Generation of syntectonic calc-alkaline, magnesian granites through remelting of pre-tectonic igneous sources – U-Pb zircon ages and Sr, Nd and Pb isotope data from the Donkerhoek granite (southern Damara orogen, Namibia)
Generation of syntectonic calc-alkaline, magnesian granites through remelting of pre-tectonic igneous sources – U-Pb zircon ages and Sr, Nd and Pb isotope data from the Donkerhoek granite (southern Damara orogen, Namibia)
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DOI：
10.1016/j.lithos.2018.04.020
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发表时间：
2018-06
期刊：
影响因子：
3.5
通讯作者：
L. Schwark;Stefan Jung;F. Hauff;D. Garbe‐Schönberg;J. Berndt
中科院分区：
文献类型：
--
作者：
L. Schwark;Stefan Jung;F. Hauff;D. Garbe‐Schönberg;J. Berndt
541 ± 4 Ma的镁质、弱过铝质、钙碱性Donkerhoek Onanis花岗岩是约400 Ma的一部分。纳米比亚达马拉造山带南部地区6000平方公里的大型Donkerhoek岩基。主、微量元素呈线性变化，MgO、FeO、Al 2 O3、CaO、K2 O、Na 2 O、Ba和Sr浓度随SiO2的增加而降低，表明这部分岩基为一个凝聚体，经历了分离结晶过程。Donkerhoek Onanis花岗岩的同位素演化（初始εNd：−4.7至−12.3，初始87 Sr/86 Sr：0.7099-0.7157）具有中等放射成因Pb同位素比值（206 Pb/204 Pb：17.26-18.22; 207 Pb/204 Pb：15.59-15.67; 208 Pb/204 Pb：37.60-38.06）。除了不均匀性所赋予的来源，轻稀土分馏和Nd同位素数据的评价表明，AFC过程也修改了一些样品。根据化学和同位素数据，Donkerhoek Onanis花岗岩不能通过它们侵入的Kuiseb组的富Al和Fe的变质沉积岩的部分熔融而形成。相反，更有可能是具有元古代地壳居住年龄的变火成岩地壳源的熔融。该地区的三个火成岩-变火成岩岩组（无匹配斜长角闪岩、来自喀拉哈里克拉通南部基底的元古代镁铁质-长英质片麻岩、同构造Salem花岗闪长岩-花岗岩）是潜在的矿源。化学和同位素数据的评估表明，早期同造山Salem型花岗岩的重熔是最有可能的过程，这也可以解释ca。Donkerhoek Onanis花岗岩中的563 ± 4 Ma锆石。Donkerhoek Onanis花岗岩与实验得出的来自中间火成岩母体的熔体成分的比较表明温度在800和850 °C之间。认为达马拉带这一部分的泛非岩浆活动是一次中温壳内事件。虽然有一些成分的相似性，在俯冲带中产生的幼年花岗岩，非放射性铅同位素比值和中等放射性锶和非放射性Nd同位素表明，再加工地壳岩石更可能的来源。先前获得的Donkerhoek Onanis花岗岩的高δ 18 O值范围为11.8 - 13.6‰，覆盖了该地区Salem型花岗岩的δ 18 O值范围（12.5-13.3‰），证实了这一观点。相反的火成岩过程沿着活跃的大陆边缘，产生年轻的岩基与钙碱性亲和力，这火成岩事件不是一个主要的地壳形成的插曲和Donkerhoek Onanis花岗岩代表再加工地壳材料。
The 541 ± 4 Ma-old magnesian, weakly peraluminous, calc-alkalic Donkerhoek Onanis granite is part of the ca. 6000 km2large Donkerhoek batholith in the Southern Zone of the Damara orogen of Namibia. Linear major and trace element variations and decreasing MgO, FeO, Al2O3, CaO, K2O, Na2O, Ba and Sr concentrations with increasing SiO2indicate that this part of the batholith represent a coherent mass and underwent fractional crystallization processes. The Donkerhoek Onanis granites are isotopically evolved (initial εNd: −4.7 to −12.3, initial87Sr/86Sr: 0.7099–0.7157) with moderately radiogenic Pb isotope ratios (206Pb/204Pb: 17.26–18.22;207Pb/204Pb: 15.59–15.67;208Pb/204Pb: 37.60–38.06). Beside heterogeneities imparted by the sources, an evaluation of LREE fractionation and Nd isotope data suggests that AFC processes also modified some samples. Based on the chemical and isotope data, the Donkerhoek Onanis granites cannot be derived by partial melting of Al- and Fe-rich metasedimentary rocks of the Kuiseb formation in which they intruded. Instead, melting of meta-igneous crustal sources with Proterozoic crustal residence ages is more likely. Three igneous to meta-igneous rock suites from the area (Matchless amphibolites, Proterozoic mafic to felsic gneisses from the southern Kalahari craton basement, syn-tectonic Salem granodiorites to granites) are potential sources. An evaluation of chemical and isotope data suggests that remelting of early syn-orogenic Salem-type granites is the most likely process which would also explain the existence of ca. 563 ± 4 Ma-old zircon in the Donkerhoek Onanis granites. Comparison of the Donkerhoek Onanis granites with experimentally derived melt compositions from an intermediate igneous parent indicates temperatures between 800 and 850 °C. It is suggested that the Pan-African igneous activity in this part of the Damara Belt was a moderate-temperature intra-crustal event. Although there are some compositional similarities with juvenile granites generated in subduction zones, unradiogenic Pb isotope ratios and moderately radiogenic Sr and unradiogenic Nd isotopes suggest that reprocessed crustal rocks are more likely sources. Previously obtained high δ18O values of the Donkerhoek Onanis granites ranging from 11.8 to 13.6‰, covering the range of δ18O values obtained on Salem-type granites from the area (12.5–13.3‰) confirm this view. In contrast to igneous processes along active continental margins that produce juvenile batholiths with calc-alkaline affinities, this igneous event was not a major crust-forming episode and the Donkerhoek Onanis granites represent reprocessed crustal material.