A COMPLEX MAGMATIC INJECTION ZONE IN THE HALF DOME GRANODIORITE, TUOLUMNE INTRUSIVE SUITE, SIERRA NEVADA BATHOLITH, CA

Previous work on the Tuolumne Intrusive Suite has shown that it formed from multiple pulses of magma. However, the timing, the size and composition of pulses, location of possible magma conduits, and physical state of the magma chamber as later pulses were emplaced remain poorly resolved.

New mapping has identified a roughly linear, semi-continuous belt of mafic schlieren near the equigranular Half Dome (Khd)-porphyritic Half Dome (Khdp) granodiorite contact but within Khd. The zone is approximately 4.5 km length (possibly longer) with variable width (tens to one hundred meters) and a 10±5° azimuth. Evidence for magma flow is apparent within the schlieren zone, and commonly later sets of schlieren overprint and disrupt earlier sets. Sparse and widely separated porphyritic andesite dikes are intruded into the schlieren belt and cut the schlieren, but are also locally mingled with the schlieren and enclosing Khd. Leucogranite sheets and dikes are intimately associated with the schlieren belt and exhibit both concordant (sharp or gradational) as well as sharp, discordant contacts with schlieren and with enclosing Khd. TIMS and SHRIMP zircon dating from a late discordant dike that cuts the schlieren gives an age of 89 to 90 Ma (age hampered by inheritance). Petrographic and whole rock geochemical data for the schlieren indicate they are partly cumulate, whereas inter-schlieren felsic sheets and discordant felsic dikes have liquid compositions. A mafic schlieren layer and an adjacent inter-schlieren felsic sheet have identical eNd_(t) (-4.8), which is also identical to Khd from outside the schlieren zone. Discordant felsic dikes have eNd_(t) of -3.4. An andesitic porphyritic dike that is mingled with schlieren has eNd_(t) of -7.0.

We suggest that the schlieren belt marks a discrete zone of flow within the Half Dome magma chamber, and that processes responsible for schlieren development occurred repeatedly in this zone. Identical eNd_(t) for Khd, mafic schlieren and concordant inter-schlieren felsic sheets suggests a closed-system relationship but other geochemical data preclude a simple unmixing scenario. The belt was probably linked to a zone of injection of intermediate magma (andesite porphyry) and felsic magma (discordant felsic dikes) that may have mixed in this region or at a deeper level to form batches of the main Khd magma.