Using hornblende geochemistry to unravel the emplacement of magmatic structures, Tuolumne Intrusive Complex, California



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It has been suggested that magmatic structures preserved in plutons may provide information about pluton emplacement and chamber construction; information that is critical for evaluating geochronological data, which indicate that large plutons are gradually emplaced by small magma batches rather than an individual large magma batch. Physical processes recorded in magmatic structures might provide information about the magma conditions prior to solidification, but due to the complexity of formation and petrogenesis of these structures, separating or identifying magma batches on the basis of contact relationships could be very difficult. In this study, major element and trace element variations of hornblende are used to interpret the origins of magmas that formed magmatic structures from Pothole Dome, Tuolumne Intrusive Complex (TIC) and to identify different magma batches and the magmatic processes that led to the complexity of the magmatic structures. Trace element compositions and the variable zoning patterns in hornblende from the magmatic structures cannot be related by fractional crystallization of a single magma batch, nor can they be related by simple magma mixing of two magmas. Instead, the diverse and distinct zoning patterns suggest involvement of at least three magma batches represented by: 1, Hornblende in sample 146B and hornblende 137i-hb4 (reversely zoned, high Ti, Zr, Y, Sr, low Sc, and no concave-up REE patterns); 2, Hornblende in sample 144B, and hornblende crystals 137ii-hb4, 140i-hb1 and 140-137-hb1 (normal zoning with slight to moderate concave-up REE patterns); 3, Hornblende from sample 137, 140i and 140-137 (reversely zoned, low Ti, Y, Zr, Sr, high Sc, and slight to moderate concave-up REE patterns).
The compatible behavior of Zr, Sr, and Ba in most of the samples is interpreted to result from crystallization of zircon, plagioclase, potassium feldspar and biotite. The concave-up REE patterns are likely to be caused by sphene crystallization.



Hornblende, Trace element, Granodiorite, Magmatic structure