| 英文摘要: | Magmatic activity associated with the convergence and subduction of tectonic plates typically results in an array of volcanic complexes in the overriding plate and their subterranean plumbing systems. Given that volcanic systems and related intrusive igneous rocks form over tens of millions of years and commonly result in large volcanic edifices that are constantly subjected to erosion and burial, reconstructing the complete timing and geochemical evolution of volcanic systems can be difficult due to large gaps in the geologic record. In this study, the principal investigators are using a trace mineral commonly found in sedimentary rocks to help fill in gaps in our understanding of the geologic evolution of the volcanic and intrusive rocks of the Sierra Nevada volcanic arc of California. The object of this study is zircon, a mineral that commonly forms when igneous crystallize and its chemical structure contains radioactive elements that can be precisely dated by isotopic methods. In addition, other chemical elements provide a means by which the chemical evolution of the magmatic system may be revealed. Zircon is an unusually resistant mineral to chemical and mechanical breakdown in fluvial systems and it is commonly preserved in sedimentary rocks. Its abundance in many sedimentary rocks ensures that it can provide a record of volcanic and igneous rocks that have long since been eroded away by analyzing their chemistry and dating them. Analysis of such zircons thus provides a means by which the history of missing parts of volcanic arcs may be reconstructed, and this information can be used to unravel details of the tectonic and geodynamic evolution of the region in which the volcanic rocks were formed. In addition to the scientific objectives of the research, the project is contributing to important societal outcomes, including the training and mentoring of undergraduate students in a STEM discipline, as well as broadening of participation of underrepresented groups in science. The project represents a four-part collaboration between academic institutions of diverse type and educational mission.
The goal of this study is to combine age and trace element geochemistry of detrital zircons as proxy records for the evolution of the Mesozoic Cordilleran magmatic arc system. Zircon is an accessory mineral in a wide variety of igneous rocks, and is resistant to recrystallization during hydrothermal alteration and sedimentation. Detrital zircon can yield time-integrated records of magmatic systems. Zircon from in situ igneous rocks and detrital zircons derived from them together have the capability of recording both precise ages and variations in melt compositions in a long-lived magmatic environment, and when paired with studies of in situ igneous rock suites, detrital zircon records may provide a relatively more complete and detailed understanding of vertical and secular variations in partially eroded and incompletely exhumed magmatic systems. The principal investigators in this study will analyze populations of arc-derived detrital zircons in order to develop detrital proxy records of (1) the age and petrologic asymmetry of this arc that was constructed across a variable basement and (2) the geochemical variability of magmas through pulses and lulls in arc magmatism. Based on observations from an initial data set derived from a retroarc foreland basin, the principal investigators hypothesize that arc-derived detrital zircons can provide a temporally-controlled record of the evolution of average melt compositions over the full life span of the Cordilleran arc. They will study detrital zircon from a series of stratigraphic sections from within the forearc, intra-arc, and retroarc regions of the Cordilleran arc. Uranium-Lead (U-Pb) isotopic dates and trace element geochemistry of detrital zircons from these sections will allow them to evaluate the latitudinal and longitudinal variations in arc magmatism, and secular variations in zircon geochemistry will in turn help them test hypotheses regarding average melt compositions during magmatic pulses and lulls. These zircon age and geochemical records will also allow us to link magmatic pulses to tectonic events, such as episodes of crustal thickening within and beneath the arc. Thus these zircon geochemical data will constitute a useful new tool, both by describing the petrotectonic evolution of this long-lived arc system and by adding new geochemical constraints to detrital zircon provenance interpretations. |