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Petrogenesis of a late-stage calc-alkaline granite in a giant S-type batholith: geochronology and Sr–Nd–Pb isotopes from the Nomatsaus granite (Donkerhoek batholith), Namibia
International Journal of Earth Sciences ( IF 2.3 ) Pub Date : 2021-04-07 , DOI: 10.1007/s00531-021-02024-w
S. Aspiotis , S. Jung , F. Hauff , R. L. Romer

The late-tectonic 511.4 ± 0.6 Ma-old Nomatsaus intrusion (Donkerhoek batholith, Damara orogen, Namibia) consists of moderately peraluminous, magnesian, calc-alkalic to calcic granites similar to I-type granites worldwide. Major and trace-element variations and LREE and HREE concentrations in evolved rocks imply that the fractionated mineral assemblage includes biotite, Fe–Ti oxides, zircon, plagioclase and monazite. Increasing K2O abundance with increasing SiO2 suggests accumulation of K-feldspar; compatible with a small positive Eu anomaly in the most evolved rocks. In comparison with experimental data, the Nomatsaus granite was likely generated from meta-igneous sources of possibly dacitic composition that melted under water-undersaturated conditions (X H2O: 0.25–0.50) and at temperatures between 800 and 850 °C, compatible with the zircon and monazite saturation temperatures of 812 and 852 °C, respectively. The Nomatsaus granite has moderately radiogenic initial 87Sr/86Sr ratios (0.7067–0.7082), relatively radiogenic initial εNd values (− 2.9 to − 4.8) and moderately evolved Pb isotope ratios. Although initial Sr and Nd isotopic compositions of the granite do not vary with SiO2 or MgO contents, fSm/Nd and initial εNd values are negatively correlated indicating limited assimilation of crustal components during monazite-dominated fractional crystallization. The preferred petrogenetic model for the generation of the Nomatsaus granite involves a continent–continent collisional setting with stacking of crustal slices that in combination with high radioactive heat production rates heated the thickened crust, leading to the medium-P/high-T environment characteristic of the southern Central Zone of the Damara orogen. Such a setting promoted partial melting of metasedimentary sources during the initial stages of crustal heating, followed by the partial melting of meta-igneous rocks at mid-crustal levels at higher P–T conditions and relatively late in the orogenic evolution.



中文翻译:

大型S型岩床中后期钙碱性花岗岩的成岩作用:纳米比亚Nomatsaus花岗岩(Donkerhoek岩床)的年代学和Sr-Nd-Pb同位素

构造较晚的511.4±0.6 Ma古老的Nomatsaus侵入岩(Donkerhoek岩床,达马拉造山带,纳米比亚)由中等钙质,镁质,钙碱性到钙质花岗岩组成,类似于世界范围内的I型花岗岩。演化岩石中的主要元素和微量元素变化以及LREE和HREE浓度表明,分级的矿物组合包括黑云母,Fe-Ti氧化物,锆石,斜长石和独居石。随着SiO 2含量的增加,K 2 O丰度的增加表明钾长石的积累。与大多数演化岩石中的小正Eu异常兼容。与实验数据相比,Nomatsaus花岗岩很可能是由可能为胶结成分的亚火成岩源生成的,这些成分在水不饱和条件下融化(XH 2O:0.25–0.50),且温度在800至850°C之间,分别与锆石和独居石的饱和温度分别为812和852°C兼容。Nomatsaus花岗岩具有中等放射成因的初始87 Sr / 86 Sr比率(0.7067-0.7082),相对放射成因的初始εNd值(-2.9至-4.8)和中等演化的Pb同位素比率。尽管花岗岩的初始Sr和Nd同位素组成不会随SiO 2或MgO含量而变化,但f Sm / Nd与初始εNd值呈负相关,表明独居石主导的分级结晶过程中地壳组分的同化作用有限。形成Nomatsaus花岗岩的首选成岩模式涉及大陆-大陆碰撞背景,堆积有地壳切片,再加上高放射性产热率,加热了增稠的地壳,从而导致了中等P /高T环境特征。达玛拉造山带的南部中部地区。在地壳加热的初始阶段,这种环境促进了沉积物源的部分熔融,随后在较高的PT条件下以及在造山运动演化的后期,在中地壳水平上的准火成岩的部分熔融。

更新日期:2021-04-08
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