EMPLACEMENT AND STRUCTURE OF THE SOUTHERN MARGIN OF THE TUOLUMNE INTRUSIVE SUITE, YOSEMITE NATIONAL PARK, CALIFORNIA

The ~88-93 Ma Tuolumne Intrusive Suite (TIS) is one of the world’s classic zoned intrusive bodies, yet its emplacement mechanisms are still under debate. The southern margin of the TIS intruded Cretaceous plutonic and Jurassic(?) meta-sedimentary rock, forming generally sharp and steep contacts around the NW corner of a large salient of host rock. In this area the TIS comprises the marginal Kuna Crest Granodiorite and the more internal Half Dome Granodiorite. Host rock units include the Mount Clark Leucogranite to the north, the central Grey Peak Leucogranite and Red Devil Granodiorite, and the Red Peak Granodiorite to the south.

Throughout the TIS and host rock, lineation pitch and foliation dip are steep. A N-S striking solid state foliation, which formed at a high angle to the E-W striking internal contacts, pervades the host rock and probably formed from regional strain. Foliations in the Kuna Crest unit trend ~NNE and are a result of solid state and magmatic deformation. Ductile shear zones in this unit are parallel to host rock contacts and display dextral and reverse displacements. The Half Dome unit has two magmatic foliations; the dominant foliation generally trends NW and probably records regional strain, while the weaker one is parallel to external contacts for distances up to 2.3 km in from the contact, and probably resulted from strain during emplacement.

Structures in the host rock and TIS provide evidence for emplacement mechanisms. The N-S striking host rock foliation is deflected to an E-W strike in a 450 m wide zone next to the northern host rock-TIS contact, presumably as a result of emplacement of the TIS. Along the western margin of the host rock salient, the N-S trending contact of the TIS discordantly cuts the E-W striking internal contacts of the host rock. This discordance is compatible with stoping. Meta-sedimentary xenoliths, blocks of diorite displaying cumulate layering, and wall rock fragments are found close to the contact with the Red Peak body, and are further evidence for stoping. The Half Dome body cuts out the Kuna Crest body along the northern corner of the contact with the host rock, also a possible result of stoping. These field data show that stoping and ductile host rock deformation operated as material transfer processes during emplacement of the southern part of the TIS.