Timescales of Pluton Construction and the Crystallization History of the Mt Stuart Batholith, North Cascades, WA
Abstract
Unraveling the magmatic and thermal evolution of arc magmatism is essential for understanding the processes of magma transport, crustal growth and evolution. We suggest the approach of using the histories of individual plutons in eroded arcs to understand the origin of magmatic diversity and magma plumbing systems. Relevant questions are:on what timescales are batholiths constructed? Can silicic magma chambers be repeatedly rejuvinated by influx of mafic magmas. How long are mafic magmas intruded into an evolving pluton? We are evaluating these questions by detailed study of the Mt Stuart Batholith, a >1400 km3 plutonic complex emplaced in the crystalline core of the 96-46 Ma North Cascades arc. By examining the construction of the MSB in detail, we may draw inferences about the construction of arc plutons in general. The MSB ranges in composition from gabbro near its NE margin to granodiorite in its core with gradational contacts between phases. Tonalite and granodiorite comprise >70% of the outcrop area. New U-Pb zircon geochronology from 9 samples indicate that the MSB was constructed over a ca. 5.4 m.y. period from ca. 96.3 Ma to ca. 90.9 Ma. Significant amounts of mafic magma were emplaced throughout this time period with crystallization dates from a diorite at ca. 96.2 Ma and two samples of gabbro at ca. 90.9 Ma and 91.2 Ma. The oldest dated phases (ca 96 Ma) occur in the NW and central part of the batholith and cover the range of observed compositions. The bulbous, SE part of the batholith comprises the youngest phases of gabbro, tonalite and granodiorite. Magmatic foliations are continuous with foliation in the host rock, are locally at high angles to pluton margins, and can be traced across contacts between different phases. These observations indicate that this foliation is not related to intrusion and suggests that the regional strain field remained constant throughout emplacement. The oldest phases remained a crystalline mush during intrusion of progressively younger phases or the younger phases partially remelted and reoriented magmatic foliation in a zone around the older phases they intrude. The new geochronological data indicate that mafic magmas were emplaced throughout the 5 Ma history of pluton growth, and that the thermal effects of a pluton may be significant over a several m.y. period. By dating multiple samples from a single batholith, we have constrained the scale of heterogeneity within a pluton, and suggest that in general it may be inappropriate to view a pluton as composed of large, discrete batches of magma. The continuity of magmatic foliation in pulses of different ages raises questions about the rheology of an evolving pluton.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2002
- Bibcode:
- 2002AGUFM.T11E..09M
- Keywords:
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- 8125 Evolution of the Earth;
- 8145 Physics of magma and magma bodies