科研之友
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摘要
Zircon from granulite-facies xenoliths can preserve unique geodynamic records of deep and mostly inaccessible levels of the continental crust within Archean cratons. We present the first such records from beneath two Archean crustal blocks that were affected by multiple episodes of regional-scale Proterozoic metamorphism, and are located within the western Churchill Province, Nunavut, Canada. Zircon U-Pb geochronology and trace element analysis were carried out on samples that originated in the deep crust at the time of kimberlite eruption, as confirmed with Raman spectroscopy. Zircon was recovered from four metabasites (grt-cpx-plg) from the Rankin Inlet area of the Chesterfield block, and five metabasites (grt-cpx-plg +/- hbl +/- opx) and three metatonalite/anorthosites (plg-cpx-plg +/- hbl +/- opx) from the Repulse Bay area of the Rae craton. SEM-CL imaging reveals several types of growth zoning diagnostic of igneous and metamorphic zircon crystallization. The latter are related to local high-temperature recrystallization and micro-fracturing, and are spatially associated with zones of apparent Pb-loss and/or intracrystalline Pb-variability. Evidence for the formation of Paleoarchean/Mesoarchean to Neoarchean crust is preserved in oscillatory zoned, igneous cores having Pb-207/Pb-206 ages of ca. 3.5 Ga and ca. 3.0-2.6 Ga. Two periods of Paleoproterozoic metamorphism and zircon growth followed, with the oldest episode at ca. 2.0-1.9 Ga in the Chesterfield block. Metamorphic zircon rims from all twelve xenoliths have an age range of ca. 1.75 to 1.70 Ga. The trace element compositions of the metamorphic rims differ from those of older igneous and metamorphic cores, with the rims characterized by lower Sigma HREE contents and low (Lu/Gd)(N), suggesting that garnet crystallization occurred syn- to pre-rim growth at ca. 1.75 Ga. The Archean and Paleoproterozoic ages correspond broadly to the upper crustal histories of the western Churchill Province; however, the prevalence of ca. 1.75-1.70 Ga metamorphic rims indicates regional-scale metamorphic reworking of the lower crust across the region, during a period of intracratonic extension and associated magmatism. Regional magmatic underplating and/or remobilization of the lithosphere at ca. 1.75-1.70 Ga, to an extent not widely appreciated, represents a terminal stage in western Churchill cratonization, including stabilization of reworked Archean crust relative to underlying diamondiferous mantle.
- 出版日期2014-2
