The growth of new crust: tracing the chemical and textural evolution of magma through multiple stages of magmatism, Chelan Migmatite Complex, North Cascades, Washingtion
Abstract
The Chelan Migmatite Complex (CMC) records multiple stages of mafic magmatism and partial melting within the deep crust (~30 km). Detailed mapping of a road cut that best represents the lithologies and textures found in the CMC document cross-cutting relationships involving at least five different periods of magmatism. The first is the coarse banded gneiss is composed of coarse grained (<1 cm) quartz, plagioclase, and biotite with hornblende-rich bands. The coarse banded gneiss is interpreted to correlate to previously dated Jurassic-Triassic metatonalite (emplaced ~150-170 Ma, partially melted and assimilated into the migmatite ~120-110 Ma). The second rock type is a finer grained (<0.5cm) quartz diorite gneiss. The quartz diorite gneiss is interpreted to correlate to previously dated meta-quartz diorite (95-105 Ma). The contact between the coarse banded gneiss and qtz diorite gneiss appears gradual. A fine-grained light grey (quartz and plagioclase rich) and dark grey (biotite and hornblende rich) mixed lithology (hereafter magma mixing unit) cuts the foliation in the coarse banded gneiss and qtz diorite gneiss, and was emplaced within the same time period as the leucosomes (see below); as some leucosomes are cross cut by the magma mixing unit, while others cross cut the magma mixing unit. The magma mixing unit has a relatively sharp contact with the older rock types and contains some angular inclusions (<4 cm) of the coarse banded gneiss. There are two types of leucosomes observed (qtz and plg only; and qtz, plg, and k-spar) and are interpreted to correlate with previously dated leucosomes (~115-90 Ma). A tonalitic dike cross cuts all units except one leucosome vein, making it likely part of the final crystallization period of the CMC. These results document multiple intrusions and presence of partial melt over ~30 Myr. Melt compositions evolve from mafic (qtz diorite gneiss) to felsic (pegmatitic tonalite and granite) consistent with evolution from mafic mantle-derived magma to intermediate or felsic crust.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFM.V43F0158K
- Keywords:
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- 1031 Subduction zone processes;
- GEOCHEMISTRY;
- 8031 Rheology: crust and lithosphere;
- STRUCTURAL GEOLOGY;
- 8150 Plate boundary: general;
- TECTONOPHYSICS;
- 8178 Tectonics and magmatism;
- TECTONOPHYSICS