Evolution of upper crustal magmatic plumbing systems: Comparisons of geochronological, petrological, geochemical and structural records in the Cretaceous Fangshan pluton, Beijing, China, the Mesozoic Central Sierra arc, CA, USA and the Miocene Silver Creek caldera, AZ, USA.

摘要

Magmatism is the most important process to form crust in Earth's history. Both volcanic and plutonic rocks are major components of Earth's current crust and their geochronologic, geochemical and structural features provide key information for rock sequence, source reservoirs, tectonic environments and emplacement mechanisms of igneous rocks. In order to better picture the igneous rocks as a whole I picked three upper crustal magmatic systems in different tectonic environments to study. Fangshan pluton is an intraplate pluton that emplaced at the depth of about 10-15; Central Sierra plutons and correlated volcanic rocks are subduction related magmatism with the plutons emplaced at about the same depth as Fangshan; Silver Creek caldera is a much shallower magmatic system with intralcaldera tuff and subvolcanic plutons. High-precision CA-TIMS (chemical abrasion thermal ionization mass spectrometer) geochronology of Fangshan pluton rocks indicated a much longer history of the magmatic system below the pluton than what people realized before. The existence of antecrystic zircons shows the intense recycling of crystals by multiple magma pulses. The same technique was used on Silver Creek plutonic and volcanic rocks. No antecrystic zircons have been found so far in both the volcanic and subvolcanic plutons, which indicates a relatively simpler history for this magmatic system and the existence of possible filtering mechanisms of older zircons. Considering all single zircon ages from oldest to youngest, the lifespan of this system is only about 1.5 m.y. that is a lot shorter than those deeper magmatic systems. Regional geochemistry study done on the Central Sierra Mesozoic igneous rocks show a close link between the two rock types and the important roles magma fractionation, mixing and mingling played during the formation of these magma bodies. The magma source below this area did not change much through the whole Mesozoic though. In order to see the rock fabrics in Silver Creek caldera, AMS study was carried on for 23 samples from this caldera. The results show extremely weak fabrics in both shallow intrusions and volcanic rocks. Although the randomly oriented magnetic lineations in the intracaldera tuff can't be well connected with the outflow magnetic lineations, it does agrees well with the typical caldera forming model of mechanically collapsing of the magma chamber roof after the chamber drained. Magnetic fabrics in the plutonic rocks do not form any noticeable pattern. More data need to be collected to make sure whether there is pattern or not. Multiple data sets from different magmatic systems do show the complications involved in this process and correlated study of both plutonic and volcanic rocks of the same system is a powerful way to understand any single magma plumbing system. There is still some fundamental difference between the plutonic and volcanic rocks, so the correlation of them needs careful evaluations still.