科研之友
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摘要
The Archean (ca. 2.97 Ga) Fisken ae sset layered intrusion, southwestern Greenland, consists of an association of anorthosite, leucogabbro, gabbro, hornblendite, pyroxenite, peridotite and dunite. The intrusion is characterized by well-preserved igneous layering, cumulate texture and primary igneous minerals including olivine, pyroxene, plagioclase, hornblende and chromite. We use new whole-rock (n = 36) and mineral (n = 32) oxygen isotopic data for all major lithologic units from the best preserved stratigraphic section of the Fisken ae sset Complex at Majorqap qava to revisit geodynamic and petrogenetic hypotheses proposed for the origin of Archean terranes. The Fisken ae sset Complex has modern mantle-like whole-rock O-isotope compositions (delta O-18 = 5.8 +/- 0.5 parts per thousand). Average delta O-18 values increase from peridotite (delta O-18 = 5.0 parts per thousand), through hornblendite (delta O-18 = 5.7 parts per thousand), gabbro (delta O-18 = 5.8 parts per thousand), pyroxene hornblendite (delta O-18 = 6.0 parts per thousand) and leucogabbro (delta O-18 = 6.3 parts per thousand), to anorthosite (delta O-18 = 6.3 parts per thousand). These whole-rock isotopic compositions reflect the approximate modal abundances of olivine (average delta O-18 = 4.9 parts per thousand), hornblende (average delta O-18 = 5.7 parts per thousand), clinopyroxene (average delta O-18 = 6.4 parts per thousand) and plagioclase (average delta O-18 = 6.4 parts per thousand) in each rock type, as a consequence of mineral fractionation in the magma chamber(s). Field relationships and the absence of crustal contamination suggest that the Fisken ae sset Complex formed in an oceanic setting. Subduction zone-like whole-rock trace element signatures and mantle-like delta O-18 and initial epsilon(Nd) values are consistent with formation of these rocks in a juvenile oceanic island arc setting. Field and geochemical data from the Fisken ae sset region and adjacent terranes suggest that the origin of Archean crust in southwestern Greenland is consistent with Phanerozoic-like plate tectonic processes rather than density-driven sinking, delamination and diapiric processes requiring formation of greenstone belts and anorthosite complexes on pre-existing continental crust. Mantlelike delta O-18 values in the ca. 2.97 Ga Fisken ae sset Complex are inconsistent with substantial recycling of continental crust, indicating that the Fisken ae sset rocks originated distal from continental sources.
- 出版日期2014-6-15
