Keller Duncan S, Lee Cin-Ty A, Peck William H, Monteleone Brian D, Martin Céline, Vervoort Jeffrey D, Bolge Louise
Department of Earth, Environmental and Planetary Sciences, Rice University, Houston, TX 77005, USA.
Department of Earth and Environmental Geosciences, Colgate University, Hamilton, NY 13346, USA.
Sci Adv. 2024 Aug 16;10(33):eadn3976. doi: 10.1126/sciadv.adn3976. Epub 2024 Aug 14.
Massif-type anorthosites, enormous and enigmatic plagioclase-rich cumulate intrusions emplaced into Earth's crust, formed in large numbers only between 1 and 2 billion years ago. Conflicting hypotheses for massif-type anorthosite formation, including melting of upwelling mantle, lower crustal melting, and arc magmatism above subduction zones, have stymied consensus on what parental magmas crystallized the anorthosites and why the rocks are temporally restricted. Using B, O, Nd, and Sr isotope analyses, bulk chemistry, and petrogenetic modeling, we demonstrate that the magmas parental to the Marcy and Morin anorthosites, classic examples from North America's Grenville orogen, require large input from mafic melts derived from slab-top altered oceanic crust. The anorthosites also record B isotopic signatures corresponding to other slab lithologies such as subducted abyssal serpentinite. We propose that anorthosite massifs formed underneath convergent continental margins wherein a subducted or subducting slab melted extensively and link massif-type anorthosite formation to Earth's thermal and tectonic evolution.
地块型斜长岩是侵入地壳的巨大且神秘的富含斜长石的堆积侵入体，仅在10亿至20亿年前大量形成。关于地块型斜长岩形成的相互矛盾的假说，包括上升地幔的熔融、下地壳的熔融以及俯冲带上方的弧岩浆作用，阻碍了人们就何种母岩浆使斜长岩结晶以及为何这些岩石在时间上受到限制达成共识。通过硼、氧、钕和锶同位素分析、全岩化学成分分析和成岩模拟，我们证明，作为北美格林维尔造山带经典实例的马西和莫林斜长岩的母岩浆，需要来自板片顶部蚀变洋壳衍生的镁铁质熔体的大量输入。这些斜长岩还记录下了与其他板片岩性（如俯冲的深海蛇纹岩）相对应的硼同位素特征。我们提出，斜长岩地块在汇聚大陆边缘之下形成，在那里，俯冲或正在俯冲的板片发生广泛熔融，并将地块型斜长岩的形成与地球的热演化和构造演化联系起来。
