岩化作用以外,还存在古元古代与Columbia超大陆聚合过程相关的一期混合岩化作用,为目前已知的大别造山带内最早一期混合岩化作用。此外,该套混合岩原岩为太古宙岩石,且对应模式年龄高达3.6 Ga,这扩展了目前已知的大别造山带最古老岩石信息范围,表明大别造山带内太古宙古老地壳物质可能不仅局限于黄土岭一带,还在北大别更广泛地区出露。

The northern Dabie ultrahigh pressure(UHP)metamorphic belt is one of the key regions to investigate the evolution of the basements of the Qinling-Dabie-Sulu orogenic belt.Migmatites are widely distributed in the northern Dabie belt.Based on previous geochronological studies they have long been regarded as having been formed during the Mesozoic.Here,we report zircon U-Pb ages and Hf isotopic compositions of migmatites from the Tuanfeng area in the northern Dabie belt.CL images reveal that the zircons can be divided into two groups.The first magmatic zircons yield an upper intercept age of 2850±86 Ma,dating magma emplacement of the protolith.The remaining zircons are metamorphic zircons.They gave a weighted mean 207 Pb/206 Pb age of 2011±12 Ma,which is interpreted as the age of migmatization.Most magmatic zircons have negativeεHf(t)values(-8.1~2.2),corresponding to two-stage Hf isotopic model ages(TDM2)of 3.6~3.0 Ga,indicating that the protoliths might have formed by remelting of the Paleoarchean crustal rocks in the Dabie orogen,with involvement of minor mantle-derived melts.By contrast,metamorphic zircons have positiveεHf(t)values(0.3~8.2),corresponding to TDM2 of 2.7~2.2 Ga,revealing an open Lu-Hf isotopic system during the Paleoproterozoic migmatization.Our study suggests a record of Paleoproterozoic migmatization during the assembly of the Columbia supercontinent,except for the Mesozoic migmatization.The protoliths of the Tuanfeng migmatites were formed in Neoarchean,with their TDM2 ages up to 3.6 Ga.It is thus inferred that the Archean basement might be widely distributed in the Dabie orogen.