DOI: 10.1016/j.epsl.2020.116084
论文题名: Testing emplacement models for the Rustenburg Layered Suite of the Bushveld Complex with numerical heat flow models and plagioclase geospeedometry
作者: Robb S.J. ; Mungall J.E.
刊名: Earth and Planetary Science Letters
ISSN: 0012821X
出版年: 2020
卷: 534 语种: 英语
中文关键词: Bushveld Complex
; diffusion
; layered intrusions
; numerical simulations
; thermal modeling
英文关键词: Binary alloys
; Computer simulation
; Cooling
; Diffusion
; Feldspar
; Geochronology
; Igneous rocks
; Lead alloys
; Mica
; Uranium alloys
; Zircon
; Zoning
; Alternative hypothesis
; Bushveld Complex
; Conventional models
; Crystallization temperature
; Layered intrusions
; Plagioclase crystals
; Thermal model
; Thermal perturbations
; Numerical models
; cooling
; crystallization
; emplacement
; heat flow
; igneous intrusion
; magma chamber
; plagioclase
; ultramafic rock
; zircon
; Bushveld Complex
; North West Province
; Rustenburg
; South Africa
; Calluna vulgaris
英文摘要: Conventional models of the genesis of the Rustenburg Layered Suite (RLS) of the Bushveld Complex postulate that it is the product of a single multiply-replenished magma chamber that cooled as a single massive unit, i.e., a monolithic magma chamber. This concept has been challenged by alternative hypotheses involving out-of-sequence emplacement of some of its major layers as sills. Both scenarios have partial support from high-precision U-Pb geochronological data. To test these contradictory hypotheses, new zircon geothermometric data and compositional data for plagioclase from the Upper Critical Zone are presented and the available geochronological data are tested against the predictions of the thermal evolution of the entire RLS for both emplacement and cooling scenarios. Crystallization temperatures of zircon within units of the Upper Critical Zone were estimated using Ti-in-zircon thermometry and range between 799±45 °C and 865±64 °C. The data are used in conjunction with a detailed numerical model of heat flow in the Bushveld Complex that is used to approximate the duration and extent of thermal perturbations felt throughout the system and delimit the time intervals during which zircon crystals could have grown at the measured crystallization temperatures. The results show that prolonged cooling histories (>1 Myr), that would be experienced near the centre of a large, slowly cooled, plutonic magma chamber, would erase signs of plagioclase zoning at 4 kbar and moderate fH2O; however, plagioclase crystals with zoning profiles spanning at least 10 mole% An are common in the Upper Critical Zone. In contrast, emplacement of the major units of the RLS at widely separated intervals, as recorded by the existing high-precision U-Pb dataset, would impose transient thermal effects on the system against a backdrop of generally much lower ambient temperatures throughout its evolution. In this scenario, apparent ages of zircon crystals would record the time of cooling through temperatures near the water-saturated biotite granite solidus, corresponding to the final intercumulus mineral assemblage in both mafic and ultramafic macrolayers. Subsequent reheating of these rocks would be unable to reset primary zircon crystallization dates but might, in some examples, result in some amount of remelting or regrowth of zircon. Comparison of the evolving temperature fields of the two types of emplacement and cooling histories shows that the existing geochronological data and preservation of plagioclase zoning are consistent with out-of-sequence emplacement and rapid cooling of individual ultramafic sills into previously solidified lithologies of the RLS but not with the conventional view of a single monolithic magma body. © 2020 Elsevier B.V. Citation statistics:
资源类型: 期刊论文
标识符: http://119.78.100.158/handle/2HF3EXSE/165241
Appears in Collections: 气候变化与战略
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作者单位: Department of Earth Sciences, Carleton University, 1125 Colonel By Drive, Ottawa, ON K1S 5B6, Canada
Recommended Citation:
Robb S.J.,Mungall J.E.. Testing emplacement models for the Rustenburg Layered Suite of the Bushveld Complex with numerical heat flow models and plagioclase geospeedometry[J]. Earth and Planetary Science Letters,2020-01-01,534