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秦岭岩群被认为是出露于北秦岭地体内最古老的前寒武纪基底岩石,记录了北秦岭造山带的地壳形成和演化历史。本文报道丹凤-西峡地区五件秦岭岩群片麻岩锆石U-Pb年龄结果,限定其形成和变质时代,探讨北秦岭地体的构造归属。定年结果表明,岩浆成因锆石颗粒的年龄集中在1400～1600Ma左右和850～950Ma左右,记录两期主要岩浆活动。6粒锆石具有变质成因特征,低Th/U比值(<0.03),206Pb/238U年龄变化在510～465Ma之间,加权平均值477±18Ma。这一古生代变质叠加时代与北秦岭地体南北缘高压变质作用时代基本一致,说明秦岭岩群遭受到北秦岭造山带俯冲-碰撞造山过程的变质作用。秦岭岩群主要形成于中元古代晚期至新元古代早期,基底岩石缺乏早元古代和太古代岩浆活动的记录。在岩浆作用时代上,北秦岭地体与广泛发育新元古代中-晚期岩浆作用的扬子陆块北缘有差别,也不同于晚太古代-早元古代的华北陆块南缘,可能是中-新元古代形成的独立微陆块。
The Qinling Group is considered to be the oldest Precambrian basement exposed in the North Qinling and recorded the crustal formation and evolution history of the North Qinling orogenic belt. This paper reports the zircon U-Pb ages of five gneiss from the Qinfeng Group in Danfeng-Xixia area, defining their age of formation and metamorphism and discussing the tectonic belts of the North Qinling terrane. The dating results show that the age of magmatic zircon particles is about 1400 ~ 1600Ma and about 850 ~ 950Ma, and the two major magmatic activities are recorded. Six zircons have the characteristics of metamorphism, low Th / U ratio (<0.03), 206Pb / 238U age ranged from 510 to 465Ma, and the weighted average was 477 ± 18Ma. This Paleozoic metamorphism superimposed era is basically consistent with the high-pressure metamorphism of the northern and southern Qinling terranes, indicating that the Qinling rock mass was subjected to metamorphism during the subduction-collision orogeny of the North Qinling orogenic belt. The Qinling Group mainly formed in the late Mesoproterozoic to early Neoproterozoic. The basement rocks lack records of the Early Proterozoic and Archean magmatic activities. In the age of magmatism, there is a difference between the North Qinling terrane and the northern margin of the Yangtze block where the Neoproterozoic mid-late magma was extensively developed, and also different from that of the Late Archean-Early Proterozoic southern margin of the North China Block. - Neoproterozoic formation of independent microclimate blocks.