当前位置: X-MOL 学术Lithos › 论文详情
Our official English website, www.x-mol.net, welcomes your feedback! (Note: you will need to create a separate account there.)
Using apatite to resolve the age and protoliths of mid-crustal shear zones: A case study from the Taxaquara Shear Zone, SE Brazil
Lithos ( IF 3.5 ) Pub Date : 2020-12-01 , DOI: 10.1016/j.lithos.2020.105817
B.V. Ribeiro , J.A. Mulder , F.M. Faleiros , C.L. Kirkland , P.A. Cawood , G. O’Sullivan , G.A.C. Campanha , M.A. Finch , R.F. Weinberg , O. Nebel

Abstract Shear zones accommodate strain and facilitate migration of hydrothermal fluid and magma through the crust. Unravelling the deformation history of shear zones requires correspondence between the closure temperature of mineral geochronometers and the temperature of deformation. Here, we adopt apatite U–Pb-trace element analysis as a tool for dating deformation and tracing the protoliths of mid-crustal shear zones through a case study of the Taxaquara Shear Zone (TSZ), a major transpressional shear zone in the southern Ribeira Belt of SE Brazil. Apatite from mylonites in the TSZ yield U–Pb ages of 558–536 Ma, considering uncertiainties, which slightly overlap with 40Ar/39Ar ages of 538 ± 2 Ma from muscovite in the lower limit. The closure temperature of apatite is estimated at 500–460 °C, which is slightly higher than that estimated for syn-kinematic muscovite (445–420 °C). Apatite from shear zone mylonites has Sr/Y and LREE systematics typical of apatite from S- and I-type granitoids, suggesting the adjacent and undeformed Pilar do Sul and Piedade granites are the likely protoliths of the mylonites. This interpretation is supported by new U–Pb ages of ca. 605 Ma from pre-kinematic zircon and titanite from mylonites, which corresponds closely with new U–Pb apatite ages and previously published U–Pb monazites ages from the Pilar do Sul Granite. We suggest the U–Pb system of apatite in the TSZ was reset via volume diffusion during rapid cooling given that it preserves the igneous geochemical signatures. Moreover, this interpretation is consistent with the lower apatite closure temperature (500–460 °C) relatively to the temperature of deformation (530–480 °C). The revised ~560–535 Ma age for the TSZ demonstrates that it post-dates the collisional phase of the Ribeira Belt (620–595 Ma and 595–565 Ma), indicating protracted strain accommodation during the Brasiliano–Pan African orogeny, and supports correlation with the 600–550 Ma and 570–550 Ma transpressional Dom Feliciano and Kaoko Belts. This study demonstrates that apatite is a powerful tool for unravelling the history of mid-crustal shear zones as it is stable in a wide range of lithotypes, has trace element compositions that are sensitive to the environment of formation, and Pb closure temperatures typical of mid-crust conditions. U–Pb-trace element analysis of apatite provides a robust means to date shear zones that can be complimentary to, or independent of, more traditional 40Ar/39Ar analysis of mica or amphibole.

中文翻译:

使用磷灰石解析中地壳剪切带的年龄和原岩:巴西东南部 Taxaquara 剪切带的案例研究

摘要 剪切带适应应变并促进热液流体和岩浆穿过地壳的运移。解开剪切带的变形历史需要矿物测时仪的闭合温度与变形温度之间的对应关系。在这里,我们采用磷灰石 U-Pb 痕量元素分析作为工具,通过对里贝拉南部主要的挤压剪切带 Taxaquara 剪切带 (TSZ) 的案例研究,确定变形年代和追踪中地壳剪切带的原岩巴西东南带。考虑到不确定性,TSZ 中糜棱岩的磷灰石产生 558-536 Ma 的 U-Pb 年龄,这与下限白云母的 538 ± 2 Ma 的 40Ar/39Ar 年龄略有重叠。磷灰石的闭合温度估计为 500–460 °C,这略高于同运动白云母的估计值(445-420°C)。剪切带糜棱岩的磷灰石具有 S 型和 I 型花岗岩类磷灰石典型的 Sr/Y 和 LREE 系统学特征,表明相邻且未变形的 Pilar do Sul 和 Piedade 花岗岩可能是糜棱岩的原岩。这种解释得到了大约的新 U-Pb 年龄的支持。来自糜棱岩的前运动锆石和钛石的 605 Ma,这与新的 U-Pb 磷灰石年龄和先前发表的来自 Pilar do Sul 花岗岩的 U-Pb 独居石年龄密切对应。我们建议 TSZ 中磷灰石的 U-Pb 系统在快速冷却过程中通过体积扩散重置,因为它保留了火成岩地球化学特征。而且,这种解释与相对于变形温度 (530–480 °C) 的磷灰石闭合温度 (500–460 °C) 较低是一致的。修正后的 TSZ 约 560-535 Ma 年龄表明它晚于 Ribeira 带的碰撞阶段(620-595 Ma 和 595-565 Ma),表明在巴西-泛非造山运动期间存在长期应变调节,并支持与 600-550 Ma 和 570-550 Ma 压压 Dom Feliciano 和 Kaoko 带的相关性。这项研究表明,磷灰石是解开中地壳剪切带历史的有力工具,因为它在各种岩型中都很稳定,具有对形成环境敏感的微量元素组成,以及典型的中地壳剪切带的铅闭合温度。 -地壳条件。
更新日期:2020-12-01
down
wechat
bug