Common lead, Sm-Nd, and U-Pb constraints on petrogenesis, crustal architecture, and tectonic setting of the Penokean orogeny (Paleoproterozoic) in Wisconsin
Common lead, Sm-Nd, and U-Pb constraints on petrogenesis, crustal architecture, and tectonic setting of the Penokean orogeny (Paleoproterozoic) in Wisconsin
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共同铅、Sm-Nd 和 U-Pb 对威斯康星州佩诺克造山运动（古元古代）岩石成因、地壳结构和构造环境的限制
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发表时间：
1997
期刊：
影响因子：
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通讯作者：
C. Johnson
中科院分区：
文献类型：
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作者：
N. V. Wyck;C. Johnson
在威斯康星州Penokean同造山期侵入岩套的形成过程中，Nd2+和Pb2+直接参与了古老陆壳的形成。深成岩的初值显示出明显的ϵ-ND增加趋势，从−6向南增加到+4，直到彭宾-沃索和马什菲尔德地体之间的缝合处，表明古老地壳的污染量正在减少。穿过太古宙马什菲尔德地体，迅速向下移动到较低的ϵND值(−1至−7)。同造山岩体中的长石普通铅一般定义了太古宙地壳和原始弧壳之间的混合阵列，其铅同位素组成与元古界地幔相似。基于NdPb浓度和同位素比值的混合模型表明，同造山侵入岩是通过与太古宙地壳相互作用而获得其同位素比值的，而不是通过沉积物俯冲对地幔源区的污染而获得的。此外，Pembine-Wausau地体裸露的基底直接测年首次表明这里存在太古宙地壳(U/Pb锆石年龄为2607±22 Ma)。联合的Nd和Pb同位素数据表明，区分前寒武纪造山带中地幔的混染和重熔的地壳基底对形成旧的ND模式年龄具有重要意义。这种区别对于评估前寒武纪造山地体的地壳增长估计至关重要。Penokean造山作用的首选构造模式是在Penokean造山的主要时期，南部Superior省的弧后裂谷作用，随后是马什菲尔德地体向南的碰撞，后者也受到Penokean时代岩浆的侵入。除了这里提供的同位素和年代学数据外，这个模型还得到了重力数据的支持，这些数据表明尼亚加拉断裂以南存在低密度物质(可能是太古宙地壳)；这一特征被解释为优势省南缘埋藏在彭宾-沃索地体之下的延伸。在Penokean造山作用期间，同造山侵入岩通过与太古代地壳的直接相互作用获得了其同位素组成。
Nd and Pb isotopes indicate significant direct involvement of older continental crust during the generation of the Penokean synorogenic intrusive suite in Wisconsin. Initial values for plutons show a clear trend of increasing ϵ Nd , from −6 to +4 southward from the Superior Province to the suture between the Pembine-Wausau and Marshfield terranes, indicating decreasing amounts of contamination by older crust. Crossing into the Archean Marshfield terrane, there is a rapid shift back down to lower ϵ Nd values (−1 to −7). Feldspar common Pb from synorogenic plutons generally defines a mixing array between Archean crust and primitive arc crust, which has Pb isotope compositions that are similar to those of the Proterozoic mantle. A mixing model based on Nd and Pb concentrations and isotopic ratios indicates that synorogenic intrusive rocks acquired their isotopic ratios through interaction with Archean crust, rather than through contamination of a mantle source region by sediment subduction. In addition, direct dating of exposed basement in the Pembine-Wausau terrane indicates, for the first time, that Archean crust is present here (U/Pb zircon age of 2607 ± 22 Ma). The combined Nd and Pb isotope data demonstrate the importance of distinguishing between contamination of the mantle and remelting crust basement for the origin of old Nd model ages in Precambrian orogens. Such distinctions are critical for evaluating crustal growth estimates in Precambrian orogenic terranes. The preferred tectonic model for the Penokean orogeny involves back-arc rifting of the southern Superior Province during a major period of Penokean orogenesis, followed by collision of the Marshfield terrane to the south, which was also intruded by Penokean-age magmas. In addition to the isotopic and geochronology data presented here, this model is supported by gravity data which indicate the presence of low-density material (probably Archean crust) south of the Niagara fault; this feature is interpreted to be a buried extension of the southern margin of the Superior Province beneath the Pembine-Wausau terrane. During the Penokean orogeny, synorogenic intrusive rocks acquired their isotope compositions through direct interaction with Archean crust.