A Nd Isotopic Study of Early Proterozoic Crustal Growth in Southern Wyoming

Sm-Nd同位素系统学将应用于怀俄明州南部古老的地壳缝合带夏延带(CB)内及其邻近的前寒武纪变质沉积岩和变质火成岩，约束了元古代陆壳的成因模式和这些地壳段向先前存在的太古代大陆克拉通边缘的吸积。夏延带是北美一个暴露良好的太古宙大陆边缘的罕见例子，显然没有直接被一大段“混合”的元古界地壳所覆盖，即在元古代产生的与太古宙地壳物质混合的地壳。该提案将集中于确定怀俄明州克拉通南缘没有这种地壳是否反映了这样一个事实，即这种地壳从未在该地区(与太古宙克拉通边缘的其他部分不同)积聚，或者是否沿着CB的构造运动已经移走并混合了可能原本存在的地壳。将开展三个相互独立但又相互关联的项目：1)CB以北的元古界微地斜沉积岩的Sm-ND同位素系统学。目的是利用这些沉积物的Nd同位素组成的时间变化来确定由新形成的年轻或“混合”地壳物质组成的元古界岛弧是否最初接近古大陆边缘，2)CB内部构造块体中副片麻岩和正片麻岩的Sm-Nd同位素体系，为了确定这些岩石是否代表最初可能在怀俄明州克拉通南缘沉积的混合地壳的残留物，以及3)CB以南科罗拉多省元古代火成岩和变质沉积岩的同位素系统学，以确定该地体最北端是否存在“混合”地壳，并确定变质沉积岩是否记录了该地体最初撞击的地壳的同位素特征(太古宙或“混合”地壳)。所有这三个项目的成果都将与现有的地质和构造数据相结合，试图构建怀俄明州太古宙克拉通南缘前寒武纪构造演化的模型。

Sm-Nd isotope systematics will be applied to Precambrian metasedimentary and metaigneous rocks in and adjacent to the Cheyenne Belt (CB), an ancient crustal suture zone in southern Wyoming, constrain models for the genesis of Proterozoic continental crust and for the accretion of these crustal segments to the margins of preexisting Archean continental cratons. The Cheyenne Belt represents a rare example of a well exposed Archean continental margin in North America which is apparently not directly mantled by a large segment of "mixed" Proterozoic crust, that is, crust produced in the Proterozoic that intermixed with Archean crustal material. The proposal will center on determining whether the lack of such crust at the southern margin of the Wyoming craton reflects the fact that such crust was never accreted in this region (unlike other segments of Archean cratonal margins) or whether tectonic movement along the CB has removed and mixed crust that may have originally been present. Three separate, but interrelated projects will be undertaken 1) Sm-Nd isotope systematics of Proterozoic miogeoclinal sedimentary rocks north of the CB. The purpose will be to use the temporal variations in the Nd isotopic compositions of these sediments to determine whether Proterozoic island arcs composed of newly formed juvenile or "mixed" crustal material initially approached the ancient continental margin, 2) Sm-Nd isotopic systematics of para-and orthogneisses in tectonic blocks within the CB itself, to determine if these rocks represents remnants of "mixed crust that might have initially accreted against the southern margin of the Wyoming craton, and 3) the isotope systematics of Proterozoic igneous and metasedimentary rocks of the Colorado province south of the CB, to determine if "mixed" crust is present in the northernmost portions of this terrane, and to determine if the metasedimentary rocks record the isotopic characteristics (Archean or "mixed" crust) of the crust against which this terrane initially impinged. The results of all three projects will be integrated with existing geologic and tectonic data in an attempt to construct a model for the Precambrian tectonic evolution of the southern margin of the Wyoming Archean craton.