MULTIPLE POPULATIONS OF MAGMATIC EPIDOTE AND MAGMATIC GARNET FROM THE NORTH CASCADE MOUNTAINS, WASHINGTON, U.S.A.: GEOCHEMICAL INDICATORS OF EARLY MAGMATIC PROCESSES

Two weakly deformed epidote-bearing pegmatitic trondhjemite intrusions, representing part of the high-pressure magmatic system of the northwestern U.S. Cretaceous-volcanic arc, occur in the North Cascade Mountains, Washington. These intrusions, containing quartz, albite/oligoclase, muscovite, magmatic epidote, and rare magmatic garnet and apatite, were emplaced within an orthogneiss metamorphosed at amphibolite facies conditions. Whole-rock geochemistry suggests that the melts were derived from lower crustal mafic rocks, likely decompression melts. Laser ablation inductively coupled plasma mass spectrometry and electron microprobe has been used to collect major, minor, and trace element concentrations for whole rock samples and individual minerals.

Significantly, the epidote occurs in seven texturally and chemically distinct populations, with individual crystals up to 13 cm. The larger epidote crystals are interpreted as liquidus crystals due to their euhedral internal zoning and lack of evidence of mafic-mineral + melt reaction, as other workers have previously noted. This detailed geochemical study allows for insight into early high-pressure magmatic processes.

Epidote element concentrations were used in SPSS software to statistically correlate textural and chemical populations. Differences between these populations were used to geochemically test hypotheses of early magmatic processes. Two processes were indicated to be dominant during epidote crystallization. Magma differentiation resulting from fractionation of early magmatic minerals led to decreasing rare earth element (REE) concentrations in later epidote populations and breakdown of heavy REE (HREE) rich mineral(s) significantly altered the HREE geochemical patterns of subsequent epidote populations.

These tests, coupled with the overall heterogeneity of the trace element geochemistry between populations, indicate that early magma formation likely involved multiple melt pods that coalesced during magma transport and ascent. Experimental work on high-pressure felsic magmatic systems containing epidote and garnet show the magmas were transported at least 17 km; this coupled with the geochemical research of this study allows for a model of magma formation and transport to be developed.