ABSTRACT

Ten samples of felsic plutonic rocks from the Eastern Segment of the Sveconorwegian Orogen in southern Sweden, previously dated by ID-TIMS on zircon, have been dated anew using SIMS spot analysis of individual zircon grains, leading to more reliable and in most cases also more precise revised magmatic crystallization ages. A gneissic monzonite within the Protogine Zone in Småland yields a revised U-Pb age of ca. 1725 Ma, four samples of orthogneiss from Skåne all yield revised ages between 1700 and 1690 Ma, while two samples of coarse-grained granitic gneiss in the same region yield ages between 1690 and 1680 Ma. These revised ages are between 15 and 250 m.y. older than previously obtained TIMS ages. Two samples of the Gumlösa-Glimåkra granite along the Protogine Zone in northern Skåne and one sample of related syenite yield ages around 1220 Ma, similar but more precise compared to the previous ages. The U-Pb zircon data have been complemented by Hf isotope analysis by LA-ICP-MS on the same grains, and previously obtained initial Sr and Nd whole-rock isotope data have been recalculated to the revised crystallization ages. The Sr isotope data scatter, while the revised initial ε_Nd values fall between +1 and +2 for the older rocks, and close to 0 for the younger intrusives along the Protogine Zone. Initial ε_Hf in magmatic undisturbed zircons shows relatively little spread within each sample, between 2 and 4 Epsilon units, disregarding a few outliers, with average values for the 1725 to 1680 Ma rocks between +3 and +5.5 and for the 1220 Ma rocks at ca. +1.5. Covariation between initial ε_Nd and initial ε_Hf in the older rocks suggests either mixing between two isotopically distinct magma sources or one magma source which was isotopically heterogeneous. The isotopic signatures of the 1220 Ma intrusive rocks along the Protogine Zone are indicative of juvenile mantle input to their magmas, rather than pure crustal melting.

摘要

来自瑞典南部 Sveconorwegian 造山带东部段的 10 个长英质深成岩样本（之前由 ID-TIMS 对锆石进行了测年），已使用单个锆石颗粒的 SIMS 点分析重新确定了日期，从而导致更可靠，在大多数情况下也更多精确修正的岩浆结晶年龄。Småland 的 Protogine 带内的片麻岩二长岩产生了修正后的 U-Pb 年龄，约为 1725 Ma，Skåne 的 4 个正片麻岩样品的年龄都在 1700 到 1690 Ma 之间，而同一地区的两个粗粒花岗片麻岩样品的年龄在 1690 到 1680 Ma 之间。这些修改后的年龄比之前获得的 TIMS 年龄大 15 到 250 岁。斯科讷北部 Protogine 带沿线的两个 Gumlösa-Glimåkra 花岗岩样品和一个相关正长岩样品的产量年龄在 1220 Ma 左右，与以前的时代相比，相似但更精确。U-Pb 锆石数据已通过 LA-ICP-MS 对相同晶粒的 Hf 同位素分析进行​​了补充，并且先前获得的初始 Sr 和 Nd 全岩同位素数据已重新计算为修正的结晶年龄。Sr 同位素数据分散，而修正后的初始 ε_{较老岩石的Nd}值介于 +1 和 +2 之间，而沿 Protogine 带的较年轻侵入岩的 Nd 值接近 0。岩浆原状锆石的初始 ε _Hf在每个样本中显示出相对较小的分布，在 2 到 4 个 Epsilon 单位之间，忽略一些异常值，1725 到 1680 Ma 岩石的平均值在 +3 到 +5.5 之间，1220 Ma 岩石的平均值在约 +1.5。_{较旧岩石中初始 ε Nd}和初始 ε _Hf之间的协变表明两种同位素不同的岩浆源之间的混合或一种同位素异质的岩浆源之间的混合。沿 Protogine 带的 1220 Ma 侵入岩的同位素特征表明了幼年地幔输入到它们的岩浆中，而不是纯粹的地壳熔融。