Collaborative Research: Influence of a Proterozoic Southern Laurentia long-Lived Convergent Orogen In the Lake Superior region, USA

PI建议采用最新的地质年代学技术来完善他们对南苏必利尔湖地区元古代地质的理解和模型。他们的目标有两个：1)利用U-Pb单氮石地质年代学（离子探针和电磁脉冲）来记录和直接确定Penokean和随后的变质事件（即Yavapai， Mazatzal和Wolf River变质）的年代，试图将这些事件与Penokean造山运动后的持续俯冲联系起来；2)利用Ar-Ar激光微探针粒度测年，评价多地壳事件对区域热记录的影响。他们的研究结果最终将用于在一个长期存在的汇聚造山带模型的背景下建立这个历史地区的元古代地质（Karlstrom等人，2001）。变质geochronometry。最近由美国国家科学基金会支持的研究（Holm和VanSchmus）对后佩诺肯花岗岩起源的研究结果表明，长期以来被认为只发生在1760 Ma的岩浆活动，实际上发生在1800、1775和1750 Ma的三个独立的、通常是东南迁移的脉冲中。范·施莫斯、霍尔姆和他们的合著者推测，这些岩浆脉冲可能与大陆向南生长的向北俯冲有关。在一种可能的情况下，在1800 - 1750 Ma之间，亚瓦派时代第17代洋板块向北俯冲的板块回退导致大陆弧岩浆活动普遍向东南方向移动。随着岩石变陡，它就会脱离上覆过厚的地壳。压缩应力的减少和热输入的增加使得造山带的伸展崩塌和随后的但短暂的地壳稳定成为可能。PI建议进行详细的变质研究，以验证广泛的变质作用（而不仅仅是广泛的冷却）发生在迁移弧岩浆活动中，并且可能在空间上与构造挤压（即崩塌）结构有关的假设。他们对变质独居石的初步SHRIMP和EMP定年似乎支持这一点。这些数据将首次为苏必利尔湖南区佩诺肯期和后佩诺肯期变质作用提供直接的时间限制。变质thermochronometry。在20世纪90年代，美国国家科学基金会资助的（霍尔姆）Ar-Ar矿物年龄数据被用来记录造山带冷却和后来热再加热的时间和程度。例如，丰富的1760-1720 Ma高原冷却年龄被解释为与快速的地壳塌陷/稳定期有关，1630-1600 Ma高原年龄被解释为与南部的马扎塔造山运动有关的再加热/变形。然而，初步的氩离子激光微探针年龄数据记录了白云母晶体之间存在显著的年龄梯度（在一个案例中为500毫微米）。这些结果值得进一步审查现有的热年代学数据。具体而言，1630-1600年Ma Ar-Ar高原年龄是否反映了广泛变形和温和再加热的时间；Ii)代表与第14期岩浆活动有关的部分重置；Iii)或者可能代表缓慢冷却？Ar-Ar白云母激光探测年龄测绘将使PI能够更好地约束该地区的中温历史，特别是将其与热再加热地区较年轻的套印事件联系起来。本研究收集的数据对于正确解释新的火成岩年龄（由Van Schmus， Holm等人获得）和现有的Ar-Ar年龄以及在长期会聚造山带模型背景下评估南苏必利尔湖地区元古代地质至关重要。

The PI's propose to employ recent advancements in geochronometric techniques to refine their understanding and models of the Proterozoic geology of the southern Lake Superior region. Their objectives are two-fold: 1) to use U-Pb monazite geochronometry (both ion probe and EMP) to document and directly date Penokean and subsequent metamorphic events (i.e., Yavapai, Mazatzal and Wolf River metamorphism) in an attempt to link these episodes to continued subduction after the Penokean orogeny; and 2) to use Ar-Ar laser microprobe grain-scale age mapping to evaluate the influence of multiple crustal events on the regional thermal record. Their results will be used ultimately to establish the Proterozoic geology of this historical region in the context of a long-lived convergent orogen model (Karlstrom et al., 2001). Metamorphic geochronometry. Results of recent NSF-supported research (Holm and VanSchmus) on the origin of post-Penokean granites show that magmatism long thought to be solely1760 Ma, actually occurred in three separate, generally southeastward-migrating pulses at 1800,1775, and 1750 Ma. Van Schmus, Holm and their co-authors have postulated that thesemagmatic pulses could correlate with northward-directed subduction associated with southwardgrowth of the continent. In one possible scenario, geon 17 Yavapai-age slab rollback of anorthward subducting oceanic plate caused continental arc magmatism to step generallysoutheastward between 1800 and 1750 Ma. As the slab steepened, it would have disengagedfrom the overlying, overthickened crust. The reduced compressional stresses and increasedthermal input allowed for extensional collapse of the orogen and subsequent but short-livedcrustal stabilization. The PI's propose to carry out a detailed metamorphic study to test the hypothesis that widespread metamorphism (and not simply widespread cooling) occurred with the migrating arc magmatism and might be linked spatially to tectonic extrusion (i.e., collapse) structures. Their preliminary SHRIMP and EMP dating of metamorphic monazites appear to support this. These data will provide the first direct timing constraints on both Penokean and post-Penokean metamorphism in the southern Lake Superior region.Metamorphic thermochronometry. In the 1990's, NSF-funded (Holm) Ar-Ar mineral age datawere used to document the timing and extent of orogenic cooling and later thermal reheatingacross the Penokean orogen. For instance, abundant 1760-1720 Ma plateau cooling ages areinterpreted to be associated with a rapid crustal collapse/stabilization period and 1630-1600 Maplateau ages are interpreted to indicate reheating/deformation associated with the Matzatalorogeny to the south. However, preliminary Ar-ion laser microprobe age data documents thepresence of significant age gradients across muscovite crystals (500 m.y. in one case). Theseresults merit further scrutiny of the existing thermochronologic data. Specifically, do the 1630-1600 Ma Ar-Ar plateau ages i) reflect the time of widespread deformation and mild reheating; ii)represent partial resetting associated with geon 14 magmatism; iii) or perhaps represent slowcooling? Ar-Ar muscovite laser probe age mapping will allow the PI's to better constrain the medium-temperature history of the region and, especially to relate it to younger overprinting events in the thermally reheated areas. The data collected in this study are critical to properly interpret the new igneous ages (obtained by Van Schmus, Holm, and others) and existing Ar-Ar ages and to evaluate the Proterozoic geology of southern Lake Superior region in the context of a long-lived convergent orogen model.