The velocity of seismic S-waves through a rock is given by Vs = √(μ/ρ), where μ is the shear modulus (rigidity) and ρ is the density. A rock's S-wave speed depends on the balance between these two properties — a higher shear modulus increases speed, while higher density decreases it.
Gabbro is a mafic rock and is denser than granodiorite (a more felsic, intermediate rock), which on its own would tend to slow down S-waves...
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