Petrology and Sr-Nd-Pb isotope geochemistry of Late Cretaceous continental rift ignimbrites, Kap Washington peninsula, North Greenland

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• 作者：Sigurjon B. Thorarinsson^a ; ^{st@avannaa.com} ; Paul M. Holm^a ; Helene I. Duprat^a ; Christian Tegner^b
• 关键词：Kap Washington ; North Greenland ; Continental rift ; Peralkaline ; Ignimbrite
• 刊名：Journal of Volcanology and Geothermal Research
• 出版年：2012
• 期刊代码：53_03770273
• 类别：ear
• 出版时间：15 March, 2012
• 卷：219-220
• 期：Complete
• 页码：63-86
• 文件大小：4634 K

摘要

The Late Cretaceous-Palaeocene (71-61 Ma) Kap Washington Group (KWG) volcanic sequence is exposed at the north coast of Greenland. The sequence is bimodal and was erupted in a continental rift setting during the opening of the Arctic Ocean. The succession exposed on the Kap Washington peninsula, which forms the bulk of the KWG sequence (> 5 km thick), has been sampled along four traverses with a combined stratigraphic thickness of ca. 1500 m. The sampled sequence is dominated by silicic ignimbrites (69-79 wt.%SiO₂) showing geochemical features typical of ferroan, A-type granitoids. The ignimbrites range from sparsely phyric, mildly peraluminous compositions [ASI = Al₂O₃/(CaO + Na₂O + K₂O) = 1.05-1.20] to feldspar + quartz 卤 sodic amphibole 卤 Fe-Ti oxide phyric peralkaline compositions [PI = (Na₂O + K₂O)/Al₂O₃ = 1.00-1.40]. The peraluminous ignimbrites appear to overlie the peralkaline ignimbrites, although stratigraphy is complicated by faulting. Fiamme imbrication indicates that both types were erupted from a vent area located north of the Kap Washington peninsula.

The peralkaline ignimbrites have Sr-Nd-Pb isotopic compositions which overlap with the compositions of KWG basalts, indicating a dominantly basaltic source. The more peralkaline compositions were generated by up to ca. 50%fractional crystallisation of alkali feldspar-quartz-dominated assemblages from mildly peralkaline parental magmas, themselves probably derived by fractionation of trachytic magmas. The peraluminous ignimbrites have slightly negative 蔚_Nd(i) and more radiogenic ²⁰⁷Pb/²⁰⁴Pb_i and ²⁰⁸Pb/²⁰⁴Pb_i. Modelling indicates that they are not cogenetic with the peralkaline ignimbrites and they are inferred to have originated by partial melting of hybridised mafic crust. Petrographic evidence suggests that magma mixing was an important process and variations in Nd-Pb isotopes and trace element ratios indicate mixing between peralkaline and peraluminous magma batches.