Browsing by Subject "Laser ablation inductively coupled plasma mass spectrometry (LA ICP-MS)"
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Item Assembly and evolution of the Wooley Creek Batholith: Evidence from mineral compositions and U-Pb geochronology(2012-08) Coint, Nolwenn; Barnes, Calvin G.; Yoshinobu, Aaron S.; Hetherington, Callum J.; Ridley, Moira K.Modalities of batholith assembly within the crust are still debated. Numerous studies conducted over the past 60 years suggest that processes such as fractional crystallization, assimilation, or mixing can account for a lot of the chemical variations associated with intrusive suites. Recent U-Pb geochronology data obtained on zircon show however that some of the cordilleran batholith can b emplaced over several millions of years. Emplacement of large intrusion is now thought to be an incremental process, based on the thermal models which indicate that even large batches of magma cannot remain above their solidus more than a couple million of years. In these models, small magma batches are emplaced in the crust through a long period of time, limiting the possible interactions between these batches and prohibiting the development of large magma tic reservoirs capable of feeding large eruptions. This goal of this study was to reconstruct the assembly of one of these cordilleran batholith, the Wooley Creek batholith and the Slinkard pluton, which are part of the same magmatic system, in order to characterize and map the size of the magma batches that were once capable of chemically and physically interacting. Two main approaches were taken. The first one, pretty common in the literature, is to use TIMS U-Pb geochronology on zircon to date the various part of the intrusion. Preliminary results indicate that the main part of the Wooley Creek batholith and Slinkard pluton system crystallized over less than 3 m.y. These results do not prohibit the development of a sustainable magma chamber. The second approach consisted on using trace element in rock forming minerals (augite and hornblende in this case) to characterize different magma batches and track their evolution through time. While the lower part of the system is constituted of small magma batches that evolved individually, the upper zone was more homogeneous and the consistency in the hornblende composition throughout the whole unit indicate that it was once a single large batch of magma. Upward zoning with more felsic rocks cropping out at the top of the system can be accounted for by percolation of the differentiated melt through the mush. The presence of roof dikes associated with the upper zone indicates that this part of the system was once eruptible. This study introduce a new technique to study magmatic evolution and characterize the size of the magma batches in the system, which combined with U-Pb geochronology, give a powerful tool to understand pluton assembly and the development of magmatic reservoirs. In the case of the Wooley Creek batholith and the Slinkard pluton it shows that modalities of assembly can differ from one part of the intrusion to another, despite a similar timescale of magma input. Results associated with assimilation processes indicate that the preserved record depends on the size of the magma batch interacting with the assimilated material. In the upper zone, the effect of assimilation is diluted within the large volume of magma, whereas in the central part, where batches are much smaller, it can be easily characterized.