Dating syn-orogenic exhumation of subducted continental crust: The case of the Northern Apennines

IF 2.9 2区 地球科学 Q2 GEOCHEMISTRY & GEOPHYSICS
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Abstract

Dating deformation is key to unravel the evolution of orogens. Unfortunately, this is commonly challenged by the occurrence in the rock record of multiple overprinting deformation stages that lead to repeated blastesis and form multiple fabrics during both prograde and retrograde evolutions. Additionally, recrystallization is not always complete, which causes a mixture of inherited and neoblastic mineral phases. Therefore, an approach integrating age dating with microstructural and petrographic analyses is necessary to extract structurally constrained ages from the rock record. We used this approach to contribute to the long-lasting debate about exhumation of continental metamorphic units subducted to high-pressure conditions. We studied continental metasedimentary sequences of the Tuscan Metamorphic Units of the Northern Apennines (Italy) from two localities: the Island of Giglio to the west and the Monticiano-Roccastrada Unit to the east. We obtained two analytically distinct 40Ar/39Ar age ranges: an older 21–16 Ma age cluster is related to syn-orogenic top-to-the E contractional deformation occurring at ca. 1 GPa and 350 °C. A younger 15–11 Ma age group is related, instead, to exhumation to shallower structural levels and retrogression of these units to greenschist facies conditions. The interpretation of the results is based on white mica chemical compositions and rock fabrics. By integrating our results with published literature, we conclude that the investigated high-pressure continental metasedimentary sequences experienced early syn-orogenic exhumation from blueschist to greenschist facies conditions predominantly by a combination of extrusion and out- and in-sequence thrusting, with only minor contributions by extensional shearing during regional crustal thinning. Therefore, this study highlights the role of contractional fabrics in accommodating significant exhumation amounts of deeply subducted continental rocks.

俯冲大陆地壳同步成因掘起的年代测定:北亚平宁山脉案例
对变形进行定年是揭示造山运动演化过程的关键。遗憾的是,岩石记录中通常会出现多个叠压变形阶段,导致反复爆破,并在顺行和逆行演化过程中形成多种构造,这就给我们提出了挑战。此外,再结晶并不总是完全的,这就造成了继承矿物相和新生矿物相的混合。因此,有必要将年代测定与微结构和岩石学分析结合起来,以便从岩石记录中提取有结构约束的年代。我们利用这种方法,为有关大陆变质岩单元俯冲到高压条件下的掘起这一长期争论做出了贡献。我们研究了北亚平宁山脉托斯卡纳变质岩单元(意大利)的大陆变质岩序列,这些变质岩单元来自两个地方:西部的吉利奥岛和东部的蒙蒂西亚-罗卡斯特拉达单元。我们获得了两个在分析上截然不同的 40Ar/39Ar 年龄范围:较早的 21-16 Ma 年龄群与发生在约 1 GPa 和 350 °C的同步成因顶向东收缩变形有关。1 GPa 和 350 °C。而较年轻的 15-11 Ma 年龄组则与这些单元向较浅构造层位的掘起以及向绿岩面条件的逆退有关。对结果的解释基于白云母化学成分和岩石结构。通过将我们的研究结果与已发表的文献进行整合,我们得出结论:所调查的高压大陆变质岩序列经历了从蓝岩到绿岩面条件的早期同步成因蜕变,主要是通过挤压、向外和向内推力的组合作用,只有在区域地壳减薄过程中的延伸剪切作用起到了微小的作用。因此,这项研究强调了收缩构造在容纳深俯冲大陆岩石大量掘出方面的作用。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
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来源期刊
Lithos
Lithos 地学-地球化学与地球物理
CiteScore
6.80
自引率
11.40%
发文量
286
审稿时长
3.5 months
期刊介绍: Lithos publishes original research papers on the petrology, geochemistry and petrogenesis of igneous and metamorphic rocks. Papers on mineralogy/mineral physics related to petrology and petrogenetic problems are also welcomed.
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