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
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
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DOI：
10.1007/s00531-021-02024-w
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发表时间：
2021-04
影响因子：
2.3
通讯作者：
S. Aspiotis;S. Jung;F. Hauff;R. Romer
中科院分区：
文献类型：
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作者：
S. Aspiotis;S. Jung;F. Hauff;R. Romer
晚构造511.4±0.6 ma的Nomatsaus岩体（纳米比亚Damara造山带Donkerhoek岩基）由中等过铝质、镁质、钙碱性至钙性花岗岩组成，与世界上的i型花岗岩相似。演化岩石的主微量元素变化和轻稀土、重稀土含量表明，分选矿物组合包括黑云母、铁钛氧化物、锆石、斜长石和独居石。随着sio2的增加，K2O丰度的增加表明钾长石的富集；在大多数演化的岩石中有一个小的正Eu异常。与实验数据相比，Nomatsaus花岗岩可能是由变火成岩形成的，其成分可能为英灰质，在水欠饱和条件下（xh2o: 0.25 ~ 0.50），温度在800 ~ 850℃之间熔化，与锆石和monazite的饱和温度分别为812℃和852℃相适应。Nomatsaus花岗岩具有中等放射性成因的初始87sr /86Sr比值（0.7067 ~ 0.7082），相对放射性成因的初始εNd值（−2.9 ~−4.8）和中等演化的Pb同位素比值。虽然花岗岩的初始Sr和Nd同位素组成不随sio2或MgO含量的变化而变化，但fSm/Nd和初始εNd值呈负相关，表明在单氮石为主的分步结晶过程中，地壳成分的同化作用有限。Nomatsaus花岗岩形成的首选成岩模式是大陆-大陆碰撞环境，地壳片层叠加，高放射性产热率加热增厚地壳，形成达马拉造山带中南部中温中温环境特征。这种环境在地壳加热初期促进了变质沉积岩源的部分熔融，随后在较高的P-T条件下和相对较晚的造山演化阶段促进了中地壳水平变质火成岩的部分熔融。
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 SiO2suggests 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 initial87Sr/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 SiO2or MgO contents,fSm/Ndand 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.