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

PI‘s建议利用地质计时技术的最新进展来完善他们对南部苏必利尔湖地区元古界地质的理解和模型。他们的目标有两个：1)使用U-Pb独居石测年(离子探针和EMP)来记录和直接测定Penokean及其后续变质事件(即Yavapai、Mazatzal和Wolf River变质作用)的年龄，试图将这些事件与Penokean造山后的持续俯冲联系起来；以及2)使用Ar-Ar激光显微探针颗粒年龄填图来评估多个地壳事件对区域热记录的影响。他们的结果最终将被用来在长寿汇聚造山带模型(Karlstrom等人，2001年)的背景下建立这一历史地区的元古界地质学。变质地质计时学。最近由NSF支持的关于后Penokean花岗岩成因的研究(Holm和VanSchmus)的结果表明，长期以来被认为是唯一的1760 Ma的岩浆活动，实际上发生在1800 Ma、1775 Ma和1750 Ma三个独立的、大致向东南迁移的脉冲中。Van Schmus、Holm和他们的合著者假设，这些岩浆脉冲可能与大陆向南生长相关的向北俯冲有关。在一种可能的情况下，在1800至1750 Ma之间，雅瓦派17岁的板块向正向俯冲的大洋板块的回滚导致大陆弧岩浆活动总体上向东南走。当板块变陡时，它就会从上面加厚的地壳中脱离出来。挤压应力的降低和热输入的增加允许造山带的伸展坍塌和随后的但短暂的地壳稳定。PI建议进行一项详细的变质研究，以检验广泛的变质作用(而不仅仅是广泛的冷却)与迁移的弧岩浆作用发生，并可能在空间上与构造挤压(即坍塌)构造联系的假说。他们对变质独居岩的初步SHRIMP和EMP测年似乎支持这一观点。这些数据将首次为南湖优势区的Penokean和Penokean后变质作用提供直接的时间约束。变质热年代学。上世纪90年代，S利用美国国家科学基金会(HOLM)的Ar-Ar矿物年龄数据，记录了佩诺基山造山带的造山冷却和后来的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地壳快速塌陷/稳定期有关，1630-1600 Ma高原年龄被解释为与南部的马扎塔尔造山作用有关的重新加热/变形。然而，初步的Ar离子激光微探针年龄数据证明，在白云母晶体中存在显著的年龄梯度(500m.y。在一个案例中)。这些结果值得对现有的热年代学数据进行进一步的审查。具体地说，1630-1600 Ma的Ar-Ar高原年龄：i)反映了大范围变形和温和再加热的时间；ii)代表了与14号岩浆活动有关的部分重置；iii)或者可能代表了缓慢冷却？Ar-Ar白云母激光探头年龄测绘将使PI能够更好地约束该地区的中温历史，特别是将其与热再热地区较年轻的叠印事件联系起来。这项研究收集的数据对于正确解释Van Schmus和Holm等人获得的新的火成岩年龄和现有的Ar-Ar年龄，以及在长寿汇聚造山带模型的背景下评价南部优越湖地区的元古界地质是至关重要的。

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.